摘要:

Abstract. Published data on carbenium ions containing C:C bonds both directly conjugated with the carbenium centre and separated from it are surveyed and described systematically. Ammonium, diazonium, iminium, phosphonium and iodonium cations containing alkynyl groups, which can be regarded as heteroanalogues of alkynylcarbenium ions, are also considered. The bibliography includes 283 references. I.Introduction The chemistry of unsaturated carbenium ions is constantly attracting the attention of researchers. The range of objects containing unsaturated carbenium ions is fairly broad: they vary from stable salts used as reagents with unique properties to extremely reactive intermediates the stability of which is compa- rable to that of the methyl cation. Unsaturated carbocations with a variety of structures are known.However, all of them can be classified as two main types: (1) cations with a tricoordinated carbenium centre (trisubstituted ions) and (2) cations with a dicoordinated carbenium centre (disubstituted ions). In the former type of cations, the multiple bond is located in the a-position in relation to the positively charged atom (or is farther removed from it along the chain), whereas in the latter type of cations, an atom forming the multiple bond acts as the carbenium centre.The first type of unsaturated cations comprises allyl cations 1 1±7 including functionally substituted ones 8, 9 and their hetero- analogues � 2-azaallenium ions 2,10 ± 15 iminocarbenium ions 3 16, 17 and a-oxocarbenium ions 4.17 ± 21 The second type includes vinyl (5) 22 ± 27 and phenyl (6) 17,26 ±29 cations as well as acylium (7) 2, 26, 30 and nitrilium (8) 26, 31, 32 cations.The species, which arise when a carbenium centre is generated near a triple carbon ± carbon bond, occupy a special place among unsaturated carbocations. They can be related to either of the two types mentioned above, because in some cases, these species behave as destabilised (compared to the dimethylvinyl- and even ethyldimethyl-carbenium) tertiary cations (alkynylcarbenium ions 9),17 whereas in some other cases, they behave as vinyl ions stabilised by conjugation (allenyl ions 10).23 It is obvious that in reality, the charge in the three-carbon chain is substantially delocalised, the degree of delocalisation and, hence, the structure and the behaviour of the cations 11 being dependent on the nature of the substituents R1, R2 and R3 and the reaction medium.These cations have been considered in numer- ous studies, the results of which have not yet been surveyed. The stabilised species 10 are discussed in several reviews devoted to vinyl cations.22, 23, 26, 33 However, in our opinion, it is more correct + + 1a 1b N + + N + 2a 2b 2c N N + + 3a 3b N O + + 4a 4b O + + 5 6 C R O + R C + 7a 7b C O R1 N R2 + R1 C R2 + 8a 8b C N + R3 + + R1R2C C C R3 R1R2C C 9 10 C R1R2C 11 C C R3 SMLukyanov, A V Koblik, L A Muradyan Institute of Physical and Organic Chemistry, Rostov State University, pr.Stachki 194/2, 344104 Rostov-on-Don, Russian Federation. Fax (7-863) 228 56 67 Received 12 January 1998 Uspekhi Khimii 67 (10) 899 ± 939 (1998); translated by Z P Bobkova UDC 547.312 Alkynylcarbenium ions and related unsaturated cations SMLukyanov, A V Koblik, L A Muradyan Contents I.Introduction 817 II. Alkynyl carbocations in transformations of acetylene derivatives 818 III. Spectroscopic studies of alkynyl carbocations 822 IV. Quantum-chemical calculations 824 V.Synthetic applications of alkynyl carbocations 825 VI. Alkynyl carbocations with a remote triple bond 845 VII. Hetero-analogues of alkynyl carbocations 849 Russian Chemical Reviews 67 (10) 817 ± 856 (1998) #1998 Russian Academy of Sciences and Turpion Ltdto regard them as acetylene derivatives in which the triple bond is activated by a cationic substituent. This approach has been proposed in studies 34, 35 dealing with the reactions of acetylene derivatives with nucleophiles.The use of acetylene derivatives with cationic substituents proved to be more efficient than the introduction of neutral electron-withdrawing groups 36 ± 38 or the use of acetylene derivatives of the push ± pull type (for example, compounds 12 and 13 39, 40). In this review we consider the ions 11 as alkynylcarbenium ions the properties of which are determined by the degree to which the multiple bond in these species retains the nature of a triple carbon ± carbon bond.II. Alkynyl carbocations in transformations of acetylene derivatives Alkynylcarbenium ions have been postulated as intermediates in acid-catalysed transformations of functionally substituted acety- lene derivatives.First of all, this refers to the Meyer ± Schuster and Rupe rearrangements (see a review 41 and studies 42 ± 44). Long before alkynylcarbocations were detected experimentally (see Section III), it had been concluded that acid-catalysed propargyl rearrangements 45 ± 47 can occur via the intermediate formation of carbocations the structure of which allows realisation of two reaction pathways.48 These pathways usually compete, the pre- dominance of either of them being determined by the structure of the initial a-hydroxyacetylene 14.the Rupe rearrangement In both cases, a,b-unsaturated carbonyl compounds 18 and 19 are formed. The main difference between these two routes is determined by the carbon atom in the acetylenic fragment which is hydrated and, hence, forms the carbonyl group.The Rupe rearrangement occurs via alkynylcarbenium intermediates 15, which are capable of being deprotonated to enynes 17. Protona- tion of the b-carbon atom in the enyne 17 with the addition of water to the a-carbon atom is energetically the most favourable route. However, when the cationic intermediate is stabilised by, for example, aryl groups, participation of the allenyl cation 16 in the reaction becomes more favourable.This cation is hydrated at the b-carbon atom of the acetylenic fragment (the Meyer ± Schuster rearrangement). Other functional derivatives of acetylene also can enter into this type of rearrangement. a-Chlorinated alkynes 20 are con- verted into allene compounds 21 or 22, depending on the reaction conditions.48, 49 In the latter case, the reaction also gives products 23 and 24.49 3-Methoxy-1,3,3-triphenylprop-1-yne 25 rearranges under various conditions, and the structure of the rearrangement prod- ucts implies that the reaction occurs predominantly via the most stable cationic intermediates.50 In the presence of copper and silver salts, the acetates of tertiary alkynols 23 are converted exclusively into a,b-unsaturated aldehydes 27.46 It has been suggested that alkynyl carbocations can arise even under Grignard reaction conditions.51 When hydroxy ketones of the acetylene series 28 react with dry HCl in CCl4, they undergo an unusual rearrangement to give vinyl-substituted a,b-diketones 29.52 Me2N C C NO2 12 C C Ph Ar Ph Ar + 7 13 a b R1C C C CR4 R2 R3 OH H H+ 7H2O + + R1C C C CR4 R2 R3 H R1C C C CR4 R2 R3 H 14 15 16 16 7H+ H2O a b R1C C C CR4 R2 R3 H OH R1C C CH CR4 R2 R3 H O 19 R1C C C CR4 R2 R3 R1C C C CHR4 R2 R3 OH H+, H2O a b 17 15 7H+ R1C C C CH2R4 R2 R3 O 18 the Meyer ± Schuster rearrangement C C CH R1 R2 Cl CuCl C C CHCl R1 R2 20 21 20 AgOAc AcOH C C CHOAc R1 R2 + 22 C C CH R1 R2 OAc + 23 C C R1 R2 OAc CH2OAc 24 (a) MeOH, 100% H2SO4; (b) HCl.+ C Ph Ph OMe CPh 25 a C Ph Ph O+Me CPh C Ph Ph CPh H + C Ph Ph CPh MeOH C Ph Ph C Ph OMe + H + CH Ph Ph C Ph OMe CH Ph Ph CPh O 26 25 b C Ph Ph C Cl Ph + 26 (23%) (60%) 23 22 R1R2C CHCHO 27 R1=Me: R2=(CH2)2CH=CMe2, (CH2)2C(Me)=CMe2; R1, R2=(CH2)4, (CH2)5. + C R OAc . MgI2 R CH C C CH R R C C CHI R R C R Me R CH + C C CHMe R R MeMgI I7 R=Me, Et, Pri.RC C C CMe2 OH O RC C CH CMe2 O O HCl(gas) CCl4 28 29 818 S MLukyanov, A V Koblik, L A MuradyanA series of studies describe 53 ± 57 the transformations of 1-ethynyl-1-hydroxyphthalans 30 yielding the corresponding a,b-unsaturated ketones 31. R1=Me, Ph; R2=Me, But, Ph, 4-MeC6H4. Alkoxyalkynols 32 ± 34 are converted into a,b-unsaturated esters 35 ± 37 under mild conditions.58 ± Four possible mechanisms, including those involving alkynyl- carbenium ions, have been discussed for these transformations.59 It has been noted that none of the schemes provides a satisfactory explanation for the formation of the cis-isomers of the compounds 36 and 37, which possess higher energies than the corresponding trans-isomers.Acid-catalysed transformations of a large series of symmet- rical and asymmetrical alkynediols 38 have been described in detail.61 The pathway of these reactions, which occur presumably with participation of alkynylcarbenium ions, depends on the structure of the substrates and the nature of the acid catalysts and the solvents.The structures of the products formed in these transforma- tions reflect the typical reactions of carbenium intermediates,2 namely, deprotonation, addition of nucleophiles, alkyl migrations and electrophilic aromatic substitution.61 The same typical reac- tions are observed in many other cases.Thus tertiary b-alkynols 39 undergo the Rupe rearrangement.41 On treatment with dilute H2SO4, bitertiary 1,2-diols of the acetylene series 40 undergo not only the Meyer ± Schuster rear- rangement yielding enones 43 (or subsequent dehydration to dienones 44) but also the pinacolic rearrangement to give alky- nones 42.Undoubtedly, this process occurs via the alkynyl carbocations 41. The transformation of the compounds 40 affords as well stable dihydrofurylium salts 45.62, 63 The use of the Meyer ± Schuster and Rupe rearrangements, accompanied by sextet rearrangements with group migrations and ring transformations, for the synthesis of compounds that are difficult to prepare by other methods has been described in a review.41 These transformations are possible when tertiary or, at least, secondary alkynyl carbocationic intermediates are fairly stable. In this connection, the formation of the vinylic cation 46, which rearranges to give dihydrofuran 47, deserves attention.64 Based on dehydration of hydroxyacetylenes 49, an interesting preparative approach has been developed for the transformation of alicyclic ketones 48 into diketones 51 and furans 52.65, 66 O R1 R1 HO C CR2 O R1 R1 CHCOR2 AcOH, C6H6 30 31 C R2 R1 OH COR3 H+ CH R2 R1 COOR3 C R2 R1 COR3 OH 32 35 HO C COEt MeO CO2 EtOH MeO CHCOOEt 36 33 HO C COEt COOEt H 34 37 R3 O R1 R2 R4 R3 Hal Hal R2 R1 Hal R2 R1 R3 R4 Hal C C R2 R1 R3 R4 O R1 R2 R4 R3 O C C R2 R1 OH R3 R4 HO 38 R4 R3 OH O R1 R2 R 4 = Ph C R2 R1 Hal R4 OH R3 R2 R1 Hal O R4 R3 H O O H R3 R4 H R1 R2 Solvent: THF, EtOH, AcOH, Me2CO, CCl4, AcOH ± dioxane.R1±R4=Me, Et, Pri, Bui , But, Ph, 4-MeC6H4, PhCO, 2-pyridyl, (CH2)5; Hal=Cl, Br, I; Acid catalyst: H2SO4, HCl, HBr, HI, HI7I2, AcCl, HCOOH; + CH2 C CH R2 R1 OH H+ CH2 R2 R1 C CH 7H+ 39 CH COCH3 R2 R1 CH C CH R2 R1 H2O + R2 R1 R3 C OH HO CR4 R2 R1 R3 C HO CR4 40 41 H+ 7H2O 7H+ O R1 C R3 R2 CR4 R2 R1 R2 C HO CR4 R3 + 42 H2O 7H+ R2 R1 R3 CH OH R4 O H+(R2=Me) 7H2O R2 CHCOR4 H2C R1 43 44 + 43 44 H+ 7H2O H+ R3 CHCOR4 R2 R1 O R3 R4 R1 R2 + 45 Me3Si C C C CH2OH Me Me BF3 . 2AcOH + C C CH CH2OH Me3Si Me 46 Me Me3Si OH Me Me + O SiMe3 Me Me 47 Alkynylcarbenium ions and related unsaturated cations 819Tertiary alcohols of the acetylene series 53 have been used to synthesise enynes 54 and 55.67 Dehydration of secondary alkynols of the general formula 56 occurring on treatment with various acidic reagents (HCl, CF3COOH, polyphosphoric acid) affords unusual enyne furan derivatives 57.68 ± 72 On heating in aqueous dioxane (60 8C) in the presence of toluene-p-sulfonic acid, the alcohols 56 recyclise to cyclopent- enones 59, 60, which can serve as valuable starting compounds for the synthesis of prostanoids.72 In early experiments, the yields of these products were very low (*10%), due to substantial resin- ification of the reaction mixtures; however, later, the yields were brought to 80% as a result of stabilisation of the intermediate alkynyl carbocations by complex formation (see Section V).The Rupe rearrangement in the presence of the solid superacid `Nafion-H' has been proposed as an improved method for the synthesis of alicyclic and linear a,b-unsaturated ketones 61 73 (see also Ref. 74). The Meyer ± Schuster rearrangement has been used to prepare fulvenes 63 (which are intermediates in the synthesis of spiro compounds) based on 3-methyl-2-azafluorenone 62.75 A new method for the preparation of S-phenyl thioesters 65 consists in the reaction of phenylthio-substituted alkynols 64 with the trime- thylsilyl ester of polyphosphoric acid.76 The specific structure of the initial hydroxyacetylene may result in an unusual route of the propargyl rearrangement. Thus treatment of ethynyl-containing alcohols 66, 67 and 69 with a 35% solution of HClO4 in THF gave rise to dienones 68 and 70, which are difficult to prepare by other methods and which are used in the synthesis of di- and tri-terpenoids.77 Perfluorobutyl-substituted alkynols 72 have been found 78 to behave unusually under the Rupe reaction conditions: dehydra- tion does not occur at all; nevertheless, a carbonyl group appears at the a-carbon atom of the multiple bond.The a-hydroxy ketones 73 thus formed are converted into a mixture of hydroxy ketones 74 and furanones 75 upon prolonged (60 h) refluxing inHCOOHas a result of the acyloin rearrangement involving the perfluorobutyl group. X O X C HO CR X C CR X C CR O 48 49 50 X O RCH2 O HCOOH H2O HgSO4, H2SO4 Me2CO 51 (6% ± 76%) X O R 52 (17% ± 95%) R=Me, Ph, 3,4-(MeO)2C6H3, 4-NO2C6H4; X =CH2, CHMe, CHPh, CPh2 (51, 52); R=4-MeOC6H4, 2,4-(NO2)2C6H3; X = SO2, NTs, NCO2Et (52).Et C Me OH CH Ac2O, TsOH 1207130 8C MeCH C C Me + H2C C C 53 54 55 Et CH CH O R1 H HO C CR2 R3OH, H+ O R3O R1 CH C CR2 (Z,E)-57 (33% ± 80%) 56 56 H+ 7H2O O R1 H C CR2 H2O H+ + O R1 HO CH C CR2 H+ R1 HO OH C CR2 + 58 7H+ R1=H, Me: R2=H, Pr, Bu, C5H11, C6H13, CH2OMe, C:CPh, Ph; R3=Et, But.O C CR2 OH R1 + 59 O C CHR2 OH R1 60 R1=Me, Et; R2=Me; R1, R2=(CH2)n, n=3±6; R3=H, Me. C R1 R2H2C OH CR3 Nafion-H R1 O CH2R3 R2 61 (60% ± 88%) N Me O 62 N Me C HO CPh 70% H2SO4 40 8C, 15 min N Me CHCOPh 63 PhS C C C R2 R1 OH PPSE PhS C CH CR1R2 O 64 (E)-65 R1=Et, Pri, Bu, But, PhCH2CH2, BuCH2, PrCH=CH, PhCH=CH, BuC:C; R2=Bu; R1, R2=(CH2)n, n=476.PPSE is the trimethylsilyl ester of polyphosphoric acid. Me C HO Me Me CH 66 Me C Me Me CH OH 67 H2SO4 HClO4 THF Me C Me Me CH2 O 68 H Me HO 69 HClO4, THF 5 h CH CH2 Me O C CH Me OH + 70 (74%) 71 (8%) C CH 820 S MLukyanov, A V Koblik, L A MuradyanMention should be made of the unusual pinacolic rearrange- ment of 1,2-glycol derivatives of the acetylene series 76, which gives rise to a,b-unsaturated alkynones 77.These compounds are used as chiral synthons in syntheses of biologically active com- pounds.79 This stereospecific 1,2-rearrangement occurs due to the fact that the ability of the ethynyl substituent for migration to the carbenium centre is much lower than that of the alkenyl sub- stituent activated by the Me3Si group.79 Acid-catalysed esterification of alcohols 78 resulting in the formation of mixed orthoesters 79 and the subsequent Claisen rearrangement of ketene acetal 80 afford allene esters 81.80 This type of transformation of orthoesters is known 81 to involve cationic intermediates.These reactions have been used to synthe- sise compounds 82 and 83, analogues of juvenile hormones 82 and sex pheromones 83 of insects. The Meyer ± Schuster rearrangement initiated by SOBr2 has been used to carry out stereospecific synthesis of chiral 1,3- disubstituted bromoallenes (R)-85 from alkynols (R)-84 84 and to synthesise the a,b-unsaturated aldehydes 27 in the presence of polyvanadiumorganosiloxanes.85, 86 Areaction of cyclopropenylium salts which is accompanied by rearrangement with cyclopropene ring opening and gives alkynyl- carbenium dications 87 has been described;87 the dications 87 were converted into tricarbenium ions 88 stabilised by electron- donating substituents.The principle of coupling of positively charged building blocks used here seems promising for the syn- thesis of extended polycationic systems consisting of allyl-cationic and/or cyclopropenylium fragments stabilised by electron-donat- ing groups.Treatment of ethynyl ketones 89 with PCl5 affords 1,3- dichloroallenes 91 via intermediate chloro(ethynyl)carbenium ions 90.88 Participation of ethynyliminium ions 93 has also been assumed in the transformations of azomethines of the acetylene series 92.89 HCOOH, 8 h 72 73 MeC C R HO O CH2C4F9-n MeC C CC4F9-n R HO HCOOH, 60 h 72HF RC C O OH CH2C4F9-n Me + O O R Me n-F7C3 74 (15%) 75 (85%) R=Me, Et.Me H MsO OH C Me3Si R3 R2 CR1 C O Me SiMe3 R3 R2 CR1 H Me3Al Ar, 745 8C CHCl37MePh 76 77 R1=Ph, ButMe2Si, Ph(CH2)3; R2=H, Bu, PhCH2OCH2; R3=H, Bu. R1=H, Me; R2,R3=H, Me, Pr, (CH2)5; R4=H, Me. R1 C C R3 OH R2 R4CH2C(OEt)3 EtCOOH, 140 ± 150 8C R1 C C R3 O R2 R4CH2C(OEt)2 H+ 7EtOH 79 78 R1 C C R3 O R2 R4CH OEt 80 R1 C C R3 R2 R4CHCOOEt 81 (34% ± 63%) CHO C OH CMe H+ MeC(OEt)3 .COOEt 82 n-C8H17CH(OH) C CH n-C8H17 C COOEt 83 (95%) MeC(OEt)3 EtCOOH RC C CH(OH) (R)-84 But C CH But (R)-85 RBrC R1 C C CH OH R2 (Z,E)-27 R1=H, Me; R2=4-PriC6H4, 4-ButC6H4, 4-MeOC6H4, 4-MeC6H4, 2,4-Me2C6H3. R=Me, But.+RC C BrCH But SOBr2 ClO¡4 Cl Me2N Me2N + ClO¡4 + Li R2N R2N + ClO¡4 Cl Me2N Me2N NR2 NR2 + 86 NR2 R2N C Me2N C NMe2 + Cl7 ClO¡4 87 86 7LiCl + + NMe2 NMe2 NMe2 Me2N C Me2N Me2N + 2ClO¡4 HClO4 H NMe2 Me2N R2N R2N NR2 NR2 + + + 3ClO¡4 88 R=Me, Pri, Ph. Ar=2-MeC6H4, 4-ClC6H4. 90 C CH Cl + + RC C CH Cl RC RC C CHCl Cl 91 (100%) 89 RC C CH O PCl5 Me3SiC C CH NAr MeSCl 92 Me3SiC C C H Ar + MeSCl Cl7 93 NSMe Me3SiC C C H Ar Cl SMe + Cl7 NSMe Alkynylcarbenium ions and related unsaturated cations 821Ionic hydrogenation of alkynols 94 (treatment with trialkylsi- lanes in the presence of trifluoroacetic acid) involves reduction of the hydroxyl group.90 The mechanism proposed for this reaction includes transformation of the alcohol into carbocation 95, which adds the hydride ion detached from the silane.91 R1=Me, Et, Ph; R2=H, Me, Et, Pri, But; R3=Me, Et, Bu, Ph.In the greater part of the publications listed above, the formation of alkynylcarbenium ions was merely postulated based on the general considerations about the mechanisms of acid-catalysed propargyl rearrangements and transformations of functional derivatives. However, alkynylcarbenium ions have ceased to be hypothetical species long ago.They have been repeatedly detected by spectroscopy (Section III); numerous reliable examples of stable alkynylcarbenium salts have now been described (Section V). III. Spectroscopic studies of alkynyl carbocations The first attempts to detect experimentally alkynylcarbenium ions were based on the study of the kinetics of solvolysis. The few studies devoted to this topic are surveyed in several publica- tions.33, 92, 93 However, they provided only kinetic evidence for the participation of carbocationic intermediates in solvolysis but gave no information about the structure of these species.It has been noted that the allenyl cations 10 are contributing structures in relation to the the propargyl cations 9 and that the ions in question can be generated by solvolysis of both haloalkynes 96 and haloallenes 97.In general, the experimental data obtained for the solvolysis of both series of derivatives under a variety of conditions indicate that these reactions follow the SN1 mechanism and that the halides 96 react much faster than the corresponding allenyl isomers 97.92 ± 95 Attempts to elucidate the nature of unsaturated cationic intermediates have also been made in studies dealing with electro- philic addition to cumulenes.Thus, it has been found that hydro- chlorination of penta-1,2,3-triene 98 in aqueous ethanol gives 1,2- and 1,4-addition products (100 and 101, respectively) in 1 : 4 ratio, whereas in a 4 : 1 sulfolane ±CH2Cl2 mixture, the ratio of these products is 1 : 1.93 The relative stability of the ethynyl carbocations with respect to the stabilities of secondary, tertiary and allylic cations can be derived based on the rates of solvolysis of the series of derivatives 102.The results obtained indicate that substituents R can stabilise the cationic intermediate 103 (the rate of solvolysis for R=Hwas taken to be unity).17 R H C:CH CH2CH3 CH=CH2 krel 1 102 104 3.46106 More definite information about the structure of the unsatu- rated cations in question can be obtained by NMR spectroscopy.The first direct observation of alkynyl carbocations 105 was reported by Richey et al.95 in 1965. These cations aroused particular interest due to the fact that they could be regarded as a sort of vinylic cations (see Section I).The 1H NMR spectrum of the solution resulting from extraction with CCl4 of the product formed upon careful addition of 1,1-di(p-methoxyphenyl)but-2- yn-1-ol 104 to concentrated H2SO4 proved the formation of the cations 105 under these conditions. Ar=4-MeOC6H4 . Dilution of this solution by water or aqueous alkali results in the recovered initial alcohol 104 together with a minor amount of the rearranged product 106 (i.e.k2 k3). The signal for the methyl protons of the propynyl fragment (d=2.6 ppm) in the cation 105 is substantially shifted downfield in relation to this signal for the initial alcohol 104 (d = 1.87 ppm); this is due to the substantial charge delocalisation between the tertiary carbenium centre and the multiple bond. It was shown in the same study 95 that the cation 108a is generated both from alcohol 107 and from a mixture of dienynes 109 and 110; this was proved by the data of 1H NMR spectra in which the signals for the methyl protons at the ends of the unsaturated chain (Mea,Meb) almost coincide.A large series of ethynyl- and propynyl-carbenium ions 9a ± h, 105 and 108a,b have been obtained by mixing solutions of the corresponding tertiary alcohols in liquid SO2 with a solution of + C C Ph C OH R1 R2 CF3COOH CH2Cl2 C C Ph C R1 R2 95 94 R33 SiH C C Ph CH R1 R2 (20% ± 84%) 9 R1R2C C CR3 Hal 96 7Hal7 R1R2C C CR3 + + R1R2C C CR3 Hal 97 7Hal7 R1R2C C CR3 10 MeCH C C CH2 H+ slowly 98 MeCH C Me + + Me C C MeCH 99 Cl7 C MeCH C C Me Cl MeCH C C Me Cl + 100 101 Cl Me Me R 7Cl7 Me R Me + 102 103 Ar C Ar OH CMe k1 k2 Ar C Ar CMe + k3 Ar Ar H Me O 104 105 106 Mec Mec Meb C CMea 108a + H2SO4(conc.) Me Me Me OH C CMea 107 Me Me H2C C CMea + CMea 109 Me Me Me C 110 Compound Chemical shift, d /ppm Mea Meb Mec 107 1.78 108a 2.65 2.76 1.10 109 1.93 110 1.93 822 S MLukyanov, A V Koblik, L A Muradyanantimony pentafluoride in fluorosulfonic acid (the ratio SbF5 :FSO3H = 1 : 3 by volume) at 778 8C.Table 1 presents the 1H NMR spectra of these mixtures recorded at760 8C.96 The chemical shifts of the methyl group protons in the compounds 9a ± d, 105 and 108a and the ethyne protons in 9e ± h, 108b as well as the downfield shifts of these signals (Dd) with respect to the signals of the corresponding protons in the initial alcohols point to substantial deficiency of the electron density on the carbon atoms of the acetylenic fragment.This deficiency becomes more pronounced as the carbenium ions become less stable, i.e. the ability of the substituents at the tertiary carbenium centre to delocalise the charge decreases. More comprehensive data on the 1H NMR spectra of the above cations and some other alkynyl carbocations can be found in reviews.33, 97 Olah et al.98 attempted to generate alkynylcarbenium ions by treating tertiary alkynyl chlorides with SbF5 or a solution of SbF5 in SO2. However, these experiments were unsuccessful due to the large amount of side products.Therefore, the researchers 98 used again the corresponding tertiary alcohols, which were treated as described in the study cited above. 96 As a result, they recorded the 1H NMR spectra of the cations 9b and 9i ± n (Table 2). Several conclusions were drawn from analysis of the 1HNMR spectra of these cations.98 Thus the downfield shift of the signal of the protons of the methyl group bound to the tertiary carbenium centre increases in the sequence 9m < 9n < 9j < 9k < 9l, i.e. following the decrease in charge delocalisation by the other two substituents.The positive charge in the ion 9l (dMe=3.67 ppm) is markedly redistributed, with respect to that in the tert-butyl cation (dMe=4.35 ppm),99 with participation of the propynyl fragment; this is also manifested as a downfield shift of the signal of the methyl protons in the cation 9l. Nevertheless, an alkynyl group is unable to delocalise the charge efficiently (at least, as efficiently as does the vinyl group), because the attempts to generate alkynyl carbocations from the secondary alcohols RCH(OH)C:CPh (R=Ph, Me) failed.99 Under the conditions reported, instanta- neous polymerisation occurred even at760 8C.The alkynoyl cations RC:C±C+=O (R = H, Me, Ph) are quite stable; for these species, 1Hand 13CNMRspectra have been recorded 100 and substantial charge delocalisation over the con- jugation chain has been demonstrated. Tris(tert-butylethynyl)me- thanol 111 readily reacts with strong acids even at room temperature.However, a satisfactory 1H NMR spectrum of cation 112 was obtained only upon cooling a solution of the alcohol 111 in chlorosulfonic acid below 3 8C. The spectrum exhibits a singlet with d = 1.86 ppm due to the tert-butyl group protons (note for comparison that the protons of the alkyl group in the initial alcohol 111 are responsible for a signal with d=1.70 ppm).101 It is of interest that bright colouring of acidic solutions of tris(alkynyl)methanols was mentioned in publications back in the 1920s (see Ref. 101 and references therein). The alkynyldihydroxycarbenium ions RC:C±C+(OH)2 (R=H, Me, Ph) arising upon protonation of the corresponding propiolic acids by FSO3H in liquid SO2 (778 8C) or in SO2ClF (7120 8C) have been detected by NMR spectroscopy.102 A fairly comprehensive study of the 13C NMR spectra of a series of phenylethynylcarbenium ions (together with benzyl- and naphthylcarbenium cations) has been reported by Olah et al.103 They analysed in detail the degree of positive charge delocalisation as a function of the nature of the substituents at the carbenium centre.In particular, the 13C NMR spectra of protonated car- bonyl compounds 113 were discussed. Olah et al.104 have analysed the ability of an alkynyl group to delocalise positive charge in comparison with this property of other groups.This was done using a series of carbenium dications 114 generated from the corresponding diols or dihalo-derivatives on treatment with FSO3H, SbF5 ± FSO3H or SbF5±SO2ClF. According to the data of 1H and 13C NMR spectra, para- phenylene and cyclopropyl groups possess the best capacity for charge delocalisation. The ±CH=CH± group occupies an inter- mediate position, and the ±C:C± group exhibits the poorest charge delocalisation capacity. In the tetraphenyl-substituted dications 114 (R=Ph), the charge is mainly concentrated on the carbenium centres and also in the ortho- and para-positions of the terminal phenyl groups.104 The 13C NMR spectra of dications of the type 114 were compared with those of the diphenylpropynylcarbenium (9b) and diphenylallyl (115) cations.100, 104 The relationship between the chemical shifts of the carbon atoms implies that in symmetrical dications 114 (R=Ph, X=±CH=CH±, ±C:C±), the charge is delocalised to a lesser extent than in the corresponding ions 115 and 9b.When the initial alcohol molecule contains no electron- + C R2 R1 CR3 9a7h, 105 Me Me Me C CR3 108a,b + (ButC C)3COH 111 ClSO3H (ButC C)3C+ 112 + PhC C C OH X 113 X=H, Me, OH, OEt, NH2.X R R R R + + Ph Ph CH CH2 + Me Me Me Me HO OH 114 115 116 C C R=Me, Ph; X=7CH2CH27, , ,7C:C7, . H H Table 1. Chemical shifts of the protons of substituent R3 in the 1H NMR spectra of ethynyl- and propenylcarbenium ions.96 Compo- R1 R2 R3 d(R3) Dd und /ppm 9a Me 2.47 0.63 9b Ph Ph Me 2.88 1.06 9c Ph Me Me 2.94 1.12 9d MeC:C7 MeC:C7 Me 2.76 0.89 9e H 5.58 3.35 9f 4-MeOC6H4 4-MeOC6H4 H 5.70 3.05 9g Ph Ph H 6.96 4.35 9h Ph Me H 7.24 4.76 105 4-MeOC6H4 4-MeOC6H4 Me 2.60 0.73 108a7 7 Me 2.63 0.85 108b7 7 H 6.32 4.00 Table 2.Chemical shifts of methyl group protons in the cations 9b, i ± n.98 Compound R1 R2 R3 d/ ppm 9b Ph Ph Me 2.88 9i Ph Ph Ph 7 9j Ph Me Ph 3.14 9k Me Me Ph 3.39 9l Mea Mea Meb 3.67(a), 3.13(b) 9m Mea 4-MebC6H4 Mec 3.05(a), 2.45(b), 2.59(c) 9n Mea Ph 3.11(a), 2.82 ± 3.00(b, c) Meb Mec Alkynylcarbenium ions and related unsaturated cations 823releasing groups attached to the potential carbenium centre and capable of charge delocalisation, dications of the type 114 (R= Me) are not formed.Attempts to ionise 2,5-dimethylhex-3-yne- 2,5-diol 116 result in a complex mixture of carbocations.As has already been mentioned, systems of this sort (38, Section II) readily undergo various types of isomerisation such as the Rupe and Meyer ¡À Schuster rearrangements. Other spectroscopic methods have also been used to study alkynylcarbenium ions. Stable alkynylcarbenium salts have been characterised by IR spectroscopy (Section V). Electronic spectro- scopy has been used to detect cationic intermediates and to measure their lifetimes.33, 95 The dependence of the electronic spectra of cations 117 on the nature of the substituent in the arylethynyl fragment has been studied in order to develop new ethynyl analogues of triphenylmethane dyes, which intensely absorb light in the region 700 ¡À 800 nm.105 It has been found that introduction of a triple carbon ¡À carbon bond in the structure of a triarylmethane dye makes this cationic species planar, because one of the aryl groups moves away from the trigonal unit; this is accompanied by a strong bathochromic shift of the absorption band to the visible region of the spectrum.In addition to the decrease in the steric strain between the ortho- hydrogen atoms in the aryl groups, it becomes possible to influence the electron density distribution over the whole cation by varying the nature of substituent R.Degradation of the molecular ions of allene hydrocarbons has been proposed to give the propargyl cation (HC:C¡ÀCH2 +).7 The most intense peaks (about 100%) in the mass spectrum of iodoallene 118 are the peaks of fragment ions with m/z 107, 91, 79 to which structures of the ethynylcarbenium ions 119, 122 and 120, respectively, were assigned.106 The use of various spectroscopic techniques for identification of alkynylcarbenium ions is discussed in Section V using partic- ular synthetic examples.IV. Quantum-chemical calculations The mechanism of the Meyer ¡À Schuster rearrangement has been studied theoretically using ab initio calculations for O-protonated 3-methylbut-1-yn-3-ol 124 and the product of its rearrangement, 3-methylbuta-1,2-dien-1-ol 125.Based on the results of these calculations, it was concluded that the transition state of the 124!125 rearrangement is a complex consisting of a carbocation and a water molecule acting as the migrating species.107 To rationalise the predominant 1,4-addition to penta-1,2,3- triene 98, which could not be explained based merely on the data of 1H and 13CNMRspectra of alkynyl carbocations (Section III), ab initio calculations of the charge distribution in three cations (126 ¡À 128) have been performed.93 The calculations showed that in these three cations, the charge is mostly concentrated on the propargyl sp2-hybridised centre. As the number of methyl groups in the cations 126 ¡À 128 increases, nucleophilic attack at this position becomes more favourable.93 Since the stability of alkynyl carbocations is insufficient for them to be directly observed by spectroscopic methods (Section III), it is difficult to follow the variation of the stability of these species over a wide range of structures starting from the simplest one and to estimate the contributions of the resonance structures 9 and 10 as functions of the degree of substitution.Valuable information on the geometrical parameters of a series of alkynylcarbenium ions 129 and charge distribution in these species found by ab initio calculations has been reported by Dorado et al.108 For cations 129, the lengths of the Ca¡ÀCb, Cb¡ÀCg, Ca¡ÀX, Ca ¡ÀY and Cg ¡À Z bonds, bond angles and the relative energies have been calculated. In general, it was noted that the propargyl form 9 predominates in those cases where the substituent X, substituent Y or both of them either withdraw s-electrons from the Ca atom or donate p-electrons to the unsaturated fragment. Conversely, the same effects of the substituent Z increase the contribution of the allenyl structure 10.These effects are especially pronounced for fluorinated cations. The change in the ratio of theCa¡ÀCb andCb¡ÀCg bond lengths (and, correspondingly, in the bond orders) depending on the number and positions of substituents in cations 129 presents particular interest. Upon introduction of any substituent other than hydrogen (Me, F, NH2) to the Cg atom, the Ca¡ÀCb bond becomes shorter, while the Cb¡ÀCg bond becomes longer than the corresponding bonds in the cation 129 in which X=Y=Z=H.Conversely, accumulation of substituents at Ca causes the oppo- site tendency. The average Ca¡ÀCb and Cb¡ÀCg bond lengths for trisubstituted cations have been determined. For X=Y=Z=Me, these values are 1.381 and 1.204 A �º , and for X=Y=Z= F, they are 1.399 and 1.196 A �º , respectively.Thus, introduction of substituents into the Ca position makes the Ca¡ÀCb bond `more single' and the Cb¡ÀCg bond `more triple'. The values for charge distribution determined in the study cited 108 are consistent with the fact found by Olah et al. 100 that the positive charge on the Ca atom is approximately twice as large as that on the Cg atom.A detailed theoretical study of a series of alkynylcarbenium ions including the unsubstituted propargyl cation and methyl- and phenyl-substituted ions of the type 129 has been reported by Mayr et al.109 As in the previous publication,108 ab initio calculations for HClO4 NMe2 117 R C Me2N C + ClO¡¦4 C OH Me2N NMe2 C R R=H, Cl, NO2, MeO, NMe2. C C H I 118 C C H I + 7I7 7HI C CH + 119 7C2H4 C CH + 120 C CH + 121 7CH3 C CH + 122 C CH + 123 7H + C Me Me CH O H H 124 C Me Me H O H H + 125 126 127 128 + H2C C CMe + MeCH C CMe + Me2C C CMe + C Y X C Z a b g 129 X, Y, Z=H, Me, F, NH2.C 824 S MLukyanov, A V Koblik, L A Muradyanthe bond lengths, charge distributions and stabilisation energies for methyl- and phenyl-substituted allenyl cations were carried out.The bond lengths calculated by the two groups of research- ers 108, 109 and the conclusions based on them virtually coincide. Thus the Ca±Cb bond in the nonsubstituted propargyl cation (129, X=Y=Z=H) (1.360 A Ê ) is somewhat shorter than the length of the single C±C bond in propyne (1.484 A Ê ) but longer than the double C=C bond in allene (1.288 A Ê ); at the same time, the Cb±Cg bond in this cation (1.215 A Ê ) is longer than the triple bond in propyne (1.170 A Ê ) but shorter than the double bond in allene.Phenyl substituents at the tertiary Ca atom elongate the Ca±Cb bond and simultaneously shorten the Cb±Cg bond more substantially than methyl substituents. The calculations performed 109 made it possible to predict the reactivity of the alkynyl carbocations 11 with respect to nucleo- philic reagents.The propargyl structure 9 is responsible for the addition to Ca and the allenyl form 10 provides the attack at Cg. Hard nucleophiles add predominantly to the Ca position, because the positive charge on this atom is always greater than that on the Cg atom; therefore, under conditions of kinetic control, propargyl derivatives 130 are mostly formed.The formation of allenyl derivatives 131 can be explained by assuming thermodynamic control of the addition. Soft p-nucleophiles should also add predominantly to the Ca atom, because, according to the results of calculations, this site is characterised by the highest LUMO coefficients.109 However, as substituents R1±R3 stabilising the cations 11 are introduced in the molecule, the addition to the Cb±Cg moiety becomes more and more pronounced, because these substituents stabilise intermediate 132 but not 133.The formation of the allylic cations 132 can be demonstrated by reactions of alkynylcarbe- nium ions with alkenes and dienes (the Diels ± Alder reaction). V. Synthetic applications of alkynyl carbocations The first systematic studies on the possibility of practical applica- tion of alkynylcarbenium ions as reactive intermediates for build- ing various cyclic and polyfunctional systems were carried out in the late 1970s and the early 1980s by a group of German scientists headed by Mayr.110 Alkenes and dienes of various structures were used as nucleophilic substrates. 