ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. Published data on the preparation, structure, physical and chemical properties, and reactivity of nitrosoalkenes are described systematically. The influence of the structure and stereoelectronic effects of substituents at the double bond on the stability of nitrosoalkenes is analysed. Primary attention is paid to concerted cycloaddition of nitrosoalkenes to miscellaneous multi- ple bonds, in particular, to the problems of regio-, stereo-, and enantioselectivity of [4+2]-cycloaddition of nitrosoalkenes to alkenes.The bibliography includes 183 references. I. Introduction The nitroso group is a powerful electron-withdrawing substituent; this has a crucial effect on the chemical properties of all con- jugated systems containing this group. In particular, the nitroso group is known to promote efficiently nucleophilic substitution in benzene derivatives, and in some cases its activating influence is even more pronounced than that of the nitro group.1, 2 The main methods for synthesis, physical properties, struc- tures, and reactivities of conjugated nitrosoalkenes are considered in only one review covering publications which appeared before 1983.3 However, more recently, some important aspects of the chemistry of these compounds have been further developed, most of all, owing to the efforts of the research groups headed by Reissig and Gilchrist.In the present review, we generalise and describe systematically the data published up to the end of 1997 including those concerning participation of nitrosoalkenes in concerted processes, primarily, in [4+2]-cycloaddition to alkenes.The formation of nitrosoalkenes as reactive intermediates was hypothesised as early as 1898;4 however, it was not until 1960 that synthesis of the first stable compound of this type, trifluoroni- trosoethylene, was reported.5 Since then, only a few stable nitro- soalkenes have been prepared; they usually contain bulky aryl, tert-alkyl, or halo substituents at the b-carbon atom.Nitrosoal- kenes in which the b-carbon atom is included into a heterocyclic system can also be put into this group. However, there is strong evidence indicating that many nitro- soalkenes can exist in solutions as unstable intermediates. They comprise a large number of b-alkyl- and b-aryl-substituted nitro- soalkenes, several halo-substituted derivatives, and only few compounds containing OR or NR2 groups at the b-position. Some of these intermediates have been identified by spectroscopy, while others have been detected on the basis of their characteristic blue colour.The lifetimes of these compounds vary a over wide range. Finally, there exist nitrosoalkenes, which cannot be detected due to their instability.The formation of these compounds is judged based on experiments on their trapping. Thus all the known intermediates of the general formula CH2=C(R)NO belong to this group. The properties of typical representatives of nitrosoalkenes 1 are listed below in Table 1. This review consists of three parts. The first part is devoted to the methods for the synthesis of nitrosoalkenes, and the second one deals with their physical properties and structures.The third part describes chemical transformations of nitrosoalkenes, classi- fied according to the type of reaction. II. Methods for generation of nitrosoalkenes 1. 1,4-Elimination in a-substituted oximes under the action of bases In recent years, the chemistry of nitrosoalkenes 1 (Table 1) has been developing especially successfully, mostly owing to the elaboration of a simple and reliable method for their generation from available precursors, a-substituted oximes.This method has been used to prepare all the compounds listed in Table 1, except for trifluoronitrosoethylene 1a. The chloride ion is the most frequently used leaving group; however, other halides are also suitable.Since nitrosoalkenes are sensitive to nucleophilic attacks, it is very important to choose appropriate conditions for their generation. The reaction should be carried out using weakly nucleophilic solvents and bases, except for those cases where they are used to trap nitrosoalkenes. Stable nitrosoalkenes have been synthesised using tertiary amines, Et3N6 and 1,5-diazabi- 7BH,7X7 N R1 R3 R2 X O H B7 R2 R3 R1 NO 1 IMLyapkalo, S L Ioffe N D Zelinskii Institute of Organic Chemistry, Russuan Academy of Sciences, Leninskii prosp. 47, 117913 Moscow, Russian Federation. Fax (7-095) 135 53 28. Tel. (095) 135 53 29 Received 30 July 1997 Uspekhi Khimii 67 (6) 523 ± 541 (1998); translated by Z P Bobkova UDC 547.331 Conjugated nitrosoalkenes y IMLyapkalo, S L Ioffe Contents I.Introduction 467 II. Methods for generation of nitrosoalkenes 467 III. Structure and physical properties of nitrosoalkenes 472 IV. Reactions of nitrosoalkenes 473 y The review is dedicated to Professor Seebach in connection with his 60th birtday. Russian Chemical Reviews 67 (6) 467 ± 484 (1998) #1998 Russian Academy of Sciences and Turpion Ltdcyclo[4.3.0]non-5-ene (DBN).7 Unstable intermediates are better generated by treatment of a-halo oximes in non-nucleophilic organic solvents with insoluble inorganic bases such as Na2CO3 17, 18 or Ca(OH)2.10 These conditions ensure maintenance of a very low steady-state concentration of the highly reactive intermediates 1 and, as a consequence, suppression of the unde- sirable side processes involving more than one nitrosoalkene molecule, for example, polymerisation.Denmark et al.19, 20 proposed a very interesting and useful modification of this method, namely, generation of nitrosoalkenes under `neutral' conditions by treatingO-trialkylsilyl derivatives of a-halo oximes with fluoride ions in weakly nucleophilic aprotic solvents. The best results were obtained with CsF and KF in MeCN;20 however, AgF or Bun4 NF can also be used.One of the synthetic modifications of the general method for the generation of the compounds 1 is rupture of the C±O bond in the isoxazoline ring of compound 2 as a result of deacetylation of the oxyimino group on treatment with a base and subsequent elimination of Br7.21 Since a-halo oximes are important precursors of nitrosoal- kenes, we considered it pertinent to outline here the three main methods for the synthesis of these compounds.The first method is based on the reaction of the corresponding halo ketones with hydroxylamine. In particular, a-chloro oximes are prepared from a-chloro ketones and hydroxylammonium chloride in protic solvents.19 To synthesise a-bromo oximes, hydroxylammonium sulfate is used in order to avoid halogen exchange.22 It is undesirable to use alcohols as solvents, because they can substitute the halogen if the reaction time is long.16 According to the Denmark modification,19 O-trialkylsilyl deriva- tives of a-chloro oximes are prepared by condensation of the corresponding chloro ketones with O-trialkylsilylhydroxylamine in CHCl3 in the presence of 4 A molecular sieves.The second method includes interaction of nitro alkenes with SnCl2 23 or TiCl4.8 It is used to prepare some a-chloro oximes or a- chlorohydroxyiminoyl chlorides, respectively. The third method is also applicable only to the synthesis of chloro oximes. It is based on the addition of nitrosyl chloride to alkenes,24 ± 29 and was first proposed more than a hundred years ago 26 for the synthesis of crystalline derivatives of terpenes.In the 60s, this reaction was extended to a broad range of nucleohilic and electrophilic alkenes mostly in the studies by Ogloblin et al. 27 ± 29 It is a convenient pathway to chloro oximes of the type ClCH2C(R)=NOH [R = C(O)H, Ac, C(O)Ph, and CN]. The reactions of alkenes with NOCl yield initially b-chloro nitroso- compounds, which can subsequently dimerise to give compound 3 and/or rearrange into a-chloro oximes 4.The latter route is possible if the carbon atom bound to the nitroso group carries a hydrogen atom (Scheme 1).24, 25 The reactions with ordinary alkenes can be classified as electrophilic substitution, and the adduct is formed in conformity with the Markovnikov rule.24, 25 The fact that nitrosyl chloride adds unexpectedly rapidly to some a,b-unsaturated carbonyl compounds 27 ± 29 might be due to preliminary coordination of the nitrosonium cation (NO+) to the oxygen atom of the carbonyl group.Scheme 1 If there are substituents at the C=C bond of the substrate molecule, able to stabilise efficiently the carbocationic site result- ing from the attack by the NO+ cation, the reaction product has an ionic structure. The reactions shown in Scheme 2 are interesting examples of the generation of ionic intermediates of this type.30 [1,5]-shift of hydrogen N O Me Br O Ph 7OH 2 N H Me Ph CO2H O NOH Me CO2H Ph +NOCl C C C NO C Cl O7 + + C N O7 N 3 C Cl C Cl C C NOH 4 C Cl Table 1.Typical representative nitrosoalkenes 1. Alkene R1 R2 R3 Properties Ref. 1 a F F F blue gas, 5 stable up to 100 8C b a H Mes Me blue 6 crystals, m.p. 47 ± 58 8C c But But H blue 7 crystals, m.p. 38 8C d Cl Cl Me can be 8 isolated at room temperature e Cl Cl CCl=CCl2 m. p. 44 8C 9 at 0.02 mm Hg f H 7(CH2)47 blue 10 crystals at750 8C, decomp. above730 8C g a H Cl Cl long-lived 8 in solution h Cl Cl H the same 8 i a H Ph Me detected in 11 solution (colour) j a Me Mor b Ac the same 12 k H H H lifetime in the 13 gas phase 30 s l H H Ph the existence 14, 15 was confirmed by trapping and by kinetic studies m H H Ac detected only 16 by cycloaddition n H H CF3 the same 17 a The stereochemistry at the C=C bond was not studied; b Mor= N O 468 IMLyapkalo, S L IoffeScheme 2 The nitrosation of the double bond in compound 5 by nitrosyl chloride or the acid-catalysed reaction of alkyl nitrate with the immonium cation 6 give compounds 7a,b.To prepare nitro- soalkene 8, the nitrosation product is treated with a base. During the transformations presented in Scheme 2, the sterically hindered radical site remains intact.30 The type 3 dimers of b-chloro nitroso-compounds have also been used as precursors of some nitrosoalkenes.31 ± 34 Thus the reaction of Bun3 N with dimers of chloro nitroso-compounds prepared from styrenes affords only orange dimers of b-nitroso styrenes 9.33 There are only few examples of generation of nitrosoalkenes from oximes with leaving groups other than halide ions.Thus products of the addition of nitrosyl sulfate (NO+HSO¡4 ) to alkenes have been used as the initial compounds for the prepara- tion of nitrosoalkenes.35 The nitrite ion can also serve as the leaving group.7 Yet another interesting variant of this approach is cleavage of the oxirane rings in a-epoxy ketoximes on treatment with the cuprate reagent to give compound 10.36 The acid-catalysed dehydration of sulfoxide 11 involves presumably the intermediate formation of the nitrosoalkene 1o.37 However, as far as we know, the direct dehydration of a-hydroxy ketoximes has not been used as a method for gener- ation of nitrosoalkenes. 2. Reactions of nitrogen(II) oxide with vinylic radicals Trifluoronitrosoethylene 1a was obtained by photolysis of tri- fluoroiodoethylene 12 in the presence of nitrogen(II) oxide.5 It has been assumed that the reaction product is formed upon recombination of trifluorovinyl radical with NO.To date, the compound 1a is the only vinylic nitroso derivative prepared in a pure state using this type of reaction; however, in a number of other syntheses, nitrosoalkenes have been postulated as reaction intermediates. Thus acetylene reacts with hydrogen atoms and NO to give HCNand formaldehyde resulting apparently from rearrangement and fragmentation of the intermediate nitroso ethylene 1k.38 Several more reactions of this type have been reported.38, 39 For example, nitrosoalkene 13 was postulated as an intermediate in the reaction presented below:39 3. 1,3-N,C-elimination of trialkylsilanols from silyl nitronates The reactions of 1,4-elimination of HX from a-substituted oximes under the action of bases is the most important and the most general method for the generation of nitrosoalkenes 1.It is curious to note, however, that the `alternative' possibility of 1,3-elimination of HX from compounds like 14 as a route for generation of nitrosoalkenes has not been specially considered until recently. The possibility of this transformation was first hypothesised by Seebach et al.40 They postulated 2-nitrosohept-1-ene 1p as an intermediate in the reaction of tert-butyldimethylsilyl nitronate 14a with excess organolithium reagent.The first equivalent of RLi reacts as a base facilitating the abstraction of tert-butyldimethyl- silanol, and the second equivalent reacts with the resulting nitro- soalkene 1p as a nucleophile, and this leads to the final reaction product, dialkyl ketoxime 15.40 5 N N O NOCl MeOH AmONO, HCl HO7 X=Cl (a), OSO2OMe (b). 7a,b + N N NOH X7 O N N NO O 8 + 7 6 N N OSO2OMe O Bun3 N + + Ar Cl O7 N O7 N Cl Ar + + 9 Ar O7 N O7 N Ar N OH O NO OLi Me2CuLi Me2CuLi Et2O,725 8C N OH OH Me 10 S But Me SMe NOH 11 NO MeS But MeS 1o O H+ I F F F 12 +NO NO F F F 1a hv 7I HC CH H NO H H H 1k H H H NO H2C O+HCN. O N H H H +HCN. O N O O O O O H NO O H NO 13 1 NO R1 R3 R2 7HX + 14 N R1 R3 R2 H X O7 + Me(CH2)4 Me OSiMe2But N O7 14a RLi, THF Conjugated nitrosoalkenes 469Later, in studies by Ioffe et al.,41 ± 44 b-functionally substituted nitrosoalkenes 1 have been considered as the most probable intermediates for trimethylsilylation of nitro compounds contain- ing electron-withdrawing groups at the b-position (Scheme 3).In the authors' opinion, these substituents facilitate spontaneous fragmentation of the intermediate trimethylsilyl nitronates 14.The fate of the resulting nitrosoalkenes 1 depends on their structure, in particular, on whether or not protons or electron- withdrawing groups are present at the g-position of the initial nitro compound.44 The final reaction products are either O-tri- methylsilyl-derivatives of a,b-unsaturated oximes 16 [pathway (a)] or N,N-dialkenyl-N-trimethylsilyloxyamines 17 [pathway (b)].The reaction follows pathway a if the molecule 1 contains two electron-withdrawing groups (R2, R4) such as CN or COOMe. In the latter case [pathway (b)], silyl nitronates 14 not only serve as the source of nitrosoalkenes 1 but also react with them according to the pattern of 1,2-addition to the nitroso group.Recently, the generation of silyl nitronates 14 and then of nitoso alkenes 1 during the formation of type 17 products was confirmed by selective trapping of these intermediates in the silylation of methyl 3-nitropropionate.45 The above sequence of reactions confirms the possibility of generating nitrosoalkenes 1 from silyl nitronates 14.This is fundamentally important, since the compounds 14 are readily accessible and can prove to be convenient precursors of nitro- soalkenes, especially, of b-functionally substituted ones, which have scarcely been studied. 4. Nitrosation of C=C bonds enriched in electrons When dithiafulvenes 18 are treated with amyl nitrite, nitroso- compounds of the type 19 are readily produced.46 The ease of nitrosation is due to the presence of the electron- donating heterocyclic fragment, which efficiently stabilises the positive charge on the endocyclic C(2) atom by means of aroma- tisation. The reaction is accelerated by catalytic amounts of acids and inhibited by tertiary amines; therefore, it is believed that at the first stage, the nitrosonium cation is generated.However, in the opinion of the researchers cited, 46 the electrophilicity of the NO+ cation is insufficient to accomplish nitrosation by an electrophilic mechanism; therefore, as the most probable reaction route, they proposed a scheme involving single-electron transfer. Selenium- 47 and tellurium-containing 48 analogues have been synthesised in a similar way. Nitration or nitrosation of trithiapentalene 20a has given unsaturated nitroso-derivative 21a, the first representative of 2,4-epidithio-1-nitrosobuta-1,3-dienes.49 RLi, THF 1p Me(CH2)4 NO 15 Me(CH2)4 NOH R MeO2C NO2 BSA O N EtO CO2Me N O OSiMe3 CO2Me MeO2C [3+2]-cyclo- addition CO2Me 1q CO2Me O N 14b OSiMe3 MeO2C + N O7 7Me3SiOH [4+2]-cyclo- addition OEt R1=Ph, R2=H (a); R1=R2=CO2Me (b). AmONO CH2Cl2 S S R1 R2 Ph NO 19a,b S S R1 R2 CHPh 18a,b 19+AmOH+NO+ + S S R1 R2 Ph NO AmONO 18 + S S R1 R2 Ph NO NO+ X=Se, Te.AmiONO CH2Cl2 X X Ph CHPh X X Ph Ph NO BSA NOH R3 R2 R4 N O H R2 R4 R3 NO2 R1 R3 R2 BSA 7Me3SiOH 1 NO R1 R3 R2 a R2 N R1 OSiMe3 R1 R2 NO2 7Me3SiONO + R2 N R1 7O H R2 R1 b R3=H 1 R2 N R1 OH R1 R2 BSA BSA is N,O-bis(trimethylsilyl)acetamide; (a) R1=CH2R4; R2, R4 are electron-withdrawing groups; (b) 1,2-addition of 14. 14 + N R1 R3 R2 O7 OSiMe3 16 NOSiMe3 R3 R2 R4 17 R2 N R1 OSiMe3 R1 R2 Scheme 3 470 IMLyapkalo, S L IoffeAt R1 = R2 = Ph, the introduction of a nitroso group is accompanied by the transformation of the thiobenzoyl substituent into benzoyl (compound 21a).49 The formation of 2,4-epidithio-1- nitrosobuta-1,3-dienes 21b ± d from the corresponding trithiapen- talenes 20b ± d occurs with retention of the thiocarbonyl frag- ment.49, 50 The CS2Me group in the compound 21b is removed on treatment with mercury(II) acetate.49 The nitrosation of the corresponding 2,4-epidithiobuta-1,3-dienes yields the compounds 21a 49 and 21f.51 5.Other methods Other methods are as yet of limited utility. Examples of generation of nitrosoalkenes from substrates having some specific structural features have been reported; in some cases, the arguments for the intermediate formation of nitrosoalkenes are based on the analy- sis of the final reaction products and, hence, they are not always convincing.For example, 2-azidopyridineN-oxides 22 have been shown to undergo a number of transformations on exposure to radiation or on heating above 85 8C.52 ± 54 It is assumed that upon ring opening and concerted elimination of N2, the compounds 22 can be converted into conjugated nitroso dienes, which then cyclise.When a substituent is present at the 3-position, two types of products can be formed, namely 6H-1,2-oxazines 23 and cyclic nitrones, 2H-pyrrole N-oxides 24. When R1 = Cl or Br, the compounds 23 can undergo subsequent transformations in the presence of nucleophilic solvents.53 When R1 = Me or H, the oxazines 23 are kinetically controlled products, whereas the nitrones 24 are thermodynami- cally controlled products of cyclisation of the intermediate nitroso dienes.54 Similar transformations have also been observed for azidopyrazine 1-oxides 25 52 and azidoquinoxaline 1,4-dioxides 26.55 In the opinion of Abramovich et al.,54 in the latter case, a cyclic nitrone of type 24 is the kinetically controlled product, and 1,2-oxazine of type 23 is the thermodynamically controlled product.The generation of nitrosoalkenes 27 is accompanied by isomerisation of the carbon skeleton of the substrate.56 One more particular example of formation of nitrosoalkenes is retro-[2+4]-cycloaddition, which is facilitated by the formation of the stable aromatic structure of substituted indole.57 Apparently, the reactions of sulfonium ylides with nitrile oxides also proceed via nitrosoalkenes, which then react with nucleophilic ylides according to the addition ± elimination reac- tion pattern or addition followed by cyclisation.34, 58 ± 61 An example of this type of reaction is presented below: Indian researchers 62 have shown that the addition of gaseous NOCl in CH2Cl2 at 0 8C to the triple bonds in methyl alkynoates affords the corresponding b-chloro nitrosoalkenes.They found that nitrosoalkenes of type 28a can isomerise to the corresponding chloro ketene oximes 29. However, it should be mentioned that this isomerisation has never been observed for other nitrosoal- kenes.(a) HNO37AcOH; (b) [HONO]; (c) Hg(OAc)2. R1=Ph: R2=Ph (a), SMe (b), NMe2 (c); R1=R2=SMe (d). c R1=Ph, R2=SMe 20a7d S S S R1 R2 a or b R1=R2=Ph b 21b7d 21a S . . . . . S O N Ph O Ph S S R Ph b R=PhCO (a), NO2 (f); S . . . . . S O N R1 S R2 21a,f S S N Ph R . . . . . O 21e S Ph S N . . . . . O + N R1 N3 O7 R2 22 D or hv R2 N CN R1 O N O R1 CN R2 23 + N O7 R1 CN R2 24 R1=H R2 N CN OH 25 +N N N3 O7 26 + + N N N3 O7 Me O7 MeO N O7 Br MeO NO 27 X=COMe, CO2Et.N + Me N N O X Me 1408C NO X + Me2S CH¡2 O 7 PhC O +N + O S N O7 Ph Me Me 7DMSO + NOH Ph N O Ph 1l NO Ph + Me2S CH¡2 O +S O N Me Me O7 Ph 7DMSO R=H (a), Alk (b); R1=(CH2)nCO2Me. 28a,b RC CR1 NOCl RC(Cl) C(R1)NO+R1C(Cl) C(R)NO R=H C R1 Cl NOH 29 Conjugated nitrosoalkenes 471Condensation of oximes 30 with allylamine 63 or benzyl- amine 64 makes it possible to isolate moderately stable nitro- soalkenes 31 as highly coloured crystals.The structure of the compound 31a was confirmed by X-ray diffraction analysis.64 Methylation of 5-hydroxyiminomethyl-1,2,3-thiadiazole fol- lowed by treatment with a base affords a stable nitrosoalkene in a low yield.65 Oxidative dimerisation of a,b-unsaturated steroid oxime 33 gives rise to nitrosoalkene 34, which is formed apparently by a radical mechanism.66 Deoxygenation of perchloro-2-nitrobuta-1,3-diene upon treatment with R1P(OR)2 results in the formation of perchloro- 2-nitrosobuta-1,3-diene 1e (see Table 1).The yield of this product is not indicated in the study cited.9 Gas-phase thermolysis of nitroethylene on Fe or Ni at 883 K yields nitrosoethylene 1k, together with MeCN.67 There exist two groups of compounds that can serve as close precursors of nitrosoalkenes, owing to their structural features.The first group comprises N,N-bis(lithioxy)enamines 35, which were first described by Seebach et al.,68 and the second group includes N,N-bis(trialkylsilyloxyenamines) of type 36.It was shown 68 that treatment with benzoyl chloride of 1-[N,N-bis(li- thioxy)]amino-2-phenylethylene, the product of double deproto- nation of 1-nitro-2-phenylethane, gives a polymer. This was explained by assuming the generation and subsequent polymer- isation of b-nitrosostyrene under the reaction conditions. How- ever, no additional evidence supporting this hypothesis was presented.68 Moreover, b-nitrosostyrene had been described ear- lier 33 as a stable crystalline dimer.N,N-Bis(trialkylsilyloxy)enamines 36 were first described 69 as products of double silylation of aliphatic nitro compounds. The thermal instability of the compounds 36 (Alk = Me) observed in some cases was explained by possible easy elimination of (Me3Si)2O giving nitrosoalkenes, which then polymerise.How- ever, the attempt to trap nitrosoethylene 1k as the corresponding adduct with cyclopentadiene during decomposition of the enam- ine 36 (R1=R2=H, Alk=Me) failed.69 Nevertheless, compounds 37 which have become quite avail- able by now, are of interest as initial compounds for the generation of nitrosoalkenes via anionic intermediates of the semiacetal type by treatment with reagents possessing nucleophilic properties with respect to the trialkylsilyl group (see Sheme 3).Previously this route for the construction of a nitroso group has been realised during ring opening in N-trimethylsilyloxyisoxazolidines.41, 71 III. Structure and physical properties of nitrosoalkenes As was to be expected, stable nitrosoalkenes, the properties of which are listed in Table 1, have been characterised most compre- hensively by spectroscopic data.Spectroscopic characteristics of long-lived nitrosoalkenes have been described systematically.8 The blue colour of these compounds is due to absorption in the visible region (lmax = 675 ± 795 nm) with low extinction coefficients (e=20 ± 60).19 Absorption is also observed between 250 and 350 nm (in some cases, two maxima can be distinguished).IR spectra of nitrosoalkenes contain two absorption bands in the region 1420 ± 1660 cm71. A band in the range 1420 ± 1480 cm71 corresponds to the stretching vibrations of the N=O bond (for 2,2-di-tert-butylnitrosoethylene 1c 7 this band occurs at 1485 cm71). The same region contains the absorption bands for the N=O bonds in other monomeric nitroso compounds.72 The absorption at shorter wavelengths (1500 ± 1600 cm71) was assigned to stretching vibrations of the C=C bond.8 In the 1H NMR spectra of the compounds 1b and 1f (see Table 1), the vinylic protons account for signals at d 9.13 ppm 6 and 8.68 ppm,10 respectively.In the case of the nitrosoalkene 1c, the signal due to the proton at the carbon atom bound to the nitroso group manifests itself in a substantially higher field (d 6.30 ppm ).7 Thus, spectral data attest to the occurrence of appreciable p-conjugation between the C=C bond and the nitroso group.The stability of nitrosoalkenes markedly increases in the presence of halogen atoms or bulky aryl or tert-alkyl groups (see Table 1), as well as upon formation of strong intramolecular hydrogen bonds (compound 31b) 63 or complexes with transition metals (compound 38).73 Ar=4-ClC6H4: R=Ph (a), CH=CH2 (b).O +H2N R 7H2O HON ArNH O Me 30 HON ArNH O N Me R 31a,b NH Me O R ArNH N O 32 S N N O N Me 1. Me3O+BF¡4 2. K2CO3, H2O S N N NOH 33 NOH AcO Pb(OAc)4 PhH, D N O 34 ON NO R1=Ph, R2=H, R3=PhCO (37a); SiAlk3 (37b).polymer R2 N O R1 35 R2 N OLi OLi R1 R2 N(OSiAlk3)2 36 R1 R2 N OR3 O7 R1 37a,b R3Cl F7 or AlkO7 472 IMLyapkalo, S L IoffeThe high stability of 2,4-epidithio-1-nitrosobuta-1,3-dienes 21, the nitroso compound 32, the nitroso fulvenes 19, and their selenium- and tellurium-containing analogues is due to the fact that the b-carbon atom of the nitrosoalkenyl fragment and the electron-donating substituents attached to it are incorporated into a common 6p-electron heterocyclic system.The structure of the nitrosobutadienes 21a,f was unambigu- ously determined based on X-ray diffraction analysis. It was found that the S . . .O interatomic distance is much shorter than that expected on the basis of the van der Waals radii of the S andO atoms.74 ± 76 The abnormally short distance between the sulfur and oxygen in the nitrosobutadienes 21 is explained by the ability of the formally divalent sulfur atom to be involved into additional binding, which can be accomplished either with participation of the sulfur d orbitals or due to the formation of a three-centred electron-enriched S ± S ±O bond.75 It is noteworthy that, accord- ing to X-ray diffraction data, the S .. .O interatomic distance formed upon coordination of the nitroso group to sulfur is markedly shorter than that arising upon coordination to a carbonyl 76 or nitro group.74 The configuration of the C=C bond in the compounds 21 is determined by the fact that sulfur coordinates preferentially to the oxygen atom of the nitroso group rather than to other groups (C=S, C=O, or NO2).51 In terms of the valence-bond method, the S .. .O binding can be represented by the following canonical structures: The CNDO/2-SCF-MO calculations 77 for a hypothetical structure of aldehyde 39 demonstrated the possibility of this binding. The X-ray diffraction 78 and 15N NMR spectroscopy 79 data for 1,2,3-thiadiazole 40 also imply a thiapentalene character of this structure. Mass spectra of 2,4-epidithio-1-nitrosobuta-1,3-dienes 21b,e,f have been studied.80 Unstable nitrosoalkenes have scarcely been investigated by spectroscopy. The structure of the s-trans-rotamer of nitroso- ethylene 1k generated by pyrolysis of chloroacetaldoxime has been studied by microwave spectroscopy in the gas phase.13 The molecular orbitals of 1k have been calculated by the HuÈ ckel 56 and CNDO81 methods.The optimised structures, bond orders, and electrostatic potentials for the compound 1k and fluoronitroso- ethylenes have been calculated by the ab initio (SCF-MO) method.82 It was found 81 that the s-trans-rotamer of 1k is some- what more stable than the s-cis-rotamer. IV. Reactions of nitrosoalkenes Due to the presence of theC=C±N=Ofragment, nitrosoalkenes are highly reactive compounds; they readily undergo intramolec- ular rearrangements, react with nucleophiles, and enter into concerted cycloaddition reactions. 1.Unimolecular reactions Two types of intramolecular rearrangements of nitrosoalkenes have already been mentioned here; these are cyclisation to 1,2- oxazetines followed by fragmentation (see Section II.2) and cyclisation of nitroso-dienes to 6H-1,2-oxazines and 2H-pyrrole N-oxides (see Section II.5). Taking into account the reaction conditions described in Ref. 30, the transformation of the nitro- soalkene 8 into the final product 42 upon treatment with acetyl chloride occurs most probably via an intermediate 1,2-oxazetine rather than according to the haloform decomposition pattern, as the authors cited 30 believe.The intramolecular route of fragmentation of nitrosoethylene 1k involving intermediate formation of 1,2-oxazetine was con- firmed subsequently by ab initio (SCF-MO) calculations.83 Oxazetines have also been isolated after thermal rearrange- ment of the nitrosoalkene 1c 7 and after the reaction of bromo oxime 43 with bases.In the latter case, the intermediate formation of the nitrosoalkene 1o was assumed.37 It has been suggested 8 that cyclisation to 1,2-oxazetines followed by fragmentation is the main route of decomposition of many other nitrosoalkenes, which do not isomerise to unsaturated oximes without external nucleophiles. The examples of formation of PhCN and Ph2CO upon treatment of oximes 44 and 45, respectively, with Na2CO3 in CH2Cl2 were presented as evidence supporting this hypothesis.8 31b C6H4Cl-p N O N Me H O .. . H N . . . Ni N O Me Me O Ac Me N N 38 S O N S S O N S S O N S 40 S . . . . . N N O N Ph Me 39 S . . . . . O N O H S s-trans-1k N O s-cis-1k O N AcCl, Et3N CHCl3 8 N N O NO +N Cl7 N O NO Ac N N O NO Ac 7AcCN 41 N N O O N Ac 42 N N O O 7HCl DBN is diazabicyclononane.DBN O N But MeS MeS 43 NOH But Br MeS MeS 220 8C 240 8C O N But But 1c NO But But But O+HCN But 1o NO But MeS MeS NOH Ph Cl 44 F F NOH Cl Cl 45 Ph Ph Conjugated nitrosoalkenes 473Yet another characteristic type of intramolecular rearrange- ments is [1,5]-shift of a hydrogen atom. This is the main route of transformation of internal nitrosoalkenes having allylic hydrogen atoms.This rearrangement results in the formation of the corre- sponding unsaturated oximes in 28%± 72% yields.31 Thus it has been assumed 35 that the [1,5]-shift of hydrogen in 2-methyl-1- nitrosocyclohexene 46 gives unsaturated oxime, which has been identified 3 as having the structure 47. During the reaction, the blue colour, typical of solutions of the nitrosoalkene 46, disap- pears.When chlorinated oxime 48 is treated with Na2CO3,84 [1,5]- shift of hydrogen results in the formation of unsaturated oxime 49. The same compound is formed as a side product in cycloaddition reactions of the nitrosoalkene MeCH=C(Ac)NO 16 (see Section IV.3). It has been found 19 that the rearrangement giving unsaturated oximes can occur not only as an intramolecular transformation but also as a base-catalysed one.Thus nitrosoalkene 50 for which concerted hydrogen [1,5]-shift is impossible due to steric reasons, is much less stable than compound 51, which contains no hydro- gen atoms at C(3) (the half-life t1/2 for the compound 50 in a dilute solution is 12 min, while that for the compound 51 is 900 min).19 The scheme of transformation of the isoxazolinone 2 into the corresponding a,b-unsaturated oxime (see Section II.1) 21 and the synthesis of O-trimethylsilyl derivatives of the a,b-unsaturated oximes 16 (see Scheme 3) 44 have also been assumed to include stages involving the [1,5]-shift of hydrogen. 2. Reactions with nucleophiles In the nitroso compounds 1, the C=C double bond is conjugated with the nitroso group and is thus activated. The reactivity of nitrosoalkenes obviously resembles those of other activated alkenes, for example, conjugated nitroalkenes.The presence of p-conjugation accounts for the high electrophilicity of the b-car- bon atom in nitrosoalkenes; therefore, these compounds are especially prone to react with nucleophiles. The reactions of nitrosoalkenes with nucleophiles can be conventionally divided into two groups: (1) 1,2-addition at the N=O bond and (2) conjugate 1,4-addition. Only two examples of reactions of the former type are known.These are addition of silyl nitronates to the nitroso group in b-substituted vinyl nitroso compounds [see Scheme 3, pathway (b)] and intramolecular condensation of substituted nitrosoal- kenes 52 to the corresponding vinylimidazoles 53.63 Conjugate 1,4-nucleophilic addition is much more typical of vinyl nitroso compounds and largely determines their synthetic value.When a-halo oximes are used as synthetic precursors of the substituted oximes 54, two routes are, in principle, possible: (1) 1,4-elimination±addition with intermediate formation of nitro- soalkenes and (2) direct substitution of the halogen by a nucleo- phile by the SN2 mechanism.Is it possible to determine which of the mechanisms is realised in each particular case? This important question has been answered 55 based on a semiquantitative study of the rates of N-alkylation of azoles 55a±e by model a-bromo oximes 56a,b. The results of this study are presented in Table 2. It can be seen from the data of Table 2 that imidazole 55a and 3,5-dimethylpyrazole 55b react with a-bromo oxime 56a much faster than their less basic analogues. On passing to the bromo oxime 56b, the rate of the reaction with the compounds 55a,b N O H H H 46 47 NOH 48 NOH Ac Cl Na2CO3 Me H H2C 49 CH C NOH Ac 51 NO Me (CH2)3 50 NO Me H MeCN, D 7H2O NH N O Me R O N R O NH Me 53 52 C C N O Nu7 C C N O7 Nu BH 7B7 54 C N OH Nu C N N Me Me H 55b +56a,b NOR N N CO2Et Me Me 57c,d N N Me H 55c +56a NOH N N CO2Et Me 57e NOH N CO2Et N Me + 57f N N H 55d +56a NOH N N CO2Et 57g NOH N CO2Et N N 57i N NH 55e +56a N + NOH N CO2Et N N 57h R=H (56a, 57a), CMe2OMe (56b, 57b).NOR N N CO2Et 57a,b N N H 55a C(COOEt)CH2Br 56a,b +RON Table 2. Effect of the basicity of the azoles 55a ± e on the duration of N-alkylation with a-bromo oximes 56a,b.Substrate pKa a Duration of the 55 reaction (h) with the oximes 56a 56b a 7.10 0.2 24 b 4.12 0.5 48 c 3.32 24 b 7 d 2.52 72 7 e 2.27 72 7 Note. An azole (2.0 mmol) and a bromo oxime (1 mmol) in 25 ml of MeCN, 20 8C. a For aqueous solutions of conjugated acids. b The ratio 57e : 57f=2 : 1. 474 IMLyapkalo, S L Ioffemarkedly decreases and becomes comparable with the rates of reactions of weakly basic azoles. Evidently, the azoles 55a,b are sufficiently basic to react with the oxime 56a via the 1,4-elimination ± addition pattern involving intermediate formation of the corresponding nitrosoalkene, act- ing simultaneously as bases and as nucleophiles.Bromo oxime 56b cannot react according to the elimination ± addition scheme.In this case, the reaction occurs as direct SN2 substitution. The conclusion about the formation of ethyl a-nitrosoacrylate 1r during this reaction was confirmed 85 by its trapping. The product of reaction of the nitrosoalkene 1r with 2,5-dimethylfuran 58 was obtained in 50% yield. The reaction of the oxime 56a with a weaker base, pyrazole 55d, in the presence of 2,5-dimethylfuran yields only the corresponding N-alkylation product, cycloadduct of type 58 being totally missing.When Na2CO3 is added to a solution of pyrazole 55d and bromo oxime 56a under standard conditions, the reaction is completed over a period of 10min giving the correspondingN-alkylation product in a high yield.85 Based on these data, it was concluded that the reactions of the azoles 55c ± e with the oxime 56a and of the compounds 55a,b with the oxime 56b occur as direct SN2 substitution.The facts presented in the study cited 85 to justify the assump- tion that the mechanism of N-alkylation changes upon a decrease in the azole basicity seem quite convincing. However, in our opinion, the reaction mechanism depends not only on the basicity of the nucleophile but also on the acidity of the preceding oxime, which varies as a function of the substituent X at the C=N bond.There are weighty arguments that the reactions of the dimers of b-chloro nitroso compounds 3 with nucleophilic bases occur by the elimination ± addition mechanism. These compounds react with MeONa faster than with piperidine; however, when both bases are present, a-piperidino oximes 59 are the major reaction products.33 The experimental results rule out the direct substitution mechanism.It is obvious that the ratio of the reaction products is determined by the fact that at a fast reaction step, piperidine which possesses stronger nucleophilic properties, adds preferen- tially to the nitrosoalkene 1. Study of the reactions of syn- and anti-a-bromoacetophenone oximes with morpholine in an aqueous buffer solution also points to the intermediate formation of 1-nitroso-1-phenylethylene 1l.Reactions with both bromo oximes give anti-a-morpholino oxime 60.15 Presumably, bromo oximes undergo fast deprotonation and then eliminate Br7 in the rate-determining step yielding the intermediate 1-nitroso-1-phenylethylene 1l. Morpholine adds to the nitrosoalkene 1l, which occurs in the transoid conformation.In the majority of other cases, the mechanisms of reactions of nucleophiles with a-halo oximes and with dimers of b-chloro nitroso compounds have not been studied. Alkylation of the anions derived from 1,1-dinitroalkanes 86 or tertiary phos- phines 87, 88 with a-halo oximes proceeds apparently as the direct SN2 substitution of the halogen atom.In view of the relatively low basicity of these nucleophiles, the intermediate formation of nitrosoalkenes during their alkylation seems unlikely. Below we discuss only those examples in which either the reaction conditions strongly imply the occurrence of the elimina- tion ± addition mechanism or independent evidence for the inter- mediate formation of nitrosoalkenes exists.Examples of interaction of N-, O-, and S-nucleophiles with nitrosoalkenes are presented in Table 3. O N O CO2Et Me Me 58 N NH Me Me O Br CO2Et NOH 56a CO2Et N O 1r C C N X7 Hal O C NOH X C Hal 7H+ C X C Hal N O7 (a) (b) MeO7. NH; 3 + N O7 + N O7 C H C Cl C H C Cl a, b 1 C C NO a, b 59 + N C C NOH C NOH MeO C Ph N HO O Ph N s-cis-1l Ph N 7O Br Ph N OH O HN N 60 Ph N OH O s-trans-1l O N Ph Br Ph N O7 Br Br Table 3.Addition of N-, O-, and S-nucleophiles with nitrosoalkenes. Nitrosoalkenes Types of nucleophiles Ref. CH2=C(Me)NO NH3, R2NH 4 CH2=C(Pr)NO RCH(NH2)CO2Et 89 CH2=C(Ph)NO (1l) NH3 90 R2NH 15 SCN7 91 CH2=C(Ac)NO (1m) (H2N)2CS, H2NCH2CN 91 RCH(NH2)CO2Et 92 Me(CH2)4OH 57 CH2=C(CO2Et)NO (1r) ArSH 93 R1R2C=C(R3)NO NH3, RNH2, R2NH, 94 R1=H, Alk; NH2OH, NO¡2 , N¡3 R2, R3=Alk, cyclo-Alk R2NH, ROH, ArSH 31, 33, 35, 95 R3N 11 ClCH=CHNO RNH2, ArNH2 96 Cl2C=CHNO (1h) ArNH2 97 ArCH=C(R)NO R2NH, ROH, ArSH 31, 33 R3N 11 SCN7, EtOCS¡2 98 CH2=C(CF3)NO (1n) Et2NH, H2O 17 Conjugated nitrosoalkenes 475The reaction follows the general Scheme 4; however, the primary adducts often undergo subsequent transformations.For example, treatment of chloroacetone oxime with aqueous ammo- nia affords tertiary amine 61.4 Reactions of this type have also been observed for 1-haloalkyl phenyl ketoximes.90 The dichlor- oacetaldehyde oxime reacts with primary amines to give imines 62 resulting from elimination of HCl from the primary adduct 63;96 the chloral oxime reacts in a similar way.96 The reactions with thiocyanate ions yield 2-aminithiazole N-oxides 64 formed upon cyclisation of the primary adducts.31, 91, 98 The interaction of nitrosoalkenes with enamines and other electron-donating alkenes will be considered in Section IV.3.b among cycloaddition reactions.In this section, we shall discuss reactions of nitrosoalkenes with carbanions and also electrophilic substitution in aromatic substrates.Vinyl nitroso compounds are convenient reagents for alkyla- tion of nucleophilic carbon atoms under mild conditions. Reactive carbanions or their equivalents serving as nucleophiles in these reactions are taken in a twofold (or larger) molar excess. The first mole of a nucleophilic compound reacts as a base with liberation of an intermediate nitrosoalkene, and the second mole acts as the nucleophile.A typical example of these reactions is the generation of nitrosoalkenes followed by nucleophilic addition of organo- metallic substrates to them, which gives rise to the compounds 10 36 and 15.40 EtMgBr,11 PhMgBr,94 and EtC:CLi 99 have also been used in similar reactions. a-Bromo oximes have been used to alkylate lithium enolates prepared from substituted cyclopentanones.100, 101 The reactions of nitrosoalkenes with other stabilised carban- ions are presented in Table 4.In many cases, EtONa and EtOH are used as the base and the solvent, respectively; in some cases, Na2CO3 in CH2Cl2 93 or in ButOMe17 or piperidinium acetate in THF96 are also employed. The reaction products are 1 : 1 adducts; they exist in solutions as mixtures of linear tautomers and five- or six-membered cyclic species, as for example, the adduct 65 (Scheme 5).17 Scheme 5 Some of the addition products can undergo subsequent trans- formations.Thus the adducts obtained from the nitrosoalkene ClCH=CHNO eliminate HCl under the reaction conditions giving rise to a,b-unsaturated oximes R1R2C=CHCH=NOH.96 Some of the adducts can be converted into the corresponding 1-hydroxypyrroles by treatment with an acid.102 ± 105, 107 An exam- ple is provided by the formation of 3-acetyl-1-hydroxy-2,5-dime- thylpyrrole 66 from the adduct of chloroacetone oxime and acetylacetone.102 Reactions of sulfonium ylides yielding isoxazolines and a,b- unsaturated oximes have been described above (Section II.5).34, 58 ± 61 It seems very attractive to use nitrosoalkenes as electrophiles in aromatic substitution reactions, because they are generated under mild conditions in the absence of acids.This reaction would permit introduction of various functional groups. However, in practice, only highly electrophilic nitrosoalkenes in combination with highly nucleophilic aromatic and heteroaromatric systems are suitable for this purpose.Thus alkylation of 1,3-dimeth- oxybenzene with electrophilic nitrosoalkenes is one of the few NOH Br NOCH2Ph EtC CLi 778 8C NO NOCH2Ph EtC CLi 720 8C (85%) NOH NOCH2Ph Et R=(CH2)3CH=CH2. OLi Me OLi Me H2O Br R NOH (92%) N O Me R OH N O R O O Me Me Na2CO3 + Br CF3 NOH 65 O Me O Me CF3 HON O H O NOH Me CF3 Me O HO N O CF3 Me Me EtOH, HCl 66 OH N Ac Me Me Ac NOH Ac Me Cl NOH Cl 7RNH3Cl RNH2 7HCl 62 RN NOH Cl NO RNH2 63 NOH RNH Cl NH3 7NH4Cl Cl NOH Me 61 N Me NOH Me NOH Me HON Me NO NH2 Me NOH N Me NOH H Me NOH N Me O NH3 Me NO N OH NH R1 R2 NO SCN7 R1 R2 NOH CN S H R2 R1 NHOH S C N S R1 R2 64 + S N R1 R2 O7 NH2 Scheme 4 476 IMLyapkalo, S L Ioffereported examples of this type of reaction.16 The ratio of the isomers 67 and 68 formed in this process is 1 : 4.Alkylation of 1,4-dimethoxybenzene, N,N-dimethylaniline, and 2-naphthol with 3-nitrosobut-3-en-2-one 1m gives the corre- sponding products 69 ± 71 in low yields; less electrophilic ethyl a-nitrosoacrylate 1r does not alkylate 2-naphthol. The reactions of nitrosoalkenes with heteroaromatic systems (furans, benzofurans, pyrroles, and indoles) are more general.In this review, we classify these reactions as electrophilic substitu- tion. However, this classification is quite arbitrary (see, for example, Refs 16, 17, and 108), because for these heterocycles it is often difficult to decide between electrophilic substitution and [4+2]-cycloaddition reaction mechanisms. The reactions of pyrroles with ethyl a-nitrosoacrylate 1r involve only the 2-position.108, 109 The data about the formation of 3-substituted pyrroles 3, 16 as side products have not been supported later.108 Depending on the presence and the positions of substituents, pyrroles give 1 : 1 adducts of two types, 72 and 73.This is apparently due to proton migrations in the adducts formed initially. Unfortunately, the attempt to involve ethyl a-nitro- soacrylate 1r in the reaction with 1,2,5-trisubstituted pyrroles, which would rule out the possibility of these migrations, was unsuccessful.Ethyl a-nitrosoacrylate also did not react with less electron-donating N-(toluene-4-sulfonyl)pyrroles and pyrrole-2- carbaldehyde.108 Unlike pyrroles, indoles are alkylated by nitrosoalkenes; the reaction involves the 3-position and yields two types of adducts, 74 and 75, depending on whether or not this position contains substituent.In the case of electron-deficient nitrosoalkenes � CH2=C(CO2Et)NO (1r),16, 93, 110 CH2=C(Ac)NO (1m),16 and CH2=C(CF3)NO (1n) 17 � the corresponding adducts are formed in high yields (65% ± 88%) even from equimolar amounts of bromo oxime and indole.16 However, at elevated temperatures, the yields decrease, because the reaction becomes noticeably reversible.57 When 1-nitroso-1-phenylethylene 1l reacts with N- methyltetrahydrocarbazole, cyclic nitrone 76 and 2 : 1 adduct 77 are formed together with the main product 74.57 Alkylation of 2-substituted indoles by the nitrosoalkene CH2=C(CO2Et)NO (1r) is the key stage in the synthesis of several tryptophan derivatives.111, 112 A series of studies 113 ± 115 have been devoted to alkylation of 3-substituted indoles with ethyl a-nitro- soacrylate aimed at the synthesis of biologically active tryptophan derivatives.In some cases, the primary adducts were found to be unstable and to rearrange readily.113 ± 115 The reactions with less aromatic substrates such as furan,16, 18, 93 2-metyl-,116 2,5-dimethyl-,16, 18, 116, 117 and benzo- furan 16 always afford cycloadducts 78 and 79, which readily undergo selective cleavage on treatment with acids 16, 118 or bases 116 to give either products of oxazine ring opening or products of more extensive transformations.116, 118 3.Cycloaddition reactions Nitrosoalkenes can participate in cycloaddition reactions either as 2p-electron systems (the reaction involves the nitroso group) or as 4p-electron conjugated heterodienes (1,2-dihetero-1,3-diene sys- tems).No examples of reactions involving only the C=C bond have been found yet. However, some researchers 119, 120 have suggested that nitrosoalkenes enter into [4+2]-cycloaddition reactions with 1,3-dienes as 2pC=C components, which are fol- lowed by fast [3,3]-sigmatropic rearrangement of the resulting adducts into the final products.For example, cycloaddition of nitrosoalkenes to cyclopentadiene to give adduct 80 can be considered within the framework of this hypotheses; however, no facts supporting it have been obtained. a. Nitrosoalkenes as 2p systems in cycloaddition reactions Cycloaddition to a nitroso group acting as a 2p-electron system is typical of nitroso compounds of many classes.121 ± 123 In the case of nitrosoalkenes, this type of reaction is limited to b,b-dihalo- and b-halo-b-alkyl-derivatives.8 For example, trichloronitroso- ethylene 1s reacts as heterodienophile with various cyclic and OMe OMe + NO C(O)R 67 OMe C(O)R MeO NOH + C(O)R MeO NOH OMe 68 R H Me OEt Yield of 67+68 (%) 41 63 11 R1, R2=H, NMe2. 69 Ac MeO NOH OMe 70 Ac R1 NOH R2 Ac OH NOH 71 R=Me, Ph (72); R=H, Me (73). 72 N O N R CO2Et R 73 N CO2Et NOH R 75 N NOH R2 R1 X 74 N N O R1 R2 R3 X R, R=(CH2)4. 