1. Reactions of alkynyl carbocations with unsaturated compounds The alkynylcarbenium (allenyl) cations 11 are ambident electro- philic species.Five variants of the addition of unsaturated substrates (alkenes 134 and 1,3-dienes 135 both linear and cyclic) can be conceived theoretically. Figure 1 shows the structures of cationic adducts A±Q arising upon the reaction of the ions 11 with unsaturated substrates. The carbon atoms of the substrates 11, 134 and 135 participating in the formation of new covalent bonds are designated by letters.The alkenes 134 and the 1,3-dienes 135 are considered to be symmetrical, i.e. positions C(1) and C(2) in the substrate 134 and positions C(1) and C(4), C(2) and C(3) in the substrate 135 are equivalent. The addition of alkynyl carbocations of the type 11 to the alkenes 134 and the dienes 135 can yield linear carbenium ions (A, C) or allylic (B, E, O, P, Q) or vinylic (D, F, G, H, I ±N) cations.The reactions of alkynyl carbocations with cyclopentadiene (CPD) have been studied most comprehensively.110 ± 116 The ions were generated by treatment of the corresponding halides with silver trifluoroacetate in pentane in the presence of CPD.110 After hydrolysis of the reaction mixture, obtained from 3-bromopro- pyne 20a, with aqueous ammonia, 4-(prop-2-ynyl)cyclopent-2-en- 1-ol 138a was isolated as the major product containing a slight amount of bicyclic ketone 139a.Under the same conditions, the transformation of 3-chloro-3-methylbut-1-yne 20b gave three products 138b, 139b and 140 in nearly equal amounts.110, 113 C R2 R1 CR3 + 11 X7 X7 C C C R2 R1 C X R3 131 C C R2 R1 CR3 X 130 C R2 R1 CR3 + 11 + C 133 CR3 R1 R2 C C + + CR3 R1 R2 C C 132 CF3COOAg C R R X CH 20a,b 7AgX C R R CH + CF3COO7 9o,p + + C C C a b g 11 Ca(a), Cg(b), Ca+Cb(c), Ca+Cg(d), Cb+Cg(e).C1(m), C1+C2(n). C C 1 2 134 1 2 3 4 135 C1(x), C1+C2(y), C1+C4(z). + A (11a+134m) C C B (11a+135x) C + C (11b+134m) + + C C D (11b+134n) C + C E (11b+135x) + + C F (11b+135y) G (11b+135z) H (11c+134n) C C + C I (11c+135y) + J (11c+135z) + K (11d+134n) + L (11d+135y) C or + + + M(11d+135y) N (11d+135z) O (11e+134n) P (11e+135y) + Q (11e+135z) Figure 1.Variants of cycloaddition of alkynylcarbenium ions to alkenes and dienes. The letters designate the fragments of molecules formed upon the reaction. Alkynylcarbenium ions and related unsaturated cations 825Since cyclopentadiene is highly sensitive to acidic reagents, in more recent studies,111, 112 the researchers employed a milder catalyst, namely, the efficient homogeneous catalytic system ZnCl2±Et2O±CH2Cl2, which is formed upon dissolution of zinc chloride (1 mole) in diethyl ether (1.5 moles) and dilution of the resulting solution with dichloromethane.113 It has been noted 110 that the reaction route depends on the structure of the alkynylcarbenium ion.The reaction conditions are also fairly significant. In the presence of an ethereal solution of ZnCl2, the halides 20b,c mentioned above do not react with CPD but instead, they are converted into allenyl halides.113 Trisubstituted haloacetylenes 141 react with CPD to give products of [4+2]- and [4+3]-cycloaddition 147 and 148, respec- tively.111, 113 In some cases, side products, enynes 149 and 150, have been isolated from the reaction mixtures.Apparently, these com- pounds result from dehydrochlorination of the initial halo- acetylenes 141. Another side product, compound 152, forms upon the addition of two CPD molecules to dimethyl-(phenyl- ethynyl)carbenium ion 9k.112, 113 It is assumed that [2+2]-cyclo- addition of the ethynyl fragment of the ion 9k to one of the C=C bonds in CPD giving allylic cation 151 also occurs to a small extent.This cation is sufficiently stable, so that it has time to add one more CPD molecule. The subsequent rearrangements of the cations give the compound 152. The reaction of 3-chloro-1,3-diphenylpropyne 153 with CPD gives three isomeric products of [4+2]-cycloaddition 154 ± 156 in a total yield of 28%.113 The structures of the products 147, 148 and 154 ± 156 formed in these transformations indicate that the addition of CPD to alkynyl carbocations having geminal substituents (or a phenyl group at the secondary carbenium centre) involves the sp2 carbon atom (Ca) and one of the sp atoms (Cb or Cg).This can give only the vinylic cations 143 ± 146 (Fig. 1, ions I, J, L, N). Allylic cations of the types P and Q (Fig. 1), which are energetically more favourable, are not formed. Of the four possible routes of trans- formation of the cations 142, only two are realised, namely those in which the ions 145 and 146 are formed intermediately. This is due to the fact that, according to calculations,113 the bicyclic vinylic cations 143 and 144 are characterised by much higher energies.It is noteworthy that the alternative pathway to the vinylic halides 147 consisting in the isomerisation of the initial haloalkynes 141 to haloallenes followed by their reaction with CPD has been rejected,113 because it was found that 1-halo-3- methylbuta-1,2-dienes do not react with CPD under these con- ditions.R =H(9o, 20a), Me(9p, 20b); X=Cl, Br. OH CH R R C 136a,b + CF3COO7 + R R CF3COO7 137a,b + H2O 20 H2O + Cl Me Me 139a,b 140 O R R OH CH R R C 138a,b X=Cl (20b), Br (20c). C Me Me X CH 1. ZnCl2 2. NH4Cl C C C Me Me H X 20b,c C R1 R1 X C R2 ZnCl2 C R1 R1 C R2 + 141 R1 R1 C CR2 b g 1 2 3 4 + 142 R1 R1 R2 + Cb+C(2) Cg+C(2) 143 R2 R1 R1 + 144 C R1=Me, Et, (CH2)4; R2=Me, Et, Ph, 4-MeOC6H4, 4-BrC6H4; X=Cl, Br.Cb+C(4) Cg+C(4) R2=Ar [4+2] R2=Alk [4+3] R1 R1 C+ R2 R1 R1 X7 R2 X 145 147 (20% ± 65%) 146 X7 R1 R1 R2 X R1 R1 R2 + 148 (28% ± 36%) 142 + 141 ZnCl2 7HCl R1=Et, R2=Ph R1=7(CH2)47, R2=Ph Ph C C C CHMe Et 149 (4%) Ph C C 150 (7%) 141 ZnCl2 R1=Me, R2=Ph C C Ph Me Me [2+2] + + Ph Me Me Ph Me Me [3+4] Cl7 151 Cl H Ph Me Me 152 (6%) 9k C H Ph Cl C Ph ZnCl2 C H Ph C Ph + Ph Cl Ph H Ph Cl H Ph Cl Ph Ph H + + 154 (12%) 155 (8%) 156 (8%) 153 826 S MLukyanov, A V Koblik, L A MuradyanAs has already been noted (Section III), not only functional acetylene derivatives but also the corresponding allene derivatives can serve as the precursors of the alkynyl carbocations 11.In fact, the reaction of cyclopentadiene with 1-bromo-3-methylbuta-1,2- diene in the presence of silver trifluoroacetate affords a mixture of polymers from which allenylcyclopentenols 158 and 159 (ratio 83 : 17, total yield*15%) and traces of the compounds 138b and 161 were isolated.113 Thus, 3-chloro-3-methylbut-1-yne 20 and isomeric bromoal- lene react with CPD under the same conditions to give absolutely different products.Although in the latter case, the yields of the reaction products are too low for reliable conclusions to be made, it should be noted that the CPD molecule adds to that carbon atom in the alkenylcarbenium ion precursor from which the halogen atom is eliminated. In all probability, this situation is due to the very low stability of ethynyldimethyl carbocations of the type 9 with a terminal triple bond.Alkynyl carbocations with three aryl substituents are much more stable (see also Sections III and IV). The attempts to generate triphenylethynyl carbocations by treatment of chlorotri- phenylallene 162 with the ZnCl2±Et2O±CH2Cl2 catalytic system resulted in complex product mixtures of an unknown composi- tion. Therefore, silver trifluoroacetate was used, which ensured that transformations of the cationic intermediates were irrever- sible, because the cations were bound by the trifluoroacetate anions.114, 115 Treatment of a mixture of chlorotriphenylallene 162, CPD and CF3COOAg in pentane with an alcoholic solution of KOH gave a mixture of the compounds 26, 164, 166, 167, 172 and 173 in a total yield of 95%.However, it can be seen from the above scheme that only 9% of the alkynyl carbocations 9i are bound by trifluoroacetate anions yielding the a,b-unsaturated ketone 26 as the final product. The prevailing process is addition of CPD to the cation 9i. It was shown in special experiments 114, 115 that the tertiary alcohol 172 rearranges to the secondary alcohol 173 on treatment with toluene-p-sulfonic acid.Nevertheless, according to 1H NMR spectroscopy, dissolution of allenylcyclopentenol 164 and the tertiary alcohol 167 in a mixture of FSO3H and liquid SO2 at 770 8C affords only the bicyclic vinylic cation 166. Thus, the allylic cations 170 are intermediates on the pathway to the alcohols 172 and 173, while the cations 163 react with CPD to give the ions 166.These results led the investigators to the conclusion that the [2+2]-cycloadditions 9i!163, 9i!166 are stepwise processes, whereas the [4+2]-cycloaddition 9i!170 is a concerted process. The addition of CPD to the terminal sp-hybridised carbon atom in the cation 9i might be a result of the structure of the initial chloroallene 162. However, when phenyl-substituted alkynol 174 was made to react with CPDin the presence of fluorosulfonic acid, compounds 167 and 168 were obtained as the major products after hydrolysis of the reaction mixture (the ratio 167 : 168 : 26 : 175 = 9 : 4 : 4 : 3, total yield*35%).114, 116 The tricyclic ketone 175 is formed in a high yield on treatment of the alcohols 172 and 173 or specially synthesised compound 176 with acids.A mechanism for this process was proposed, which includes electrocyclic ring closure in the cationic intermediate 170 and acid-catalysed rearrangements of compound 177.114, 116 + C5H6, CF3COOAg C Me Me H Br C Me Me H 157 H2O C Me Me H HO 158 + C Me Me H OH 159 H Me Me 157 [4+2] 160 + H Me Me OH H2O 161 + 162 CF3COOAg n-C5H12 Ph C Ph Ph C 9i C Ph Ph Ph Cl + C Ph Ph Ph 163 + OH7 C Ph Ph Ph OH 164 (23%) C Ph Ph Ph HO 165 (15%) + 163 [2+2] Ph Ph Ph 166 + Ph Ph Ph OH endo-167 (7%) OH Ph Ph Ph exo-168 (10%) H2O FSO3H 9i 172 OH Ph Ph Ph + 173 (29%) Ph Ph Ph HO CF3COO7 C Ph Ph OCOCF3 Ph 169 H2O CHCOPh Ph Ph 26 (9%) + 170 Ph Ph Ph 171 Ph Ph Ph H + H2O Ph2C C C Ph OH 174 1.C5H6, FSO3H, SO2 (liq) 2.OH7 167+168+26+ O Ph H Ph 175 172 Ph C(Ph2)OH 176 173 FSO2H 170 Ph Ph + Ph Ph + 7H+ Ph Ph H+ H2O Alkynylcarbenium ions and related unsaturated cations 827Treatment of chlorotriphenylallene 162 with SbCl5 affords dark-red crystals of hexachloroantimonate 178.Hydrolysis of this salt gives rise to a mixture of the alcohol 174 and the ketone 26 in 11 : 14 ratio. The reaction of the salt 178 with CPD (liquid SO2, 730 8C) and subsequent alkaline hydrolysis yields the products 167, 168 and 177 described above in a ratio of 4 : 5 : 1 (total yield *30%).116 Thus, the data presented here permit the conclusion that the triphenyl-substituted ethynyl carbocation 9i always adds a CPD molecule at the terminal sp-hybridised carbon atom, irrespective of the nature of the preceding compound.The cycloaddition involves only the Cb and Cg atoms, i.e.only allylic cations 166 and 170 (type P and Q cations, see Fig. 1) are generated. Unlike cyclopentadiene, which enters into both [2+4]- and [3+4]-cycloaddition reactions with dialkyl-substituted alkynyl carbocations,111, 113 cyclohexadiene and cycloheptadiene react only according to the [2+4]-cycloaddition pattern in the presence of ZnCl2, and cyclooctadiene gives only 1,4-addition products 181a,b (Scheme 1).117, 118 Scheme 1 This difference in the behaviour of alicyclic 1,3-dienes is due to different energies of both the corresponding cationic intermedi- ates and the final products, which result from different degrees of strain in the rings.The reaction of 4-chloro-4-methylbut-2-yne 182 with anthra- cene in the presence of ZnCl2 gives rise to the product resulting formally from [4+2]-cycloaddition to the isomeric chloroallene 183.107 The reaction of furan with the same cation yields acyclic product 184.107 The addition of linear 1,3-dienes 186a ± g and 1,2-dimethyle- necyclohexane 186h to the propynylcarbenium ions 11a ± c in the presence of the ZnCl2±Et2O±CH2Cl2 system occurs only at the sp2-hybridised carbon atom and gives linear allylic cations 187. Subsequent transformations of the cations 187 are largely deter- mined by the structures of both reactants.119, 120 R1=R2=R3=Me (185a, 11a); R1=R2=Me, R3=Ph (185b, 11b); R1=R3=Ph;R2=H(185c, 11c);R4=R5=R6=R7 (186a);R4=Me, R5=R6=R7=H (186b); R4=R5=R6=H, R7=Me (186c); R4=R5=Me, R6=R7=H (186d); R4=R5=H, R6=R7=Me (186e); R4=R7=Me, R5=R6=H (186f); R4=R6=R7=Me, R5=H(186g); R47R5=(CH2)4, R6=R7=H(186h). Ph Ph H HO H+ 177 175 + 162 SbCl5 C5H12 C C Ph Ph C Ph SbCl6 7 1.C5H6; 2. HO7 167+168+177 178 H2O 174+26 + C Me Me Cl C R Me Me C R 179 9k,l C (CH2)n Me Me Cl R 180a-d (CH2)n ZnCl2 C Me Me C R Cl 181a,b ZnCl2 7HCl C Me Me C R C Me Me Cl C Me ZnCl2 9l 182 9l+ O O H C Me Me C Me + 1,3-H H H Me Me Me Cl 183 (8%) + O C CH C Me Me Me Cl7 O C CH C Me Me Me Cl 184 (30%) R3 C C C CH2 R2 R1 C C C R7 R6 R5 R4 187 + C R2 R1 Cl C R3 ZnCl2 C R2 R1 C R3 + R4 R5 R7 R6 186a ± h 185a ± c 11a ± c R3 C C R1 R2 R4 R5 R6 R7 Cl 188 Cl7 Cl7 R3 C C R1 R2 R4 R5 R6 R7 Cl 189 187 R4 R1 R2 C+ R7 R6 R5 R3 Cl7 R4 R1 R2 R7 R6 R5 Cl R3 190 191 C Me Me Cl C Ph 185b + Me Me H C Me Me H + C Ph ZnCl2 186e 1. 186e; 2. Cl7 Cl7 Me Me Me Me Ph Me Me Cl H 192 Me Me Cl C Me Me C Ph 193 Compound R n E/Z Yield (%) 180a Me 2 1 : 1 22 180b Ph 2 only E 54 180c Me 3 1 : 1 22 180d Ph 3 only E 50 181a Me 4 7 10 181b Ph 4 7 29 828 S MLukyanov, A V Koblik, L A MuradyanMost often, allyl chlorides 188, which are thermodynamically more stable than their isomers 189, are formed as the major products of these reactions. Upon the reactions of the chlorides 185a,b with buta-1,3-diene 186a and the reaction of chloride 185c with penta-1,3-diene 186c, isomeric allyl chlorides 189 were also isolated (the 188 : 189 ratios were 4 : 1, 1 : 1 or 2 : 1, respectively). Cyclic vinyl chlorides 191 were the only products formed upon the reaction of the chloride 185b with 1,2-dimethylenecyclohexane 186h and the reaction of 185c with the diene 186f.It should be noted that the formation of six-membered cyclic vinyl halides 191 is of special interest, because the allyl-cation intermediates 187 are alkynylcarbenium ions in which the triple carbon ± carbon bond is separated from the carbenium centre by one methylene unit (see Section VI). It was shown 119, 120 that dimethylpropargyl chloride 185c does not add to buta-1,3-diene 186a or isoprene 186b.When the chloride 185a is made to react with the diene 186e or the chloride 185b reacts with the dienes 186e,f,g, the reaction mixtures poly- merise. Only the reaction of the chloride 185b with the diene 186e did give bicyclic diene 192 (yield 23%) which resulted from the addition of two molecules of the diene 186e to the cation 11b.Four cyclisation pathways can, in principle, be conceived for the alkynylallylic cations 187: those leading to four- (a), five- (b), six- (c) and seven-membered (d) vinylic cations. Obviously, pathways a and b are energetically unfavourable, because 194 contains a strained four-membered ring, and in 195, the geometry of the vinyl cationic fragment differs substantially from the linear geometry inherent to it.120 The latter statement is also true for the isomer 196. Only the cation 191 in which the carbenium centre is exocyclic and is stabilised by the neighbouring phenyl substituent is energetically favourable.This is especially clearly seen for the diene 186h, which reacts with the chloride 185b yielding the species 187 [R4, R5=(CH2)4] the geometry of which is rigidly fixed by the cyclohexane ring.Considerable synthetic and theoretical interest is aroused by rearrangements of alkynylcarbenium ions 197 in which the multi- ple bond and the carbenium centre are separated by one or more sp3-hybridised carbon atoms. p-Cyclisation of cations of the type 197, where n = 1 (homopropargyl rearrangement 22 ± 24, 26, 27, 121) and n=3 (see Ref. 22, for more details, see Section VI) is a well- known reaction. The reactions of alkynyl carbocations with dienes described above are examples of cyclisation of cations of the type 197 with n = 2; the reactions with alkenes also belong to this type.122 Chlorobenzylidenecyclobutanes 201 and chlorocyclopentenes 203 are formed upon [2+4]- or [3+4]-cycloaddition of the alkynyl carbocations 11a ± e to alkenes 198a ± c.122 A stepwise mechanism with participation of carbocationic intermediate 199 was proposed for this reaction.The formation of this intermediate was proved by the isolation of enynes having mainly structure 204 as side products. The two routes for cyclisa- tion of the intermediates 199 are determined by the stabilising influence of the phenyl substituent, which is attached directly to the vinylcarbenium centre in structure 200.When the molecule contains no phenyl substituent, the reaction affords product 202 having a less strained structure.122 The reaction of the triphenyl-substituted cation 9i with alkenes follows a fundamentally different pathway 123 leading to [2+2]-cycloaddition products. + + + R1 R2 C R4 R5 R7 R6 R3 194 C C R4 C R1 R2 R7 R6 R5 R3 1 2 b g 187 R1 R2 R3 R4 R5 R7 R6 195 a b c d + R1 R2 R4 R5 R6 R7 R3 196 (a) Cb+C1; (b) Cg+C1; (c) Cb+C2; (d) Cg+C2.R1 R2 R6 R7 R5 R4 C+ R3 191 + C n C C R C 197 + C n C C R + C C C R C n R4=R5=H(198a); R4=H,R5=Me (198b); R4=R5=Me (198c). 185a-c ZnCl27Et2O CH2Cl2 11a7c Me Me R5 R4 198a-c C R5 R4 R1 R2 Me Me C R3 199 R3=Ph R4=R5=H Ph C C C Me Me CH2 Me 204 (20%) + + R3=Me Me Me Me R4 R2 R1 R5 202 203 (26% ± 38%) Me Me Me R4 R2 R1 R5 Cl Cl7 + R3=Ph Cl7 R1 R2 R4 R5Me Me C Ph 200 201 (53% ± 62%) R1 R2 R4 R5Me Me Ph Cl Ph R5 R4 R7 R6 Ph Ph 206 + R4=H 7H+ Ph R4 R7 R6 Ph Ph 207a,b,d,f (41% ± 84%) 174 C CPh + 9i R6 R7 R5 R4 198a7f C Ph Ph Ph R5 R4 R7 R6 + 205 Ph Ph FSO3H CH2Cl2 R7 R6 R5 Ph Ph R4 + 206 R6 R5 Ph Ph R4 R7 7H+ + R6 R5 Ph Ph R4 R7 208b (72%), c, e (78% ± 86%) R4=R5=H, R6=R7=Me (a); R4=H, R5=R6=R7=Me (b); R4=R5=R6=R7=Me (c); R4=H, R5=R6=Me, R7=H (d); R4=R5=H, R6=Ph, R7=Me (e); R6=Me, R47R7=(CH2)4 (f).Alkynylcarbenium ions and related unsaturated cations 829The alkenes 198a,b,d,f are converted into methylenecyclobu- tenes 207a,b,d,f, while in the case of tetramethylethylene 198c, a-methylstyrene 198e and trimethylethylene 198b (with excess FSO3H), fast electrocyclic opening of the four-membered ring occurs, resulting in the formation of bicyclic structures 208b,c,e.When the alcohol 174 reacts with vinyl ether 209, some of the arising ethoxycarbenium ions 210 undergo dealkylation, typical of this type of species,124 to give aldehyde 211. However, most of the ions 210 have time to cyclise yielding a new ethoxycarbenium ion, which is converted into aldehyde 212.Turning back to Fig. 1, which shows all the possible variants of cationic intermediates, one can conclude that the studies described above have provided experimental evidence for the formation of eleven of these ions, namely, A (199), B (136, 142, 187), C (205, 210), E (157, 163), H (200), J (145, 191), K (202), N (137, 146), O (206), P (166) and Q (160, 170). Cyclopropylvinyl cations (D, F) have been reported 22, 23, 97 to be quite stable. As regards structures (G, I, L and M), they are energetically so unfavourable that their formation is considered 113 to be doubtful irrespective of the structure of the initial compounds and reaction conditions.It is obvious that the increase in the stability of the ion 11 upon introduction of two aryl substituents to the sp2-carbe- nium centre increases the contribution of the alkynylcarbenium structure 9.This is consistent with the data obtained by spectro- scopy and quantum-chemical calculations (see Sections III, IV), which indicate that the Ca±Cb bond in the cations 129 is elongated, while the Cb±Cg bond is shortened.The attack on these cations by nucleophilic agents is mainly directed at the sp- carbon atom remote from the cationic centre, as in acetylene derivatives activated by electron-withdrawing substituents, while cycloaddition follows the [2+2] pattern involving the two sp- carbon atoms. Conversely, the introduction of substituents, able to delocalise positive charge, into the acetylenic fragments results in the predominance of the allenyl form of the cation 10 and causes the nucleophile to attack the sp2-carbon atom.It has been shown above for phenylethynylcarbenium ions (9i ± k, n, 117, 132, 142) that a similar influence can be exerted by the vinyl group. Thus the reaction of halogenated enynes 213 with buta-1,3-dienes 186c,i in the presence of a homogeneous catalytic system, ZnHal2±Et2O± CH2Cl2 (Hal = Cl, Br), affords alkenylidenecyclohexenes.125 A valuable product formed in this reaction is the perfume d-dam- ascone 214.Similarly to allylic cations, alkynylcarbenium ions can act as efficient dienophiles in low-temperature Diels ± Alder reac- tions.126 Thus the reaction of 1,1-diphenylprop-2-yn-1-ol 215 with 2,4-dimethylpenta-1,3-diene 186g gave tricyclic compound 217 in 70% yield.126 Presumably, this product is formed in two successive cyclisations via the ethynyl carbocation 9g and the cyclic allylic cation 216 (corresponding to structure Q, see Fig. 1). However, the reaction of the cation 9g with cyclohexadiene does not stop at the stage of formation of the 1 : 1 tricyclic adduct 218.This adduct undergoes one more [2+2]-cycloaddition to give alcohol 219 (the total yield after 20 s at 25 8C is 80%). The proposed reaction scheme was confirmed by isolation of the compound 218 (yield 62%) from this reaction carried out with a tenfold excess of cyclohexadiene and its subsequent transforma- tion into the final product 219 under the same conditions.126 Neither styrene nor the indene derivative 217 reacts with the cation 9g under the described conditions.The high reactivity of the compound 218 was explained 126 by the strained character of the double bond in the five-membered ring.126 Cyclisation of alkynylcarbenium ions 220 and 222 was used to prepare alkynyl derivatives of adamantane 221 127 and fused heterocycles 223.128 + 174+ Me Me OEt 209 FSO3H C Ph Ph Ph Me Me H EtO 210 C Ph Ph Ph Me Me H O 211 (22%) Me Me H EtO Ph Ph Ph + Ph Ph Ph Me Me H OEt + Ph Ph Ph Me Me H O 212 (47%) C R2 R1 X C CH CHR3 213 H2C CH CH CHR4 + 186c,i ZnHal2 CH2 R1 C R2 C CH CHR3 CH CH CH(Hal)R4 ZnHal2 R1, R2, R3, R4=Me, Et; Hal=Cl, Br.R2 R1 CHR3 Hal R4 H H2O R17R4=Me Me Me O Me H Me 214 C Ph Ph OH CH 215 CF3SO3H 25 8C, 20 s + C Ph Ph CH 9g TfO7 Ph Me Me Me H H + Ph Me Me Me + 216 186g Ph Me Me Me 217 (70%) 9g+ Ph + 7H+ Ph 218 9g + Ph Ph Ph H 7H+ H2O Ph Ph Ph H OH 219 CH2 C CMe + H2O 7H+ CH2 C CMe OH 221 C C C H+ C C CH2 C Me + 220 CH2 CH2 830 S MLukyanov, A V Koblik, L A MuradyanThe participation of the sp2-hybridised carbenium atom in the cyclisation is due to the fact that the greatest charge is concen- trated on this atom and also to the geometry of the intermediate 222, which hampers the attack of the sp-hybridised carbon atom.Heteroatoms (O, S, N) adjacent to a carbenium centre are known to exert a stabilising influence on carbocations.124 In the presence of acids (HCOOH, CF3COOH), aldehydes of the acety- lene series 224 are converted into alkynylhydroxycarbenium ions 225, which can react with cyclic dienes according to three path- ways: (1) the Diels ± Alder reaction giving cycloaddition product 227, (2) nucleophilic addition of the C:C bond to the diene molecule giving compounds of the type 228 and (3) the Michael condensation of two furan molecules with one aldehyde mole- cule 129, 130 affording alkynes 229.It has been shown 129, 130 that cyclopentadiene and cyclohexa- 1,3-diene react with the aldehydes 224a,c to give only cyclo- addition products 227a,c in high yields.The reactions of anthra- cene and diphenylfulvene with the same aldehydes 224a,c give rise to mixtures of compounds 227 and 228 in which the Diels ± Alder reaction products predominate. The third pathway, which is similar to the known reaction of aldehydes with dimedone,131 occurs only for the aldehydes 224d,e,g ± i.The products 229h,i were obtained in only moderate yields (20% and 55%, respec- tively). The route of the reaction of the aldehydes 224 with furan depends on the nature of the acid used. For example, the aldehydes 224d,e react with furan in the presence of HCOOH giving rise to mixtures of the compounds 227d,e and 228d,e.In the presence of CF3COOH, no cycloaddition products 227 are formed; instead, mixtures of compounds 228 and 229 are pro- duced. This difference in the reaction pathways is due to the fact that in trifluoroacetic acid, which is stronger than formic acid (pKa 0.23 for CF3COOH and pKa 3.75 for HCOOH132), acylox- ycarbenium ions 230 can be formed, which are less stable than the hydroxycarbenium ions 225.32 As a result, the diene adds to the sp2-hybridised atom of the alkynyl carbocation. The addition of acetylenic esters 231 to alkenes is catalysed by Lewis acids and occurs either as nucleophilic addition at the triple bond or as [2+2]-cycloaddition.133, 134 The reaction presumably involves the intermediate formation of complexes 232, which can be regarded as stabilised 124 dialkoxycarbenium ions.True ethynyldialkoxycarbenium ions 234 generated, for example, from orthoester 233 proved, as expected, to be extremely reactive dienophiles.135 They add to dienes at low temperatures (0 to778 8C) to give cyclic products 235 or 236 in good yields. These examples demonstrate successful application of alkyl propiolates in Diels ± Alder reactions.135 Ionisation of acetals of the acetylene series having a phenyl- selenyl substituent (237) affords alkoxycarbenium cations 238, which add silyl ethers of enols 239a,b at the carboxonium frag- ment.136 The alkylation of the acetals 237 with organoaluminium or organozinc compounds occurs in a similar way giving rise to ethoxyalkylacetylenes 241 or 242.X=O, S, NMe, NPh. R C Ar Ph Cl X R ZnCl2 or SnCl2 + R C Ar CPh X R 222 223 X R C Ar C Ph X C C C H OH H R + 226 7H+ X R CHO 227a,c R C C CHO H+ 224a,c R C C C H + 225 X OH O R CHO 227d,e + O C R CHCHO 228d,e 224d,e O HCOOH 224d,e O CF3COOH R C C C H OH O H + 7H+ R C C C H OH O R C C H O + C O 7H+ R C C CH 229 H+ 7H2O O 2 224d ± i R C C OCOCF3 OH CH CF3COOH O 7CF3COOH R C C OCOCF3 H + 229 (10% ± 54%) 230 R=CHO (a), (EtO)2CH (b), COOEt (c), CONMe2 (d), CN (e), H (f), Bu (g), Ph (h, 54%), 4-ClC6H4 (i, 20%).R=H, Cl, COOMe. AlCl3 R C C C O OMe 231 R C C C O OMe AlCl3 232 7 + R OMe O + R COOMe HC C C(OEt)3 CF3SO3Si(Me)3 233 HC C C OEt OEt + TfO7 234 R1 H2C R2 R4 R3 (CH2)n COOEt R1 R2 R3 R4 COOEt 235 (57%, n=1; 78%, n=2) 236 (37% ± 73%) (CH2)n + R17R2=(CH2)4, R3=OSiMe3 (a); R1=Ph, R2=H, R3=OSiMe3 (b).PhSe C C CH(OEt)2 BF3 . Et2O PhSe C C CH OEt R3 R1 R2 239a,b 237 238 PhSe C C CH CH C R1 O R2 OEt 240a,b (42% ± 61%) Alkynylcarbenium ions and related unsaturated cations 831A series of studies devoted to reactions of acetals of acetylenic aldehydes with CH acids in the presence of boron trifluoride etherate deserve attention.137 ± 140 Acetone, methyl ethyl ketone, 137 ethyl acetoacetate, methyl acetoacetate,138, 140 acetyl- acetone and cyclohexane-1,3-dione 139 have been used as sub- strates in the reactions with alkynal acetals 244 and butynedial tetraethyl acetal.140 Compound R1 R2 R3 Yield (%) 245a H H H 71 245b H Me H 26 245c H CO2 Et H 75 245d H CO2 Et Ph 75 245e H COMe H 37 245f Me CO2Me H 18 245g Me CO2Me Ph 18 The researchers cited claim 137 ± 140 that in these reactions, acetals are not converted into alkoxycarbenium ions like 238 and propose at least six hypothetical schemes with four- and six-centre cyclic transition states.However, none of the schemes consider the role of the catalyst, boron trifluoride etherate. In addition, in our opinion, some of the observations made by the authors rather attest to the participation of alkoxycarbocations.Thus alkynal diethyl acetals are converted into ethoxycyclohexenone 246, and dimethyl acetals give the corresponding methoxy derivative. When a mixture of acetals is used, a mixture of methoxy- and ethoxy-cyclohexenones is formed. The idea of activating acetylene derivatives by cationic sub- stituents attached directly to the triple bond has found synthetic implementation in cycloaddition reactions involving iminium derivatives of the acetylene series.Viehe et al.34, 141 have developed a method for the synthesis and studied the properties of ethynyl- substituted amidium salts 249 unknown previously. These salts proved to be substantially more efficient dienophiles than other acetylene derivatives activated by electron-withdrawing substitu- ents.The salts 249 were prepared by O-alkylation of the amides 248 synthesised using the Viehe salts (phosgene iminium chlor- ides) 247.34 The amidium salts 249 remain unchanged in air and are much more stable than their dialkoxycarbenium heteroanalogues 234. A systematic study of their 13C NMR spectra showed that the carbon ± carbon triple bond is substantially polarised; this decreases the energy demands and considerably facilitates the Diels ± Alder and 1,3-dipolar cycloaddition reactions.The salts 249a ± e readily react with cyclopentadiene at room temperature according to the [4+2]-cycloaddition pattern to give stable salts 250. The reaction of the ethynyl-substituted salts 249c,d with tetraphenylcyclopentadienone under the same con- ditions gives rise to polysubstituted benzene derivatives resulting from aromatisation of the adduct 251 formed initially.34 The ethynylcarbenium salts 249d,e readily enter into 1,3- dipolar cycloaddition at room temperature.Thus the reaction with ethyl diazoacetate yields pyrazole derivatives 252, and the addition of the salt 249d to the azomethine 1,3-dipolar fragment of munchnone 253 opens up the way to imidates of the pyrrole series.34 PhSe C C CH OEt Et 237 Et3Al or Et2Zn C6H13C C AlEt2 241 (76% ± 81%) PhSe C C CH OEt C C H13C6 +241(25%) 242 (20%) MeCOCHR1R2+R3C CCH(OEt)2 243 244 BF3 MeCOC CH C CR3 R1 R2 OEt 245a7g 244+ O OH BF3 R3=H, Ph.OH HO H R3 O O + O O O H R3 + O OEt 246 CH C C CH EtO EtO OEt OEt +2MeCOCH2COOEt EtOOC OEt H H Me O C C COOEt EtO H H Me O O O COOEt Me EtOOC Me R2 R1COMe+ N Cl Cl R3 R2 + 247 Cl7 Cl R1 N Cl R3 + Cl7 NaHCO3 H2O Cl R1 N O R3 R2 MeO7 MeOH R1 C C C O N R3 R2 248 Et3O+BF¡4 CH2Cl2 R1 C C C OEt N R3 R2 249a7e + BF¡4 R1=Ph, R2=R3=Me (a); R1=But, R2=R3=Me (b); R1=H: R2=R3=Me (c); R2=H, R3=Me (d); R2=R3=H (e).HC C C N OEt R Me + BF¡4 + CH2Cl2, 20 8C, 24 h Ph Ph Ph Ph O R= H, Me. 251 (83%) Ph H Ph Ph Ph OEt N R Me + BF¡4 R1=H, But, Ph; R2, R3=H, Me. 249a7e CH2Cl2, 25 8C R1 OEt NR2R3 + BF¡4 250 R=H, Me. 249d,e+N2CH2COOEt N N H O EtO NHR EtO + BF¡4 252 249d+ O Me Ph O7 Ph + 253 7CO2 N Me Ph N EtO H Me Ph + BF4 7 Na2CO3 H2O N 832 S MLukyanov, A V Koblik, L A MuradyanAmong the alkynes containing resonance-stabilised cationic substituents, the best known are those containing a propynimi- nium fragment of the general formula R1=H, Alk, Ar; R2=H, Alk, Ar, NR3R4, OR5, SR5, Hal; R3, R4=H, Alk, (CH2)2O(CH2)2.These are propynyliminium (R2=H, Alk, Ar), propynylami- dinium (R2=NR3R4) and propynylamidium (249) ions.142 The propynyliminium ions 256 can be prepared by treatment of easily accessible enaminones 254 with trifluoromethanesulfonic anhy- dride.142 Elimination of TfOH from the iminium triflate 255 is carried out either thermally or in the presence of a tertiary amine. This procedure is suitable only for the aroyl-substituted enamines, because enamines containing an aliphatic substituent in the acyl fragment are deprotonated to give diene amines of the type R1C(NR22 )=CH¡À C(OTf)=CR32 .The IR spectra of the salts 256 exhibit intense absorption bands in the region of 2190 ¡À 2200 (C:C) and 1590 ¡À 1630 (C=N) cm71.The 13C NMR spectra of these triflates have been studied.142 Propynyliminium salts similar to 256 react with enaminones 257 according to the [2+2]-cycloaddition pattern and with diphenylisobenzofuran and anthracene according to the [2+4]- cycloaddition scheme.142 The propynylamidium salts 249 form no cycloaddition prod- ucts with anthracene.34 In turn, the propynyliminium salts 256 do not react with tetraphenylcyclopentadienone.142 2.Reactions of alkynyl carbocations with nucleophiles The propynylamidium cation arising upon O-alkylation of the amide 259 has been used to prepare 2-(2-chlorophenylethynyl)- benzimidazole 260 and 2-amino-3H-1,5-benzodiazepine 261.143 Treatment of 1,3-dichlorotrimethinecyanine 262 with ali- phatic amines gives ynamine amidines 264 via unstable chlorimi- nium cations 263.144 Methods for generation and properties of another type of alkynyl carbocationsDalkynylvinyl cations 267Dwere simulta- neously described by Japanese 145 and German 146 scientists, working independently.The former researchers 145 studied sol- N Me Ph NMe EtO Ph C C C R2 NR3R4 R1 + R1 = H (a), Me (b), Ph (c); R2=Me, (CH2)2O(CH2)2; Tf =CF3SO3.PhCOCH C R1 NR22 254 Tf2O PhC CH C R1 OTf +NR22 TfO7 255 MeCN 7TfOH PhC C C R1 NR22 + 256a7c TfO7 + + O N Ph C C C Ph 256c TfO7 + R1COCH C Me NR22 257 CH2Cl2 20 8C + + O Ph N NR22 H Me R1CO Ph TfO7 O R1 H Ph Ph N Me R22 N O TfO7 OH7 O N Ph R22 N Ph R1 TfO7 7OH Ph O R22 N Ph R1 R = H (a), Me (b), Ph (c). 256a7c+ COR Ph 258a7c 1. MeCN, 807110 8C 2. K2CO3 256b,c+ O Ph Ph 68% ¡À 74% R1=Me EtPri 2N R1=Ph NaOH Ph O Ph O Ph Ph O CH2 N Ph Ph Ph O R1 N Ph O Ph Ph + TfO7 C CCONH2 Cl 259 C C Cl N N H 260 C C Cl C OEt NH 259 K2CO3, H2O CH2Cl2 C C Cl C NH NH HCl, EtOH (51%) NH2 NH2 C C Cl C OEt NH2 + BF¡¦4 Et3O+BF¡¦4 NH2 NH2 C6H4NH2-o N N H H NH2 Cl7 + KOH N N NH2 261 C6H4Cl-o C6H4Cl-o N C C C N Cl H Cl Me Me Me Me + Cl7 RNH2 Et3N 262 + Cl7 N C C C N Cl Me Me Me Me 263 RNH2 264 N C C C Me Me NR NMe2 Alkynylcarbenium ions and related unsaturated cations 833volysis of 2-bromo enyne 265 and 1-halobuta-1,2,3-triene 266 in aqueous ethanol.Ethoxycumulene 268 was detected in the reac- tion mixtures. Unlike the well-known ethynylvinyl derivatives with terminal functional groups,147 the isomeric 1-alkynylvinyl compounds of the type 269 have been little studied.Hanack et al. 146 proposed a method for the synthesis of alkynyl triflates 269, which can serve as the precursors of a-alkynylvinyl cations of the general form 270. The method consists in treatment of ketones of the acetylene series 271a ± c with trifluoromethanesulfonic anhydride in the presence of a non- nucleophilic sterically hindered base, 2,6-di-tert-butyl-4-methyl- pyridine 272.A somewhat different approach, based on silylated ethynyl ketones, has been proposed by Stang et al.148 These compounds are converted into ethynylvinyl triflates 269a and 273a,b upon successive treatment with trifluoromethanesulfonic anhydride in the presence of the base (272) and with potassium fluoride in methanol.The reaction mixtures resulting from solvolysis of the triflate 269b in 80% aqueous ethanol and 80% trifluoroethanol have been found to contain products 274 ± 278 the formation of which has been explained by the participation of the cationic intermediate 270b.146 Alcohol Yelds of solvolysis products (%) 274 275 276 277 278 EtOH 30 15 4 3 0 CF3CH2OH 24 0 20 0 28 The composition of the products of solvolysis depends on the alcohol used. These results imply that the cation 270b reacts mainly in the form of the alkynylvinyl cation.This is consistent with kinetic measurements. It was shown 146 that the propynyl- vinyl triflate 269b is solvolysed in 50% aqueous ethanol 70 times faster than the triflate MeC(OTf)=CH2 and 35 times faster than the triflate PrC(OTf)=CH2;146 this points to a stabilising influ- ence of the ethynyl fragment on the neighbouring vinyl cationic centre.The above synthesis of 1-(1-alkynyl)vinyl triflates 269 from ethynyl ketones 271 146 involves intermediate formation of alky- nylacyloxycarbenium cations 279, 281. The route of their depro- tonation to the final products 269 depends on the nature of the substituentsR1 andR2.149 It was shown that only in the case of the acetylenic ketones 271a ± c, does the reaction occur unambigu- ously to give the compounds 269a ± c.The alkynyl ketones 271d and 271e form complex mixtures of products. Thus the the ketone 271d gave the expected triflate 269d in 70%± 75% yield. In addition, the reaction mixture contained the isomeric triflate 280 (25% ± 30%, the ratio E :Z=1 : 1).Treatment of the ketone 271e with trifluoromethanesulfonic anhydride affords mainly the triflate 280 (85% ± 90%, the ratio E :Z=1 : 1) and only 10%± 15% of the triflate 269d. 267 267 Ph Ph C Br C 265 C6H4OMe-p C C Ph Ph Hal 266 (Hal=Cl, Br) + C C Ph Ph 7Br7 C C6H4OMe-p C6H4OMe-p C EtOH + C C C Ph Ph C C Ph Ph OEt 268 (40%) C6H4OMe-p C C6H4OMe-p + X=OR, SR, NR2.R1 C C C C X R3 R2 7X7 269 R1 C C C C R3 R2 R1 C C C C R3 R2 + 270 R = H (a), Me (b), But (c). R C C COCH3 271a ± c CH2Cl2, 25 8C, 5 ± 24 h R C C C CH2 OTf 269a7c (70% ± 80%) a (a) Tf2O, (272). N But But Me R2CH C O Cl + Me3Si C C SiMe3 AlCl3 CH2Cl2 CH2Cl2 R2CHCO Tf2O, 272 KF.2H2O MeOH C C CSiMe3 R2C OTf C C SiMe3 R = H (269a), Me (273a), Ph (273b).C C CH R2C OTf 269a, 273a,b 269b Me C C C CH2 OTf + + Me C C C CH2 Me C C C CH2 270b Me C C C CH 274 Me C C C CH2 OEt 7H+ EtOH H2O F3CCH3OH 275 Me C C COCH3 276 + MeCO CH C CH2 277 Me C C C CH2 OCH2CF3 278 PrCH2 C C COMe Tf2O 271d + 279 PrCH2 C C C Me OTf TfO7 + PrCH2 C C C Me OTf TfO7 7H+,7TfO7 7H+,7TfO7 PrCH2 C C C CH2 OTf Me C C C C TfO H Pr 269d + (E)-280 Me C C C C TfO Pr H (Z)-280 834 S MLukyanov, A V Koblik, L A MuradyanIt can be seen from the above scheme that in both cases, the major reaction products result from deprotonation of the inter- mediates in the alkynyl carbocation form 279, 281.The addition of anhydrous HCl to 3-chloro-3-methylbut-1- yne 282 affords 16 reaction products. The main products are (E)- 1,3-dichloro-3-methylbut-1-ene 285, 1,1,3-trichloro-3-methylbu- tane 286 and (Z)- and (E)-1,3-dichloro-2-methylbut-2-ene 284.The intermediate formation of dimethylethynyl carbocation 283 was postulated.150 In the presence of H3PO4, the alcohol 285 reacts with 2- and 1-naphthols to give products of O-alkylation (287, 289), aromatic electrophilic substitution (290) and cyclisation (288, 291) (Scheme 2).151 When the reaction mixture is kept in the presence of an acid, the compound 287 isomerises to 288, and the compound 289 isomerises to 291; apparently, isomerisation involves dissociation of the thermodynamically less favourable naphthol ethers under the action of acids.When triarylchloroallenes 292 are irradiated with a mercury lamp at 730 8C in CH2Cl2 solutions of alcohols, alkoxycumu- lenes 293 are formed.152 This reaction is believed to proceed via alkynylcarbenium ions.A ferrocenyl residue is known to stabilise efficiently a carbe- nium centre adjacent to it.153 The treatment of ferrocenyl(pheny- lethynyl)-methanol 294a or -ethanol 294b with acids resulted in the synthesis of stable phenylethynylcarbenium salts 295a,b. They react with nucleophiles at the sp2-hybridised carbon atom to give charged or neutral ferrocenyl derivatives of the type 296 ± 299.154 Bis-ferrocenyl cations in which the two ferrocene fragments are linked by one or two acetylene (or butadiyne) bridges have been studied by spectroscopy.155 3.Alkynyl-substituted heterocyclic cations The cationic fragment of the alkynyliminium cations of the type 256 described above can be a part of an aromatic heterocycle.These cations are the most stable due to the charge delocalisation with participation of the aromatic system. The protonation or alkylation of the nitrogen atom in alkynyl- substituted pyridines and quinolines proved to be an efficient method for activating the acetylene fragment towards reactions with various nucleophiles.Thus 2-phenylethynylquinoline 300 readily adds water in acid media.156 Me C C COCH2Pr Tf2O 271e + 281 Me C C C CH2Pr OTf TfO7 Me C C C CH2Pr OTf TfO7 + 7H+,7TfO7 7H+,7TfO7 269d (E)-280 + (Z)-280 + C Me Me Cl CH 7Cl7 C Me Me CH 282 283 + 283 7H+ H2C C C CH Me H+ H2C C C CH2 Me Cl7 H2C C C CH2 Me Cl ClCH2C CCl Me Me (Z,E)-284 HCl Me2C(Cl)CH CHCl 285 HCl Me2C(Cl)CH2CH2Cl2 286 283 Cl7 CH2Cl2 hn, ROH C C C Ar Ar Cl Ar 292 C C C Ar Ar OR Ar 293 R = H (a), Me (b); X=BF4, ClO4.Fc is ferrocenyl, Py is pyridine. + X7 HX Fc C C C Ph OH R 294a,b Fc C C C Ph R 295a,b H2O, Na2CO3 PPh3 2Et2NH Me2S Py 295a 294a + Fc CH C C Ph PPh3 BF¡4 296 Fc CH C C Ph NEt2 297 + Fc CH C C Ph SMe2 BF¡4 298 + Fc CH C C Ph N BF¡4 299 OH OCMe2 287 OCMe2 289 + C OH Me Me C CH CH2 H3PO4 C Me Me C CH CH2 285 286 OH C C CH CH2 C C CH CH2 + OH Me2C 290 C C CH CH2 + O Me Me CH CH CH2 288 + O Me Me CH CH CH2 291 Scheme 2 Alkynylcarbenium ions and related unsaturated cations 835The alkylation of the quinoline 300 with dialkyl sulfates or with esters of toluene-p-sulfonic acid yields quaternary salts 301, which are quite stable in alkaline solutions and are fairly reactive towards S- and N-nucleophiles and CH-acids.156 N-Alkylated salts of nitrogen-containing heterocycles readily add to phenylethynylquinolinium salts giving rise to cyanine dyes 302.156 Phenylethynylpyridinium salts 303 ± 305, which add nucleo- philes at the C:C bond, have been obtained by N-methylation of the corresponding ethynylpyridines.157 Yet another method for the synthesis of the alkynylquinoli- nium salt 301 is based on the interaction of N-methylquinolinium iodide with propynylmagnesium bromide. The subsequent oxida- tion in the presence of HClO4 affords the corresponding perchlo- rate.158 The IR spectrum of this salt contains an intense absorption band at about 2230 cm71 corresponding to the triple carbon ± carbon bond; this confirms the salt structure.As in the case of quinolinium salts with a phenylethynyl substituent, the addition to the propynyl group is also directed at the end of the triple bond that is more distant from the heterocycle. This gives enamine 306. Cyanine dyes containing quinaldine (307), 2-methylbenzo- thiazole (308), 2,3,3-trimethylindolenine (309) and N-methylrho- danine (310) residues are synthesised in a similar way.N C CPh 300 H2SO4 N C CPh + H HSO¡4 H2O N CH2COPh + H HSO¡4 H N Me Ph N H2C (CH2)n + X7 n=1,4; X=ClO¡4 , I7 NaSH CH2 (CH2)n N + 300 Me2SO4 N C CPh Me MeSO¡4 301 N CH Me C Ph S PhNH2 N Me Ph NHPh + MeSO¡4 MeCOCH2COR N Me Ph COR COMe R=Ph, EtO R=Me, Et; X=MeSO¡4 , I7, ClO¡4 ; N R Ph Het [MeHet]+X7 + 302 N C CPh R + X7 Me Het= , , , . S N Me Se N Et N Me Me N Me + X7 N C CPh Me 303 NaSH X=MeSO¡4 NH X=ClO¡4 N Me CH Ph S N Me N Ph ClO¡4 + + X7 N Me C Ph X=ClO¡4 NH X=MeSO¡4 NaSH 304 N Me S Ph N Me N Ph + ClO¡4 305 ClO¡4 N Me C PhC C CPh + NH ClO¡4 + N Me Ph N Ph N N Me C CMe + ClO¡4 301 N Me N Me + NH ClO¡4 306 Fe2(SO4)3 HClO4 N Me + I7 MeC CMgBr N Me H C CMe N Me Me N Me + ClO¡4 307 + ClO¡4 308 N Me Me N S Me + ClO¡4 309 N Me Me N Me Me Me + ClO¡4 310 N Me Me N S Me O S 836 S MLukyanov, A V Koblik, L A MuradyanAcetylenic analogues of cyanine dyes containing a carbon ± carbon triple bond in the conjugated chain between two hetero- cycles can be prepared by different methods.Thus perchlorate 312 was synthesised by dehydrochlorination of meso-chlorocarbocya- nines 311.159, 160 It has been shown by electronic spectroscopy and X-ray diffraction analysis that the alkyne structure 312a is energetically more favourable than the cumulene structure 312b.Study of a series of similar conjugated systems containing different hetero- cycles showed that in this case, an equilibrium of the type 313a.313b is established; the isomer in which the positive charge is concentrated on the more basic heterocycle (for example, on a benzimidazole rather than benzothiazole ring) predominates in the mixture.The experimental results and spectroscopic data were confirmed by quantum-chemical calculations.160 Dyes with similar structures 316 and 319 have been obtained by condensation of the chlorovinylquinolinium salt 314 with betaine 315 161 or by condensation of benzothiazolium salts 317 and 318 with each other.162 Synthesis, spectral characteristics and chemical properties of dyes including compounds 320 in which an acetylenic fragment connects two heterocyclic cations have been reported.163 3-Azaindolizine derivatives 323 are formed upon cyclisation of 2-ethynylpyridine-N-imides 322.The latter, in turn, are obtained by N-amination of 2-alkynylpyridines 321 followed by treatment with potassium carbonate in the presence of DMF.164, 165 Similar cyclisation of 2-alkynylquinoline 324 and alkynylisoquinolines 325, 326 yields pyrazolo[2,3-a]quinolines 327 and pyrazoloisoquinolines 328, 329.165 Transformations of 2-alkynyl-N-phenacylpyridinium salts 330 follow different pathways depending on their structures and reaction conditions.166 The monosubstituted 2-alkynylpyridines 321 are converted into the alkynylpyridinium salts 330 on treat- ment with phenacyl bromide.Refluxing of 330 in benzene in the presence of a base (potassium tert-butoxide, 1,5-diazabicy- 311 7HCl S N CH Et C CH Cl S N Et + Cl7 S N CH Et C CH S N Et + ClO¡4 312a S N C Et C CH S N Et ClO¡4 312b + , , . R1, R2= , , , N N N N Et Et + N N Cl Cl Et Et + N S Et + N S Et + Et + + N Et R1 C C CH R2 313a R1 CH C CH R2 313b N S + + ClO¡4 + N Me Cl Me Et N SO¡3 Me Et 314 315 Et3N CHCl3 + N C Me Et C CH N Me Et ClO¡4 316 + + S N Me CH CCl2 BF¡4 317 + S N Me CH2F BF¡4 318 1. 4-MeC6H4NEt2 2. NaI + S N Me C C C F Me S N BF¡4 319 Et N CH CH Br Br + 2I7 Py N O Et + N Et C + 320 C 2I7 N O Et + C CR 7 7 R=H, Me, Bu, CH2OH, Ph. N C CR 321 H2NOMes CH2Cl2 N NH2 + MesO7 322 (75%± 90%) K2CO3 DMF 7 +N C CR NH N N R + N N R H + N N R 323 (38% ± 98%) N C CR 324 N N R 327 (32% ± 78%) N C CR 325 N N R 328 (36% ± 67%) 326 329 (46% ± 98%) R=Bu, CH2OH, Ph. N C CR N N R Alkynylcarbenium ions and related unsaturated cations 837clo[5.4.0]undec-5-ene) affords the corresponding ylides, which cyclise to 3-benzoylindolizines 331; in boiling acetic acid, the salts 330 are converted into 1-benzoylindolizines 332.On treatment with bases, 6-methylpyridinium salts 333 give 10%± 15% of 2-phenylcyclazines 335 in addition to 3-benzoylin- dolizines 334.166 Unlike the examples described above, 6-amino-2-alkynylpyr- idines 336 are converted into 1-azaindolizinium salts 337 just upon treatment with phenacyl bromide; treatment of the salts 337 with a solution of sodium carbonate yields 1-azacyclazines 338.166 Data on these and other related reactions have been surveyed in a review.167 Synthesis of a reverse transcriptase inhibitor via a 2-alkynylpyridinium intermediate has been described in a recent study.168 An interesting reaction is the conjugated addition of organozinc-copper reagents to (2-propynylidene)morpholinium triflate 339.Hydrolysis of the addition products gives a,b-unsa- turated ketones 340 and 341, respectively.169 In recent years, extensive studies on cyclic oxonium ions, pyrylium cations containing alkynyl substituents, have been carried out. The first representative of these stable alkynylcar- boxonium ions, namely, 2,6-diphenyl-4-phenylethynylpyrylium perchlorate, was prepared by the reaction of 2,6-diphenylpyry- lium perchlorate 342 with lithium phenylacetylenide and subse- quent oxidative dehydration of the pyran intermediate.170 Later, other lithium acetylenide derivatives have also been involved in this reaction.171 Three possible structures, 343 ± 345, have been proposed for the reaction intermediate;170 based on the data of IR spectroscopy, the allene species 344 was preferred.The IR spectrum of salts 346 contains an absorption band at 2210 cm71 typical of acetylene derivatives.170 R=Me, Bu, Ph. ButOK PhH, D 321 PhCOCH2Br C CR CH2COPh + C CR CHCOPh + Br7 7 330 N N N O Ph R + 7 O Ph R + 7 N O Ph R 331 (60% ± 70%) N 330 AcOH, D Ph HO R OAc + CH2 COPh R OAc + 7 N N N COPh OAc R 7AcOH N COPh R 332 (50% ± 70%) R=Me, Bu, Ph; DBU is diazabicycloundecene.+ N O Ph R Me 334 (60%) C CR CHCOPh Me + 7 N N R Ph 335 C CR CH2COPh + 7 N H2C DBU C CR CH2COPh Me + N 333 N H2N C CR 336 PhCOCH2Br H2N C CR CH2COPh + Br7 Br7 N C CR Ph + N N R=Me, Bu, Ph. N N C CR Ph . HBr 337 (85% ± 95%) N C CR Ph 7 + N N N R Ph 338 (80% ± 90%) O N R C CPh + 339 TfO7 1. a 2. H2O 1. b 2. H2O Ph R O X 340 CH2CH2 Ph R O (CH2)nX 341 (a) IZnCu(CN)C6H4X; X=2-CN, 4-CN, 3-COOEt; (b) IZnCu(CN)CH2CH2(CH2)nX; n=0, 1; X =CN, COOEt.O Ph Ph C CHR H Ph 344a7c + Ac2O, HClO4 O Ph Ph C CR ClO¡4 346a7c 346a7c Ph Ph R=Ph (a), MeOCH2 (b), Me2(MeO)C (c). O 345a7c + O Ph Ph ClO¡4 342 + RC CLi Et2O 7LiClO4 O Ph Ph C H CR 343a7c 838 S MLukyanov, A V Koblik, L A MuradyanIt has been found that the outcome of this reaction depends on the nature of the solvent.172 When the reaction between the salt 342 and lithium phenylacetylenide was carried out in diethyl ether, only 4-phenylethynylpyran 343a was formed.The replacement of ether by tetrahydrofuran resulted in the formation of the [2+2]- cycloaddition product 347. The researcher cited believe 172 that it is formed via dimerisation of allenylpyran 344a.Later, this difference between the routes of nucleophilic addition to the pyrylium salt 342 was explained 173 by different conditions of metallation of phenylacetylene. In fact, Doney et al.172 used an equimolar amount of a standard solution of n- butyllithium, whereas other investigators 170, 171 employed an excess of butyllithium, which then remained in the reaction mixture and acted as a base during the isomerisation of the compound 343a to 344a.173 Study of dyes containing carbon ± carbon triple bonds have led to the synthesis of acetylenic analogues of triarylmethane dyes, which are able to acquire an alleno-quinoid structure and, hence, they exhibit interesting photochemical properties.Alcohols 350 can be synthesised by the reaction of organometallic derivatives of arylacetylenes 348 with the corresponding esters 347.The com- pounds 350 were converted into the corresponding 4-arylethynyl- pyrylium salts 352 by treatment with 70% HClO4 in a 2 : 1 C6H6 : EtOH mixture.174 The IR spectra of the salts 352 exhibit absorption bands at about 2170 cm71. It has been assumed that under the reaction conditions, the trialkynylcarbenium ions 351 undergo the Meyer ± Schuster rearrangement to give arylethynyl-substituted pentene-1,5-diones, which then form the pyrylium ring.175, 176 This was confirmed using simpler systems. Thus treatment of alcohol 353 with an acid gives rise to the 4-acetonylpyrylium cation 355 via alkynylcarbenium intermediate 354.177 Recently it has been shown that non-symmetrical 2,4-diphe- nypyrylium perchlorate 356 adds lithium phenylacetylenide at the free a-position yielding unstable 2H-pyran intermediate 357.Oxidative dehydration of the latter affords extremely explosive 2-phenylethynylpyrylium perchlorate 358.178 Conversely, 2H- pyran intermediates 360, obtained analogously from 2-unsubsti- tuted 1-benzopyrylium salts 359, are stable and can be isolated. 4-Phenylethynyl-1-benzopyrylium salts 363 were synthesised in the same way from 1-benzopyrylium salts 362 unsubstituted at the 4-position.178 The IR spectra of the salts 358, 361 and 363 exhibit absorption bands in the region of 2180 ±2200 cm71 typical of C:C triple bond vibrations. This means that in phenylethynylpyrylium salts, this bond is retained, the positive charge being concentrated in the heteroaromatic ring.This conclusion has been confirmed by MINDO/3 quantum-chemical calculations 178 according to which the C(1) ± C(2) bond length in species 364 (0.1209 ± 0.1210 nm) is close to the length of a C:C triple bond, whereas the C(2) ± C(3) bond (0.1429 ± 0.1437 nm) is substantially longer than the corresponding bond in an allenyl cation 109 (see Section IV).The above reactions of the pyrylium salts 342, 356 and 1-benzopyrylium salts 359, 362 with lithium phenylacetylenide are, in essence, examples of nucleophilic substitution in the aromatic nucleus, little studied so far. Processes of this type involving heteroaromatic cations as the substrates are known mainly for azinium salts.179 Study of nucleophilic aromatic sub- stitution with pyrylium salts as substrates is of interest because it does not require substrate activation.In addition, the low nucle- ofugal activity of hydrogen in the non-charged intermediates (i.e., pyran systems) permits conducting the SHN (AE) process 179 in two separate stages. 343a 344a H Ph Ph H O O Ph Ph Ph Ph 347 2 R1 C CM+R2OCO C C R1 THF 778 8C 348 349 R1 C C COH HClO4 350 3 R1=H,Me2N; R2=Me, Et; M=Li, MgBr.(MeC C)3COH H2SO4 (MeC C)3C+ 353 H2O O Me Me CH2COMe + 355 354 R1 C C C+ ClO¡4 351 3 O C C + 352 (58% ± 68%) ClO¡4 C6H4R1-p C6H4R1-p p-R1C6H4 O R C CPh + ClO¡4 361 (76% ± 82%) R1=Ph, But; R2=H, Me, Et. + O R O R H C CPh 359 360 Ph3C+ClO¡4 PhC CLi + + 2. Ph3C+ClO4 7 O R2 R1 ClO¡4 362 O R2 R1 C CPh ClO¡4 363 (57% ± 95%) 1. PhC CLi Ph3C+ClO¡4 O Ph Ph + ClO¡4 356 O Ph Ph H C CPh 357 PhC CLi + 358 (22%) O Ph Ph C CPh ClO¡4 C C O C C C C C H R H H R + 1 2 3 364 R=H, Me.Alkynylcarbenium ions and related unsaturated cations 839Besides the nucleophilic substitution of a hydrogen atom in a pyrylium cation by an alkynyl group, alkynylpyrylium salts can also be synthesised by forming a pyrylium ring. Two examples of this type, namely, cyclisation of trialkynyl carbocations 351 and 354, are shown above.174, 177 However, methods for the synthesis of pyrylium salts involving two or three components 175, 176 are not very suitable for the preparation of alkynyl-substituted deriva- tives, because of the high probability of involvement of the triple bond in condensation.Only few examples of synthesis of 4-phe- nylethynylpyrylium salts such as 365 and 366 by the reaction of 3- phenylprop-2-ynal diethyl acetal with cyclic ketones like 1-tetra- lone or cyclohexanone have been reported.180 Alkynylpyrylium salts 368 are obtained by oxidative dehy- drogenation of the corresponding 4H-pyrans 367 on treatment with triphenylmethyl perchlorate (yields 92%± 95%), an Ac2O± HClO4 mixture (yields 53%± 71%) or BF3 .Et2O (yields 43%± 82%),181 or by cyclisation of difficultly accessible 3-alkynylpen- tane-1,5-diones 369 on treatment with HClO4 in acetic anhydride, acting as dehydrating and dehydrogenating agent (yields 78%± 84%).182 The combination of the highly stable heteroaromatic cation and the alkynyl fragment in the molecules of 368 makes these compounds fairly useful for organic synthesis.Thus the presence of the cationic pyrylium fragment efficiently activates the C:C bond towards nucleophilic substitution reactions. Nucleophilic reagents, irrespective of their structure, attack the triple bond in the alkynyl substituent at the end that is remote from the hetero- cycle. It should be emphasised that the reactivity of the pyrylium fragment in the compounds formed in these reactions remains virtually unchanged.Therefore, after the nucleophilic reaction of the alkynyl group, diverse transformations of the oxygen-contain- ing heterocycle can be performed by treatment with the same or other nucleophilic reagents. For example, 2,6-diaryl-4-phenylethynylpyrylium perchlo- rates 368 (R1=R2=H) easily add hydrogen halides to give halo-derivatives 370.181 It might be expected that the reactions of 368 with water or alcohols would afford 4-phenacylpyrylium derivatives 371 (or the corresponding alkyl ethers) upon hydration of the triple bond or addition of alcohols.However, refluxing of the salts 368 with water, methanol or ethanol resulted in the isolation of mono- methinecyanines 374 the structures of which were confirmed by IR and 1H NMR spectroscopy and by X-ray diffraction analysis of the pyridinium analogue 376 (Ar= Ph) prepared by the reaction with NH3.183 The pathway to these monomethinecyanines includes hydration of the triple bond in the initial salts 368 yielding benzoylmethylenepyrans 372.However, the reaction does not stop at this stage, and the hydration products 372 react with the 4-phenylethynylpyrylium salts 368 as C-nucleophiles according to the [2+2]-cycloaddition pattern.The final products 374 result from electrocyclic ring opening in the cyclobutene intermediates 373. The 4-phenacylpyrylium salt 378 and benzoylmethylenepyran 379 have been obtained only when the 3,5-dimethyl-substituted 4-phenylethynylpyrylium salt 368 (R1=R2=H, Ar=Ph) was treated with alcohols and the resulting vinyl ethers 377 were dealkylated.184 Apparently, the methyl groups decrease the elec- O R +PhC C CH(OEt)2 HClO4 Ac2O O R R C CPh + ClO¡4 365 (28% ± 52%) O +PhC C CH(OEt)2 HClO4 Ac2O C CPh CH C CPh CH C PhC ClO¡4 366 R1, R2=H, Me; Ar=Ph, 4-MeC6H4, 4-MeOC6H4. + O H C R2 Ar R1 Ar CPh 367 HX X=ClO¡4 , BF¡4 O C R1 R2 Ar Ar CPh 368 X7 369 H C R2 Ar R1 Ar CPh O O HClO4, Ac2O X=ClO¡4 Ar=Ph, 4-MeOC6H4; Hal=Cl, Br. 368 HHal MeCN O Ar Ar C ClO¡4 370 (76% ± 93%) HClO4 Ph Hal + O Ar Ar CH Ph Hal + d7 O Ph O Ar Ar ClO¡4 371 7H+ O Ar Ar H Ph O 372 368 368 H2O Ar=Ph, 4-MeOC6H4. + 373 374 (60%) O O Ar Ar Ph Ph O Ar Ar + ClO¡4 O Ar Ar O Ph Ph O Ar Ar ClO¡4 PhNH2 NH3 376 N Ar Ar O Ph Ph O Ar Ar N Ar Ar O Ph Ph O Ar Ar Ph + ClO¡4 375 840 S MLukyanov, A V Koblik, L A Muradyantron-withdrawing influence of the pyrylium ring on the alkynyl fragment and thus hamper [2+2]-cycloaddition. However, it was shown that these groups do not create steric hindrance, because refluxing of benzoylmethylenepyran 379 with 2,6-diphenyl-4- phenylethynylpyrylium perchlorate 368 (Ar = Ph) gave mono- methinecyanine 380 in 97% yield. This result confirms the validity of the reaction scheme presented above.The pyrylium salt 381, which contains only one methyl substituent, like the salts 368, is directly converted into monomethinecyanine 382.184 It should be noted that the monomethinecyanine 374 (Ar = Ph) was prepared from tris(phenylethynyl)methanol 353 back in 1957, and its structure was then established by virtue of cumber- some chemical transformations.177 The suggestion of Koblik et al.183 that this and related compounds are produced via a cyclo- butene intermediate was based on some analogies found in the literature, for example, on the formation of benzoazepines in reactions of indoles with esters of acetylenedicarboxylic acid.185 However, the reaction of indoles 383 with 4-phenylethynylpyry- lium salts 368 yields indolevinylpyrylium salts 385 rather than benzoazepines.186 The structure of these products was proved by IR and 1H NMR spectroscopy, and by mass spectrometry and X-ray diffraction analysis of one of the pyridine derivatives 386 (Ar=Ph, R1=R2=H, R3=Me).This reaction is believed 186 to occur by a stepwise mechanism.The first stage is addition of the enamine fragment of the indole 383 to the distant end of the triple C:C bond. The trans- formations of the resulting allene intermediate 384 can follow two routes: proton transfer from the C(3) atom of the indole residue to the sp-hybridised carbon atom (pathway a) and the nucleophilic attack by the C(2) atom of the indole fragment on the sp-hybridised carbon atom of the allene fragment (pathway b). It is obvious that pathway a is energetically more favourable, because it is accompanied by aromatisation of two heterocycles, resulting in a conjugated chain with the maximum charge deloc- alisation possible in this case.The formation of the cyclobutene intermediate in the reaction of the salts 368 with the benzoylmethylenepyrans 372 is appa- rently due to the fact that the pyran and pyrylium rings can approach each other as a result of p ± p donor-acceptor interac- tion, and this creates favourable conditions for [2+2]-cycloaddi- tion.When the salts 368 react with indoles, the reactants do not come close to each other in this way, and, hence, intramolecular proton transfer occurs. This interpretation is quite consistent with the outcome of the reaction of the pyrylium perchlorate 368 (R1=R2=H, Ar=4-MeOC6H4) with vinyl ether 388 involving a similar proton transfer.186 The structure of the reaction product 389 was confirmed by the 1H NMR spectrum.The 4-phenylethynylpyrylium salts 368 are also able to enter into [2+4]-cycloaddition reactions. Thus norbornadiene deriva- tives 390 containing a positively charged heteroaromatic ring were + ClO¡4 368 O Me Me Ph Ph Ph OR ClO¡4 377 (83% ± 86%) ROH CF3COOH + O CH2COPh Me Me Ph Ph ClO¡4 378 (98%) +H+ 7H+ O CHCOPh Me Me Ph Ph 379 (81%) MeCN + O C Me Me Ph Ph CPh + O Ph Ph C CPh R=Me, Et, Pri.+ + O C Me Ph Ph CPh ClO¡4 381 EtOH O Me Ph Ph Ph Ph O O Ph Ph Me ClO¡4 382 + O Me Me Ph Ph Ph Ph O O Ph Ph ClO¡4 380 + ClO¡4 384 O Ar Ar R2 R1 C Ph N R3 H 383 N R3 + + ClO¡4 368 O Ar Ar R2 C R1 CPh R1, R2, R3=H, Me; Ar =Ph, 4-MeOC6H4.O Ar Ar R2 R1 CH Ph N R3 ClO¡4 + a NH3 N Ar Ar R2 R1 Ph R3 N 386 385 ClO¡4 387 + O Ar Ar R2 R1 N Ph R3 b O Ar Ar C CPh + ClO¡4 +H2C OEt Ph 368 388 Ar ClO¡4 + O C Ar Ph C Ph OEt H H ClO¡4 + O Ar Ar Ph OEt Ph 389 ClO¡4 + O Ar Ar Ph Ph OEt H Alkynylcarbenium ions and related unsaturated cations 841obtained for the first time by the reaction of 368 with cyclo- pentadiene.187 When solutions of the norbornadiene derivatives 390 are exposed to the light of a mercury lamp (lmax = 546 nm) or to sunlight, they undergo fast photochemical transformation into quadricyclanes 392.However, these products are readily hydrated under the reaction conditions, which unfortunately precludes the possibility of thermally induced back reaction.The pyrylium salts 368 add alcohols and aromatic amines (aniline, p-toluidine, N-methylaniline) as well as tert-butylamine to the sp-hybridised carbon atom that is remote from the pyrylium ring; this gives enaminopyrylium salts 394.184 It should be emphasised that in the case of pyrylium salts containing no ethynyl substituent, the reactions with ammonia and amines involve the pyrylium fragment and are used typically to prepare pyridines and pyridinium salts.176 Unlike normal pyrylium salts, the compounds 394 are deprotonated on treatment with ammonia (at R4=H) to unsaturated imines 395 rather than exchange the oxygen atom in the ring for nitrogen.However, ammonium acetate and methyl-, butyl- and benzyl-amines attack the pyrylium ring in the salts 368, which gives rise to alkynyl-substituted pyridine derivatives 396 and 397.184 The acetylenic fragment in the 4-phenylethynylpyridinium salts 397 is still sufficiently activated to add one more amine molecule; this affords aminovinylpyridinium salts 398.Unlike monocyclic pyrylium salts, 1-benzopyrylium salts do not tend to exchange the ring oxygen atom for nitrogen.188 Therefore, it comes as no surprise that 4-phenylethynyl-1-benzo- pyrylium salts 363a,b are converted into aminovinyl derivatives 400a,b on refluxing with an equimolar amount of an aliphatic amine or benzylamine followed by treatment with HClO4.189 However, when the salt 363b is mixed with excess amine at room temperature, the products of the amine addition at the 2-position of the ringD2-alkylaminoflavenes 401Dare rapidly formed. On treatment with HClO4 they easily decompose giving the initial salt 363b.Thus, this addition is reversible and kinetically controlled. Refluxing of the salts 363a,b with a two- or three-fold excess of an amine in acetonitrile for 1 ¡À 5 h affords 4-(2-hydroxyphenyl)pyr- idinium salts 402.The same products are formed under similar conditions from the aminovinyl salts 399a,b, which are evidently formed as intermediates in the transformation of the 4-phenyl- ethynyl-1-benzopyrylium salts 363. It is obvious that the 2-aminoflavenes 401 are not formed as intermediates in this thermodynamically controlled recyclisation. However, the formation of aminoflavenes proved to be the key stage in the transformation of the 4-phenylethynyl-1-benzopyry- lium salts 363a,b into 2H-chromeno[3,4-b]pyrroles 404.189 These products are formed in 36%¡À 75% yields upon refluxing of the R1, R2=H, Me; Ar=Ph, 4-MeOC6H4. 390 O Ph Ph R2 R1 Ph OH + ClO¡¦4 H 393 H2O CH2Cl2 O Ph Ph R2 R1 Ph + ClO¡¦4 392 hn Ar=Ph 368 MeCN O Ar Ar R2 R1 Ph + ClO¡¦4 AcONH4 AcOH N Ar Ar R2 R1 Ph 391 390 + O R2 R1 Ar Ar NR3R4 Ph ClO¡¦4 394 368 R3NHR4 R1, R2=H, Me; Ar=Ph, 4-MeOC6H4; R3=But, Ph, 4-MeC6H4; R4=H, Me.O R2 R1 Ar Ar NR3 Ph 395 NH3 R4=H R1, R2=H, Me; Ar=Ph, 4-MeOC6H4; R3=Me, Bu, PhCH2. 368 AcOH R3NH2 AcONH4, + N Ar Ar R2 R1 C CPh 396 N Ar Ar R2 R1 C CPh R3 397 ClO¡¦4 + N Ph Ph Me Me HC CNHMe Me Ph 398 ClO¡¦4 MeNH2 Ar=Ph, R1=R2=Me R=Me, Bui, PhCH2.+ Ph Me C CPh ClO¡¦4363b 401 CPh RNH2 (excess), MeCN, 20 8C HClO4, AcOH O Ph Me C NHR O + + R2NH2 MeCN, 82 8C, 20 min O Ph R1 Ph NHR2 ClO¡¦4 399a,b O Ph R1 C CPh ClO¡¦4 363a,b 1. OH7 2. HClO4 R1 = H (a), Me (b); R2, R3=Me, Bui, PhCH2. + N R1 Ph Ph R3 402 C6H4OH-o R1 = H (a), Me (b), R2=PhCH2, But, Bui . O Ph R1 Ph NR2 400a,b 363a,b R2NH2 MeCN, D 399a,b R3NH2 7H+ O Ph R1 NHR3 Ph NHR2 OH Ph NHR2 R1 NR3 Ph N R1 Ph Ph NHR2 R3 H+ 7R2NH2 C6H4OH-o 842 S MLukyanov, A V Koblik, L A Muradyansalts with excess primary amines in ethanol.Presumably, ethanol acts as a competing nucleophile and suppresses the formation of the 4-alkylaminostyryl salts 399. The unstable adducts 401 have enough time to undergo ring cleavage yielding phenol intermedi- ates 403, which again cyclise to chromenopyrroles 404.The structure of one of the tricyclic products 404 was established by X-ray diffraction analysis.189 Recently, unexpected recyclisations of the 3-methyl-2-phenyl- 4-phenylethynyl-1-benzopyrylium perchlorate 363b, which occur on its refluxing with a two- or three-fold excess of aromatic amines, have been reported.190 Treatment of the salt 363b with aniline in dichloroethane or acetonitrile gives 4-anilino-2-phenyl- 1-benzopyrylium salt 405a.On treatment with p-toluidine under the same conditions, the pyridinium (402b) and 4-(p-toluidino)-1- benzopyrylium (405b) perchlorates are formed. It has been noted above that the compounds 402 result from recyclisation of the 4-aminovinyl-1-benzopyrylium salts 399.It has been suggested and confirmed experimentally 190 that the compounds 399 can arise as intermediates in the recyclisation of the salts 363b to the 4-(R-amino)-1-benzopyrylium salts 405a,b. 4-Phenylethynylthiopyrylium salts 406 have been obtained recently from 4-phenylethynyl-4H-pyrans 367 (yields 30%± 40%) and from 3-phenylethynylpentane-1,5-diones 369 (yields up to 80%) on treatment with hydrogen sulfide in the presence of boron trifluoride etherate.191 Bis-thioxanthene cumulene 407 has been prepared within the framework of studies aimed at the synthesis of p-electron-donat- ing systems for charge transfer complexes possessing metallic conduction.When this product is treated with concentrated sulfuric acid, it is readily oxidised to acetylenic dication 408, but does not undergo the expected triple bond protonation.192 Development of a simple method for the synthesis of 3-acyl- methylene-2,3-dihydrofurans from diacetylenic vicinal diols 193, 194 has led to the preparation of the dihydrofurylium salt 413 containing a conjugated enyne fragment.Apparently, the transformation of the diol 409 to the furan 412 occurs via the intermediate formation of dialkynyl carbocation 410 and cyclic alkynyl carbocation 411, which then undergoes the Meyer ± Schuster rearrangement. 363a,b R2NH2 EtOH, D O C CPh R1 Ph NHR2 401 O H R1 Ph N C C Ph R2 403 R1=H, Me; R2=Me, Bui, PhCH2. N 7 O R2 Ph R1 Ph H O N R1 Ph R2 H Ph 404 + + 363b RNH2 O Ph Me Ph NHR ClO¡4 399 N Me Ph Ph R ClO¡4 RNH2 7H+ 402a,b C6H4OH-o N RHN Ph R Me Ph H2O OH Ph NHAr CH Me COPh RHN RNH3ClO4 72RNH2, 7PhCOEt C6H4OH-o R=Ph (a), 4-MeC6H4 (b).+ O Ph NHR ClO¡4 405a,b H2S, HCl BF3 . Et2O BF3 . Et2O H2S + 406 BF¡4 S C CPh R R Ph Ph O C CPh R R Ph Ph 367 O O R R C Ph Ph CPh 369 (b) SnCl2, HCl, Et2O,780 8C. (a) 1. BuLi, THF, 778 8C; 2. ,740 8C S O S S HO C CH OH C C HO S a b + S C C S 407 H2SO4 S C C S+ 408 Me Me C C HO OH CPh CPh 409 HgCl2, HCl MeOH Me Me C C OH CPh CPh + 410 HClO4 O Ph CH Me Me C C CPh OH Ph ClO¡4 + CPh C Ph Ph CH Me Me O 413 O C Ph Me Me CPh + 411 PhC CMgBr Et2O O Ph CHCOPh Me Me 412 (66% ± 84%) Alkynylcarbenium ions and related unsaturated cations 843Comparison of the electronic spectra of the cation 413 (lmax=509 nm) with those of some other 4-styryldihydrofury- lium salts containing H, Me, Et or Ar instead of the phenylethynyl group has demonstrated that the bathochromic shift caused by the PhC:C group is twice as large as that caused by Ph.This is due to the fact that the triple bond efficiently participates in charge delocalisation.194 To summarise the foregoing, it can be concluded that all the methods that are, in principle, suitable for generation of carbe- nium ions have been used to generate alkynyl carbocations.2 The most widely used procedure is detachment of a nucleofu- gal species from functional acetylene derivatives of the propargyl type containing one, two or three heteroatomic functions at the sp3-hybridised carbon atoms [Eqns (1) ± (3)].135, 136 Methods based on abstraction of nucleofuges from functionally substituted enynes, allenes and cumulenes [Eqns (4) ± (6)] are more difficult for practical implementation and, therefore, are used more rarely.113 ± 115, 128, 145, 146.In the two latter cases [Eqns (5), (6)], the appearance of a carbenium centre results as well in the formation of a C:C triple bond. The oxidative dehydrogenation of ethynylpyran systems 171, 181 [Eqn (7)] also belongs to this type of reactions.The general approach to the generation of carbocations consisting in the addition of a cationic electrophile to an unsatu- rated system has also been used for the preparation of alkynyl- carbenium ions. Compounds used most often as the precursors are alkynones 88, 129, 130, 146 [Eqns (8), (9)], amides 34, 143 [Eqn (10)], linear imines 8 and nitrogen-containing heterocycles 156 ± 158, 165 ± 166 [Eqns (11), (12)].The protonation of terminal cumulenes [Eqn (13)] is represented by only one example.127 Electrophilic reactions of enynes deserve a more detailed discussion, which is presented in the last paragraphs of this Section. One more fundamental approach to the generation of alky- nylcarbenium ions is formation of a carbon ± carbon triple bond in cationic structures.Known examples are deprotonation of imi- nium salts and quaternised nitrogen-containing heterocycles fol- lowed by elimination of a nucleofuge (Hal7, CF3SO¡3 ) 142, 144, 159 ± 161 [Eqn (14), (15)]. Several studies have been reported in which a carbenium centre near a triple bond was generated via 1,2-migration during the pinacolic rearrangement;79 an example of two-electron oxida- tion of cumulene to a dication, which is a rarely encountered type of process in the preparative chemistry of carbocations, has also been described.192 In the case of conjugated enynes, electrophilic reactions of enynes, which have been mentioned among the methods for generation of alkynyl carbocations, proceed ambigously.4, 195 Therefore, generation of alkynylcarbenium ions from these sub- strates is problematic.However, it is known that cations of the type 9, which are not detected even in superacidic media (see Section III), can be readily prepared or even isolated as dicobalt- hexacarbonyl (DCHC) complexes of the type 414.196 The DCHC complexes 415 derived from a number of enynes, namely, iso- propenylacetylene, 1-ethynylcyclohexene, 1-ethynylcyclopentene, have been used to conduct a large series of AdE reactions according to a two-stage pattern.The natures of the electrophile X+ and the nucleophile Y7 in these reactions can be varied independently, due to the high stability of the alkynylcarboca- tionic intermediates 416.196 ± 198 + + + X=Hal, OH, OAlk, OAc. (2) (3) X=Br, TfO. (5) (6) (7) (8) E=H, CF3SO2.