76 + N Me R N Ph O7 R 77 + N N O N Me O7 Ph R Ph R O N O 79 Ac O N O R1 R R2 78 R=Ar, C(O)R. [3,3] fast 80 N O R R N O + 1 ON R [4+2] slow Conjugated nitrosoalkenes 477acyclic nucleophilic dienes including butadiene,8 1-methoxybuta- diene,8 cyclopentadiene,8 cyclohexadiene,8 and benzene oxide.124 Adducts formed from cyclic dienes are thermally unstable and readily rearrange at room temperature to epoxyaziridines, sim- ilarly to the [4+2]-cycloadducts of 1,3-dienes with singlet oxygen, which are converted into 1,3-diepoxides.125, 126 The structure of the adduct derived from b,b-dichloronitroso- ethylene 1t and cyclopentadiene 81 was confirmed by recording its low-temperature NMR spectrum.127 It should be emphasised that the rearrangement to epoxyazir- idines is typical only of adducts of halo-substituted nitrosoalkenes with cyclic dienes.Cycloadducts of nitrosoalkenes with acyclic dienes,8 as well as the products of [4+2]-cycloaddition of cyclic 1,3-dienes to the nitroso groups of other types of nitroso com- pounds,121, 127 do not undergo this rearrangement.This regularity has not been explained yet. It has been shown that tetrafluoroethylene and chlorotri- fluoroethylene add to the nitroso group of trifluoronitrosoethy- lene 1a according to the [2+2]-cycloaddition pattern to give 1,2- oxazetidines.5 b. Nitrosoalkenes as conjugated heterodienes in the cycloaddition to C=C bonds The [4+2]-cycloaddition to alkenes is perhaps the most important reaction of nitrosoalkenes, which has mainly determined the vigorous development of the chemistry of these compounds in the last decade.In this reaction, nitrosoalkenes 1 act as electron- withdrawing 4p-components (conjugated heterodienes) and give rise to 5,6-dihydro-4H-1,2-oxazines 82, which are valuable inter- mediate compounds in the synthesis of various cyclic and linear polyfunctional compounds.This type of addition was observed for the first time in the reaction of cyclopentadiene 18 with 1-nitrosphenylethylene 1l. Later, this reaction route has been extended to other alkenes and nitrosoalkenes; as a rule, the latter contain unsubstituted b-carbon atoms (R1=R2=H),8, 14, 16 although this type of cycloaddition is also possible 8, 16, 45, 128 for some b-substituted nitrosoalkenes, for instance, MeCH=C(Ac)N=O(1u) 16 and MeO2CCH=CHN=O (1q).45 The reaction is highly regio- and stereoselective; in the adducts of type 80 derived from cyclopentadiene, the oxygen atom of the nitroso group is always attached to the allylic carbon atom of the cyclopentene fragment.Other linear and cyclic conjugated dienes add to nitrosoalkenes of the general type CH2=C(R)NO in a similar way.8, 14, 18, 120, 129 Another type of nucleophilic dienes, methoxyallenes 83, read- ily react with nitrosoalkenes to give 6-methoxy-substituted cyclo- adducts 84 with high regioselectivity.130, 131 The successful synthesis of these compounds opened up new opportunities for using the oxazine ring in various transformations.132 ± 136 Unfortunately, the attempt to extend cycloaddition to allene, phenylallene, and trimethylsilylallene was unsuccessful, indicating that the electronic requirements to allenes are stricter than those to alkenes.131 Electron-deficient alkenes, diethyl fumarate,16 trans-1,2- dichloroethylene,16 and maleic anhydride 137 do not form [4+2]- cycloaddition products with nitrosoalkenes.Simple alkenes, oct-1-ene,14, 16 cis-cyclooctene,16, 138 and cyclohexene,16 react only with highly electrophilic nitrosoalkenes 1 (R1 = R2 =H), containing substituents such as Ac, CHO, or 4-NO2C6H4 at the a-position; these reactions can give mixtures of regioisomers. Thus 1-nitroso-1-(4-nitrophenyl)ethylene 1v reacts with oct-1-ene giving rise to a mixture of 6- and 5-n-hexyl- substituted 1,2-oxazines 82 in a ratio of 85 : 15.14 However, the reactions of nitrosoalkenes with conjugated terminal mono- and disubstituted alkenes result in the formation of only 6-mono- and 6,6-disubstituted regioisomers 82 (R4=R5=H; R6=NR2, OR or Ar; R7 = H, Alk, or Ar).14, 18, 120 The interaction with 1,2- disubstituted alkenes follows a more complex route.Thus the reaction of trans-1-phenylpropene with nitrosoalkenes affords both regioisomers with low regioselectivity. The introduction of an electron-donating methoxy group into the para-position of trans-1-phenylpropene markedly increases the regioselectivity of this reaction; in this case, regioisomers containing the aryl group at the 6-position are formed predominantly.14 The cycloaddition of nitrosoalkenes to 1,2-disubstituted alkenes, containing anORorNR2 group as one of the substituents at the C=C bond, always yields 6-alkoxy(or silyloxy-) or 6-dia- lkylamino(or disilylamino)-substituted 1,2-oxazines, respec- tively.117, 120, 139 ± 144 The versatility and high regioselectivity of the cycloaddition of nitrosoalkenes to vinyl ethers or enamines have stimulated investigations of the synthetic potential of the reaction products, namely, 5,6-dihydro-4H-1,2-oxazines contain- ing OR or NR2 groups at the 6-position.93, 107, 117, 118, 138 ± 140,145 ± 152 It is noteworthy that allylsilanes proved to be convenient ene components for [4+2]-cycloaddition to nitrosoalkenes (triallyl- butylstannane reacts in a similar way 120).The reaction is highly regioselective and results in the formation of 6-(1-trimethylsily- lalkyl)-1,2-oxazines,120, 151, 152 which can be converted into val- uable compounds by modification of the oxazine ring 155 and also proved to be convenient starting compounds for the synthesis of g,d-unsaturated ketones and amines.120, 153 ± 155 Using the method of competing reactions, it has been found 16 that the rate of [4+2]-cycloaddition of alkenes to 3-nitrosobut-3- en-2-one 1m increases with increase in the electron-donating capacity of the substituents at the C=C bond in the alkenes; in terms of the reaction rate, the alkenes studied can be arranged in the following sequence: Reissig et al.128 studied the relative reactivities of various alkenes in more detail.They used mainly trimethylsilyl ethers of enols and chose 1-nitroso-1-phenylethylene 1l as the model nitro- soalkene.128 The following relative rate constants (k) were obtained for cycloaddition of various alkenes to 1-nitroso-1- phenylethylene:128, 156 Cl Cl Cl N O 1s + O N Cl Cl Cl O N Cl Cl Cl + 1t Cl Cl N O 81 Cl Cl N O 1 O N R1 R2 R3 + R6 R4 R5 R7 82 O N R4 R1 R2 R3 R7 R6 R5 1 2 3 4 5 6 83 OMe R1 R2 N O 84 O N MeO R2 R1 Ph Me OEt . 478 IMLyapkalo, S L IoffeThe rate constant (k) values presented above indicate that the EtO group possesses higher activating ability than the Me3SiO group. It is also noteworthy that the reactivity of 1,1-disubstituted trimethylsilyl enol ethers is much lower than that of 1,2-disub- stituted derivatives.For example, the relative reactivity of E-(1- trimethylsilyloxy)propene is *100 times higher than that of (2- trimethylsilyloxy)propene, and the enol silyl ethers containing Pri, Ph, or But groups at the a-position do not react with the nitro- soalkene 1l at all. The absence of the reaction in the latter case is due to the steric influence of substituents and, hence, it confirms indirectly the concerted mechanism of [4+2]-cycloaddition of nitrosoalkene.If the reaction occurred by a stepwise mechanism, these substituents would efficiently stabilise the positive charge or the radical site in an intermediate.128 The substantially higher reactivity of the E-isomers of vinyl ethers compared to that of the Z-isomers also deserves atten- tion.128 The ratios of the reaction rates for E- and Z-isomers [k(E)/ k(Z)] are the following: Compound The k(E)/k(Z) ratio MeCH=CHOSiMe3 26 PriCH=CHOSiMe3 25 PhCH=CHOSiMe3 >50 Me3SiOCH=CHOSiMe3 >50 MeCH=CHOEt 6 This relationship has found application in the development of an elegant method for the kinetic resolution of mixtures of the E- and Z-isomers of trimethylsilyl vinyl ethers,157 based on the difference between the rates of their cycloaddition to 1-nitroso-1- phenylethylene 1l and diphenyl ketene.157 1-Nitroso-1-phenyl- Compound (E)-85a (Z)-85a (Z)-85b Yield (%) 66 43 99 Degree of enrichment (%) >98 >99 >99 ethylene 1l generated in situ reacts selectively with the E-isomers of the silyl ethers 85a,b, whereas diphenyl ketene reacts with the Z-isomers; as a consequence, the initial mixtures of trimethylsilyl enolates become enriched in the less reactive isomers.Although alkyl ethers of enols are generally more reactive towards nitrosoalkenes than silyl ethers,128 the reaction of dihy- drooxepines 86a,b with the nitrosoalkene 1n gives fused 1,2- oxazines 87a,b as the only reaction products.This unexpected result has been explained by assuming that specific conforma- tional features of the dihydrooxepine ring prevent the lone electron pair of the endocyclic oxygen atom from overlapping efficiently with the C(5)=C(6) double bond, and hence, the C(2)=C(3) bond becomes more reactive.158 The high stereospecificity of cycloaddition confirms its con- certed mechanism.It has been established reliably that the formation of mixtures of cis- and trans-1,2-oxazines from cis- alkenes, observed in some cases, is due to some amounts of trans- isomers present in the initial alkenes. Since the rate of cyclo- addition of nitrosoalkenes to trans-alkenes is markedly higher, and the typical experimental procedure involves the use of a large excess of the alkene, the content of trans-substituted 1,2-oxazines in the reaction products can be quite noticeable.14, 16, 17, 128 The nitrosoalkenes 1 with R1 = R2 =H are extremely unstable (see Table 1); therefore, their relative reactivities can hardly be estimated.Nevertheless, some qualitative conclusions have been drawn. For example, the yields of cycloaddition products obtained from 1-nitroso-1-(4-nitrophenyl)ethylene 1v were generally higher than those obtained from 1-nitroso-1-phenylethylene 1l.There- fore, it was concluded that the former is more reactive.14 3-Nitro- sobut-3-en-2-one 1m, which is more reactive, forms adducts with cyclohexene and benzofuran, whereas ethyl a-nitrosoacrylate 1r does not react with these substrates. This reflects the general tendency of the reactivity to decrease upon replacement of the acetyl group by CO2Et.16 As noted above, sterically hindered alkenes like CH2=C(R)OSiMe3 (R = Pri, Ph, But) do not react with 1-nitroso-1-phenylethylene 1l or ethyl a-nitrosoacry- late;120, 128 however, more reactive 1,1,1-trifluoro-2-nitrosopro- pene 1n does form cycloadducts with these alkenes, although in low yields.17 The higher reactivity of the nitrosoalkene 1n com- pared to 1-nitroso-1-phenylethylene 1l was also confirmed by MNDO calculations.17 Nitrosoethylene,18 2-nitrosopropene,8, 120 and some other 1-alkylnitrosoethylenes 120 form cycloadducts in extremely low yields or do not react at all even with the most reactive dienophiles, cyclopentadiene and ethyl vinyl ether.Recently, calculations have been carried out for some cyclo- addition reactions of nitroso ketene 159 and nitrosoethylene.160, 161 It is of interest that trichloronitrosoethylene 1s readily reacts with various alkenes as a conjugated heterodiene,162 although as has been noted above, it behaves as a typical dienophile towards conjugated dienes.(E,Z)-85a,b R OSiMe3 (E)-85b Me OSiMe3 (Z)-85a,b OSiMe3 R Me OSiMe3 Ph O Ph N O Ph R OSiMe3 + + a b R=Me (a), Ph (b); (a) NOH, Na2CO3, Et2O; (b) Ph2C=C=O. Ph ClCH2 R=H (a), R=Me (b).Na2CO3 ButOMe, 20 8C 87a,b O N O CF3 R OSiMe2But N OH Br CF3 1n N O CF3 ButMe2SiO O R 86a,b Alkene k Alkene k 17 11.5 8.5 3.5 2.2 1.2 1.0 0.95 0.6 0.33 0.2 0.19 0.13 0.11 0.09 <0.01 Me OSiMe3 OEt OSiMe3 OMe OSiMe3 Ph OSiMe3 OEt Me OSiMe3 Me SiMe3 OSiMe3 Me OSiMe3 Me Me Me OSiMe3 (R=Pri, Ph, But) R OSiMe3 Conjugated nitrosoalkenes 479The reactions of 1-nitroso-1-phenylethylene 1l with norbor- nene and norbornadiene yield mainly exo-adducts.120 The diastereoselectivity of cycloaddition of alkenes to 3,4- dihydro-1-nitrosonaphthalene 88 has been studied.128 It was found that trimethylsilyloxyethylene affords almost exclusively the exo-adduct.When there are one or two alkyl substituents at the double bond, the proportion of the endo- adduct increases. In the case of 1-trimethylsilyloxycyclopentene, the endo-adduct 89 predominates among the reaction products. Evidently, in this case, the exo-approach in the transition state is sterically hindered. For methoxyallene, the steric obstacles to the exo-approach prove to be insignificant, and consequently, this alkene, like trimethylsilyloxyethylene, forms predominantly the exo-adduct (the exo : endo ratio is 92 : 8).131 The fact that the reaction of 1-nitroso-3,4-dihydronaphthalene 88 with allyltrime- thylsilane is less diastereoselective than its reaction with trime- thylsilyloxyethylene has been explained 128 by different reactive conformations of these molecules.Intramolecular [4+2]-cycloaddition of the nitroso-alkene fragment to the vinyl-ether fragment in compound 90 has been studied.20 An equimolar mixture of the E- andZ-isomers of 90 was converted into a mixture of endo- and exo-adducts 91 in a ratio of 3.4 : 1 (total yield 35%). Based on these data, it was concluded that the endo-approach is preferable.However, in our opinion, this selectivity is actually due to the fact that the Z-alkene fragment is much less reactive than the E-fragment. Arnold and Reissig 163 were the first to use nitrosoalkenes in enantioselective synthesis. They studied [4+2]-cycloaddition to optically active vinyl ethers with various optically active frag- ments and alkenyl residues. The highest diestereoisomeric excess (up to 90%) was observed for the reaction of 1-nitroso-1-phenyl- ethylene 1l with isomeric bulky norbornane derivatives 92a,b.163 Further studies along this line have shown 164 that readily available diacetone glucosyl is the most versatile optically active fragment. Compounds with the general formula 93, where R are various alkenyl groups (including the allenyl group) ensure high diastereoisomeric yields in [4+2]-cycloaddition to terminal nitro- soalkenes.The above characteristic features of the cycloaddition of nitrosoalkenes to alkenes lead to the clear idea that this is a concerted Diels ± Alder reaction with inverse electron demand.165 ± 168 The concept of this reaction, first proposed by Gilchrist,3, 14 is based on interaction of the lowest unoccupied molecular orbital (LUMO) of nitrosoalkene, acting as an elec- tron-withdrawing conjugated heterodiene, with the highest occu- pied molecular orbital (HOMO) of the alkene, which serves as the electron-donating dienophile.The regioselectivity of this reaction is determined by theHOMOcoefficients of the atoms in the alkene (the greatest coefficient characterises the b-C atom) and the LUMO coefficients of the atoms in the nitrosoalkene (the greatest coefficient belongs again to the b-C atom) (Fig. 1); the greater these coefficients, the higher the cycloaddition rate. It should be emphasised that no examples of participation of nitrosoalkenes in any Diels ± Alder reactions of the standard type have been found so far. The attempts to carry out their cyclo- addition to alkenes with electron-withdrawing substituents 16, 137 failed.It would be expedient to systematise the available arguments supporting the concerted mechanism of cycloaddition of nitro- soalkenes to alkenes: (1) there are no indications of formation of any long-lived dipolar 14 or diradical 128 intermediates; (2) the process is highly stereospecific; (3) the relative rate constants for the cycloaddition of E- and Z-isomers of alkenes are substantially different; (4) the rates of cycloaddition substantially decrease on passing to alkenes like CH2=C(R)OSiMe3; (5) there are no rearrangements during cycloaddition to strained alkenes: norbornadiene,120 norbornene,120 128, 156 and benzvalene.128, 156 In a more detailed investigation of the reaction mechanism, it was noted 128 that, despite the concerted character of the reaction, the transition state can be asymmetrical; in particular, the formation of the C±O bond can lag behind the formation of the C±C bond.This assumption was confirmed by PM3 calculations. It has also been suggested 131 that in special cases, this reaction can follow a two-step mechanism involving formation of 1,6-zwitter- ion intermediates.This occurs, for instance, when 1-nitroso-1- phenylethylene 1l adds to highly sterically strained 1-(hydroxydi- phenylmethyl)-1-methoxypropa-1,2-diene.131 89 N O 88 R2 R1 H R3 + N O R1 R2 H R3 Ratio exo-89 : endo-89 >97: 3 64 : 36 18 : 82 1 : 1 R1 R2 R3 H H OSiMe3 Me H OSiMe3 (CH2)3 OSiMe3 H H CH2SiMe3 (E)-90 N O MeO Me endo-91 N O H Me OMe endo-approach (Z)-90 N O OMe Me exo-91 N O H Me OMe exo-approach 92a 92b O CHPh2 O CHPh2 R=CH=CHMe (E, Z) (a); CH=CMe2 (b); CH=C=CH2 (c). 93a ± c OR O O O O O + + b a Z a + 7 O N Z X X + b 7 + O N Figure 1. Reaction of nitrosoalkenes with alkenes in terms of the MO LCAO method.3, 14 480 IMLyapkalo, S L IoffeThe closest analogues of nitrosoalkenes in [4+2]-cycloaddi- tion reactions are vinylnitrosonium cations 94 169 ± 172 and con- jugated nitro alkenes.173 ± 177 Apparently, the range of cycloaddition reactions of nitro- soalkenes to C=C bonds is not exhausted by the data presented above.Thus Gilchrist et al.18, 93 believe that the reaction of a-halo oximes with enamines in the presence of Na2CO3 involves generation and subsequent [4+2]-cycloaddition of nitrosoalkenes to electron-rich C=C bonds.18, 93 However, the alkylation of enamines with a-bromo oximes 152, 153 in the absence of bases should apparently be regarded as direct SN2 substitution of bromine, because the intermediate immonium salts can be isolated. The structures of these intermediates have been established by IR, UV, and 1H NMRspectroscopy.151 It was found that on treatment with bases, the immonium salts cyclise to 6-(N,N-dialkylamino)-5,6-dihydro- 4H-1,2-oxazines.This sequence of reactions is considered below in relation to the alkylation of 1-pyrrolidinocyclohexene 95 with a-bromoacetophenone oxime.151 [4+2]-Cycloaddition of nitrosoalkenes can be accompanied by the formation of side products. Thus the reactions of 1-nitroso- 1-(p-nitrophenyl)ethylene with styrene, a-methylstyrene, and a-phenylstyrene, in addition to 1,2-oxazines, affords b,g-unsatu- rated oximes 96 14 (the formation of similar products has also been reported in another study 152).c. [3+2]-cycloaddition of nitrosoalkenes to alkenes [4+2]-Cycloaddition of 1-nitroso-1-phenylethylene 1l and 1-nit- roso-1-(4-nitrophenyl)ethylene 1v to 2-methoxypropene giving rise to the corresponding 1,2-oxazines is accompanied by [3+2]- cycloaddition, which yields cyclic nitrones 97 as minor products 14 (see also Section IV.2, compound 76).It was shown in special experiments that the ratio of the [4+2]- to [3+2]-cycloadducts does not depend on the E- or Z-configuration of the a-bromo oximes, precursors of nitrosoalkenes.14 In Gilchrist's opinion,3 for concerted [3+2]-cycloaddition to occur, the nitrosoalkene molecule should assume a reactive conformation in which the lone electron pair of the nitroso- group nitrogen atom lies in a plane parallel to the plane of the pC=C bond, according to the 1,3-dipole type (Fig. 2). At the same time, it was noted 14 that the minor products 96 and 97 are formed in those cases where the possible dipolar intermediates are efficiently stabilised. However, in both cases, the product ratio proved to be insensitive to an increase in the polarity of the solvent.14 The [3+2]-cycloaddition of 1-nitroso-1-phenylethylene 1l to o-methylstyrene gives nitrone 98.178 The reaction of nitrosoalkenes considered in this Section resembles the known [3+2]-cycloaddition of azines to multiple bonds.179 d.Nitrosoalkenes in cycloaddition to C=N, C=S, and C:C bonds [4+2]-Cycloaddition of nitrosoalkenes is a general method for functionalisation of alkenes. However, cycloaddition to nitro- soalkenes is not restricted to the reactions with carbon ± carbon double bonds. Nitrosoalkenes are able to add to C=N bonds in some cyclic 1,2-oxazines to give nitrones 99 119, 180 (see also Section IV.2, compound 77) and fused 1,2,5-oxadiazines 100,181 their yields ranging from low to moderate.This reaction is not a general transformation of the oximine fragment.180 Recently, it has been shown that nitrosoalkenes react smoothly with amidines of type 101 and 102 according to the [3+2]-cycloaddition fashion giving rise to the corresponding imidazoline N-oxides 103 and 104.182 This reaction is general and, in the opinion of the researchers cited,181 occurs via formation of a stabilised dipolar intermediate. The use of the C=S bonds of thiocarbonyl compounds as a heterodienophilic 2p-component in [4+2]-cycloaddition to sub- stituted a-nitrosostyrenes has been reported.182 The reaction of trichloronitrosoethylene 1s with Ph2C=S is an example.162 + C N 94 R C O 95 N Ph NOH Br 7BH;7Br7 B7 NOH Ph +N Br7 O N Ph N R1=Ph: R2=H, Me, Ph; R1=Me, R2=Ph.R2 R1 96 N NO2 OH X=H, NO2. 97 +N X O7 Me MeO 98 + N Me Ph O7 99 + O N N O7 X Ph 100 O N Ph O N X Ar N O 103 + N N Ar N Ar 0 O7 Ph R Me2N + 101 R N + Ph NAr 0 NMe2 Ar N O 102 Me2N NAr 0 104 + N N Me2N Ar 0 Ar O7 X=H, OMe, NO2; R1, R2=Ar, PhS, R3Si. N O p-XC6H4 + p-XC6H4 N S R1 O R2 R1 S R2 N O Figure 2.Conformation of nitrosoalkene during [3+2]-cycloaddition. Conjugated nitrosoalkenes 481Trichloronitrosoethylene is capable of reacting with methyl propargyl ether to afford 4H-1,2-oxazine 105;162 however, the attempts to involve various alkynes in cycloaddition to nitroso alkynes of the type CH2=C(R)NO were unsuccessful.3, 17, 120 e. Other reactions The known chemistry of nitrosoalkenes is virtually exhausted by the information presented here.At present, there are no reliable data concerning radical addition or ene reactions of these com- pounds, although these are typical reactions of other types of nitroso derivatives. It can only be added that 1-nitrosocyclohex- ene 1f 10 and trifluoronitrosoethylene 1a 5 can undergo electro- philic addition.The addition of HCl to some nitrosoalkenes has been reported.10 Trifluoronitrosoethylene 1a is able to polymerise by a radical mechanism in solution.5 The reduction of the nitro- soalkene 8 with NaBH4 gives rise to the corresponding oxime. 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(Rome) 50 1635 (1960) 106. M Ohno, N Naruse Bull. Chem.Soc. Jpn. 39 1125 (1966) 107. T L Gilchrist, G M Iskander, A K Yagoub J. Chem. Soc., Chem. Commun. 696 (1981) 108. T L Gilchrist, A Lemos J. Chem. Soc., Perkin Trans. 1 1391 (1993) 109. T L Gilchrist, A Lemos Tetrahedron 48 7662 (1992) 110. H C J Ottenheijm, R Plate, J H Noordick, D M Herscheid J. Org. Chem. 47 2147 (1982) 111. R Plate, R J F Nivard, H C J Ottenheijm J. Chem. Soc., Perkin Trans. 1 2473 (1987) 112.J P Li, K A Newlander, T O Yellin Synthesis 73 (1988) 113. R Plate, H C J Ottenheijm, R J F Nivard J. Org. Chem. 49 540 (1984) 114. R Plate, A W G Theunisse, H C J Ottenheijm J. Org. Chem. 52 370 (1987) 115. R Plate, H C J Ottenheijm Tetrahedron Lett. 27 3755 (1986) 116. T L Gilchrist, D Hughes,W Stretch J. Chem. Soc., Perkin Trans. 1 2505 (1987) 117.E J T Chrystal, T L Gilchrist, W Stretch J. Chem. Res. (S) 180 (1987) 118. R Faragher, T L Gilchrist J. Chem. Soc., Perkin Trans. 1 258 (1979) 119. K Beck, S Hunig Angew. Chem. 99 694 (1987) 120. C Hippeli, H-U Reissig Liebigs Ann. Chem. 217 (1990) 121. J H Boyer, in The Chemistry of Nitro and Nitroso Groups (Ed. H Feuer) (New York: Wiley, 1969) Ch. 5 122. G W Kirby Chem.Soc. Rev. 6 1 (1977) 123. J Streith, A Defoin Synthesis 1107 (1994) 124. E Francotte, R Merenyi, H G Viehe Angew. Chem., Int. Ed. Engl. 17 936 (1978) 125. K H Schulte-Elte, B Willhalm, G Ohloff Angew. Chem., Int. Ed. Engl. 8 985 (1969) 126. W R Adams, D J Trecher Tetrahedron 27 2631 (1971) 127. H G Viehe, R Merenyi, E Francotte,M Van Meerssche, G Germain, J P Declercq, J Bodart-Gilmont J.Am. Chem. Soc. 99 2340 (1977) 128. H-U Reissig, C Hippeli, T Arnold Chem Ber. 123 2403 (1990) 129. G M Iskander, V S Gulta J. Chem. Soc., Perkin Trans. 1 1891 (1982) 130. R Zimmer, H-U Reissig Angew. Chem. 100 1576 (1988) 131. R Zimmer, H-U Reissig Liebigs Ann. Chem. 553 (1991) 132. C Unger, R Zimmer, H-U Reissig, E-U Wurthwein Chem Ber. 124 2279 (1991) 133. 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Chem. 709 (1992) 148. K Paulini, H-U Reissig Chem Ber. 127 685 (1994) Conjugated nitrosoalkenes 483149. R Zimmer, T Arnold, K Homann, H-U Reissig Synthesis 1050 (1994) 150. K Paulini, A Gerold, H-U Reissig Liebigs Ann. 667 (1995) 151. P Bravo, G Gaudiano, P P Ponti, A Umani-Ronchi Tetrahedron 26 1315 (1970) 152. S Nakanishi, Y Shirai, K Takahashi, Y Otsuji Chem. Lett. 869 (1981) 153. S Nakanishi, M Higuchi, T C Flood J. Chem. Soc., Chem. Commun. 30 (1986) 154. C Hippeli, H-U Reissig Synthesis 77 (1987) 155. H-U Reissig, C Hippeli Chem Ber. 124 115 (1991) 156. H Henneberger, Ph.D. Thesis, University of Wursburg, 1981 157. C Hippeli, N Basso, F Dammast, H-U Reissig Synthesis 26 (1990) 158. B Hofmann, H-U Reissig Chem Ber. 127 2337 (1994) 159. S Ham, D M Birney Tetrahedron Lett. 35 8113 (1994) 160. B S Jursic, Z Zdravkovski J. Org. Chem. 60 3163 (1995) 161. D Sperling,A Mehlhorm, H-U Reissig, J Fabian Liebigs Ann. 1615 (1996) 162. 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S E Denmark, C B W Senanyahe, G D Ho Tetrahedron 46 4857 (1990) 177. S E Denmark,M E Schute J. Org. Chem. 56 6738 (1991) 178. M K Pillay, R Jeyaraman, R K Kumar J. Indian Chem. Soc. 69 24 (1992); Chem. Abstr. 117 212 255 (1992) 179. G Tennant, in Comprehensive Organic Chemistry Vol. 2 (Ed. I O Sutherland) (Oxford: Pergamon, 1979) 180. D Mackay, K N Watson J. Chem. Soc., Chem. Commun. 775; 777 (1982) 181. A K Sharma, S N Mazumdar,M P Mahajan Tetrahedron Lett. 34 7961 (1993) 182. B F Bonini, E Foresti, G Maccagnani, G Mazzanti, P Sabatino, P Zani Tetrahedron Lett. 26 2131 (1985) 183. B G Gowenlock, K G Orrell, V Sik, G Vasapollo, M V Lakshmikantham, M P Cava Polyhedron 13 675 (1994) a�Russ. J. Gen. Chem. (Engl. Transl.) b�Russ. J. Org. Chem. (Engl. Transl.) c�Russ. Chem. Bull. (Engl. Transl.) 484 IMLyapkalo, S L

ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. Data on palladium carbonyl complexes are described systematically and subjected to a critical analysis. The methods of synthesis and data on the structures and chemical properties of mono- and poly-nuclear carbonyl-containing palladium com- plexes in which the formal oxidation state of the metal varies from 0 to +2 are discussed. Virtually all the publications on this question up to 1997 are examined.The bibliography includes 241 references. I. Introduction The interest in the chemistry of palladium carbonyl complexes is to a large extent determined by the activity of palladium and its compounds in the catalysis of reactions involving CO. Such reactions include in the first place the catalytic oxidation of CO to CO2, the synthesis of alkyl oxalates and alkyl carbonates, reactions involving the carbonylation of alcohols, saturated hydrocarbons, and aromatic nitro-compounds, etc.1 The study of the properties and reactivities of carbon-containing palladium complexes is an essential stage in the investigation of the mecha- nisms of the reactions occurring with participation of palladium catalysts.2 The carbonyl complexes of palladium and other platinum metals are of undoubted interest from the standpoint of the theory of coordination compounds, especially the nature of the bond in complex many centre systems of clusters containing the metal in the lowest oxidation states.The susceptibility of PdCl2 to reduce by carbon monoxide to the metal, i.e. PdCl2+CO+H2O=Pd+CO2+2HCl was observed more than 100 years ago.3¡¾ 5 Palladium carbonyl chlorides were among the first platinum metal carbonyl complexes synthesised.6 However, owing to their higher reactivity compared with other platinum metal carbonyls, they remained for a long time represented by a few poorly characterised compounds.The available information about their structures and reactivities was based solely on indirect data (mainly by analogy with compounds of other platinum metals).Data on the chemistry of palladium carbonyl complexes have been partly surveyed in a monograph 7 in which none of the compounds of this class have been reliably characterised. A review 8 presents the structural characteristics of two palladium carbonyl complexes�¢[Pd(dam)Cl]2CO,9, 10 where dam=bis(di- phenylarsino)methane, and Pd4(CO)4(OAc)4.11, 12 Since the end of the 1970s, the number of studies on palladium carbonyls has increased sharply. The classification of the compounds constituting the subject of the present review is based on the formal oxidation state (FOS) of palladium.The formal oxidation state of the metal atom in a complex is defined as the average charge of the metal atoms which they possess after the removal of the ligands.13, 14 It is assumed that in the leaving ligands (for example, such as H7, Cl7, O27, OH7, NH27, NR¢§2 , NR27, PR27, SnCl¢§3 , and CH¢§3 ) the electron shell of the atom linked to the metal corresponds to the shell of the nearest inert gas atom; the ligands are removed in the form of neutral molecules (CO, NO, N2, CO2, C2H2, C2H4, etc).As regards certain p-linked organic groups (allyl C3H5, cyclopenta- dienyl C5H5), they are regarded as anions, like the s-linked C3H¢§5 and C5H¢§5 groups.Mainly mononuclear compounds are considered in classical coordination chemistry. The stable oxidation states 0 and +2 are attributed to palladium. However, the study of polynuclear, mainly cluster, compounds has shown that many of them have an oxidation state of +1; there are also compounds in which the formal charge per metal atom is nonintegral.Palladium carbonyl hydride complexes, for which the exact stoichiometry of the polyhydrides can be established in by no means all cases which precludes in its turn the determination of the FOS of the metal, constitute a separate group. A separate Section has been devoted to the palladium carbonyls isolated in zeolite cells, which may function as polyanions.T A Stromnova, I I Moiseev N S Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii prosp. 31, 117907 Moscow, Russian Federation. Fax (7-095) 954 12 79. Tel. (7-095) 952 12 03 (T A Stromnova) Received 23 December 1997 Uspekhi Khimii 67 (6) 542 ¡¾ 572 (1998); translated by A K Grzybowski UDC 541.49 : 546.98 Palladium carbonyl complexes c T A Stromnova, I I Moiseev Contents I.Introduction 485 II. Activation of carbon monoxide in the coordination sphere of the transition metal and the strength of theM¡¾CO bond 486 III. Palladium(0) carbonyl complexes 486 IV. Palladium(+1) carbonyl complexes 493 V. Palladium(+2) carbonyl complexes 502 VI.Palladium carbonyl complexes containing the metal in fractional oxidation state 504 VII. Palladium carbonyl hydride complexes 508 VIII. Palladium carbonyl complexes in zeolites 509 IX. Conclusion 509 { Commemorating Professor Peter M Maitlis on the occasion of his 65th birthday. Russian Chemical Reviews 67 (6) 485 ¡¾ 514 (1998) #1998 Russian Academy of Sciences and Turpion LtdA systematic treatment of heteronuclear, palladium-contain- ing complexes in accordance with the above principles is in many cases difficult.The FOS of palladium can be comparatively readily inferred, provided that the structure of the heterometal- containing species forming part of the complex remains unchanged on formation of the compound. For example, the reaction 15, 16 of the benzonitrilepalla- dium(+2) complex PdCl2(NCPh)2 with Na[Co(CO)4] affords, after the substitution of Cl7 by [Co(CO)4]7, the trinuclear cluster Pd[Co(CO)4]2(NCPh)2, in which the palladium atom is believed to retain a FOS of +2.However, on interaction of this cluster with dppm [dppm=bis(diphenylphosphino)methane], which leads to PdCo2(CO)5(m-dppm)2, not only are the benzonitrile ligands displaced but also three carbonyl groups are lost by the two anionic carbonylmetallates [Co(CO4]7.The CO-containing com- plexes formed ultimately have to analogues in classical coordina- tion chemistry and this hinders the overall classification. The present review deals with individual heteronuclear palla- dium-containing complexes, the assignment of which to a partic- ular Section of the review is not open to doubt.Since the aim of the present review is the analysis of the methods of synthesis and the characteristics of the structures and reactivities of molecular palladium complexes with carbonyl ligands, we shall deal only briefly with data on the palladium carbonyls formed on the surface of oxide carriers. II. Activation of carbon monoxide in the coordination sphere of the transition metal and the strength of theM±CO bond In the complexes of transition metals carbon monoxide effectively stabilises their lowest oxidation states.The carbonyl complexes of each metal from the first series of transition elements have been well characterised.17 The composition of the compounds obeys the `18 electron' rule, which predicts monomeric compounds for Group IV metals, for example, Cr(CO)6, Fe(CO)5, and Ni(CO)4.The metals in odd Groups form paramagnetic [V(CO)6] or dimeric [Mn2(CO)10 or Co2(CO)8] carbonyls. Mononuclear complexes are not very characteristic of transition metals in the second and third series. These metals afford preferentially compounds in which the number of metal atoms is from 2 or 3 to several hundreds.18 ± 21 From a formal point of view, features, structure and chemical properties) common to mononuclear carbonyls and a metallic surface coated by adsorbed CO molecules are characteristic of these clusters.22 A considerable number of communications (see, for example, Refs 23 ± 27) have been devoted to the question of the stability of monomeric transition metal, especially palladium, carbonyls.It has been established on the basis of quantum-chemical calcula- tions that, among the transition metal carbonyls M(CO)6 (M= Cr, Mo, W), M(CO)5 (M=Fe, Ru, Os), and M(CO)4 (M=Ni, Pd, Pt), the metal ± carbonyl bond energy and the dissociation energy of the first carbonyl ligand are a minimum for Pd(CO)4. Calculations led to the conclusion that the p-back-donatn from the ligand to the metal makes a much greater contribution in the stable M(CO)m systems than the s-donor bond. Thus the results of quantum-chemical calculations have shown the palladium carbonyls should have a minimum thermal stability and a maximum kinetic lability among transition metal carbonyls.These properties of the Pd ±CO bond have probably in fact been responsible for the activity of palladium and its compounds in the catalysis of processes occurring with participa- tion of carbon monoxide. However, the same properties have apparently `inhibited' the development of the chemistry of palla- dium carbonyl complexes compared with other transition metal complexes.The catalytic activation of carbon monoxide entails a change in the molecular orbitals of this ligand due to coordination to the metal atom.Carbon monoxide is a weak s-donor ligand, but it can bind fairly strongly to the metal atom. The overall mechanism of the binding of carbon monoxide to a metal atom is presented in Fig. 1. According to this scheme, apart from the ligand ± metal interaction determined by the s-donor properties of the ligand, the back-donation from the occupied d orbitals of the metal to the occupied antibonding p orbitals of the ligand takes place.Evi- dently, these two bonding modes should lead to opposite effects in the resulting transfer of electron density between the metal and the ligand.28 Until recently the view that the p-acceptor properties of the ligand predominate in the formation of the metal ± carbonyl bond was dominant.This was used to account for the weakening of the C±O bond in the carbonyl ligand itself, observed on coordination of CO to metals in the lowest oxidation states, and hence for the appreciable decrease in the nCO stretching vibra- tional frequencies in the IR spectra of the compounds. However, on coordination of CO to metals in high oxidation states, for example on formation of Pd(+2) carbonyls, the predominance of the s-donor properties of the ligand over their p-acceptor properties was noted, which leads in its turn to an increase in nCO to values equal to and even exceeding the stretching vibrational frequencies of CO in free carbon monoxide.29 III.Palladium(0) carbonyl complexes All the Pd(0) complexes described hitherto have been described by convention in two groups � binary carbonyls (compounds containing only CO molecules as the ligands) and complexes with additional stabilising ligands (as a rule N- or P-donor ligands). 1. The binary carbonyls The binary carbonyls are characteristic of many Group VIII metals and are readily formed even at room temperature when the metal interacts with carbon monoxide.In contrast to these metals, palladium does not form binary carbonyls on direct interaction with CO at room or elevated temperature. The binary palladium carbonyls began to be synthesised simultaneously by two groups of investigators.30 ± 34 The method involving the joint condensation of metal vapour and CO at 4.2 ± 27 K with isolation of the carbonyl in a crystalline argon or CO matrix was employed. At 4.2 ± 10 K and for a CO content in the gas phase ranging from 100% to 10%, Pd(CO)4 was obtained.On raising the temperature and on reducing the partial pressure of CO, the formation of Pd(CO)3, Pd(CO)2, and Pd(CO) was observed. All the conclusions concerning the structure of the binary palladium carbonyls are based on the results of the study of their vibrational spectra.Analysis of the Raman and IR spectra of carbonyl molecules containing isotope-substituted CO fragments (12C16O, 13C16O, 12C18O) showed that the Pd(CO)4 molecule is tetrahedral like Ni(CO)4 (Td symmetry).33 The frequency of the stretching vibration active in these spectra (the T2 band) is 2066 cm71; the frequency of the AI vibration, active in the Raman spectrum, is 2122 cm71 and the force constant kCO=17.48 mdyn A71 in the Cotton ± Kraihanzel approxima- tion.It is of interest that the force constant kCO for the more stable Ni(CO)4 molecule is virtually the same (17.23 mdyn A71). The M±C stretching vibrational frequency and force constant are more sensitive to the nature of the central atom. The nM7C=258 cm71 for palladium tetracarbonyl, whereas the + 7 M 7 + + 7 + 7 7 + O C + Figure 1.Schematic illustration of the bonds between carbon monoxide and a metal atom. 486 T A Stromnova, I I Moiseevcorresponding band has been observed at 435 cm71 for nickel tetracarbonyl.34 Spectroscopic data have led to the conclusion that the Pd(CO)3 molecule is planar (D3h symmetry), while the Pd(CO)2 (Dh symmetry) and Pd(CO) (Cn symmetry) molecules are linear.The frequencies of the IR-active vibration are respectively 2060, 2044, and 2050 cm71 for these molecules, but the force constant kCO varies from 17.35 to 16.96 mdyn A71.32 The decrease in the force constant is correlated with the decrease in the stability of the carbonyls in the sequence Pd(CO)4>Pd(CO)3>Pd(CO)2> Pd(CO).33 Apparently the palladium mono-, di-, and tri-carbon- yls are mononuclear, the CO groups are terminal, and there is no metal ± metal interaction.31, 33 The stretching vibrational fre- quency of the terminal CO group in the mononuclear complexes is in the range 2050 ± 2066 cm71.The formation of binary carbonyls stable at temperatures up to 190 Khas been noted in the deposition of palladium from a gas phase containing carbon monoxide onto an Al2O3 surface.35 These carbonyl compounds have been characterised under high vacuum conditions by thermal desorption spectroscopy (TDS), diffraction of slow electrons (LEED), UV photoelectron spectro- scopy (UPS), and X-ray electron spectroscopy (ESCA).Another type of binary palladium carbonyls � the cationic complexes [Pd(CO)4]2+�is considered below. 2. Palladium(0) carbonyl complexes containing stabilising ligands The binary Pd(0) carbonyls are extremely unstable and decom- pose at a temperature above 20 K. The insertion into the coordi- nation sphere of palladium of additional ligands having low-lying unoccupied orbitals capable of accepting the d electrons of the central atom (for example, N- and P-donor bases) made it possible to obtain relatively thermally stable Pd(0) complexes. Carbonyl-containing Pd(0) complexes are synthesised mainly by three methods: by the reduction of Pd(+2) complexes under a CO atmosphere; by the substitution of ligands by carbon mon- oxide in preformed Pd(0) complexes; by the mild oxidation of coordinated ligands (for example, by the oxidation of phosphines to phosphine oxides, which leads to the `liberation' of coordina- tion sites and hence the formation of clusters with a greater nuclearity).Only the first method or the first method combined with the third have been used to obtain Pd(0) carbonyl complexes. When the first method was used, carbon monoxide served only in rare instances both as a reductant and as a stabilising ligand. As a rule, more vigorous reductants have been used in the reduction of Pd(+2) to Pd(0); the products of such reduction were small clusters.In order to obtain clusters with a greater nuclearity (Pd10 and above), `mild' oxidation (i.e. oxidation of only some of the isotypical ligands) has been used, leading to the liberation of coordination sites and the formation of new metal ± metal bonds. Mainly Pd(+2) and Me3N+O7 have been used as the oxidants in this instance. a.Mononuclear palladium(0) complexes The reduction of Pd(+2) in the presence of carbon monoxide and additional stabilising ligands leads as a rule to di- and poly-nuclear complexes; the number of isolated and reliably characterised mononuclear Pd(0) complexes is extremely small (Table 1).The first mononuclear Pd(0) complex was obtained by the reduction of palladium acetylacetonate with triethylaluminium in toluene at 223 K.36 Together with mononuclear complexes, the formation of the polynuclear complexes Pd3(CO)3(PPh3)3 2 and Pd3(CO)3(PPh3)4 3 has been observed. Complex 1 has also been obtained by the reduction of the phosphinepalladium(+2) chloride complex in methanol at 228 K by the reaction.37 The interaction of PdCl2(PPh3)2 and a mixture of methanol with primary or secondary alkylamine also results in the formation of complexes 1 and 3.38 However, one cannot rule out the possibility that in this instance the amine itself exhibits reducing properties.When a primary or secondary amine is replaced by a tertiary amine, a methoxycarbonyl derivative of divalent palladium, (PPh3)PdCl(COOMe), is formed.38 This com- plex has also been synthesised by Zhir-Lebed' et al.39 The formation of complex 1 has been noted in the carbon- ylation of [(Z3-allyl)PdCl]2 in the presence of PPh3 in solution in methanol or ethanol (PCO=4 atm; 293 K).40 The divalent palladium acetate complex (PPh3)2Pd(OAc)2 is not reduced by carbon monoxide in methanol under mild con- ditions.41 Only at 323 K and a CO pressure of 20 ± 50 atm is Pd(+2) reduced to Pd(0) with formation of a mixture of complexes 1 and 3.In all the studies described, complex 1 was characterised solely by elemental analysis and IR spectroscopic data. The stretching vibration frequency of the carbonyl group is fairly low (1955 cm71), which precludes an unambiguous determination of the mode of coordination of this ligand (bridging or terminal carbonyl) and it is therefore impossible to estimate the reliability of the conclusion that this compound is mononuclear.However, later investigations of the complexes Pd(CO)L3 made it possible to regard complex 1 as mononuclear with a terminal carbonyl ligand. Thus, when carbon monoxide is passed through a benzene solution of Pd(NP3) [NP3=tris(2-diphenylphosphinoethyl)- amine N(CH2CH2PPh2)3], Pd(NP3)CO 4 was obtained.42 Com- plex 4 is stable only under a CO atmosphere.Its IR spectrum contains an intense band at 1930 cm71. The presence of a band at 2820 cm71, assigned to the stretching vibrations of the CH±N bond, showed that the nitrogen atom is not linked to the metal atom.The 31P{1H} NMR spectrum indicated the equivalence of all the phosphorus atoms. On the basis of the entire set of data obtained and comparison with the data for the analogous nickel complex characterised by X-ray diffraction,43 the structure assigned to compound 4 consists of a tetrahedron in the centre of which is the palladium atom and whose vertices are occupied by the carbon atom of the carbonyl ligand and the three phosphorus atoms of tris(phosphino) amine.The complex Pd[MeC(CH2PPh2)3](CO) 5, obtained later and characterised by X-ray diffraction, has precisely this structure.44 Pd(acac)2+AlEt3+PPh3+CO Pd(CO)(PPh3)3+... 1 PdCl2(PPh3)2+NaBH4+CO 1+... Table 1. Mononuclear palladium(0) complexes. Complex Physical methods nCO Ref. used to characterise /cm71 the substance Pd(CO)(PPh3)3 elemental analysis, 1955 36 ± 38 IR spectroscopy 40, 41 Pd(CO)(NP3) a elemental analysis, 1930 42 IR spectroscopy, 31P{1H} NMR Pd(CO)[MeC(CH2PPh2)3] elemental analysis, 1919 44 IR spectroscopy XRD Ba[Pd(CN)(CO)]2 elemental analysis, 1780, 45 IR spectroscopy 1720 [RPh3P]+[Pd(CN)(CO)]2 the same 1790 45 aNP3= tris(2-diphenylphosphinoethyl)amine N(CH2CH2PPh2)3.Palladium carbonyl complexes 487Complex 5 is formed on interaction of Pd(DBA) (DBA=di- benzylideneacetone) and the triphosphine ligand MeC(CH2PPh2)3 in toluene under a CO atmosphere. It has been isolated in the form of yellow needles and characterised by IR spectroscopy (nCO=1919 cm71) and X-ray diffraction. In the molecule of compound 5, the palladium atom is linked to the carbon atom of the terminal CO group and the phosphorus atom of the triphosphine, forming a trigonal pyramid.Carbon monoxide is capable of substituting CN7 in Pd(CN)2¡ 2 , obtained by the reduction of Pd(CN)2¡ 4 .45 On treat- ment with oxygen-free dry CO in liquid ammonia (196 K), the following reaction takes place: Complex 6 has been isolated 45 in the form of a salt with PRPhá3 (R=Me or Ph) and also in the form of a barium salt and has been characterised by elemental analysis and IR spectroscopy.The stretching vibrational frequencies, which are extremely low for a terminal carbonyl ligand (nCO=1790 cm71 for the phos- phonium salts and nCO=1780 and 1720 cm71 for the barium salt), provide grounds for the assumption that in these compounds the CO group is a bridging structure between two or three metal atoms and the compounds themselves are at least dinuclear.The ability of carbon monoxide to reduce Pd(+2) in aqueous solutions has been used to obtain carbonyl complexes stabilised by 1,10-phenanthroline (Phen) or a,a0-bipyridyl (Bipy).46 When a solution of PdCl2 containing Phen or Bipy is treated with CO containing 5% of O2, a bright-red solution is obtained, from which the substance (L)Pd(CO) . 