(9) (1) C C X C C 7X7 C C CH(OR)2 C C OR H H+ 7ROH H+ 7ROH C C C(OR)3 C C OR OR (4) C C X 7X7 C C C + C + C C C Hal C C 7Hal7 + C C C C Hal C C C 7Hal7 + 7H7 O C H C O C C 7H7 O C C H + C C O E+ C C OE + C C O PCl5 C C Cl (10) C C N O Et3O+ 7Et2O C C N OEt + (11) C C N RCl 7Cl7 R N C C + C C N RX 7X7 C C N R + (12) C C C CH2 H+ C C Me + (13) N H X + CH N CH Br Br + B 7HX B 72HBr C N C + (14, 15) C C C OR Co2(CO)8 C C C OR Co2(CO)6 H+ 7ROH 414 C C C C Co2(CO)6 C C C Co2(CO)6 + H+ + HC C C CH2 Me Co2(CO)6 X+Y7 HC C C CH2X Me Co2(CO)6 Y7 415 416 Nu7 844 S MLukyanov, A V Koblik, L A MuradyanThe vinylacetylene complex 417 regioselectively adds C-elec- trophiles and C-nucleophiles to the double bond to give alkyne- 1,5-diones 419, which are difficult to obtain by other methods.199 The same process involving an a,b-unsaturated acyl cation and methanol as the nucleophile gives rise to the alcohol 420.Thermal cyclisation of 420 yields bicyclic ketone 421 (as a mixture of stereoisomers).200 The reaction of allyl alcohol acting as a nucleophile with ethynyl cation 422 gives spirotricyclic products 423.200 The alkynylcarbenium ions of the type 418, 422 stabilised by complex formation have been generated and successfully used in a number of syntheses.71, 72, 201 ± 208 For example, from the DCHC complex of enyne 424, hydroxycyclopentanone 425a has been prepared in a quantitative yield 71 and arylaminocyclopentanones 426 have been prepared in yields of up to 93%.208 VI.Alkynyl carbocations with a remote triple bond Finally, a special place is occupied by the reactions of alkynyl carbocations of the type 9 with alkenes giving new cations with structures A and B (see Fig. 1). These species are, in principle, alkynylcarbenium ions in which the carbenium centre is separated from the acetylenic fragment by one or more carbon atoms. They are fairly reactive, and their transformations are being vigorously studied.In 1965, Hanack et al.209 discovered that solvolysis of acety- lene derivatives 427 yields cyclopropyl ketones 430 and cyclo- butanones 431. Later, it was shown by detailed experiments and quantum-chemical calculations that cyclopropylidenemethyl cat- ions 428 and cyclobut-1-enyl cations 429 are formed as intermedi- ates during this reaction.22, 23, 26, 27 This transformation has been called `homopropargyl rear- rangement'.Normally it occurs quantitatively if the solvolysis is carried out in a highly polar and weakly nucleophilic solvent (for example, in HCOOH or CF3COOH) and the initial compound 427 contains a good leaving group (tosylates, nitrobenzene-p- sulfonates, nonaflates and triflates). The ratio of the rearrange- ment products 430 and 431 depends on the nature of the substituent R.When R = H or Alk, the cyclobutanones 431 are the main reaction products, because in this case, the cations 429 are more stable than 428. When R = Ar or cyclopropyl, the ketones 430 predominate in the product mixture. Homopropargyl derivatives of type 432 are mostly converted into cyclobutanones 433.210 The mechanism of the homopropargyl rearrangement has been determined based on the investigation of the solvolysis of functionally substituted alkylidenecyclopropanes 22 and cyclobu- tenes 211 as well as various open-chain and cyclic propargyl derivatives.212, 213 Studies of isotope effects involved in the sol- volysis 121 and stereochemical aspects of the rearrangements of chiral pentynyl triflates 214 have also been invoked.The results of these studies led to the conclusion that the solvolysis directly proceeds to the cations 428 and 429 via a transition state typical of SN2 type reactions.27, 121, 210 However, it has been shown for the solvolysis of cyclobut-1-enyl nonaflates 434 in trifluoroethanol that the presence of substituents stabilising the cationic centre at the 3-position of the ring is favourable for the formation of RBF4 (R=But, 1-adamantyl), ArSbF6 (Ar=4-ClC6H4, X+Y7=RCOBF4 (R=Me, But, MeCH=CH), 4-MeC6H4), NO2BF4; Nu=OH, MeO.HC C C CH2X Me Nu Co2(CO)6 [O] (65% ± 90%) HC C C CH2X Nu Me HC C CH CH2 Co2(CO)8 HC C CH CH2 Co2(CO)6 417 R1CO+BF¡4 + HC C CH CH2COR1 Co2(CO)6 BF¡4 418 H2C C(OSiMe3)R2 [O] HC C CH CH2COR1 CH2COR2 419 (54% ± 97%) HC C CH CH2COR1 CH2COR2 Co2(CO)6 R1=Me2CHCH2, cyclo-C4H7, MeCH=CH, 1-adamantyl; R2=Me, Ph, cyclo-C3H5.R=Et, Bui; n=1, 2. MeOH 417 MeCH CHCO+BF4 7 Co2(CO)6 + O BF¡4 418 Co2(CO)6 O MeO MeMgI Co2(CO)6 OH MeO 420 SiO2, 60 8C O OMe H OH 421 (CH2)n Co2(CO)6 (CH2)n COR Co2(CO)6 + 422 OH RCO+BF¡4 (CH2)n COR O Co2(CO)6 60 8C (CH2)n COR O O 423 R1=Ph (a), Pr (b), C6H13 (c); R2=H, Et; R3=COOH, NO2, I.O CH H EtO R1 Co2(CO)6 424a7c H3O+ dioxane, 80 8C R2NHC6H4R3 O OH 425a Co2(CO)6 Ph O R2NC6H4R3 426 Co2(CO)6 R1 R + 428 R + 429 427 R C C CH2 CH2X 7X7 C R1 O R2 433 432 R1 C C CH2 CHX R2 7X7 R O 431 R C O 430 Alkynylcarbenium ions and related unsaturated cations 845secondary homopropargyl cations 437. The latter are converted into the corresponding ethers 438.27, 212, 213, 215 Compound R1 R2 Yield (%) 438 439 434a Me H 3 95 434b H Me 63 34 434c (CH2)6 68 32 The homopropargyl rearrangement is important from the synthetic viewpoint as a facile and convenient method for the preparation of cyclobutanones containing diverse substituents including those fused with carbocycles.The processes of cyclisation of alkynyl carbocations 197 (n=2) with two methylene units between the triple bond and the carbenium centre are described in detail in Section V.1.It is noteworthy that upon the solvolysis of pent-4-ynyl triflates R± C:C± (CH2)2CH2OTf, no cyclic products have been detected.26 This is due to the extremely high strain in the five-membered cyclic vinylic (i.e. cyclopent-1-enyl) cations (see structures K, Fig. 1, and 202). Meanwhile, the cyclobut-1-enyl cations 429 and 435 are quite stable owing to delocalisation of the positive charge over three carbon atoms across the ring.27, 216 Therefore, the cation 441 is, in essence, a bridged non-classical ion in which the C(3) atom is equidistant from the ends of the double bond.The cyclopent-1-enyl cations 202 can arise during this process if the cationic intermediates are sufficiently stabilised by appro- priate substituents. Thus protonation of 1,2,4-trimethyl-3-meth- ylene-4-(prop-2-ynyl)cyclobut-1-ene 442 affords bicyclic ketone 445.The reaction scheme includes the formation of cyclobutenyl- allyl cation 443, which cyclises to bicyclic vinylic cation 444.217 The structure of the cation 443 was confirmed by its 1H NMR spectrum (in HSO3F±SO2FCl,7120 8C).A variation of the degree of substitution in the substrate sharply changes the reaction route. Under the same conditions, cyclobutenes 446 substituted in the side chain afford polymethy- lated styrenes 450, presumably according to the following scheme:218 The spectral characteristics of the cationic intermediates formed in this process have been described in detail.219 It is clear that the cations 443 and 447 correspond to structure B, the cations 444 and 448, to structures L and M and the ion 449, to allylic cation Q (see Fig. 1). Two other theoretically possible cation structures with participation of a C:C triple bond are also formed during this transformation. Protonation of propynyl-substituted cyclohexadienols 451 and 452 and methylenecyclohexadienes 453 results in the forma- tion of cations 454.The subsequent [1,2]-migration or two successive migrations of the propynyl fragment give rise to substituted benzenes.220 Evidently, these rearrangements are energetically favourable because they lead to ring aromatisation. In alkynyl carbocations 456 in which the carbenium centre and the acetylenic fragment are separated by three carbon atoms, due to the close arrangement of groups in space, conditions are created for the formation of cations 457 and 459.The predom- inance of one of these ions is determined by their relative stability, which depends on the substituents present in the initial acetylene derivative. R2 R1 ONf R3OH 434a7c R2 R1 + 435 R1 R2 + 436 C R3OH 438+ R2 R OR3 439 434 435 OR3 R2 CH2 + 437 CH C C R1 R2 CH CH2 438 C C R1 Nf=C4F9SO2; R3=CF3CH2.C R1 R2 OR3 434 436 440 C C C C 1.734 1.291 1.791 1.477 1.433 441 + OPO3H2 Me Me CH2C Me H2C CH 85% H3PO4 Me Me CH2C Me Me CH H2PO¡4 + 443 442 Me Me Me Me + H2PO¡4 444 Me Me Me Me Me Me Me Me O 445 Me Me Me CMe2C H2C CR 446 H3PO4 Me Me Me CMe2C CR Me + 447 Me Me Me Me R Me Me + 448 449 Me Me Me R Me Me Me + R Me Me Me Me Me Me + R=H, Me.Me Me R Me Me Me Me + 7H+ Me CH2 R Me Me Me Me 450 [3,4] + [1,2] 7H+ 454 H+ 7H2O 7H2O H+ H+ H HO 451 452 H HO 453 454 + [1,2] 846 S MLukyanov, A V Koblik, L A MuradyanThus solvolysis of terminal hex-5-ynyl triflate 455a in tri- fluoroethanol results in the formation of an open-chain ether (X=OCH2CF3, 58%) and cyclohexene derivatives 458a (12%) and 458f (X=OCH2CF3, 24%).22 Conversely, the methyl homo- logue 455b is converted by approximately 85% into cyclopentane derivatives 460a (67%) and 460f (17%).The introduction of a terminal phenyl group (compound 455c) stabilises the exocyclic vinylic cation 459 to even a greater extent; therefore, the trans- formation of 455c gives almost exclusively methylenecyclopen- tane derivatives 460g (X=OAc).The secondary tosylates 455d,e behave in a similar way.22 The alkynylcarbenium (456) and vinylic (457, 459) cations can arise as intermediates in cascade cyclisations used in elegant syntheses of steroids. Within the framework of a general approach to the preparation of testosterone, biomimetic syntheses of some of its precursors have been reported.221 ± 223 The treatment of thioketal 461 with SnCl4 has given a mixture of cyclisation products 462 ± 464 in 12%, 55% and 32% yields, respectively.224 The synthetic potential of an approach to the synthesis of steroids, which, in principle, imitates the transformation of squalene into polycyclic products occurring in living organisms, has been discussed.221 ± 225 It was noted that the propynyl group plays a special role, since it makes possible the formation of both five- and six-membered rings via two vinyl cationic intermediates.Using this scheme, linear compounds can be converted in one stage into tetracyclic systems with seven chiral centres, this reaction being highly stereoselective (see also relevant reviews 226, 227). Cyclisations of alkynylvinyl cations 466 formed upon solvol- ysis of vinyl triflates 465 afford vinylic cations 467 and 469, which are then converted into cyclopentenyl ketones 468 and cyclo- hexenones 470, respectively.These ketones are unstable under the solvolysis conditions and rearrange into the corresponding a,b- unsaturated ketones.228, 229 The dependence of the reaction route on the mutual orienta- tion of the carbenium centre and the acetylenic fragment can be clearly followed for the solvolysis of cycloalkynyl esters.Thus cyclodec-5-ynyl derivatives 471 are mostly converted into deca- lone 473 via cation 472. Only traces of bicyclo[5.3.0]decan-2-one 474 were found in reaction mixtures.22, 26, 230, 231 Solvolysis of 4-nitrobenzoate 475 containing a nine-mem- bered ring yields almost quantitatively bicyclic ketone 476,22, 230 whereas in the case of isomeric ester 477, the triple bond does not participate in the solvolysis at all.230 + C CH CR1 X R2 455a7d C CHR2 CR1 7X7 456 + R1 R2 + R2 C R1 457 459 X7 X7 R1 R2 X 458a,f R2 R1 X 460a,f,g R1=R2=H, X=OTf (a); R1=Me, R2=H, X=OTf (b); R1=Ph,R2=H, X=OTs (c); R1=H,R2=Me, X=OTs (d); R1=R2=Me, X=OTs (e).OH O O EtNO2,725 8C, 30%; (c) Cl3CCOOH, Me2CHNO2, 0 8C, 45%± 65% (a) CF3COOH, EtNO2,778 8C, 80%; (b) CF3COOH, Me3N, a N CHMe OH O O b N CHMe HO O ON CHMe c S S SnCl4 CH2Cl2 SH S Cl Me 462 (12%) 461 + + SH S Cl Me 463 (55%) + SH S Me Cl 464 (32%) Me Me + 467 Me Me O 468 Me Me O C + 470 Me Me 469 Me Me O Me Me O C Me OTf C Me CF3CH2OH 120 8C C C Me Me + 465 466 C X=OTs, 4-NO2C6H4COO.O + + 472 473 O 474 C C X 471 Alkynylcarbenium ions and related unsaturated cations 847Further progress of the studies described above resulted in the development of a new approach to the generation of fairly unstable phenyl cations of the type 6.27, 229, 232 Suffice it to say that these cations are 20 ± 25 kcal mol71 less stable than the propen-2-yl cationH2C=C+CH3,29 and it was not until 1985 that they were obtained for the first time by solvolysis.28 The inter- mediate formation of the phenyl cations 480 was confirmed by the detection of functional derivatives of benzene 481 and 482 among the products of solvolysis of conjugated dienyne triflates 478.Benzyl ethers 483 result from transprotonation of the cationic intermediate 479. Solvolysis of cyclic dienyne vinyl triflate 484 in the presence of LiBr gives bromo-substituted tetrahydronaphthalene 485.233 Naphthyl cations 487 have been generated in the same way from enol triflate 486.27, 234 When the carbon ± carbon triple bond is further removed from the carbenium centre, the cyclisation becomes virtually im- possible.22, 26 Another, barely studied, type of cations with a triple carbon ± carbon bond, namely the alkynyl cations R±C:C+, should also be mentioned.These species are extremely unstable; their energy is 130 kcal mol71 higher than that of primary vinyl cations or phenyl cations described above.235 Phenylethynyl cations were detected for the first time in the mass spectra of arylazoethynylbenzenes 488.236 Alkynyl cations can be generated in solution only by the nuclear-chemical method 235 (see also Ref. 237). Reactions of 1,4- diethynylbenzene 489 containing tritium atoms with benzene and brominated hydrocarbons were studied. The b-decay of tritium yields a helium atom, which is the best leaving group. The intermediate formation of alkynyl cation 490 was proved by chromatographic isolation of the products of its reactions 491, 492.235 Recently, in order to find pathways to ethynyl cations, nitro- sation of enamines, bis-silylated at the nitrogen atom, has been studied.238 In the multicomponent reaction mixtures formed upon these reactions, some products that could have arisen with participation of the ions in question were detected.It has been proposed to use alkynyllead triacetates 494, which are formed in situ on treatment of alkynyltrimethylstannanes 493 with lead tetraacetate, as synthetic equivalents of alkynyl cations; these compounds permit easy introduction of an alkynyl group C C OOCC6H4NO2-4 477 CF3CH2OH C C OCH2CF3 C C OOCC6H4NO2-4 475 + O 476 CF3CH2OH R=H, Me.X=H, Et, CF3CH2. Me C OTf Me C R 478 XOH 7TfO7 + Me C CMe C R 479 + Me Me 480 Me Me CH2OX 483 (8% ± 41%) R LiBr, CF3CH2OH R=H 482 (14% ± 26%) Me Me Br Me Me OX 481 (17%) R H + HC C CH2 CR Me 479 R=Me Me Me CH2 + XOH C C OTf LiBr C C + 484 + CF3CH2OH Br 485 (16%) C Me OTf CMe 486 CF3CH2OH Me Me + 487 OCH2CF3 Me Me + OTf Me Me (15%) (6%) (17%) Br Me Me LiBr Ar N N C C Ph + 7Ar PhC CN2 + 7N2 PhC C+ 488 7e7 TC C C C T 489 73He TC C C C 3He + 490 TC C C C+ C6H6 ButBr 491 (98%) TC C C C TC C C CBr 492 (30%) 848 S MLukyanov, A V Koblik, L A Muradyaninto 1,3-dicarbonyl compounds and nitronates.The reaction products are formed in 47%± 87% yields.239 VII. Hetero-analogues of alkynyl carbocations The principle of activation of a carbon ± carbon triple bond by means of a positively charged substituent can be realised in compounds containing an acetylene fragment and a heteroatomic cation.Unlike alkynylcarbenium ions, onium cations 495 con- taining an ethynyl group cannot have allenyl structure like 10. An obvious merit of these systems is the possibility of remov- ing the activating heterocationic fragment from the molecule after a transformation of the acetylene group.35 1. Alkynylammonium and alkynyldiazonium ions Alkynylammonium ions 496 are usually regarded as intermediates in the formation of ynamines 497 from alkynyl halides.36 As a rule, these species are unstable, although in some cases, they have been isolated as salts and characterised.Thus the ethynylammonium salt 499 was prepared from methyl bromopropiolate 498. The IR spectrum of this salt exhibits an absorption band typical of acetylene derivatives at about 2180 cm71.240 Organyl chloroe- thynyl sulfides 500 react with tertiary amines to give quite stable salts 501, which are soluble in water and alcohols.The acetylene absorption band in the IR spectra of these salts (2205 cm71) is shifted to shorter wavelengths with respect to this band in the spectra of the initial sulfides (2170 cm71).241 During studies on nucleophilic substitution at an acetylenic carbon atom, fairly unstable ethynyltriethylammonium bromide 502 has been isolated.Upon mixing with an alkaline solution of KI and HgCl2, this salt was converted into stable mercurioethy- nylammonium iodide 503.242, 243 An unusual type of hydration of a C:C triple bond without a catalyst has been performed 244 by irradiation of 4-ethynyl-N,N- dimethylaniline 504. This reaction was assumed 244 to occur via the formation of radical cation 505.Published data on alkynylammonium cations are scarce. A comprehensive review of the available information can be found in a paper by Katrizky 245 devoted to the synthesis and spectro- scopic characteristics of heterocyclic ynammonium salts. Since these salts cannot be prepared from highly nucleophilic 4-di- methylaminopyridine, acridone 506 was used as the heterocyclic substrate.This permitted the N-alkylation and quaternisation stages to be conducted separately. N-Alkynylacridinium salts 508, 510 were prepared via two routes, namely, by O-acylation of N-alkynylacridones 507 and by dehydroxylation of pseudo-bases 509 with trifluoromethanesulfonic acid.245 In the 13C NMR spectra of the salts 510, the signal of the triple-bond carbon atom, more distant from the heterocycle, is shifted downfield (on the average, by 12 ± 14 ppm) in relation to its position in the spectra of the precursors 507, 509.On treatment with nucleophiles, the salts 510 are converted into stable ynamines of the type 509, the attack of the nucleophile being directed at the 9-position of the acridinium ring.245 After several unsuccessful attempts to prepare alkynyldiazo- nium salts, Hanack et al. 246 described the synthesis of phenyl- ethynyldiazonium salt 512 from a-bromo-1-phenylacetyl chloride 511. The salt 512 obtained at 730 8C was made to react with nucleophiles without isolation. It was found that nucleophiles first add to the C:C triple bond. The alkenyldiazonium salts 513 thus formed eliminate a nitrogen molecule.The initial salt 512 itself cannot abstract a nitrogen molecule, because the energy of the alkynyl cation that would result from this process is extremely high (see Section VI). RC C SnMe3+Pb(OAc)4 CHCl3 [RC C Pb(OAc)3] , 493 494 COOEt O O , COOEt C CR 494 O Me O 494 O COMe C CR , MeCO CH COOEt Me 494 MeCO CH COOEt , Me C CR R=H, Me(CH2)5, Ph, Me3Si.NO2 Na+ Me Me 7 494 C Me Me NO2 CR , NO2 Na+ 7 494 C NO2 CR + X=N, P, Hal. R1 C C X(R2)n 495 + + R1=Et, Bu; R2, R3, R4=Et, (CH2)5. R C CCl C C NMe3 Cl7 R Me3N Me3N 7MeCl R C C NMe2 , 496 497 Br C C COOMe Et3N Et2O,730 8C 498 R1S C C Cl +NR2R3R4 500 R1S C C NR2R3R4 Cl7 501 + Et3N C C COOMe Br7, 499 + HC CBr Et3N HC C NEt3 Br7 502 KI HgCl2 I + Hg C C NEt3 I7, 503 p-HC 504 CCl4 7CCl3,7Cl7 p-HC 505 C NMe2 C6H4 C C6H4 + NMe2 p-HC H2O p-MeCO C NMe2 Cl C6H4 NMe2 C6H4 R=Me, Ph, 2,4,6-Me3C6H2.N O H 506 N O C CR 507 + N OCOBut C CR 508 PhLi ButCOCl, AgClO4 509 510 + N C CR OH Ph CF3SO3H N Ph C CR CF3SO¡3 Alkynylcarbenium ions and related unsaturated cations 849The IR spectrum of a solution of the diazonium salt 512 (CH2Cl2, 720 8C) contains absorption bands due to the triple bond (2150 and 2255 cm71) and the diazo group (2295 cm71).The researchers were unable to isolate this salt in the solid state. 2. Alkynylphosphonium cations Unlike ynammonium salts, alkynylphosphonium salts of the type 514 are fairly stable, and by now, they have become significant from the synthetic viewpoint. These compounds were discovered in 1962 by Viehe et al.,247 who studied reactions of alkynyl halides with nucleophiles.An important method for the synthesis of the salt 514 consists in treatment of alkynyl halides with triphenyl- phosphine (more rarely, with trialkylphosphines) in aprotic sol- vents (for example, in ether).248 ± 250 Arylethynylphosphonium salts 516 and 517 are prepared by acylation of hexaphenylcarbo- diphosphorane 515.251 Yet another approach to the alkynylphosphonium salts of the type 514 is based on interaction of aroylmethylenetriphenylphos- phoranes 518 with triphenylphosphine dibromide.252 The attempts to prepare salts 519 with aliphatic substituents R by this method failed. However, ethoxycarbonyl- (518a) and phen- oxycarbonyl-methylenetriphenylphosphoranes (518b) were con- verted into the alkoxyethynylphosphonium salts 519a,b, andN,N- dialkylcarbamoylmethylenephosphoranes 518c ± f were converted into the aminoethynylphosphonium salts 519c ± f, which represent a new type of push ± pull acetylenes.The alkynylphosphonium salts 519 with alkylamino or aryl groups as the substituentsRare quite stable on heating and do not tend to be oxidised by atmospheric oxygen.As a rule, alkynyl- phosphonium bromides are solids, whereas the corresponding chlorides are normally oils. The IR spectra of these salts exhibit absorption bands of moderate intensity at about 2150 ± 2200 cm71. In aqueous solutions, anion exchange is possible, for example, bromide can be replaced by iodide,253 and chloride can be replaced by chloroplatinate.249 On heating with water or acetic acid, the acetylene fragment in the salts 519 is hydrated giving a carbonyl group at the carbon atom furthest from phospho- rus.252, 253 Recently, preparation of alkynylphosphonium salts 521 by treatment of alkynyliodonium salts 520 with polycyclic trialkyl- phosphines has been described 524 (see Section VII.3).The addition of nucleophilic reagents to alkynylphosphonium salts is directed at the triple-bond carbon atom that is furthest from the phosphonium group.249, 253 Some of these reactions are presented below The addition of tributylphosphine is accompanied by dis- placement of the triphenylphosphine residue, because tributyl- phosphine is a stronger nucleophile, and affords bis- tributylphosphonium styrene derivative 522.Triphenylphosphine adds to the compound 523 to yield mixed bis-phosphonium salt 524. + PhCH COCl Br TsNHNH2 511 PhCH CO Br NHNHTs (79%) PCl5 PhCH C Br NNHTs Cl (78%) Et3N (91%) PhCH C N Cl NTs SbCl5 PhC C N NTs PhCH N NTs SbCl¡6 + 7HSbCl6 SbCl5 C PhC N N SbCl5Ts7 512 C X, Y=OH, OMe, Cl. 512 HX X H N2 Ph + 513 SbCl5Ts7 7N2 X Ph H + HY 7H+ X H Y Ph C R=EtO (a), PhO (b), Et2N (c), Ph2N (d), Pri 2N (e), PhMeN (f).R=Ar, Alk3N. R C CH PPh3 OPPh3 Br + Br7 RCOCH PPh3+[Ph3PBr]+Br7 518a ± f Et3N + RC C PPh3 Br7+Ph3PO+Et3NHBr 519a ± f + 519 H2O RCOCH2 PPh3 Br7 Ar=Ph (a), 4-MeOC6H4 (b), a-C10H7 (c). + + Ph3P C C C C PPh3 , 2Cl7 517 (71%) 515 COCl ClOC + R1 C CX+R23 P R1 C C PR23 X7, 514 ArCOCl+Ph3P C PPh3 C6H6 ArCO PPh3 PPh3 + Cl7 110 8C 7Ph3PO 515 + ArC C PPh3 516a ± c (56% ± 68%) R1C C I Ph TfO7 + R23 P 7C6H5I 520 R1C C PR23 TfO7 + 521 + O Ph Br7 H2O H+ + CHPPh3 N O Ph Br7 + CHPPh3 f PhCOCH2PPh3 Br7 N Ph Br7 + CHPPh3 H+, H2O e d c b a + CH2PPh3 PhN Ph Br7 + CHPPh3 Ph2P Ph Br7 + CHPPh3 PhS Ph Br7 + Ph C C PPh3 Br7 516a (a) PhSH; (b) Ph2PH (72%); (c) PhNH2 (80%); (d ) AcCH2COOEt, Et3N (72%); (e) (95%); (f) .HN CH PPh3 Ph Ac COOEt N O 850 S MLukyanov, A V Koblik, L A MuradyanA method for the synthesis of a new family of heterocyclic ylides 525 based on reactions of alkynylphosphonium salts with sodium azide has been developed.250 Phenylethynylpyridines 528 have been obtained by the reaction of the salt 516a with pyridine N-oxides.255 The use of alkynylphosphonium salts in the synthesis of heterocycles comprising the addition of nucleophiles to a triple C:C bond and subsequent cyclisation with the removal of the activating triphenylphosphine fragment has been demonstrated 35 in relation to reactions of the salt 516a.Numerous syntheses based on propynylphosphonium salts have been described in a number of studies 256 ± 258 and have been surveyed in a review.259 3. Alkynyliodonium ions Alkynyl derivatives of tricoordinated iodine have been known since 1965.The interest in the chemistry of tricoordinated iodine, which has sharply increased in recent years,260 has stimulated studies of its ethynyl derivatives. Phenyl(phenylethynyl)iodonium chloride 529 was synthesised in a yield of up to 20% by the reaction of lithium phenylacetylenide with l3-iodanylbenzene dichloride.The IR spectrum of this salt 261 contains a character- istic absorption band in the region of 2170 cm71. On storage for several hours at room temperature, the salt 529 spontaneously decomposes to give iodobenzene and chloro(phenyl)acetylene. Phenyliodanyl bis(trifluoroacetate) 531, capable of oxidising terminal triple bonds in compounds 530, proved to be a more efficient reagent.262 The reagent 531 was used for efficient transformation of alkynols and arylalkynols 532 into 1,3-dihydroxy 2-carbonyl derivatives 534 via alkynyliodonium intermediates 533.263 The first method, really suitable for the preparative-scale synthesis of alkynyliodonium salts, is based on the reaction of alkynes with hydroxy(tosyloxy)iodanylbenzene 535.Short-term refluxing of the reagents in CHCl3 gave alkenyl (536a ± e) and alkynyl (537d,f ± h) iodonium salts.264, 265 R Yield (%) R Yield (%) 536 537 536 537 Pr (a) 58 Bui (e) 29 33 Bu (b) 52 Bus (f) 50 n-C5H11 (c) 26 But (g) 74 Pri (d) 11 15 Ph(h ) 61 Alkylethynyliodonium salts 538 can be prepared only in those cases where the alkyl group in the alkyne (Pri, But ) creates steric hindrance hampering nucleophilic attack on the triple bond.266 This strategy was also employed to synthesise 4-alkoxyphenyl- ethynyliodonium salts 539 containing long alkyl chains.267 + + PhC CPPh3 Br7 516a Bu3P, MeCN PhC CHPBu3 2Br7 522 PBu3 + + + 523 PhC CPBu3 Br7 Ph3P, MeCN PhC CHPBu3 2Br7 524 PPh3 + Ar=Ph (a), 4-ClC6H4 (d). N O +516a CHCl3, 60 8C R + N O Ph PPh3Br7 R HO7 7HBr N + PPh3 O7 Ph 527 1807200 8C R + N PPh3Br7 COPh 526 R + + ArC C PPh3 X7+NaN3 DMF 7NaX 516a,d N N N PPh3 Ar 7 525 N C CPh 528 R 526 OH7 7HBr + N PPh3 O7 Ph R =4-Cl 1807200 8C 528 R N O Ph X=NH, S; Y =CH, N; Z=O, NH.O Ph NH2 516a, NaH, HMPA N Ph Ph + N N Ph Ph Y X Z Ph + Br7 Y XH ZH 516a Y X ZH CH Ph PPh3 + 7MePh3P Br7 R3=H, CF3CO. PhC CH BuLi PhC CLi PhICl2 075 8C PhC C 529 + I PhC CCl+PhI HOCH2CO COCH2OH Ph Cl7 R1 OH C CH 531 532 R1 OH C R2 C 533 + IPh H2O R1 OH R2 O OR3 534 R2 C C CH+ HC PhI(OCOCF3)2 530 531 CHCl3 C C C C 2CF3COO7 (64%) + I Ph + IPh H2O + + CH TsO R I Ph TsO7+RC C IPh TsO7 536a ± e 537d,f,h RC CH+Ph I OTs OH CHCl3 535 Alkynylcarbenium ions and related unsaturated cations 851The use of hydroxy(mesyloxy)-l3-iodanylbenzene 540 made it possible to produce alkynyliodonium mesylates 541.268 However, this synthetic approach has a substantial limitation: due to the high nucleophilicity of the counter-ion, the reaction affords alkenyliodonium salts of the type 536 as side products.This decreases the yield and purity of the target products. Therefore, a new approach has been developed; it includes interaction of readily accessible iodosylbenzene 542 with silylated alkynes 543 in the presence of an equimolar amount of BF3 .Et2O and subse- quent treatment of the reaction mixture with an aqueous solution of sodium arenesulfonate.269 In addition, two methods for the synthesis of alkynyliodonium salts 546 and 547 have been described; one of them is based on the use of iodonium salt 545, 270 and the other involves the use of alkynylstannanes.271 ± 273 The IR spectra of the alkynyliodonium salts 546 and 547 contain absorption bands at 2155 ± 2190 cm71 due to the acety- lenic fragment.X-Ray diffraction analysis of phenyl(phenylethy- nyl)iodonium tosylate 537h have demonstrated that the cation has an angular structure 268 and that Ph ±C:and C:C bond lengths are close to the typical values for the corresponding single and triple bonds.At the same time, the acetylenic fragment is sub- stantially activated towards efficient interaction with diverse nucleophiles. In recent years, alkynyliodonium salts have acquired large synthetic significance; numerous examples of the preparation and synthetic application of these compounds have been described in detail in recent reviews.274 ± 278 In conclusion, we would like to mention heteroanalogues of alkynylcarbenium ions containing heteroatoms in the unsaturated fragment.Cyanocarbenium ions of type 548 have scarcely been studied. The results of quantum-chemical calculations for these systems and study of solvolysis of a number of tertiary a-cyanoalkyl tosylates and related compounds have been surveyed in a review.17 Finally, 1-oxa-3-azabutatrienium cations 553 present consid- erable interest from the synthetic viewpoint.The chemistry of these species is being vigorously studied by Jochims et al.11, 279 ± 283 These cations are prepared by ionisation of a-chloroalkyl iso- cyanates 552, by treatment of silylated cyanuric acid 550 with chlorocarbenium ions 280, 281 or by the reaction of ketones with chlorocarbonyl isocyanate 551.12 Similarly to alkynylcarbenium ions, these cations add nucle- ophiles at the carbonyl carbon atom and the nitrogen atom, for example, in reactions with ketones.11 The salts 553 and their thio-analogues have been used to develop a number of procedures for the synthesis of various nitrogen- and oxygen-containing heterocyclic and polyfunctional compounds; these methods deserve generalisation in a separate review.This review was supported by the Russian Foundation for Basic Research (Project No. 97-03-33127). 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ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. The published data of the last decade concerning the mechanism of the Kabachnik ± Fields reaction and its significance for the chemistry of organophosphorus compounds as a method for the synthesis of a-amino phosphonates and their numerous functionally substituted derivatives and analogues, such as phos- phinates and phosphine oxides, are generalised and systematised. The review discusses the classical version of the Kabachnik ± Fields reaction, its modifications with the use of phosphorus chlorides, neutral esters and inorganic phosphorus acids, as well as chemical processes simulating separate steps of the reaction, viz., hydrophosphorylation of imines and amination of a-hydroxy phosphonates. Data on the practical application of a-amino phosphonates are presented. The bibliography includes 253 references.I. Introduction The Kabachnik ± Fields reaction corresponds to the classical methods for the synthesis of organophosphorus compounds. It was discovered in 1952 independently by Kabachnik and Medved'1 and Fields.2 The reaction occurs in a three-component system consisting of a hydrophosphoryl compound, a carbonyl compound (aldehyde or ketone), and an amine and results in a-aminoalkylphosphonates commonly named as a-amino phos- phonates (AP).During the subsequent two or three decades, no intense studies in this field were carried out. However, after the discovery in the late 1960's that AP possess practically useful properties, mainly biological activity (see Ref. 3 and references cited therein), and isolation of phosphorus-containing amino acids from natural sources,4 the areas of chemistry, biology and medicine related to the synthesis and study of the properties of AP show a real renaissance.As a confirmation it can be noted that more than 5000 studies on this topic had been published by 1993.5 By now, this number has increased greatly. The prospects of using AP in various areas of science and engineering stimulated the develop- ment of new ways for the synthesis of AP, including polyfunc- tional ones.However, in our opinion, none of numerous approaches to the synthesis of AP is as general as the Kabach- nik ± Fields reaction. However, no generally accepted viewpoint on the mechanism of this reaction has been developed until recently. The data concerning this matter were isolated and often contradictory.Only in recent years have kinetic and other special studies aimed at a deep study of the Kabachnik ± Fields reaction and the establish- ment of its mechanism been carried out. As a result, the contra- dictions were resolved to a considerable extent; however, the reviews generalising and analysing these studies are lacking.On the contrary, the synthetic aspects of the Kabachnik ± Fields reaction have been developed rather widely and discussed in monographs6 ±12 and reviews.13 ± 19 However, new and impor- tant results that deserve thorough consideration have also been obtained in this area during the last five years. The purpose of the present review is to systematise, analyse and generalise the data published basically in the 1990's and related to the study of the mechanism of the Kabachnik ± Fields reaction and its use for the synthesis of AP.Earlier publications will be cited only to the extent required for the understanding of the essence of the problems under discussion. In order to rank the Kabachnik ± Fields reaction, estimate its significance among numerous other methods for the synthesis of AP and reveal its synthetic potential, we considered it necessary to present data on the basic methods for the synthesis of AP.Due to the abundance of data in this field, the topic is inevitably covered superficially, and we confine ourselves to the analysis of only the latest publications. Whenever necessary we have to refer the reader to the literature cited in these papers.Chemical processes that simulate separate steps of this reac- tion and its versions and involve other derivatives of three- coordinate phosphorus, such as neutral esters, chlorides, inor- ganic acids and their silyl derivatives instead of hydrophosphoryl compounds, are discussed in more detail. We believe that the reactions of the silyl derivatives with carbonyl compounds and amines or amides in a three-component system may also be rightfully attributed to reactions of the Kabachnik ± Fields reac- tion type.As will be shown below, reactions of P(III) derivatives often produce hydrophosphoryl compounds that react exactly in the same way as those in the `classical' system. P(O)H+ C O+HN P(O) C N +H2O . R A Cherkasov and V I Galkin Kazan State University, ul.Kremlevskaya 18, 420008 Kazan, Russian Federation. Fax (7-843) 238 09 94. Tel. (7-843) 231 51 86. E-mail: rafael.cherkasov@ksu.ru (R A Cherkasov) Received 23 February 1998 Uspekhi Khimii 67 (10) 940 ± 968 (1998); translated by S S Veselyi UDC 547.230241 The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism { R A Cherkasov, V I Galkin Contents I.Introduction 857 II. Application areas and principal methods for the synthesis of a-amino phosphonates 858 III. The mechanism of the Kabachnik ± Fields reaction 864 IV. Synthetic potential of the Kabachnik ± Fields reaction 867 V. Conclusion 879 { The review is devoted to the memory of Academician M I Kabachnik (1908 ± 1997). Russian Chemical Reviews 67 (10) 857 ± 882 (1998) #1998 Russian Academy of Sciences and Turpion LtdSpecial attention is given to the hydrophosphorylation of imines.On the one hand, this represents one of the steps of the Kabachnik ± Fields reaction and, on the other hand, a version of the Pudovik reaction, which was described in 1952 20 almost simultaneously with the pioneer research by Kabachnik and Medved' (and published in the same volume of Doklady Chem- istry).In addition, we found it necessary to present brief informa- tion on the main areas where AP are used. II. Application areas and principal methods for the synthesis of a-amino phosphonates 1. Application areas of a-amino phosphonates It is believed 3 that aminomethylphosphonic acid was first men- tioned in the literature in 1943.21 Being structural analogues of a-aminocarboxylic acids 1, a-aminoalkylphosphonic acids 2 (APA) possess diverse biological activities.For this reason, they attract steady attention of chemists, biologists, pharmacologists, physicians and other specialists who study biologically active compounds.3, 11, 16, 18, 22, 23 Organophosphorus analogues of almost all proteinogenic amino acids have been synthesised. APA isostructural to tyrosine have been isolated from natural sources (see Ref. 24 and references cited therein). APA resulting from replacement of the carboxy groups in `normal' amino acids by the phosphonate groups inhibit the enzymes or receptors to which natural amino acids normally bind. Therefore, APA are their antagonists (see Refs 3, 25 and references cited therein).The inhibitory action of APA predeter- mines their physiological activity as antibacterials, neuroactive agents, antibiotics, carcinostatic, cytotoxic and other compounds of pharmacological importance.3 Much attention is given to the use of AP (the acids themselves, i.e., APA, their mono- and diesters and some other derivatives) in peptide synthesis.3, 26 ± 31 Phosphonopeptides represent models of the tetrahedral transition states (activated complexes) in the hydrolysis of natural peptides.32 ± 34 In addition, they exhibit diverse and rather high physiological activities, such as pesticide,35 fungicide 36 and growth-controlling 37 ones.Phosphonopeptides are inhibitors of thrombin 38 and pepsin 39 and are used as haptens of antigens affecting the production of antibodies.40 They also manifest other kinds of physiological activities.3 Numerous studies have been devoted to the synthesis and study of APA possessing herbicidal properties. Of these, N-(phos- phonomethyl)glycine, 7HO3PCH2N+H2CH2COOH is known under the trade name glyphosate and used most widely (see Ref. 41 and references cited therein). Its numerous analogues have also been obtained.42 A series of studies 43 ± 45 have dealt with the development of methods for the synthesis of a-aminofluorenylphosphonates, -phosphinates and -phosphine oxides.As structural analogues of a synthetic phytohormone morfactin, they represent an interesting class of plant growth regulators and herbicides. Wide prospects of practical application of this type of AP stimulated in-depth spectroscopic studies of their structures.45 Attempts at establish- ment of quantitative structure±biological activity relationships were also made.46 It should be noted that the relation between the structure of a number of neutral AP and their antimicrobial 47 and fungicide 48 activities is well described by correlation equations. Although the biological activity is certainly a very valuable and the most attractive property of AP, this by no means confines the prospects of their practical use.APA themselves, as well as their derivatives and analogues, correspond to polyfunctional organophosphorus compounds. They can form complexes, in which they act as mono-, bi- and polydentate ligands.49 The electron-donating atoms of nitrogen and phosphoryl oxygen serve as coordination centres, the acid groups of APA form the corresponding salts, and the suitable functional groups specially introduced to nitrogen and phosphorus atoms and a-carbon atoms can provide additional bonding to metal ions.The same properties are used as a stereocontrolling factor in enantioselec- tive synthesis of AP (see below). Kabachnik was one of the first to study the complexing properties of AP,50 ± 52 using linear and cyclic (cyclopendant) AP as the complexones.Recently, the first X-ray characteristics of an AP metallocomplex, viz., a coordination compound of Cu(II) with O-ethyl hydrogen a-(2-pyridyl)-N-isopropylaminomethylphos- phonate, have been obtained.53 Complexes of chromium with fluorenylphosphonates have been synthesised.54 In recent years, complexes of AP with lanthanides and actinides have been proposed for diagnostics with the use of theNMRtechnique.55, 56 Extraction processes involving AP have been studied.It was shown that APA can be used in hydrometallurgy.57 Processes for gold extraction with neutral AP and efficient separation of Au(III) from the commonly accompanying ions, Fe(III) and Cu(II),58 as well as extraction of Pd(II) in strongly acidic media have been reported.59 The acid ± base properties ofO,O-dialkyl N,N-dialkyl- aminoalkylphosphonates have been studied.As expected, they appeared to be weaker bases than amino acids. The pKBH values (in aqueous propan-2-ol) are 9.2 ± 9.7, whereas pKBH of histamine is 5. Substituents at the a-carbon atom and nitrogen do not affect essentially the AP basicity.Due to the ability of AP to complexation, they can serve as carriers for a-hydroxy and a-amino acids through lipophilic liquid membranes,60 which is a new promising direction for their utilisation. N-Bornyl-AP,61 (d)-N-a-methylbenzyl-AP and other linear and cyclic AP,62, 63 as well as calix[4]arenes with a-amino phosphonate fragments in the bottom and top rings were chosen for molecular recognition of substrates.64 Generally, the latter derivatives appeared to be more efficient transport agents than their acyclic analogues.The flux of a-hydroxy acids is most strongly affected by the nature of substituents at the a-carbon atom in AP. It is remarkable that the AP studied can also be used for the separation of D- and L-forms of a-hydroxy and a-amino acids.60 The results of these studies are in good agreement with the data obtained in a study of the thermochemistry of AP.65 ± 67 Specific features of dissolution of neutral AP in solvents of various nature (hexane, CCl4, CHCl3) characterise these compounds as distinct proton acceptors. However, with respect to sufficiently strong bases, for example, pyridine, AP behave as proton donors.Calculations of the enthalpy of solvation showed the predominant effect of steric factors of the substituents on the liability to biphilic reactions of this kind.68 Finally, it is worthy of mentioning that analytical methods for the determination of enzyme inhibitors, including APA esters, have been developed.69 2.Principal methods for the synthesis of a-amino phosphonates No simple and versatile methods for the synthesis of AP existed before the discovery of the Kabachnik ± Fields reaction. Single attempts at the preparation of AP mentioned in the early studies 1 ± 3, 70, 71 did not satisfy the researchers because of multi- step syntheses and low efficiency. The credit for the intense progress in studies on the chemistry and biology of AP in the last decade should be given to the development of a series of highly efficient one-pot or two- or three-step methods for the synthesis of AP.The Kabachnik ± Fields reaction occupies the leading posi- tion among these methods. It is now believed that the Kabach- nik ± Fields reaction and its constituting separate steps are the most convenient methods for the synthesis of AP, including homochiral ones (see Section IV).The basic problem in the synthesis of AP is the formation of the aminoalkylphosphonate fragment, P(O)7C7N. Therefore, H2N C OH R O 1 H2N P OH R O OH 2 858 R A Cherkasov, V I GalkinScheme 1 the synthetic routes shown in Scheme 1 are of prime interest. However, prior to the discussion of the synthetic versions pre- sented in the scheme, it should be mentioned that the stereo- chemical aspects of these reactions are very important.As in case of a-amino acids, the biological activity of AP is largely determined by the absolute configuration of the stereo- genic a-carbon atom. Of the four possible diastereomers of the antibiotic alaphospholine 3, the (S,R)-diastereomer manifests the greatest activity against pathogenic microorganisms. The three other stereoisomers are much inferior to this compound in activity (see Ref. 72 and references cited therein). The first synthesis of an optically active APA was carried out in 1972.73 The most comprehensive information on the methods for the stereoselective synthesis of AP is presented in the review by Kukhar' et al.18 containing the data published before the middle of 1992.Since then, a large number of publications describing new ways for the enantioselective synthesis of AP have appeared; the main attention in the present review is given to these methods. The strategy for the synthesis of AP that envisages the formation of the amino phosphonate framework, P(O)7C7N, is based mainly on the use of the Kabachnik ± Fields reaction in a three-component system `phosphite ± carbonyl compound ± amine' and its various versions [Scheme 1, pathway (a)].As will be shown below, the Pudovik reaction, i.e., catalysed or non- catalysed addition of hydrophosphoryl compounds to imines [pathway (b)], as well as nucleophilic amination of hydroxy phosphonates [pathway (c)], represent separate steps of the two- (or more)-step Kabachnik ± Fields reaction. We shall consider these three most important methods for the synthesis of AP separately in Section III.In the present section, we pay the main attention to the strategy designated as pathway (d ) in Scheme 1 (C-, N-, P- modification). Numerous methods for the synthesis of AP based on this approach assume that a preliminarily created combination of phosphoryl, a-carbon and amine fragments is used as the key synthon.The synthesis of the target AP, including peptides, homochiral APA, etc. is carried out by introduction of the desired functional groups to the a-carbon atom (C-modification), to the nitrogen atom (N-modification) and to the phosphorus atom (P-modification). It should be noted that this strategy is being developed intensely in recent years.Special attention has been paid to the stereocontrolled functionalisation of AP at all the reaction centres specified above. Data published before the middle of 1992 have been systematised in a review (Ref. 18). Here we present only the studies that have not been mentioned there and in subsequent publications.a. C-Modification It is known that the carbon atom bonded with the phosphoryl group, which has considerable electron-withdrawing properties, possesses a pronounced anionoid character. This circumstance is widely used in organic and organometallic synthesis, for example, for functionalisation of the a-carbon centre or for olefination of carbonyl substrates by the Horner ± Wittig ± Emmons reaction.74 Generation of carbanions from AP and their subsequent C-func- tionalisation is a popular approach for the synthesis of diverse polyfunctional AP by reactions of a-phosphoryl amino carban- ions with appropriate electrophilic reagents.In these processes, the following starting compounds are used: APA or their esters containing the P(O)CH(R)N moiety with a terminal primary, secondary or tertiary amino group or imines incorporating the C=N7CH(R)P(O) fragment.The latter are usually obtained by the reaction of AP having a primary amino group with aldehydes or ketones.75 Other procedures, e.g., the reaction of a-oxo phosphonates with primary amines 76 or the reaction of dialkyl phosphites with hexahydrotriazine, have also been reported.77 C-Alkylation of acyclic AP [Scheme 2, pathway (a)] is used rather seldom, probably because of their rather low CH-acidity. The a-phosphoryl carbanion is probably insufficiently stable due to the presence of an electron-withdrawing phosphoryl group and an electron-donating amino group.Nevertheless, easy introduction of a methyl, phenylthio, and other heteroatomic groups to the a-carbon atom of AP 4 (THF, 770 8C) by treating phosphoryl carbanions with appropriate halogen-containing reagents of the RX type has been described recently.78 However, in the series of heterophosphacyclanes containing endo- and exocyclic nitrogen atoms, the method for the synthesis of AP discussed above [Scheme 2, pathway (a)] is much more popular.A scheme for the synthesis of APA by alkylation of a five-membered cyclic ephedrine derivative 5 can be given as an example.79 P H O R1 R2 + C O H + N N P C O R1 R2 P H O R1 R2 + N C OH + H N P C O R1 R2 a 7H2O b c d R1, R2=Alk, Ar, AlkO, ArO, H, OH, OSiR3 ; (a ± d )�see the text.P C O N R1 R2 P OH OH O N H2N H O (4S,1R)-3 (EtO)2P N O 1. LDA 2. RX (EtO)2P N O R LDA=LiNPri 2 ; 4 Ph Ph R=SiMe3 (79%), SnBu3 (52%), SPh (60%), Me (70%).H2N P O 1. B 2. R2X a b C O 1. B 2. R1X H2N P O R1 1. B 2. R2X H2N P O R1 R2 N P O C 1. B 2. R1X N P O C R1 N P O C R1 R2 H+, H2O 7 C O H2N P O R1 R2 Scheme 2 The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 859Homochiral 2-benzoylaminomethyl-3,4-dimethyl-2-oxo-5- phenyl-1,3,2-oxazaphospholanes 5a and 5b (`chiral phosphorus'), epimeric at position 2, were separated by column chromatography and subjected to C-alkylation under mild conditions to give enantiomers 6a and 6b.Subsequent hydrolysis of compounds 6a and 6b resulted in the APA (S)-7 and (R)-7, respectively. Later,80 this strategy was used for the synthesis of APA 7 from enantio- meric oxazaphospholanes with the exocyclic chloromethyl group.In recent years, functionalisation of methylphosphonates has been carried out according to the electrophilic amination scheme (see Ref. 72 and references cited therein). Strictly speaking, these processes do not conform to the strategy of C-alkylation of AP [Scheme 2, pathway (a)], as the amino group is not `yet' present at the a-carbon atom of the starting phosphonates.However, since we are speaking of reactions of a-phosphoryl carbanions, we considered it appropriate to mention here this method for the synthesis of AP. The reaction of oxazaphospholanes 8 with PriLi and then with azodicarboxylate results in products of the addition to the N=N bond. Their subsequent hydrolysis gives the APA 7.81 The same reaction was carried out with bicyclic phosphon- amide 9.82 In the reaction with oxazaphosphorinane, aryl azide serves as the aminating reagent.83 In all cases, high diastereoselectivity of amination and high chemical yield of APA 7 were achieved (see also Ref. 18). Successful use of carbanions stabilised by 1,3,2-dihetero- phosphacyclane substituents in the C-modification is, probably, largely caused by the rather high CH-acidity and, accordingly, high stability of these systems.It is known that incorporation of a four-coordinate phosphorus atom in a ring increases its electron- withdrawing properties.74 This is particularly pronounced in the chemical behaviour of 1,3,2-diheterophospholanes.84 A similar effect is achieved in the case of iminomethylphos- phonates [Scheme 2, pathway (b)]. Charge delocalisation by two acceptor substituents, viz., the imino and phosphoryl groups, lowers the energy of a carbanion.Enantioselective syntheses of AP by C-modification, i.e., alkylation of an a-imino carbanion with alkyl halides, carbonyl compounds and unsaturated electrophilic reagents, have been performed successfully with camphor derivatives 10 and a number of similar structures.+ N P O Me Ph Me O NHCOPh 5b (26%) Ph N PCl2 H O O N P O Ph O NHCOPh 5a (30%) Me Me a N P O Me Ph O NHCOPh R 6a (67% ± 82%, 73%± 92% de) Me d, e H R H2N P(OH)2 O (S )-7 (89% ± 92%) 5a b, c N P O Me Ph Me O NHCOPh R 6b (67% ± 82%, 58%± 84% de) R H H2N P(OH)2 O (R)-7 (89% ± 92%) 5b b, c d, e (a) Ephedrine, NEt3, THF; (b) BuLi, THF, 770 8C; (c) RX,770 8C; (d) conc. HCl, 110 8C, 20 h; (e) O Me ; de is diastereomeric excess. Pht is phthaloyl.N P O Ph Me O Cl R N P O Ph Me O Cl R0 R 1. 2BuLi 2. R0X PhtNK, KI NH2 H R0 (HO)2P O 7 HCl N P O Ph Me O R0 NPht R NH2NH2 NPht H R0 (HO)2P O O P N O R2 CH2R1 8 1. PriLi 2. (NCOOR3)2 O P N O R2 CH N NH COOR3 COOR3 R1 (HO)2P O C(R1)HNH2 7 N P N Me Me O Me 9 (HO)2P Me H NH2 O 7a 1. PriNLi, THF,7100 8C 2. ButOOCN=NCOOBut 3. CF3COOH (a) BuLi; (b) 2,4,6-Pri 3C6H2SO2N3; (c) Ac2O.N P O R2 R1 Ph O R3 a, b, c NR3 P O R2 R1 Ph O N Ac N NSO2C6H2Pri 3-2,4,6 Al/Hg, EtOH P Ph NH2 O HO HO 7b H+, H2O N P O R2 R1 Ph O R3 NHAc H+, H2O N P(OR)2 O R0 1. BuLi, THF 2. R0X 10 N P(OR)2 O 860 R A Cherkasov, V I GalkinIt was shown recently that high diastereomeric excess (de) in C-modification can be achieved due to the chelation effect 85 provided by the introduction of suitable functional groups capa- ble of forming intramolecular hydrogen bonds, as, e.g., in structure 11.The carbanions generated from phosphorus-containing Schiff's bases 10 (R=Et) enter into the Michael reaction.86 Adducts 12 undergo cyclisation in acidic medium to 5-oxo-2- pyrrolidinylphosphonate subsequent reduction of which results in diethyl 2-pyrrolidinylphosphonate.Secondary processes occurring after the electrophilic step of C-modification of AP expand the synthetic potential of phos- phorylated imines. More often, however, the alkylation product is subjected to acid hydrolysis in order to obtain APA.87 It has to be noted that the carbonyl compounds from which imine synthons are obtained can be regenerated by acid hydrolysis of phosphorylimines.18 This strategy for the synthesis of AP has been expanded88 for isocyanomethylphosphonate 13.(1-Aminocyclopropyl)phosphonates 16 were obtained by alkylation of the phosphonate 13-derived carbanion with epox- ides to give the hydroxy phosphonate 14. The mesylate 15 obtained from the latter undergoes cyclisation in the presence of bases to give a cyclopropane intermediate subsequent methanol- ysis of which in acidic medium results in the target product 16.Other examples of similar reactions are described in Ref. 18. b. N-Modification This type of AP reactions is responsible for their inhibitory effect with respect to enzymes and enables the involvement of APA and their esters in peptide synthesis. These processes were analysed in the reviews cited above,3, 16, 34, 38, 39 and their examination is beyond the scope of our review; we shall describe only some of the results obtained in this field lately.Fluorenylmethyl chloroformate was used as an acylating agent in the synthesis of a-(9-fluorenylmethoxycarbonyl- amino)alkylphosphinates.89 a-Aryl-(N-benzyl)aminomethylphosphonates react with b-tri- phenylgermylpropionic acid to afford germylated AP derivatives.90 Germylated AP manifest high antitumr activity that sur- passes the efficiency of nongermylated AP derivatives in assays with sarcoma-180.90 Acylation of AP containing a primary amino group with asymmetrical anhydride 17 results in a mixture of both possible N-acylated products.91 H2N P(OR)2 .O R0 7 7 N N O H O H O P OEt OEt E+ 11 AcOH, H2O 10 1.BuLi 2. R1CH=CR2CO2Me NCHP(OEt)2 O CHR1 HC(R2)CO2Me 12 N R1 P(OEt)2 R2 H O LiBH4, BF3 . OEt2 N O R1 P(OEt)2 R2 H O 7 Ph Ph N P(OBut)2 O E+ Ph Ph N P(OBut)2 O E 1. HCl 2. O H2N P(OH)2 O E CN P(OEt)2 O 13 a, b HO P(OEt)2 R1 R2 NC O c 14 MsO P(OEt)2 R1 R2 NC O d 15 (c) MsCl, Et3N; (d) ButCH2OK; (e) HCl, MeOH. (a) BuLi or ButCH2OK; (b) O R1 R2, BF3 .OEt2; R1 P(OEt)2 H NC R2 H O e R1 P(OEt)2 H NH2 R2 H O 16 Fmoc = CH2OCO . 4N NaOH, dioxane FmocN P R O H OH H + FmocCl H2N P R O H OH 7 O Ph3GeCH2CH2C OH + PhCH2 NH CH P(OR)2 C6H4X-p O THF DCC, HOBt Ph3GeCH2CH2C N CH P(OR)2 C6H4X-p O CH2Ph 50 ±79% O DCC is dicyclohexylcarbodiimide; HOBt= N N N OH . XNHCHCNH R2 O C P(OR5)2+ R4 R3 O R1OCNH O C P(OR5)2 R4 R3 O XNHCHCOCOR1+ R2 O O 17 H2N C P(OR5)2 R4 R3 O The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 861The readily occurring reaction of APA with ethyl orthofor- mate 92 results in a mixture of (N-formyl)amino- and (N-ethoxy- methylene)imino derivatives of alkylphosphonates.The well-known method to increase the nucleophilicity of the nitrogen atom by introduction of a trialkylsilyl group (`silyl activation') was successfully used in peptide synthesis in the N-acylation of APA 7b with benzyl chloroformate.93 It should be noted that this synthetic technique has also been used in the synthesis of phosphorus analogues of glycine contain- ing the dimethylphosphine oxide group.94 N-Silylated AP add to acetone, benzaldehyde and p-bromo- benzaldehyde at room temperature.95 Distillation of (N-methyl-N-silyloxyalkyl)phenylmethylphos- phonates results in their dissociation to the original AP and carbonyl compounds.Nucleophilic reactions involving the nitrogen atom of AP are not limited to the substitution processes. Other reactions charac- teristic of primary, secondary and tertiary amines are also typical of AP.Among these, reactions of quaternisation and especially reactions of addition to unsaturated compounds are well studied. An unusual sigmatropic [2.3]-rearrangement of N-allyl- ammoniomethyl phosphonates occurs in the presence of bases.78 In this case, quaternisation of the tertiary amine centre is followed by the Cope rearrangement into the corresponding C-substituted AP, viz., 1-aminobut-3-enylphosphonate.Of addition reactions, those of AP with aryl isocyanates have been studied in detail.96 ± 100 Comparative study of the kinetics of addition of amines and their a-phosphorylated analogues, i.e., AP, to phenyl isocyanate revealed similarity of the addition mechanisms. In both cases, a concerted mechanism of addition is postulated with a four-centred transition state in which the C7N bond is formed in the activated complex somewhat earlier than the N7H bond.96 ± 98 The small activation enthalpies and the high negative activa- tion entropies are in good agreement with this conclusion.As was to be expected, AP are appreciably less active in this reaction than their aliphatic analogues, i.e., dialkylamines. This is due to a decrease in the nucleophilicity of the amino group in the AP molecule due to the electron-withdrawing effect of the phosphoryl group.In a series of phosphorylated nucleophiles, good agree- ment between their reactivities and steric effects of the substituents at the phosphorus atom is observed, as follows from high correlation factors of the corresponding equations.96 Reactions ofAP with perfluoro-N-methylformimine probably occur according to the addition ± elimination scheme.101 The synthetic result of this reaction, which occurs under very mild conditions, depends on the degree of substitution at the a-amino group. Primary a-phosphorylalkylamines are transformed to the corresponding a-(N0-trifluoromethylcarbodiimido)alkylphos- phonates.Secondary amines react with the elimination of one molecule of hydrogen fluoride and form a-(perfluoro-N-methylform- amidino)alkylphosphonates under the same conditions.c. P-Modification In the chemistry of AP, this type of reactions is normally represented by saponification and esterification of phosphorus acid derivatives. Though these reactions play an important role in the synthesis ofAPAand their esters, they are of no special interest in the context of this review.Of recent studies, we shall mention only Ref. 102. Selective mono-esterification of APA occurs with sterically hindered alcohols in the presence of bromo[tris(dimethyl-amino)]- phosphonium hexafluorophosphate (BroP) or N,N,N0,N0-bis(te- tramethylene)chloronium tetrafluoroborate (TPyCIU). H2N P(OH)2 R HC(OEt)3 O HOCNH P(OEt)2 R O + EtO N P(OEt)2 R O H2N PO3H2 Ph 7b Me3SiCl, Et3N Me3SiNH P(OSiMe3)2 Ph O 1. PhCH2OCOCl 2.H2O PhCH2OCNH PO3H2 Ph O Me SiMe3 +RR0C O (EtO)2PCHN O Ph 20 8C D (EtO)2PCH O Ph N CRR0 OSiMe3 Me (PriO)2P O CH2NEt2 R2 R3 R1 BrCH2 NEt2 Br7 R2 R1 R3 CH2 (PriO)2P O + 7HB+,7Br7 B NEt2 O (PriO)2P CH R3 R2 R1 NEt2 R2 R1 R3 O (PriO)2P CH + 7 R1=Alk, Ar; R2=H, Alk, Ar, CH2P(O)(OR4)2; R3=Ar.R3HNC O NR1R2 HNR1R2+R3N C O R3 N C O H N R1 R2 = (RO)2P C(R0) O Me NH2+CF2 NCF3 2KF,730 to710 8C 72HF.KF (RO)2P C(R0) O Me N C NCF3 (R3O)2P C(R0) O Me NHR2 +CF2 NCF3 KF 7HF.KF (R3O)2P C(R0) O Me NR2CF NCF3 NH P O OR OH Z Ph NH P O OH OH Z Ph +ROH BroP or TPyCIU 862 R A Cherkasov, V I Galkind. Other methods The simplest and rather common method for the synthesis of AP is reduction of nitrogen-containing organophosphorus compounds that already have a phosphonate framework with the a-nitrogen atom under the action of various hydrogenating agents. For example, a-nitro phosphonates can easily be transformed to AP even at room temperature in almost quantitative yields.103 Reduction of hydroxyiminophosphonate with sodium boro- hydride occurs in the presence of MoO3 or NiCl2 (see Ref. 104). Reductive amination of a-oxo phosphonates in the presence of triacetoxyborohydride gives the corresponding AP in moderate yields.76 This reaction occurs in two steps: Schiff's bases are formed in the first step and reduced in the second step. Other reactions of unsaturated organophosphorus com- pounds containing, or devoid of, a potential amino group were used in the synthesis of AP.Ethyl (diethoxyphosphoryl)diazoace- tate is obviously inserted as a carbene into the N7H bond of amines and amides.105 The reaction takes place under drastic conditions, viz., reflux- ing in toluene in the presence of palladium salts. Diethyl azidoalkylphosphonate undergoes 1,3-dipolar cyclo- addition with alkynes to give 4,5-disubstituted 1-(1-diethoxy- phosphorylalkyl)-1,2,3-triazoles in high yields (85% ± 92%).106 Reactions of unsaturated organophosphorus compounds with nitrogen-containing 1,3-dipoles are well studied and are discussed in a review.107 Vinylphosphonates are attractive synthons when choosing a strategy for the synthesis of AP.Recently,108 a convenient method to involve them in the reaction with N-tosyliminophenyl-l3- iodane in the presence of Cu(II) or Cu(I) triflate has been described.Catalytic hydrogenation of the aziridine intermediate occurs regiospecifically to give the AP 18. The solvent used in this reaction affects essentially the rate of the process. For example, a 70%± 80% yield of compound 18 is reached in 10 min in acetonitrile, in 1 ± 3 h in CH2Cl2, and in 76 ± 90 h in benzene.Enantioselective synthesis of AP from vinyl-phosphonates and -phosphinates was also carried out with the use of nickel com- plexes of natural amino acids. As a result, the latter undergo condensation at the terminal carbon atom of the phosphorus reagent.18, 109 Polyfunctional AP can be obtained by functionalisation involving other unsaturated fragments as well. For example, bisphosphonates 19 were synthesised 110 which represent the Diels ± Alder adducts of maleic anhydride with furan derivatives of AP.Yet another promising strategy for the synthesis of AP has found quite a limited use so far. This is phosphorylation of unsaturated compounds containing the nitrogen atom at the a-position to the double bond, for example, enamines and similar compounds.The so-called electrophilic version of the Pudovik reaction makes an exception. It is known10 that hydrophosphorylation of enamines under classical conditions of the Pudovik reaction, which is base- catalysed addition, could not be carried out until the late 1980's. Asimple one-step method for the synthesis of AP was created only after the discovery of the electrophilic version of the Pudovik reaction, which occurs with vinyl ethers, enamines and other electron-excess alkenes and five-membered cyclic alkylene- phosphorous acids characterised by an increased proton-donor ability.111 ± 115 Synthetic, kinetic and thermochemical studies 8, 111 ± 115 of this reaction together made it possible to establish its mechanism.It consists of pre-equilibrium transfer of a proton from the phos- phorous acid to the enamine and addition of the cyclophosphite anion to the a-carbon atom.As is frequently observed in electro- philic addition of proton-containing reagents, the addition occurs according to the Markovnikov rule. However, this method was found to be unsuitable for the hydrophosphorylation of amino- butenones.116 Enamino ketones with N-methyl and N-phenyl groups react without participation of the carbon ± carbon double bond.Dibutylphosphine oxide adds to the ketone carbonyl group, (R0O)2P R NO2 O LiBH4, Me3SiCl (R0O)2P R NH2 O R P(OMe)2 O O 1. H2NCHPh2 2. NaBH(OAc)3 R P(OMe)2 HNCHPh2 O (30% ± 60%) EtOC P(OEt)2 O O N2 +H2NR 7N2 EtOC P(OEt)2 O O HNR R=Alk, Ar, Ac, COOBut etc. N N N R2 R3 (EtO)2P R1 O + N N N R3 R2 P(OEt)2 R1 O (EtO)2P CH(R1)N3 O +R2 C C R3 (EtO)2P Ar O PhI NTs, 20 8C [Cu] (EtO)2P Ar O NHTs 18 H2, Pd/C, MeOH HCO2NH4, 24 h N (EtO)2P Ar O Ts O (EtO)2(O)P P(O)(OEt)2 CH NH X NH CH O O O O O O O O O P(OEt)2 CH NH X NH CH O O O O (EtO)2P O 19 PH O + C C N P O N C CH R1 C CHR2 NR32 + P(O)H R4O R5O P R4O R5O O C(R1)CH2R2 NR32 R1=Alk, Ar; R2=H, COOEt, COMe etc.; R32 =(CH2)5, CH2CH2OCH2CH2, Me2; R4, R5=CH(Me)CH(Me), CH(Me)CH2, CMe2CMe2, CH2CMe2CH2 , o-C6H4; R4=R5=CHF2CF2CH2 , ClCH2CH2 .The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 863and the second phosphine oxide molecule replaces the amino group to give the diphosphorylated unsaturated alcohol 20. It is rarely possible to employ the classical nucleophilic Pudovik reaction for the synthesis of AP.117 For example, the addition of diethyl phosphite to N-acetylaminomethylidene- malonic ester gave the AP 21, which served as the starting reagent in the synthesis of phosphonohistidine 22.This reaction was successful due to the presence of three strongly electron-withdrawing substituents in the unsaturated electrophilic reagent, viz., two ethoxycarbonyl groups and an acylamino group.Finally, we shall note the unusual synthesis of AP through 1,2-phosphorylotropic migration in a series of N-acylated deriv- atives of a-aminoalkyl compounds of three-coordinate phosphorus.118 The a-amino alcohol formed in the first step of the reaction esterifies the P(III) halide to give the P(III) alkoxy derivative, which then undergoes isomerisation to the corresponding AP.The material presented in this section shows that essential progress in the creation of new types of AP has been achieved during the search for practically useful compounds. The ever- increasing complication of the AP structures demands a wider use of diverse approaches for the functionalisation of the starting substances. However, the analysis of the results obtained shows that the main problem in AP chemistry, viz., the formation of the a-alkylaminophosphinyl fragment P(O)7CR27N, can be best solved using the Kabachnik ± Fields reaction. This explains, first of all, the keen interest of researchers in the studies of various aspects of the Kabachnik ± Fields reaction in the last decade. This primarily corresponds to the investigations into the reaction mechanism, as the desired synthetic result can only be achieved on the basis of in-depth understanding of the factors governing this complex process.III. The mechanism of the Kabachnik ± Fields reaction As noted above, the Kabachnik ± Fields reaction, i.e., a three- component reaction of hydrophosphoryl compounds with ammo- nia, amines or other NH-compounds and aldehydes or ketones, is a unique method for the formation of the AP framework [N7C7P(O)].However, a common viewpoint on the mechanism of this reaction has not been worked out until recently, as the conclusions were based mainly on synthetic studies. Spectral and kinetic data were obtained later. The main problem in the understanding of the mechanism of the Kabachnik ± Fields reaction is the determination of the sequence of separate steps of this process.In their early studies, Kabachnik and Medved' believed that it was similar to the Rodionov reaction employed for the synthesis of b-amino acids or the Zelinsky reaction, i.e., the formation of amino nitriles upon treatment of aldehydes with ammonium cyanide (see Refs 1, 119 ± 121 and references cited therein).Studies of the reaction of hydrophosphoryl compounds with aldehydes 1 and ketones 119 made it possible to suggest a scheme of aminoalkylation of hydrogen phosphites and phosphonites,120, 121 which involves the addition of a hydrophosphoryl compound to the C=O bond with subsequent replacement of the hydroxy group of the a-hydroxy phosphonate by the amino group.Fields 2 used primary and secondary amines instead of ammo- nia in a three-component mixture containing dialkyl phosphites and carbonyl compounds. For example, exothermic reaction of diethyl phosphite with diethylamine and formaldehyde givesN,N- diethylaminomethylphosphonate in 94% yield. Fields 2 believed that the reaction occurred as the Mannich reaction, viz., the reaction of an amine with an aldehyde first produces an a-amino alcohol, which reacts with a hydrophosphoryl compound to give finally an AP.In the opinion of Fields,2 the reaction with primary amines might also occur through intermediate imines. An imine formed in the first step, i.e., the product of dehydration of an a-amino alcohol, readily added a hydrophosphoryl compound. Thus, in the studies of the Kabachnik ± Fields reaction,2 the important assumption has been made that aminoalkylation of hydrophosphoryl compounds could occur through a step of formation of an imine and its subsequent involvement in the Pudovik reaction. The experimental fact that the methoxy group in methoxyme- thylphosphonate is not exchanged for the amino group on heating with ammonia provides evidence in support of the `imine' path- way of the Kabachnik ± Fields reaction.122 In addition, hydroxy phosphonates, i.e., addition products of dialkyl phosphites to carbonyl compounds, do not react with ammonia under the mild reaction conditions used by Fields.2 The hydroxy group can be replaced by the amino group only in the presence of sodium alkoxide.It is believed to cause the decomposition of a hydroxy phosphonate into the original compounds, viz., hydrogen phos- phite and a carbonyl compound.122 The process subsequently goes through a step of formation of an imine, which adds a hydro- phosphoryl compound according to the Pudovik reaction scheme.R=Me (1%), Ph (28%). MeCCH O CHNHR+Bu2PHO 7RNH2 Bu2P C(Me)CH CH O OH 20 PBu2 O AcNH (EtO)2PHO EtONa, EtOH, 60 8C CO2Et CO2Et N N R AcNH (EtO)2(O)P 22 AcNH (EtO)2(O)P 21 CO2Et CO2Et R1=CCl3, H(CF2)4 , H(CF2)6; R2=PhCO, P(O)(OEt)2 , P(O)Ph2; R3=R4=AlkO, Alk2N, Ar.R1CHNHR2 PR3R4 O R1CHNHR2 OPR3R4 1,2-migration of P R1CHO R2NH2 R1CHOH NHR2 R3R4PCl RCHO +NH3+CH2(COOH)2 RCH(NH2)CH2COOH +H2O+CO2 , RCHO +HCN RCH(CN)OH NH3 RCH(NH2)CN+H2O NH3 P O C NH2 +H2O PHO+ C O P O C OH NH+ C O N CH OH PHO 7H2O N CH P O C NH OH NH C P O 7H2O C N PHO 864 R A Cherkasov, V I GalkinThe reversibility of the formation of hydroxy phosphonates received a convincing experimental confirmation in the studies by Gancarz et al.5, 123 ± 125 It was found that hydroxy phosphonates obtained from aliphatic and aromatic aldehydes and ketones decompose into the original hydrophosphoryl and carbonyl com- pounds under certain conditions in the presence of primary or tertiary amines (butylamine, triethylamine), as well as amines in the presence or in the absence of proton donors (ethanol). Hence, the formation of hydroxy phosphonates under conditions of the Kabachnik ± Fields reaction does not necessarily imply that the formation of the ultimate AP should definitely occur according to the `hydroxy phosphonate' pathway.As shown above, the phos- phite and carbonyl compound formed upon decomposition of hydroxy phosphonates can give the final AP according to the `imine' pathway, viz., through condensation of a carbonyl com- pound and an amine into an imine and its subsequent hydro- phosphorylation. At first glance, this viewpoint does not explain the participa- tion of secondary amines in the Kabachnik ± Fields reaction.However, according to the data obtained by Petrov et al.,122 in this case either the addition of hydrophosphoryl compounds to iminium salts or the Mannich reaction occur. The possibility of the latter pathway is confirmed by the formation of AP in the reaction of dialkyl phosphites with N,N,N0,N0-tetra- ethylmethylenediamine or with ethoxymethyldiethylamine.On the other hand, it was found123±125 that hydroxy phospho- nates are not only formed reversibly from aromatic ketones under conditions of the Kabachnik ± Fields reaction but also are irrever- sibly converted to phosphates according to the well-known phosphonate ± phosphate rearrangement scheme. The fastest step of the reaction is the formation of a hydroxy phosphonate.Its rearrangement to a phosphate occurs six times faster than the decomposition to the original compounds. It is necessary to emphasise that both Kabachnik with Medved' and Fields have made their conclusions on the sequence of the reaction steps without carrying out special mechanistic studies. Therefore, they did not contrast the two possible routes of the aminoalkylation of hydrophosphoryl compounds, viz., the `hydroxy phosphonate' and `imine' pathways.The experimental data reported in their and other studies cited above2, 119 ± 121 as well as in some later publications 122, 126, 127 allow one to assume that, depending on the nature of the reactants (hydrophosphoryl and carbonyl compounds and amines), the reaction mechanism and the sequence of steps can vary.An assumption was made128 ± 130 that the formation of AP via hydroxy phosphonates occurs with amines whose basicity is sufficiently high (pKa>6). Otherwise (pKa<6), the `imine' mech- anism operates. This conclusion is based on the shift of signals in the 31P NMR spectrum of diethyl phosphite in its mixtures with amines.The