4H2O (L=Phen or Bipy) was isolated on treatment with acetone. Certain properties of this complex (good solubility in water, inability to dissolve in organic solvents resistance to the action of oxygen, etc.) provide grounds for the assumption that it contains ionogenic groups and that the oxidation state of palladium is greater than zero.b. Polynuclear palladium(0) complexes Polynuclear (cluster) Pd(0) complexes consist mainly of com- pounds containing carbonyl and phosphine ligands. Clusters comprising metals having a large number of valence electrons (nickel, palladium, and platinum) are capable of coordinating a smaller number of ligands than clusters of the preceding transition metals, which leads to the instability of their carbonyl deriva- tives.50 The coordination of tertiary amines, which produce the same number of valence electrons as carbon monoxide but ensure, by virtue of steric factors, more effective shielding of the surface of the metal core, leads to the formation of fairly stable compounds.The methods of synthesis structures and reactivities of phosphine palladium carbonyl clusters are considered below.The formation of the dinuclear Pd2(dppm)2(CO) is described in a communication 47 devoted to the synthesis of catalysts for the hydrogenation of carbon dioxide. The complex, believed to contain a bridging carbonyl group, was obtained by the reduction of PdCl2(dppm)2, with sodium tetrahydroborate in ethanol under a CO atmosphere.There are only elemental analytical and IR spectroscopic data (nCO=1820 cm71) and the conclusions con- cerning its structure are therefore insufficiently reliable. Complexes which are at least dinuclear and contain sulfide ligands in addition to the carbonyl group have been obtained.48, 49 The reduction of palladium acetate with carbon monoxide in an aqueous solution of trifluoroacetic acid in the presence of an excess of dialkyl sulfides R2S (R=Me, Et, Prn, Bun) afforded complexes with the overall composition Pd(R2S)3(CO).48 The band at 1828 ± 1838 cm71 (nCO) in the IR spectra of the complexes indicates the bridging coordination of the carbonyl ligand and hence the di- or poly-nuclearity of the complexes.Compounds with the overall composition {Pd2[RS(CH2)mSR]2(CO)(HOAc)n} have been obtained similarly from palladium acetate in the presence of sulfides having the general formula RS(CH2)mSR, where m=1, 2, or 3 and R=C3H7, C7H15, Ph, or Ph ±CH2.49 The IR spectra of all the complexes isolated contain an intense band in the region of 1800 cm71, which has been assigned to the stretching vibrations of the bridging carbonyl ligand.Measurements of the molecular masses of the complexes, whose solubility is low, did not allow a choice to be made between dinuclear and tetranuclear complexes. 3. The trinuclear complexes Pd3(CO)3L3 and Pd3(CO)3L4 Clusters with the composition Pd3(CO)3L3 are known only for very bulky ligands (L=PBut 3, PBut 2Ph). Such clusters were obtained by the interaction of coordinatively unsaturated com- plexes PdL2 and carbon monoxide.51 An X-ray diffraction anal- ysis has been carried out only for the platinum analogue Pt3(CO)3[P(cyclo-C6H11)3].52 It has been established that the cluster has a metal core shaped as an equilateral triangle (in which the Pt7Pt band length is 2.665 A), to the sides of which the bridging CO groups are coordinated.All the phosphorus atoms of the terminal phosphine ligands, one of which is coordinated to each metal atom, lie in the Pt3 plane.The interaction of M[M0(NR2)2]3 and CO afforded 53 the heteronuclear clusters M3[m-M0(NR2)2]3(CO)3 7 (M=Pd or Pt, M0=Ge or Sn, R=SiMe3), which were characterised by IR spetroscopy and NMR involving various nuclei. X-Ray diffrac- tion study of the structure of complex 7 withM=Pd andM0=Sn showed that it has a planar metal core, whose base consists of the Pd3 triangle.The bridging Sn(NR2)2 ligands are coordinated symmetrically to the sides of the triangle. The carbonyl groups are terminal (nCO=2038 cm71), one being coordinated to each palladium atom. Heteronuclear clusters of another type, Pd3Sn2(Z2-acac)4(- CO)2(PR3)3 8 (R=Ph, Et), was obtained by the interaction of the clusters Pd4(m-CO)5(PR3)4 and Sn(acac)2 in a hydrocarbon sol- ution.54, 55 Compounds 8 are sensitive to moisture and are unstable in solution. In the region of the vibrational frequencies of the carbonyl groups, their IR spectra contain only bands due to the bridging CO groups in the range 1770 ± 1890 cm71.According to X-ray diffraction data, the palladium atoms in clusters 8 (R=Ph) form an isosceles triangle, the sides of which are spanned by bridging CO groups, while the base is coordinated to two Sn(Z2- acac)2 bridging ligands. One phosphine ligand is linked to each palladium atom.In principle, the Pd3Sn2 metal core may be regarded as a `propeller' with a Pd ± Pd axis and triangular wings made up from a Pd atom and two Sn atoms.The trinuclear clusters Pd3(CO)3L4 are obtained by reducing PdCl2L2 (L=PPh3) 36 or palladium acetate with carbon mon- oxide in the presence of NaOAc and two equivalents of the phosphine ligand (L=PPh3, P(C6H4OMe-p)3].56 The structure of clusters of this type has been established in relation to Pd3(CO)3(PPh3)4.57 The molecule of the cluster consists of an isosceles triangle, to the sides of which m-CO groups are coordi- PPh2 CH2 C CH2 CH3 H2C Ph2P PPh2 Pd OC 5 Pd(CN)2¡ 2 +CO=Pd(CN)(CO)7+CN7. 6 Pd Ph3P OC CO Pd Sn(acac)2 Pd PPh3 PPh3 (acac)2Sn 8 488 T A Stromnova, I I Moiseevnated.Each metal atom in the base of the triangle is linked to one phosphorus atom, while the atom at the apex of the triangle is linked to two phosphorus atoms of the phosphine ligands.Such coordination of the phosphines leads to the nonequivalence of the carbonyl groups, which is reflected in the IR spectra: two intense bands are observed in the region of the stretching vibrations of the CO groups (1820 ± 1880 cm71). 4. The tetranuclear clusters Pd4(CO)5L4 and Pd4(CO)6L4 It was noted above that the coprecipitation of Pd and CO in vacuo at low temperatures leads to mononuclear complexes which decompose even on a slight increase in temperature.On the other hand, if hexafluorobut-2-yne F3CC:CCF3 (HFB) is added to Pd and CO, then precipitation in vacuo at 7196 8C results in the formation 58 of a volatile substance having the tentative composition Pd(CO)2(HFB), which is converted into the cluster Pd4(CO)4(HFB)3 on slow heating.The substance has been characterised by osmometric data obtained in acetone (molecular mass), IR spectroscopy (nCO=2138 and 2122 cm71), and 19F NMR spectroscopy. Pd(OAc)2 is usually employed for the synthesis of tetranuclear carbonyl clusters with phosphine ligands, but the conditions used in the preparation of compounds with aliphatic and aromatic phosphines are different.The tetranucler cluster Pd4(CO)5(PPh2Me)4 was obtained 58 by reducing Pd(NO2)2(PPh2Me)2 with carbon monoxite in meth- ylene chloride. The cluster Pd4(CO)5(PPh3)4 9 was obtained later in a similar reaction.59 X-Ray diffraction analysis of Pd4(CO)5(PPh2Me)4 58 and compound 9 59 showed that the metal cores in both clusters have the form of a tetrahedron with one elongated edge (`open' tetrahedron or `butterfly').All the car- bonyl groups are involved in m-type coordination on the edges, each metal atom being linked to one phosphorus atom of the phosphorus ligand. The IR spectra of the complexes in the region of the stretching vibration of the CO groups are characterised by one intense band. The Pd4(CO)5L4 clusters are not very stable in solution and are capable of adding additional ligands.Thus, slow crystallisa- tion of Pd4(CO)5(PBu3)4 under a CO atmosphere results in the addition of another CO molecule with formation of Pd4(CO)6(PBu3)4 10.60 According to X-ray diffraction data, the cluster has a tetrahedral metal core, to which bridging CO groups are coordinated along all the edges and each palladium atom is linked to one terminal phosphine ligand.The interaction of the Pd4(CO)5L4 clusters with organomer- cury compounds takes place virtually without change in the metal core. Thus 8-(a-bromomercurioethyl) quinoline, both parts of the molecule of which may assume the role of an additional ligand (substituted quinoline as a N,C-chelating ligand and mercury as a metalloligand) reacts with the Pd4(CO)5(PEt3)4 cluster to form a heteronuclear compound.61 There is no information about the `fate' of the quinoline component of the molecule.Formally, the two HgBr species generated as a result of the homolytic dissociation of the C± Hg bond, substitute one carbonyl group. However the coordination modes of CO and HgBr are different. According to X-ray diffraction data, the metal core is retained in cluster 11 in the form of an `open' tetrahedron or a `butterfly', to which external triangular faces the HgBr ligands are coordinated. 5. Pentanuclear palladium clusters The pentanuclear carbonyl-containing platinum clusters are fairly numorous (see, for example, the review of Eremenko et al.50). The methods of their synthesis include either the reductive carbon- ylation of platinum(+2) compounds or the interaction of plati- num dicarbonyl with phosphines.Similar approaches have proved ineffective for the carbonyl-containing palladium clusters. Com- pounds of this group were obtained 62 as a result of ligand substitutions in the presynthesised pentanuclear cluster Pd5(m- SO2)2(m3-SO2)(PR3)5 12.63 Thus, when CO was passed through a solution of compound 12 (R=Ph or C6H4OMe-p) in dichloromethane, the correspond- ing complexes having the general formula Pd5(SO2)2(CO)2(PR3)5 were obtained.They were characterised by 31P NMR and mass spectrometry (fast atom bombardment method, FAB). If the initial complex contained another phosphine ligand, for example PR3=PMe2Ph, the analogous reaction led to the complex Pd5(SO2)3(CO)2(PMe2Ph)5 13, characterised by X-ray diffraction data. The metal core of compound 13 consists of a tetrahedron,to one edge of which a fifth palladium atom (Pdbridge) is m-coordi- nated.The Pd ± Pd distances are in the range from 2.73 to 2.97 A. The bridging SO2 groups are coordinated to two Pdbridge ± Pd bonds and to the edge of the tetrahedron most remote from Pdbridge. The carbonyl ligands are coordinated on the opposite faces of the tetrahedron and their coordination may be regarded as highly distorted m3-coordination. 9 PPh3 Ph3P PPh3 Ph3P OC CO Pd Pd Pd Pd CO C C O O Pd OC C C Pd Pd Pd C C PBu3 CO Bu3P PBu3 O O O O 10 PBu3 Pd4(CO)5(PEt3)4+C9H8NCH(Me)HgBr CO+Pd4Hg2Br2(CO)4(PEt3)4. 11 Hg Pd Pd OC Pd Br Hg PEt3 CO Pd PEt3 C O O PEt3 C PEt3 Br 11 CO Pd Pd Pd SO2 Pd CO Pd SO2 SO2 13 Palladium carbonyl complexes 4896. Hexanuclear palladium clusters The first cluster containing six palladium atoms with the compo- sition Pd6(m3-CO)4(PMe3)7 14, was obtained by the interaction of Pd(PMe3)4 and carbon monoxide (PCO=1 atm) at 323 K.64 The cluster has a distorted octahedral metal core, on whose faces four carbonyl ligands are m3-coordinated. The even phosphine ligands are linked to each of the palladium atoms.The reduction of potassium hexachloropalladate K2PdCl6 with sodium tetrahydroborate in the presence of dppm under a CO atmosphere in a toluene ± ethanol mixture afforded the com- plexes Pd2(m-CO)(m-dppm)2Cl2, [Pd3(m3-CO)(m-dppm)3Cl]. .Cl, Pd2(m-dppm)3, and Pd6(m-CO)6(m-dppm)3 15. The IR spectrum of the last compound contains bands characteristic of the stretch- ing vibrations of the bridging CO groups (nCO= 1818 ± 1884 cm71).Accordlng to X-ray diffraction data,65 the palladium atoms in cluster 15 form two almost equilateral triangles in which the length of one side is 2.658 ± 2.714 A. The triangles are linked by three bridging diphosphine ligands. One carbonyl group is coordinated to each side of each triangle.The coordination of the diphosphine ligands is such that they form almost planar trimeric Pd3(m- CO)3P3 species, which are linked by the7CH27bridges belong- ing to the dppm ligand. The Pd3 triangles are almost parallel and have rotated relative to one another by 108. Thus the metal core of the cluster can be regarded as a slightly distorted trigonal prism, the distances between the bases of which (along the Pd ± Pd edges) are 3.030, 3.010, and 2.948 A. 7. The heptanuclear Pd7(m3-CO)4(m-CO)3(PMe3)7 cluster The heptanuclear Pd7(m3-CO)4(m-CO)3(PMe3)7 16 cluster has been obtained by the reduction of the complex Pd(Z1,Z3- C8H12)(PMe3) with carbon monoxide and is the only example of palladium clusters with the above degree of nuclearity.66 The IR spectrum of the complex contains absorption bands in the region characteristic of the stretching vibrations of the bridging CO group (l685 ± 1825 cm71).The structure of the cluster has been established by X-ray diffraction. The metal core consists of an octahedron with one `capping' palladium atom. The additional palladium atom is located asymmetrically above the face of the octahedron: one of the interatomic `nonbonding' Pdcap ± Pdoctahe- dron (3.17 A) is appreciably longer than the other two bonds (2.73 A).The metal ± metal bond lengths in the octahedron are 2.75 ± 2.81 A; these are typical values for distances over which direct metal ± metal interaction usually takes place. 8. The octanuclear Pd8(m3-CO)2(m-CO)6(PMe3)7 cluster The syntheses of phosphinepalladium(0) carbonyl are carried out under similar conditions.The formation of clusters with different nuclearities can be accounted for by the extreme sensitivity of the process leading to the formation of the cluster metal core to the nature of the reaction medium and the source of the palladium atoms. Thus, when a solution of Pd(DBA)2 (DBA=dibenzylide- neacetone) was treated with carbon monoxide (PCO=1 atm) in dry CH2Cl2 containing PMe3, Pd8(CO)8(PMe3)7 17 was obtained in a high yield at room temperature.The cluster was isolated in the form of dark reddish-black plates and was characterised by IR spectroscopy (nCO= 1763 ± 1878 cm71) and X-ray diffraction data.67 According to the X-ray diffraction data, six of the eight palladium atoms in the cluster form a regular octahedron. The remaining two palladium atoms behave as `caps' on two opposite faces of the octahedron and are linked to the same apical palladium atom not joined to the terminal phosphine ligand. 9. The decanuclear clusters Pd10(CO)12L6 and Pd10(CO)14L4 a. Clusters with aliphatic phosphines The synthesis of palladium clusters with aliphatic phosphines has been studied in greatest detail.The compounds are formed on reduction of palladium acetate by carbon monoxide in the presence of CF3COOH. In these syntheses, the acid generates the optimum equilibrium concentration of the free phosphine, the excess of which is bound in the phosphine salt [HPAlk3]+[CF3- COO]7.60, 68, 69 Superstoichiometric amounts of PAlk3 and the presence of a strong acid are necessary in the syntheses of the complexes Pd10(CO)12(PEt3)6 18, Pd10(CO)12(PBu3)6 19, and Pd10(CO)14(PBu3)4 20. On the other hand, in weakly acid and neutral media, it is necessary to reduce the concentration of PAlk3 to give stoichiometric ratios.68 The compounds which could be isolated at a low PAlk3 concentration in a neutral medium proved to be noncrystalline.Clusters 18 ± 20 were characterised by IR spectroscopic data (the bands in the range 1810 ± 1920 cm71 were assigned to the stretching vibrations of the bridging CO groups) and 31P NMR Pd Pd Pd Pd Pd Pd C CO PMe3 PMe3 PMe3 PMe3 PMe3 PMe3 PMe3 C O O OC 14 Pd Pd C dppm C Pd Pd Pd dppm OC OC dppm Pd O O CO CO 15 P Pd CO CO C O C OC P P P P P P CO Pd OC Pd Pd Pd Pd Pd O 16 Me3P Me3P Me3P CO OC Me3P CO Pd CO OC C Pd Pd Pd Pd Pd Pd Pd PMe3 PMe3 PMe3 C C O O O 17 490 T A Stromnova, I I Moiseevspectroscopic data.60, 68 ± 71 The 31P NMR spectra of clusters 18 and 19 are characterised by two signals with an intensity ratio of 2 : 1 and d=711.0 and 73.1 ppm for cluster 18 and d=74.0 and +3.3 ppm for cluster 19.The 31PNMRspectrum of cluster 20 contains one singlet at d=75.0 ppm.69, 70 The spectroscopic data are consistent with the structure of the clusters determined by X-ray diffraction analysis.Thus cluster 19 is an octahedron, over all the nonadjacent faces of which `capping' palladium atoms are located.69 The palladium atoms of the metallopolyhedron can be subdivided into equatorial, apical (in the octahedron), and `capping'. Since the Pdcap ± Peq distance (2.694 ± 2.722 A) is much shorter than the Pdcap ± Pdapic distance (3.301 ± 3.421 AÜ, it would be more logical to regard the `capping' atoms not as atoms m3-linked to the Pd3 face of the octahedron but as bridging atoms on the Pdeq ± Pdeq edges.The Pd ± Pd distances in the inner octahedron are in the range from 2.791 to 2.851 A. Eight m-CO groups behave as bridges on the Pdcap ± Pdeq edges and fourCOgroups are m-coordinated to the four free faces of the octahedron.The coordination of these CO groups is also asymmetric, but, in contrast to the `capping' metal atoms, the carbon atoms of the m3-CO groups are displaced towards the apical palladium atoms. The six phosphorus atoms of the tributylphosphine ligands are bound to four capping and two apical palladium atoms.Treatment of complex 18 with the isocyanide XylNC (Xyl=C6H3Me2-2,6) leads to the displacement of some of the coordinated CO groups and the formation of the cluster Pd10(m- CO)6(m3CO)2(m-CNXyl)2(PEt3)6 21. According to X-ray diffrac- tion data 72 cluster 21 retains the same metal core as cluster 18. b. Clusters with aromatic phosphines Aromatic phosphines [for example, PPh3, PMePh2, and P(OMe)Ph2] are very weak bases and a very high content of hydrogen ions is required to regulate their concentration.Attempts have been made 69 ± 71 to synthesise these compounds under neutral conditions with vigorous control of the concen- tration of the initial phosphine. The clusters Pd10(CO)12L6 [L=PPh3, PMePh2, and P(OMe)Ph2] were isolated in the solid state and identified by elemental analysis and IR spectroscopy. 10. The hexadecanuclear cluster Pd16(CO)13L9 The use of the traditional procedure � oxidation of CO by trimethylamine N-oxide 73 � made it possible to obtain 74 the 16-nuclear cluster Pd16(CO)13(PEt3) 22 by the reaction of Pd4(CO)5(PEt3)4 and Pd10(CO)12(PEt3)6. X-Ray diffraction study demonstrated the presence in this cluster of the centred icosahedron Pd13(m3-CO)7(PEt3)6, overlaid by three bridging Pd(m-CO)2L fragments.These fragments impart chirality to the centred icohedral metal core of the cluster, reducing its idealised symmetry Ih to D3. The centrosymmetric racemic crystals of the clusters contain equal numbers of the optically active isomeric molecules. Compound 22 is the first example of a polynuclear palladium complex with icosahedral packlng of the metal atoms. 11. Tricosahedral clusters The use of Pd(OAc)2 as a mild acceptor of phosphine ligands made it possible to convert decanuclear carbonyl ± phosphine clusters into the compounds with a high nuclearity. Thus the interaction of Pd10(CO)12(PEt3)6 and Pd(OAc)2 (molar ratio 1 : 1) in benzene at 293 K for 24 h and subsequent addition of heptane afforded the cluster Pd23(CO)22(PEt3)10 23.75 A slight change in the conditions of the synthesis (molar ratio Pd : Pd=1 : 1.5; methanol ± ether mixture, 303 K, 2.5 h) leads 76 to another 23-nuclear cluster having the composition Pd23(CO)20(PEt3)8 24.Both clusters have been characterised by X-ray diffraction data (Fig. 2a). Pd Pd Pd Pd Pd Pd C CO OC O Pd OC Bu3P Bu3P PBu3 CO OC PBu3 CO Pd Pd OC Pd CO PBu3 CO CO PBu3 O C 19 22 P P P P P P P P P Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd a b Figure 2.The structure of the metal cores of the Pd23(CO)22(PEt3)1) and Pd23(CO)20(PEt3)8 (b) clusters according to X-ray diffraction data.75, 76 The crosses designate the positions of the carbon atoms of carbonyl ligands.Palladium carbonyl complexes 491The metal core of the centrosymmetric molecule of cluster 23 can be regarded as a cubooctahedron (designated by thickened lines in Fig. 2a) with one metal atom at the centre. This cuboocta- hedron has an idealised Oh symmetry. Six `capping' palladium, atoms are m4-coordinated to each square face of the cubooctahe- dron while the remaining four metal atoms are symmetrically m- coordinated to the four edges of the cubooctahedron.The metal ± metal distances beetween the central and peripheral metal atoms (on average 2.847 A) and between pairs of peripheral metal atoms (2.849 A) in the cubooctahedron are virtually equal. All the other metal ± metal distance in the metallopolyhedron are in the range from 2.720 to 2.920 A.Clusters 23 and 24 have a 22-atomic metallopolyhedron and one endopolyhedral metal atom. However, in contrast to complex 23, containing a fragment with cubic close packing of metal atoms, the metallopolyhedron in complex 24 has a different structure. In the immediate environment of the endopolyhedral palladium atom, there are not 12, as in complex 23, but 14 metal atoms; the average Pd ± Pd distance (2.918 A) exceeds by almost 0.17 A the metal ± metal distance in metallic palladium.The central palla- dium atom with metal atoms in its immediate vicinity is approx- imately regarded as a fragment with body-centred cubic packing (BCC), although the appreciable distortions of the idealised BCC packing observed in complex 24 are more likely to indicate that the polyhedron is not so regular in its shape.This polyhedron, made up of 15 metal atoms, is overlaid by eight `caps' comprising palladium atoms, which are also coordinated to eight terminal phosphine ligands. The composition of the coordination sphere also includes three m3-CO and seventeen m-CO ligands. 12. The cluster Pd34(CO)24(PEt3)12 The 34-nuclear cluster Pd34(m-CO)20(m3-CO)4(PEt3)12 25 has been obtained as a by-product of the elimination of phosphine ligands from the decanuclear clusters Pd10(CO)12(PEt3)6 and Pd10(CO)14(PEt3)4, leading to the formation of a 38-nuclear cluster with `irregular' packing of atoms in the quasi-spherical metal polyhedron (see below).According to X-ray diffraction data,77 the metal polyhedron of the cluster contains four inner palladium atoms located at the apices of a distorted octahedron.Each of these atoms has a distorted icosahedral environment comprising 12 palladium atoms. This environment does not occur in the crystal lattice of the bulk metal but is encountered in the metal cores of cluster with finite dimensions. The observed metal ± metal distances in the metal core of cluster 25 indicate unambiguously an icosahedral type of packing of the metal atoms (Fig. 3). Histograms of the metal ± metal distances in large palla- dium clusters based on X-ray diffraction data have been compared with atomic radial distribution curves calculated from EXAFS data (Fig. 4).78 13. The cluster Pd38(CO)28(PEt)12 .Me2CO The cluster Pd38(CO)28(PEt)12 .Me2CO 26 was obtained, like the 23-nuclear cluters, by the reaction of the Pd10(CO)12(PEt3)6 with Pd(OAc)2 (molar ratio 1 : 2) in acetone in an argon atmosphere.The complex was isolated in the form of crystals sensitive to atmospheric oxygen and characterised by X-ray diffraction data (Fig. 5).79 The cluster metal core has an incompletely regular structure with noncrystallographic symmetry close to D2. The flattened inner Pd4 tetrahedron with two long (2.958 and 2.970 A) and four short (2.539, 2.610, 2.603, and 2.514 A) Pd ± Pd bonds is sur- rounded by twenty metal atoms, located in four approximately parallel layers, and two metal atoms located outside these layers.All this corresponds to the 4 : 8 : 2 : 8 : 4 configuration. The 26- Figure 3. The structure of the metal core of the cluster Pd34(m-CO)20(m3- CO)4(PEt3)12 according to X-ray diffraction data (the dark lines designate the inner metallotetrahedron).77 a b c 2 3 4 5 6 7 8 Figure 4.Histograms of the distribution of interatomic distances in the Pd34 metal core of the cluster Pd34(m-CO)20(m-CO)4(PEt3)12 with partic- ipation of only four intracluster atoms (a) with participation of all the remaining distances in the Pd30 metal `shell' (b), and the theoretical dis- tribution for the standard (idealised) icosahedral packing of the metal (c).Figure 5. The structure of the metal core of the cluster Pd38(CO)28(PEt)12 (the filled circles identify the inner metal atoms).79 492 T A Stromnova, I I Moiseevnuclear metal core thus obtained is linked to twelve `capping' palladium atoms, while each `capping' metal atom is linked to the phosphorus atom of the terminal phosphine ligand.The average Pd ± Pd distance for 118 metal ± metal bonds is 2.775 A. Among the 28 carbonyl groups, four are m3-coordinated, whilst the others are m-coordinated. Analysis of the structures of the polynuclear phosphinepalla- dium(0) carbonyl complexes showed that the `tendency' to form a spherical metallopolyhedron, leading to the minimisation of the surface of the molecule, is characteristic for these compounds, as in the case of the platinum and nickel clusters (Table 2) The structures of clusters 24 ± 26 provide a good illustration of the characteristic feature of the structure of the entire class of large clusters: the possibility of the rearrangement of the metallopoly- hedron following a change in the ligand environment and the `tendency' to achieve close packing of the metal atoms. In order to investigate the structure and nature of the chemical bonding in the palladium(0) carbonyl clusters, an attempt has been made 80 to apply an incorporation method as the basic procedure for the `construction' of large clusters by the addition of `caps' to the triangular and/or rectangular faces of the metal- lopolyhedron.The method is based on the graph theory. Thus the structures of clusters 16, 17, 19, and 20 are produced by the addition of one, two, three, and four palladium `caps' respectively to the central Pd6 octahedron. However, for the description of the Pd23 and Pd38 clusters, this approach requires the introduction of additional concepts and appears somewhat artificial. The results of the determination of the metal ± metal bond length in a series of phosphinepalladium(0) carbonyl complexes from X-ray diffraction and EXAFS data agree well.78 This gives rise to the hope for a wider application of the EXAFS method in the study of the structures of cluster compounds. 14. Infrared spectra of palladium(0) carbonyl complexes Analysis of the IR spectra (Table 2) of structurally characterised carbonylphosphine clusters 81, 82 permitted the conclusion that the characteristic differences in the region of the sretching vibrations of the carbonyl group are observed only as the coordination mode of the carbonyl ligands is varied.Thus the vibrations of the terminalCOgroups are manifested in the short-wavelength region (2000 ± 2050 cm71), those of the m-CO groups are in the middle region (*1850 ± 1950 cm71), and those of the m3-groups are shown in the longest-wavelength part of the spectrum.The regions corresponding to absorption by the last two types of CO ligands overlap appreciably. Complexes of different nuclearity, contain- ing the same type of CO ligand, have no stable distinctive features in the IR spectra.On the other hand, the presence at one metal atom of only one terminal group and three n-donor atoms {for example, in the complexes Pd(CO)(PPh3)3, Pd(CO)(NP3) and Pd(CO)[- MeC(CH2PPh2)3], see Table 1} leads to a decrease in nCO to values characteristic of the stretching vibrations of the bridging CO groups, i.e. the region of the stretching vibrations of the bridging and terminal carbonyl groups overlap. It is noteworthy that in all other complexes where the number of n-donor atoms at each metal atom is smaller (for example, two or one phosphorus atoms), the stretching vibrational frequencies of the terminal CO group lie above 2000 cm71.IV. Palladium(+1) carbonyl complexes The coordination compounds in which palladium is present in the formal oxidation state +1 have been frequently postulated as active intermediates in palladium-catalysed reactions.84 ± 86 This applies to a considerable extent to reactions occurring with participation of carbon monoxide.Palladium(+1) intermediate Table 2. Characteristics of the polynuclear palladium(0) complexes containing carbonyl ligands.Compound Coordina- nCO /cm71 Metal core Pd7Pd /A Ref. tion of CO Pd3Sn2(Z2-acac)4(CO)2(PPh3)3 m 1890 ± 1770 equilateral Pd3 triangle 2.71 ± 2.80 54, 55 Pd3(CO)3(PPh3)4 m 1820 ± 1880 the same 2.670, 2.741 2.777 57 Pd4(m -CO)5(PPh2Me)4 m 1840, 1820 `open' Pd4 tetrahedron (`butterfly') 2.74 ± 2.77 3.20 58 Pd4(m -CO)5(PPh3)4 m 1858 the same 2.74 ± 2.78 3.19 59 Pd4(m -CO)6(PBu3)4 m tetrahedron 2.77 ± 2.79 60 Pd4Hg2Br2(CO)4(PEt3)4 m 1897, 1855 `open' tetrahedron (`butterfly') 2.69 ± 3.01 3.43 61 Pd5(SO2)3(CO)2(PMe2Ph)5 m 1908 tetrahedron with one Pd bridge 2.73 ± 2.97 62 Pd6(m3-CO)4(PMe3)7 m3 1730, 1708 distorted octahedron 2.705 ± 2.806 64 Pd6(m-CO)6(m-dppm)3 m 1884, 1876, 1841, distorted trigonal prism 2.658 ± 2.724 65 1826, 1818 Pd7(m3-CO)4(m-CO)3(PMe3)7 m, m3 1825, 1800, 1785, octahedron with one Pd cap 2.75 ± 2.81 66 1770, 1750, 1685 Pd8(CO)8(PMe3)7 m, m3 1878, 1843, 1810, octahedron with two Pd caps 2.73 ± 2.87 67 1803, 1775, 1763 Pd10(CO)12(PBu3)6 m, m3 1920 ± 1810 octahedron with four Pd caps 7 69 Pd16(CO)13(PEt3)9 m, m3 centred icosahedron with three 7 74 Pd caps Pd23(CO)22(PEt3)10 m, m3 1863, 1847, 1823 centred cubooctahedron with 2.720 ± 2.920 75 six m4-coordinated and four m-coordinated Pd caps Pd23(CO)20(PEt3)8 m, m3 1865, 1839, 1815 fragment of the BCC structure with 2.819 a 76 eight Pd caps Pd34(m-CO)20(m3-CO)4(PEt3)12 m, m3 1892, 1865, 1837, fragment of the icosahedral type 77, 78 1809 of packing Pd38(CO)28(PEt3)12 m, m3 1899, 1885, 1849, a not very regular structure with 2.775 a 79, 83 1842, 1817, 1778 noncrystallographic symmetry a Average distance.Palladium carbonyl complexes 493complexes could arise either in the reduction of Pd(+2) by carbon monoxide or in the oxidation of Pd(0), for example, during oxidative addition in a medium containing CO as the stabilising ligand. These postulates constitute the basis of the syntheses of palladium(+1) carbonyl complexes, although other products of the oxidation ± reduction reactions [except Pd(+1)] have by no means been characterised in any way and discussed in all cases.The first among palladium(+1) carbonyl complexes was the carbonyl chloride Pd(CO)Cl obtained in 1926.87 The structure of this complex and in the first place the coordination mode of the carbonyl group remained obscure for many years.It was not until more than half a century later that X-ray diffraction studies on the structures of palladium carbonyl complexes were published. [Pd(m-dam)Cl]2(m-CO) 9, 10 and Pd4(m-CO)4(m-OAc)4 11, 12 were investigated. X-Ray diffraction data with IR spectroscopic data provided answers to a number of questions concerning the structure of palladium carbonyl chloride, its analogues, and derivatives. 1. The syntheses and structures of palladium(+1) carbonyl complexes a. Palladium(+1) carbonyl halides and their anionic derivatives The carbonyl chloride Pd(CO)Cl was first obtained by the reaction of solid PdCl2 with carbon monoxide saturated with methanol vapour at 0 8C and atmospheric pressure.87 A more convenient method of synthesis of the carbonyl halides involved the reaction of PdX2 (X=Cl, Br) with CO at atmospheric pressure in a concentrated aqueous solution of the corresponding hydrogen halides. The formation of diamagnetic powdered Pd(CO)Cl and Pd(CO)Br deposits was observed when an aqueous solution of the corresponding palladium salt was treated with carbon monoxide or after treatment with formic acid.88 Palladium carbonyl chloride has also been obtained by carbonylation of a suspension of PdCl2 in the presence of acetyl chloride 89 or small amounts of H2O90, 91 and also by the reaction of PdCl2 and allyl chloride withCOat both atmospheric and elevated pressures. 91, 92 The benzonitrile complex (PhCN)2 . PdCl2�a convenient reagent which has proved very useful in the preparation of alkene palladium p-complexes 94 � has been employed as the starting material for the synthesis of Pd(CO)Cl.91, 93 The carbonyl chloride Pd(CO)Cl (nCO=1975 cm71) has also been obtained 95 by treating the palladium(+2) carbonyl complex Pd2(CO)2Cl4 with carbon monoxide (Ac2O, 25 8C, 24 h, PCO= 1 atm).In contrast to the binary palladium(0) carbonyl compounds, which are not formed from metallic palladium at temperatures above 323 8C, the palladium(+1) carbonyl chloride can be produced from powdered palladium at a CO pressure of 170 atm and 190 8C in a phosgene-containing solution in o-dichloroben- zene.91 The palladium(+1) carbonyl fluoride or carbonyl iodide could not be obtained by any of the methods indicated above. The elemental composition of palladium(+1) carbonyl chlor- ide specimens obtained by different methods 88 ± 93 as a rule corresponds to the formula Pd(CO)Cl: the IR spectra of three specimens agree satisfactorily.A powdered amorphous or very fine-crystalline substance, which does not dissolve in organic solvents and is thermally stable up to 60 8C but readily decom- poses on treatment with water or even atmospheric moisture, was obtained in all cases. 2Pd(CO)Cl+H2O=2Pd+CO2+2HCl+CO. The reaction is strongly inhibited by H+ or Cl7 ions, as a result of which the carbonyl chloride dissolves almost without decomposition and is stable in concentrated hydrochloric acid.The properties of the thermally more stable (Tdecomp=120 8C) carbonyl bromide Pd(CO)Br are similar. This too is stable in concentrated aqueous HBr solution.88 Both palladium(+1) car- bonyl halides are relatively stable in air in the absence of H2O.Thermolysis of the carbonyl halides leads to the elimination of CO and the formation of palladium metal. Probably as a result of slight decomposition the colour of the Pd(CO)X specimens obtained by different workers varies from yellow-green to dark- violet. The stretching vibrational frequency of the CO groups in these compounds varies in the range from 1850 to 2000 cm71.96, 97 It was postulated in early investigations that the compounds Pd(CO)X are polymers with halide 88 or carbonyl 98 bridges.On dissolution in the corresponding hydrogen halide solution, depo- lymerisation takes place with formation of mono- or di-nuclear anions, for example: 2Pd(CO)X+2X7=[Pd2(CO)2X4]27. The crystalline substance with the composition (NH4)2[Cl2Pd(CO)]2 has been isolated from a solution of (NH4)2PdCl4 in concentrated HCl, treated with carbon monoxide for several days, while in the presence of enH2á 2 and [Pt(NH3)4]2+ cations the crystalline salts of the latter with the [Pd2(CO)2Cl4]27 anion were obtained.It was assumed that the anion contains Pd(+3) atoms linked by bridging CO groups to form a dimer.99 Without going into the question of the electronic structure of the anion, we may note that, since the COgroup is electrically neutral, the formal oxidation state of palladium is in this instance +1.One cannot rule out the possibility that a metal ± metal bond is present in the dimeric anion. The diamagnetism of the cesium rubidium, and tetraalkylammonium salts of the anions [Pd2(CO)2Cl4]27 and [Pd2(CO)2Br4]27 obtained by a similar method 88, 98 agrees with this hypothesis.Treatment of the complex Pd(CO)Cl with an equimolar amount of NH2Et2Cl afforded the anionic complex [NH2Et2]2[Pd2(CO)2Cl4], characterised by elemental analysis and IR spectroscopy (nCO=1916 cm71).95 The structure of the anionic carbonyl halide complex [Bun4 N]2[Pd2(CO)2Cl4], synthesised by Goggin and Mink,98 has been investigated by X-ray diffraction.100 The 1966 and 1906 cm71 frequencies correspond to the stretching vibrations of the CO groups in the complex in solution in CH2Cl2.palladium atoms in the anion of the complex are linked by two carbonyl bridges, each metal atom being bound to two terminal chloride ligands.The metal ± metal distance (2.697 A) indicates the presence of the Pd ± Pd single bond. Each palladium atom has a highly distorted tetrahedral environment, the PdCl2 plane making a dihedral angle of 8 ± 108 with the Pd2(m-C)2 plane. The planar configuration of the Pd2(m-C)2 species agrees well with the IR spectrum of the solid complexes (the spectrum contains only the nCO=1903 cm71 band due to the asymmetric vibrations).The presence in the spectrum of the solution in CH2Cl2 of a band at 1966 cm71 assigned to the symmetrical vibrations of the CO group, indicates the distortion of the planar configuration of the Pd2(m-C)2 species in solution. Thus the carbonyl ligand in virtually all the reliably charac- terised neutral and anionic palladium(+1) carbonyl halide com- plexes is a bridge between two metal atoms.The only exception is the complex [Pd(CO)2(m-Cl)]2 27 obtained whenCOis passed (6 h, 60 8C) through a reaction mixture containing PdCl2 andNaHCO3 in anhydrous methanol in the presence of 2-(1,5-dimethylhex-4- enyl)-3-hydroxy-5-methyl-1,4-benzoquinone.101 Cl Cl Pd Pd C C Cl Cl O O Cl Pd Pd Cl OC CO 27 CO OC 494 T A Stromnova, I I MoiseevComplex 27 has been isolated from n-hexane in the form of crystals stable in air.Unfortunately, the very brief relevant publication 101 contains no comments whatsoever concerning the fairly unexpected choice of reagents and the role of sodium bicarbonate or the substituted benzoquinone. The IR spectrum of the complex contains bands characteristic of terminal CO groups (2103, 2082, 2024, and 1997 cm71).According to X-ray diffraction data, the two palladium atoms of the complex, at a distance of 3.114 A, are joined by two chloride bridges cis-posed relative to the Pd...Pd axis (the dihedral angle of the Pd2Cl triangle is 124 8). Each metal atom is also joined to two terminal CO groups, which builds up their environment to a distorted planar square.b. Palladium(+1) carbonyl halides containing n-donor stabilising ligands Neutral molecules containing an element E (E=N, P, As, Sb), which is capable of donating an unshared electron pair, are usually employed as additional stabilising ligands. In the chemistry of carbonyl-containing palladium(+1) compounds, nitrogen-con- taining ligands are relatively rare,whilst phosphorus-containing ones (mono- and di-phosphines) are very common.The complexes [(PdXL)2CO]n, where X=Cl, Br, or I and L=PPh3, PPh(p-MeOC6H4)2, PPh2(C6H4COOH), PPh2(C6H4F), PPh2(C6H4Cl) or PPh2(C6H4Me), have been obtained by the reactions of the corresponding phosphine with a solution of H2Pd2(CO)2Cl4 in concentrated HCl.102 The IR spectra of the solid complexes contain two bands in the range 1860 ± 1962 cm71, which have been assigned to the stretching vibrations of the bridging CO groups. On the other hand, if a solution of H2Pd2(CO)2Cl4 in concentrated HCl is acted upon initially by a solution of SnCl2 and then triphenylphosphine in concentrated HCl, all the acido-ligands are displaced from the initial carbonyl chloride and [Pd2(PPh3)2(SnCl3)2]CO (nCO=1800 ± 2000 cm71) is formed.103 The complexes [(PdXL)2CO]n, containing stabilising ligands of the type of arsine (L=AsPh3) or stibine (L=SbPh3) have been obtained on partial substitution of carbonyl and acido-ligands in palladium(+1) carbonyl halides and carbonyl acetate complexes by organoele- ment ligands.104 The complexes have been characterised by elemental analysis, ESCA, and IR spectroscopy.Judging from the n=1710 ± 1890 cm71 region, the carbonyl group in these compounds is involved in bridging coordination.The dinuclear complex Pd2(CO)Cl2L3 (L=PEt2Ph) 28 has been obtained 59 in a fairly high yield (49%) yield by the reaction of CO with Pd(NO2)2L2 in dichloromethane (293 K, PCO=1 atm, 48 h) and subsequent precipitation of the com- pound formed on treatment with CO-saturated hexane at 273 K.In the formation of compound 28, the source of chloride ligands could be dichloromethane itself, although the authors did not rule out also the possibility of the interaction of the initial Pd(NO2)2L2 and HCl or Cl2, present in. the solvent, with formation of PdCl(NO2)L2. In order to confirm this hypothesis, trans- PdCl(NO2)L2 was obtained. Its treatment with CO also affords the dimer 28.According to X-ray diffraction data, the two palladium atoms in the molecule of compound 28 (the Pd ± Pd distance is 2.652 A) are linked asymmetrically by the coordinated semibridging car- bonyl ligand (the Pd ±C distances are 1.874 and 2.110 A, while the Pd ±C± Pd angle is 83.38.105 They have nonequivalent environ- ments. The palladium atom involved in the shorter bond with the carbonyl group is also linked to the second metal atom, one chloride ligand, and one phosphine ligand.It has a highly distorted 4-coordinated environment. There are five atoms in the environment of the second palladium atom: one Cl atom, two phosphorus atoms of the phosphine ligands, palladium, and a somewhat remote carbon atom of the carbonyl ligand.Unfortunately the authors do not quote spectroscopic data for complex 28. Complexes containing the Pd2(m-Q)2 moiety, where Q=diphosphine or diarsine ligands having the composition R2E(CH2)nER2 (E=P or As, n=1 ± 3), constitute a special group. As a rule, such complexes are formed by the insertion of CO in the metal ± metal bond in the dinuclear palladium(+1) complexes Pd2(m-Q)2X2 (X=Cl, Br, I), which are formed in their turn in the reaction of the polymeric [Pd(CO)X]n (or its anionic derivatives) with the bidentate ligand Q.In the interaction of [Pd(CO)X]n and dppm, the coordinated CO group is fully dis- placed with formation of the complexes [Pd(dppm)X]2.88 In the case of dam, the reaction product depends on the nature of the halogen: if X is chlorine or bromine, [Pd(m-dam)X]2(m-CO) 29 is formed, whereas in the case where it is iodine the product is [Pd(m- dam)I]2.9, 10, 105 In complex 29 (X=Cl) palladium atoms are bound to two dam molecules and the carbonyl bridge.The Pd ± Pd distance (3.274 A exceeds twice the covalent radius. Nevertheless, the compound is diamagnetic, so that the spins are apparently paired via the carbonyl bridge, which in this case is fairly unusual: the Pd ±C± Pd angle is 1198 and not 85 ± 898 as in the majority of transition metal carbonyl complexes. When [Pd(m-dppm)X]2 (X=Cl, Br) is treated with CO in dihloromethane, [Pd(m-dppm)X]2(m-CO) (nC0=1704 cm71) is formed;106; it liberates CO reversibly on heating in solution in dichloromethane.The complex Pd2(m-CO)(m-dppm)2Cl2 has been obtained as a result of the addition of oxalyl chloride ClC(=O)- C(=O)Cl to Pd2(dppm)3 in dichloromethane.107 The kinetics of the reversible insertion on of carbon monoxide in the Pd ± Pd bond of the complex Pd2(dppm)2X2 with formation of Pd2(m-CO)(m-dppm)2X2 have been investigated spectrophoto- metrically.108 It was established that the reaction obeys a first- order equation with respect to both the metal and carbon mon- oxide.The reaction of the polymer [Pd(CO)Cl]n with dmpm (dmpm=bis(dimethylphosphino)methane] in dichloromethane at 198 K leads to the formation of Pd2Cl2(dmpm)2 30. Treatment of compound 30 with carbon monoxide results in the formation of [PdCl(m-dmpm)]2(m-CO) 31 with a bridging carbonyl ligand (nCO=1710 cm71). According to X-ray diffraction data, the distance between the metal atoms in the molecule of compound 31 (the Pd ± Pd distance is 3.169 A) exceeds the sum of the covalent radii of palladium and the environment of each metal atom consists of an almost planar square.109, 110 In contrast to analogues containing dppm, an unusually high solubility and stability in aqueous solution were characteristic of complex 31.On the basis of IR, Raman, and NMR spectroscopy involving different nuclei and electrical conductivity data, it has been shown that the following reactions take place readily in water: PhEt2P PEt2Ph Cl Cl PhEt2P Pd Pd C 28 O As As Pd As As Pd C Ph Ph Cl Cl Ph Ph Ph Ph Ph Ph O 29 Cl Pd(m-dmpm)2Pd Cl+H2O HO Pd(m-dmpm)2Pd OH, Palladium carbonyl complexes 495The interaction of [Pd(m-dppm)X]2 and SnCl2 followed by treatment of the resulting solution with carbon monoxide afforded 111 in situ the complex Pd2(m-dppm)2Cl(SnCl3)(m-CO) (nCO=1688 cm71).The principal characteristic feature of the reaction involving the insertion of small molecules in the Pd ± Pd bond is the increase in the metal ± metal distance on the formation of bridges.Despite the fact that the palladium ± palladium distance increases to a value greatly exceeding the sum of the covalent radii, the com- pounds containing palladium in the formal +1 oxidation state remain diamagnetic. The probable causes of the diamagnetism of these compounds will be discussed below. c. The binary palladium(+1) carbonyl The only compound in which the Pd(+1) atom is linked to carbonyl groups only is the cationic complex [Pd2(m- CO)2](SO3F)2.The reductive decomposition of the divalent palladium car- bonyl Pd(CO)2(SO3F)2 in solution in fluorosulfonic acid HSO3F at 298 K leads to the formation (according to IR spectroscopic data) of a mixture of complexes containing both bridging and terminal CO groups.112 Slow crystallisation (for about three weeks) of the mixture from fluorosulfonic acid leads to the precipitation of needle-like crystals.It has been suggested that the reduction of Pd(+2) to Pd(+1) includes the elimination of the SO3F . radicals and their dimerisa- tion to S2O6F2 with simultaneous liberation of one mole of CO in accordance with the equation which is probably followed by the oxidation of CO to CO2: The formation of S2O5F2, in the reaction mixture has been confirmed by 19F NMR (signal at d=47.5 ppm).The complex [Pd2(m-CO)2](SO3F)2 has been characterised by elemental analysis IR spectroscopy (nCO=1977 cm71), and X-ray diffraction data. Its molecular structure consists of cyclic, almost planar four-membered Pd2(m-CO)2 species, in which the palladium atoms are linked together as a result of direct metal ± - metal interaction (the Pd ± Pd distance is 2.698 A) and by two symmetrical carbonyl bridges.The individual metallocycles are joined by symmetrical bridging bidentate fluorosulfate groups, forming a planar polymer chain. Thus the coordination sphere of each palladium atom incorporates the two carbon atoms of two cis-disposed carbonyl groups, two oxygen atoms of two fluoro- sulfate groups, and a neighbouring palladium atom.If the metal ± metal bonding is ignored, the geometrical environment of each palladium atom can be regarded as a slightly distorted square. d. Other palladium(+1) complexes containing carbonyl and organic ligands The interaction of the complex Pd2(m-Cp)(m-Br)(PR3)2 and CO in toluene leads to the displacement of the bromide and cyclo- pentadienyl bridges to axial positions and to the formation of Pd2(m-CO)(Z5-Cp)(Br)(PR3)2 32 [R=Et (32a), R=Pri (32b)].113 The analogous initial compound containing an acetate instead of a bromide bridge reacts with CO also with formation of complex 32.114 Complexes 32a,b have been characterised by IR spectro- scopy (nCO=1804 and 1828 cm71 respectively) and NMR spec- troscopy on different nuclei.The. structure of complex 32b has been investigated by X-ray diffraction. The metal atoms in the molecule of the complex are linked both to one another (direct metal ± metal interaction, the Pd ± Pd distance is 2.67 A) and with the bridging carbonyl. The phosphine ligands are trans-located relative to the metal ± metal axis. It has been shown that the carbonyl bridge in complex 32 is readily substituted by other small molecules, for example SO2 or CH3NC.The dinuclear complexes (PR3)2Pd2(m-CP)(m-X) (X=Cl or CH3COO) readily react with NaCpM (CO)3 (M=Cr, Mo, W) or NaCo(CO)4. Such interac- tion affords the heteronuclear clusters (PR3)2Pd2(m-Cp)[m- M(CO)3Cp)] 33 and (PR3)2Pd2(m-Cp)[m-Co(CO)4] 34 respec- tively.115 These complexes have been characterised by spectro- scopic methods.It has been established that the cyclopentadieny and carbonylmetallate ligands function as bridges between the two palladium atoms ,while the phosphines occupy the axial position. The attempt to substitute the cyclopentadienyl ligand in complexes 33 and 34 by a second carbonylmetallate in order to obtain tetranuclear clusters was unsuccessful, but such clusters have been obtained by another method.The interaction of trans-Pd(PEt3)2Cl2 and [Z5-CpM(CO)3]7 results in the formation 116, 117 of the tetranuclear clusters Pd2M2(Z5-Cp)2(m3-CO)2(m-CO)4(PEt3)2 (M=Cr, Mo, W) 35. All three complexes have been characterised by X-ray diffraction data. The metal atoms in complex 35 (M=Mo) are in the same plane and form a rhombus with a short Pd ± Pd diagonal.One m3-CO group is linked to each Pd2Mo triangle (nCO=1772 cm71), while a m-CO group is linked to each Pd ±Mo edge (nCO=1842 and 1801 cm71). A similar cluster with another phospine ligand, Pd2Mo2(Z5- Cp)2(m3-CO)2(m-CO)4(PBzPh2)2 (the Pd ± Pd and Pd ±Mo distan- ces are 2.58 and 2.86, 2.82 A respectively), has been synthes- ised.118 Palladium(+1) complexes with terminal carbonyl groups have been obtained 119 in a study of the reactions of a dimeric univalent palladium complex with phosphide bridges as a result of the following transformations in solution in dimethoxyethane: where X7=CF3SO¡3 or BF¡4 .Complexes 36 and 37 have been characterised by NMR and IR spectroscopy (nCO=2080, 2055, and 2045 cm71 for complex 37) and also by X-ray diffraction data (the Pd ± Pd distances are 2.611 and 2.682 A for complexes 36 and 37 respectively).The NMR spectra have shown that complex 37 exists in the form of a mixture of isomers differing in the relative HO Pd(m-dmpm)2Pd OH+13CO HO Pd(m-dmpm)2(m-13CO)Pd OH. Pd C C Pd O Pd* O O S F O O Pd* Pd* O O 2Pd(CO)2(SO3F)2 2Pd(CO)SO3F+2CO+S2O6F2, HSO3F CO+S2O6F2 HSO3F CO2+S2O5F2.M OC CO Pd Pd OC CO M PEt3 Et3P C C O O Cp 35 Cp 36 [R2HP (H)Pd(m-PR2)Pd(PR2) PHR2]+X7, R2HP Pd(m-PR2)2Pd PHR2+HCl 37 [R2HP (CO)Pd(m-PR2)Pd(CO) PHR2]+X7, 36+CO 496 T A Stromnova, I I Moiseevdisposition of the carbonyl and phosphine ligands. Complex 37 is stable only under a carbon monoxide atmosphere; it is readily decarbonylated in vacuo, on passing nitrogen, on heating, or on UV-irradiation.e. Palladium(+1) carbonyl carboxylates All the carbonyl-containing univalent palladium complexes pre- sented above are as a rule dimers or linear polymers. The tetranuclear carbonyl carboxylates having a planar cyclic metal core are an exception. The structure and chemical properties of compounds of this group have been thoroughly investigated.The complexes containing simultaneously both carbonyl and carboxylate ligands are usually obtained by reduction of palla- dium(+2) carboxylates with carbon monoxide in a carboxylic acids or in an inert solvent containing the corresponding carbox- ylic acid. The second method of synthesis involved the substitu- tion of ligands in the presynthesised palladium(+1) carbonyl carboxylates. The cluster Pd4(m-CO)4(m-OAc)4 38 was obtained 11, 12 by the first methods, i.e.by carbonylating (PCO=1 atm) palladium acetate in glacial acetic acid at 323 K. 4Pd(OAc)2+8CO Pd4(CO)4(OAc)4+2AcO+4CO. The cluster crystallises with two molecules of acetic acid to give Pd4(m-CO)4(m-OAc)4 . 