d 31P value is the higher the more basic the amine. Furthermore, it was considered 129, 130 that the increase in the yield ofAP in the Kabachnik ± Fields reaction with weakly basic amines upon addition of crown ethers confirmed the scheme discussed above. The idea that the mechanism of the Kabachnik ± Fields reaction displays certain dualism seems quite reasonable.How- ever, not all of the statements given in Ref. 129 appear to be convincing, and some of the postulates seem unlikely. Thus the suggestion that weakly basic amines can protonate the phosphoryl oxygen atom of a phosphite and the amide anion thus formed adds to a carbonyl compound to form methylolamine and/or imine 129 is wrong, in our opinion, as the amines are too weak acids to protonate the weakly basic oxygen atom of the P=O group.A confirmation that the Kabachnik ± Fields reaction can follow both of the pathways discussed here, viz., `hydroxy phosphonate' and `imine', was obtained 131 ± 139 by the authors of the present review in a complex study including kinetic, spectro- scopic and preparative methods. We studied three-component systems containing a hydro- phosphoryl compound, a carbonyl compound, and an amine, in which the nature of each reagent and the reaction conditions (solvent, additives of various types) were varied widely.We also studied the possibilities of bimolecular processes in the following pairs: hydrophosphoryl compound ± carbonyl compound, hydro- phosphoryl compound ± amine, hydroxy phosphonate ± amine, and hydrophosphoryl compound ± imine.It was found that each of the reaction partners affects somewhat the reaction efficiency and its pathway. A kinetic study of the reaction in the system dialkyl(methyl or pentyl) phosphite ± benzaldehyde ± aniline and reactions in the pairs PhNH2+PhCHO, dipentyl phosphite+PhCHO, and dipentyl phosphite+PhN=CHPh has made it possible to estab- lish that the `imine' mechanism operates in this case.131 UV spectroscopic study of the reaction kinetics permitted detection of an increase in the optical density of the mixture due to the formation of an imine chromophore in the beginning of the process.The formation of an imine is the fastest reaction of all reactions possible in this system. It is catalysed by a dialkyl phosphite.The reaction of phosphites with benzaldehyde in the absence of acidic additives (formic acid or aniline hydrochloride) does not occur at all. Pyridine, whose basicity is equal to that of aniline, but which cannot serve as a proton donor and does not participate in the reaction, does not catalyse it. Hence, the basicity of aniline is insufficient to catalyse the addition of dialkyl phosphites to benzaldehyde.This rules out the intermediate formation of hydroxy phosphonates. Thus, due to low basicity, aniline manifests proton-donor properties and forms a pre-reaction complex of the type 23 with the phosphite. The reaction of the complex 23 with benzaldehyde results in an imine hydrophosphorylation of which gives the target AP. A study of this system in solvents of different nature, viz., in benzene (neutral medium) or propan-2-ol (a potential proton donor), in the presence and in the absence of acidic admixtures made it possible to reveal the effect of acid catalysis on the rate of each step of this reaction.The use of a strong proton donor, such as HCOOH, always accelerates the reaction by the general acid catalysis mechanism. This effect is most pronounced in a polar solvent, viz., propan-2-ol.Propan-2-ol itself is inefficient as a N C P O PHO+ C N RONa PHO+ C O NH2 7H2O C P OH O (RO)2PHO+Et2NCH2X P(OR)2 Et2N O +HX X=NEt2, OEt. C O R1 R2 +HP(O)(OR3)2 C P(O)(OR3)2 R1 R2 OH k3 CH R1 R2 O P(O)(OR3)2 R1, R2=Ar; Relative reaction rate constants: k1=100, k2=1, k3=6. k1 k2 (RO)2P(O)H+PhNH2 (RO)2P O H H N Ph H d+ d7 23 PhCHO 7H2O PhN CHPh +(RO)2P(O)H (RO)2P CHPh O NHPh The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 865proton donor. It manifests only solvation effects, slowing down the reaction in some cases due to the formation of H-complexes with basic reagents.Different kinetics were observed if the relatively weak base, aniline, was replaced by the appreciably more basic cyclohexyl- amine 24.136 It was found that the reaction of amine 24 with benzaldehyde is very slow and results in benzylidenecyclohexyl- amine, which is practically insoluble both in the selected media and in the majority of organic solvents.Attempts of addition of dialkyl phosphites to N-cyclohexylbenzylideneamine even in the presence of strong acids and bases failed.These data indicate that in the system `hydrophosphoryl compound ± amine 24 ± benzaldehyde', the `hydroxy phospho- nate' mechanism operates. According to this pathway, the reac- tion starts with the addition of a phosphite to benzaldehyde catalysed by the amine 24 (the Abramov reaction), followed by the replacement of the hydroxy group by the cyclohexylamino group.A special 31P NMR study made it possible to detect the formation of the hydroxy phosphonate 25 in all cases. If a catalytic amount of the amine 24 is used, the phosphonate 25 is the sole reaction product. As the amine concentration increases, the signal intensity for the final product (AP) increases, while that for the phosphonate 25 decreases. In the system with aniline discussed above, the formation of the hydroxy phosphonate was never detected.The results of these studies lead to the conclusion on the decisive role of the amine basicity. Variation of the nature of the amine enables `switching' of the reaction from one pathway to the other. To estimate the effect of the carbonyl component on the reaction pathway, the same complex of studies was carried out in the system `dialkyl phosphite ± aniline ± substituted benzalde- hyde'.Electron-donating p-methoxy and p-dimethylamino groups and electron-withdrawing p-bromo and m-nitro groups were used as the substituents. It was found that in all cases the `imine' reaction pathway (Scheme 3) was realised and its general laws were obeyed. However, on switching from electron-donating substituents to electron-withdrawing ones, the rate of imine formation increases both in the carbonyl compound ± amine pair and in the three-component system.On the other hand, unlike benzaldehyde, its derivatives with both electron-donating and electron-withdrawing substituents undergo the Abramov reaction with phosphites to produce the corresponding hydroxy phosphonates. Moreover, simultaneous formation of both an imine and a hydroxy phosphonate was reported.The rate of the Abramov reaction for the aldehydes with electron-donating substituents exceeds the rate of imine formation by almost an order of magnitude. However, this does not suggest that the `hydroxy phosphonate' mechanism is preferable kineti- cally, since, first, the Abramov reaction is reversible, and, second, the ability of transformation of hydroxy phosphonates to AP depends on the basicity of the amine.Weakly basic amines cannot replace the hydroxy group in a hydroxy phosphonate molecule. Thus, even if the formation of the latter is preferable kinetically, thermodynamic control directs the reaction with weakly basic amines to the `imine' pathway. In some cases, e.g., for anisaldehyde, the ratio of rates of all pairwise reactions possible under the Kabachnik ± Fields reaction conditions allows both reaction pathways.However, even in this case, the reaction follows mainly the `imine' pathway, while the `hydroxy phosphonate' pathway is minor if aniline is used as the amino component.137 In the general case, the basicity of the amine is the crucial factor that determines the reaction pathway, in particular, the possibility of formation of pre-reaction complexes 23 or 26 (Scheme 3).Weakly basic amines, which can act with hydro- phosphoryl compounds as H-donors, direct the reaction to the `imine' pathway [pathway (a)]. Strong bases enable nucleophilic interaction of the pre-reac- tion complex 26 with carbonyl compounds and thus favour the `hydroxy phosphonate' direction.An important role belongs to the reactivity of reactants, which affects the ratio of rates of decomposition of the hydroxy phosphonate and replacement of the hydroxy group in it by the amino group according to the pathway 29?30. An intermediate case is possible as well: a biphilic reaction of a hydrophosphoryl compound with an amine with the formation of the complex 27.This version assumes the probability of parallel reactions through both pathways. The importance of consideration of all factors in the determi- nation of the reaction pathway is clearly seen where salicylalde- hyde is used in the reaction.138 Irrespective of the amine basicity, the reaction with salicylaldehyde occurs exclusively according to the `imine' pathway.The reason for this is the thermodynamic instability of the hydroxy phosphonate obtained from this alde- hyde in the Abramov reaction. The nature of the hydrophosphoryl compound is also of significant importance.139 As will be shown below (Section IV) and as follows from Scheme 3, the acid ± base interaction of a hydrophosphoryl compound with an amine is the crucial factor that determines the direction of the reaction.Replacement of the alkoxy groups by alkyls in a hydrophosphoryl compound, i.e., going from dialkyl phosphites to dialkyl phosphinites and espe- cially to phosphinous acids, alters the ability of a hydrophos- phoryl compound to form a pre-reaction complex with an amine. High basicity of the phosphoryl oxygen atom in dibutylphosphi- nous acid and the decrease in PH-acidity of the hydrophosphoryl compound due to the replacement of electron-withdrawing alkoxy substituents at the phosphorus atom by electron-donating alkyl groups is the reason for the `switch' of the reaction pathway from the `hydroxy phosphonate' pathway, which is typical of dialkyl phosphites, to the `imine' one if cyclohexylamine 24 is used as the amine component. H (RO)2PH+ O H2N 24 H N H d+ (RO)2P d7 O PhCHO (RO)2P CH O OH Ph 25 24 7H2O (RO)2P CH O N Ph H N CHPh+Bu2P(O)H Bu2P CH O N Ph H Bu2P O H H N H d+ d7 31 Bu2P(O)H+24 PhCHO 7H2O 23 (R2O)2P O H H N R1 H d+ d7 H NHR1 H O (R2O)2P 27 26 d7 d+ H NH2R1 O (R2O)2P 27 R1NH2 + (R2O)2P(O)H R3CHO 23 pathway (a) pathway (b) 26 R3CH NR1 28 R1NH2 7H2O 30 (R2O)2P CHR3 O OH 29 (R2O)2P(O)H (R2O)2P CHR3 O NHR1 30 Scheme 3 866 R A Cherkasov, V I GalkinAlthough phosphinous acids undergo smooth addition to carbonyl compounds, even in the absence of catalysts, the effect of the pre-equilibrium step, which involves the formation of the complex 31 and the addition of cyclohexylamine 24 to benzalde- hyde catalysed by dibutylphosphinous acid, is more essential.An alternative mechanism involving nucleophilic attack of the dibu- tylphosphinite anion on the carbonyl carbon atom is noncompe- titive in this case. In a three-component system `dibutylphosphinous acid ± salicylaldehyde ± cyclohexylamine', the hydrophosphoryl com- pound can enter into nucleophilic reactions both with an amine, which determines the `imine' mechanism of the Kabachnik ± Fields reaction, and with salicylaldehyde. However, in the latter case, the product of the Abramov reaction, viz., compound 33, does not react with cyclohexylamine, and the product of the Kabachnik ± Fields reaction, viz., aminomethylphosphine oxide 34, is not formed (Scheme 4).High stability of the hydroxy phosphonate 33 precludes its decomposition into the original components.Both addition processes resulting in the imine 32 and hydroxy phosphonate 33 are competitive, and compound 33 is the side reaction product. This result confirms once again the general scheme of the Kabachnik ± Fields reaction (Scheme 3) and explains numerous experimental observations that the purity and yield of the reaction product increase if the reaction is carried out not in a three- component system but rather in a stepwise manner, for example, with isolation of the imine.140, 141 It is difficult to interpret unequivocally the competitive reactions of two nucleophiles in a system `hydrophosphoryl compound ± carbonyl compound ± amine', viz., the phosphite (the Abramov reaction, the `hydroxy phosphonate' pathway) and the amine (the `imine' pathway), in terms of electrophilicity of the carbonyl carbon atom.142, 143 It was not possible to reveal the relationship between the preference of attack by a particular nucleophile (amine or phosphite) and the steric requirements for the formation of adducts.142 It is believed 142, 143 that the experimental results obtained from kinetic data and density functional 143 calculations are best explained from the HSAB standpoint.According to this concept, hard acids (aliphatic aldehydes and ketones) react faster with the hard bases (amines). Soft bases, i.e., phosphites, preferably react with soft acids, such as aromatic carbonyl compounds. Similar opinions have also been expressed earlier.144 We believe than these obser- vations are only estimates that cannot rationalise the diversity of factors that determine the reaction pathway (for example, its changes on varying the substituents at the phosphorus atom in a hydrophosphoryl compound139).The general mechanism of the Kabachnik ± Fields reaction presented in this section (Scheme 3) has much larger interpretative and predictive power. IV. Synthetic potential of the Kabachnik ± Fields reaction The ample synthetic potential of the Kabachnik ± Fields reaction has allowed it to occupy a deserved position in the arsenal of preparative chemistry of organophosphorus compounds.The possibility of variation of the functional groups in the carbonyl and amine components, the involvement of various types of derivatives of trivalent (three-coordinate) phosphorus in the reaction, the rather mild reaction conditions and the possibility of further one-pot modification of the resulting AP, all these attractive aspects of the three-component method for the synthesis of AP by the Kabachnik ± Fields reaction stimulated the search of newer substrates for this reaction.Here we make an attempt to classify the versions of the Kabachnik ± Fields reaction according to the starting P(III) compound. 1. The classical version a. Reactions of hydrophosphoryl compounds The scope of the Kabachnik ± Fields reaction and its applicability to various types of hydrophosphoryl compounds, aldehydes and ketones, and secondary amines and ammonia outlined in the early studies satisfied to a certain extent the needs for the synthesis of AP with rather simple structures.The results of these studies are covered in reviews.6, 12 The scope of compounds that are being used in this reaction has now expanded appreciably. First, this refers to the synthesis of AP containing heterocyclic substituents at the C- and N-centres. Pyridine, pyrrole and imidazole derivatives were synthesised by a one-pot procedure using diethyl phosphite and benzhydryl- amine.140 In the presence of hydrochloric acid, the benzhydryl protective group is removed and the process goes through condensation and hydrolysis to give the corresponding a-amino- hetarylmethylphosphonic acids.The reaction of heterocyclic aldehydes with benzylamine and dibenzyl phosphite occurs at elevated temperature and results in O,O,N-tribenzyl phosphonates.Consecutive addition of the amine and the phosphite to the aromatic aldehyde assumes the initial formation of a Schiff's base.140 The alkaloid cytisine reacts with aliphatic and aromatic aldehydes and dimethyl phosphite under mild conditions in the presence of catalytic amounts of crown ethers to give the corre- sponding AP.145 On refluxing in benzene, amino phosphonate derivatives of cytisine form internal salts. 20% HCl Het= N , N N H . HetCHO +Ph2CHNH2+HP(OEt)2 O HetCHPO3H2 NH2 . HCl HetCHO+PhCH2NH2+HP(O)(OCH2Ph)2 HetCH(NHCH2Ph)P(O)(OCH2Ph)2 PhMe, D Het= N H , O . o-HOC6H4CH Bu2P CH OH C6H4OH-o 33 H2N 34 the Kabachnik ± Fields reaction Bu2PHO+o-HOC6H4CHO+H2N 32 O N Bu2PHO Bu2P CH O N C6H4OH-o H 34 the Abramov reaction Scheme 4 The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 867It should be noted that dealkylation involving the dimethox- yphosphoryl group can complicate the Kabachnik ± Fields reac- tion, especially if weakly reactive aromatic aldehydes, that are most susceptible to this process, are used.146 Phosphorylmethyl derivatives of aminotetrazoles are readily formed in the reaction with dibutyl phosphite and p-fluoro- benzaldehyde. 5-Aminotetrazole (35a) reacts with dibutyl phosphite on the amino group at position 5 to give compound 36, whereas in 1,5-diaminotetrazole (35b) it is the amino group at the nitrogen atom that undergoes phosphorylalkylation. Taking into account the tendency of 1,5-diaminotetrazole (35b) to form Schiff's bases, the `imine' pathway of the reaction resulting in the AP 37 seems to be preferential.147 In a search for biologically active AP, aminomethyl- diphenylphosphine oxides148 as well as bisphosphonate deriva- tives of tetrahydropyran, its thio and seleno analogues149 have been synthesised recently.The use of appropriate bifunctional amines in the Kabach- nik ± Fields reaction allows the synthesis of heterocyclic com- pounds.For example, chiral phosphorylated oxazolidine is formed in a one-pot procedure from (R)-(7)-phenylglycinol, formaldehyde and dimethyl phosphite in boiling methanol.150 Heterocyclisation was successfully used in the synthesis of 3-amino-2-oxo-1,2-oxaphosphacyclanes.151 A one-pot synthesis of AP 38 from 4-benzyloxy-2-butanone, ammonia and diethyl phosphite followed by its hydrogenolysis results in alkanol 39.Intramolecular transesterification of the latter gives the 1,2-oxa- phospholane. AP used in membrane extraction were obtained recently from calix[4]arene. Modified calix[4]arene platforms with (phosphor- ylalkyl)aminoalkyl fragments in the top and bottom rings were prepared from dialkyl phosphites and the appropriate carbonyl compounds.60, 64 Regiospecific amination of calix[4]arene gave compound 40, and in the `top' rim of its platform two phosphorylalkyl groups were introduced.The amino phosphonate fragments in the `bottom' rim were obtained using a calix[4]arene platform carrying b-aminoethoxy groups. R +RCHO +(MeO)2PHO N NH O C6H6 , 25 ± 30 8C, 4 h 15-crown-5 NCHP(O)(OMe)2 R CHP O7 OMe O R N Me + Pri Ph 4-MeOC6H4 Yield (%) 90 78 80 R=H(a), NH2 (b); R0=(BuO)2P(O)CH(C6H4F-p)NH.(BuO)2P(O)H p-FC6H4C(O)H N N N N H R0 36 N N N N R0 NH2 37 R=H R=NH2 N N N N R NH2 35a, b Ph2PHO+CH2O+HNR1R2 R1=H: R2=CH2Ph, (CH2)2Ph, 2-pyridyl; R1=R2=CH2Ph. Ph2PCH2NR1R2 O H2N OH Ph HP(O)(OMe)2, CH2O N O Ph (MeO)2P O OCH2Ph H2N P Me O EtO OEt Pd/C, EtOH, HCl 40 8C 38 OCH2Ph Me O HP(O)(OEt)2, NH3 (gas) 60 8C P O O EtO H2N Me NaH, cat, DME 20 8C OH H2N P Me O EtO OEt 39 O O H2N Pr Pr O Pr 40 (EtO)2PHO, R2CO O NH2 Pr R2=Me2 , (CH2)5 .O O Pr HN CR2 Pr O Pr O NH R2C (EtO)2P O Pr P(OEt)2 O (EtO)2PHO, R2CO O NH2 O O But But But H H O But H2N H H O O But But But O HN CR2 P EtO OEt O O But NH R2C P EtO OEt O 868 R A Cherkasov, V I GalkinFour amino phosphonate groups were introduced in a similar way using a tetrakis(aminomethyl) derivative of calix[4]arene.As a result of the reaction, all phosphorus-containing fragments were located in the `top' rim. Modification of calix[4]arene with diethyl N-benzylamino- methylphosphonate gave a tetrasubstitution product in the `par- tial cone' conformation. Orthoformate was successfully used instead of carbonyl com- pounds in the classical version of the Kabachnik ± Fields reaction.In this case, two phosphoryl groups rather than one can be attached to the carbon atom. Condensation of dibenzylamine and diethyl phosphite with orthoformate (1 : 3 : 1.2, refluxing for 24 h) gives dibenzylaminomethylenediphosphonate (yield 58%).152 The reaction of benzylamine with one equivalent of HC(OEt)3 and four equivalents of diethyl phosphite at 160 8C with subse- quent acidification of the reaction mixture gave diaminomethyl- phosphonate (PhCH2NH)2CHP(O)(OEt)2 in 34% yield.152 This version of the reaction makes it possible to introduce organophosphorus groups of different structures into an AP during one chemical process.For example, a phosphite and a phosphonite participate in a one-pot synthesis of phosphonato- phosphinate 41.153 The drawback of this method is that it gives, along with asymmetrical phosphonatophosphinates, a small amount of sym- metrical bisphosphonates and bisphosphinates. This can be avoided if phosphorus-containing groups are introduced succes- sively.For example, a-phosphonoacetal, alkylphosphonite and aminopyridines react under these conditions to give compound 41 exclusively.The scope of the classical version of the Kabachnik ± Fields reaction is expanded if carbamates, amides of carboxylic and phosphorus acids and some other compounds are used as the amine component. Benzyl carbamate, an aliphatic or aromatic aldehyde and diethyl phosphite react at room temperature in the presence of a mixture of acetic acid and thionyl chloride [method (a)] or acetyl chloride [method (b)] to give moderate to high yields of AP containing an N-benzyloxycarbonyl group, which can easily be removed if necessary.154 A modification of this method was suggested for the Kabach- nik ± Fields reaction with a-halocarboxamides.155 Their conden- sation with aromatic aldehydes and hydrophosphoryl compounds occurs in the presence of an alcoholic solution of HCl or p-toluenesulfonic acid in acetic anhydride.The diastereoselectivity of condensation of chiral cyclic car- bamates and ureas according to the scheme of the Kabachnik ± Fields reaction depends largely on the nature of the cyclic compound. The highest diastereomeric excess (de) in the one-pot synthesis of AP from diethyl phosphite, benzaldehyde and com- pounds 42 ± 44a ± c containing an amino group is reached for derivatives of camphorsulfonic acid 44a ± c.156 O Pr P(O)(OEt)2 Me2C P(O)(OEt)2 CMe2 O O Pr Pr HN NH NH (EtO)2(O)P Me2C Me2C (EtO)2(O)P O Pr HN (EtO)2PHO, Me2CO O Pr O O Pr Pr H2N NH2 NH2 O Pr H2N O O O H H H PhCH2NHCH2P(O)(OEt)2, CH2O, AcOH, THF O H R=CH2Ph.OH OH RN NR (EtO)2P(O)CH2 CH2P(O)(OEt)2 OH RN (EtO)2P(O)CH2 NR CH2P(O)(OEt)2 HO (PhCH2)2NH+2 (EtO)2PHO+HC(OEt)3 (PhCH2)2NHCH[P(O)(OEt)2]2+3 EtOH N NHCHP(O)(OH)R2 R1 P(O)(OH)2 41 (a) D (7EtOH); (b) H2O, D (7R3OH); R1=H, Me; R2=Me, Bu; R3=Et, Pri.+HC(OEt)3+HP(O)(OEt)R2+HP(O)(OR3)2 a, b N NH2 R1 (EtO)2P O OEt OEt + + 41 HP O OEt R2 N NH2 R1 Ph O NH2 O +R1CHO+HP(O)(OR2)2 Ph O NH O P(O)(OR2)2 R1 a or b (a) AcOH, SOCl2, 20 8C, 20 min, D, 2±12 h; (b) AcCl, 0 ± 5 20 8C (43% ± 98%).NH O R3 Hal P(OR2)2 O R1 Ac2O, HCl, EtOH, 80 8C, 6 h CHO R1 + (R2O)2PHO + NH2 O R3 Hal The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 869If benzaldehyde is replaced by other aldehydes (anisaldehyde, a-naphthaldehyde or p-tolualdehyde) in the reaction with carba- mates 44, the de remains at about 99%.The de is small (56%) only in the case of cyclohexanecarbaldehyde. It is noteworthy that both hydrophosphoryl compounds and their salts can participate in this condensation.157 The AP 45 obtained this way were used as synthons in the development of a new approach to the synthesis of functionalised indoles.158 The reaction between diphenyl phosphite, benzyl carbamate and p-nitrobenzaldehyde in acetic acid results in the AP 46.Dephosphonylation of amino phosphonate 46 by the Horner ± Wittig ± Emmons reaction with o-nitrobenzaldehyde gives the enamine 47, which is converted in two steps to the C-substituted indole 48. The use of a-aminophosphine oxide provides a convenient method for the synthesis of thio- and selenoamides.159 In this reaction, generation of a carbanion is followed by cleavage of the P7C bond under the action of two equivalents of sulfur or selenium.The use of acetyl chloride as the condensation agent makes it possible to introduce an organophosphorus radical to the amine centre of AP if O,O0-diethyl phosphoramidite is used as the amine component.160 b. Reaction of inorganic phosphorus acids This version of the Kabachnik ± Fields reaction is also named161 the Moedritzer ± Irani reaction in the name of the researchers who were the first to use inorganic phosphorus acids of the lowest oxidation state for the synthesis of APA in a three-component system `hydrophosphoryl compound ± carbonyl compound ± amine'.71 Reaction of orthophosphorous acid with formaldehyde is known to occur almost quantitatively.Since both primary and secondary amines can be involved, the Mannich reaction mecha- nism is considered most probable for these reactions,71 although other mechanisms are not ruled out either. Attempts at expanding this version of the Kabachnik ± Fields reaction to aliphatic aldehydes and ketones failed.162 The `imine' version of the reaction is preferable for these compounds (see below).Acylamides readily undergo condensation with aldehydes and phosphorous acid under the action of acetic anhydride.163 Presumably, the condensation of an aldehyde with an amide is followed by decomposition of the condensation product to APA 49.163 The reaction of 1,2-cyclohexanediamine 50 with formalde- hyde and phosphorous acid occurs in a low yield.161 The yield of tetraphosphonate 51 increases considerably if 1,2-cyclohexane- diaminotetraacetic acid is phosphorylated with a mixture of H3PO4 and PCl3.R=N(cyclo-C6H11)2 (a), NPri 2 (b), C6H4OMe-4 (c). SO2R O NH2 O 44a ± c NH2 O 42 N Ph Me O NH2 O 43 N P(OEt)2 R H O Ph RNH2+PhCHO +(EtO)2PHO 42, 43, 44a ± c AcOH 0 8C Starting compound Yield of AP (%) de (%) 42 43 44a 44b 44c 71 84 75 73 77 14.4 34.1 99 96.4 96.2 R1CH2P O O7 H+R2CHO+ X+ PhCH2O NH2 O AcCl 0 to 20 8C PhCH2O NH O P R2 OH O CH2R1 45 R2=H, Me, Et, Bui, Ph, p-MeOC6H4 .X=H, H3N ; R1=H, Pr, N, EtOOCCH2; O O Ph O NH2+p-O2NC6H4CHO O (PhO)2PHO+ AcOH NO2 NHCO2CH2Ph C6H4NO2-p 47 H 48 N C6H4NO2-p HN O Ph (PhO)2P(O)CH C6H4NO2-p 46 o-O2NC6H4CHO O R1, R2=Me, Ph, (CH2)2O(CH2)2; R3=H, Alk, alkenyl; X=S, Se. Ph2P R3 O R1 R2 N 1.BuLi or LDA 2. X (S or Se) R3 X R1 R2 +Ph2P(O)XLi N R=Pr, Pri, Ph, p-MeC6H4, p-MeOC6H4, p-Me2NC6H4. (EtO)2PNH2+RCHO +(PhO)2PHO O AcCl 20 8C, 8 ± 12 h NH P(OPh)2 R O (EtO)2P O (10% ± 80%) R37nNHn+nCH2O +HP(O)(OH)2 R37nN[CHP(O)(OH)2]n+H2O RCHO +H2NCOR0 +H3PO3 Ac2O RCHPO3H2 HNCOR0 49 (10% ± 75%) R=Me, Pri, Ph, 3-NO2C6H4, 4-ClC6H4, 4-MeC6H4; R0=PhCOO, PhCO, Ac. 870 R A Cherkasov, V I GalkinThe reaction of 1,2-cyclohexanediamine 50 with formalde- hyde and diethyl phosphite results in a bicyclic bisphosphonate.161 Hypophosphorous acid in a mixture with formaldehyde undergoes polymer-analogous Kabachnik ± Fields reaction with polycaprolactam.This method has been used for the synthesis of materials with low flammability.164 Salts of hypophosphorous acid with (R)-(+)- or (S)-(7)-1- phenylethylamine react with aldehydes in boiling ethanol to form homochiral APA.165 The protective group at the nitrogen atom is easily removed by a standard method, and the intermediate phosphonous acid is converted into the APA.Recently, an efficient method for the synthesis of AP has been developed in which 80%± 90% aqueous solutions of hypophos- phorous acid is used as the hydrophosphoryl compound.In this case, orthoesters and acetals were used instead of carbonyl compounds, and 1,3,5-tribenzylhexahydro-1,3,5-triazine 52 was chosen as the amine component. The reaction of triazine 52 with triethyl orthoacetate and H2PO3 gives an amino phosphinate under very mild conditions and in high yield.166 A three-step synthesis of a phosphinate containing both an a-hydroxy and an a-amino group in different P-alkyl substituents simultaneously 167 involves the initial formation of a phosphonite from 2,2-dimethoxypropane and hypophosphorous acid, its sub- sequent transesterification to the more hydrolytically stable iso- butyl phosphonite 53 and the reaction of the latter with triazine 52 to give the AP.In these cases, commercial 50% hypophosphorous acid can be used following its concentration in vacuo. The efficiency of this approach in comparison with the methods based on the use of anhydrousH3PO2 is noteworthy (see Ref. 166 and references cited therein). 2. Reactions of trivalent phosphorus chlorides This version of the Kabachnik ± Fields reaction based on phos- phorus trichloride was used in the synthesis of APA in 1978.168 The method is equally applicable to alkyl(aryl)dichloro- and dialkyl(aryl)chloro-phosphines.169, 170 It is considered 170 that the process starts with condensation of an amide with a carbonyl compound.The alkylidenebisamide 54 thus formed is phosphory- lated to AP 55, hydrolysis of which results in an APA. The method was expanded to a wide set of amides, carbonyl compounds and P(III) chlorides (see Ref. 171 and references cited therein). Another interpretation of aminomethylation of P(III) chlorides is considered in detail in Ref. 171. The method based on the use of PCl3 was applied successfully to the synthesis of a-aminoarylalkylphosphonic acid 36 and its monoesters,172 as well as a large number of APA containing fluorinated aryl substituents.173 Aminomethylol synthons 174 or hexahydrotriazine derivatives were also used as aminoalkylating agents.174, 175 The latter compounds have repeatedly been used in the synthesis of AP.N,N,N-Tribenzyloxyhexahydrotriazine is reversibly formed in the condensation of formaldehyde with O-benzylhydroxylamine and undergoes conversion to O-benzyl- formaldoxime on gentle heating, which reacts with P(III) chlorides in acidic medium to give the corresponding APA.175 The mechanism of aminomethylation of P(III) chlorides is rather complex.It can be different in each particular case. For instance, in the system `benzyl carbamate ± aromatic aldehyde ± phenyldichlorophosphine ± acyl chloride',176 the addition of the amide to the aldehyde occurs first, and then acylation of the resulting acylaminoalkanol produces compound 56.N(CH2PO3H2)2 N(CH2PO3H2)2 51 CH2O, H3PO3 7H2O (20%) PCl3, H3PO3 7CO2 (82%) N(CH2COOH)2 N(CH2COOH)2 NH2 NH2 50 50+3CH2O+2 (EtO)2PHO N N P(O)(OEt)2 P(O)(OEt)2 CO(CH2)5NH +CH2O+H3PO2 CO(CH2)5N n=12 ± 14. n n CH2PO2H R=Pri , Bui , cyclo-C6H11, Ph, PhCH2 . NH Ph P R O OH H Br2, H2O 70 8C H2N OH OH O R P RCHO + NH3 H2PO2 Ph + 7 N N N CH2Ph PhCH2 CH2Ph 52 H3PO2, MeC(OEt)3 Ph N P O OEt OEt OEt Me H H P CMe2 OH O OMe 1.H2O 2. BuiOH 52 H P CMe2 OH O OBui 53 Me2C(OMe)2+H3PO2 20 8C, 5 days HOCMe2 PCH2NHCH2Ph O OBui R1 R2 NH2 PO3H2+PhCH2OH +CO2 R1 R2 O +PCl3+ Ph O NH2 O 1. AcOH 2. H2O, HCl R1 R2 O+H2NCOOR3 7H2O PCl3, AcOH R1 R2 NHCOOR3 NHCOOR3 54 R1 R2 NHCOOR3 PO3H2 55 H2O R1 R2 NH2 PO3H2 R=Alk, Cl; R0=Alk, OH.N N N PhCH2O OCH2Ph OCH2Ph H2CO+H2NOCH2Ph 40 8C H2C NOCH2Ph+RPCl2 PhCH2ONHCH2PR0 O OH AcOH HCl The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 871The reaction of the acylated carbamate 56 with the starting amide gives the diamide 57. The acetic acid evolved converts phenyldichlorophosphine into phenylphosphinic chloride.The latter cleaves the N7C bond in diamide 57 and converts it into the final amidomethylphosphinic chloride 58. A similar reaction of aryldichlorophosphines, aromatic alde- hydes and 5-amino-1,2,4-triazole occurs in the presence of cation- exchange resins (D-72, Amberlyst-15) and results in 6-phospha- 4,5,6-trihydroimidazo[2,3-e]-1,2,4-triazole. It is believed 177 that the Schiff's base is formed first and then adds arylphosphinic chloride. Intramolecular dehydrochlorination of the addition product results in the final bicycloamidophosphinate. 3. Reactions of neutral phosphites and their analogues Reactions of natural a-amino acids with trialkyl and triaryl phosphites have long and successfully been used in peptide synthesis. As a rule, the non-classical Arbuzov reaction occurs in this case (see, e.g., Ref. 178). A three-component system `phosphite ± carbonyl com- pound ± amine' was successfully used in a number of syntheses of AP derivatives of urea and thiourea. Triethyl phosphite undergoes condensation with an aldehyde and phenylurea in the presence of boron trifluoride etherate on refluxing in toluene to give the corresponding AP.179 A similar reaction of a thiourea 180 and N-substituted thiour- eas 181 occurs with triphenyl phosphite in the presence of glacial acetic acid. Under certain conditions, both amino groups of thiourea can be involved in this reaction.The stereochemistry of condensation of chiral carbamates and thioureas with benzaldehyde or aliphatic aldehydes and triaryl phosphites in the presence of acetic acid depends on the chirality of the starting amine: (+)-AP are obtained from chiral (+)-car- bamates and ureas; accordingly, (7)-AP are formed from (7)- amides.182 The reaction occurs via an immonium salt with subsequent syn- or anti-attack of the phosphite on the iminium carbon atom.The `triphenyl phosphite' version of the Kabachnik ± Fields reaction was successfully used for the synthesis of C-hetaryl derivatives of AP.183 ± 185 The mechanism of these reactions has not been discussed in detail.However, in this case the reaction begins most probably with the interaction of the aldehyde with the amino component. The smooth reaction of phosphinites and phosphonites with chloralurea 186 can probably serve as a model of the second step of the `trialkyl phosphite' version of the Kabachnik ± Fields reaction.As in the classical version of the Kabachnik ± Fields reaction, in this case it is possible under certain conditions to carry out cyclisation by the one-pot technique, for example, in the N-phos- phonoalkylation of (R)-(7)-phenylglycinol;187, 188 in the latter case glutaraldehyde replaces formaldehyde in a three-component mixture.188 The reaction can also be carried out stepwise.187 For example, N-benzylphenylglycinol reacts with an aldehyde to form oxazoli- dine with high diastereomeric excess.The latter reacts with Bnz Bnz NH O OH Ar AcCl Bnz NH2+ArCHO O NH2 O 7AcOH Bnz NH O OAc Ar 56 N H O CHAr 57 2 Bnz PhPCl2 AcOH P Ph H Cl O 57 PhCH2OCNHCHP O Ar O Ph Cl 58 NH N N NH2 +ArCHO D-72, 100 8C 7H2O NH N N N CHAr NH N N N CH Ar + P H Ar0 O Cl Ar0PCl2+H2O Ar0P(O)ClH N N N N H Ar P O Ar0 NH N N N H Ar P O Cl Ar0 7HCl O PhHNCNH2+RCHO+(EtO)3P BF3 .OEt2 PhHNCNHCHP(OEt)2 O R O RCHO +(PhO)3P+H2NCNH2 S (PhO)2P N N P(OPh)2 R R O O H H S + RHNCNH2+R0CHO O AcOH P(OAr)3 R N C N H O C H R0 H AcO7 Ar=Ph, 2-MeC6H4. RHNCNHC(R0)HP(OAr)2 O O HetCHO+PhCH2COONH2+P(OPh)3 HetCHP(O)(OPh)2 NHCOOCH2Ph AcOH 90 8C, 2 h Het= N N , N , N(CH2)n O O (n=3, 4, 5).H P R0O R O CHNHCNH2 CCl3 O RP(OR0)2+Cl3CCHNHCNH2 OH O 7R0OH Ph N O (EtO)2P O P(OEt)3+ Ph OH NH2 +OHC(CH2)3CHO MeOH D, 2 h, 58 8C Ph OH NH2 P(OMe)3 (CH2O)n Ph O (MeO)2PCH2N O (75%) 872 R A Cherkasov, V I Galkintrimethyl phosphite in the presence of SnCl4 to give an oxaza- phosphorinane. Polydentate ligands, viz., bisphosphinate triaminocyclohex- anes, were obtained by the `phosphonite' version of the Kabach- nik ± Fields reaction with diethyl methylphosphonite and paraform.189 It is possible to introduce two phosphorylmethylene groups into a 1,3,5-triamino-2,4,6-trimethylcyclohexane molecule due to the fact that all of its methyl groups occupy the equatorial positions, while all amino groups occupy the axial ones. 4. Reactions in two-component systems As discussed above, reactions in two-component systems resulting in APA and their esters and involving interactions in the pairs hydrophosphoryl compound ± imine and hydroxy phosphonate ± amine are, in essence, separate steps of the Kabachnik ± Fields reaction. It is often impossible to distinguish between three- and two-component systems.This can depend on the order of mixing the reactants. On the other hand, it has repeatedly been noted that it is often preferable to carry the reaction out in a stepwise way rather than to use a one-pot three-component process: the chemical yield and de of the APare generally higher in two-component processes. The methods that have been developed most extensively recently include enantioselective addition of hydrophosphoryl compounds to imines and replacement of the hydroxy group in hydroxy phosphonates by the amino group under the Mitsunobu reaction conditions.190 We shall give these processes the most attention in our review.a. Addition of hydrophosphoryl compounds to imines The addition of hydrophosphoryl compounds to imines, discov- ered simultaneously with the Kabachnik ± Fields reaction, corre- sponds to the Pudovik reaction type 20 and has been covered rather comprehensively in reviews.6, 10 Some examples of asym- metric synthesis of AP in the `imine' version are discussed in another review.18 However, many new studies have been pub- lished in recent years which are to some extent relevant to the problem under discussion.Important data on the mechanism of the `imine' method and the catalytic effects observed have been obtained. The `imine' method should be preferred when choosing the strategy for the synthesis of AP if it is necessary, first, to reach the maximum diversity of functional groups at the N and C atoms in the target AP and, second, to enable conditions for the maximum stereoselectivity of addition of hydrophosphoryl compounds to the C=N bond.The reaction of hydrophosphoryl compounds with the Schiff's bases studied in detail by Pudovik20 continues to draw the attention of researchers due to the simplicity of the procedure and useful properties of the AP obtained in this way. The reaction occurs in a condensed phase or in a solution in the presence of acidic and basic catalysts or without them at all, sometimes upon simple mixing of the reagents.191, 192 The reaction of hydrophosphoryl compounds with the Schiff's bases containing fluorine atoms in theN- and/or C-aryl fragments (Ar=2-, 3-, 4-FC6H4; 2-, 3-, 4-F3CC6H4; 2-, 4-F3CCOC6H4; 3,4-F2C6H3 and others) 193 ± 195 allows one to obtain biologically active fluorinated AP.The reaction occurs on heating of the starting reagents to 90 ± 110 8C without a catalyst. N-Pentafluoro- phenylaldimines add dimethyl phosphite even more readily.196 Aryl-substituted AP are formed as a mixture of diastereomers (chiral a-carbon atom).The presence of each of the stereomers is confirmed by analysis of 1Hand 13CNMRspectra. In some cases, these diastereomers could be separated by column chromatogra- phy.Bisamino phosphonates of various structures were synthes- ised by addition of hydrophosphoryl compounds to bisimines.192 ± 195 The NMR and mass spectroscopic, as well as other character- istics of bisamino phosphonates have been studied.197 ± 199 Numerous N-substituted AP derivatives, mainly N-benzyl ones, were obtained for use in peptide synthesis.The addition of hydrophosphoryl compounds to N-benzylbenzylideneimines occurs very readily. Lower dialkyl phosphites react at room temperature to give the corresponding AP in very high yields.36, 200, 201 Longer reaction time and higher temperature are needed with the lengthening of the alkyl chain in the hydrophosphoryl com- pound (R 0). For example, at R=n-C5H11, the reaction at 80 8C takes 8 h or longer.Asimilar effect is caused by replacement of the C-aryl group by C6F5 (however, there are exceptions: see Ref. 196). N-Protected alkylideneimines,202 perfluoroisopropylidene-N- acylimine,203 N-tritylmethyleneimine,75 and many other com- pounds (see the review 18 and references cited therein) also add hydrophosphoryl compounds rather easily, usually on heating in the absence of catalysts, giving AP in high yields.The `imine' method is widely used in the synthesis of hetaryl derivatives of AP. In this case, the structures of heterocyclic Ph O PhCH2N R RCHO Ph OH PhCH2NH P(OMe)3, SnCl4 12 ± 16 h PhCH2N P O Ph O OMe R (56% ± 92%) P O Me EtO OEt Me O P Me Me N Me N N NH2 Me NH2 NH2 Me Me CH2O, MeP(OEt)2 THF, 60 8C R1 N CH R2 + PHO R3 R4 R1 N H CH P(O)R3R4 R2 C6F5NHCHP(O)(OMe)2 C6H4R-p (72% ± 88%) C6F5N R=H, Me, MeO, Cl.CHC6H4R-p+(MeO)2PHO 60 8C, 8 h RCHNHXNHCHR P O P O RCH N X N CHR+2 PHO R=Alk, Ar; X=(CH2)2 , , . CHNHCH2Ph P(O)(OR0)2 R CH NCH2Ph R (R0O)2PHO The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 873fragments can be varied widely. An array ofC-pyridine derivatives have been obtained by addition of diethyl phosphite in situ to the appropriate Schiff's bases.204 1-, 2- and 3-formylpyridines readily react with amines in a toluene solution, forming the corresponding aldimines, which are hydrophosphorylated in a one-pot proce- dure. One-step syntheses of 1-(2-aminoethyl)pyridyl-1-arylmethyl- phosphonates 205 and analogous APA monoesters were carried out in a similar way.206 It is noteworthy that the addition of dialkyl phosphites to the Schiff's bases containing a pyridine fragment under mild conditions (at room temperature, in the absence of catalysts) has been reported.207 The results obtained in a study of the reaction of hydro- phosphoryl compounds with imines containing furan and thio- phene rings are also worth noting. 