2AcOH 38a, while on drying in vacuo over KOH it loses an acetic acid molecule and is converted into Pd4(CO)4(OAc)4 38b.The IR spectra of the complexes each contain intense bands corresponding to the stretching vibrations of the CO groups (1934 and 1975 cm71 for 38a and 1940 and 1975 cm71 for 38b) and bands due to the stretching vibrations of acetate groups coordinated as bidentate species. According to X-ray diffraction data, the cluster as an almost rectangular metal core with angles of 83.48 and 96.68.The sides of the rectangle, linked by acetate bridges are longer (2.909 A), while the sides linked by carbonyl bridges are shorter (2.663 A) than the metal ± - metal distance in metallic palladium (2.751 A) (Fig. 6).120 The mechanism of the formation of cluster 38 has been investigated by spectroscopic and kinetic methods. The data obtained led to the conclusion that the formation of complex 38 involves the transfer of an oxygen atom from the carboxylate group to the coordinated carbonyl group.This results in the formation of the coordinated acyl [CH3CO] and in the elimination of carbon dioxide.121 ± 123 It has been established that the coordinated acetate groups in complex 38 are substituted by other carboxylates in the reactions with the corresponding acids, for example: According to IR spectroscopic data, all the complexes contain bridging carbonyl and carboxylate groups.Comparative analysis of X-ray diffraction data for palladium carbonyl acetate and of EXAFS data for all carbonyl carboxylate complexes permitted the conclusion that they all have a similar structure�a planar metal core (rectangle, square, or rhombus) to the sides of which bridging carboxylate and carbonyl ligands are coordinated.124 Together with the existence of isomers having different metal cores, there is also a possibility of the existence of isomers with different relative dispositions of ligands: disposition of ligands in pairs (for example as in complex 38 characterised by X-ray diffraction analysis) and uniform distribution of similar ligands.The influence of substituents in carboxylated groups on the isomerism of the metal core of the carbonyl carboxylates has been examined in detail.125, 126 f. Derivatives of palladium(+1) carbonyl carboxylates with N- and P-donor ligands The interaction of complex 38 with ligands such as Py, dppe, PPh3, Bipy, and Phen leads to a series of complexes, the composition of which depends on the initial Pd/ligand(L) ratio.The palladium complexes formed with the Pd/L=2 ratio are unstable. With decrease in the ratio, more stable compounds appear. The interaction of complex 38 with Py and its alkyl derivatives (with the ratio Pd/Py=1) entails the disproportionation of Pd(+1) to Pd(0) and Pd(+2): Pd2(CO)4(OAc)4+4Py Pd(0)+2Pd(Py)2(OAc)2+4CO.The interaction of complex 38 and dppe (Pd/dppe=1) in benzene affords the complex Pd2(dppe)2(CO)(OAc)2:127, 128 According to IR spectroscopic data, the group CO in complex 40 is of the bridging type (nCO=1790 cm71). The fairly low stretching vibrational frequency in the compound where the metal atoms are linked also by a further two diphosphine bridges indicates an increase in the Pd ± Pd distances compared with the analogous distances in complex 38.Analysis of the stretching vibrational frequencies of the acetate group in the IR spectrum of complex 40 [nas(COO)=1580 cm71, ns(COO)=1380 cm71, Dn=200 cm71) permits the conclusion that monodentate coor- dination of the acetate groups takes place. Apparently the structure of complex 40 is analogous to that of palladium(+1) carbonyl halide complexes with diphosphine and diarsine groups.9, 10 The coordination sphere of each metal atom consists of two phosphorus atoms of the two diphosphine ligands, the carbon atom of the carbonyl group, and the oxygen atom of the terminal acetate group.The analogous complex [Pd(dppm)(O2CCF3)]2(m-CO) has been isolated in the course of the study of the kinetics of the interaction of the dimer [Pd(dppm)(O2CCF3)]2 and carbon mon- oxide in dichloromethane.129 The complex has been characterised by IR spectroscopy (nCO=1720 cm71) and NMR spectroscopy involving different nuclei and also by X-ray diffraction analysis.The two palladium atoms in the molecule of the complex, linked by a bridgingCOgroup, are at a distance of 2.896 A.This is less than the sum of the covalent radii of palladium (2.98 A) and is appreciably shorter than the Pd ± Pd distance (*2.7 A) charac- teristic of the Pd(+1) dimers. Each palladium atom in the molecule is surrounded by the same set of atoms: the carbon atom of the carbonyl group, two phosphorus atoms of the two diphosphine ligands, and one oxygen atom of the terminal carboxylate ligand.If the Pd ± Pd bond is excluded from consid- eration, then environment of each palladium atom consists of an almost planar square (the deviation from the plane of the square is not more then 0.02 A), whereas the environment of the second metal atom is close to being tetrahedral. The authors believed that in this case the distortion of the symmetrical structure may be caused by a redistribution of electron density in the complex and that the latter can be regarded as an intermediate in the dissoci- ation of the trifluoroacetate complex leading to [Pd2(m- dppm)2(m-CO)(O2CCF3)]+(CF3CO2)7. R=CD3, C2H5, C6H5, CF3, CCl3, CH2Cl.Pd4(CO)4(OAc)4+4RCOOH =Pd4(CO)4(RCOO)4+4AcOH, 39 Pd4(CO)4(OAc)4+4dppe 40 2Pd2(dppe)2(CO)(OAc)2+2CO.Pd Pd C Pd Pd C C O O C O O O C C O O C C O O O O O H3C CH3 CH3 H3C Figure 6. The structure of the cluster Pd4(m-CO)4(m-OAc)4 according to X-ray diffraction data.122 Palladium carbonyl complexes 497The interaction of complex 38 and dppe, taken in nine ± tenfold excess in tetrahydrofuran, leads to the formation of a cationic complex in accordance with the equation Pd4(CO)4(OAc)4+8dppe 2[Pd2(CO)(dppe)4](OAc)2+2CO. According to 31P NMR data the diphosphine ligands in the complex are involved in both bridging and terminal coordination, whereas the carbonyl group serves as a bridge between the two metal atoms.Treatment of complex 38 with PPh3 leads to the complex Pd2(CO)(PPh3)(OAc)2: Complex 41 contains a bridging CO group (nCO=1800 cm71).Depending on the conditions in the synthesis, compounds differing in the coordination of the acetate groups are formed. Thus, when the process is carried out in tetrahydrofurane a complex with acetate bridges is produced [nas(- COO)=1530 cm71, ns(COO)=1400 cm71, Dn= 130 cm71], whereas the reaction of complex 38 with PPh3 in benzene leads to a compound with terminal OAc groups [nas(- COO)=1610 cm71, ns(COO)=1350 cm71 Dn=260 cm71]. In the interaction of complex 38 and Bipy at an initial Bipy concentration>0.5 M and a concentration of complex 38 of 561073 M all the CO molecules are displaced:49, 50 Pd4(CO)4(OAc)4+2Dipy 2Pd2Dipy(OAc)2+2CO.At low Bipy concentrations (< 561073 M), only half of the CO's are displaced from complex 38: Pd4(CO)4(OAc)4+2Dipy 2Pd2Dipy(CO)(OAc)2+2CO.In contrast to Bipy, o-phenanthroline (Phen) displaces from complex 38 only half of the CO groups present even at the highest ligand concentrations. In the reaction of complex 38 with Phen, in the proportions Phen : Pd=1 : 1, in acetic acid, a tetranuclear cationic complex is formed:130 The complexes have been characterised by IR spectroscopy (nCO=1800 cm71) and X-ray diffraction data.130 In the cation 42, the four metal atoms form a tetrahedron, to the two mutually perpendicular edges of which bridging CO groups are coordi- nated.The remaining metal ± metal bonds are unbridged. One Phen molecule is coordinated as a chelate species to each palla- dium atom, while all the acetate groups are displaced to the outer sphere On passing from complex 38 to 42, all the metal ± metal distances change significantly. The Pd ± Pd distance in the Pd(m- CO)Pd fragments increases from 2.667 A in complex 38 to 2.890 A in complex 42, while the unbridged Pd ± Pd bond is shortened from 2.909 to 2.718 A to compare with the Pd(m-OAc)Pd fragment in the initial carbonyl acetate.It has been suggested that the conversion of the neutral molecule of complex 38 into the cationic cluster 42 involves the interaction of complex 38 and Phen, which results in the displace- ment of the acetate groups and the formation of the dication [PhenPd(CO)2PdPhen]2+.This is followed by the elimination of half of the coordinated CO groups and the dimerisation of coordinatively unsaturated species produced, which leads to the appearance of cluster 42.g. The structure of palladium(+1) complexes containing a bridging CO group The palladium(+1) complexes containing a bridging CO group have been reliably characterised by the results of spectroscopic and structural studies (Table 3). In the complexes where the palladium atoms are not linked to n-donor atoms (Nos 1 ± 3), the metal ± metal distances are close to that in metallic palladium, which is 2.7 A.The stretching vra- tional frequencies of the CO group in these complexes vary from 1900 to 2000 cm71. The presence of powerful electron-withdraw- ing groups, for example CF3COO (complex No. 4), increases the stretching vibrational frequency of the CO group to 2000 cm71. After the insertion of n-donor phosphorus atoms (axial phosphine ligands in complex Nos 5 and 6) into the coordination sphere of palladium, the Pd ± Pd distance does not change, but nCO falls sharply.Only the presence of chelating phenanthroline molecules, linked to one palladium atom (No. 7), or of bridging diphosphino and diarsine ligands (complexes Nos 8 ± 10) leads to a simultaneous increase in the Pd ± Pd distance and decrease in the stretching vibration frequency of the bridging carbonyl ligand.It is of interest that powerful electron-accepting ligands, for example trifluoroacetate groups, in the axial positions in complex No. 4 (Table 3) somewhat `extinguish' the effect involving the donation of n electrons by phosphorus atoms, which is probably the reason for the shortening (compared with other complexes containing diphosphines and diarsines) of the Pd ± Pd distance.As mentioned above, all the complexes discussed are diamag- netic. On the other hand, the metal ± metal distance in complexes with diphosphines and diarsines (complexes Nos 9 and 10 in Table 3) greatly exceeds the sum of the covalent radii palladium and for this reason the pairing of electrons as a result of the formation of the Pd ± Pd bond is impossible. The diamagnetism of such complexes has been explained by the exchange of charges in the Pd2(m-CO) fragment with participation of the structure Pd4(CO)4(OAc)4+4PPh3 41 Pd2(CO)(PPh3)2(OAc)2+2CO.Pd4(CO)4(OAc)4+4Phen [Pd4(CO)2Phen4](OAc)4+2CO. 42 N N N N N Pd Pd Pd Pd C C N N N O O 42 4+ + Pd C O7 Pd + Pd C O7 Pd + Pd C O7 Pd Pd C . O Pd Table 3.Certain spectroscopic and structural data for palladium(+1) complexes containing a bridging carbonyl group. No. Complex n-Do- Pd ± Pd nCO Ref. nor /A (XRD) /cm71 atom 1 [cyclo-Pd2(m-CO)2](SO3F)2 2.698 1977 35 2 [Pd2(m-CO)2Cl4]27 2.697 1903 22 3 Pd4(m-CO)4(m-OAc)4 2.663 1934, 1975 7, 8 4 Pd4(m-CO)4(m-O2CCF3)4 2.700 a 2002, 1940 45 5 Pd2(m-CO)(Cl)(PEt2Ph)3 P 2.652 7 27 6 Pd2(m-CO)(Z5-Cp)(Br)(PPri 3)2 P 2.670 1828 38 7 [Pd4Phen4(m-CO)2](OAc)4 N 2.890 1800 52 8 Pd2(m-CO)(m-dppm)2(OAc)4 P 2.980 1720 51 9 Pd2(m-CO)(m-dmpm)2Cl2 P 3.169 b 1720 32, 33 10 Pd2(m-CO)(m-dam)2Cl2, As 3.274 b 1710 5, 6, 29 a According to EXAFS data.b For comparison, the Pd ± Pd distance in Pd2(m-dppm)2Br2 is 2.699 A. 498 T A Stromnova, I I MoiseevEvidently the hypothesis of the resonance of structures with different numbers of unpaired electrons (Pd+...Pd+ and Pd2+...Pd0) conflicts with the main principles of the resonance theory.131 At the same time, the bond in the group made up of two palladium atoms and one carbon atom of the carbonyl group can be described approximately in terms of a localised three-centre system of molecular orbitals, similar to that which occurs in the allyl radical.The diamagnetism of the dinuclear palladium(+1) compounds under consideration implies, in terms of this approx- imation, that four electrons (one from each palladium atom and two from the carbonyl ligands fill the bonding and nonbonding molecular orbitals of the system (Fig. 7). Within the framework of this approximation, all the changes in the Pd ± Pd distance and nCO frequencies should be considered from the standpoint of the variation of the electron density on the metal atoms and the variation of the levels of the atomic orbitals in view of the change in the nature of the ligand atoms coordi- nated to palladium.It is easily seen that in the isomeric ylide the frequencies of the carbonyl group should be significantly reduced compared with the frequencies of the dimetallocyclopropanone.The complexes presented in Table 3 may be regarded as models of the intermediate states on the path to the isomerisation of the dimetallocyclopropanone to the corresponding dimetalloylide: The strong electron-donating influence of the ligands linked to the palladium atom may weaken the Pd ± Pd bond and increase the distance between the atoms involved.This is equivalent to the isomerisation of the cyclopropanone to an ylide with an opened ring: 2. The reactivity of palladium(+1) carbonyl complexes The available information about the chemical properties of palladium(+1) carbonyl halides is limited to data concerning their behaviour in relation to water and under thermal decom- position conditions.The reactivity of the carbonyl carboxylate complexes and their derivatives has been investigated much more fully. a. Ligand substitution reactions in palladium(+1) carbonyl carboxylates Synthesis of a carbene cluster by the substitution of CO groups When certain acido-ligands (for example, the acetate groups in complex 38) are substituted by other carboxylates, new carbonyl carboxylate complexes are formed.The substitution of neutral carbonyls has been observed in the interaction of complex 38 with diphenyldiazomethane.132 The interaction of complex 38 and Ph2CN2, which is capable to generate diphenylcarbene groups in situ, results in the displace- ment of the CO groups and the formation of a new complex (Fig. 8); Complex 43 contains bridging acetate groups.It has been established by IR spectroscopic studies that nas(COO)= 1560 cm71, ns(COO)=1400 cm71, and Dn=160 cm71. The signals due to the protons of the phenyl and acetate groups in the ratio HPh/HOAc=10/3, i.e. there is one CPh2 ligand per acetate group, have been observed in the 1H NMR spectrum. According to EXAFS data, the coordination sphere of each palladium atom in cluster 43 includes two metal atoms at a distance of 2.67 A, one metal atom at a distance of 3.65 A, and also two light atoms in each case (in all six) at distances of 1.86, 2.03, and 3.25 A.These values agree with the structure proposed by Stromnova et al.132, 133 (Fig. 8). Oxidative substitution of carbonyl ligands The coordinated carbon monoxide in transition metal carbonyl complexes may be oxidised by an outer-sphere oxidant, for example trimethylamine N-oxide: R3N=O+LnM(CO)m R3N+N+CO2+LnM(CO)m71, or by other compounds containing the N±O group.73, 134 In a study of the interaction of complex 38 with organic compounds containing the N±O group, such as nitrobenzene, nitrosobenzene, azoxybenzene, phenylhydroxylamine, and trime- thylamine N-oxide, it has been established 135 that only nitro- sobenzene and phenylhydroxylamine are capable of oxidising the coordinated carbon monoxide under mild conditions, like trime- thylamine N-oxide: Pd Pd C O + Pd Pd C O7 + Pd Pd C O7 .C C C O + C C C O7 + C C C O7 + C C C O7 . Pd4(CO)4(OAc)4+4Ph2CN2=Pd4(CPh2)4(OAc)4+4CO +4N2. 43 Pd4(CO)4(OAc)4+PhNHOH Pd4(CO)4(OAc)4+Me3NO Pd+CO2+Me3N+Ac2+..., Pd+CO2+PhNH2+Ac2+Ac2O, 2Pd (+1) C O Figure 7. The three-centre system of molecular orbitals in the group comprising two palladium atoms and one carbon atom. Pd Pd Ph2C Ph2C Pd CPh2 CPh2 Pd O O O O O O O C O C C R C R R R Figure 8. The structure of the cluster Pd4(m-CPh2)4(m-OAc)4 according to EXAFS data.132, 133 Palladium carbonyl complexes 499The complex Pd2(m-Ac)2(Ph ¡ÀN¡ÀC6H4 ¡À NO)2 44, containing a new ligand D phenyl-o-nitrosophenylamide, is formed in the reaction between complex 38 and nitrosobenzene together with organic products (azoxybenzene, azobenzene, and aniline) of the transformation of the NO-containing compound .136 This ligand is obtained by the reaction of a nitrene species (the product of the deoxygenation of nitrosobenzene when the latter is acted upon by the CO group) and a second nitrosobenzene molecule.Reactions of complex 38 with anionic carbonylmetallates Presumably, like the substitution of the OAc groups in complex 38 by carboxylate groups, the same groups might; be substituted by more complex ones, for example the carbonylmetallates Co(CO)¡¦4 , V(CO)¡¦6 , and CpMo(CO)¡¦3 . Such reactions would make it possible to obtain heteronuclear palladium-containing clusters. Indeed, the reaction of complex 38 with NaCpMo(CO)3 in tetrahydrofuran entails the formation of the octanuclear cluster Na2{Pd4[CpMo(CO)3]4} . 2THF.137, 138 X-Ray diffraction analysis has shown that the structural unit in the crystals investigated consists of the centrosymmetric anion {Pd4[CpMo(CO)3]4}27, two sodium cations, and two tetrahydro- furan molecules.The palladium atoms are located at the vertices of a square (the Pd ¡À Pd distances are 2.675 ¡À 2.691 A). Each molybdenum atom forms an almost regular triangle with the two palladium atoms (the Pd ¡ÀMo distances are 2.723 ¡À 2.741 A), and all the metal atoms lie in the same plane. The observed metal ¡À - metal distances correspond to single bonds.Three carbonyl ligands are linked to each molybdenum atom and one of the carbonyls is a m3-bridge on the Pd2Moface, while the other two are bridges on the Pd ¡ÀMo bonds (Fig. 9). The IR spectrum of the complex contains bands at 2000, 1945, 1900. 1870, and 1830 cm71 referring to the coordinated CO groups. In the course of the formation of the octanuclear cluster, not only are the acetate ligands substituted by the CpMo(CO)¡¦3 anions, but the neutral CO molecules are also displaced.The vacancies liberated in the coordination sphere of palladium in this process are occupied by the CO molecules coordinated to the molybdenum atom, which are converted into bridging ligands. However, it is significant that the process is not restricted to the substitution of ligands: during the reaction, the Pd(+1) atoms are reduced, apparently as a result of the one-electron oxidation of the CpMo(CO)¡¦3 anions.The overall process is described by the equation Pd4(CO)4(OAc)4+6Na[CpMo(CO)3]= =Na2{Pd4Mo4Cp4(CO)12}+4CO+[CpMo(CO)3]2+4NaOAc. The dimer [CpMo(CO)3]2 has been found in the reaction products If it is assumed that the molybdenum atoms incorporated in the complex retain a zero oxidation state, then the observed reaction can be regarded as the reduction of the Pd(+1) atoms to Pd(+0.5).The resulting cluster is the first example of a compound in which eight metal atoms linked by the metal ¡À metal interaction lie in the same plane. The interaction of the another carbonylmetallate D [(Z5-C5H4Me)Mn(CN)(CO)2]7 (in the form of the sodium salt) D and complex 38 leads,139 as in the preceding case, to the displacement of all the ligands and the formation of the octanu- clear cluster [(OC)Pd(m-NC)Mn(Z5-C5H4Me)(CO)2]4 45.In con- trast to the preceding reaction, in this case the formation of the cluster is not accompanied by any kind of oxidation-reduction reactions and, if it 1s assumed that the carbonylmanganate ligand retains a formal 71 change, then the formal oxidation state of palladium in cluster 45 remains +1.The structure of cluster 45 is unusual. It can be regarded as two mutually orthogonal bent metal chains Mn¡À Pd2 ¡ÀMn with short metal ¡À metal distances. The metal chains are linked to one another by four cyanide bridges. Furthermore, one terminal carbonyl group is linked to each palladium atom, while the coordination environment of each manganese atom is supplemented by the methylcyclopentadienyl ligand and two carbonyl groups (one terminal and the second behaving as a semibridge in relation to the nearest palladium atom).Pd4(CO)4(OAc)4+PhNO Pd2(m-OAc)2(Ph N C6H4 NO)2+CO2+PhN(O) NPh. N Pd N O OAc OAc O N Pd N 44 O Pd C C N Pd Mn Cp0 CO Mn C C Cp0 C O O C OC OC Cp0 Mn C C Pd N N OC CO CO Pd N O O O Cp0 C C Mn 45 Mn C N Pd N C Pd Mn Mn Pd C N Pd N C Mn Mo Pd Pd Pd Pd Mo Mo Mo Figure 9.The structure of the anion {Pd4[CpMo(CO)3]4}27according to X-ray diffraction data.137, 138 500 T A Stromnova, I I Moiseevb. Reactions of palladium carbonyl acetate with oxygen-containing nucleophiles In the presence of alkali metal acetates in solution in glacial acetic acid, complex 38 decomposes to metallic palladium in accordance with the equation Analysis of the kinetic data permits the conclusion that the reaction mechanism involves a repeated alternation of the follow- ing cycles: insertion of CO in the Pd ±OAc bond, elimination of CO2 with formation of an acyl fragment, liberation of Ac2O, and the formation of the Pd ± Pd bond.Overall, the process proceeds via the reductive condensation of the palladium clusters.124, 140 Presumably the interaction of complex 38 and water described by the equation Pd4(CO)4(OAc)4+2H2O=4Pd+4AcOH+2CO+2CO2, also takes place via a reductive condensation mechanism. The reaction of complex 38 with aliphatic C1 ±C3 alcohols proceeds simultaneously via several paths with formation of CO2 and dialkyl carbonate � the products of the oxidation of the coordinated COmolecules, as well as carbonyl compounds arising owing to the oxidation of the corresponding alcohols. It has been observed that the reaction with ethanol results in the appearance of both dialkyl carbonates and carbonyl compounds� acetalde- hyde and its acetals. In the case of isopropyl alcohol, dialkyl carbonate and carbonyl compounds are produced.The reaction of cluster 38 with methanol leads to the formation of only methyl acetate. It has been shown 141, 142 that the reactions of complex 38 with alcohols proceed via several paths with formation of carbonyl [LnM(CO)], alkoxy- (LnMOR), alkoxycarbonyl (LnMCOOR), and acyl [LnMC(O)R] derivatives of palladium.In the interaction of complex 38 with phenol,143, reactions via three paths, leading to the formation of phenyl acetate, diphenyl carbonate, and phenyl salicylate (paths A, B, and C), are also observed. c. Oxidation of the coordinated CO groups in the carbonyl carboxylate complexes The thermal decomposition of carbonyl complexes leads as a rule to the elimination of carbon monoxide.The thermolysis of the carbonyl carboxylate clusters proceeds via a fundamentally differ- ent path. It has been observed that the thermolysis of the clusters in the absence of oxygen entails the inner-sphere oxidation of the neutral ligand with formation of CO2. In the reaction, an oxygen atom is transferred from the carboxylate group to the carbonyl group, i.e.the acetate group functions as an oxygen donor (oxidant):124, 144, 145 Pd4(CO)4(OAc)4 Pd+CO2+Ac2. The oxidation-reduction process which has been put forward, in which carbon monoxide is oxidised by the oxygen of the coordinated carboxylate groups, apparently involves the insertion of CO in the metal ± oxygen bond with formation of an unstable intermediate: The decomposition of the intermediate leads to the elimina- tion of CO2 and the formation of reduced forms of palladium and coordinated acyl groups: The recombination of the two acyl ligands bound to the core of the cluster affords the biacyl: 2Pd[RCO] 2Pd+RCOCOR.If the thermolysis of the carbonyl carboxylate clusters is carried out in benzene or toluene solution, the main product of the oxidation of the coordinated CO groups is not CO2 but an arenecarboxylic acid: Presumably oxidative addition of an arene molecule to the palladium core with dissociation of the H± Ar bond, followed by the insertion of CO2 in the Pd ±Caryl bond, takes place simulta- neously with the transfer of an oxygen atom from the carboxylate to CO: The inner-sphere transfer of an oxygen atom from the carboxylate group to the neutral carbonyl group is a fairly general reaction.Complexes containing as ligands carboxylate groups having both powerful electron-donating substituents at the car- boxylate group [the acetate CH3COO and the pivalate (CH3)3COO] and powerful electron-accepting substituents (the trifluoroacetate CF3COO and the monochloroacetate CH2ClCOO) are capable of entering into this reaction.The CO2 produced by the reaction is coordinated to the palladium atoms of the decomposing cluster, exhibits unusual properties, and is capable of being inserted into the C±H bonds of aromatic compounds. Summarising the data on the synthesis, structures, and reac- tivities of the carbonylpalladium(+1) complexes, one should note that their number is appreciably smaller than that of carbon- ylpalladium(0) complexes.All the palladium(+1) complexes contain an even number of metal atoms (2 or 4). Since the main structural element of polymeric carbonyl halides is the species [X ± Pd(m-CO)2Pd ± X] and there is no metal ± metal bond between such species, it appears legitimate to regard them as dinuclear. Complexes with a higher nuclearity have not been described for Pd(+1).All the palla- dium(+1) carbonyl complexes are diamagnetic and the pairing of electrons at the two metal atoms, having the d9 electronic config- uration, takes place either as a result of the formation of a metal ± metal bond (with a Pd ± Pd distance of *2.7 A) or via the formation of a three-centre four-electron bond incorporating two palladium atoms and one carbon atom of the bridging carbonyl ligand. Pd4(CO)4(OAc)4 4Pd(0)+2CO+2CO2+2Ac2O.NaOAc [Pd(CO)(OAc)]4 + OH A OH C OH COOPh O O C O B Pd Pd O C O C O R Pd Pd O O C R C O Pd Pd O C O C O R CO2+2Pd[RCO] Pd4(CO)4(OAc)4+Ar H Pd+CO2+ArCOOH. (*10%) (*80%) Pd Pd O C O C O R +Ar H H Ar Pd Pd O C O C O R Pd[RCO] +ArCOOH. Palladium carbonyl complexes 501In complexes containing only carbonyl and halide, carbox- ylate, or fluorosulfate ligands, the Pd ± Pd distance is close to 2.7 A and the stretching vibrational frequency of the m-CO group is in the range from 1900 to 2000 cm71.Being relatively resistant to thermal decomposition, these complexes are fairly sensitive to the action of oxygen-containing nucleophiles and, when treated with, for example, water or lower aliphatic alcohols, decompose with formation of palladlum metal.The introduction of n-donor ligands into the complexes leads as a rule to an increase in the metal ± metal distance in the Pd(m-CO)Pd fragment and to a decrease in the stretching vibra- tional frequency of the m-CO group. The resistance of these complexes to reductive decomposition on treatment with nucleo- philes increases simultaneously and some of them, for example, [Pd4(CO)2Phen4](OAc)4 or [Pd(m-dmpm)Cl]2(m-CO), are readily soluble and are stable even in aqueous solutions.In contrast to palladium(0) carbonyls, the reactivity of palla- dium(+1) carbonyls is not limited to ligand substitution reactions and has been investigated much more fully. In our view, the oxidation ± reduction reactions of carbonyl carboxylate clusters is of greatest interest.In the first place, this is the reaction involving the intramolecular transfer of an oxygen atom from the carbox- ylate ligands to the coordinated carbonyl group. This reaction can serve as a model for the whole series of oxidation ± reduction processes occurring with participation of CO and catalysed by palladium compounds.V. Palladium(+2) carbonyl complexes A considerably smaller number of palladium(+2) complexes with carbonyl ligands than palladium carbonyls containing the metal in other oxidation states have been synthesised and reliably charac- terised at the present time. This can probably be accounted for primarily by the striking tendency of Pd(+2) to be reduce by carbon monoxide and hence by the high lability of Pd(+2) carbonyls in proton-donating basic solvents. 1.Binary carbonyls The first studies on the chemistry of palladium(+2) carbonyls date back as early as the past century,6 but sufficiently definite conclusions concerning the structure of the palladium(+2) car- bonyl complexes have been reached on the basis of studies carried out in the last three ± five years. In these investigations, the key compounds were characterised by X-ray diffraction. This applies primarily to the binary cationic carbonyls.A series of cationic carbonyls have been obtained quite recently in strong protic acids (HSO3F) or superacids (HSO3F . SbF5).146, 147 In the solid state, the carbonyl cations were stabilised by very weakly basic anions SO3F7 or Sb2F¡11.These are singly-charged linear cations [M(CO)n]+ [M=Au or Ag, n=1 or 2;148 ± 150M=Hg, n=1 (Ref. 151)] or square-planar cations [M(CO)4]2+ [M=Pd(46) or Pt(47)152, 153]. Complexes 46 and 47 have been obtained as a result of the reductive carbonylation of [Pd(SO3F)3]2 or Pt(SO3F)4 respectively and the subsequent solvolysis of the resulting neutral complex in liquid antimony(+5) fluoride.Treatment of the solid salt containing Pd(+2) and Pd(+4) with CO (PCO=0.5 atm, 25 8C) affords Pd(CO)2(SO3F)2 in accordance with equation According to IR spectroscopic data, when the reaction is carried out in the absence of solvent, complex 48 is formed exclusively as the cis-isomer. On the other hand, when fluorosul- fonic acid HSO3F is used as the solvent, a mixture of isomers is produced.In liquid antimony fluoride in the presence of free carbon monoxide (PCO=0.5 atm, 60 ± 80 8C), complex 48 is converted into [Pd(CO)4][Sb2F11]2 in accordance with the equa- tion Pd(CO)2(SO3F)2+8SbF5+2CO [Pd(CO)4][Sb2F11]2. Complex 47, the platinum analogue of complex 46, was obtained in the same manner. Complexes 46 and 47, isolated in the form of white powders, are thermally fairly stable (they begin to decompose at 155 and 200 8C respectively), but are extremely sensitive to moisture.The stretching vibration frequencies of the CO groups in both complexes are very high (2259 cm71 for complex 46 and 2261 cm71 for complex 47) and are higher by approximately 120 cm71 than for free carbon monoxide. These data suggest that, as in the neutral and anionic divalent palladium carbonyl halides, in these compounds the s-donor properties of the CO group greatly predominate over its p-acceptor properties Dicarbonylpalladium fluorosulfate 48 (a precursor of com- plex 46) has been obtained 154 in the form of a thermally fairly stable yellow substance (decomposition temperature 117 8C).Complex 48 is the first example of a thermally stable compound in which two monodentate carbonyls are coordinated to one Pd(+2) atom.Two other complexes of the type Pd(CO)2R2 (R=C6F5 or C6Cl5),155 where two terminal carbonyls are coor- dinated to one palladium atom, are much less stable and already begin to decompose at a temperature below 730 8C. The fluo- rosulfate group in complex 48, which has a pronounced electron- withdrawing capacity, probably enhances the electron-accepting properties of the metal atom.The stretching vibrational frequency of the CO group is 2218 cm71, which is the highest observed for all the known neutral carbonyl complexes of platinum metals. Complex 48 is the first example of a structurally characterised neutral palladium carbonyl.156 Crystals of the cis-isomer of compound 48, suitable for X-ray diffraction, have been obtained on slow recrystallisation of the complex from its saturated solution in fluorosulfonic acid.The palladium atom in the molecule of compounds 48 is surrounded by two cis-disposed terminal carbonyl groups and monodentate fluorosulfate groups located at the corners of a planar square.The C±O bond is fairly short (1.106 A), which agrees well with its high frequency nCO in the IR spectra and with the conclusion that the p-back-donation from the metal to the carbonyl group is absent or at least greatly reduced. A weak interaction of the noncoordinated oxygen atom with the carbon atom of the carbonyl groups has been noted (the C±O distance is 2.819 ± 3.172 A).In the absence of p-back- donation, secondary contacts, with participation of the electro- philic carbon atom of the CO group as an electron acceptor probably promote somewhat charge compensation and exert a stabilising influence on the entire molecule. 2. Neutral and anionic palladium(+2) carbonyl halides The synthesis of the three carbonyl derivatives of palladium PdCl2 .(CO), PdCl2 . (CO)2, and PdCl2 . (CO)3 on interaction of solid PdCl2 with carbon monoxide at 260 8C, described by Fink,6 could not be reproduced subsequently. Manchot 157 showed that complexes having this composition are actually formed by the platinum present as an impurity in the initial palladium salt. However, one of these complexes has been obtained from pure palladium chloride.The divalent palladium carbonyl Pd(CO)Cl2, which is extremely unstable when acted upon by atmospheric moisture, has been obtained 158 at 20 8C from solid PdCl2 and carbon monoxide, from which water vapour had been carefully removed and which had been saturated with anhydrous methanol vapour, and also from a suspension of PdCl2 in anhydrous methanol on treatment with dry carbon monoxide: Pd(CO)Cl2+H2O=Pd+2HCl+CO2.A similar complex Pd(CO)Br2 has been obtained by the interaction of PdBr2 and carbon monoxide saturated with meth- anol vapour and also on treatment of a suspension of K2PdBr4 in dioxane containing 0.5% ± 2.0% of H2O with carbon monoxide. Palladium(+2) bromide complexes are not carbonylated in anhy- drous dioxane.96, 158 It has been suggested 158 that the above palladium(+2) carbonyl halides are dimeric and that the anions Pd(+2)Pd(+4)(SO3F)6+5CO 2Pd(CO)2(SO3F)2+S2O5+CO2. 48 502 T A Stromnova, I I Moiseev[Pd(CO)X3]7 are also present in a solution containing an excess of halide ions. In these anionic complexes, isolated in the form of a salt with the tetetrabutylammonium cation,159 the CO molecule may be bound as a monodentate ligand, like the alkene in a p- complex of the Zeise's salt type.The complexes: listed above (both neutral and anionlc) were initially insufficiently fully characterised and the information available about their structure was tentative. However, later the structure of analogues of these compounds was established quite definitely. Thus the carbonylation of PdC12 in thionyl chloride under fairly strong conditions (PCO=50 atm, 120 8C) yielded 160 Pd2(CO)2Cl4 49: 2PdCl2+2CO Pd2(CO)2Cl4.The interaction of NH2Et2Cl and compound 49 in solution in CH2Cl2 leads to the formation of the anionic complex NH2Et2[Pd(CO)Cl3] 50 .Complexes 49 and 50 have been charac- terised by elemental analyse and IR spectroscopy (n=2167 cm71 for complex 49 and 2141 cm71 for complex 50).Complex 49 is stable only in a CO atmosphere, decomposing in the absence of CO. The terminal CO groups are readily substituted by tertiary phosphines, forming initially Pd2(m- Cl)2Cl2(PPh3)2 and then trans-PdCl2(PPh3)2. It has been suggested that complex 49 is a dimer with two chloride bridges; the two terminal carbonyl ligands are trans- disposed relative to the metal ± metal axis. This is in fact the structure of the platinum analogue Pt2(CO)2I4 isolated in a crystalline form and characterised by IR spectroscopy (nCO=2122 cm71) and X-ray diffraction data.161 This is a dimer with a nonbonding metal ± metal spacing, two bridging iodide ligands, and two trans-disposed terminal carbonyl groups.The complex to which the authors attribute the composition PdCO)Cl2(PPh2-polymer) may have a similar structure (but with carbonyl and not halide bridges). The complex has been obtained 162 by carbonylating the corresponding benzonitrile derivative `attached' to the polymeric support via a phosphide bridge. The stretching vibrational bands of the CO group in the range 1944 ± 1964 cm71 have been found in the IR spectrum of the complex, indicating that these groups are of the bridging type.On the basis of spectroscopic data for the anion of complex 50, a structure has been proposed in which the metal atom has a planar environment (three chlorine atoms and one carbon atom of the terminal carbonyl ligand). It has been found that the anion of the salt (NBun4 )+[Pd(CO)Br3]7 51 has precisely this structure. Complex 51 has been obtained by treating a solution of (NBun4 (PdBr3) in ttetrachloroethane with CO (PCO=1 atm).163 The carbonylation of PdBr2(PCO>70 atm) leads to the formation of Pd2(CO)2Br4, which, judging from the IR spectra (nCO=2145 cm71), is constructed similarly to complex 49.The iodide analogue of complex 49 could not be obtained. However, treatment of a solution of PdI2 in toluene with CO in the presence of stoichiometric amounts of I7 leads to [Pd(CO)I3]7.The latter has been isolated as a salt with (NBun4 )+ and has been charac- terised by IR spectroscopy (nCO=2015 cm71). According to X-ray diffraction data for complexes 50 and 51, the palladium atoms in the anions [Pd(CO)Cl3]7 and [Pd(CO)Br3]7 have a square-planar environment with the usual Pd ±CCO (1.87 A for both anions) and C±O (1.10 and 1.11 A respectively) distance.The stability of the palladium carbonyl halide complexes (both neutral and anionic) decrease in the sequence Cl>Br>I.163 The study of the kinetics of the exchange of the carbonyls in the carbonyl halides between [M(13CO)X3]7 and 12CO (IR and 13CNMRspectroscopy) showed that the carbonyl ligands in these complexes are bound fairly strongly and that the rate of exchange of the carbonyls is lower almost by six orders of magnitude than the rate of exchange of ethene in the analogous ethene p-comp- lexes.164 The formation of complex 49 in thionyl chloride has been investigated and the thermodynamic constants of the process have been determined, which made it possible to estimate the energy of the Pd ±CO bond (*24 kcal mol71).163 Analysis of the thermo- dynamic and spectroscopic data led to the conclusion 155, 165 that the contribution of metal-to-ligand the p-back-donation in the palladium carbonyls is slight and that the strength of the Pd ±CO bond is determined almost exclusively by the s-donor capacity of the carbonyl ligand.This conclusion appears fairly justified for the neutral and anionic palladium(+2) carbonyl halides, but the question of the contribution of the s-donor and p-acceptor interactions in organopalladium compounds with carbonyl ligands (see below) requires additional discussion.The contribution of a particular type of bonding of the carbonyl ligands may be inferred from spectroscopic data.29 As a result of the predominance of the s-donor properties over the p-acceptor properties in the coordinated carbon monoxide mole- cule, the stretching vibration frequency of the CO group becomes higher than in free CO, while the chemical shift of the carbon atom in the 13C NMR spectra is observed at d=120 ± 130 ppm, in contrast to 190 ± 220 ppm for the classical carbonyls. 3. Palladium(+2) carbonyl halides containing additional stabilising ligands Palladium(+2) carbonyl halide complexes with additional stabil- ising ligands have been obtained in both neutral and anion forms.For example, the reaction of CO with a suspension of the [Pd2Cl2(PEt3)4]2+ fluoroborate in chloroform afforded the cati- onic complex [PdCl(CO)PEt3)2][BF4], which has been character- ised by elemental analysis and IR spectroscopy (nCO=2140 cm71) and which is comparatively stable under a CO atmosphere.In the absence of CO from the gas phase over the complex a carbon monoxide molecule is evolved and the initial complex is reformed.166, 167 In this case it is reasonable to postulate that the CO groups are terminal if the palladium atom contains four ligands in the coordination sphere, as in the majority of other palladium(+2) complexes.For the complex anion [Pd(CO)(SnCl3)Cl]7 isolated 168 as a salt with NEtá4 the frequency nCO is equal 2054 cm71. The neutral complexes Pd(CO)(L)Cl2, where L=dimethyl sulfoxide, diethyl sulfoxide, or diethyl sulfide, have been obtained 169, 170 by carbonylating [PdLCl2]2. The authors attribute a mononuclear structure to these compounds, but on the basis of IR spectroscopic data (nCO=1927 ± 1977 cm71) one may postu- late that the complexes are at least dinuclear and contain bridging CO groups.The complex Pd(PhN=NPh)Cl2(CO), obtained by the reac- tion of CO and Pd(PhN=NPh)Cl2(PhCN),171 should also prob- ably be at least dimeric. The stretching vibrational frequency of the CO group in the complex is 1900 cm71, which definitely indicates a bridging coordination of the ligand.Unfortunately, none of the complexes of this type have been characterised by X-ray diffraction analysis. 4. Organopalladium compounds containing carbonyl ligands The complexes obtained by the reaction of carbon monoxide and dimeric palladium complexes containing oxime ligands may be regarded as an example of palladium(+2) compounds containing both the carbonyl ligand and an organopalladium group with a Pd ±C s-bond:172 R=Ph, Me, H. 2CO Pd N C CO X O R H 52 Pd C R N OH X 2 Palladium carbonyl complexes 503Complexes of type 52 have been isolated in the solid state and characterised by elemental analysis and IR spectroscopy (nCO=2108 ± 2136 cm71). The dimeric complexes containing an organopalladium group with a Pd ±C s-bond proved to be convenient precursors for the synthesis of heteronuclear compounds.173 Thus the cyclopalla- dated dimers with the general formula [Pd(C±N)Cl]2 [C±N=di- methylbenzylamine (dmba), 2-(dimethylamino)-toluene (dmat), or 8-methylquinoline (8-mq)] react with anionic carbonylmetal- lates M7 {M7=[Co(CO)4]7, [Fe(CO)3(NO)]7, or [M(CO)3Cp]7, where M=Cr, Mo, or W}, the carbonylmetal- lates substituting one of the bridging chloride ligands and forming thereby the heterometallic complexes (C±N)Pd(m-Cl)(m- M)Pd(C±N) 53.The structures of complexes 53a {C±N=dmba, M7=[Co(CO)4]7} and 53b [C±N=dmba, M7=Mo(CO)Cp7] have been investigated by X-ray diffraction analysis. In both molecules the palladium atoms are at a distance exceeding the sum of the covalent radii (3.442 A for 53a and 3.241 A for 53b).In complex 53a, the Co atom is linked to two Pd atoms, two carbonyl groups are bridging in relation to the Pd ± Co bonds, and two others are coordinated terminally to the cobalt atom. In complex 53b, having a similar metal core, among the three carbonyl ligands two are bridges on the Pd ±Mo bonds, whilst the third is m3-coordinated to the Pd2Mo metallotriangle. Organopalladium carbonyl complexes of another type have been synthesised 174 by ligand substitution on treatment the complexes of the type Pd(C6F5)(ClO4)L2, where L=Bipy, Phen, tetraethylenediamine, or 2PR3 with CO (P=1 atm, 0 8C, tol- uene, chloroform, or acetone).Pd(C6F5)(ClO4)L2+CO=[Pd(C6F5)(CO)L2](ClO4). The available information about structures of this type is also limited to the results of elemental analysis, the conductivity of solutions, and IR spectroscopy (nCO=2132 ± 2160 cm71).There are no structural data for the complexes, but the frequencies, nCO, in the IR spectra indicate unambiguously the terminal coordina- tion of the carbonyl. The interaction of [PPh3(CH2Ph)]2Pd(R)Cl(m-Cl)]2 and car- bon monoxide (PCO=1 atm, 20 8C, CH2Cl2) yielded 175 the complexes [PPh3(CH2Ph)][Pd(R)Cl2(CO)] [R=C6H3(2-Me)(6- NO2) (54) or C6H2(NO2)3-2,4,6 (55)]. These complexes are stable in the solid state, but in solution they eliminate CO, reconverting into the initial state.The IR spectra of both complexes contain characteristic bands of terminal CO groups 2100 cm71 for 54 and 2115 cm71 for 55).The frequencies nCO are lower than for the cationic complex [Pd(C6H5)(CO)L2]+ (2132 ± 2163 cm71) 174 or the neutral complex [Pd(C6F5)2(CO)2] (2152 ± 2186 cm71),155 but higher than the frequencies nCO for the palladium carbonyl complexes containing the metal in lower oxidation states (for example, for Pd(CO)n, where n=4, 3 or 2, nCO=2060, 2044, and 2050 cm71 respectively 33). IR spectroscopic data indicate the occurrence of the `classical' p-donation in the complexes 54 and 55.The structure of complex 54 has been investigated by X-ray diffraction. In the anion, the palladium atom has a planar environment consisting of two cis-disposed chlorine atoms, the carbon of the terminal carbonyl group (the Pd ±C bond length is 1.853 A and the Pd ±C±O angle is 175.48) and the 2-methyl-6- nitrophenyl ligand.The C±O distance, amounting to 1.145 A, has the usual value for terminal carbonyl groups. Summarising the data presented on the structure of palla- dium(+2) carbonyl complexes, one should note that all the reliably characterised homonuclear complexes (Table 4) are monomers with terminal CO groups. In complexes containing halide and alkyl ligands (No. 2 ± No. 5, Table 4) apart from the terminal carbonyl, the strength of the Pd ±CCO bond is low, which is the reason for the low thermal stability of the carbonyls. The presence of the fluorosulfate group, which possesses strong electron-withdrawing properties, in the complex Pd(CO)2(SO3F)2 (No. 1, Table 4) leads to an increase in the electron-accepting capacity of the palladium atom. The consequence of such increase is, on the one hand, the strengthening of the Pd ±CCO bond, expressed by an unusually high thermal stability of palladium complexes with a terminal carbonyl group, and, on the other hand, extremely high stretching vibrational frequencies of the CO group.In the Pd(+2) complexes where the metal atom has the d 8 electronic configuration and a much smaller ability to donate the metal p electrons to the corresponding orbitals of theCOmolecule than in the palladium(0) and palladium(+1) complexes, an enhancement of the s-donor properties of the carbonyl has been observed.The enhancement of the s-donor and the weakening of the p-acceptor properties of the carbonyl ligand are accompanied by an increase in the stretching vibration frequencies of the CO group (Table 4).VI. Palladium carbonyl complexes containing the metal in fractional oxidation state The appearance of unusual or `intermediate', including fractional, oxidation states of transition metals is associated with the devel- opment of the chemistry of polynuclear compounds, especially the chemistry of clusters. Table 4. Palladium(+2) carbonyl complexes characterised by X-ray diffraction.No. Complex No. of Mode of nCO Pd ±C carbonyl C7O Geometrical Ref. CO groups coordination /cm71 bond length bond length environment bound to Pd of CO groups of Pd atom 1 Pd(CO)2(SO3F)2 2 Z 2218 a 1.94 1.106 cis-configuration, 152, 153 planar square 2 (NH2Et)2[PdCl3(CO)] 1 Z 2141 1.87 1.100 planar square 160 3 (Bun4 N)[PdCl3(CO)] 1 Z 2132 1.87 1.100 the same 163 4 (Bun4 N)[PdBr3(CO)] 1 Z 1.87 1.110 " 163 5 [PPh3(CH2Ph)]{Pd(R)Cl2(CO)] 1 Z 2100 1.853 1.145 " 175 6 dmbaPd(m-Cl)(m-M0)Pddmba 2 Z (on Co 2018 2.17 1.150 distorted square 173 ([M0]=[Co(CO)4]7) atom, w.r.t. 1963 2.13 1.140 (disregarding the Pd7Co 1880 Pd7CO bond) bonds) 1860 7 dmbaPd(m-Cl)(m-M0)Pddmba 2 m (w.r.t. 1844 2.28 1.150 distorted octahedron 173 ([M0]=[CpMo(CO)3]7) Pd7Mo bonds) 1770 2.20 1.140 (disregarding the m3 (on the 2.39 1.130 Pd7Pd bond) Pd2Mo triangle) a For free carbon monoxide, n=2140 cm71. 504 T A Stromnova, I I Moiseev1. The synthesis and structure of clusters containing a metal in a fractional formal oxidation state p-Allylbis(carbonylchloropalladium) having the composition p- C3H5[PdCl(CO)]2 56, described as early as 1964, should probably be regarded as the first palladium carbonyl in which the the formal oxidation state of the metal atom proved to be nonintegral.176 The authors proposed a composition for the complex where there are three singly-charged acido-ligands for two palladium atoms; the FOS of the metal in the complex is +1.5.The complex was obtained by passing carbon monoxide through a solution of a mixture of PdCl2 and HCl in methanol in which p-allylpalladium chloride had been suspended.The IR spectrum of the resulting bright-yellow precipitate, which did not dissolve in the usual organic solvents, contained an intense band at 1940 cm71, which the authors attributed to the stretching vibrations of the terminal CO group. The absence of bands in the range 286 ± 360 cm71, characteristic of the vibrations of the Pd ± Cl bond, led to the conclusion that the chloride ligands were involved in bridging coordination.The following structure of 56 was proposed: However, the structure of complex 56 is somewhat more compli- cated. On the basis of current data concerning the structure of palladium carbonyls, one may postulate that the carbonyl groups in this compound are bridges, like the chloride ligands.The bridging coordination of the allyl ligands cannot be ruled out either. Similar complexes with the composition p-C3H5[PdX]2(CO) 57, where X=Cl or Br, have been obtained by passing carbon monoxide through a methanol solution of p-allylpalladium chlor- ide.89 The authors suggest that the compounds formed are tetramers, in which bridging halide and semibridging carbonyl groups alternate: The presence of three singly-charged acido-ligands in the molecule containing two metal atoms makes it possible to assign a formal oxidation state of +1.5 to palladium.Unfortunately, in both cases the substances were characterised only by elemental analysis and IR spectroscopy, so that the interpretation of their structure does not appear sufficiently reliable.However, the presence in these compounds of bridging carbonyl groups is undoubtedly reliable. Furthermore, the data on the magnetic properties of complexes 56 and 57 might prove useful in the interpretation of their structure. Thus if the proposed formulae are correct, it follows that complex 56 should be paramagnetic and complex 57 diamagnetic.The trinuclear carbonyl clusters synthesised later were more fully and reliably characterised. Thus the interaction of LiPBut 2 and the polymeric carbonyl chloride [Pd(CO)Cl]n in THF at 778 8C afforded (together with other products) the cluster Pd3(m-PBut 2)3(CO)2Cl 58, in which the metal has a formal oxida- tion state of +1.33.177 According to X-ray diffraction data, the palladium atoms in complex 58 form a triangle with Pd ± Pd distances ranging from 2.949 to 3.000 A which indicates a weak metal ± metal interaction. All the phosphide groups are bridges along the Pd ± Pd bonds, while one chloride and two carbonyl ligands are terminal.The relatively low stretching vibrational frequencies of the terminal COgroups (two bands in the IR spectrum at 2035 and 2030 cm71) compared with the nCO in the mononuclear complexes (usually above 2100 cm71) are due to the presence of phosphide species with their high electron content.Another type of carbonyl trinuclear clusters represented by the dication [Pd3(m3-CO)(m-dppm)3]2+ 59, was described for the first at time by Manojlovic-Muir et al.178 and Puddephatt 179 and was characterised in detail by Lloyd and Puddephatt 180.Cluster 59 has been obtained by the reaction of Pd(OAc)2 and dppm with carbon monoxide (PCO=1 atm) in solution in aqueous acetone containing an excess of trifluoroacetic acid and was isolated as a salt with CF3COO7 59a. The interaction of complex 59a with an excess of NH4(PF6) leads to the formation of [Pd3(m3-CO)(m- dppm)3](PF6)2 59b.Complex 59 has been characterised by IR spectroscopy (nCO=1820 cm71 and X-ray diffraction. The pal- ladium atoms in the cation form a triangle (the Pd ± Pd sides are 2.576, 2.607, and 2.610 A, to the sides of which diphosphine bridges are coordinated symmetrically, while the carbonyl group is linked to all three metal atoms (the Pd ±C distance ranges from 2.09 to 2.18 A.In the clusters of type 59 containing only neutral ligands, there are two positive charges on three metal atoms and hence the FOS of each palladium atom is +0.67. The interaction of halides or of the trifluoroacetate anion with the dication 59 is reversible, without change in the FOS of the metal atoms, and leads to the formation of [Pd3(X)(m3-CO). .(m-dppm)3]+ (X=CF3COO, Cl, Br, I) 60.It has been established that the capacity for coordination diminishes in the sequence I>Br>Cl>CF3COO and that each successive ligand can be quantitatively substituted by the preceding one.181, 182 Clusters 60 have been isolated in the form of a salt with CF3COO7 and have been characterised by X-ray diffraction analysis for X=I 180 and Cl.181 In both cases, the anionic ligand Xis m3-coordinated to the side of the Pd3 triangle opposite the side to which the m3-CO group is coordinated. It is noteworthy that the Pd ±X distances in the Pd3(m3-X) groups are fairly large (the Pd ±X distance is 2.741 ± 3.161 A for X=Cl and 2.951 ± 3.083 A for X=I) and an unambiguous conclusion concerning the incorporation of X in the coordination sphere of the cluster cannot be reached on the basis of X-ray diffraction data.However, the results of studies of the molar conductance of solutions of the clusters and of their spectra in the Cl Pd Cl Pd CO CO HC CH2 CH2 nCO=1937 cm71 (for X=Cl), nCO=1929 cm71 (for X=Br). Pd Pd CO O C X X Pd X X Pd HC CH2 CH2 CH H2C H2C O PBut 2 But 2P Cl OC C PBut 2 Pd Pd Pd 58 Ph2P CO Pd Pd Pd Ph2P Ph2P PPh2 PPh2 C H2 PPh2 CH2 H2C 59 Ph2P Pd Pd Pd Ph2P P Ph2 PPh2 P Ph2 C H2 PPh2 CH2 H2C Br C O 60 Palladium carbonyl complexes 505visible region on titration of the cluster solution with a solution of KX (X=Cl, Br, I) showed that the anionic ligand is incorporated into the coordination sphere of the palladium cluster and that the contribution of the covalent component to the formation of the Pd ±X bond greatly exceeds that of the ionic component. The mechanism of the formation of clusters 59 and 60 has been investigated 182 by 31P NMR.The NMR monitoring of the reaction solution containing initially Pd(OAc)2, dppm, and CO in an acetone ±CF3COOH±H2O mixture made it possible to identify the formation of the complexes Pd(CF3COO)2(dppm) 61, Pd2(CF3COO)2(m-dppm)2 62, Pd2(CF3COO)2(m-CO)(m- dppm)2 63, and [Pd3(m-CF3COO)(m3-CO)(m-dppm)3](CF3COO) 64.There are grounds for the postulate that a short-lived palladium(0) complex with diphosphine is formed initially and then reacts with complex 61 to form compound 62 or 63. The subsequent reaction of the palladium(0) complex containing bis(diphenylphosphino)methane with compound 62 or 63 leads to the appearance of [Pd3(m3-X)(m3-CO)(m-dppm)3]+ (X=CF3COO) 60.It can be assumed arbitrarily that the two palladium atoms in the base of the triangle, linked by a shortened Pd ± Pd bond (2.567 A), have a FOS of +1. The third metal atom at the apex of the triangle than has a zero FOS. Incidentally. such treatment should be regarded as a marked simplification, which is used only for a clear representation of a more complex situation.The bridging halide ligand in the inner coordination sphere of complex 60 is readily substituted by another halide when the complex interacts with KX in acetone. Palladium carbonyl complexes with a nonintegral FOS of the metal are few in number, are not distinguished by a structural diversity, and are represented (among reliably characterised com- plexes) only by clusters with a triangular metal core (Table 5).Whereas, the Pd ± Pd distances are fairly short (2.58 ± 2.61 A) in clusters 59 and 60, where each metal atom has a FOS of +0.67, and indicate the occurrence of a direct metal ± metal interaction, in cluster 58 (the FOS of the metal is +1.33) such interaction is weaker, which is reflected in the length of the metal ± metal bonds (2.95 ± 3.00 A).Certain heteronuclear complexes should apparently be assigned to compounds in which the metal atom has a nonintegral FOS. Thus in the octanuclear complex Na2{Pd4[Cp- Mo(CO)3]4},138 there are two positive charges on the four palla- dium atoms (the palladium atoms form a square with 2.675 ± 2.691 A sides), i.e. the FOS is +0.5. Another example is provided by the anionic cluster [Pd6Ru6(CO)24]27 64 obtained 183 by the interaction of [Ru3H(CO)11]7 and Pd(PhCN)2Cl2.The palladium atoms in the cluster form an octahedron, the ruthenium atoms serve as `caps' on the faces of the octahedron, while the carbonyl ligands are involved in terminal (12 molecules) and bridging (six molecules are m-coordinated and six are m3-coordinated) coordination.If it is assumed by analogy with the preceding cluster that the `capping' ruthenium atoms represent together with the carbonyl ligands anionic carbonylmetallates, then a formal charge somewhat exceeding +1 should be attributed to the palladium atoms forming the inner octahedron, but in principle such a procedure does not appear sufficiently legitimate.The picture becomes even more complex with increase in the number of atoms in the cluster. Thus the interaction of Pd(PPh3)2Cl2 and [Ni6(CO)12]7 afforded 184 the anionic cluster (Pd33Ni9(CO)4(PPh3)6]47 isolated in the form of a salt with [PPh4]+ and characterised by X-ray diffraction (Fig. 10). The metal atoms in the cluster form five triangular layers and the vertices of the upper, lowest, and middle triangles are occupied by nickel atoms.The carbonyl ligands (not shown in Fig. 10) consist of five terminal and 36 bridging groups the terminal carbonyl being linked to the three nickel atoms of the middle layer and the central palladium atoms of the upper and lowest layers. 2. The reactivity of clusters containing the metal in a fractional formal oxidation state The reactivity of the clusters under discussion is characterised by two distinctive features: the coordinative unsaturation of the clusters (and hence their ability to coordinate additional ligands) and the ability of clusters containing metal atoms in the lowest oxidation states to enter into oxidative addition reactions.Such features are most characteristic of the type 59 clusters [M3(m3-CO)(m-dppm)3]2+, the reactivity of which has been char- acterised in fair detail.Table 5. Palladium complexes containing the metal in fractional oxidation states. Compound FOS Physicochemical nCO /cm71 Metal core, Ref. methods Pd7Pd distances /A p-C3H5[PdCl(CO)]2 +0.67 elemental analysis, 1940 176 IR spectroscopy {p-C3H5[PdX]2(CO)}2 +1.5 the same 1937 (X=Cl) 89 (X7=Cl7 or Br7) 1929 (X=Br) Pd3(m-But 2P)3(CO)2Cl +1.33 elemental analysis, 2035, 2030 triangle, 177 IR spectroscopy, XRD 2.949 ± 3.000 [Pd3(m3-CO)(m-dppm)3]X2 +0.67 elemental analysis, 1820 triangle, (X7=CF3COO7 or PF¡6 ) IR spectroscopy, 2.576, 2.607, 2.610 178, 179, 180 XRD, 31P NMR [Pd3(m3-X)(m3-CO)(m-dppm)3]X +0.67 the same 1712 (X=CF3COO) triangle, 181, 182 (X7=CF3COO7, Cl7, Br7, I7) 2.584 ± 2.603 [Pd3(Z5-C5Me5)3(m3-CO)2]CF3COO +0.33 elemental analysis, 1769 triangle, 2.63 183 IR spectroscopy, XRD Figure 10.The structure of the metallopolyhedron of the heterometallic cluster anion (Pd33Ni9(CO)4(PPh3)6]47 according to X-ray diffraction data.184 506 T A Stromnova, I I Moiseeva. Coordination of additional ligands It has been shown that cluster 59 (M=Pd or Pt) is capable of coordinating two- and four-electron donor ligands and the newly introduced ligands can be both terminal and bridging on the M3 triangle, or on the sides of this triangle, and their coordination has an appreciable influence on the coordination of the carbonyl ligand.Since cluster 59 and the products of its interaction with various reagents are much more stable in the case where M=Pt than for M=Pd, the authors were as a rule able to characterise fairly reliably the platinum-containing products, the analogous properties and structures being merely attributed to the palladium complexes.Thus it has been shown in relation to the platinum complex by NMR involving different nuclei 185 that complex 59 can combine with a second CO molecule with formation of two isomers; the first with terminal and m3-coordinated carbonyls and the second with two m3-coordinated carbonyl ligands: Phosphines and phosphites (L) are coordinated as monoden- tate species in reactions with complex 59, forming [Pt3(m3- CO)(L)(m-dppm)3]2+.The coordination of the carbonyl group and of the diphosphines does not then change.186, 187 Treatment of cluster 59 (M=Pd or Pt) with the cyanide ion in acetone solution at 778 8C and subsequent heating of the solution to room temperature lead to the formation 188 of the adduct [M3(CN)(m3-CO)(m-dppm)3]+. According to X-ray dif- fraction data for M=Pt, the cyanide ligand in such an adduct is linked to one of the platinum atoms in such a way that the Pt ±C±N line is virtually perpendicular to the plane of the Pt3 triangle.The study of the behaviour of this adduct by 31P NMR in solution at 790 8C showed that the central ligand migrates rapidly around the Pt3 triangle. This migration is not accompa- nied by the dissociation of the carbonyl or cyanide ligand and involves in all probability an intermediate with a m3-coordinated cyanide ion. On interaction of cluster 59 (M=Pt) with isocyanides RNC,189 initially one isocyanide molecule is coordinated and then a second one is attached to the same metal atom.On coordination of a second isocyanide molecule, a CO molecule is displaced and the cluster [Pt3(RNC)2(m-dppm)3]2+ is formed. If the isocyanide molecule contains a bulky group, its reaction with complex 59 proceeds in a somewhat different manner.190 Thus, the interaction of complex 59 (M=Pd) with XyN=C (Xy=2,6-C6H3Me2) results in the formation of [Pd3(XyNC)(m- dppm)3]2+ 64, in which the m3-Z-isocyanide ligand is coordinated (according to X-ray diffraction data) over the Pd3 plane.Cluster 64, containing 42 valence electrons, combines reversibly with a second isocyanide molecule, forming the 44-electron cluster [Pd3(m3-CNXy)2(m-dppm)3]2+.The reaction of complex 59 (M=Pd) with 0.5 mole of 1,4-C=NC6Me4N=C also leads to the substitution of CO. The cationic cluster {Pd3(m- dppm)3]2(CNC6Me4NC)}4+ 65 is then formed. It was isolated in the; form of a salt with PF¡6 or CF3COO7. X-Ray diffraction analysis showed that the diisocyanide ligands in complex 65 serve as bridges between the Pd3(m-dppm)3 species.By analogy with the isocyanides, the anion SnX¡3 (X=F or Cl) initially forms the 1 :1 adduct [Pt3(m3-CO)(m3-SnX3)(m- dppm)3]+ in the reaction with complex 59 (M=Pt) and then combines with a second equivalent of SnX¡3 . This leads to the displacement of CO and the formation of [Pt3(m3-SnX3)(m- dppm)3].53, 191 It has been shown by theNMRmethod 192 that the interaction of complex 59 and NaBH4 entails the initial formation of the fragment [Pt3(m3-CO)(m3-H)(m-dppm)3]+, which affords [Pt3(m3- H)(m-dppm)3]+ after eliminating CO.The reaction of complex 59 with halide ligands proceeds similarly to the reaction with hydrides but terminates at the first stage. Complex 59 combines with a halide ligand to form [M3(m3- CO)X(m-dppm)3]+. In the case where M=Pd, it has been established by X-ray diffraction study that the halide m3-coordi- nated to the second side of the Pd3 triangle is built into the Pd3(m3- CO)(m3-X) bipyramid.180, 181 The pseudohalide SCN7 also forms 1 : 1 adducts, but it has been established by X-ray diffraction that in [Pt3(m3-CO)(SCN)(m- dppm)3]+ the thiocyanate ion assumes the role of a terminal ligand coordinated to one metal atom.193 Complex 59 does not react directly with diphosphine ligands (even in the presence of their large excess).Other bidentate ligands, such as R2NCS¡2 and R3PCS2, form 194 the complexes [Pt3(m-CO)(m-dppm)3(m- S2CNR2)] or [Pt3(m-CO)(m-dppm)3(m-S2CBr3]2+. Thus complex 59 can combine with one- or two-electron ligands or a bidentate four-electron ligand with formation of 44- and 46-electron clusters.On addition of weakly coordinated halide or trifluoroacetate, the number of valence electrons of the cluster may reach 48. In the cluster, theM3P species is planar and the M±M bond lengths vary within a very narrow range. Addi- tional ligands may bind to complex 59 via the unoccupied Pz orbitals of the metal and not the unoccupied s* orbitals of the metal ± metal bond of the cluster.This conclusion has been confirmed by a theoretical analysis.195, 196 The additional ligands can interact with one, two, or three metal centres, the first case being most frequently encountered. The degree of displacement of the carbonyl group from the symmetrical m-coordination (expressed by a change in the M±C bond lengths) reflects the donor properties of the additional ligand.It increase in the sequence m3-I<m3-Z2-CF3COO<SCN<P(OPh)3. b. Oxidative addition reaction In the cluster [M3(m3-CO)(m-dppm)3]2+ or [M3(m3-H)(m- dppm)3]+, the formal +2 charge is distributed among three metal atoms, which corresponds to a FOS of +2/3. After the removal of a further two electrons from these complexes the formal 4+ charge is distributed among three palladium atoms, which corresponds to a FOS of +4/3.In the oxidised complexes, one of the sides of the metal triangle increases to a size exceeding the sum of the covalent radii of palladium and precluding direct metal ± metal interaction. In this case, one may assume that the complexes contain the (M2)2+M2+ species, where M2+ is linked to the (M2)2+ cations only via two m-dppm groups and the outer m3-ligand introduced during the oxidation process.The oxidative addition reactions have been examined in relation to platinum compounds.197 ± 199 The complex [Pt3(m3- CO)(m-dppm)3]2+ reacts with H2E or REH (E=S or Se R=Et or Ph) with the elimination of the CO molecule and the formation of [Pt3H(m3-ER)(m-dppm)3]2+ or [Pt3H(m3-E)(m-dppm)3]+.The P M C M P P P M O P P P M C M P P P M CO O P P P M C M P P P M C O O P P +CO Pt Pt Pt C P P P Ph2P P Ph2 Ph2 Ph2 Ph2 Ph2P C H2 CH2 C H2 C O N Palladium carbonyl complexes 507palladium cluster reacts similarly, but the Pd ±H bond is more labile and the complexes formed could not be isolated in a crystalline form. The reactions described above are of interest as model processes involving the poisoning of platinum or palladium catalysts by hydrogen sulfide.The complex [M3(m3-CO)(m-dppm)3]2+ reacts with SCN7 to form [M3(m3-CO)(SCN)(m-dppm)3]+ which undergoes a thermal or photochemical rearrangement 193 with dissociation of the S ± C. bond and the formation of [M3(CN)(m3-S)(m-dppm)3]+. The mechanism of this reaction has been investigated 200 and it has been established that the principal characteristic features of the process extend also to the interaction of complex 59 and the heterocumulenes S=C=E (E=N7, O, S, or NR).In all cases, the reaction proceeds with formation of the adduct [M3(S=C=E)(m3-CO)(m-dppm)3]2+, which is followed by the oxidative addition of the heterocumulene to the C=S bond, the elimination of CO, and the appearance of [M3(m3- S)(C=E)(dppm)3]2+.VII. Palladium carbonyl hydride complexes The palladium complexes containing simultaneously both car- bonyl and hydride ligands are of great interest primarily as possible intermediates (or models of intermediates) in catalytic reactions occurring with participation of carbon monoxide and hydrogen. This is a wide range of processes including the indus- trially important alkene hydroformylation, alkoxycarbonylation, etc.reactions. The carbonyl hydride complexes are believed to be intermediates in the key stages of the processes, but the evidence for the formation of such complexes is extremely scanty or completely absent.201 ± 203 There are few reliably characterised palladium carbonyl hydride complexes.204 The first communication about the synthesis of a carbonyl hydride complex appeared as early as 1969.205 A solution of palladium chloride in 2-methoxyethanol was treated with carbon monoxide until the formation of a yellow solution. A brown diamagnetic complex was precipitated on treatment with AsPh4Cl.The sparing solubility of this complex precluded the study of its 1H NMR spectra. On the basis of the results of elemental analysis, conductimetric measurements (the compound behaved as a 1 : 1 electrolyte in solution in dimethylformamide), and IR spectroscopic data (the weak band at 1960 cm71 was assigned to the stretching vibrations of theM±H bond, while the intense band at 1900 cm71 was assigned to the stretching vibra- tions of the terminal CO group), the authors postulated that the compound is the carbonyl hydride [AsPh4][Pd(CO)HCl2].In the light of later data on palladium carbonyl and hydride complexes, taking into account the new interpretation of the IR spectra of these compounds (both bands in the IR spectra should be more correctly assigned to the stretching vibrations of bridging groups), this hypothesis appears insufficiently well founded. The complex probably has a more complicated dinuclear structure, particularly since mononuclear compounds are absent among the palladium carbonyl halides described later.On protonation of Pd(PPh3)4 with aqueous trifluoroacetic acid at 25 8C or on reduction of (PPh3)2Pd(CF3COO)2 with hydrogen in solution in aqueous CF3COOHat 70 8C, the complex [(PPh3)3PdH]+ is formed in situ, carbonylation of the complex afforded 206 ± 208 the carbonyl hydride [(PPh3)4Pd2(m-H)(m- CO)](CF3COO) in accordance with the equation Complex 66 has not been isolated in the solid state but it has been fairly completely characterised by NMR data, which agree well with those for platinum analogues.209, 210 The protonation of Pd(PPh3)4 in toluene under a CO atmos- phere or of (PPh3)3Pd(CO) in toluene on treatment with the sulfonic acid H2C(SO2CF3)2 also leads to the formation of complex 66 [X=HC(SO2CF3)2], which has been isolated in the solid state and characterised by elemental analysis and IR spectroscopy (nCO=1851 cm71), while the complex in solution in CD2Cl2 has been characterised by NMR spectroscopy involv- ing different nuclei.211 Complex 66 (X=BPh4) has been obtained 212, 213 in a study of the nature of the compounds responsible for the catalysis of the reduction of nitro-compounds by carbon monoxide in the Pd(OAc)2/PPh3/CO/PhNO2/BunOH/H2SO4 system.The only palladium carbonyl hydrate the structure of which has been investigated by X-ray diffraction has been synthes- ised 214, 215 by a fairly unusual method � the interaction of the organopalladium compound Pd(dippp)(Ph)(Cl), where dippp=1,3-bis(diisopropylphosphino)propane, and methanol in the presence of an excess of triethylamine at 60 8C in accord- ance with the equation Experiments using CH3OD showed that methanol is in fact the source of hydride ions in the synthesis of complex 67.The carbonyl groups also arise as a result of the decarbonylation of methanol.The 1HNMRspectrum of complex 67 contains a signal at d=75.17 ppm (quintet. 2JP ±H=41.1 Hz); the IR spectrum contains an intense band at 1789 cm71, which does not change when the hydride is replaced by deuterium and which has been assigned to the stretching vibrations of the CO group. The relatively low stretching vibrational frequency of the carbonyl group reflects the presence of an increased electron density on the metal atom, arising as a consequence of the high donor capacity of the chelating phosphines.According to X-ray diffraction data, the two palladium atoms in complex 67 are at a distance of 2.767 A, indicating a metal ± - metal single bond. The hydride ligand is localised and the Pd ±H distance is 1.53 A, which is within the range of distances determined for the structurally characterised palladium hydrides.216 ± 2l8 Acomplex analogous to complex 67 has also been obtained by decarbonylating methanol.219 On heating to 80 8C a solution of the acetyl complex Pd(COMe)(Cl){(S,S)-bdpp}, where (S,S)- bdpp=(2S,4S)-2,4-bis(diphenylphosphino)pentane, in a metha- nol ± toluene-d8 mixture in an inert gas atmosphere, an equimolar mixture of (bdpp)PdCl2 and [Pd2(m-H)(m-CO)(bdpp)2]+Cl7 is formed.The interconversions of the polynuclear palladium hydride and carbonyl hydride complexes have been described.2, 220 ± 226 The reduction of Pd(OAc)2 by molecular hydrogen in glacial acetic acid in the presence of L (L=Phen or Bipy) leads to the formation of X-ray-amorphous substances with the approximate composition [Pd4L(OAc)2H4]n, where n*l00.The reaction of polynuclear hydrides with carbon monoxide leads to the carbonyl hydride clusters [Pd4L3(CO)3(H)2(OAc)3]n (n=2, 3). The inter- action of CO with a 1 : 1 solution of Pd(OAc)2 and Phen in acetic acid at 20 8C leads to the formation of the complex [PhenPd(- CO)(OAc)]n, which does not contain hydrides. If the temperature of the reaction mixture is raised to 50 8C, a polymeric carbonyl hydride having the general formula [(Phen)2Pd2(CO)(H)(OAc) .AcOH]m is formed; the signal of its hydride hydrogen atoms in the 1H NMR spectrum is d=715.5 ppm. Finally, the interaction of Pd(OAc)2 and Phen X=CF3COO. 2[(PPh3)3PdH]+X7+CO=[(Ph3P)4Pd2(m-H)(m-CO)]X, 66 2Pd(dippp)(Ph)(Cl)+2CH3OH+NEt3= =2C6H6+CH2O+H2+HNEt3Cl+[Pd2(dippp)2(m-CO)(m-H)]+Cl7 67 H2C H2C Pri 2P PPri 2 CH2 CH2 CH2 H2C H Pd Pd C PPri 2 Pri 2P O 508 T A Stromnova, I I Moiseevin acetic acid under a mixed H2/CO atmosphere affords the carbonyl hydride [(Phen)3Pd4(CO)3H3(OAc)2 .AcOH]. All the carbonyl hydrides have been characterised by elemen- tal analysis and IR, 1H NMR, and EXAFS spectroscopy. On the basis of the data obtained from these compounds, a structure has been proposed.It is based on tetrahedral palladium fragments with chelating phenanthroline ligands and bridging CO groups, which are linked to one another by bridging hydrides. Thus the palladium carbonyl hydride complexes described are represented either by dinuclear singly charged cations witbridg- ing carbonyl and hydride ligands or by polynuclear compounds also containing bridging carbonyls and hydrides.In all cases, the FOS of the palladium atoms has an intermediate value between 0 and +2. VIII. Palladium carbonyl complexes in zeolites In the early studies devoted to the adsorption of carbon monoxide on palladium(0) deposited on oxide carriers,227, 228 it was estab- lished that two types of adsorption exist: weak reversible and strong irreversible adsorption.In the former case, the carbon monoxide molecules are involved in terminal coordination according to IR spectroscopic data (nCO=2085 cm71), while in the latter bridging CO groups predominate (nCO=1960 cm71). The authors of later studies 229 ± 236 obtained additional data on the structures of palladium carbonyls in the cavities of the NaY and 5A zeolites by employing modern physical research methods (Fourier transform IR spectroscopy, atomic radial distribution methods, and EXAFS), as well as data obtained for the struc- turally characterised palladium(0) carbonyls.Thus comparative analysis of the IR spectra of compounds of palladium and carbon monoxide, located in the cavities of the NaY zeolites, and data on the molecular clusters of ruthenium and iridium carbonyls in zeolites led to the conclusion that the principal form of the palladium carbonyl consists of the clusters Pd13(CO)x containing both bridging and terminalCOgroups. 237 The size of the cluster is determined by the size of the zeolite cavity and is almost independent of either the initial form of palladium or of the cluster formation conditions.230 Subsequent studies by the EXAFS method confirmed the hypothesis that the Pd13 clusters predominate.These can have a metal core in the form of both a cubooctahedron and an icosahedron.231 In the initial stage of the formation of the compound on treatment of palladium immobilised on the surface of the zeolite with carbon monoxide at room temperature, the formation of the Pd6 cluster was observed. The subsequent migration of such clusters and species with a smaller nuclearity into the zeolite cavity leads to Pd13.On raising the temperature to *200 8C, there is a possibility of the growth of the metal core right up to Pd40. However, all the conclusions concerning the nuclearity of clusters in the zeolite and their composition and nature are tentative.It is believed that, under the conditions where the Pd13(CO)x clusters are formed on the NaY zeolites, clusters having the composition Pd6(CO)x predominate on the surface of the 5A zeolite. The presence of mainly terminal and m3-bound CO groups is characteristic of Pd6(CO)x.232 The ability of various zeolites to form palladium carbonyls with different nuclearities has been used in the preparation of catalysts altering the selectivity in various processes.Thus the hydrogenation of CO on Pd/5A leads to methanol and dimethyl ether, whereas the employment of Pt/NaY leads mainly to the formation of C2 hydrocarbons.234 IX. Conclusion The organic chemistry of palladium was initiated by the study of the p-complexes and alkyl derivatives of this metal.201 The carbonyl complexes played only a modest role at this stage.The last quarter of the century has been marked by the development of a series of processes which have found industrial applications or for which prospects for an industrial application exist. Carbon monoxide plays the role of one of the substrates in this series, whilst the active component of the catalytic system consists of palladium compounds.237 Simultaneously with this field of chemistry, the chemistry of palladium carbonyl complexes developed, its importance growing far beyond the framework determined by utilitarian needs. The synthesis and study of palladium carbonyl complexes, regarded as a chemical rarity as late as 25 years ago, posed a series of problems before the theory of the chemical bond and of the reactivity of compounds.Modern ideas about the nature of the metal ± metal and metal ± organic ligand chemical bonds 238 are based in many respects on the existing factual data and the chemistry and structure of platinum group metal carbonyl com- plexes. The analysis of these data carried out in the present review has confirmed the existing hypothesis that the stability of polynuclear compounds diminishes with increase in the formal oxidation state of the central atom in palladium carbonyls and hence the number of reliably characterised compounds diminishes sharply on pass- ing from Pd(0) to Pd(+2).Thus, whereas binary carbonyls and mononuclear and polynuclear compounds with various forms of the metal core and all the forms of the coordination of carbonyl ligands have been described for palladium(0), all the known palladium(+2) carbonyls are limited by mononuclear compounds with terminal carbonyl groups.Among the fairly wide range of methods of synthesis of transition metal carbonyl complexes 239 ± 241 (direct carbonylation of the metal, reductive carbonylation, oxidative carbonylation, reduction of carbonyl complexes, substitution of ligands etc.), only some have been applied in the chemistry of palladium carbonyls, different numbers of compounds being obtained by the application of a particular method.Thus only four complexes have been synthesised by the coprecipitation of palladium metal and CO at low temperatures; the reaction involving the substitu- tion of ligands by carbon monoxide has also been successful in only isolated instances.The method involving the insertion of CO in the metal ± metal bond in dinuclear and polynuclear palladium complexes and reductive carbonylation of palladium compounds have proved much more productive. It is noteworthy that the vast majority of compounds have been obtained by the latter method and not only palladium(0) or palladium(+1) complexes but also palladium(+2) complexes.Palladium(0) carbonyls constitute the most numerous class among palladium carbonyl complexes, polynuclear carbonyls, including also large clusters with several tens of palladium atoms in the metal core, predominating. Probably one of the most important causes of such diversity of the complexes is the stability of the Pd7CO bond determined by the combination of the s- donor and p-acceptor properties of the CO molecule and the ability of the palladium atom (d 10 electronic configuration) to donate the electron density from the 3d metal orbital to the p*-orbital of the CO molecule. The stability of the Pd7CO bond in palladium(+1) com- plexes, where the metal atom has the d9 electronic configuration, is lower and hence the number of such complexes is much smaller than that of palladium(0) carbonyls.All the palladium(+1) complexes contain an even number of metal atoms � 2 or 4. They are diamagnetic and the pairing of electrons at the two metal atoms, having the d 9 configuration, occurs either as a result of the formation of a metal ± metal bond (with a Pd7Pd distance of *2.7 A) or by the formation of a three-centre four-electron bond incorporating two palladium atoms and one carbon atom of the bridging carbonyl ligand.The palladium(+1) complexes are fairly sensitive to the action of oxygen-containing nucleophiles and, when treated with, for example, water or lower aliphatic alcohols, they decompose with the formation of palladium metal.The insertion of p-donor Palladium carbonyl complexes 509ligands into these complexes promotes an increase in their resistance to reductive decompositions. In the palladium(+2) complexes where the metal atom has the d 8 electronic configuration and the possibility of the donation of electrons from the metal to the corresponding orbitals of the CO molecule is much smaller than for Pd(0) and Pd(+1), an enhance- ment of the s-donor properties of the carbonyl is observed.All the palladium(+2) carbonyl complexes are monomeric with terminal CO groups. The strength of the Pd7CO bond is as a rule small, which leads to a low thermal stability of the complexes. The presence in the palladium(+2) carbonyl complex of groups with strong electron-withdrawing properties, for example SO3F7, enhances the electron-accepting properties of the paadium atom and hence leads to an increase in the thermal stability of the complexes.There are few palladium carbonyl complexes containing the metal in fractional oxidation states. In fact, only compounds having the Pd3(m3-CO) core can be regarded as reliably charac- terised. The reactions involving the coordination of additional ligands are characteristic of these compounds and the newly introduced ligands can be both terminal or can function as m3- bridges on the Pd3 triangle.Reactions of the second type are oxidative addition reactions. As a consequence of the change in the charge on the Pd3 triangle, the metal core is altered and the charges are redistributed, which actually leads to a species with nonequivalent metal atoms.Palladium carbonyl hydrides are of enormous interest primar- ily as possible intermediates (or models of intermediates) in catalytic processes occurring with participation of carbon mon- oxide and hydrogen and catalysed by palladium and its com- pounds. The number of such complexes, which have been postulated in a large number of studies, is small. All the palladium carbonyl hydrides are represented either by dinuclear singly- charged cations with bridging carbonyl and hydride ligands or by polynuclear compounds also containing bridging carbonyl and hydride ligands.In all cases, the FOS of the palladium atoms is intermediate between 0 and +2. Summarising the data described above concerning the char- acteristic features of the Pd7CO bond in the carbonyl-containing complexes, one should note that such a bond may be described by several models distinguished by different contributions of the s- and p-interactions.The symbolism introduced by Willner and Aubke (Fig. 11) appears very clear in the simplest case where the overlap of orbitals in an individual M7CO fragment with CO groups bound as terminal species is considered.238 In this scheme, the s-donor bond arising from the 5s molecular orbital of CO is shown as an arrow directed from the carbon atom to the metal atom, while the p-acceptor bond, responsible for the donation from the ndxz or ndxy orbital of the metal to the p* molecular orbitals of CO is shown by arrows directed towards CO.The thin lines indicate a weak bond, whereas thick lines demonstrate the dominant type of bond and the boundary situations.Figure 11a illustrates the dominant p-back- donation (this involves predominantly highly reduced carbon- ylmetallates), whilst Fig 11c illustrates the predominance of s-bonding (cationic carbonyls). Figure 11b characterises the bonding in typical metal carbonyls, where the bonds of the types indicated in Fig. 11a and Fig. 11c may overlap depending on the oxidation state of the metal. Apparently there are at present no physicochemical data which would make it possible to assign the type of bonding in the carbonyl fragments solely to the `p-inter- action' or the `pure s-interaction' and both types of bonding occur in any of the existing compounds.The palladium carbonyls where the bond might be assigned to the type presented in Fig. 11b and which should have a large positive charge, for example [M(CO)4]47 (M=Cr, Mo, W) and [M(CO)4]37 M=Co, Rh, Ir) 242 have not so far been described and the possibility of the synthesis of such reduced carbonyl carbonylpalladates appears doubtful. In the vast majority of the palladium carbonyl complexes, the bond is of the type presented in Fig. 11b (the s- and p-interactions are combined). Only one complex � the palladium(+2) carbonyl complex [Pd(CO)4]2+ � may be assigned to complexes in which a bond of the Pd7CO type presented in Fig. 11c is achieved. Only in this complex is an anomalously high stretching vibrational frequency of the CO (2248 cm71) observed, exceeding the value of nCO for free carbon monoxide.In the case of all other palladium carbonyl complexes, the energy of the Pd7CO bond and the vibrational frequencies nCO, indirectly characterising this bond, depend on a whole series of ancillary factors. Thus the vibrations of the terminal CO groups are manifested in the shortest-wavelength region (2000 ± 2050 cm71), those of the m-CO groups are in the middle region (1850 ±*1950 cm71), while the vibrations of the m3-groups are in the longest-wavelength region.The regions corresponding to absorption by the last two types of CO ligands overlap appreciably. Complexes of different nuclearity, contain- ing CO ligands of the same type, do not exhibit stable distinctive features in their IR spectra. On the other hand, the presence at one atom of only one terminal carbonyl and of three n-donor atoms [for example in Pd(CO)(PPh3)3, Pd(CO)(NP3), and Pd(CO)[MeC(CH2PPh2)3] leads to a decrease in nCO to values characteristic of the stretching vibrations of the bridging CO group, i.e.the regions of the stretching vibrations of the bridging and terminal carbonyls overlap. The appearance of strong electron-withdrawing groups increases nCO to values characteristic of the stretching vibrations of the terminal CO groups.For example, nCO for the complex Pd4(m-CO)4(m-OCOCF3)4 is 2003 cm71, which is characteristic of the terminal CO groups. At any rate, bearing in mind the fore- going one should treat with caution the description of the structure of carbonyls and the estimation of the strength of the metal ± ligand bond on the basis of spectroscopic data.The reactivity of palladium carbonyl complexes is restricted to a narrow range of reactions. Thus ligand substitution, which does not lead to a serious structural rearrangement, has been described for all types of palladium carbonyl complexes. Reactions leading to a change in the metal core have been described for polynuclear complexes containing Pd(0), Pd(+1), and palladium in fractional oxidation states, and such changes can occur with decrease or increase in the nuclearity of the complex [for example, the formation of various carbonyl phosphine complexes of palla- dium(0)] and with retention of the nuclearity {for example, the formation of the tetrahedral complex [Pd4(CO)2Phen4](OAc)4 from the carbonyl acetate Pd4(CO)4(OAc)4, which has a rectan- gular metal core}.The reactivity of the palladium(+1) carbonyl carboxylate complexes has been studied much more widely than for the other complexes. Among the reactions investigated the greatest interest attaches to the oxidative-reduction reactions of the tetranuclear clusters. In the first place, this is the reaction involving the intramolecular transfer of an oxygen atom from the carboxylate ligands to the coordinated carbonyl, which may serve as a model of a whole series of oxidation ± reduction processes proceeding with participation of CO and catalysed by palladium compounds. The extensive field of the chemistry of palladium comprising the synthesis, the structure, and the reactivity of heteronuclear p p s Mn C O p p s Mn C O Mn C O p p s a b c Figure 11.A schematic model of the formation of the bond in theM7CO fragment. 510 T A Stromnova, I I Moiseevpalladium-containing complexes with carbonyl ligands has remained outside the scope of the present review. The authors hope that the compilation of data on the syn- thesis, structures, and properties of palladium carbonyl complexes presented in the review will prove useful not only to chemists occupied with different aspects of the synthesis of carbonyl complexes but also to specialists in the application of these complexes in catalysis and in the study of the details of their electronic structure.The review has been written with the financial support by the Russian Foundation for Basic Research (Project No. 96 ± 03 ± 34389a) and the Government of the Russian Federation (Project No. 96 ± 15 ± 97577). References 1. Yu V Yan, B K Nefedov Sintezy na Osnove Oksidov Ugleroda (Syn- theses Based on Carbon Oxides) (Moscow: Khimiya, 1987) 2. I I Moiseev,M N Vargaftik Usp. Khim. 59 1931 (1990) [Russ. Chem. Rev. 59 1133 (1990)] 3. R Bottger J. Prakt. Chem. 4 233 (1859) 4. G Gore Chem.News 48 295 (1883) 5. F S Phillips Z. Anorgan. Allgem. Chem. 6 229 (1894) 6. E Fink C. R. Hebd. Seances Acad. 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B Cornils, WA Herrmann (Eds) Applied Homogeneous Catalysis with Organometallic Compounds (Weinheim: VCH, 1996) 238. H Willner, F Aubke Angew. Chem., Int. Ed. Engl. 36 2402 (1997) 239. A N Nesmeyanov, K A Kocheshkov (Eds) Metody Elemento- organicheskoi Khimii. Tipy Metalloorganicheskikh Soedinenii Perekhodnykh Metallov (The Methods of Organoelement Chemistry. The Types of Organometallic Compounds of Transition Metals) (Moscow: Nauka, 1975) Vol. 1 240. V G Syrkin Khimiya i Tekhnologiya Karbonil'nykh Materialov (The Chemistry and Technology of Carbonyl Materials) (Moscow: Khimiya, 1972) 241. J E Ellis Adv. Organomet. Chem. 31 1 (1990) a�Russ. J. Inorg. Chem. (Engl. Transl.) b�Russ. Chem. Bull. (Engl. Transl.) c ± Russ. Coord. Chem. (Engl. Transl.) d�Dokl. Chem. Technol., Dokl. Chem. (Engl. Transl.) e�Russ. Organomet. Chem. (Engl. Transl.) f�Mendeleev Chem. J. (Engl. Transl.) g�Russ. J. Gen. Chem. (Engl. Transl.) h�Russ. J. Phys. Chem. (Engl. Transl.) i�J. Struct. Chem. (Engl. Transl.) j�Kinet. Catal. (Engl. Transl.) 514 T A Stro

ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. Various methods for the synthesis of open-chain poly- (organophosphazenes) are considered. The mechanism of poly- merisation of hexachlorocyclotriphosphazene and the basic prin- ciples of formation of poly(organophosphazene) macromolecules by polymeranalogous reactions of poly(dichloro-phosphazene) with various nucleophilic reagents are analysed from a new view- point. The potential of this synthetic method for targeted design of poly(organophosphazenes) of various structures is shown.The possibility of synthesising poly(organo-phosphazenes) by poly- merisation of cyclophosphazenes is also discussed. The problem of unit non-uniformity of poly(organophosphazenes) and its influence on the properties of these polymers are considered. The properties of poly(organo-phosphazenes) are considered in detail and it is shown that these polymers possess unusual valuable properties, which provide opportunities for their successful prac- tical application.The bibliography includes 276 references. I. Introduction Open-chain poly(organophosphazenes) [±PR2=N±]n± constitute a relatively new class of organoelement polymers. They occupy a peculiar place among organoelement polymers.The inorganic nature of their backbone built of alternating phosphorus and nitrogen atoms and organic framing of the phosphorus atoms determine a number of specific properties in some of these polymers, namely, flexibility combined with elasticity at low temperatures; enhanced flame resistance and low smoke forma- tion, resistance to hydrocarbon fuels and oils, low thrombogene- ity, good biocompatibility, and others.The first representatives of phosphazenes (halophosphazenes) were obtained back in the early XIX century; however, definite progress in the synthesis of poly(organophosphazenes) was achieved only in 1965 ± 1966. During the last two decades, studies dealing with poly(orga- nophosphazenes) have been vigorously pursued.High-molecular- mass poly(organophosphazenes) with side groups of various structures have been synthesised. Considerable attention is being paid to the basic priciples of polymer formation and to compre- hensive studies of the properties of poly(organophosph-azenes). The accumulated data certainly need to be generalised. Some aspects concerning the synthesis and properties of polyphospha- zenes have been outlined in several reviews.1 ±39 In this review, we consider the synthesis of various poly(orga- nophosphazenes) and the structure of their macromolecules. The properties of these polymers are discussed as functions of the molecular structure.The prospects for practical application of poly(organophosphazenes) are also presented. II. Synthesis of polyphosphazenes The search for new valuable synthetic materials has stimulated the intense development of the chemistry of polyphosphazenes during the last three decades.Considerable attention has been paid to halo-substituted polyphosphazenes, in particular poly(dichloro- phosphazene) (PDCP), which is characterised by a broad high- elasticity region (from 730 to 300 8C), low glass transition temperature (760 to 7588C), non-flammability, etc.1, 32, 38, 40, 41 However, since PDCP contains chlorine, it suffers from a serious drawback, namely, instability to hydrolysis: during storage in air, it rapidly and irreversibly ages and loses its properties.However, it was found that the high reactivity of the chlorine atoms in PDCP offers wide opportunities for synthesising new polymeric materi- als based on it by polymeranalogous transformations.Analysis of the published data demonstrates that the two-stage synthetic scheme shown below (Scheme 1) is Scheme 1 N Cl2P N PCl2 N P Cl2 cross-linked polymer ( N PCl2 ) n 3 1 2 ROX ArOX P N OR OR n P N OAr OAr n P N NR0R00 NR0R00 n R=Alk, AlkF; X=H, Na, K, Li; R0=H, R00=Alk, Ar; R0=R00=Alk.R0R00NH S V Vinogradova, D R Tur, V A Vasnev A N Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, 117813 Moscow, Russian Federation. Fax (7-095) 135 50 85. Tel. (7-095) 135 61 78 Received 10 February 1997 Uspekhi Khimii 67 (6) 573 ± 594 (1998); translated by Z P Bobkova UDC 541.64:547.241 Open-chain poly(organophosphazenes). Synthesis and properties S V Vinogradova, D R Tur, V A Vasnev Contents I.Introduction 515 II. Synthesis of polyphosphazenes 515 III. Properties of poly(organophosphazenes) 524 IV. Conclusion 530 Russian Chemical Reviews 67 (6) 515 ± 534 (1998) #1998 Russian Academy of Sciences and Turpion Ltdcurrently the most realistic way for preparing high-molecular- mass open-chain poly(organophosphazenes). In the first stage, high-molecular-mass soluble PDCP 2 is obtained by polymerisation of hexachlorocyclotriphosphazene (HCCTP) 1.The second stage is a macromolecular reaction of the soluble PDCP with various nucleophilic reagents. 1. General features of the formation of PDCP Polymerisation of cyclic halophosphazenes with ring opening is a very complicated process. Thermal polymerisation of HCCTP in the melt gives cross-linked PDCP 3 in a high yield (Scheme 1).However, this polymer is not suitable for the synthesis of poly(- organophosphazenes), because of the low degrees of conversion of the P ± Cl groups in reactions with nucleophilic reagents; hence, these reactions give poly(organophosphazenes) with low hydro- lytic stability. Allcock and Kugel 42 have shown that the cross-linked poly- mer 3 is not an immediate product of the polymerisation of HCCTP 1 but it is rather produced from non-cross-linked PDCP 2.Acompletely soluble PDCP42, 43 was obtained by restricting the degree of conversion of HCCTP (heating was terminated at the instant when a decrease in the fluidity of the polymerisation mixture was observed) under strictly controlled temperature conditions.The data listed in Table 1 indicate that raising the temperature accelerates the formation of an insoluble cross-linked polymer.44 According to the data of Table 1, polymerisation at 250 8C affords the cross-linked polymer over a period of 96 h, while at 300 8C this product is formed in 1.17 h. Numerous attempts have been made to select catalysts for the polymerisation of HCCTP that would enable conducting it under milder conditions.1, 8, 9, 32, 37 However, these attempts have not met with success so far.Therefore, thermal polymerisation of chlorocyclophosphazenes still remains the main method for the preparation of PDCP. Meanwhile, this method for the synthesis of PDCP suffers from one more serious drawback: the results of polymerisation are not reproducible.The yields of soluble PDCP obtained by various researchers under seemingly identical conditions vary over wide limits (from 5% to 70%).2, 44 ± 47 Various opinions about the structure of soluble PDCP have been expressed.1 ± 4, 32 Apparently, the polymerisation product (before gelation) is a polymer with a fairly heterogeneous struc- ture, characterised by variable and hardly controllable unit non- uniformity.2 Most likely, this accounts for the irreproducibility of the yield and the properties of both PDCP itself and poly(orga- nophosphazenes) based on it.12, 45, 48 ± 51 Therefore, it is important to study the mechanism of polymerisation of HCCTP and side reactions resulting in the formation of branched and cross-linked polymers, in order to find the ways of controlling these processes.The possible mechanisms of polymerisation of HCCTP have been discussed in earlier reviews;2, 12 however, there is still no agreement on this point.1, 2, 8, 12, 32, 46, 52 ± 54 Here we discuss the mechanism that appears most probable from our viewpoint. A special study aimed at elucidating the influence of the residual water on the polymerisation of HCCTP in a closed system has revealed interesting and fundamental features of this process.2, 45 It was found that the intrinsic viscosity of the PDCP formed depends on the experimental conditions and on the amounts of water and the evolved hydrogen chloride in the system; this implies that polymerisation might be catalysed by hydrogen chloride.2, 45, 50 It has been found 55 that hydrolysis of the P ± Cl bonds in HCCTP at 250 8C occurs only when the reaction mixture contains admixed higher chlorocyclophosphazenes, for example, octa- chlorocyclotetraphosphazene (OCCTP).Controversial opinions have been expressed in the literature about the role of this compound in the polymerisation of HCCTP. In fact, both accel- erating 56 and inhibiting 57 effects as well as the absence of any activity during the polymerisation 58 have been reported for this tetramer. Detailed study of this point has shown that the so-called `non- catalytic' polymerisation of HCCTP1 is catalysed, in reality, by the hydrogen chloride resulting from hydrolysis of the P ± Cl bonds in OCCTP by the residual water.55 There exist the optimum quantities of the tetramer and the residual water needed for successful polymerisation of the trimer.None of the reaction mechanisms proposed previously for the polymerisation of HCCTP did imply active participation of hydrogen chloride.2 It was found that highly pure HCCTP (containing *1073 mol.% of OCCTP) neither undergoes hydrolysis nor polymerises at a noticeable rate under the conditions of formation of virtually linear poly(dichlorophosphazene) (250 8C, 50 h).45, 49 The optimum quantity of OCCTP is *0.1 mol.% ± 0.2 mol.% (Table 2), which corresponds to the amount of residual water in the polymerisation vessels.Larger amounts of the tetramer markedly retard the polymerisation of HCCTP. The known fact that the tetramer is more readily hydrolysed than the trimer 1 was rationalised based on electronic effects.55 Study of the electronic structure by the extended HuÈ ckel method demonstrated that the P ± Cl bond in OCCTP is weaker than that in HCCTP (the Mulliken populations are 0.687 and 0.724, respectively).The above results provided grounds for the conclusion that OCCTP plays a dual role in the polymerisation of HCCTP: it acts as the source of the catalyst (hydrogen chloride) needed for the chain growth to start and as an inhibitor of the polymer growth.Thus, the process of HCCTP polymerisation could be controlled by varying the ratio of the residual water and OCCTP in the system.55 The mechanism of polymerisation of HCCTP in the Table 1. Influence of the temperature and duration of the polymerisation of HCCTP on the yield of soluble PDCP.Duration/ Yield of soluble Duration/ Yield of soluble h PDCP (%) h PDCP (%) 250 8C 300 8C 3 12.0 0.08 0 4 13.8 0.25 13.7 5 17.0 0.41 19.3 6 23.4 0.50 27.3 21 30.8 0.66 37.9 48 70.1 1.00 50.3 96 gel 1.17 gel Table 2. Results of polymerisation of HCCTP in the presence of various amounts of OCCTP (250 8C, 50 h). Amount of OCCTP Yield of Intrinsic mixed with HCCTP (mol.%) PDCP (%) viscosity of PDCP in toluene (25 8C)/ total specially dl g71 admixed 0.003 0 <1 7 0.004 0 <1 7 0.048 0 42 4.10 0.078 0.075 45 5.04 0.083 0.080 74 4.25 0.103 0.100 63 4.40 0.120 0 63 5.10 0.290 0 12 0.71 0.383 0.380 <1 7 0.563 0.560 <1 7 516 S V Vinogradova, D R Tur, V A VasnevScheme 2 presence of OCCTP, water, and hydrogen chloride 2, 12, 50 is presented in Scheme 2.The first step of the reaction is hydrolysis of the P ± Cl bonds in OCCTP by the residual water, which gives gaseous hydrogen chloride and hydroxyphosphazene, the latter being converted into the tautomeric oxophosphazane form. The subsequent protona- tion of the oxophosphazane ring with hydrogen chloride and the nucleophilic attack by the nitrogen atom of the phosphazene ring of HCCTP results in opening of the oxophosphazane ring and onset of the growth of a polymeric chain containing a terminal amino group.The chain growth occurs through subsequent addition of HCCTP molecules to the active site. When hydrolysis of the P ± Cl bonds does not occur but HCl is present,50 the phosphazene ring is also protonated; this yields molecules with terminal imino groups (Scheme 3).Scheme 3 This mechanism of chain initiation and growth 45 is formally consistent with the view about the ionic character of the polymer- isation of HCCTP but implies a completely different chemistry of this process. The possibility of polymerisation according to two pathways is fundamentally important, because it opens up way of controlling this process.2, 12, 50 In fact, if the reaction starts with hydrolysis of the P ± Cl bonds of the ring, this affords polymeric chains with terminal amino groups.These terminal groups can react with the P ± Cl bonds of adjacent macromolecules; this accounts for side reactions. Due to these processes and also to the participation of oxophosphazane rings in the polymer chain growth, anomalous units of types B ±G arise in the polymer, together with the predominant units of type A.Eventially, this leads to the formation of polydichlorophosphazene with a fairly high unit non-uniformity, to poor reproducibility of the exper- imental results, and to relatively low yields of the soluble polymer. It is promising to carry out the polymerisation process under conditions permitting no hydrolysis; in this case, the reaction starts with protonation of the phosphazene ring with hydrogen chloride.This permits the preparation of PDCP with more inert terminal imino groups and, hence, with a more perfect defectless final chemical structure. Elucidation of the mechanism of polymerisation of HCCTP made it possible to find a practical method for controlling the synthesis of PDCP; this method ensures good reproducibility of the results and the formation of a practically linear PDCP with molecular masses of about (5 ± 10)6106 and with a much nar- rower molecular-mass distribution (Mw/Mn < 1.4) compared to that of the polyphosphazenes described previously (Mw/Mn&7 ± 79 3, 33,59 ± 63), its yield being more than 80%.2, 12, 50 The problem was solved by preliminary removal of water from the reaction system; for this purpose, the residual water was made to react with the P ± Cl groups of chlorocyclophosphazene and then the resulting phosphazene derivatives containing hydrolysed fragments, which are the main source of side reactions, were separated from the reaction mixture.Polymerisation was carried out at 250 8C in an jointless Pyrex apparatus.49 In this case, hydrogen chloride needed to initiate the polymer chain growth is retained in the reaction system.The study of the polymerisation process has shown that the intrinsic viscosity of PDCP decreases, whereas the Huggins constant simultaneously increases following an increase in the polymerisation time.This apparently indicates that at larger degrees of conversion, branched PDCP molecules are formed.50 The content of branched molecules and the unit non-uniform- ity of the polymer depend on the experimental conditions [the extent to which the reaction tubes are filled with the monomer, quantity of the catalyst (hydrogen chloride), temperature, and reaction time]. The influence of the duration of the process in a preheated polymerisation system on the yield and intrinsic viscosity of PDCP and its fluoroalkoxy-derivatives has been studied.63 A nearly quantitative degree of conversion of the trimer (97%) is attained when the polymerisation is carried out for 130 h (250 8C), the resulting PDCP being completely soluble in benzene and toluene.The high degrees of polymerisation [(4 ± 5)610 5] and also the fact that the molecular-mass distribution curves of various PDCP samples at low degrees of conversion of the trimer virtually coincide, together with the narrow molecular-mass distribution of the polymer, imply that the chain growth reaction occurs much faster than competing side processes. The results obtained led the researchers 63 to the conclusion about a cationic mechanism of HCCTP polymerisation in the melt.The same mechanism was proposed for the polymerisation of the trimer carried out in solution in 1,2,3-trichlorobenzene in the presence of sulfamic acid, ammonium sulfamate, toluenesulfonic acid, and other catalysts.64 The polymerisation of HCCTP in the melt at 250 8C has been studied by electron microscopy.65 In the early stages of the reaction, large spherical particles were detected, indicating a high + P Cl O + N Cl2P N PCl2 N P Cl2 + Cl7 NH2Cl7 N Cl2P N PCl2 N P Cl2 H2N PCl2 N P Cl x O P N Cl Cl P N Cl OH P NH Cl O HCl P Cl O +NH2 Cl7 H2O 7HCl OCCTP HCCTP N Cl2P N PCl2 N P Cl2 +HCl N Cl2P N PCl2 P Cl2 + NHCl7 N Cl2P N PCl2 P Cl2 + N Cl2P N PCl2 N P Cl2 + NHCl7 HN PCl2 N PCl2 N PCl2 P N PCl2 N Cl2P Cl2 + Cl7 N PCl2 N PCl2 NH+ +HCl PCl2 N PCl N N PCl2 HN P Cl O A C N PCl B N P Cl O D PCl N P Cl O PCl N N G N P Cl O PCl N F PCl HN P Cl O N E (PCl2 N)m P N N Cl PCl2 (PCl2 N)n x Open-chain poly(organophosphazenes).Synthesis and properties 517rate of chain growth. It was noted that poly(dichlorophosph- azene) specimens prepared by the conventional procedure (method A) differ markedly from those obtained with preliminary removal of the residual water and the phosphazene products of hydrolysis from the reaction medium (method B).49, 50 The for- mation of an organised supermolecular structure in the polymer synthesised by method A is apparently hampered by the presence of cross-links between its macromolecules. On the contrary, in the case of the material prepared by method B, supermolecular associates of the lamellar type can be clearly seen in some parts of the electron photomicrography.Thus, the structure of PDCP in the aggregated state obtained at high degrees of polymerisation depends on the synthetic procedure used. In general, it should be noted that despite the certain progress achieved in the synthesis of PDCP suitable for subsequent polymeranalogous substitution reactions, numerous important problems still need to be solved.The search for efficient catalysts for the thermal polymerisation of HCCTP, which would permit decreasing the reaction temperature and duration and increasing the yield of soluble PDCP, appears a promising and important aspect of investigations. 2.Synthesis of poly(organophosphazenes) based on poly(dichlorophosphazene) The commonest and most available method for the synthesis of poly(organophosphazenes) is the polymeranalogous substitution of chlorine in PDCP. It should be noted that the regularities of these reactions have not yet been adequately studied, and the majority of published data on the synthesis of poly(organo- phosphazenes) reduces mostly to the description of the procedures used to prepare one or another particular polymer.An important characteristics of poly(organophosphazenes) is the possible unit non-uniformity.2, 3, 12, 14 The properties of these polymers are known to depend on the presence of a small number of anomalous units in their chains. The anomalous units can arise for several reasons during the synthesis of poly(organophosphazenes) from PDCP and have different chemical structures.First of all, these are branching points, which were present in the initial polymer. The data on the influence of branching points on the properties of poly(organo- phosphazenes) available to date are scarce, probably, since there are no sufficiently strict characteristics for them. However, this type of unit non-uniformity can be virtually eliminated by using linear PDCP in the polymeranalogous transformations.Yet another type of anomalous units arises due to incomplete sub- stitution of the chlorine atoms in PDCP. This affords non- uniform polymers, which contain partially substituted and non- substituted chain units, in addition to the completely substituted ones. The presence of even small proportions of the anomalous units (< 1 mol. %) with non-substituted P ± Cl groups substan- tially decreases thermal, hydrolytic, and thermohydrolytic stabil- ities of poly(fluoroalkoxy- and poly(aryloxy-phosphazenes) and also facilitates their decomposition during photolysis.3, 12 Hydrol- ysis proceeding as a side process can lead to the formation of anomalous partially hydrolysed units, which occur in the polymer chain as two tautomeric forms These units are even less stable and promote destruction of the polymeric chains.Due to the diversity of anomalous units arising during the synthesis of poly(organophosphazenes), it is difficult to obtain well-reproducible results. Therefore, to obtain poly(organo- phosphazenes) possessing valuable properties, one should con- duct the polymeranalogous substitution up to very high degrees of conversion under conditions ensuring almost complete suppres- sion of hydrolysis.The polymeranalogous substitution of chlorine in PDCP by fluoroalkoxy groups has been studied in most detail (see early reviews 2, 3). Sodium,3, 42, 44, 46, 47, 66 ± 76 potassium,75, 77, 78 and lith- ium 36, 74, 75, 77, 79 ± 81 alkoxides and also fluorinated alcohols (in the presence of triethylamine) have been used as nucleophilic agents.3, 48, 82 Since some regularities of the formation of poly(fluoroalkox- yphosphazenes) have been considered in our previous review,3 we do not discuss them here.We only would like to mention that the degree of chlorine substitution and the occurrence of the extremely undesirable side reactions of hydrolysis during the polymeranalogous substitution of chlorine in PDCP by sodium trifluoroethoxide, depend appreciably on the experimental con- ditions. 69, 76, 83 ± 85 The efficiency of these reactions is largely determined by the reaction medium, temperature, and duration of substitution.The amount of water in the reaction system (even at the level 10 ±2 vol.% ±10 ±3 vol.%) is also significant. Hydrolysis is suppressed most efficiently when the reaction is carried out at room temperature and lasts for no more than 3 h when poly(- fluoroalkoxyphosphazene) precipitates, for example, from a tol- uene ± tetrahydrofuran solvent mixture during the process, and when all the reactants and reaction vessels have been thoroughly dried before the process.5, 83 The latter problem has been success- fully solved, for instance, by using molecular sieves as desic- cants.83, 86 The molecular-mass characteristics of the resulting polymer remained unchanged during the subsequent high-temper- ature hydrolysis in boiling aqueous acetone, indicating that the use of molecular sieves as desiccants had ensured virtually complete suppression of hydrolysis during the synthesis of poly(- fluoroalkoxyphosphazenes). In particular, poly[bis(tri-fluoroe- thoxy)phosphazene] (PBTFEP) with a residual chlorine content of 0.04 mass% and a molecular mass of 107 was obtained in this way.The molecular-mass characteristics of the polymer did not change after boiling in 10% aqueous acetone for 10 h.86 Hydrolytic processes play a crucial role in the synthesis of poly(fluoroalkoxyphosphazenes) regarding the possibility of pro- ducing high-molecular-mass polymers.It has been noted (Table 3) that when the degree of hydrolysis of the P ± Cl groups is less than 161074%, the molecular mass of PBTFPP is practically insensi- tive to hydrolytic destruction. The increase in the degree of conversion to *161073% may decrease the molecular mass by a factor of two, and when the degree of conversion is *161072%, even by a factor of ten.14 P N R R P N Cl R P N Cl Cl l m n x P N X OH P NH .X O Table 3. Influence of the degree of conversion of P7Cl groups in side hydrolysis reactions on the molecular mass of poly[bis(tetrafluoroprop- oxy)phosphazene] (PBTFPP) according to Ref. 14. Number of Number of Degree of 106M 7 n Intrinsic macromo- P7C groups conversion viscosity lecules (%) in a macro- of P7Cl [Z] a of molecule groups PBTFPP in THF/ dl g71 0 0 0 13.2 6.94 1 1 1.1661075 13.1 6.89 10 1 1.1661074 12.0 6.40 100 1 1.1661073 6.6 3.85 100 10 1.1661072 1.2 0.90 a [Z]=6.1561076 M0.85 (Ref. 59). 518 S V Vinogradova, D R Tur, V A VasnevWhereas synthesis and properties of poly(fluoroalkoxy- phosphazenes) have been considered in a large number of papers (see reviews 1,3), non-fluorinated poly(alkoxyphosphazenes) (PAP) have been much less studied.However, since ordinary alcohols are more accessible than fluorinated ones, it was highly desirable to study the synthesis of this type of poly(organo- phosphazenes). Data on the synthesis and properties of PAP can be found in a number of publications.44, 87 ± 100 To develop methods for the synthesis of high-molecular-mass PAP, a systematic study on the condensation of high-molecular- mass PDCP with several sodium alkoxides has been undertaken.90 Data on the influence of the conditions of synthesis of PAP on the degree of conversion of the P ± Cl groups in PDCP and on the intrinsic viscosity of the resulting polymers are listed in Table 4.90, 91 It was found that the extent of substitution of the chlorine atoms in PDCP by alkoxy groups is much more dependent on the temperature and duration of the reaction, on the chemical nature of the nucleophilic reagent, and on the polarity of the reaction medium than similar substitution by fluoroalkoxy groups.Thus under the conditions optimum for the synthesis of poly(fluoroal- koxyphosphazenes),3 namely, when the reaction is carried out in a THF± toluene mixture at 25 8C for 3 h, the degree of conversion of the P ± Cl groups into P ±OC4H9 groups is only *80%. This process was found to be best conducted in a mixture of toluene with the corresponding alcohol.In particular, in a mixture of toluene and butanol, the degree of conversion of the P ± Cl groups at 110 8C is as high as >99.98% over a period of 3 h and, what is especially important, these results are well reproducible. It should be taken into account that in the synthesis of PAP, the problem of side reactions of hydrolysis proves to be much more complicated.Unlike the acidity of fluorinated alcohols, that of ordinary alcohols (pKa&16 ± 18, in water) is nearly equal to or less than the acidity of water. Therefore, the reaction of the residual water with alkoxides affords NaOH, which causes the side hydrolysis. The efficiency of hydrolysis is determined by the amount of residual water in the reaction system and by the ratio of the main and side reaction rates.The side reactions occur to the least degree when all the reactants and equipment have been thoroughly dried prior to the reaction and when the experimental conditions employed ensure a high rate of the main reaction.90 The set of identified regularities made it possible to synthesise successfully high-molecular-mass PAP with a low content of defective units (P ± Cl, P ± OH, P=O)90 ± 93 as a result of the high degree of conversion of the P ± Cl groups in the initial PDCP. The conditions used to synthesise PAP are presented in Tables 4 and 5.90 ± 92 A significant factor influencing the rate and the degree of substitution in the alcoholysis of PDCP is the reactivity of the P ± Cl groups in the polyphosphazene, which can vary during the reaction under the influence of converted groups:14, 101, 102 the neighbouring group at the same phosphorus atom (`close neigh- bour' effect) and groups at the adjacent phosphorus atoms (`far neighbour' effect).The effects of neighbouring groups determine the structure of anomalous non-substituted units in the chain. Thus it has been found that fluoroalkoxy groups exert an accel- erating `close neighbour' effect in the alcoholysis of PDCP.102 Consequently, the resulting poly(fluoroalkoxy-phosphazenes), in addition to the completely substituted units, contain the original units with two chlorine atoms attached to phosphorus (± PCl2=N±) even at high degrees of conversion of the P ± Cl groups (>99.9%).The results of quantum-chemical calculations led to X=Cl, OAlk.NaOH 7NaCl P X O NaOH NH P X OH N P X Cl N ONa +H2N P X O Table 4. Conditions of synthesis and characteristics of some poly(alkoxyphosphazenes) (PAP). Nucleophilic Reaction medium T/ 8C [Z] of PDCP/ dl g71 Characteristics of PAP agent (toluene, 25 8C) [Z]/ dl g71 Cl contenta content of (THF, 25 8C) (mass%) P7Cl groupsb (mol.%) C3H7ONa CH3C6H5 ±C3H7OH 95 4.10 3.30 <0.01 <0.02 C4H9ONa CH3C6H5 ±THF 25 3.50 0.36 6.85 17.21 C4H9ONa CH3C6H5 25 5.74 0.53 5.26 11.40 C4H9ONa CH3C6H5 ±C4H9OH 110 5.74 4.03 <0.01 <0.02 C7H15ONa CH3C6H5 ±C7H15OH 110 4.10 5.62 <0.01 <0.03 C2H5OC2H4ONa CH3C6H5 25 5.74 0.98 0.70 1.80 C2H5OC2H4ONa CH3C6H5 ±C2H5OC2H4OH 60 4.40 7.90 1.23 3.80 C2H5OC2H4OLi CH3C6H5 ±C2H5OC2H4OH 100 4.40 Insoluble <0.01 <0.03 Note.Duration of the synthesis � 3 h; amount of sodium alkoxide � 3 moles per mole of P7Cl groups, concentration of PDCP in toluene � 0.17 mol litre71, concentration of the alkoxide in alcohol�2.5 mol litre71. a Determined by coulometric titration. b Quantity of P7OH groups in the samples <0.1 mol.%. Table 5. Conditions of synthesis and characteristics of poly(alkoxy- phosphazenes) [7P(OR)2=N7]n .OR Conditions of [Z] /dl g71 Content /mol.% synthesis (THF, T/ 8C t/ h 25 8C) P7Cl a P7OHb OCH3 60 6 2.93 0.04 2.0 OC2H5 70 5 6.50 0.06 2.7 OC3H7 80 3 3.30 0.02 0.2 OC3H7 c 80 3 1.36 0.02 2.1 OC4H9 110 3 3.70 0.03 0.1 OC4H9 c 110 3 2.70 0.03 2.7 OC5H11 110 3 4.08 0.03 0.1 OC6H13 110 3 5.62 0.03 0.1 OC7H15 110 3 4.50 0.04 0.1 OC8H17 110 3 6.07 0.04 0.1 Note.The reaction medium wasCH3C6H5 ±ROH(0.18 mol of RONa and 72 ml of ROH; 0.02 moles of PDCP in 116 ml of toluene); the intrinsic viscosity of PDCP in toluene at 25 8C was 4.1 dl g71. a Calculated from the content of chlorine found by coulometric titration. b Found by 1H NMR. c Content of water in the system was >361073 vol.%; in other cases, it was <361073 vol.%.Open-chain poly(organophosphazenes). Synthesis and properties 519the suggestion that the accelerating influence of the neighbouring fluoroalkoxy groups during the polymeranalogous substitution in poly(dichlorophosphazene) is mostly due to polar effects.102 Unlike fluoroalkoxy groups, ordinary alkoxy groups exert a decelerating `close neighbour' effect; as a result, the PAP formed contain partially substituted units of the type ±PCl(OAlk)=N±.Thus, for the same degree of conversion of the P ± Cl groups in PDCP, the number of anomalous units in PAP is twice that in poly(fluoroalkoxyphosphazenes). In addition, extensive substitu- tion of chlorines in PDCP by ordinary alkoxides requires more drastic conditions including elevated temperatures.90 Due to the high activity and equal reactivity of chlorine atoms in the initial PDCP, the `far neighbour' effect in the alcoholysis reactions is less pronounced than the `close neighbour' effect.Like the behaviour in alcoholysis, the hydrolytic stability of the anomalous units with non-substituted P ± Cl groups and, hence, the stability and repro- ducibility of the properties of poly(organophosphazenes), also depend on the effect of neighbouring groups.14 The microstructures of three specimens of PBTFPP with different degrees of conversion of the P ± Cl groups, prepared at molar ratios of the P ± Cl groups to sodium 2,2,3,3-tetrafluoro- propoxide of 1 : 0.25, 1 : 0.50, and 1 : 0.75, have been studied by 31P NMR spectroscopy.101 It was found that the distribution of the ±PCl2=N± (A) and ±P(OR)2=N± (B) units along the chain can be sketched as follows: (a) for 25% degree of conversion ...AAAAAAAA... ...BAAABAAA...BBAAAAAA... , (b) for 50% degree of conversion ...AAAAAAAA... ...BBAABBAA... , (c) for 75% degree of conversion ...AAAAAA... ...BBBABBBA... . When the degree of conversion is 25%, the polymer consists mostly of AAA triads and B units, small proportions of BB diads and AAAAAA hexads being also present. In the case of 50% conversion, alternation of AA and BB diads predominates in the chain.At 75% conversion, triads BBB are separated by the initial A units. The presence of the initial units A in poly(fluoroalkoxy- phosphazenes) even at high degrees of conversion of the P ± Cl groups is in good agreement with the data103 on the photolysis of poly[bis(trifluoroethoxy)phosphazene]. According to these data, the destruction products contain chloro(trifluoroethoxy)- cyclotriphosphazene with two geminal chlorine atoms.103 The course of the polymeranalogous substitution of chlorine in PDCP depends appreciably on the chemical nature of the substituting reagent.80, 81, 101 ± 109 Fig. 1 presents the results of interaction of PDCP with lithium 2,2,3,3-tetrafluoropropoxide, phenoxide, and o-allylphenox- it can be seen that a 96% degree of substitution of the chlorine in PDCP by tetrafluoropropoxy groups is attained as soon as after 15 min.The replacement of chlorine by phenoxy groups under similar conditions occurs efficiently during the first 2 h up to 50% degree of conversion. After 24 h, the degree of substitution increases only to 60%, and then the reaction sharply slows down.When lithium o-allylphenoxide is used in this reaction, the process is decelerated as soon as after a 25% ±30% degree of substitution.105, 106A31PNMRstudy of the nucleophilic substitution of the chlorine atoms in PDCP by the phenoxy groups has shown that one chlorine atom at each phosphorus atom is replaced first, and the polymer formed after 24 h is mostly built of [± N=PCl(OC6H5) ±] units.The substantial decrease in the reaction rate observed with phenoxides is apparently due to the fact that the chlorine atoms, the close environment of which consists of phenoxy groups, are less reactive towards subsequent nucleophilic attacks of both the phenoxide groups and other nucleophiles.In fact, unlike the initial PDCP, poly[chloro-(phe- noxy)phosphazene] is relatively stable to hydrolysis.105 When it is made to react with lithium tetrafluoropropoxide under the con- ditions outlined above (reaction time 24 h), the polymers formed contain much more chlorine (6.64%) than poly(tetrafluoropro- poxyphosphazene) (chlorine content <1.00%).This indicates that chlorine in poly[chloro(phenoxy)-phosphazene] is less reac- tive than that in poly[chloro(tetrafluoro-propoxy)phospha- zene].105 When poly(organophosphazenes) are synthesised by simulta- neous addition to PDCP of a mixture of lithium tetrafluoroprop- oxide and lithium phenoxide, taken in a molar ratio of 1 : 1, a large portion of lithium phenoxide remains unreacted, and according to elemental analysis, the repeating unit of the resulting polymer can be represented as NP(OC6H5)0.5(OCH2CF2CF2H)1.3Cl0.2.Thus, under the conditions of competitive nucleophilic substitution, the probability of formation of the ±N=P(OCH2CF2CF2H). .(OC6H5) ± units or, especially, the ±N=P(OC6H5)2 ± units dra- matically decreases,80, 105 and thus the polymer becomes enriched in units containing the more reactive nucleophilic reagent; in this particular case, it is tetrafluoropropoxide.Based on the data of 31P NMR spectroscopy,80 it can be assumed that competitive sub- stitution yields a random heterounit polymer containing phos- phorus atoms with different environments. It was shown 80 that the final structure of poly(organo- phosphazenes) depends on the order in which nucleophilic reagents are introduced in the reaction medium.To prepare a poly(organophosphazene) with mixed phosphorus environments of a specified composition, the less reactive reagent should be introduced first in the reaction area, and the process should be carried out until the desired degree of substitution is attained. Then the more reactive component should be added (the first equation in Scheme 4).Let us consider a particular aspect concerning the synthesis of poly(organophosphazenes) with mixed structures. Recently it has been reported 107 that the reaction carried out in the presence of tetra-n-butylammonium bromide or 15-crown-5 ether is compli- cated by the fact that the phenoxy groups in the ±NP(OCH2C- F3)(OC6H5) ± fragments formed can be replaced by trifluoroethoxy groups and that an excess of sodium trifluoroeth- oxide results in replacement of the phenoxy groups in the ± NP(OCH2CF3)(OC6H5) ± units but not in ± NP(OC6H5)2 ±.This OR=OCH2CF2CF2H, OAr=OC6H5. [ P(OR)(OAr) N ]k [ P(OR)2 N ]m [ PCl(OAr) N ]n Reaction time hours days 0 20 40 60 80 100 2 4 1 5 10 15 20 Proportion of substituted chlorine (%) 1 2 3 Figure 1.Dependence of the degree of substitution of chlorine atoms in PDCP (THF solution, concentration 0.5 mol litre71) on the duration of its reaction with a threefold molar amount of lithium tetrafluoroprop- oxide (1), phenoxide (2), and Ñ-allylphenoxide (3) (THF solutions, con- centration 2 mol litre71) at 20 8C. 520 S V Vinogradova, D R Tur, V A VasnevScheme 4 exchange is markedly retarded after a certain number of phenoxy groups have been substituted.The same investigators noted that tetra-n-butylammonium bromide can be successfully used as an agent accelerating the synthesis of poly(organophosphazenes) with bulky side groups or in those cases where chlorine is substituted by weak nucleophiles. Carborane-containing poly(organophosphazenes) have been synthesised using lithium derivatives of phenyl-o-carborane and - m-carborane, m-carboranethiol, lithium o-carboranylmethoxide, and lithium o-carboranylmethylphenoxide, as well as bis(hydrox- ymethyl)-o-carborane and its lithium derivative as the nucleo- philic reagents.108 ± 112 The reactions involving lithium derivatives of phenyl-o-carborane and -m-carborane yielded products, the structure of which could not be determined, since they were highly unstable.108 The use of lithium m-carboranylthiolate, lithium o- carboranylmethoxide, and lithium o-carboranylmethylphenoxide (see Scheme 4) resulted in the synthesis of carborane-containing polyphosphazenes in 60%± 70% yields.108 The highest degree of chlorine substitution attained in these reactions was only 50%, evidently, due to the steric influence of the carborane substituents.The chlorine atoms remaining in poly(chlorophosphazenes) containing m-carboranylthio and o-carboranylmethoxy groups in Scheme 5 the side chains are hydrolytically unstable.108 During the isolation of these polymers, the chlorine atoms contained in them are partially or completely hydrolysed (Scheme 5, polymer 4).The chlorine atoms in polymer 5 are not hydrolysed under similar conditions. When lithium o-carboranylmethoxide and lithium tetrafluor- opropoxide are added successively to PDCP, a polymer with carboranylmethoxy and tetrafluoropropoxy groups attached to the same phosphorus atom is produced (polymer 6). The reaction of bis(hydroxymethyl)-o-carborane with PDCP was carried out in THF in the presence of triethylamine at 20 ± 50 8C or by the alk- oxide method at 20 ± 22 8C.109, 110, 112 The results obtained indi- cate that the two chlorine atoms at the same phosphorus atom in PDCP can, in principle, be replaced by a carborane-containing group yielding cardo-type polymers; this is accomplished using bis(hydroxymethyl)-o-carborane as the nucleophilic agent.It should be mentioned that a higher degree of substitution of the chlorine atoms in PDCP is achieved in the presence of lithium alkoxide. However, in this case, the o-carborane rings are partially converted into dicarbaundecaborate groups, which are unstable with respect to hydrolysis and oxidation. Therefore, triethylamine appears to be the reagent of choice for trapping the hydrogen chloride evolved during the synthesis of carborane-containing polyphosphazenes. Replacement of the remaining chlorine atoms in these polymers by tetrafluoropropoxy groups (without isola- tion of the product formed in the first stage) has given poly(- organophosphazenes) in which the surrounding groups are represented by fluoroalkoxy groups in addition to the cardo carborane groups.110 Poly(dichlorophosphazene) is a highly reactive initial com- pound for the synthesis of insoluble polymeric catalysts contain- ing oligooxyethylene fragments.14, 113 ± 115 The reaction is carried out with stirring in a mixture of anhydrous THF with toluene.PDCP has also served as the starting compound in the synthesis of graft copolymers.Thus copolymer 7 was synthesised by the reaction of PDCP with a polyarylene sulfone oxide, prepared from 2,2-bis(4-hydroxyphenyl)propane and 4,40-dichlorodiphenyl sulfone, and containing a reactive terminal hydroxy group.116, 117 RF=CH2CF2CF2H, CH2CF3; Ar=C6H5, C6H4Cl, C6H4CH27CH=CH2; M=Na, Li. N P Cl Cl N P ORF ORF x N P OAr ORF y N P OAr OAr z N P Cl OC6H4CH2 N P Cl OCH2 N P Cl SCB10H10CH HSCB10H10CH B10H10 B10H10 RFOH, ArOM MOCH2 B10H10 MOC6H4CH2 B10H10 n N P Cl OC6H4CH2 5 N P Cl Cl n B10H10 n N P OH OCH2 4 B10H10 , H2O LiOCH2 B10H10 LiOC6H4CH2 , H2O B10H10 N P OCH2CF2CF2H 6 n OCH2 B10H10 N P Cl Cl n LiOCH2 B10H10 + + LiOCH2CF2CF2H N P Cl Cl n + B10H10 H2C CH2 HO OH 2xEt3N 72xEt3N.HCl H2C CH2 O O P N x n7x N PCl2 B10H10 N P Cl Cl n + B10H10 H2C CH2 O O P N n + 2nLiCl B10H10 H2C CH2 LiO OLi P N Cl Cl x +Na(OCH2CH2)xONa+H(OCH2CH2)xONa RONa R=CHF2CF2CH2.H (OCH2CH2)x O P N (OCH2CH2)x O P N N OR P OR O P N (OCH2CH2)x P N P O N OR HO N OR OR O (CH2)2 (OCH2CH2)x72 O O (CH2)2 N P N Open-chain poly(organophosphazenes). Synthesis and properties 521The results obtained attest that this macromolecular reaction is fairly complicated.However, the fact that it yields graft copolymer, the composition of which is close to the specified one and the amount of which is no less than 45% of the total amount of the initial compounds indicates that this method can be used to prepare graft copolymers based on PDCP. Since the copolymer 7 obtained in this way proved to be hydrolytically unstable, an attempt was made to replace the remaining chlorine atoms in the phosphazene moiety (one-pot procedure) by trifluoroethoxy groups in the presence of triethyl- amine and lithium chloride.It was found that the polyarylene sulfoxide fragments present in the macromolecule of the graft copolymer 7 hamper the attain- ment of high degrees of the additional substitution of chlorine by the trifluoroethoxy groups. The substitution product 8 in which the residual chlorine content does not exceed 3 mass% is much more resistant to hydrolysis than the copolymer 7.Graft polyphosphazenes of yet another type have also been reported. Free radical reaction of poly[bis(4-ethylphen- oxy)phosphazene] with maleic anhydride carried out in the presence of peroxides or with heating 118 afforded a graft copoly- mer, poly[bis(4-ethylphenoxy)phosphazene-g-succinic anhydride, containing up to 14% of grafted succinic anhydride groups.As shown in Scheme 1, amines can also be used as reagents replacing the chlorine atoms in PDCP. This yields poly(amino- phosphazenes).1, 9, 10, 17, 25 ,32, 42, 91, 119 ± 126 Non-branched primary amines substitute the chlorine atoms relatively easily; however, in this case, it is difficult to obtain a soluble polymer because of the possible cross-linking of macromolecules.A detailed study of the aminolysis of linear PDCP by secondary amines made it possible to find conditions for the formation of poly(di-n-alkylaminophosphazenes) possessing highly chemically uniform structures with low contents of defec- tive units and molecular masses of more than 106.91, 119, 120 In particular, a homologous series of these polymers, [±N=P(NHCnH2n71)2 ±]x , where n = 1 ± 7, was successfully synthesised in this way.119 3.Synthesis of poly(organophosphazenes) by polymerisation of cyclophosphazenes Preparation of poly(organophosphazenes) by polymerisation of completely substituted organocyclophosphazenes would make it possible to avoid the undesirable side processes, which occur when these polymers are synthesised from PDCP because of the hydro- lytic instability of chlorine.In addition, this method would permit preparation of poly(organophosphazenes) with a regular arrange- ment of substituting groups at the phosphorus atoms along the polymeric chain. However, as has been noted in a number of reviews,1, 3, 10, 11, 32 most of the numerous attempts to synthesise poly(organophosphazenes) by this approach were unsuccessful.The attempts to accomplish thermal polymerisation of alkyl-, aryl-, alkoxy-, and aryloxy-substituted cyclotriphosphazenes at 150 ± 350 8C either failed or resulted in the formation of only low- molecular-mass polymers.127 ± 131 The completely substituted flu- oroalkoxycyclotriphosphazenes containing pentafluoropro- poxy 132 and trifluoroethoxy 133 groups could not be involved in thermal polymerisation at 200 ± 300 8C, or at 200 8C under an elevated pressure.64 Attempts have been made to carry out polymerisation of fluoroalkoxycyclotriphosphazenes 64, 133, 134 in the presence of various catalysts.However, only in the presence of an equimolar amount of BCl3, did this give a polyphosphazene with a molecular mass of 23 000 ± 33 000; however, in this case, too, half of the fluoroalkoxy groups were replaced by chlorine.134 Nevertheless, chlorocyclophosphazenes with various degrees of substitution of chlorine by organic groups do enter into polymerisation and copolymerisation.132, 133, 135 Thus a polyphos- phazene with a molecular mass of 110,000 was obtained by copolymerisation of hexakis(trifluoroethoxy)cyclotriphosph- azene and hexachlorocyclotriphosphazene.133 A rubber-like poly- mer has been prepared by copolymerisation of a mixture of trichlorotrimethylcyclotriphosphazene and tetrachlorotetrame- thylcyclotetraphosphazene.135 Thermal bulk polymerisation of geminally substituted cyclotriphosphazenes C6H5(X)P3N3Cl4 (X = Cl, Br, CH3, C4H9) has been studied.136 At 250 8C, the compound withX=Cl polymerises over a period of 20 ± 56 h, and that withX=Br reacts over a period of 4.5 ± 24 h.The products of polymerisation of the monomers with X=CH3 and C4H9 com- prise a broad range of substances including both oligomers and polymers. Allcock and Turner 137 have considered the thermal behaviour of substituted cyclotriphosphazenes of complex structures.It was found that thermolysis of these compounds at temperatures above 200 8C in the absence or in the presence of hexachlorocyclotri- phosphazene, which could initiate polymerisation, affords only low-molecular-mass poly(organophosphazenes). Below we present a number of substituted cyclic phospha- zenes, products of their polymerisation and copolymerisation with hexachlorocyclotriphosphazene, and also poly(organo- phosphazenes) based on them; the latter were prepared by polymeranalogous substitution of the residual halogen atoms in the polymerisation products by the trifluoroethoxy groups.138 Polymerisation of