5-Hydroxymethylfurfural readily reacts with ethyl- and benzylamines to give the corre- sponding aldimines. Subsequent addition of diethyl phosphite gives (in the case of chiral amines) a diastereomeric mixture of AP in 2 : 1 ratio.208 When pure enantiomeric amines are used, the resulting imines react with dibenzyl phosphite to give a mixture of AP, from which individual stereoisomers can be isolated by chromatography.The same scheme was used for the synthesis of (2-furyl)ami- nomethylphosphonic acid.209 N-Benzylated imines add hydrogen phosphites in toluene without catalyst or in acetonitrile with a catalytic amount of CF3COOH. N-Tritylated imines give an AP just upon mixing the reagents. It was reported that an attempt at detritylation of an AP resulted in destruction of the AP with cleavage of the P7C bond. Similar fragmentation of AP has also been observed in other cases.203, 204, 210 Bis(amino phosphonates) containing terminal 2-furyl groups were obtained and used as monomers for the synthesis of polymeric materials with low flammability.211, 212 The addition of the hydrophosphoryl compound occurs at room temperature in the presence of catalytic amounts of sodium alkoxide.Important observations related to the activation of addition of dialkyl phosphites to hetarylimines with an N-thiophene or N-pyrrole fragment were obtained.213, 214 The addition of a hydrophosphoryl compound can occur on prolonged heating (72 ± 96 h) of a mixture of the reactants at 60 8C. The process is accompanied by formation of side products.Attempts to optimise conditions of the synthesis of AP by carrying out the reaction in various solvents failed in general: this only increased the time of conversion. On the other hand, sonochemical (ultrasonic) activation of the addition reaction was found to be rather efficient. Both in a condensed phase and in a solution, even short sonication increases abruptly the temperature of the reaction mixture, hence the yield of AP increases and the reaction time is decreased to 5 ± 120 min.This phenomenon was explained 213 using EPR spectroscopy with a spin trap: phosphinyl radicals were detected during sonochem- ical activation. The uninitiated reaction occurs by a concerted mechanism with a four- or five-centred transition state. This mechanism agrees with the stereochemical result of addition to furfuraldimine.208 A method for the introduction of heterocyclic fragments into an AP molecule by addition of a hydrophosphoryl compound to the endocyclic C=N bond in unsaturated five- and six-membered cyclic imines was developed.In these cases, the amino phospho- nate fragment C7N is included in the heterocycle. Chiral 2H-5,6- dihydro-1,3-thiazines react with dialkyl phosphites to give dia- stereomeric AP in a ratio from 52 : 48 to 88 : 12, depending on the substituents in the imine and in the phosphite.215 R=Bu, Ph, CH2Ph, CHPh2 .N CH P(O)(OEt)2 NHR N N (EtO)2PHO 110 8C CHO CH NR RNH2 C6H5Me O HOCH2 CHO RNH2 O HOCH2 NR (R0O)2PHO MeCN O HOCH2 CHP(O)(OR0)2 NHR R=CH2Ph, O CH2 , But, (R)- and (S )-CHMePh; R0=Et, CH2Ph. O CH NR + (R0O)2PHO PhMe or MeCN, CF3COOH R=CH2Ph, CPh3; R0=Et, CH2Ph.O CHNHR (R0O)2P O H+ O CHNH2 (HO)2P O O CH N CH2 O CH N CH2 H (RO)2P O 2 (RO)2PHO NaOR, ROH 2 (RO)2PHO activation (RO)2P O S S HCP(O)(OR)2 CH NMe NMe C N + P O H C P H O C N + P OH N N C P O H C P NH O S N Me Me R1 R2 (R3O)2PHO ligroin, D, 18 h S NH Me Me R1 R2 (R3O)2P O R1=H, Me, Et; R2=Me, Et, But, CMe2CH2Cl; R1, R2=(CH2)4, (CH2)5; R3=Me, Et. 874 R A Cherkasov, V I GalkinThe addition of hydrophosphoryl compounds to 2H-1,4-ben- zothiazines and 2H-1,3-oxazolidines occurs under the same conditions.100 Hydrophosphorylation of cyclic imines catalysed by chiral lanthanide catalysts results in a high de (up to 98%).216 In the presence of chiral titanium catalyst, the reaction is nonstereospe- cific. The mechanism of asymmetric induction of cyclic imines has not been discussed. Hydrophosphorylation of oximinium salts, which is a version of the `imine' method forAP synthesis, has been performed in only a few cases.In our opinion, this is rather a promising method. The addition of diphenyl phosphite to the C=N+ bond of a nitrone is carried out in the presence of an alkylating agent, viz., triethy- loxonium tetrafluoroborate, at room temperature in 3 h.217 The processes is nonstereospecific; however, rather high enantioselec- tivity is observed for chiral nitrones [R1=CH(Me)Ph].The addition of alkyl phosphinites to phosphonyliminium salt 59 also occurs nonstereospecifically.218 In this case, two chiral centres are induced in the AP, viz., the a-carbon atom and the phosphinate phosphorus atom.Broad diversity of possibilities for the synthesis of AP by the `imine' pathway is due to the variability of the substituents not only in the imine but also in the phosphorus-containing addend. Inorganic phosphorus acids and various silylated organophos- phorus compounds have been used for the synthesis of APA. It has already been noted 162 that attempts to use phosphorous acid in the classical three-component version of the Kabachnik ± Fields reaction was not always successful.The `imine' method turned out to be more efficient.162 The addition to the intracyclic bond of 3,4-dihydro- isoquinoline, resulting in 1,2,3,4-tetrahydroisoquinoline-1-phos- phonic acid, occurs successfully.162 It is believed that the reaction occurs in an autocatalytic mode with preliminary protonation of the nitrogen atom.Under certain conditions, phosphorous acid can be a reducing agent with respect to an imine. In this case, it is not the AP that is formed as the final product but rather an isomeric amido phosphate, which is believed 162 to result from the reaction of the intermediate amine with metaphosphoric acid.The phosphorus analogue of histidine 62 was obtained by the addition of hypophosphorous acid to the oxime 60. The phos- phinic acid 61 obtained in 19% yield was transformed in an almost quantitative yield into the target product 62 by oxidation with sulfuryl chloride in glacial acetic acid.219 The addition of inorganic phosphorus acids to both C=N bonds of bisimines was carried out.The reaction of geminal diimine 63 with hypophosphorous acid 220 gives, contrary to expectations, bisphosphinic acid 64 rather than a cyclic phos- phonic acid. A similar reaction of diimine 65, which is carried out by prolonged storage of the reaction mixture (20 8C, 20 ± 25 days), gives a five-membered cyclic phosphinic acid 66 221 and a linear product 67 formed due to addition to one of the C=N bonds.It is of note that the ratio of products 66 : 67 in a crystalline state is 1 : 4, whereas their ratio is 1 : 1 in dioxane solution, according to 31P NMR data.221 The possibility of mutual trans- formation of compounds 66 and 67 during dissolution and isolation from the solution cannot be ruled out. Silyl ethers of phosphorus acids of low oxidation states behave in reactions with imines similarly to phosphorous acid, its esters and other hydrophosphoryl compounds.For example, tris(silyl) phosphite in the presence of Lewis acids (ZnCl2, BF3 . OEt2) or strong protic acids (CF3COOH, p-MeC6H4SO3H) forms a prod- uct of phosphonosilylation of imines.222 On refluxing in methanol, desilylation of the adduct occurs, resulting in the corresponding APA isolated as a hydrochloride.S N R1 R2 (R3O)2PHO S NH R1 R2 (R3O)2P O N Me Me Me Me cat (5 ± 20 mol.%) (MeO)2PHO NH Me Me Me Me (MeO)2P O R1=Me, CH(Me)Ph; R2=Ph, MeOC6H4; R1, R2=(CH2)3 . N R1 O7 R2 H + NCHP(OPh)2 R1 EtO O R2 Et3O+BF¡4 7Et2O + N R1 OEt R2 H BF¡4 (PhO)2PHO R1=Me, Ph; R2=Et, Bu. (EtO)2P O CH P(OR2)R1 NMe2 O (EtO)2P O CH NMe2 + 59 Cl7 R1(R2O)PHO (EtO)2P O CHNMe2 OMe SOCl2 R1N CHR2+(HO)2PHO R1NHCHR2P(O)(OH)2 R1=Me, Et, But, PhCH2; R2=Pri, Ph, 4-ClC6H4, 2-HOC6H4.N +(HO)2PHO NH H P(O)(OH)2 C N +H3PO3 CH NH +[HPO3] CH N P(OH)2 O N N CH2CH H NH2 61 SO2Cl2 AcOH N N CH2CHP(OH)2 H NH2 62 O N N CH2CH H NOH H3PO2 60 P O H OH 64 PhCH(N CHPh)2 +H2P(O)OH 63 NHCHP O OH H PhCH 2 PhCH N N CHPh +H2P(O)OH 65 NH HN P Ph Ph O OH 66 + PHCHNH2N CHPh Ph O O + 7 67 The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 875This strategy has been widely used later in the synthesis of AP.For example, dialkylsilyl phosphites can be easily added to imines. It is believed 223 that this involves concerted 1,2-addition with formation of a five-centred activated complex.224 The easily hydrolysable adducts obtained by addition of cyclic silyl phosphites to benzylideneaniline (80 8C, 2 h, benzene) could not be isolated in the pure state.Their hydrolysis products 68 are identical with the AP synthesised from an imine and cyclic hydro- gen phosphites.225 Enimines containing aromatic substituents at the C and N atoms react with silyl phosphites to give 1,2-adducts, i.e., products of addition to the C=N bond (see Ref. 226). The addition of silyl phosphites to aliphatic enimines under the same conditions gives a mixture of products of 1,2-addition to the C=N bond and 3,4-addition to the C=C bond containing no silyl groups, obviously due to hydrolysis. For imines with R1, R2=Me and R3=Bun or Pri, the isomer ratio is 95 : 5 in favour of the 1,2-adduct.In the case of a sterically hindered enimine (R3=But), the 3,4-adduct predominates in the reaction products (ratio 35 : 66). A detailed study of the reaction of enimines with hydro- phosphoryl compounds showed that it always involves the imine fragment of the enimine. Kinetic 227, 228 and thermochemical 65 studies made it possible to establish the hydrophosphorylation mechanism.The fast formation of the pre-reaction H-complex is followed by the slow step of addition of the hydrophosphoryl compound via a four-centred transition state. Bis(trimethylsilyl) hypophosphite almost instantly adds to imines even at 0 8C, giving adducts which easily undergo hydrol- ysis and oxidation in air. Based on spectral studies, the NH-phos- phonite structure with three-coordinate phosphorus is assigned to the adduct formed.207, 229 The reaction of bis(trimethylsilyl)hypo- phosphite withN-tritylimines is believed to occur 230 with cleavage of the Si7O bond to give phosphinate 69.In this case, the structure of the adduct was not studied. It is remarkable that the reaction of weakly basic bis(trime- thylsilyl) hypophosphite with N-benzylideneaminobenzoic acid can involve the attack by the P(III) atom both on the carbon and nitrogen centres of the imine.231 It has repeatedly been noted that the possibility to carry out a stereocontrolled reaction is of great importance when one selects the strategy for the synthesis of APA and their derivatives.Obviously, the `imine' method, which assumes the formation of a concerted cyclic transition state, is the method of choice.Sometimes a high de is reached in other methods as well.18 Catalytic methods to control the stereochemical result of the hydrophosphorylation of imines have been suggested in recent years. The effect of electronic, steric and external (catalyst, medium) reaction conditions on the stereoselectivity of hydrophosphoryla- tion of imines have been revealed using molecular mechanics methods232 and by analysis of structural and other factors.233 R=Me, Pri, Ph.RCH NCH(Me)Ph+P(OSiMe3)3 RCH NHCH(Me)Ph . HCl P(OH)2 O RCH N(SiMe3)CH(Me)Ph P(OSiMe3)2 O MeOH, D P C O N Si P OSi + C N P C N Si O O X O P OSiMe3+PhCH NPh O X O P CH N SiMe3 Ph Ph O O X O P CH NHPh O Ph 68 H2O O X O PHO+ PhCH NPh 68 X=CH(Me)CH(Me), . NR3 R1 R2 R1, R2=Ph, Me; R3=Pri, Bun, But, Ph, 4-MeOC6H4, 4-O2NC6H4, 2,4,6-Me3C6H2. (EtO)2POSiMe3 20 8C, CH2Cl2, 5 ± 15 h NHR3 R1 R2 P(O)(OEt)2 N R3 R1 R2 (EtO)2POSiMe3 1,2-adduct + (EtO)2P N R1 R3 R2 3,4-adduct O NH R1 R2 P(O)(OEt)2 R3 R=cyclo-C6H11.P O H +MeCH CHCH NR fast P O H NR CH CH CHMe slow P O H CHCH NR MeCH = CHCH NHR MeCH P O (Me3SiO)2PH+PhCH NPh PhCH NHPh P(OSiMe3)2 CPh3 R N (Me3SiO)2PH THF or CHCl3 20 8C, 12 h N CPh3 Me3Si P R O OSiMe3 H H+ P O OH H H2N R 69 R=H, Me, CH2CH2CH2OH, Pri, Ph, N .PhCH NC6H4COOH (Me3SiO)2PH PhCH NH C6H4COOH P(OSiMe3)2 +PhCH2 N P(OSiMe3)2 C6H4COOH 876 R A Cherkasov, V I GalkinFor example, the substituents in the benzene ring weakly affect both the chemical yield and the de in the reaction of imines obtained from 1-phenylethylamine and para-substituted benzal- dehydes with dialkyl phosphites 234 (Scheme 5).Scheme 5 The increase in the volume of substituents at the phosphorus atom decreases the chemical yield but increases the de The yield and de are considerably affected by catalysts used for this purpose, such as Lewis acids. A negative de upon changing the catalyst means the predominance of the other enantiomer, i.e., the change in the direction of stereoinduction.In the general case, a pronounced effect of catalysis is observed: the degree of conversion increases in comparison with non-catalysed addition. However, a catalyst basically affects the ease of the reaction. A catalysed process occurs at room temper- ature, while a non-catalysed one requires heating of the reaction mixture to 140 8C.The different effect of AlCl3 and BF3 on de, on the one hand, and ZnCl2 and TsOH, on the other, is explained 234 by the different character of the donor-acceptor interaction between the imine and the catalyst. Whereas Lewis acids form A-type chelate complexes due to the transfer of n- and p-electron density to the vacant orbitals of the central atom with s-cis- arrangement of the C=N fragment and aryl substituent, ZnCl2 and TsOH interact only with the imine nitrogen atom, causing a P-nucleophile to attack an imine in conformation B.The effect of catalysts 72 and of the way of the phosphite anion generation 235 on the stereochemistry of the hydrophosphoryla- tion of imines containing a complementary functional group that can provide chelation has been studied in detail.The imine 71 adds diethyl phosphite without a catalyst in 120 h. In the presence of Lewis acids or CF3COOH, the reaction occurs faster. In all cases, a mixture of diastereomers 72a and 72b with low de is formed. Catalyst Time /h Yield (%) de (%) ± >120 87 38 ZnCl2 7 84 48 MgCl2 >48 40 38 CF3COOH >48 48 14 It is believed that the stereocontrolling role of Lewis acids consists in the formation of chelates.However, this effect appears to be insufficient to ensure high diastereoselectivity of the hydro- phosphorylation step. At the same time, high diastereoselectivity is reached if lithium diethyl phosphite is used. The chelate structure of the intermediate with trans-arrange- ment of the nucleophile and the stereo-controlling phenyl group is responsible for this impressive stereochemical result.The yield of the adduct depends on the reaction conditions and the method of generation of the phosphite anion. The best result was achieved using butyllithium as the base. The stereochemically individual imine 73 obtained by the condensation of the appropriate amine with cyclohexanecarbal- dehyde adds lithium diethyl phosphite to give the R,R-isomer of an N-protected AP with de>95%.236 MeCHN Ph CH X+(RO)2PHO Cat, CH2Cl2 MeCHNHCH X Ph P(O)(OR)2 70 Yield of 70 (%) 80 84 72 79 77 80 de (%) 61 54 60 56 54 56 X H Me OMe Cl Br F R Bun Bui Ph Me Et Pri Yield of 70 (%) 74 72 78 85 85 61 de (%) 68 71 80 43 61 83 Catalyst AlCl3 BF3 .OEt2 ZnCl2 TsOH 7 Yield of 70 (%) 77 80 75 72 65 de (%) 70 61 730 750 716 C N H Ph C H Me M P A P B C N H Ph C H Me M R=cyclo-C6H11 .BuO2C N Ph (+)-72a (major product) P(OEt)2 + R O H BuO2C N Ph (7)-72b (minor product) H P(OEt)2 R O BuO2C N R Ph (7)-71 (EtO)2PHO, cat C6H6, 25 8C R1O O N R3 R2 Lewis acid N M O R1O R3 R2 PHO 72a+72b 72a+72b (7)-71 (EtO)2POLi Li O C N Ph ButO H R O P(OEt)2 R=cyclo-C6H11 . (EtO)2PHO, ButOLi, Conditions CH2Cl2,770 to 25 8C de (%) 94 Yield (%) 38 (EtO)2PHO, BuLi, THF,778 to 25 8C 94796 75780 R=cyclo-C6H11.R,R>95% LiPO3Et2 MeOCH2 N Ph R 73 MeOCH2 NH2 Ph RCHO MeOCH2 N Ph P(OEt)2 O R H The Kabachnik ± Fields reaction: synthetic potential and the problem of the mechanism 877The reaction of the same aldehyde with diethyl phosphite and (S)-a-phenylethylamine is less stereospecific: a mixture of diaster- eomeric AP (ratio S,R: S,S=5 : 1) is formed (see Ref. 236). High stereospecificity of reactions was achieved by addition of phosphites to bis-silylated imine 74 in the presence of lithium ions (Scheme 6).237 Enantiofacial preference of attack on the imine carbon by the P(III) atom is due to the formation of a highly- organised cyclic transition state 75 due to chelation by a Li+ ion.The addition of dialkyl phosphites to the imine 74 catalysed by BF3 is not accompanied by formation of a cyclic transition state and/or intermediate. In this case, the attack by a phosphite in acyclic complex 76 can occur on the two sides with equal probability. This causes the formation of a diastereomeric mixture of products. The absence of stereoselectivity was also noted in the case of boron trifluoride-catalysed addition of chiral cyclic diamidophos- phite 77 to imines.238 The reaction occurs under very mild conditions; it is also possible to use TiCl4 as the catalyst, while SnCl4 chlorinates the cyclophosphite to the corresponding chloro- phosphate. Catalytic asymmetric synthesis of AP with the use of a wide range of imines and dimethyl phosphite was carried out in the presence of an asymmetric lanthanide-based catalyst.33 The reaction occurs with excess of a hydrophosphoryl com- pound in THF at room temperature or on gentle heating (50 ± 60 8C).A chelate heterobimetallic complex involving a lanthanide, the chelating ligand BINOL33 and potassium ions (potassium carbonate) is used as the catalyst.The catalytic cycle is presented in Scheme 7. Scheme 7 The first step of the process is deprotonation of the phosphite and generation of potassium dimethyl phosphite, which is coor- dinated by the lanthanide with formation of the complex 78; the latter reacts with the imine to give the optically activeKsalt of AP 79. Replacement of the K+ ion by a proton at the amine centre results in the complex 80.In the final step, the product of catalytic asymmetric synthesis 81 is liberated from the coordination sphere. In this process, the lanthanum complex acts as a polyfunctional asymmetric catalyst of AP synthesis with high de (96%). A drawback of the method is the rather small number of catalytic cycles. This is probably due to the too high acceptor ability of the lanthanum complex with respect to AP.b. Reaction of hydroxy phosphonates with amines As noted above, the reaction of hydroxy phosphonates with amines is a model of the `hydroxy phosphonate' pathway of the Kabachnik ± Fields reaction. It has been studied in much more detail than the alternative `imine' route, i.e., hydrophosphoryla- tion of imines. Relatively little convincing evidence has been obtained that the Kabachnik ± Fields reaction occurs through the step of formation of a-hydroxyalkylphosphoryl intermediates.Moreover, as already mentioned, in certain cases the formation of hydroxy phosphonates is undesired for successful use of the Kabachnik ± Fields reaction for preparative purposes. Nevertheless, there are many examples of transformation of hydroxyalkylphosphoryl synthons to the corresponding amino- alkylphosphoryl products by the reaction of hydroxy phospho- nates with amines (see Refs 13 and 122 and references cited therein). In recent years, conditions for carrying out this process in a preparative and stereocontrolled way have been found.RCHO R=cyclo-C6H11; (a) (S )-PhCH(Me)NH2 , MeOH; (b) 3 HP(O)(OEt)2 , 3 days, 25 8C.a, b Ph N P(OEt)2 R H O N P N CH2Ph CH2Ph O H 77 775 to760 8C, 3 ± 17 h R1CH NR2 N P N CH2Ph PhH2C O NHR2 R1 R1=Me, Et, Pri, PhCHMe, n-C5H11 , cyclo-C6H11 ; R2=Tr, CHPh2, 4-MeOC6H4. R1 H N R2 HP(O)(OMe)2 cat R1 P(O)(OMe)2 HN R2 (MeO)2PHO La O O O O O O K K K R1 P(OMe)2 HNR2 O 81 * * O K O O O HNR2 O O K O La P(OMe)2 R1 K 80 La O O O O O O K K O KP(OMe)2 H 78 O K O O O KNR2 O K O La P(OMe)2 R1 H 79 * O R1 H N R2 Li N OSiMe3 Me SiMe3 Me3Si O P 75 N SiMe3 Me OSiMe3 Me3Si H P O + H Me OSiMe3 N P O Me3Si H N H Me OSiMe3 Me3Si H P O H N Me3Si Me OSiMe3 74 POSiMe3, Li+ P(O)H, BF3 + 6à Me3Si N BF3 H Me H HP O PH O O F3B SiMe3 76 Scheme 6 878 R A Cherkasov, V I GalkinThe condensation under the action of the azodicarboxylate± triorganylphosphine system (the Mitsunobu reaction 190) has found use for replacement of the hydroxy group by the amino group in a one-pot procedure.239 ± 242 Hydroxy phosphonates and phosphine oxides obtained by the Abramov reaction from the appropriate hydrophosphoryl and carbonyl compounds by treat- ment with the Mitsunobu reagent 82 and HN3 are converted with inversion of the configuration to a-azido phosphonate 83.The reaction of triphenylphosphine with phosphonate 83 by the Staudinger reaction results in phosphimide intermediate 84. The latter is hydrolysed to the target AP. In the one-pot procedure, the intermediate azide and phos- phimine are not isolated. Since the Mitsunobu reaction with secondary alcohols, including the above hydroxy phosphonates, occurs stereospecifically by the SN2 mechanism, the formation of the azido phosphate involves inversion of configuration of the a-carbon atom.Subsequent phosphimination and hydrolysis do not involve this centre. The Mitsunobu reaction described here, which enables con- version of hydroxy phosphonates to the corresponding azides, has been used 243 for the stereoselective conversion of monosilylated a,b-dihydroxy phosphonates.Optically active silyloxy phospho- nates result from the reaction of threo-a,b-dihydroxy phospho- nates with chlorosilane in the presence of bases. Depending on the nature of the substituents R and R0 and the base used, the ratio of b- and a-silylated products 85 : 86 is from 1 : 1 to 99 : 1. Most often, the regioisomer 85 predominates (>90%).Its amination 239 involves exclusively the carbon atom carrying the hydroxy rather than the silyloxy group and is accompanied by inversion of configuration. The Mitsunobu reaction is used in the synthesis of potential haptens, viz., N-hydroxy-a-amino phosphonates, for the conden- sation of hydroxy phosphonates with N-phenoxycarbonyl-O-tert- butoxycarbonylhydroxylamine.244 As a rule, the reaction goes only in one direction, viz., with the formation of AP (40% ± 75%).a-Hydroxy-2-phenylethyl- phosphonate is an exception, it undergoes intramolecular (rather than intermolecular) dehydration to give 62% of styrylphospho- nate PhCH=CHP(O)(OCH2Ph)2. V. Conclusion Despite the impressive progress in the synthesis and studies of properties of AP, not all the problems have been solved.The problem of stereocontrol in the `imine' method for the synthesis of AP is still urgent, to say nothing about the enantioselective synthesis using a three-component system in the one-pot version. It is easy to predict that the basic efforts in studying the AP chemistry will be concentrated in this direction, and the Kabach- nik ± Fields reaction is promising for solving these problems.The prospects of chemical modification of AP with introduc- tion of new, more and more complex groups, including those with definite configurations, to C-, N- and especially P-centres are far from exhausted. This aspect ofAP chemistry is closely related to the problem of the effect of phosphorus-containing groups on the adjacent reaction centre.96, 97 The attention to a-functionalised organo- phosphorus compounds has increased abruptly in recent years.This is evident, for example, from recent publications on the chemistry of a-phosphorylated alcohols (a-hydroxy phospho- nates),97, 245 a-halogeno phosphonates,246 a-oxo phospho- nates,247 and a-phosphonoalkyl phosphites.248 Reactions that occur at the a-carbon centre and formally do not involve the phosphorus atom are still largely controlled by the phosphorus- containing fragment.It not only determines the type and rate of the reaction but also, in some cases, changes the reaction rate by a few orders (see, for example, Ref. 249) and even changes the reaction mechanism.97 A large number of studies deal with the problem of the quantitative estimation of the effect of phospho- rus-containing groups (see Refs 250 ± 252 and references cited therein).This problem still remains the focus of attention of experts in quantitative organic and organometallic chemistry.253 The quantitative estimation of the dependence of the reaction pathway both of AP and components of the Kabachnik ± Fields reaction on the structure is an important problem which is waiting for solution.One may believe that the area of the application of AP will be much expanded. In addition to the search for new biologically active compounds of high efficiency among APA, their derivatives and analogues, an abrupt growth of interest should be expected to the prospects of practical application related to their complex- forming properties. These applications may primarily include the use of AP as recognising agents (synthetic receptors) and chiral ligands in metal complexes, as well as selective highly-efficient complexones, extraction agents and analytical reagents. Analysis of the data presented above allows one to consider the discussed area of chemistry as not only rather topical but also having wide prospects in the future.This study was financially supported by the Russian Founda- tion for Basic Research (Project No. 96-03-32864a). References 1. MI Kabachnik,TYaMedved' Dokl. Akad. Nauk SSSR 83 689 (1952) a 2. E K Fields J. Am. Chem. Soc. 74 1528 (1952) 3. P Kafarski, B Lejczak Phosphorus Sulfur Silicon Relat. Elem. 63 193 (1991) 4. 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ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Abstract. The review is concerned with the electronic and molec ular structure and physicochemical properties of all types of technetium clusters with acido ligands described to date. As a typical cluster-forming metal, technetium possesses a number of specific, anomalous cluster-forming properties that can be inter- preted in terms of greater capability of outer diffuse 5s(5p) AOs to participate in the formation of additional M7M bonds in technetium acido clusters as compared to analogous clusters of other transition d-elements.Theoretical interpretation of the electronic and molecular structure and properties of technetium clusters has been confirmed by experimental data (X-ray analysis, magnetochemistry, and EPR, optical, X-ray photoelectron and X-ray emission spectroscopy). The experimentally observed increased stability of technetium clusters with odd numbers of `metallic' electrons and a decrease in the effective charge on Tc atoms upon formation of the M7M bonds are discussed.Recently found `anomalous' properties of technetium and rhe- nium compounds with ferrocenium cations (in particular, unusual low-temperature electron emission on X-ray irradiation) are also considered.The bibliography includes 101 references. I. Introduction Compounds with metal ± metal bonds (clusters) were first isolated and structurally characterised early in the 1960s. Several thou- sands of clusters are known to date.1 Among all compounds of transition elements belonging to this type, only about 80 techne- tium-containing clusters with acido ligands have been described.2 This is mainly due to difficulties encountered in the synthesis of complexes of radioactive elements.However, a great interest has been expressed in such technetium clusters, because they possess a set of `anomalous' structural and other physicochemical proper- ties.3 In the present review, generalisation of experimental data on the known technetium acido clusters { has been made with primary emphasis on studies of physicochemical characteristics of these compounds (optical spectra in the visible, UV, and IR regions, magnetochemistry, X-ray emission spectra, X-ray photo- electron spectra, 57Fe MoÈ ssbauer spectra, electric conductivity and other properties) and comparison with those of mononuclear technetium complexes.All fundamental physicochemical proper- ties of technetium acido clusters insufficiently covered in previous reviews 1 ±3 are considered in particularly great detail. All technetium acido clusters known to date have been systematised according to their electronic and molecular structure and to the method of their synthesis.2 The nuclearity of the cluster, i.e., the number of metal atoms forming direct metal ± metal (M7M) bonds in the metal core of a cluster, is the basis of classification. In accordance with this principle, all technetium clusters are subdivided into di- and polynuclear clusters.Further classification is based on the electronic structure of the M7M bonds in the metal core of clusters and their molecular structure (e.g., dinuclear d 4 ± d 4 clusters of the `lantern' structural type or trigonal-prismatic hexanuclear clusters with metal ± metal bonds of high order).The main structural types of technetium clusters are shown schematically in Fig. 1. The most important physico- chemical properties of these clusters will be considered below. II. Optical spectra of technetium clusters in the visible, UV, and IR spectral regions 1.Electronic spectra No information on systematic studies of the optical spectra of technetium clusters has been reported to date. At best, spectral parameters of particular compounds were reported, and their qualitative assignment was made by analogy, e.g., with rhenium compounds.4±7 This is first of all due to difficulties in recording and interpreting the spectra, since this requires rigorous quantum- chemical calculations and precise spectroscopic measurements on single crystals in polarised light. The most precise spectroscopic studies have been carried out only for two clusters, viz., K3[Tc2Cl8] . 2H2O8 for which scattered-wave self-consistent field (SCF ±Xa-SW) quantum-chemical calculations 9 have been per- formed, and for [Tc2(2-PyO)4Cl] (where 2-PyO is the 2-hydrox- ypyridine alcoholate) 10 for which it has been possible to obtain suitable single crystals with all molecules oriented along the z axis of the tetragonal unit cell.The results of these studies have been described in detail;1 therefore, it is hardly reasonable to repeat S V Kryutchkov Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii prosp. 31, 117915 Moscow, Russian Federation. Fax (7-095) 335 17 78. Tel. (7-095) 335 20 04 Received 16 October 1997 Uspekhi Khimii 67 (10) 969 ± 992 (1998); translated by AMRaevsky UDC 546.718 Physicochemical properties of technetium acido clusters S V Kryutchkov Contents I. Introduction 883 II. Optical spectra of technetium clusters in the visible, UV, and IR spectral regions 883 III.Magnetochemistry of technetium oxo and halide complexes 889 IV. X-Ray photoelectron study of the structure of technetium complexes 894 V. Chemical shifts in X-ray Ka emission spectra of technetium compounds 898 VI. Compounds with ferrocenium cations 900 VII. Conclusion 903 { Here, we are dealing with compounds with metal ± metal chemical bonds and weak crystal field ligands that are anions of mineral acids: Cl7, Br7, I7, SO27 4 , etc.Russian Chemical Reviews 67 (10) 883 ± 904 (1998) #1998 Russian Academy of Sciences and Turpion Ltdthem in this review. We consider in brief only the most important conclusions drawn in these studies. The optical spectra of the cluster K3[Tc2Cl8] . 