chlorocyclophosphazenes was carried out at 210 ± 250 8C, while that of fluorine-substituted cyclophosph- azenes was conducted at 250 ± 300 8C. The molecular masses of the resulting polymers amounted to (2.8 ± 12.0)6105.138 HO C Me Me O , Et3N S O O 7Et3N.HCl P N Cl Cl O C Me Me O S O O 7 N Cl P C Me Me O S O O +CF3CH2OH P N Cl O R Et3N, LiCl 7Et3N. HCl P N OCH2CF3 O R 8 R = + H2NR NR P P N N 72HCl N P 2 Cl Cl Cl Cl P N P N P N R Cl Cl Cl R Cl P N P N P N R F F F F F 9, 10 11, 12 13, 14 P N P N P N Me Cl Cl R R Cl R=But (9), Ph (10), Me (11, 13), Et (12, 14).N P N P N P N P Cl Me Me Cl Cl Me Me Cl 15 522 S V Vinogradova, D R Tur, V A VasnevAllcock and Ngo 139 have synthesised a number of new phosphazene polymers by thermal polymerisation of 1,1-bis- (trichlorophosphazo)tetrachlorocyclotriphosphazene followed by replacement of the residual chlorine atoms in the polymer- isation product by various organic pendent groups.Polymer- isation was carried out at 150 8C at a reduced pressure for 2 h. Thermal polymerisation of phenylpentafluorocyclotriphospha- zene at 300 8C for 48 ± 72 h was used to synthesise poly(phenyl- fluorophosphazene) [ ±N=PPhF±N=PF2 ±N=PF2 ±]n.140 Polymeranalogous substitution of fluorine in this compound resulted in the synthesis of poly(organophosphazenes) with Me3SiCH2 side groups and poly(organophosphazenes) with mixed phosphorus environments containing trifluoroethoxy groups in addition to Me3SiCH2 groups.Mono- (16), bis- (17), and tris- (18) o-carboranyl-substituted cyclotriphosphazenes have been prepared in order to synthesise carborane-containing polyphosphazenes.110, 141, 142 This polymer is formed under milder conditions than those resulting from polymerisation of hexachlorocyclotriphosphazene and previously known carboranecyclophosphazenes.143 To increase the hydrolytic stability of this carborane-containing polymer, the remaining chlorine atoms were substituted by tetrafluoropropoxy groups.141 Only polymerisation of mono-substituted derivative 16 gave a polymer (yield 60%) completely soluble in THF and having a reduced viscosity of 0.52 dl g71 in this solvent.141 Polymerisation of this monomer was carried out in the melt at 220 8C for 2 h.The data obtained by 31P NMR spectroscopy suggested an irregular arrangement of phosphorus atoms carrying the carboranyl groups.Apparently, the macromolecule of this polymer contains the following fragments, which are randomly distrubuted along the chain: This polymer is formed under milder conditions than those resulting from polymerisation of hexachlorocyclotriphosphazene and previously known carboranecyclophosphazenes.143 To increase the hydrolytic stability of this carborane-containing polymer, the remaining chlorine atoms were substituted by tetra- fluoropropoxy groups.141 The synthesis of cyclic and polymeric phosphazenes with methyl- and phenyl-o-carboranyl groups has also been reported.144, 145 Thermal polymerisation of (methyl-o-carbor- anyl)- or (phenyl-o-carboranyl)-pentachlorocyclophosphazenes was conducted at 250 8C for 120 h.The chlorine atoms remaining in the polymer and susceptible to hydrolysis were replaced by trifluoroethoxy groups.Let us discuss one more type of polyphosphazenes, namely, poly(thiaphosphazenes), the polymeric chains of which incorpo- rate sulfur-containing fragments in addition to the ±P=N± units. Poly(chlorothiaphosphazene) has been prepared by polymerisa- tion of cyclic thiaphosphazene N3P2SCl5.146 The process was carried out in Pyrex tubes at 90 8C or at room temperature.It is of interest that polymerisation of a freshly distilled specimen requires several days at 90 8C, whereas a speci- men that has been first kept overnight at room temperature polymerises over a period of 4 ± 5 h. The yield of the polymer CH2 O O Cl Cl Cl Cl P N P N P N CH2 B10H10 16 CH2 O O CH2 H10B10 B10H10 CH2 N P N P N P Cl Cl O O CH2 17 CH2 O O H2C H10B10 B10H10 CH2 N P N P N O O CH2 18 CH2 O O P H2C B10H10 n=1 ± 4, m=1 ± 2.N m [ PCl2 N]n CH2 O O P H2C B10H10 N Cl2P N PCl2 N S Cl 90 8C S N P N P N Cl Cl Cl Cl Cl n n N P Cl Me n NaOCH2CF3 7NaCl N P OCH2CF3 Me N P N P N P F F F F F n NaOCH2CF3 7NaF R N P N P N P OCH2CF3 OCH2CF3 OCH2CF3 OCH2CF3 OCH2CF3 R n R=But, Ph. R=But, Et. N P N P N P Cl Cl Cl R Cl R n NaOCH2CF3 7NaCl N P N P N P OCH2CF3 OCH2CF3 OCH2CF3 OCH2CF3 R R n N P N P Cl Cl Cl Me x y n NaOCH2CF3 7NaCl N P N P Cl Cl Cl Et x y n NaOCH2CF3 7NaCl x N P N P OCH2CF3 OCH2CF3 OCH2CF3 Me y n x N P N P OCH2CF3 OCH2CF3 OCH2CF3 Et y n Open-chain poly(organophosphazenes).Synthesis and properties 523after two reprecipitations was 32%. Poly(chlorothiaphosphazene) proved to be readily hydrolysable. It was involved in polymer- analogous transformations with various aryloxy-containing nucleophiles.It should be noted that some of the chlorine atoms in polymers of this type still remain unsubstituted. Their molecular masses are >105. Thus, the data presented above demonstrate that despite the numerous attempts to prepare poly(organophosphazenes) directly from completely substituted organocyclophosphazenes, conditions for their successful polymerisation have not yet been found.More reassuring results have been obtained when halocy- clophosphazenes, in which chlorine has been partially substituted by organic groups, were subjected to polymerisation. However, due to hydrolytic instability of the resulting polymers, subsequent replacement of the residual halogen atoms by organic groups of one or another type is required.Certainly, studies on the polymer- isation of organo-substituted cyclophosphazenes require further development. Currently the two-stage procedure considered above is undoubtedly the most efficient method for the synthesis of poly(organophosphazenes). III. Properties of poly(organophosphazenes) 1.Chemical properties In a number of publications,1 ± 3, 9 it has been noted that poly(- organophosphazenes) are chemically stable; however, their stabil- ity depends substantially on the structure and defectiveness of their chemical structure. Thus the presence of anomalous units in poly(organophosphazenes) markedly decreases their hydrolytic stability. Completely substituted poly(organophosphazenes) with inert alkoxy and aryloxy pendent groups are stable to atmospheric moisture and to weak acids and bases.1, 4, 9 Poly(diphenoxy)- phosphazene remains unchanged after refluxing in 1 M HNO3 or 1 M aqueous KOH for 48 h.147 Data concerning the behaviour of poly(fluoroalkoxy- phosphazenes) on treatment with various acids are contradictory. It has been reported that in the solid state, these polymers are resistant to concentrated sulfuric 67 and glacial acetic 1 acids at low temperatures. However, it has also been reported that strong inorganic acids or glacial acetic acid diminish their mechanical properties.148 Cured poly(fluoroalkoxyphosphazenes) have been noted to be stable on treatment with boiling water 67, 148 and to remain unchanged when in contact with concentrated solutions of KOH or NaOH for many months in the cold.1, 67, 148 The hydro- lytic stability of poly(fluoroalkoxyphosphazenes) at moderately elevated temperatures depends on the overall contents in the polymer of all the anomalous units with P ± Cl, P ± OH, and P(O) ±NH groups (Table 6).3, 66, 76 Table 6 presents some characteristics of poly[bis(trifluoro- ethoxy)phosphazene].Two specimens of this polymer are consid- ered: one of them (specimen A) contained <0.18 mol.% of anomalous units, while the other one (specimen B), 0.2 mol.%<x< 1.0 mol.%. For polymer B, the intrinsic viscos- ity substantially decreases (from 1.64 to 0.14 dl g71) after a 20-h refluxing in aqueous acetone, while in the case of polymer A, it remains virtually the same.79 This indicates that at a particular content of anomalous units, polyphosphazene undergoes hydro- lytic destruction, which can occur, for example, as follows: Poly[bis(trifluoroethoxy)phosphazene] containing50.2 mol.% of residual chlorine is so hydrolytically unstable that even at room temperature, it becomes insoluble in organic solvents, apparently, due to cross-links between macromolecules arising from its chemical transformations.3, 66, 76 Poly(organophosphazenes) can undergo various chemical transformations under appropriate conditions.Reactions of pol- y[bis(4-methoxyphenoxy)phosphazene] and poly[bis(benzyl-oxy- phenoxy)phosphazene] with BBr3 to give the polymer [±N=P(OC6H4OH)2 ±]n have been reported.149 The pendent substituents in this polymer contain freeOHgroups, which enable its subsequent chemical modification.The possibility of exchange reactions of poly[bis(trifluoro- ethoxy)phosphazene] with esters and hydroxyl-containing com- S N P N P N Cl Cl Cl Cl Cl n NaOAr1 NaOAr2 NaOAr3 S N P N P N OAr1 Cl OAr1 Cl OAr1 n S N P N P N OAr2 OAr2 OAr2 Cl OAr2 n S N P N P N OAr3 OAr3 OAr3 OAr3 OAr3 n Ar2= Ar3= Ar1= , , .S N P N P N O O O O O n C6H4 C6H4 C6H4 C6H4 C6H4 Ph Ph Ph Ph But p-NaOC6H4Ph S N P N P N O Cl Cl Cl Cl n C6H4But-p S N P N P N Cl Cl Cl Cl Cl n p-NaOC6H4But dioxane, 12 8C P NH H2O P OH+NH2 . O OAlk O OAlk Table 6. Some characteristics of poly[bis(trifluoroethoxy)phosphazene] containing predominating units 7P(OCH2CF3)2=N7 and anomalous units 7P(OCH2CF3)Cl=N7, 7PCl2=N7, 7P(OCH2C- F3)(OH)=N7, 7P(O)(OCH2CF3)7NH7, 7P(OH)Cl=N7, and 7P(O)Cl7NH7.Characteristics Specimen A B Total contents of anomalous <0.18 0.2<x<1.0 units x, mol.% Molecular mass 106106 36106 [Z] in acetone/ dl g71 after the synthesis 3.70 1.64 after 20 h 3.63 0.14 after 3 years 3.68 0.65 sr a/ kgf cm72 after the synthesis 1920 810 after 3 years 2260 80 eb/% after the synthesis 680 510 after 3 years 760 13 a sr is the tensile strength of a nonoriented film; b e is relative elongation at break. 524 S V Vinogradova, D R Tur, V A Vasnevpounds has been studied.150 It was found that the substitution of trifluoroethoxy groups by alkoxy groups carried out in a saturated solution of polyphosphazene and dibutyl adipate containing tetrabutoxytitanium at 240 8C occurs by 10% over a period of 20 h.When polyphosphazene is made to react with m-cresol and undecyl alcohol at temperatures below 240 8C, the exchange hardly takes place. The replacement of trifluoroethoxy groups in polyphosphazenes by alkoxy groups occurs successfully only when the corresponding metal alkoxides are used. Thus treatment of the polyphosphazene [=NP(OR)2 ± ]n withR0ONa,whereRandR0 are monovalent radicals of the type F(CF2)xCH2 or H(CH2)yCH2 (x and y vary from0 to 9) yielded polymers with mixed environments, [=NP(OR)(OR0) ± ]n. 107, 151 ± 153 The reaction was carried out by mixing solutions of the initial compounds in dry THF at 25 8C followed by refluxing the mixture for 4 h.152, 153 The use of bifunc- tional alcoholates, for example, the disodium hexafluoropentane- diolate, permits curing of polyphosphazene.151 Poly(organophosphazenes) with mixed environment contain- ing free hydroxyl groups in some of the pendent substituents, which can be cured by diisocyanates, dicarboxylic acid anhy- drides, etc., have been described.154 Poly(organophosphazenes) containing 0.5 mol.% ± 35 mol.% of organic radicals with NH bonds, in addition to side alkoxy, fluoroalkoxy, or aryloxy groups, can be cured at room temperature by cross-linking reagents containing epoxy, an- hydride, or NCO groups.155 Poly(fluoroalkoxyphosphazenes) containing a small amount (0.1% ± 5%) of OC6H4Alk-o groups in the lateral chain can be cured in compositions with various filling agents by peroxides and sulfur on exposure to high-energy radiation giving tough compounds.156 Poly(organophosph- azenes) with the residues of allyl, n-butenyl, and isobutenyl alcohols 157 ± 159 and allyl hydroxybenzoate 160 ± 162 as the P-sub- stituents have been synthesised.Under appropriate conditions, these polyphosphazenes can be cured or involved into copolymer- isation with unsaturated monomers.162 The attempts to cure poly(fluoroalkoxyphosphazenes) by procedures used in vulcanisation of fluorinated rubbers have not been a success.163 However, the polymers obtained upon intro- duction of reactive o-allylphenoxy groups, [±N=P(CH2CF2CF2H)27x(OC6H4CH2CH=CH2 ±)x] where x = 0.02 ± 1.00, acquire the ability to undergo thermal curing in the presence of initiators as well as radiation-induced and chemi- cally induced curing.163 It was found that organosilicon hydrides of various structures can serve as the curing agents for these polyphosphazenes. Curing with these agents was accomplished in the presence of hydrogen hexachloroplatinate both in solution and in films at 20 ± 65 8C for 0.25 ± 10 h.163, 164 The curing process may occur according to Scheme 6.However, the fact that polyphosphazenes are cured by orga- nosilicon monohydrides suggests that interaction by a free radical mechanism is also possible.163 Scheme 6 A method for the preparation of hybrid polyphosphazene copolymers has been developed. The method includes the reaction of poly(tetrafluoropropoxyphosphazenes) containing 2 mol.% and 5 mol.% of o-allylphenoxy groups, or poly(chlorophenoxy- phosphazene) containing 2 mol.% of o-allylphenoxy groups with polysiloxanes.165 The role of the latter component was played by a poly(dimethyl-)(or methylphenyl-) (or methylvinyl)siloxane rub- ber (the content of methylphenylsiloxane units was 8.0 mol.%, that of methylvinylsiloxane units was 0.3 mol.%) and also by a siloxane polymer of the composition [OSiHEt]m, where m=10 ± 15.In the former case, the reaction was carried out by heating films containing 2 mol.% of benzoyl peroxide at 90 8C.After 5 h of heating, the content of the insoluble fraction amounted to *30%± 70% depending on the particular polyphosphazene used. The reaction of polyphosphazenes with siloxanes containing terminal Si ±H groups was carried out in films in the presence of hydrogen hexachloroplatinate, by heating the films to 65 8C.In some cases, as soon as after 2 h, the content of insoluble fraction in the reaction products was *90%. Judging from thermomechan- ical testing, polyphosphazene ± siloxane hybrid systems of this type surpass poly(organophosphazenes) in elasticity and heat resistance.165 Poly(aryloxyphosphazenes) are capable of self- extinguishing in air with only moderate smoke evolution.33, 166 The limiting oxygen index of their inflammability is 24 ± 65.Poly(fluoroalkoxyphosphazenes) do not burn in air; the oxygen index of their inflammability is 47 ± 65, and in some cases, it is as high as 80.3, 77, 167, 168 It has been found that the probability of decomposition of polyalkoxy- and polyaryloxyphosphazenes during photoly- sis 103, 169 depends on the content of unsubstituted P ± Cl groups.In the absence of these groups, poly(organophosphazenes) possess high light resistance. Data on the influence of high temperatures on poly(organo- phosphazenes) have been reported in numerous publica- tions.1, 3, 8, 9, 11, 32,34, 36,44, 47, 104, 121 ± 123, 131, 170 ± 180 Allcock 1 has noted that heat treatment of poly(organophosphazenes) is accom- panied by depolymerisation and decomposition and that the size of the pendent groups attached to phosphorus has an appreciable influence on the polymer stability with respect to depolymerisa- tion.Thus poly(aryloxyphosphazenes) are more thermally stable than alkoxy-substituted compounds. The latter completely decompose at 550 8C, whereas aryloxy-derivatives loss only 50% of their weight when the temperature reaches 600 8C (for a heating rate of 10 deg min71).47 The decomposition of poly(aryloxy- phosphazenes) affords mostly linear and cyclic oligomers.104, 170 Tables 7 and 8 contain thermal characteristics of some poly- (diaryloxyphosphazenes) and poly(diarylaminophosphazenes).N P O OCH2CF2CF2H CH2CH CH2 + HSi R O SiH Me Me Me Me O Si Me Me N P O OCH2CF2CF2H CH2CH2CH2 Si R Me Me CH2CH2CH2 N P O OCH2CF2CF2H R= O Si Me Me 0710 Table 7.Thermal characteristics of some poly(diaryloxyphosphazenes) [7N=P(OAr)27]n.34, 122 Ar Decomposition temperature/ 8C under argon under oxygen C6H5 380 400 p-MeC6H4 310 310 m-MeC6H4 350 340 p-ClC6H4 400 385 m-ClC6H4 380 375 p-EtC6H4 285 250 p-MeOC6H4 340 233 p-ButC6H4 350 7 3,4-Me2C6H3 315 7 3,5-Me2C6H3 340 7 Note.Thermogravimeteric analysis was carried out at a heating rate of 10 deg min71. Open-chain poly(organophosphazenes). Synthesis and properties 525The data presented demonstrate that the temperatures of the onset of decomposition of arylamino derivatives are lower than those for poly(aryloxyphosphazenes). According to viscosimetric measurements, destruction of many poly(organophosphazenes) starts in the temperature range 100 ± 200 8C.9, 69, 104, 170, 177 Thus a decrease in the molecular masses of poly(diphenoxy- and poly(di- m-chlorophenoxy-phosphazenes) can be observed even at 130 ± 160 8C.170, 177 Apparently, the destruction processes in poly(orga- nophosphazenes) are strongly influenced by the end groups in the polymer chains as indicated, for example, by the fact that the rate of destruction of poly(dibutoxyphosphazene) at 200 8C in vacuo increases with a decrease in the molecular mass of the polymer and with an increase in the concentration of the P=O and P ±OH groups.9, 180 Some studies contain results of thermogravimetric analysis and thermal stability of poly(fluoroalkoxyphosphazenes); how- ever, these data are sometimes contradictory.3, 70, 171 Thus it has been reported 70 that the weight loss of poly[bis(trifluoro-ethox- y)phosphazene] starts at *200 8C, whereas other research- ers 5, 6, 34, 171 give values of 350 ± 360 8C.Molecular-mass characteristics of these polymers have been estimated.69, 172 ± 174 It was found that the molecular mass of poly(fluoroalkoxy- phosphazenes) decreases relatively rapidly at 150 ± 200 8C, prob- ably due to the presence of defective units in their macromole- cules.An attempt has been made to elucidate more precisely the nature of the defective groups and the active sites, which initiate destruction of the polyphosphazene chain in poly[bis(trifluoro- ethoxy)phosphazene].176 It was shown that thermal depolymer- isation of this polymer corresponds to a depolymerisation model with two-stage initiation at which normal phosphazane units are first transformed into defective ones.The latter may be repre- sented by oxophosphazene units like ± P(O)(CH2CF3) ±NH± and ± P(O)(CH2CF3) ± N(Cl) ±. They induce the appearance of active sites for depolymerisation, and, hence, the polymer in question undergoes complete thermal destruction even about 200 8C.Defective oxophosphazane units can arise during the synthesis of the polymer upon hydrolysis of P ± Cl bonds and subsequent tautomeric rearrangement; they also can appear upon thermal transformations of normal phosphazene groups at temperatures about 400 8C. It has been suggested 176 that this mechanism of depolymerisation is common to all poly(alkoxy- and aryloxyphos- phazenes).It order to elucidate the possibility of increasing the thermal stability of poly(organophosphazenes), polymers of this type containing carboranyl groups have been synthesised.108 ± 111, 141 It was found that a5%decrease in the weights of these polymers in air is observed at moderate temperatures (200 ± 270 8C).How- ever, the positive influence of the carboranyl groups is manifested at higher temperatures. The coke residue of these polymers in air at 500 8C amounts to 50%± 80% and that in a helium atmosphere can be as high as 100% even at 900 8C, whereas the initial poly(dichlorophosphazene) and some poly(organophosph- azenes) completely decompose even at 500 ± 570 8C.121 Figure 2 compares thermogravimetric curves for a carborane- containing polyphosphazene (3 and 4) with those for polymers containing no carborane groups (1 and 2).It can be seen that during heating in air, the weight of carborane-containing poly- mers changes to a much lesser degree than that of the polyphos- phazenes [±N=P(OCH2CF3)(OC6H4Cl) ± ]n and [ ±N=P. .(OC6H5)Cl ± ]n. Heating in a helium atmosphere does not result in any change in the weight of the carborane-containing polymer up to very high temperatures. Thus, thermal stability of poly(organophosphazenes) depends on a number of factors, namely, on the chemical structure of groups linked to the phosphorus, on the presence of defective fragments in the polyphosphazene chain, on the terminal groups in the macromolecules, and on the molecular mass.The thermal stability of poly(organophosphazenes) can be changed by varying the above factors or by adding special stabilising compounds to materials based on these polymers.3, 8, 9, 181 ± 185 2. Physical properties Physical properties of poly(organophosphazenes) are intimately connected to their chemical structure. The data of Table 9 give an idea of the influence of chemical structure of poly(organo- phosphazenes) on their solubility.It can be seen that some poly(organophosphazenes) are readily soluble in ordinary organic solvents, which provides opportunity for investigation of their hydrodynamic, rheological, and molecular-mass characteristics and other properties, which have been dealt with in a number of publications.1, 3, 9, 33, 48, 59 ± 63, 87, 91, 121 ± 126, 177, 186 ± 193 Studies of the molecular-mass distribution of polyphosphazenes have led to ambiguous results.Some researchers have noted broad molecular- mass distributions.1, 3, 60 ± 62, 121 ± 126, 177 Thus polydispersity fac- tors of about 7 ± 79 have been reported for poly(fluoroalkoxy- phosphazenes).3, 60 ± 62 However, in relation to poly[bis(tetra-fluo- ropropoxy)phosphazene], it was shown that the synthesis carried out under controlled conditions can lead to poly(fluoro-alkox- yphosphazenes) with narrow molecular-mass distributions (Mw/ Mm = 1.03, 1.39).63 A relatively low polydispersity factor, equal to 1.09, has been reported for poly[bis(octafluoropentyl-oxy)- Table 8.Thermal characteristics of some poly(diarylaminophosphazenes) [7N=P(NHAr)27]n.123 Ar T/ 8C Ar T/ 8C p-MeC6H4 250 p-MeOC6H4 266 m-MeC6H4 262 p-ClC6H4 265 p-EtC6H4 245 m-ClC6H4 253 m-EtC6H4 243 p-FC6H4 260 p-BunC6H4 253 m-FC6H4 249 Note.Thermogravimeteric analysis was carried out at a heating rate of 20 deg min71. 0 20 40 60 80 100 Mass of residue (%) 1 2 3 4 200 400 600 800 T/ 8C Figure 2. Thermogravimetric curves for polyorganophosphazenes with mixed environments recorded in air (curves 1, 2, 3) and under helium (4); heating rate 4.5 deg min71., , . N P OCH2CF3 OC6H4Cl n (1) N P Cl OC6H5 n (2) N P Cl OC6H4CH2 n (3, 4) B10H10 526 S V Vinogradova, D R Tur, V A Vasnevphosphazene].13 In another study, it was noted that for poly(ar- yloxyphosphazenes), the polydispersity factor can be as high as 30.121 According to the results of gel-permeation chromatogra- phy, these polymers are typically characterised by bimodal molecular-mass distributions.9, 177, 191 The polydispersity factor of poly[bis(phenylamino)phosphazene] is 13.4.124 For a number of poly(fluoroalkoxy- and poly(alkoxy- phosphazenes), the parameters of the Mark ± Kuhn ± Houwink equations and dependences of the sedimentation constants in THF solutions on their molecular masses have been determined (Table 10).13, 91, 192 For the same polymers, conformational parameters were calculated and it was shown that the statistical Kuhn segment values increase with an increase in the length of the pendent substituent.91, 192 Thus the Kuhn segments for the polyphospha- zenes with R = ± (CH2)2CH3, ± (CH2)3CH3, and ± (CH2)5CH3 amount to 5361078, 7261078, and 11061078 cm, respectively.The same values for poly[bis(fluoroalkoxy)phosphazenes] with R = ±CH2CF3, ±CH2(CF2)2H, and ±CH2(CF2)4H are equal to 4461078, 12261078, and 16061078 cm, respectively. These results permitted the conclusion that the equilibrium rigidity of poly(alkoxyphosphazene) macromolecules is much lower than that of poly(fluoroalkoxyphosphazene) macromolecules, pro- vided that the lengths of the lateral groups are comparable.The replacement of the oxygen atom in the side chain of poly(dihex- yloxyphosphazene) by an NH group increases the Kuhn segment by a factor of about three; thus the Kuhn segment of poly(dihex- ylaminophosphazene) is 35061078 cm.91 In several studies 2, 3, 48, 62, 186, 193, 194 dealing with the concen- tration dependence of the reduced viscosity of solutions of poly(alkoxy- and poly(fluoroalkoxy-phosphazenes), an abnormal character of this dependence has been observed, namely, the reduced viscosity increases upon dilution, as is normally observed for dilute solutions of polyelectrolytes.Viscosity, electrical con- ductivity, and dielectric properties of dilute THF solutions of four poly[bis(fluoroethoxy)phosphazene] samples were studied.193 The following unusual properties were discovered: (1) the reduced viscosity markedly depends on the shear rate; (2) the reduced viscosity increases upon a decrease in the polymer concentration in the solution; (3) the magnitude of the reduced viscosity of a polymer solution decreases and the pattern of its concentration dependence changes in the presence of various amounts of a salt (KI); (4) the magnitude of the reduced viscosity and sedimentation constants S0 of solutions decrease on passing from a more polar solvent to a less polar one.This type of combination of hydrodynamic properties is quite typical of linear polyelectrolytes.195 The polyelectrolyte character of some poly(organo- phosphazenes), for example, poly[bis(trifluoroethoxy)phosph- azene], is caused by anomalous units such as ± PCl(OCH2CF3)=N± and ± P(OH)(OCH2CF3)=N± present in the polymer chain.2,186, 193, 194 The appearance of ionised groups can be explained by intra- or intermolecular interactions between two different units, for example, by formation of quaternary nitrogen atoms with chlor- ide anions as counter-ions.186 According to the results obtained by electron microscopy for dilute solutions of polymers in polar and moderately polar solvents such as acetone and tetrahydrofuran, ionised groups in these polymers result mostly from intramolec- ular interactions.196 Several studies 197 ± 200 have been devoted to the Kerr effect in solutions of poly(fluoroalkoxyphosphazenes).Thus investigation of dynamic and electrical birefringence of solutions of the poly- phosphazenes ± P[OCH2(CF2)2H]2=N±, ± P[OCH2(CF2)4H]2=N±, and ± P(OCH2CF3)2=N± showed that the specific Kerr constants for these polymers in ethyl acetate are 4.861078, 5.761078, and 761078 cm5 g71 (300 V)72, respectively. It was shown that polyphosphazene molecules are oriented in an electric field owing to their permanent dipole moment, which is collinear with the vector connecting the ends of the macromolecule.This accounts for the substantial contribu- tion of large-scale motion to the orientation of macromolecules of polyphosphazene in an electric field. The heat resistance of poly(organophosphazenes) depends appreciably on their chemical structure.Previously, glass transi- tion temperatures for a number of poly(fluoroalkoxy- phosphazenes) have been reported; for many of these polymers, these values are fairly low.3 For example, the glass transition temperature of {±N=P[OCH2(CF2)2OCF3]2 ±}n determined by the thermomechanical method is799 8C.77 Tables 11 ± 14 contain data on the glass transition and melting temperatures of a series of poly(organophosphazenes) with different substituents. These data show that poly(alkoxy- and poly(fluoroalkoxy-phosphazenes) possess the lowest glass transi- tion temperatures.On passing to aryloxy substituents, glass transition temperatures of polyphosphazenes increase; finally, poly(diarylaminophosphazenes) possess the highest glass transi- tion temperatures (Table 14).It can be seen from Table 13 that the glass transition temperatures of polyphosphazenes with mixed environments can be varied by changing both the chemical nature of substituents at the phosphorus atom and their ratio. The heat resistance of polyorganophosphazenes with reactive side groups can also be changed by their chemical modification,163 which is illustrated by Fig. 3.Data on the physical state, in particular the phase composi- tion, of poly(organophosphazenes) have been presented in numer- ous publications (see, for example, Refs 3, 19, 34, 34, 70, 77, 93, 94, Table 9. Solubility of polyorganophosphazenes [7N=PR27]n. R Soluble in Insoluble in Ref. OEt alcohols, benzene, water, ethanol, 1, 32, ethers, ketones aliphatic 44, 92 hydrocarbons OBu hexane, benzene, methanol, water 90 ± 92 chloroform, THF, cyclohexane, ethyl acetate OCH2CF3 acetone, ethyl acet- ethanol, diethyl 1, 3, 4, ate, THF, dimethyl ether, aromatic 42, 44, ether, DMF, ethy- and aliphatic 48, 143 lene glycol, methyl hydrocarbons ethyl ketone OCH2C2F4H THF, acetone aliphatic 1, 3, 16 hydrocarbons OCH2C7F15 no solvents 3, 47 were found OPh benzene, toluene, acetone, hexane, 1, 32, DMF, dioxane, ethanol, water, 44 chloroform, chloro- DMSO benzene, THF NHC6H13 THF methanol 120 Table 10.Parameters of the equations [Z]=kZMa and S0=kSM17b for polyphosphazenes [7N=P(OR)27]n. R kZ a 1014 kS 17b Range Ref. 1073 xSZ CH2CF3 1.7761074 0.59 35.20 0.47 45.0 ± 122.1 13, 192 CH2(CF2)2H 6.1061076 0.85 1.17 0.39 3.6 ± 63.1 13, 192 CH2(CF2)4H 6.7461077 1.00 2.95 0.33 7.1 ± 23.7 13, 192 (CH2)2CH3 2.0361074 0.65 27.00 0.45 4.8 ± 20.9 91 (CH2)3CH3 1.1761074 0.70 33.30 0.43 2.7 ± 28.7 91 (CH2)5CH3 9.3661076 0.88 56.00 0.37 4.0 ± 16.1 91 Open-chain poly(organophosphazenes).Synthesis and properties 527100, 121, 201 ± 246). Studies of physical transformations of poly(- organophosphazenes) carried out by scanning calorimetry, ther- moanalytical methods, nuclear magnetic resonance, and X-ray diffraction 3, 19, 34, 70, 77, 121, 202 ± 208 demonstrated that some of them exhibit two temperature transitions, despite the fact that they are flexible-chain polymers and contain no mesogenic groups.The lower-temperature transition T1 corresponds to melting of the crystalline phase with retention of a certain degree of ordering, i.e.at T1 the polymer passes into a mesomorphic state. The upper temperature at which it transforms into an isotropic liquid is the true melting temperature (Tmelt). For some poly(- organophosphazenes), these two transitions are separated by an unusually broad temperature range; in some cases, it reaches 150 ± 250 8C or even more.6, 34 For example, T1 and Tmelt for poly[bis(- trifluoroethoxy)phosphazene] are approximately *90 8C and 240 ± 250 8C;13, 34 those for poly[chloro(phenoxy)-phosphazene] are 1698C and 356 8C.34 The physical state of poly(dialkoxyphosphazenes) has been studied in a number of publications.92 ± 94, 244 The influence of the length of the pendent alkoxy groups (ranging from methoxy to octyloxy) in polyphosphazenes on their glass transition temper- atures, crystallisation, and the ability to form mesophases has been analysed (see Table 11). For poly(dibutoxyphosphazene), it was found that a mesophase is formed only when the total content of defective units in the polyphosphazene is <0.1 mol.%.93, 94 Thus, the mesomorphic properties of poly(organophosphazenes) are crucially dependent of the chemical homogeneity of their structure.The transition of poly(butoxyphosphazene) from the mesomorphic to the isotropic state occurring at 200 8C is accom- panied by a small thermal effect (1.3 J g71). It was shown for poly(dipentyloxyphosphazene) that the formation of the meso- phase depends not only on the content of defective units in the polymer but also on the conditions under which the sample was prepared: when the solvent (benzene, THF, chloroform) is slowly evaporated from a polymer solution, the mesophase is formed, whereas fast evaporation or precipitation of the polymer yields an amorphous phase.94 IR and Raman spectra of poly(dialkoxyphosphazenes) [±N=P(OCmH2m+1)2 ± ]n, where m = 1 ± 9, have been studied over a wide temperature range (from 7100 to 100 8C).247, 248 It was suggested that the backbone has a helical structure.Some temperature-induced spectral changes were attributed to confor- mation transitions in the side chains. When the temperature is lowered, the content of trans-isomers increases, whereas that of gauche-isomers decreases. Table 11. Glass transition and melting temperatures of poly(dialkoxy- phosphazenes) [7N=P(OR)27]n.92 R Physical state of Tgl /8C Tmelt /8C polymers at 208C Me amorphous 777 does not crystallise Et " 797 7 Pr mesomorphic 7107 7 Pr amorphous 7107 7 Bu mesomorphic 7107 7 Bu amorphous 7108 7 C5H11 mesomorphic 7110 7 C6H13 amorphous 7104 7 C7H15 " 796 717 C8H17 " 777 4 Note.Glass transition and melting temperatures were determined by differential scanning calorimetry (DSC).Table 12. Melting and glass transition temperatures of some poly(diar- yloxyphosphazenes) [7N=P(OAr)27]n. Ar Tgl/ 8Ca Tmelt/ 8Cb Tmelt/ 8Cc C6H5 71 (5.5) 160 390 p-CH3C6H4 3 (0.3) 152 340 m-CH3C6H4 725 (725) 90 348 p-ClC6H4 5 (4) 167 365 m-ClC6H4 737 (724) 66 370 m-C2H5C6H4 730 (718) 43 7 p-CH3OC6H4 6 (13) 125 7 aTgl were determined both by DSC and by thermomechanical analysis (values in parentheses);122 bTmelt were determined by DSC at a heating rate of 20 deg min71 (see Ref. 122); c Tmelt were determined by the penetration method.34 Table 13. Glass transition temperatures of poly[(aryloxy)fluoroalkoxy- phosphazenes] [7N=P(OR0)(OR00)7]n. OR0 OR00 Content of Tgl/8C Ref. groups (%) a OR0 OR00 OC6H5 OC6H5 100 7 78 1 OC6H5 OCH2CF3 41 46 730 b 81 OC6H5 OCH2CF2CF2H 37 53 750 b 81 OC6H4Cl-p OC6H4Cl-p 100 7 712 121 OC6H4Cl-p OCH2CF3 78 8 720 b 81 OC6H4Cl-p OCH2CF3 52 47 725 b 81 OC6H4Cl-p OCH2CF2CF2H 59 33 740 b 81 OCH2CF2CF2H OCH2CF2CF2H 7 100 760 b 81 OCH2CF3 OCH2CF3 7 100 766 1 a According to elemental analysis.b According to thermomechanical tests at a load applied to the sample of 0.8 kgf cm72 and a heating rate of 1 deg min71.The Tgl was taken to be the temperature of 2% deformation of the specimen. Table 14. Glass transition temperatures of some poly(diarylamino-phos- phazenes) [7N=P(NHAr)27]n.36, 123 Ar Tgl /8C Ar Tgl /8C C6H6 105 p-BunC6H4 53 p-MeC6H4 97 p-MeOC6H4 92 m-MeC6H4 76 p-ClC6H4 85 p-EtC6H4 88 m-ClC6H4 80 m-EtC6H4 61 m-FC6H4 80 Note. Tgl were determined using a thermal analyser at a heating rate of 10 deg min71. 100 50 750 0 50 100 150 200 250 T/ 8C 1 2 e (%) Figure 3.Thermomechanical curves for poly(tetrafluoropropoxy)(Ñ- allylphenoxy)phosphazene ±N=P(OCH2CF2CF2H)1.96(OC6H4CH2. .CH=CH2)0.047. (1) before curing; (2) after curing with tetramethylhydrodisiloxane in a film at 65 8C for 4 h in the presence of hydrogen hexachloroplatinate.The load applied to the specimen was 0.8 kgf cm72, the heating rate was 1 deg min71. 528 S V Vinogradova, D R Tur, V A VasnevVarious aspects concerning the physical state of poly(fluor- oalkoxyphosphazenes) have been studied. 6, 13, 34, 201, 208, 210 ± 232, 244, 245 For the homologous series of poly(fluoroalkoxyphos- phazenes) with various lengths of the pendent groups, 231, 245 it was shown that the glass transition temperatures of the polymers increase with increase in the length of the side group, whereas the temperatures of the transition to the mesomorphic state, characterised by a hexagonal packing of the macromolecule backbones, scarcely change.The increase in the range of existence in the mesomorphic state for polymers with longer pendent groups is due to higher temperatures at which the polymer passes into the isotropic melt.It has been noted in a review 13 that poly(fluroalkoxy-phos- phazenes) are characterised by labile structures, which depend on the conditions of preparation of the polymer and its thermal prehistory. The main reason for the formation of the mesomor- phic state in these polymers is specific interaction between the backbone and the pendent groups containing a large number of electronegative fluorine atoms.Primary attention was devoted to poly[bis(trifluoroethoxy)phosphazene]. It was noted that a pecu- liar structure of the mesophase of this polymer accounts for its ability to flow like a liquid in the mesomorphic state. The structure of the isotropic melt of the polyphosphazene retains the main features of the mesophase structure but is distinguished by a coil conformation of macromolecules.212 In the 453 ± 493 K temper- ature range, rheological properties and structural characteristics of the mesophase melt of the polymer substantially change, which is accompanied by a thermal effect.213 It is believed that this change corresponds to a conformational transition of macro- molecules into a structure intermediate between one-dimensional layered and two-dimensional pseudo-hexagonal structures.The mesophase in poly[bis(trifluoro-ethoxy)phosphazene] was found to be highly sensitive to external pressure; when the pressure increases (up to 400 MPa), the temperature (T1) of the polymer transition from the crystalline state into the mesophase increases, the range of existence of the mesophase is sharply extended, and the mesophase becomes more ordered.211 Temperature transitions and the orientational order in the near-surface layers of poly[bis(trifluoroethoxy)phosphazene] films have been studied.230 The glass transition temperature of this polymer is equal to 770 8C, the temperature of transition of the mesophase into a new modification is 120 8C, and the temper- ature of isotropisation and transition of the polymer into the amorphous state is 225 8C.It was found that the segment optical anisotropy of poly[bis(trifluoroethoxy)phosphazene] in the bulk is twice that in solution. This points to a strong orientational interaction of the macromolecules, resulting in the higher thermo- dynamic rigidity of polymer chains in the bulk than that in solution.It was noted that the structure of uniaxially oriented fibres of poly[bis(trifluoroethoxy)phosphazene] is fundamentally modified upon annealing at a temperature above the mesomor- phic transition. The type of packing changes, namely, non- equilibrium a-phase passes into a thermodynamically stable g- orthorhombic form.216 It has been suggested that macromolecules of this polymer spontaneously straighten out in the mesomorphic state. The phase transitions in poly(organophosphazenes) with partially fluorinated alkoxy groups {±N=P[OCH2..(CF2)xCF2H]2 ±}n, where x = 1, 3, 5, from the crystalline phase to the mesophase, which can be regarded as a conformationally disordered crystal, have been studied in detail by various physical methods.217 ± 220, 224 The temperatures of phase transitions in these polymers decrease linearly following an increase in the length of the pendent groups.The structure of poly[bis(pentafluoro- propoxy)phosphazene] has been studied over a broad temperature range.221, 222 It was found that the polymer macromolecules in the mesomorphic state undergo orientation during extrusion.The studies confirmed the condis-crystalline type of the mesophase with a pseudo-hexagonal packing.222 It was established that poly[bis(heptafluorobutoxy)phosphazene] is capable of existing in the condis-crystalline and mesomorphic states. The mesophase of this polymer possesses high orientational and positional orders in the basis plane but is conformationally disordered.The physical structure of poly[bis(halophenoxy)- phosphazenes] has been considered in a number of stud- ies.208, 233, 243 It was noted that the structures of their mesophases belong to the pseudo-hexagonal type. The physical structures of these polymers are markedly influenced by the thermal prehistory of a particular specimen.243 The mesophase state can also occur in poly(organophosphazenes) with the following side groups: 4-n-C5H11OOCC6H4 ±, 4-n-C4H9OOCC6H4 ±, and 4-n- C10H21OOCC6H4 ±.234 Study of the structures and phase transitions of poly(dialky- laminophosphazenes) has demonstrated that linear highly chemi- cally homogeneous polyphosphazenes of this type form thermally reversible, ordered phases.119 An additional driving force for the self-organisation of these polymers is the formation of hydrogen bonds between the NH groups.244 A number of publications contain data on gas permeability of poly(fluoroalkoxyphosphazenes).235 ± 240, 246 In particular, it was noted that gas transfer through poly[bis(trifluoroethoxy)- phosphazene] is characterised by high selectivity parameters and, hence, this polymer is a promising material for gas separating membranes, for example, for separation of CO2-containing mix- tures.235 The penetrability factor of this polyphosphazene in relation to the CO2/CH4 pair is *10.Of particular interest is the mesophase of poly[bis(trifluoroethoxy)phosphazene], which can ensure production of membranes operating reliably at elevated temperatures.235 The physical state of poly(organophosphazenes) has a con- siderable influence on the properties of composite materials based on these polymers.Blends of poly[bis(trifluoroethoxy)- phosphazene] with polyethylene13, 241, 242 and with the ABS plastic (a ternary copolymer of acrylonitrile, butadiene, and styrene) have been studied.215 It was found that the unusual rheological proper- ties of isotropic and mesomorphic polyphosphazenes can be used to modify the technological properties of ultra-high-molecular- mass polyethylenes.Polyphosphazenes added in small amounts sharply decrease the viscosity of polyethylene during extrusion and permit production of extrudates with good surface quality.241 A decrease in the melt viscosity by approximately 40% was observed for mixtures of the ABS plastics with 10% of poly[bis(- trifluoroethoxy)phosphazene].The increase in the elasticity of samples, formed from these mixtures, at 225 ± 240 8C (processing temperatures) is presumably due to the orientation effect induced by the mesomorphic character of polyphosphazene in this temper- ature range. Many poly(organophosphazenes) possess good film-forming properties; naturally, this requires that these polymers should have high molecular masses and homogeneous chemical struc- tures.In particular, it has been noted 83 that high tensile strengths (about 2000 kgf cm72) of poly[bis(trifluoroethoxy)phosphazene] films are realised only when the molecular mass of the polymer is about 106106. In addition, the data of Table 6 clearly demon- strate the influence of the unit non-uniformity of the polymer on the strength of polyphosphazene films.Films of poly[bis(tri- fluoroethoxy)phosphazene] with high molecular masses and chemically homogeneous macromolecules were found to possess long-term climatic stability under conditions of cold (Yakutsk, Russia) 249 and tropical 250 climates. Thus the service life of these films in the Arctic environment was found to be about 18 years, whereas for polyethylene, this period is only 2 years. 3. Medical and biological properties The high reactivity of poly(dichlorophosphazene) provides broad opportunities for preparing diverse poly(organophosphazenes) with specific biological properties, which can be used for the development of materials meant for medical purposes.They include thromboresistant,14, 251, 252 self-resolvable,253 biologically inert,35, 36 and biologically active materials, 254 ± 260 medi- Open-chain poly(organophosphazenes). Synthesis and properties 529cines,35, 254 ± 256 etc. In particular, biocompatibility of eight different poly(organophosphazenes) was tested in vivo in rat muscles.36 It was found that the influence of these substances on a living organism is as minor as that of the widely used silicone materials.These poly(organophosphazenes) can be applied as film coatings onto parts to be implanted, or introduced into various composi- tions. Especially interesting results have been obtained in a study of medical and biological characteristics of poly[bis(trifluoro-ethox- y)phosphazene] 14, 251, 261 ± 263 with a molecular mass of about 166106 and the minimum unit non-uniformity (the concentration of anomalous units with non-substituted P ± Cl groups was as low as 5.861074 mole fraction).251 Thromboresistant properties of more than 100 poly(organophosphazene) samples as films and sutureless tubes have been assayed ex vivo by platelet tests and the dynamics of parietal thromogenesis in dogs.14, 262 The results presented in Fig. 4 indicate that in thromboresistance (the number of platelets adhered to the surface of the polymers studied upon contact with blood), poly[bis(trifluoroethoxy)phosphazene] sub- stantially surpasses poly(tetrafluoroethylene) and polyethylene meant for medical purposes.14, 251 Polyphosphazene is also supe- rior to polyethylene as regards the dynamics of parietal thrombo- genesis.The strength characteristics of poly[bis(trifluoroethoxy)- phosphazene] films (Table 15) remain at a good level even after a 2-year contact with blood serum (the results were obtained in vitro).261 Toxicological assays carried out with an isolated frog heart, male genital cells of animals, and isolated erythrocytes have also demonstrated that films of this polyphosphazene are free from general toxicity; they are highly chemically stable and exert no toxic effect on organs.262 Biological assays of a polyphosphazene 264 as monofilament threads was carried out in vivo for 6 months in 12 rats, 4 rabbits, and 2 dogs.Suturing polyphosphazene threads were used in surgical operations in various tissues. These sutures do not cause inflammation in tissues; they are quickly covered by a conjunctive tissue and do not adsorb the tissue liquid.In addition, they are antiseptic and non-toxic and are resolved in living tissues at a controllable rate (from 2 weeks to more than 6 months).264 A fluorinated alcohol, phosphate, and ammonia are the final prod- ucts of destruction of polyphosphazene filaments. It was con- cluded that the set of medical and biological properties of polyphosphazene suturing materials permit them to be widely employed in surgery. Polyphosphazene filaments can also be employed to produce a special reinforced jersey fabric for suture- less tubes used in endoprosthetic appliances for the restorative surgery of the upper respiratory tract and for other purposes.IV. Conclusion Nowadays, the most widespread method for the synthesis of poly(organophosphazenes) is the polymeranalogous substitution of the chlorine atoms in poly(dichlorophosphazene) by various organic groups. This opens up way for targeted design of polymeric chains and, hence, for the preparation of polymers with diverse properties. However, this synthetic route is charac- terised by poor reproducibility of experimental results.Never- theless, these difficulties can be overcome provided that the regularities of the formation of poly(dichlorophosphazene) and polymeranalogous substitution are known. In this case, the method can be used successfully to prepare high-quality poly(- organophosphazene) materials with long service lives. Poly(organophosphazenes) possess a broad range of specific properties and may be of interest for various practical pur- poses.1, 3, 8, 9, 12, 14, 24, 31, 33, 35, 36, 265 ± 276 There are prospects for using them as materials operating at low temperatures (non- freezing elastomers, lubricants, etc.).Vulcanisates of poly(fluoro- alkoxyphosphazene) elastomers are resistant to fuels, oils, and hydraulic fluids; they are also highly stable with respect to oxygen and ozone and can operate over a wide temperature range, from 765 to 175 8C.Regarding their resistance to repeated low- R=CH2CF3 (19), C6H4Me-p (20), C6H4Bui-p (21), C6H4But-p (22) R=Ph (23), C6H4Me-p (24), C6H4(CH2)3Me (25), C6H4C6H4Cl-m (26) OR P OR N n 19 ± 22 NHR P NHR N n 23 ± 26 Table 15. Mechanical properties of nonoriented poly[bis(trifluoro-ethox- y)phosphazene] films after contact with blood serum at 40 8C.Duration [Z]/ dl g71 stc / str / e E/ of the contact (acetone) kgf cm72 kgf cm72 (%) kgf cm72 (months) 0 4.00 190 1160 530 725 3 2.90 215 1400 580 910 6 2.70 230 1370 505 1000 8 3.02 170 840 395 1120 12 2.73 160 840 415 1600 16 2.60 140 700 390 1550 24 2.68 146 653 347 1200 Note.stc and str are conventional and real tensile stresses related to the initial and ruptured cross-sections of the specimen, respectively; e is the relative elongation at break; E is elasticity modulus. a Average number of platelets/8000 mm2 b 7 14 22 5000 2500 t/ min 2 1 thrombotic masses/ mg cm72 Amount of 38 36 6 4 2 0.4 6.1 38.1 Poly- phosphazene Teflon Polyethylene Figure 4.Thromboresistant properties (a) and dynamics of parietal thrombogenesis (duration of the contact of the polymer with blood � 1 min) (b) of 100 samples of poly[bis(trifluoroethoxy)phosphazene] (M=166106, proportion of anomalous units 0.06 mol.%) tested in 10 mongrels. (1) polyethylene, (2) polyphosphazene. 530 S V Vinogradova, D R Tur, V A Vasnevtemperature deformations, these materials surpass vulcanised materials based on fluorine-containing and fluorosilicone rub- bers.266 They can be used to manufacture antivibration and packing gaskets, rings and sleeves, fuel pipes, and other parts for air and space engineering, petrochemical industry, and other branches of industry.3, 4, 14, 31, 33, 148, 168, 265, 266, 268 In regions with a cold climate, poly(fluoroalkoxyphosphazenes) can be used both by themselves andas low-temperaturemodifiers for othermaterials.249 Some poly(organophosphazenes) can be useful as flame-retard- ant articles.33 Thus poly(aryloxyphosphazenes) are highly fire- resistant, and simultaneously they evolve only little smoke.These elastomers appear to be promising as insulating materials for closed spaces such as submarines and planes; they can be used to insulate electric cables and to produce insulating foam plastics.265 ± 267 There are data on using poly(organophosphazenes) as elastic hermetic adhesives for inert materials 164 and for impregnation of textiles in order to impart water- and soil-repelling properties to them.272 Being used to modify other polymers, polyethylene and ABS plastics,13, 241, 242 polyvinylphenylsiloxane rubber,270, 271 pol- yesters, phenolic and epoxide polymers,10, 24 etc., poly(organo- phosphazenes) improve in some cases technological properties, elasticity, flame resistance, and thermal stability of these materials.Poly(organophosphazenes) present considerable interest as polymers used for specific non-trivial purposes, for example, in polymeric catalytic systems,14, 113, 254 as solid electrolytes,273, 275 or as electrodes in chemical cells.274 Poly(organophosphazenes) are undoubtedly promising for medico-biological purposes, for example, as self-resolvable suture materials meant for diverse purposes in surgery, anaesthetic preparations, and medicines,14, 35, 3 251 ± 264, 276 thromboresistant materials for reliably operating artificial organs (prosthetic blood vessels, heart valves, and ventricles of heart, endoprosthetic respiratory tracts, etc.) This work was supported by the Russian Foundation for Basic Research (Project No. 95-03-09470). 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ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. A group of fungicidal antibiotics, b-methoxyacrylic acid derivatives (strobilurins, oudemansins, and myxothiazols), their producers, and mechanisms of action are considered. The fungi- cidal activity of these compounds is based on the suppression of cell respiration of fungi in the bc1-complex of cytochromes. They also manifest other biological activities that are not always coupled with inhibition of respiration.Studies of the structure of the natural methoxyacrylates has made it possible to create a novel class of synthetic agricultural fungicides with enhanced stability, high activity, and a broad spectrum of action. The main regularities of the structure ± activity relationship and meth- ods of synthesis of these compounds are discussed. The bibliog- raphy includes 159 references.I. Introduction Some antibiotics produced by various microorganisms (strobilur- ins, oudemansins, and myxothiazols) were found to inhibit the growth of a broad range of fungi.1 Mucidin or strobilurin A (1a) was the first to be discovered in this group of compounds. Later, several other compounds of similar structure that differed from mucidin only in the number of double bonds and the presence and position of substituents, were identified. These compounds were called strobilurins or oudemansins depending on the structure of their aliphatic chains.All of them are secondary metabolites of various fungi. Myxothiazol 2 has a bacterial origin and its struc- ture differs substantially from that of strobilurin A, although both compounds comprise analogous structural elements.The distinctive feature of this group of compounds is the presence of a fragment of b-methoxyacrylic acid (as methyl ester or amide) linked through an a- or b-carbon atom to the rest of the molecule. They possess identical mechanism of action, which consists in the inhibition of cell respiration at the region of the bc1 complex of electron-transport cytochromes (complex III).Such an inhibition mechanism made possible wide application of these compounds as a biochemical tool for the study of oxidative phosphorylation. These compounds also served as models for creation of a basically new class of pesticides currently known as `strobilurin analogues'. The electron-transport chain of respiratory enzymes plays a crucial role in supplying cells of all aerobic organisms (from bacteria, fungi, and plants to higher animals) with energy.The structure of components of the respiratory chain that are inter- mediates in the electron transfer from NADH to oxygen, varies insignificantly on passing from species to species. However, substantial differences are observed in the resistance of enzymes of the cytochrome system of various living organisms to specific inhibitors, which is used by certain organisms in the competition struggle.2, 3 Natural differences in the structure of respiratory enzymes can be used in the design of agrochemical preparations for achieving their high selectivity with respect to plants and low toxicity for mammals.2 Owing to the wide occurrence of redox processes involving the cytochrome system, the respiration inhibitors can be used for suppressing the vital activity of a great variety of organisms.Thus many synthetic strobilurin analogues possess not only fungicidal, but also insecticidal, acaridical, and nematocidal activities. Other natural compounds that incorporate a b-methoxya- crylic acid fragment as a structural element are also known.For instance, a b-methoxyacrylate fragment in dihydrokawain (5,6- dihydro-4-methoxy-6-phenylethyl-2H-pyran-2-one), which has been recently isolated from kava Piper methysticum 4 and pos- sesses fungicidal activity, includes a hydropyranone ring. How- ever, there is no evidence that this compound inhibits respiration. The recently discovered derivatives of mesoxalic acid amido- nitrile O-methyloxime (e.g., cymoxanil 3) are structurally similar R1=R2=H(a); R1=OMe, R2 = H (b), Cl (c); R1=H,R2=OMe (d); R1=OH, R2= H (e); R1=Me2C=CHCH2O, R2=H(f); R1=OH, R2=Me2C=CHCH2O (g).R1 R2 Me MeOOC OMe 1a7g Me Me Me S N N S OMe Me MeO CONH2 2 V V Zakharychev, L V Kovalenko D I Mendeleev Russian Chemico- Technological University, Miusskaya pl. 9, 125047 Moscow, Russian Federation.Fax (7-095) 200 42 04. Tel. (7-095) 948 54 63 (V V Zakharychev), (7-095) 496 58 37 (L V Kovalenko) Received 10 October 1997 Uspekhi Khimii 67 (6) 595 ± 605 (1998); translated by R L Birnova UDC 577.158.8.04 : 632.952 Natural compounds of the strobilurin series and their synthetic analogues as cell respiration inhibitors V V Zakharychev, L V Kovalenko Contents I.Introduction 535 II. Natural methoxyacrylate-type inhibitors 536 III. Biological activity of the natural compounds 537 IV. Synthetic strobilurin analogues and structure ± activity relationships 538 V. Synthesis of strobilurin analogues 541 VI. Conclusion 543 Russian Chemical Reviews 67 (6) 535 ± 544 (1998) #1998 Russian Academy of Sciences and Turpion Ltdto some strobilurin analogues.However, their fungicidal activity is associated with blocking of RNA synthesis in fungal cells.5 II. Natural methoxyacrylate-type inhibitors The high fungicidal activity of the new antibiotic mucidin (1a) isolated from the cultural medium and mycelium of the fungus Oudemansiella mucida was first discovered by Musilek et al.6 ± 8 in the mid-1960's. Two fungicidal antibiotics called strobilurins A and B were isolated from mycelium of the basidiomycete Strobi- lurus tenacellus in 1977.9 Structural elucidation of these com- pounds revealed that strobilurin A and mucidin are identical and represent methyl (2E,3Z,5E)-2-methoxymethylene-3-methyl-6- phenylhexa-3,5-dienoate.10, 11 The E,Z,E-configuration of the double bonds of strobilurins was confirmed by chemical and spectroscopic studies 12 as well as by stereospecific synthesis.13 The spectral properties of a synthetic E,E,E-isomer of stro- bilurin A13 differed from those of the natural compound.Later, several other secondary metabolites of various fungi were isolated and characterised. They are strobilurinAderivatives containing substituents in the benzene ring or in the aliphatic chain (compounds 1b ± g, 4 ± 8).Oudemansins (9a ± c) possess similar structure. Formally, they can be regarded as the products of addition of methanol at the C(3)7C(4) double bond of strobilurins. The known strobilurins and oudemansins together with their producers are listed in Table 1. Considering the trivial names of methoxyacrylate-type anti- biotics, it should be noted that the letter symbols A, B, andCin the names of strobilurins and oudemansins refer to substituents in the aromatic ring and their positions.The same letters are used for the same substitution pattern [cf. the structures of strobilurin B (1c) and oudemansin B (9b)]. The early name of strobilurin A, mucidin, was preserved in the biochemical literature. Strobilurin X was termed by analogy with oudemansin X,1 because the investigators who first discovered this compound did not give it any trivial name.Two strobilurins F were identified and named almost simultaneously by independent groups of investigators; therefore, they bear additional numerical indices (in the order of publication).1 Myxothiazol 2 (or myxothiazol A) mentioned above produced by the myxobacteria Myxococcus fulvus is also a derivative of b-methoxyacrylic acid.14, 26, 27 In contrast with strobilurins and oudemansins, the b-methoxyacrylate fragment of the myxothiazol molecule is represented by an amide rather than by an ester and is substituted at the b-carbon (but not at the a-carbon) atom.Nevertheless, the mechanism of biological action of this antibiotic is close to that of the natural esters of methoxyacrylic acid.28, 29 For a long time, myxothiazol had no analogues among b-methoxyacrylate-type compounds.However, a great number of related compounds were discovered recently. At present, 33 myxothiazols have been isolated, and the structures of 24 of them have been given in a review.1 Unfortunately, comprehensive information about the structure of myxothiazols is still absent in the current literature.It is known only that in myxothiazols B7I, K7O (10a) and Q, X, and Y (10b), R2=MeCO, MeCH(OH), or an (E,E)-nonadienyl residue containing hydroxy, carbonyl, and epoxide groups. Myxothiazol J does not exist. Myxothiazol P (11) has only one thiazole ring. In myxothiazols R7W (12), the b-methoxyacrylic fragment is absent and R is an oxygenated three-, five-, or six-carbon-atom chain.1 3 O EtNHCONH CN NOMe X=H (a), OH (b).O O X MeOOC OMe Me Me O Me Me 4a,b O O Me MeOOC Me O O Me Me Me Me 5 O O Me Me Me Me 6 OMe 7 OMe O O Me Me O H2C Me Me 8 Me MeOOC OMe Me MeOOC OMe Me MeOOC OMe R1=R2=H (a); R1=OMe, R2=Cl (b); R1=H, R2=OMe (c). OMe Me MeOOC OMe R1 R2 9a7c Table 1. Natural strobilurins and oudemansins.Struc- Name Producing organism Ref. ture 1a Strobilurin A Oudemansiella mucida, 6 ± 9, (mucidin) Strobilurus tenacellus, 15 ± 17 Bolinea lutea etc. 1c Strobilurin B S. tenacellus 9 1f Strobilurin C Xerula longipes, 16 X. melanotricha 1d Strobilurin X O. mucida 18 4b Hydroxystrobilurin D Mycena sanguinolenta 19 5 Strobilurin E Crepidotus fulvotomentosus 20 4a Strobilurin D Cyphellopsis anomala 21 1e Strobilurin F-1 Cyphellopsis anomala 21 1g Strobilurin F-2 Bolinea lutea 15, 22 6 Strobilurin G B.lutea 15, 22 1b Strobilurin H B. lutea 15, 22 7 9-Methoxystrobilurin Aa Favolaschia spp. 23 8 9-Methoxystrobilurin Ka Favolaschia spp. 23 9a Oudemansin A O. mucida 24 9b Oudemansin B Xerula longipes, 16 X. melanotricha 9c Oudemansin X O. radicata 25 a Trivial nomenclature is retained. 536 V V Zakharychev, L V KovalenkoStrobilurins A and B, all of oudemansins,1 and myxothia- zol A30 have been synthesised. The generalised data concerning natural methoxyacrylates can be found in a comprehensive review.1 The conformer of strobilurin A (1a) with a minimum energy was calculated using the molecular mechanics method and the molecular orbital theory.It was found that the phenylpentadienyl and methyl b-methoxyacrylate fragments are planar, but are arranged in perpendicular planes. However, according to these calculations, the energy barrier to rotation around the C(2)7C(3) bond is low, therefore enantiomers cannot be isolated at room temperature.31 The structure of oudemansin A 9a in the crystalline state was determined by X-ray diffraction analysis.24 It was found that its conformation closely resembles the conformer of strobilurin A with the minimum energy.31 Apparently these conformations of strobilurin and oudemansin are close to the forms of these molecules bound to cytochrome.31 III.Biological activity of the natural compounds Low concentrations of strobilurins, oudemansins, and myxothia- zols inhibit the growth of diverse fungi.Their mechanism of action is associated with violation in electron transport in complex III of the mitochondrial membrane, which results in the inhibition of cell respiration.28, 29, 32 ± 34 Complex III (ubiquinone ± cytochrome c oxidoreductase or the bc1-complex) is an intermediate link in the chain of respiratory enzymes of bacteria and mitochondria (plant chloroplasts contain a structurally homologous b6 f-complex).It catalyses the electron transfer from hydroquinone to cytochrome c. The energy of the redox reaction is thereby converted into a chemiosmotic mem- brane potential through the translocation of 2e7 and 4H+ across the membrane. According to the Mitchell Q-cycle concept,3, 35, 36 complex III has two quinone reaction centres localised on opposite sides of the membrane.One of them, the so-called Qo-centre (also designated asQp orQz), is the site of hydroquinone oxidation by the reaction: 2QH2 2Q+4e7+4H+. Two electrons are transferred by a high-potential route via the FeS protein and cytochrome c1 to reduce cytochrome c. The other two electrons move to the opposite side of the membrane by a low- potential route formed by cytochromes bH and bL, to the second quinone centre, the so-called Qi-centre (also designated as Qn, Qc, and Qr) where the quinone is reduced: Q+2H++2e7 QH2 .The inhibitors binding to the Qi-centre induce changes in the absorption spectrum of cytochrome bH (b562). Antimycin is the most specific of these inhibitors. Methoxyacrylates were among the first to be discovered in a group of compounds that block respiration by binding to the Qo-centre.This binding results in a shift in the absorption band of cytochrome bL (b566).29, 32, 33, 37 ± 42 Since all tissue respiration inhibitors of this type contain a residue of b-methoxyacrylic acid (MOA) as a structural fragment, which seems to be the key element for manifestation of biological activity, they were termed MOA-inhibitors.33 The binding site of MOA-inhibitors is not identical with the quinone site, since in the presence of these compounds ubiquinone remains bound with cytochrome b.This may be accompanied by changes in the relative position of the ubiquinone molecule at the reaction centre due to a conformational rearrangement of the cytochrome and the resulting violation of the electron transport to the FeS protein.43 ± 45 Thus the concentration of myxothiazol required for 50% inhibition of bovine heart cytochrome b activity is 0.58 mol per mol of cytochrome.29 At high concentrations, MOA-inhibitors can displace the quinone from the site of its binding with the Rieske FeS-protein.41 Apparently, the binding of inhibitors is reversible, since they can replace one another in the Qo-centre.39 Like methoxyacrylates, other compounds can also bind with the Qo-centre.Pyricidine (which inhibits also complex I), 6-hydroxy-5-n-undecyl-4,7-dioxobenzothiazole,2 dibromothy- moquinone,2, 41 and the chromone antibiotics stigmatellins A and B2, 41, 44, 46 also possess inhibitory activity. Despite the recip- rocal competition of these compounds, their binding sites in the Qo-centre are other than those of MOA-inhibitors.1, 2 The structure of the bc1-complex and the Qo-centre as well as the mechanisms of resistance to MOA-inhibitors are discussed in a number of publications.3, 35, 36, 47 Strobilurins and oudemansins manifest nearly identical high activity in vitro against a wide array of fungi,9, 15, 16, 18, 19 ± 21, 23 ± 25, 48, 49 but are inactive against bacteria.Strobilurin F-1 (1e), which manifests much lower activity, is an exception.21 For example, the minimum inhibitory concentrations of strobilurin A (1a) for Candida albicans, C. crusei, C. para- poilosic, C. tropicalis, Cryptococcus neofarmans, Trichophyton mentagrophytes, Epidermophyton êoccosum, and Microsporum canis lie in the range 0.1 ± 12.8 mg ml71, whereas the inhibitory concentrations for Aspergillus spp.and Scopulariopsis spp. range from 100 to 1000 mg ml71 (see Ref. 48). Owing to its antimycotic activity, strobilurin A has been used in clinical and veterinary medicine under the commercial name of Mucidermin Spofa.1 Myxothiazol A 2 is one of the most active MOA-inhibitors.32 It manifests not only fungicidal properties, but inhibits also the growth of some Gram-positive bacteria,1 reversibly inhibits the late G1/S phase of the cell cycle of lymphoblastic T-cells,50 and possesses insecticidal activity.1 In addition, myxothiazol 2 can bind to the phylloquinone site in photosystem I (Kd= 9.561076 mol litre71).However, this inhibitor does not mani- fest any activity in vivo due to the higher affinity of phylloquinone for this site.51, 52 Myxothiazol 2 inhibits also complex I of mitochondria.2 Other myxothiazols rank below myxothiazol A in fungicidal activity.1 OudemansinA(9a),24 strobilurinsA(1a), B (1c),9 E (5),20, 49D (4a),21 and G (6)15 inhibit the growth of human tumour cells, the latter three compounds manifesting the highest activity. 9-Methoxystrobilurins (7, 8)23 and strobilurin G (6)15 also display cytostatic activity.Strobilurins D (4a)21 and E (5)20, 49 possess antiviral properties. Strobilurins A (1a) and B (1c) as well as oudemansin A (9a) inhibit chitin synthase.53 Strobilurins A (1a), B (1c), C (1f), and X (1d) as well as oudemansins A (9a) and B (9b) are relatively non-toxic for mice.According to different data, the peroral lethal dose, LD50, for strobilurin A is 500 mg kg71 (see Ref. 8) or 825 mg kg71 (see Ref. 18). Intraperitoneal LD50 is 250 mg kg71 (see Ref. 8); that for oudemansin A exceeds 300 mg kg71 (see Ref. 31). However, myxothiazol 2 is highly toxic for all animals tested (peroral LD50 for mice is 2 mg kg71).27 R1=H (a), Me (b).N S R1 OMe 10a,b MeO Me CONH2 N S R2 OMe N S O H2N 11 MeO Me CONH2 Me Me Me S N 12 S N R Natural compounds of the strobilurin series and their synthetic analogues as cell respiration inhibitors 537Despite the variety of useful properties, natural MOA-inhib- itors have found only limited use as medicinal drugs or biochem- ical tools for the study of cell respiration. They cannot be used as agricultural pesticides because they have some serious disadvan- tages.For example, strobilurin A, which possesses high activity against a wide variety of fungi in vitro, is fairly inactive in greenhouse studies. This may be due to the low photochemical stability and a relatively high volatility of this compound. Hence, it rapidly evaporates from the surface of leaves of the treated plants.The time of the photochemical loss of the first 50% of strobilurin A in thin film experiments (t50) is 1 min.31 Never- theless, this group of antibiotics presents substantial interest. The relative simplicity of their structure, their ability to retain high activity irrespective of significant changes in their structure, and a basically new mechanism of their action, which entails the absence of cross-resistance in pathogens insensitive to the currently used fungicides, stimulate the synthesis of biologically active analogues of the natural substances.Although respiration inhibitors may be hazardous for homoiothermal organisms, the low toxicity of individual strobilurins and oudemansins suggests that synthetic analogues may exist which differ substantially in toxicity with respect to fungi and homoiothermal organisms.1 IV.Synthetic strobilurin analogues and structure ± activity relationships Although the first attempts to synthesise analogues of natural MOA-inhibitors were undertaken only in the early 1980's, over 200 patents for the synthesis of these compounds, their practical applications, and composite mixtures, have been pub- lished.31, 54, 55 Synthetic analogues of strobilurins have been patented as broad-spectrum agricultural and industrial fungicides, nemato- cides, insecticides and acaricides, plant growth regulators, anti- tumour and antiviral preparations.The undeniable leaders in this area are the British company ICI and the German company BASF who have succeeded in creating experimental fungicides ICI-5504 (13) and BAS-490F (14).Two fragments can be distinguished in the molecules of natural MOA-inhibitors and their synthetic analogues, viz., a toxophore or a pharmacophore, and a carrier group 56 (or a ballast group 54 or backbone 47). Thus in strobilurins, the b-methoxyacrylate fragment plays the role of a toxophore and a substituted (E,E)-phenylpentadienyl residue is the backbone.It is the toxophore that is responsible for the binding with the methoxyacrylate site of the enzyme, although it is inactive if it is not bound with the backbone. In turn, the backbone not only imparts lipophilicity to the compounds, which is needed for the transport of a pharmacophore into the quinone centre, but also enables matching of the shape of the inhibitor molecule and that of the Qo-centre cavity.47 Both factors must be taken into consideration in design of strobilurin analogues.The structure of strobilurins can be modi- fied in two independent ways: (1) substitution of the (E,E)- phenylpentadienyl chain of strobilurins for various aryl, hetero- cyclic, and other groups; (2) substitution of the toxophore element, a b-methoxyacrylic acid fragment, for isosteric fragments of crotonic, methoxyiminoacetic, and analogous acids.Synthetic analogues of the natural MOA-inhibitors are compounds with the general formula 15. In synthetic analogues, Q usually stands for a bulky hydro- phobic substituent, which mimics a labile phenylpentadienyl residue (the backbone). This largely determines the biological activity, photostability, selectivity, and systemic properties of a compound.The rest of the molecule is a toxophore. The elements V, X, Y, and Z can vary rather independently. More or less active are esters, amides, and thioamides of appropriately substituted acrylic 57 and crotonic 58 acids, b-alkoxy-,59 b-alkylthio,60 b-ami- noacrylic acids,61 O-alkyloximes,62, 63 S-alkylthiooximes,64 and alkylhydrazones of glyoxalic acids 65 and unsaturated acids of analogous structure.Toxophore groups with different structures are also known. For example, compounds 15 devoid of ester or amide groups (R1=H, Me, X=CH2, Y=O, Z=N, V=O, R2=Me) are known to inhibit respiration.47 However, these compounds rank below esters and amides of the above-mentioned acids in activity.After the discovery of a large variety of compounds that are not derivatives of b-methoxyacrylic acid but are able to inhibit cell respiration, the term `synthetic MOA-inhibitors' is no longer a correct definition for this group of biologically active substances. It was therefore suggested that such compounds be called `syn- thetic strobilurin analogues'.66 The pioneering studies in the field of synthesis of strobilurin analogues were aimed at increasing their photostability.The effects of structural changes on the photostability of strobilurin analogues were considered by some authors more comprehen- sively.31, 56 Greenhouse studies of oudemansin activity revealed that the presence of a b-methoxyacrylate group did not decrease the photochemical stability of these compounds.31, 56 The simplest strobilurin A analogue 16 with partially hydro- genated double bonds and the oudemansin A analogue 17 did not manifest any useful activity in hothouse studies, although com- pound 17 shows fungicidal properties on agar cultures of fungi.31 One of the first synthetic analogues of natural MOA-inhib- itors was the so-called MOA-stilbene 18.57, 67 ± 70 This compound possesses fungicidal and insecticidal properties and surpasses its natural prototypes in activity.1 X-Ray analysis of a monocrystal of compound 18 showed that the planar methyl b-methoxyacry- late and stilbene fragments are nearly perpendicular to each other as is the case with the strobilurin molecule.31 Stilbene 18, like strobilurin A, is a potent inhibitor of mitochondrial respiration (I50 for stilbene is 0.04 mM, that for strobilurin A is 0.11 mM31).This compound is photochemically more stable than its natural analogue (t50 for irradiation in thin film is 3 min) and less volatile.56 The photostability of MOA-stilbene increases consid- erably in the presence of photostabilisers.56 In the series of b-alkoxy-a-arylacrylates 19, even methyl (E)-b- methoxy-a-phenylacrylate 19a (without substituents in the ben- zene ring) manifests a pronounced fungicidal activity (Table 2). N N O O CN MeOOC OMe 13 14 O Me N MeOOC OMe Y R1X Z Q VR2 X=O, S, NR3; Y=O, S; Z=CH, N; V=CR4, O, S, NR5; R17R5=H, Alk; Q =Ar, Het, ArX7, HetX7, etc. 15 MeOOC OMe 16 MeOOC OMe Me 17 538 V V Zakharychev, L V KovalenkoOn the whole, phenylacrylates 19 are more active for R1=R2=Me than in those cases where R1 or R2 are represented by other alkyl substituents or hydrogen atoms.E-Isomers are usually more active than Z-isomers; it is probable that the latter, being very weak inhibitors of mitochondrial respiration, acquire the fungicidal activity following isomerisation.31 Substituents in the nucleus can both strongly enhance the activity and decrease it.Compounds having a bulky substituent similar to the styryl fragment in the strobilurin molecule at position 2 of the phenyl ring are usually more active. The activity of naphthylacrylates strongly depends on the position of a toxophore: a-naphthylacry- late 20 (X=H) is much more active than the b-isomer 21 (see Table 2). Although substituted MOA-stilbenes manifest high fungicidal activity in hothouses, in field tests they are moderately active due to their low photostability.1 However, owing to their availability MOA-stilbenes, together with natural compounds, have found wide use as tools in biochemical studies.Of particular interest are stilbenes 22, which are analogues of strobilurin E and possess antiviral and antitumour properties.71 In order to increase the photostability of such compounds, it was suggested to hydrogenate the double bond between two benzene rings 72 and to replace it by other spacer groups, such as CH2O, SCH2, O, S,57, 73 ± 75 SO, SO2,57 CH2SO, CH2SO2,76 etc.56, 77 Methyl (E)-methoxyimino-[2-(2-methylphenoxymethyl)- phenyl]acetate (14) (BAS-490F), an experimental fungicide pro- duced by BASF and possessing a protective, curative, and eradicating activities is an example. Its standard doses vary from 50 to 350 g ha71 (see Ref. 66). Acrylate 23a, a diphenyl ether derivative, is also highly efficient against a great number of fungi.57, 78 This compound is sufficiently resistant to light (t50 for irradiation in thin film is 30 h).56 In addition, this compound possesses systemic fungicidal activity.1, 54 Compound 23b, which contains three benzene rings, is even more active,54, 77 but is devoid of systemic properties.Besides, a serious disadvantage of this derivative is that it caused severe damages to some cultures in field tests.54 The reasons for phytotoxicity of such compounds have not been finally estab- lished.79 A change in the position of a phenoxy group in the molecule results in a sharp decrease in the fungicidal activity.Thus the isomer 23c is a weak fungicide. In heterocyclic analogues of the ester 23b, the high level of fungicidal activity is often associated with their ability to migrate in the plant. However, the biological properties of compounds change in an unpredictable manner depending on the number and position of heteroatoms in the rings: sometimes this activity drops drastically, and the action spectrum is narrowed or the phytotoxicity is enhanced.54 Among derivatives of the heterocyclic series, methyl (E)-2-{2-[6-(2-cyano- phenoxy)pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate (13) (ICI-A5504), turned out to be the most efficient. This compound is an experimental fungicide and exerts systemic and translaminar activities against asco-, basidio-, deutero-, and oomycetes.80, 81 MeOOC OMe 18 R1OOC OR2 X 19a7j X=H, Ph, Hal, Alk etc.X MeOOC OMe MeOOC OMe 20 21 22 O O MeOOC OMe O O R3 R2 R1 R=H (a), 3-PhO (b), 4-PhO (c). O MeOOC OMe R 23a7c Table 2. Comparative activities of acrylates 19, 20, and 21.57 Com- R1 R2 X Subject of study a pound P.r.b E.g.c V.i.d P.o.e C.a.f P.v.g E-19a Me Me H 4 4 4 0 2 4 Z-19b Me Me H 2 0 0 0 0 0 E-19c Et Me H 0 0 0 0 0 4 E-19d Me Et H 0 0 1 0 0 0 E-19e Et Et H 0 0 0 0 0 1 E-19f Me Me 2-(E-PhCH=CH) 4 4 4 3 h 4 4 E-19g Me Me 3-(E-PhCH=CH) 0 0 0 0 0 3 E-19h Me Me 4-(E-PhCH=CH) 0 0 0 0 0 2 Z-19j Me Me 2-(¦-PhCH=CH) 4 4 4 3 h 4 4 E-19k Me Me 2-Cl 0 4 0 0 4 4 20 7 7 H 4 3 4 4 4 4 21 7 7 7 3 0 2 2 7 4 Note.The infected plants were sprayed with emulsions (the concentration of active substance was 100 ppm). a Designations: 4�undamaged plants; 3�up to5%damage; 2�6%± 25% damage; 1�26% ±59% damage; 0�60% ± 100% damage in comparison with nontreated plants. b P.r.ìPuccinia recondita (on wheat); c E.g.ìErysiphe graminis f. sp. hordei (on barley); d V.i.ìVenturia inaequalis (on apple- tree); e P.o.ì Pyricularia oryzae (on rice); f C.a. ì Cercospora arachidicola (on peanut); g P.v. ì Plasmopara viticola (on grapes); h spraying with an emulsion with the concentration of the active substance of 25 ppm. Natural compounds of the strobilurin series and their synthetic analogues as cell respiration inhibitors 539In a series of compounds of the formula 24 the toxophoric group of which is attached to an aryl or a heterocyclic residue not directly, but through O and S atoms or through NR or CR2 groups, the derivatives with meta-substituents in the aromatic ring display the highest activity (Table 3).A vast array of structurally diverse synthetic analogues of strobilurin are presently known. Thus biological activities of a great number of compounds with the general formula 15 (whereQ designates various substituted carbo- and hetero-cycles), such as derivatives of indole,83 pyrazole,84 ± 86 pyrrole 87 ± 89 (methoxya- crylate derivatives of pyrrole have been described in the review 55), triazine,90 dibenzo[b,e]-1,4-dioxin,91 thiazole,92 ± 94 isoxazole,92 pyridine,95 ± 98 pyrimidine,99 etc., have been assayed.The structur- e ± activity relationships for this group of compounds are nearly the same as those for a-phenylacrylates.Thus the activity of monocyclic compounds increases as certain substituents are introduced at the position adjacent to the toxophoric group. As in the case of a-phenylacrylates, the b-styryl fragment and the radicals having similar bulk and shape, enhance the activity.Methyl methoxyacrylates derived from amides of aromatic and heteroaromatic acids 25 possess fungicidal properties.100, 101 In some cases, esters 26 manifested higher activity than the corresponding derivatives of glyoxylic acid.102 Fungicidal activity was also found in compounds 27 with an unusual structure of the toxophoric group.103, 104 Certain oximes 28105, 106 and 29 107 ± 119 show enhanced fungi- cidal activity in comparison with their analogues that do not contain an oxime fragment in their backbone.The spatial isomer- ism about the C=N bond may be crucial for the manifestation of activity of these compounds.120 In the series of synthetic analogues of the natural MOA- inhibitors, the experimental fungicides ICI-A5504 (13) and BAS- 490F (14) have been studied in most detail.Like the natural compounds, these analogues bind with the Qo-centre of cyto- chrome b and actively inhibit cell respiration.66, 80, 81, 121, 122 Both substances have low toxicity in rats (LD50>5000 mg kg71). The reason for such a selectivity are the peculiarities of their biokinetic properties rather than their selective affinity for the molecular target.Thus the inhibition constants of complex III from various living organisms were determined using various methoxyacrylates and their analogues.121 The pI50 values for MOA-stilbene (18), BAS-490F (14), and myxothiazol (2) are given in Table 4. The oxidase from cereals was the least sensitive to all of the inhibitors tested. The difference in the pI50 values is less pronounced for enzymes from other species.Structural changes in compounds that enhance their activity against fungi, induced analogous changes in the activities against other species. Analysis of biokinetic properties of a series of compounds revealed that such properties as absorption, transport, and metabolism are more important for their selective toxicity. For example, the high activity of compound 14 against surface-located powdery mildew is due to its diffusion through the gaseous phase and the low level of its absorption by leaves in which it is rapidly metabolised.In contrast, an analogue of this preparation in which the ester group is replaced by the methylamide group is expected to be highly active against endoparasitic fungi.123 A recent report deals with N-methyl-(E)-methoxyimino-(2- phenoxyphenyl)acetamide (code SSF-126) 124 (30), a new prepa- ration of this series, which possesses a broad spectrum of fungicidal activities and is being tested.It is assumed that the Y MeOOC OMe X 24a7i T is the toxophore R=Ar, Het. O N R Me MeOOC OMe 25 X R 26 X 27 O T R2 R1ON 28 Het is oxazol-2-yl, 1,3,4-oxadiazol-2-yl, etc. NOMe MeOOC NOMe Het T 29 O N R2 R1 Table 3.Activities of acrylates 24a ± i.82 Com- X Y Object of study a pound P.r. E.g. V.i. P.o. C.a. P.v. 24a 2-Ph CH2 1 0 3 0 0 0 24b 3-Ph CH2 4 3 4 0 3 4 24c 4-Ph CH2 0 0 0 2 0 0 24d H NMe 0 4 4 4 1 4 24e 3-Ph NMe 4 4 1 3 4 4 24f H O 0 0 2 0 0 0 24g 3-Ph O 4 0 4 3 3 4 24h H S 0 0 0 3 0 3 24i 3-Ph S 4 4 4 2 4 4 Note. The infected plants were sprayed with emulsions (the concentration of the active substance of 100 ppm).a For designations see footnote to Table 2. Table 4. pI50 Values for inhibitors with respect to complex III of various organisms.121 Compound Yeast Botrytis Fly Rat Corn MOA-stilbene (18) 7.3 7.4 7.8 7.6 6.3 BAS-490F (17) 7.9 7.8 6.9 6.3 6.0 Myxothiazole (2) 8.0 7.9 8.3 7.8 6.5 54arychev, L V Kovalenkomain area of its application is the fight against paddy rice diseases.125 Some strobilurin analogues can also bind with the b6 f-com- plex of chloroplasts in the Qo or Qi centre.Thus compounds 18 and 23a,b bind with the Qi centre, whereas compounds 24b,e,g, with the Qo centre. No binding was observed for compounds 20 (X=Ph) and 25 (Ar=2- or 4-ClC6H4).126 V. Synthesis of strobilurin analogues Syntheses of b-alkoxyacrylates are usually based on esters of substituted acetic acid 31.Condensation with formates in the presence of a base gives aldehydoesters 32.31, 54, 56, 81 The reaction with dimethylformamide dimethylacetal gives compounds 34, which also yield aldehydoesters 32 following hydrolysis or alco- holysis. Alkylation of enolates (compounds 32) results in b-alkox- yacrylates 33 (Scheme 1).Scheme 1 b-Methoxyacrylates are often synthesised from glyoxalates 36 by the Wittig reaction 31, 56 (Scheme 2). Scheme 2 Synthesis of the corresponding derivatives of acetic (31) or glyoxylic (36) acids is carried out by conventional methods. Alkylation of nitrogen-containing heterocycles with methyl bromoacetate in the presence of bases was used, e.g., in the synthesis of pyrrole derivatives 37.55, 58 Reaction of benzyl halides and their ortho-substituted deriv- atives 38 with sodium cyanide results in phenylacetonitriles. Their subsequent hydrolysis and esterification yield esters of the corre- sponding phenylacetic acids.128 Esters of glyoxylic acids 40 were obtained by oxidation of alkyl mandelates 39 with sodium hypochlorite.129 Alternatively, they are synthesised from isatins 41.130 To this end, substituted isatins are hydrolysed with a solution of NaOH on heating; the amino group is then substituted for iodine by treatment of the corresponding diazo compound with potassium iodide and copper bronze.The resulting substituted o-iodophe- nylglyoxylic acids 42 are esterified with methyl chloroformate in the presence of triethylamine.In compounds 43, iodine can be exchanged for other substituents to give a wide range of sub- stituted glyoxylic acids. Synthesis of compounds of the type 31 or 36 was also carried out by other methods.65, 74, 131 ± 133 Thus MOA-stilbene 18 was synthesised from o-bromobenzaldehyde. This was coupled with benzylmagnesium bromide. The resulting alcohol was dehydrated in the presence of an acid to give 2-bromostilbene (44).The Grignard reagent prepared from the latter was slowly added to an excess of dimethyl oxalate. The ester arylglyoxylic acid 45 that formed was introduced into the Wittig reaction with (methox- ymethylene)triphenylphosphorane. The target product 18 con- tained a small admixture of the Z,E-isomer.57 Yet another method to obtain compound 18 is the bromina- tion of (E)-2-(2-methylphenyl)-b-methoxyacrylate (46) with N-bromosuccinimide. The reaction of the resulting bromomethyl derivative with trialkyl phosphite and subsequent condensation of methoxyacrylate 47 with benzaldehyde by the Wittig ± Horner ± Emmons reaction results in stilbene 18.70 O N MeNHOC OMe 30 a c QCCOOR1 CHOH 32 e (a) HCOOR, NaH; (b) Me2NCH(OMe)2, Py .TsOH; c) H2O, H+; (d) MeOH, H+; (e) B, R2X (X=Hal, SO3OR2); R1, R2=Alk; Q=Ar, Het, ArX7, HetX7, etc. 34 Q CHCOOR1 CH(OMe)2 35 c d QCH2COOR1 31 31 QCCOOR1 HCNMe2 b 34 QCCOOR1 CHOR2 33 QCOCOOR MeOCH2P+Ph3Cl7, B7 QCCOOR CHOMe 36 N H BrCH2CO2Me, NaH DMF N CH2COOMe 37 X=H, Hal, OAr; X CH2Br (a) NaCN, EtOH, H2O; (b) HCl; (c) MeOH. 38 X CH2COOMe a, b, c X=Alk, Ar, Het.CH(OH)CO2Me X COCO2Me X NaOCl 39 40 N O O H X a b X COCO2H I 41 42 (a) 1. aq. NaOH; 2. NaNO2, H2SO4, H2O; 3. KI, Cu; (b) ClCO2Me, Et3N, CH2Cl2, 108C; X=H, Alk, AlkO, Hal, O2N, NC. X COCO2Me I 43 2-BrC6H4CHO a Br Ph b 44 (a) 1. PhCH2MgCl; 2. aq. HCl; 3. H3PO4, D; (b) 1. Mg; 2. (CO2Me)2; (c) MeOCH2P+Ph3Cl7, NaH. COCO2Me Ph 45 MeOOC OMe Ph 18 c (a) 1.NBS; 2. (RO)3P; (b) NaH, PhCHO. Me OMe MeOOC a b (RO)2PCH2 OMe MeOOC O 18 46 47 Natural compounds of the strobilurin series and their synthetic analogues as cell respiration inhibitors 5417-Substituted 1-naphthylacrylates 20 that are promising fun- gicides are synthesised in several steps from substituted benzenes.131, 134 ± 136 First, benzene or its derivative is condensed with succinic anhydride under conditions of the Friedel ± Crafts reaction. The 40-substituted 4-oxo-4-phenylbutyric acid 48 thus obtained is esterified and the keto group is reduced.After hydrolysis of the ester, 4-phenylbutyric acid 49 is converted into an acid chloride, which is further cyclised into a 7-substituted a-tetralone 50. Its condensation with methyl bromoacetate in the presence of zinc gives a mixture of isomeric methyl esters 51a,b, which is dehydro- genated with sulphur on heating to give 7-substituted methyl naphthylacetates 52.The latter are converted into the target compounds 20 by condensation with ethyl formate followed by methylation with dimethyl sulfate. It was proposed to synthesise compound 13 from methyl a-(o- benzyloxyphenyl)-b-methoxyacrylate 53,137 which is first hydro- genated to remove the protective benzyl group. The resulting phenol is introduced into the reaction with 4,6-dichloropyrimi- dine in the presence of potassium carbonate to give compound 54.An alternative procedure is as follows: 3-methoxymethylene- benzo-2-(3H)-furanone (55)132 is introduced into reaction with 4,6-dichloropyrimidine in the presence of sodium methoxide.The chlorine atom in pyrimidine 54 synthesised by either of these methods is replaced by heating with 2-cyanophenol inDMFin the presence of potassium carbonate to give compound 13. Since the synthesis of b-methoxacrylates according to Schemes 1 and 2 usually gives a mixture of E- and Z-isomers, it was proposed to use isomerisation of the double bond by treat- ment with gaseous HCl, Hg(OAc)2,138 some radical reagents (halogens, N-halosuccinimide),139 various O-, S-, C-, Hal-, or N- nucleophiles 140 to yield (E)-b-methoxyacrylates possessing higher biological activity.Methylthioacrylates 56 can be obtained from the correspond- ing b-hydroxyacrylates 57, e.g., via tosyl derivatives, which are introduced in the reaction with NaSMe.141 It was also proposed to synthesise methylthioacrylates 56 from esters of glyoxalic acids 58 by the Wittig reaction, i.e., by analogy with the synthesis of methoxyacrylates.133 Esters of acrylic and crotonic acids and their homologues are obtained in a similar way from esters of glyoxalic acids 58 by the Wittig reaction 60, 61, 142, 143 or by condensation with paraformaldehyde.60, 143, 144 Esters and amides of methoxyaminoacetic acids are prepared by the reaction of O-methylhydroxylamine with the correspond- ing esters or amides of glyoxylic acid.62, 63, 145 The target E-isomer is formed from a mixture of isomeric esters in the presence of acids.146 Similarly, alkylhydrazones of glyoxylates are obtained from alkylhydrazines.65 It was proposed to synthesise glyoxylic and methoxyimino- acetic acid amides by aminolysis of the corresponding acid chlorides,147 ± 149 esters,97, 150 ± 153 or nitriles (with subsequent deamination of the amidines formed with nitric acid).154 The synthesis of N-methylamides of substituted phenylglyoxylic acids 59 has been patented.155 The latter are used as intermediates in the synthesis of N-methylamides of methoxyiminoacetic acids.This method consists in the reaction of substituted benzoyl chlorides with methylisocyanide followed by hydrolysis of the product formed. C6H5X a b X O (CH2)2COOH 48 c X (CH2)3COOH 49 X O 50 d X CHCOOMe + 51a X CH2COOMe 51b X CH2COOMe 52 f 20 (a) AlCl3, ; (b) 1. EtOH; 2. CF3CO2H, Et3SiH; O O O 2. Me2SO4; X=H, Ph, Hal, Alk. 3. KOH, aq. EtOH; 4.HCl; (c) 1. SOCl2; 2. CF3SO3H; (d) BrCH2CO2Me, Zn, THF; (e) S, 2208C; ( f ) 1. HCO2Et, NaH; e PhCH2O MeOOC OMe 53 O CHOMe O 55 a b O N N Cl MeOOC OMe 54 c O N N PhO MeOOC OMe 13 (a) 1. H2, Pd/C; 2. 4,6-dichloropyrimidine, K2CO3, DMF; (b) NaOMe, 4,6-dichloropyrimidine; (c) 2-NCC6H4OH, K2CO3, DMF. (a) 1. TsCl; 2. NaSMe; (b) MeSCH2P+Ph3Cl7, ButOK. QCCOOMe CHOH 57 QCOCOOMe 58 a b QCCOOMe CHSMe 56 R1, R2=H, Alk.QCOCOOMe R1R2C PPh3 58 QCCOOMe CR1R2 QCCOOMe CH2 59 (CH2O)n, K2CO3, Bu4NI QCH2COOMe 542 V V Zakharychev, L V KovalenkoSome S-methylmethoxyiminothioacetates that are sometimes more active in trials than compound 14 can be obtained by hydrolysis of methoxyiminoacetic acid esters and subsequent reaction of free acids with sodium methanethiolate 156 in the presence of carbonyldiimidazole.Methyl O-esters and amides of methoxyiminothioacetic acids are obtained by the reaction of esters and amides of metoxyimino- acetic acids with Lawesson's reagent.157 ± 159 VI. Conclusion The modern approach to the synthesis of pesticides that makes use if the natural compounds for identification of biological targets and for modelling new biologically active substances is well known and has often been employed, e.g., for elaboration of pyrethroid insecticides, nereistoxin analogues, juvenoids, neon- icotinoids, 4-hydroxycoumarine derivatives, etc.Strobilurin ana- logues also provide an illustrative example of how bioisosterism can be applied for directed enhancement of properties of natural compounds. Despite high biological and particularly fungicidal activity of methoxyacrylate-type antibiotics, their application for plant protection is impeded due to their high sensitivity to light.Nevertheless, it is on the basis of the natural methoxyacrylates that synthetic agrochemical preparations with a basically new mechanism of action have been obtained. Analogues of the natural MOA-inhibitors have indisputable advantages over other systemic fungicides because of the lack of natural resistant microbial strains.Thus, ICI-A5504 (compound 13) efficiently inhibits fungi that are resistant to inhibitors of C-14- demethylase, phenylamides, dicarboxyimides, and benzimida- zoles.80 Strobilurins and their analogues constitute a large group of compounds that are hardly inferior to triazole fungicides in structural diversity.They represent a new class of plant-protecting agents that meet all the demands that are made nowadays for pesticides. Intensive studies aimed at a search for novel bio- logically active pesticides are currently under way by different manufacturers. However, these studies are still in their infancy and so far only three fungicides have been produced by ICI, BASF, and Shionogi.Probably, original products will be offered very soon by Bayer and Roussel UCLAF. 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年代:1998

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