2H2O in the region of the d ± d* transition have been recorded at 300 and 3.7 K.8 They belong to the lowest-energy electronic spectra.Fine structure associated with vibronic interaction and caused by progression in the n0(M7M) frequency (320 cm71 for the elec- tronically excited state), i.e., n0, 2n0, 3n0, etc., is observed in these spectra. Since the M7M bond in the excited state is weaker than that in the ground state, the frequency n0 is about 50 cm71 lower than the frequency of the ground electronic state n(M7M).It should be noted that the n(M7M) frequency is observed in the Raman spectra and is symmetry-forbidden in the IR spectra. The spectra of the [Tc2(2-PyO)4Cl] single crystal at 5 Kin the region of the d ± d* transition as well as in other regions appear to be much more complicated, since not only a progression based on the n0(M7M) frequency (339 cm71), but also vibrations of the Tc7O and Tc7N bonds with frequencies n02, and n03 (264 and 298 cm71, respectively) are observed.10 Thus, the O7O band is followed by peaks corresponding to the n01, n02 and n03 frequencies. Moreover, not only the expected extension of progressions to all possible overtones of frequencies n01 and n02, but also progressions based on combination frequencies are observed in the spectra.Thus, Cotton et al.10 have identified vibrations with frequencies 5n02, 4n02+n01, 3n02+n01, 2n02+3n01, 2n02+4n01, and 5n01 in the fifth group of peaks (see also Ref. 8). Electronic absorption spectra of solutions of potassium octa- chloroditechnetate in hydrochloric acid in the UV and visible regions have been studied,4, 11 and the results (the dependence of the extinction coefficients emax on the HCl concentration) are listed in Table 1.As can be seen, the HCl concentration affects strongly the positions of the lines and the extinction coefficient in spectra of octachloroditechnetate ions, which is due to additional complexation and hydrolysis (see below). Table 2 presents the assignment of bands in the spectra of K3[Tc2Cl8] . 2H2O and [Tc2(2-PyO)4Cl].8, 10 An appreciably larger splitting between d and d* MOs due to shortening of the Tc7Tc bonds in the oxopyridine complex is the major feature of the electronic spectra of these compounds. Selected parameters of the electronic absorption spectra of technetium clusters are listed in Table 3. These data are far from being complete and thus can be used mainly for qualitative analysis.It is likely that the only exception is the p ± d* transition (lmax&600 ± 700 nm) that can be identified for all d4 ± d5 com- plexes by analogy with the [Tc2Cl8]37 ions. The absorption bands of this optical transition correlate well with the quantity 10Dq 4 characterising the strength of the crystal field of the ligand environment in the dinuclear complexes in question.It should be noted that an analogous tendency is also observed for most of other bands. The electronic spectra of polynuclear technetium clusters are characterised by the absence of clearly defined absorption bands and by a monotonic increase in the optical density with increase e a b c D4h D4h D4d d D3h D2h f Oh Figure 1.The main structural types of technetium acido clusters:2 (a) [Tc2X8]n7 (X=Cl, Br; n=2, 3); (b) {[Tc2L4]Xm}n7 (L=SO27 4 , AcO7, But COO7, 2-PyO7; X=Cl, Br, H2O, m=1, 2; n=0, 1, 2); (c) [Tc2X6]27 (X=Cl, Br); (d) [Tc6(m-X6)X6]n7 (X=Cl, Br; n=0, 1, 2); (e) [Tc8(m-X8)X4]n7 (X=Br; n=0, 1); and (f) [Tc6(m3-Br)5Br6]27. The positions of bridging or axial ligands are shown by dashed lines.Table 1. Dependence of the extinction coefficients of [Tc2Cl6]37 on the HCl concentration.4, 11 [HCl] lmax 103 emax lmax, 103 emax / mol litre71 / nm / litre mol71 cm71 / nm / litre mol71 cm71 11.4 647 0.33 322 3.6 10.2 643 0.32 320 3.5 7.6 640 0.29 320 3.6 6.5 638 0.29 320 3.0 5.9 635 0.26 318 2.7 4.8 630 0.25 318 2.5 3.0 618 0.19 315 2.2 Table 2. Electronic spectra of [Tc2Cl8]37 and [Tc2(2-PyO)4Cl] and the band assignment.1, 8, 10 [Tc2Cl8]37 [Tc2(2-PyO)4Cl] Transition 103 nmax emax a 103 Ecalc 103 nmax Irel / cm71 / cm71 / cm71 5.9 630 6.0 13.0 m d ± d* 13.6 35 15.8 18.0 vw, br p ± d* 15.7 172 15.8 17.0 w, br d* ± p* 20.0 10 17.7 7 7 d* ± dx2 ± y2 20.2 d* ±s* 21.3 d ± p* 23.0 d ± dx2 ± y2 31.4 3900 28.3 25.0 s LMCT 29.1 LMCT 31.2 p ± p* 32.5 p ± dx2 ± y2 37.2 5600 42 7 7 LMCT 41 LMCT 44 LMCT Note.Ecalc is the calculated transition energy, Irel is the relative intensity, and LMCT denotes the bands with ligand to metal charge transfer. The bands with electric dipole transitions are underlined. a Measured in litre mol71 cm71. 884 S V Kryutchkovin energy.4 This is due to a large number of allowed electronic and vibronic transitions close in energy in the molecules of these compounds, which precludes their experimental observation because of limited resolution of spectrometers. Despite the small number of reliable theoretical assignments of electronic spectra of technetium clusters, they are successfully employed by experimentalists, e.g., in studies of the mechanism and kinetics of reactions in solutions.2, 4, 11, 15, 17 Typical changes in the absorption spectra of potassium octachloroditechnetate in hydrochloric acid solutions in the presence and in the absence of atmospheric oxygen are shown in Fig. 2. Based on these data, the kinetics and mechanisms of acid hydrolysis, complexation, dis- proportionation with the cleavage of M7M bonds, oxidative addition of atmospheric oxygen to a multiple M7M bond and cycloaddition of multiple M7M bonds with the formation of polynuclear clusters occurring in the system under consideration and characterising the stability of octahalotechnetate ions in solutions of hydrohalogenic acids have been studied.Each reac- tion dominates under particular conditions depending on the chemical nature of the solvent, concentration of dinuclear com- plexes and complexing agent, temperature, etc.Based on kinetic experiments on the stability of octahaloditechnetate ions in solutions of hydrohalogenic acids, it has been possible to calculate the effective rate constants for some of the reactions listed above and, in particular, to establish that the stability of these ions in solutions and their thermal stability in the solid state is determined by the strength of the M7M bonds and decreases in the series Cl>Br>I.2 2.Vibrational spectra The vibrational spectra of technetium clusters have been studied somewhat more thoroughly. However, most of them are not very informative, since the fundamental vibrational modes of cluster fragments themselves are observed in the long-wave region. Moreover, the most interesting n(M7M) vibrations are forbidden in the IR spectra due to the centrosymmetric structure of most of the clusters and are allowed in the Raman spectra that cannot be recorded in many cases because of an intense dark colour of the compounds. Thus, only the IR spectra containing bands corre- sponding to stretching and bending vibrations of the atoms in cations or to characteristic vibrations of the Tc7X bonds (X is a ligand) can be observed for most of the technetium clusters.In this case, the Tc7X type of vibrations can be isolated from a set of more complex normal vibrations only by convention, since these can hardly be considered as characteristic ones because of large masses and strong mutual influence of the ligands. As has been mentioned in the previous Section, vibronic spectra can serve as yet another source of information on the cluster vibrational modes.However, vibrational progressions observed in this case will be characteristic of the electronically excited state of the cluster rather than of its ground state.18 Table 3. Electronic absorption spectra of technetium clusters. Cluster lmax / nm Note Ref.(emax a or Irel) [Tc2(m-O)2(H2EDTA)2] 500 (2000), aqueous 14 600 (<200) solution [Tc2(m-O)2(H2EDTA)2]7610 (s) ditto 14 [Tc2(m-O)2(TCTA)2]27 510 (s) " 14 [Tc2(m-O)2(TCTA)2]37 592 (s) " 14 [Tc2Cl8]27 680 (w), 391 (m), TBA salt 5 303 (m), 238 (s) in CsCl pellet at 10 K [Tc2Cl8]37 676 (w), 388 (w, sh) ditto 5 337 (m), 308 (w, sh) [Tc2I8]37 760 (250) solution in HI 4 (conc.) [Tc2Br8]37 690 (400) solution in HBr 15 (conc.) [Tc2(AcO)4Cl2]27 620 (200) solution in 4 AcOH [Tc2(SO4)4]27 700 (300) solution in 16 H2SO4 [Tc2Cl6]27 460 (600) solution in 4 HCl [Tc2Br6]27 480 (600) solution in 4 HBr Note.TBA is tetra-n-butylammonium, EDTA is ethylenediaminetetraa- cetate and TCTA is 1,4,7-triazacyclononane triacetate. a Measured in litre mol71 cm71. & D 1 2 3 4 5 6 7 8 7 8 1 2 3 4 5 6 a 8 1 7 6 5 4 3 2 1.2 1.4 0.8 0.4 0 & 8 7 6 5 4 3 2 1 8 1 7 6 5 4 3 2 b 1.2 1.4 0.8 0.4 0 1.2 0.8 0.4 0 300 350 400 450 500 600 700 l /nm 1 2 5 3 4 6 7 8 9 10 & 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 8 79 10 8 1 7 6 5 4 3 2 c Figure 2.Typical changes in the absorption spectra of solutions of K3[Tc2Cl8] .H2O in hydrochloric acid in the absence (a) and in the presence of atmospheric oxygen (b, c) at initial concentrations of octa- chloroditechnetate(II, III) ions of less than 561074 mol litre71 (a, b) and more than 561073 mol litre71 (c).The duration of the oxidation of [Tc2Cl8]37 with oxygen (b, c) and of disproportionation of [Tc2Cl8]37 (a) increases uniformly in the order of increasing the numbers of spectra in the series (1 ± 10).4, 17 Physicochemical properties of technetium acido clusters 885Thus, all available information on the vibrational spectra of technetium clusters is reduced mainly to characteristic atomic vibrations in the cations in the region 4000 ± 400 cm71 and to vibrations of cluster anions (or molecules) themselves in the region 400 ± 50 cm71.Since the anions of different chemical nature have little effect on the cations, the absorption bands corresponding to characteristic vibrations of the same cations in different com- pounds differ insignificantly in most of spectra.In this connection, the IR spectra in the region 4000 ± 400 cm71 can be used mainly for qualitative analysis, and it makes no sense to consider them in this review, since they are rather characteristic of the cations themselves.Apparently, the IR spectra of differently hydrated hydroxonium cations and certain bridging bidentate (e.g., carbox- ylate or sulfate) ligands are the exceptions, since they can contain essential information on the formal charge and on the electronic and molecular structure of the cluster fragments. Let us consider the results obtained from these spectra in more detail.The absorption bands corresponding to stretching and bend- ing vibrations of the OH groups in technetium clusters with differently hydrated hydroxonium cations 19 are listed in Table 4. The results obtained indicate that the pattern of the IR spectra depends on the character of bonding of the OH groups in crystals of the compounds in question.For instance, since only isolatedH3O+ions are present in the crystals of compound 7, only two bands corresponding to the n(OH)H3Oá and d(H7O7H)H3Oá vibrations are observed in its IR spectrum (see Table 4). Only water molecules of crystallisation are present in the cluster 1; therefore, only absorption bands due to the n(OH)H2O and d(H7O7H)H2O vibrations are observed in the IR spectrum of this compound. As can be seen, their positions differ considerably from those of corresponding absorption bands of hydroxonium cations.Several bands due to stretching and bending vibrations of the OH groups are simultaneously observed in the IR spectra of compounds 2 ± 5 and 8 (see Table 4). This is probably due to the fact that the bridging hydrogen atoms in the [H(H2O)2]+ and [H3O(H2O)3]+ cations are closer to one of the oxygen atoms while to the other oxygen atom they are bonded by a strong hydrogen bond.Therefore, the absorption bands corresponding to vibra- tions of both H3O+ ions and H2O must be observed in the IR spectra of hydrated hydroxonium cations. It should be noted that in this case the frequencies of d(H7O7m-H) absorption bands must be intermediate between those of the d(H7O7H)H2O and d(H7O7H)H3Oá vibrations.A much larger number of bands corresponding to bending vibrations must be observed in the case of partially deuterated differently hydrated hydroxonium ions (see Table 4). Thus, IR spectra, as well as the experiments on thermal dehydration and magnetic measurements, can be used to estab- lish 19 the structure of clusters if their crystals contain differently hydrated hydroxonium cations, whereas X-ray analysis provides no unambiguous solution of this problem.Yet another example of this type is provided by the IR spectra of compound (Bun4 N)2[Tc6(m3-Br)5Br6] in which no absorption bands corre- sponding to vibrations of theOHgroups are observed. In contrast to the initial assumptions,20 this indicates that the [Tc6(m3- Br)5Br6]27 anion contains no OH groups.The IR spectra of m-acetato dinuclear d 4 ± d 5 technetium complexes have been studied and all absorption bands have been assigned.21 Previously,7 the IR spectra of analogous dinuclear d4 ± d4 technetium and rhenium clusters have been studied; however, no complete assignment of the absorption bands has been performed.From comparison of the results obtained it follows that, in contrast to analogous dinuclear complexes of other elements (e.g., copper acetates), the splitting (D) between the frequencies of symmetric ns(C7O) and antisymmetric nas(C7O) vibrations of the bridging acetate anions in technetium clusters is extremely small (*20 cm71). It should be noted that a tendency for the D value to decrease is observed on going from monodentate acetate ions to bidentate ones and that the anomalously small value of this splitting is probably due to a specific influence of strong Tc7Tc bonds. The frequencies of the n(Tc7X) vibrations are appreciably shifted towards the long-wave region relative to analogous frequencies of other halide technetium complexes without bridging ligands.This indicates appreciable weakening of the Tc7X bonds in m-acetato technetium complexes as compared to analogous bonds in other halide Tc complexes (Table 5). From these data altogether it has been suggested 21 that the compounds under consideration have the `lantern' type of structure (see Fig. 1 b), viz., two technetium atoms form a strong Tc7Tc bond, four bridging acetate ions additionally bind Tc atoms to one another, while two halide ions are in axial positions with respect to the M7M bonds and are in essence ionically bonded to the technetium atom.This assumption has been completely confirmed by X-ray analysis.22 ± 24 Using the IR spectra, the structures of sulfate dinuclear technetium complexes have also been determined.16 The param- eters of the IR spectra and the assignment of bands of two sulfate dinuclear technetium complexes, K2[Tc2(SO4)2] . 2H2O and K4(H3O)2[Tc2(SO4)6], are listed in Table 6. The IR spectrum of the former compound contains absorption bands in the region 330 ± 3530 cm71 with maxima at 3450 and 1655 cm71 corre- sponding to stretching and bending vibrations of coordinated water molecules, respectively.Four groups of bands each split by the crystal field are observed in the regions of both stretching and Table 4. Frequencies and relative intensities of the bands of bending (d) and stretching (n) vibrations ofOHgroups in the IR spectra of technetium clusters (in KBr pellets).19 Compound d(H7OH) / cm71 n(OH) / cm71 d(H7OH)H3O+ d(HO7m-H) d(H7OH)H2O H3O+ H2O {[Tc8(m-Br)8Br4]Br} . 2H2O (1) 7 7 1620 (m) 7 3520 (br, s) [H(H2O)2]{[Tc8(m-Br)8Br4]Br} (2) 1030 (w) 1410 (m) 1620 (m) 3150 (vw) 3540 (br, s) [H(H2O)2]2{[Tc8(m-Br)8Br4]Br2} (3) 1040 (m) 1420 (m) 1620 (m) 7 3550 (br, s) [H3O(H2O)3]2[Tc6(m3-Br)5Br6] (4) 1045 (m) 1400 (m) 1618 (m) 7 3550 (br, s) [H3O(H2O)3]2[Tc6(m3-Br)5Br6] . 4H2O (5) 1035 (m) 1420 (s) 1620 (s) 7 3550 (br, s) [H3O(H2O)3]2[Tc6(m3-Br)5Br6] .nH2O (6) a 980 (s) 1322 (m) 1390 (m) 7 2800 (vw) 1040 (s) 1350 (m) 1620 (s) 7 2930 (w) 1420 (s) 3020 (w) (H3O)2[TcBr6] (7) 1030 (m) 7 7 3150 (m) K03¡xK00 6(H3O)x[Tc2Cl8]3 . nH2O (8) b 7 7 1610 (m) 2965 (w) 3550 (br, s) 2930 (w) 2865 (w) a Partially deuterated specimen. bK0 denotes K+ ions in special three-fold position in the structure (on the two-fold axis) and K00 denotes K+ ions in general position in the structure. 886 S V Kryutchkovbending vibrations of sulfate ions; this corresponds to distortion of the tetrahedral SO27 4 ion to the idealised C2u symmetry due to the formation of bidentate bridging bonds between the sulfate ions and the Tc6á 2 fragment,25 which is analogous to those found in the structure of the [Re2(SO4)4]27 anion.26, 27 The IR spectrum of compound K4(H3O)2[Tc2(SO4)6] is char- acterised by a more intense absorption band in the region *2950 cm71, which can be assigned to the n(OH)H3Oá vibrations (see Table 6).Three groups of broad bands are observed in the region of stretching and bending vibrations of the S7O bond in the spectrum of this compound; this is characteristic of mono- dentate sulfate ions with C3u symmetry.25 Later, the assumptions 16 concerning the structure of com- pound K2[Tc2(SO4)4] . 2H2O made on the basis of IR spectra, the results of chemical analysis and data of thermogravimetric anal- ysis and other physicochemical methods were confirmed by X-ray study of a single crystal synthesised by the author of this review.{ The assignment of the absorption bands of a number of di- and poly-nuclear halide technetium clusters is given in Table 5 and the long-wave IR spectra of several polynuclear technetium clusters are shown in Fig. 3. The following major tendencies are observed for compounds of similar structure, viz., the absorption bands corresponding to the n(Tc7X) and d(Tc7Tc7X) vibra- tions are, as a rule, shifted towards the low-energy region as the formal oxidation state of technetium decreases and the replace- ment of ligands in the series Cl, Br and I occurs.As has already been noted, vibrations of the M7M bonds are symmetry-for- bidden and are not observed in the IR spectra of most of dinuclear clusters; however, in polynuclear clusters this exclusion is removed, and certain n(Tc7Tc) and d(Tc7Tc7X) vibrations can also be observed in the IR spectra. Thus, the stretching vibrations of the M7M bonds of F1u; E 0 and A00; and B1u, B2u and B3u symmetry types are expected to be observed in the spectra of octahedral, trigonal-prismatic, and tetragonal-prismatic clus- ters, respectively. For lack of sufficient experimental data, unam- biguous assignment of all vibrational frequencies for polynuclear clusters is currently impossible.Therefore, their long-wave IR spectra can be used mainly for qualitative analysis and determi- nation of the structure. Thus, a large number of relatively narrow absorption bands in this spectral region is almost unambiguous evidence for the polynuclear nature of the clusters, while the positions of these lines can be tentative evidence for the formal oxidation state of technetium.The use of IR spectra of technetium and rhenium complexes with ferrocenium cations for the solution of analytical and structural problems has been described.28, 29 The composition Table 5. Frequencies of absorption bands (cm71) in the long-wave IR spectra of technetium halide clusters and their assignment.28, 29 Cluster n(M7M), n(M7X), d(XMY), d(MMX) [Tc2(AcO)4Cl] 7 180 7 [Tc2(AcO)4Br] 7 120 7 [Tc2(2-PyO)4Cl] 383 a (337 b) 10 7 7 [Tc2Cl8]27 7 362, 342 179, 170 [Tc2Br8]27 323 a 5 269, 249 5 7 [Tc2Cl8]37 370 a (320 b) 1 335, 310 184, 157 [Tc2Cl6]2n¡ n 7 310, 285 183, 175 [Tc6Br12] (see c) 324, 302, 298 234, 170 141, 120. 106 [Tc6Br12]7 (see c) 349, 335, 298 230, 181 100 [Tc6Cl12]7 (see c) 349, 336, 298 253, 233 189, 176, 153, 122 [Tc6Cl12]27 360 (br) 303 (br) 195, 153 254 (br) [Tc8Br12] (see c) 355, 290, 266 232, 213 120, 96 [Tc8I12] (see c) 342, 283, 243 187, 113 80, 61, 54 [Tc6Br11]27 330, 292 266, 238 (w) 164 (w), 120, 76 a Data of Raman spectra.b Data of vibronic spectra. c Axial ligands are omitted. Table 6. Frequencies (cm71) and relative intensities of bands in the IR spectra of sulfate dinuclear technetium complexes.16 Compound n(OH) d(H7O7H) n(S7O) n(Tc7O) d(O7S7O) Other bands K2[Tc2(SO4)4] .2H2Oa 3450 (s, br) 1655 (w) 1295 (s), 1230 (w); 698 (m) 698 (s); 381 (m), 335 (s, br) 1185 (sh), 1155 (s); 600 (m), 585 (m); d(O7Tc7O), 1070 (m), 1041 (m); 532 (sh, w), 522 (w); 250 (sh, br), 915 (s), 850 (m) 465 (m) 200 (m), 181 (w), 158 (m), 152 (w), 118 (m), 78 (s) K4(H3O)2[Tc2(SO4)6] b 2950 (sh, br) 1230 (sh, br), 1155 (s); 675 (m) 605 (s); 290 (s, v br) d(O7Tc7O) 1045 (s); 535 (w); 156 (s, v br), 965 (s), 850 (sh) 483 (m) 80 (w, br) a Sulfate ions have C2u symmetry in the complex K2[Tc2(SO4)4] . 2H2O. b Sulfate ions have the C3u symmetry in the complex K4(H3O)2[Tc2(SO4)6]. { X-Ray analysis has been done by P A Koz'min,M D Surazhskaya, and T B Larin and the results of these investigations have been described in Ref. 2. a Intensity (rel. units) 500 400 300 200 100 n /cm71 b c d Figure 3. The long-wave IR spectra of polynuclear technetium clus- ters:28, 29 (a) (FeCp2)2[Tc8Br14]; (b) (FeCp2)3[Tc6Cl14]; (c) (Bun4 N)3. .[Tc6Cl14]; and (d) [H(H2O)2]2[Tc8Br14]. Physicochemical properties of technetium acido clusters 887and structure of these complexes suggest that the lines corre- sponding to vibrations of ferrocenium cations, technetium or rhenium anions, and hydrated hydroxonium cations {in the case of compound (FeCp2)4[H3O(H2O)4]2 .[TcCl6]3} must be observed in their IR spectra. According to the published data,30 if ferrocene and ferrocenium have the D5d(D5h) point symmetry, then vibra- tions of A2u(A00 2) and E2u(E 00 2) symmetry types must be active in their IR spectra.It has been shown 28, 29 that all most intense vibrational lines of ferrocene are also observed in the IR spectra of technetium and rhenium compounds with ferrocenium cations. The only exception are the absorption bands in the region 480 ± 820 cm71, where only one composite line in the region 670 ± 560 cm71 is observed instead of very intense bands corre- sponding to the nonplanar rCH (A2u) vibrations and n(Fe7C- p) (E1u) stretching vibrations characteristic of pure ferrocene; this line is probably due to the interaction between the above two vibrations. It is known 30 that interaction between these types of vibrations [as well as a shift of n(Fe7Cp) towards the high- frequency region] characterises an increase in covalency of the Fe7Cp bonds.It should be noted that neither absorption bands corresponding to the n(Fe7Cp) (E1u) vibrations in the IR spec- trum of compound (FeCp2)2[Tc8Br14] nor the bands correspond- ing to bending skeletal n(Cp7Fe7Cp) (E1u) vibrations in the spectra of all compounds studied are observed. All bands in the IR spectra of compounds containing [Tc6Cl14]37 and especially [Tc8Br14]27 anions are considerably broadened and diffuse (see Fig. 3).Analogous effects have been found for certain specimens preliminarily frozen to *78 K. It has been pointed out that this phenomenon can be due to the effects of vibronic interaction in the molecules.29, 30 As to the absorption bands corresponding to stretching and bending vibrations of isolated hydroxonium cations in the cluster [Fc(Cp2)]4[H3O(H2O)4]2[TcCl6]3, it is difficult to identify them because of the superposition of more intense lines of ferrocenium cations.Nevertheless, certain bands in the region of d(H7O7H) bending vibrations (at 1620, 1555, 1510, and 1058 cm71) can be identified unambiguously. The conclusion that the effect of ferrocenium cations on the vibrational spectra of anions is not so significant as the reverse effect of various anions on the vibrations in ferrocenium cations is yet another important result.28, 29 Never- theless, this effect persists and manifests itself mainly as a shift of frequencies of these vibrations.Similar shift of the frequencies of stretching and bending vibrations of technetium anions due to the effect of various cations has been found;31 it has been shown that the absorption bands of the anions are shifted towards the low- frequency region as the average size of cations increases. For pertechnetate ions, an increase in the cation size is accompanied by a decrease in the splitting of the n(Tc=O) stretching band.Comparison of published data (see Refs 28, 29 and 31) shows that the extent to which the anions are affected by the ferrocenium cations, the effective radius (Reff) of which is larger than 3 A, approaches that of (Me4N)+ and (Et)4N+ having similar Reff.It should be noted that the absorption bands in the long-wave IR spectra of compound (FeCp2)2[Tc8Br14] are also considerably broadened and some of them are missing (see Fig. 3), as is the case of the short-wave IR spectra.This may also be associated with vibronic interactions.28, 29 The IR spectroscopic studies of ferrocenium salts with Tc- and Re-containing anions 28, 29 have shown that: 1. All compounds correspond to the expected composition and structure. 2. The absence of additional absorption bands in the IR spectra of the compounds obtained (except for the bands corre- sponding to anionic and cationic vibrations) indicates that no formation of new types of directed chemical bonds occurs except for those expected from the composition and structure of these compounds. 3. Strong mutual influence of the ferrocenium cations and the technetium (rhenium) anions is observed, which increases in the series of anions [TcO4]7<[Re2Br8]27&[Tc6I14]37<[TcCl6]27< <[Tc6Cl14]37<[Tc8Br14]27 and is probably due to the strength- ening of vibronic interaction in this series.The assignment of absorption bands in the long-wave IR spectra of [Tc2X8]n7 (X=Cl, Br; n=2, 3) dinuclear technetium clusters with a simpler structure has been performed.1, 4, 5, 8, 32 Thus, the IR spectra of these anions contain two absorption bands with Eu and A2u symmetry corresponding to the n(Tc7X) stretching vibrations and one A2u band corresponding to the d(X7Tc7X) vibrations, while their Raman spectra contain two absorption bands with A1g symmetry corresponding to the n(Tc7Tc) and n(Tc7X) vibrations and one band with Eg sym- metry.The frequencies of these bands in the spectra of different anions are listed in Table 5.Complete normal coordinate analysis for the [Tc2X8]n7 dinu- clear complexes (X=Cl, Br; n=2, 3) has been performed, the bands in their IR and Raman spectra (including those of isotope- substituted complexes) have been assigned, and ten force con- stants have been calculated.6 The structural characteristics of these ions used in the calculations and the force constants obtained are listed in Table 7.The assignment of bands in the IR and Raman spectra of analogous rhenium and osmium dinuclear complexes with D4h and D4d symmetry, respectively, has also been performed.6 The calculations reproduce well the major regular- ities expected from the data of X-ray studies. Thus, from Ref. 6 and the data in Table 7 it follows that for all the complexes in question the bond stretching force constants fd (M7M) and fd (M7X), as a rule, decrease as the ligands are replaced in the series F, Cl, Br and I and n increases. It should be noted that these Table 7.Bond lengths (d /A), bond angles (a /deg) and force constants (f /mdyn A71) for [Tc2X8]n7 (X=Cl, Br; n=2, 3) ions.6 Parameter [Tc2Cl8]27 [Tc2Cl8]37 [Tc2Br8]27 [Tc2Br8]37 d (Tc7Tc) 2.15 2.11 2.15 2.13 d (Tc7X) 2.32 2.36 2.47 2.51 a (Tc7Tc7X) 103.80 104.60 103.90 104.00 fd (Tc7Tc) 3.79 4.86 2.67 3.15 fd (Tc7X) 1.79 1.47 1.48 1.31 d 2(X7Tc) fa(X7Tc7X) 0.67 0.64 1.55 1.60 d (X7Tc) d (Tc7Tc)fa(X7Tc7Tc) 1.09 0.88 1.37 1.15 fdd(TcTc7TcX) 0.11 0.24 0.05 0.13 fdd(TcX7TcX) 0.42 0.40 0.32 0.36 d (Tc7X) fda(TcX7XTcX) 0.03 0.02 0.02 0.01 d (Tc7X) fda(TcTc7XTcTc) 0.30 0.30 0.17 0.19 d (Tc7X) d (Tc7Tc) faa (XTcTc7XTcTc) 0.25 0.15 0.23 0.05 Note.The subscripts d and a at the force constants f correspond to stretching and bending vibrations, respectively; the direction of the vibrations is given in parentheses. 888 S V Kryutchkovchanges and changes in analogous interatomic distances due to ligand replacement and the decrease in the formal oxidation state of the metal occur in the opposite directions.Of all the complexes studied, the largest fd (M7M) value was obtained for the [Tc2Cl8]37 anion with a formal M7M bond order of 3.5 for which the shortest M7M distance has been found experimen- tally.2 In conclusion of this Section, it should be noted that many problems associated with the assignment and interpretation of the optical spectra of technetium clusters remain unsolved due to the complex structure of these compounds, the lack of sufficient experimental data (which is often due to the impossibility of obtaining them with currently available instruments) as well as to the absence of reliable theoretical calculations.However, the available data can well be used for qualitative structural analysis.III. Magnetochemistry of technetium oxo and halide complexes Among experimental methods of studying the electronic structure of coordination compounds, magnetochemical measurements occupy a particular place since they make it possible to determine the number of unpaired electrons in the molecule, the magnitude of intermolecular exchange interactions, and in some instances can even serve as immediate evidence for the relative order of some MOs in the molecules of these compounds.Sometimes, magneto- chemistry can be used as an analytical and, to some extent, as a structural investigation technique.33 However, no information on systematic studies of magnetochemical properties of technetium complexes has been reported so far.34 The results available 35, 36 have not been systematised, contain no data on diamagnetic corrections for technetium and thus are unreliable.In connection with the foregoing, in this review we discuss the magnetic proper- ties of technetium complexes in which Tc atoms have different oxidation states and which have been studied by the static magnetic susceptibility method and by EPR spectroscopy. The data of magnetic measurements for technetium com- pounds are listed in Table 8.Since no diamagnetic corrections for technetium ions are known, they were obtained by extrapolat- ing the data on diamagnetic corrections for neighbouring elements in the Periodic Table (Mn, Re, Mo and Ru).34 The results of calculations of the electronic structure of technetium oxo and halide complexes are shown in Fig. 4. As can be seen, in most cases the experimental data are in good agreement with theoretical predictions. Let us consider the most typical cases in detail. Pertechnetates are tetrahedral d 0 complexes. The recently found distortion of these complexes 37 ± 44 has virtually no effect on their diamagnetic properties, except for weak temperature- Table 8. Magnetic properties of technetium oxo and halide complexes.18, 28, 34 Compound Tc Electron geff meff /mB y /deg Number oxidation configura- of unpaired state tion 80K 300K electrons M[TcO4] (M=K, NH4, Bun4 N) +7.0 d 0 7 see a see a 7 0 (FeCp2)[TcO4] +7.0 d 0 2.01 2.78 3.88 7185 1.95 ± 3.00 M2[TcOCl5] (M=K, NH4) +5.0 d 2 7 see a see a 7 0 (Bun4 N)[TcOX4] (X=Cl, Br, I) +5.0 d 2 7 see a see a 7 0 Tc2O5 .nH2O +5.0 d 2 7 see a see a 7 0 [TcO(NO3)3] .H2O +5.0 d 2 7 see a see a 7 0 K3[Tc2Cl8O2] +4.5 d 2 ± d 3 7 3.69 4.15 740 3 M2[TcCl6] (M=K, NH4) +4.0 d 3 7 2.79 3.44 768 3 1/3 {[FeCp2]4[H3O(H2O)4]2[TcCl6]3} +4.0 d 3 2.015 6.94 6.80 9 6.02 ± 5.87 (Bun4 N)2[TcCl6] +4.0 d 3 7 3.78 3.80 0 3 M2[TcBr6] (M=K, NH4) +4.0 d 3 7 3.21 3.45 724 3 M2[TcI6] (M=K, NH4) +4.0 d 3 7 2.79 3.34 760 3 (Me4N)2[TcI6] +4.0 d 3 7 3.68 3.67 0 3 TcO2 +4.0 d 3 7 1.0 1.5 200 1 (Bun4 N)2[Tc2X8] (X=Cl, Br) +3.0 d 4 ± d 4 7 see b see b 7 0 [Tc2(RCOO)4Cl2] (R=Me, But) +3.0 d 4 ± d 4 7 see b see b 7 0 K2[Tc2(SO4)4] .2H2O +3.0 d 4 ± d 4 7 see b see b 7 0 M3[Tc2Cl8] .nH2O (M=K, NH4, C5H5NH, C9H8NH) +2.5 d 4 ± d 5 2.09 1.75 1.75 0 1 M3[Tc2Br8] . 2H2O (M=K, NH4) +2.5 d 4 ± d 5 2.09 1.74 1.74 0 1 [Tc2(AcO)4X] (X=Cl, Br) +2.5 d 4 ± d 5 2.09 1.75 1.73 0 1 K[Tc2(AcO)4Cl2] +2.5 d 4 ± d 5 2.09 1.75 1.75 0 1 1/2 [Tc4O5] .nH2O +2.5 d 4 ± d 5 7 1.77 1.75 0 1 M2[Tc2Cl6] (M=K, NH4; X=Cl, Br) +2.0 d 5 ± d 5 7 see b see b 7 0 (Et4N)2{[Tc6(m-Br)6Br6]Br2} +2.0 6d 5 7 0.85 1.16 0 0.1 M3{[Tc6(m-X)6X6]X2} (X=Cl, Br,M=Me4N, Bun4 N) +1.83 (3) 5d 5 ± d 6 2.00 (gk), 1.5 1.7 1 1 1.63 (g?) (FeCp2)3{[Tc6(m-Cl)6Cl6]Cl2} +1.83 (3) 5d 5 ± d 6 2.007 5.75 6.01 0 4.85 ± 5.13 (Bun4 N)3{[Tc6(m-Cl)6Cl6]Cl2} +1.83 (3) 5d 5 ± d 6 2.00 (gk), 1.50 1.77 0 1 1.63 (g?) (Me4N)2[Tc6(m-Cl)6Cl6] +1.66 (6) 4d 5± 2d 6 1.97 0.8 1.1 760 0.2 {[Tc8(m-Br)8Br4]Br} . 2H2O +1.625 5d 5± 3d 6 7 1.12 1.57 120 1 (Bun4 N)3{[Tc6(m3-Br)5Br6] +1.5 3d 5± 3d 6 2.0 0.73 0.91 0 0.1 [H3O(H2O)3]2[Tc6(m3-Br)5Br6] .nH2O (n=0, 4) +1.5 3d 5± 3d 6 2.0 1.0 1.2 780 0.2 [H(H2O)2]n{[Tc8(m-Br)8Br4]Brn} (n=1, 2) +1.5 4d 5± 4d 6 7 see b see b 7 0 (Bun4 N)2{[Tc8(m-X)8X4]I2} (X=0.5 Br+0.5 I) +1.5 4d 5± 4d 6 7 see b see b 7 0 (FeCp2)2{[Tc8(m-Br)8Br4]Br2} +1.5 4d 5± 4d 6 7 4.82 5.87 764 3.83 ± 4.96 a Compounds are diamagnetic. b Temperature-independent paramagnetism. Physicochemical properties of technetium acido clusters 889independent paramagnetism (TIP) } observed in some cases (see Table 8 and Fig. 4). It should be noted that according to Fig. 4, the [TcO4]27 ions formed in the one-electron reduction of pertechnetates, must possess weak paramagnetism corresponding to one unpaired electron, which is observed experimentally.45, 46 Technetium(V) oxohalide complexes are characterised by the d2 electronic configuration of the central atom.The ground electronic state of the molecules of these compounds must be spin-paired because of a symmetry breakdown in the nearest environment of technetium (e.g., to C2u for [TcOX4]7, see Ref. 47); in fact, these compounds also possess diamagnetism or TIP (see Table 8, Fig. 4).34 Technetium(IV) halide complexes are characterised by the d 3 electronic configuration and octahedral structure of the nearest coordination sphere of the Tc atoms.According to theoretical predictions (see Fig. 4), these compounds have a triply degenerate t2g highest occupied molecular orbital (HOMO) and, hence, they must possess paramagnetism corresponding to the spin 3/2 (meff&3.87 mB). From the data in Table 8 it can be seen that only for several technetium(IV) compounds are the magnitudes of the effective magnetic moments meff close to the pure spin value; in most cases, they are much smaller. Previously,48 similar effects of a decrease in meff of analogous rhenium halide complexes have been rationalised by intermolecular exchange interactions occurring along the Re07X0 .. .X007Re00 chains.It has also been shown 48 that similar antiferromagnetic interactions can occur at X0 . . .X00 distances up to *5 A. Thus, antiferromagnetic exchange interactions in technetium(IV) complexes are also the most plausible explanation of the decrease in their effective magnetic moments. In fact, as can be seen from the data in Table 8, a decrease in the size of the outer-sphere cation and an increase in the anion size (halide ion) leads to an increase (in the absolute value) in the Weiss constant characterising the strength of antiferromagnetic exchange interactions.However, in the case of rather bulky tetraalkylammonium cations, the [TcX6]27 anions are `pushed apart' by these cations to such an extent that the exchange interactions along the Tc07X0 .. .X007Tc00 chains become impos- sible (the X0 . . .X00 distances between adjacent [TcX6]27 octahe- dra are *6 A).49 An even greater decrease in meff is observed for technetium dioxide (see Table 8). According to the published data,50 it is assumed that the metal ± metal (Tc7Tc) bonds can be formed in the TcO2 molecule; this may be an additional reason for the decrease in meff compared to its pure spin value for technetium(IV).As can be seen from the data in Table 8, the magnetic proper- ties of most of the compounds considered above are first of all due to the electronic configuration of the Tc atoms, the symmetry of their nearest ligand environment and intermolecular exchange interactions. The magnetic properties of all other technetium compounds listed in Table 8 cannot be explained by these reasons only, since their electronic structures are strongly affected by the M7M bonds localised on pairs of Tc atoms or delocalised over the polynuclear metal core.In these complexes, the M7M interactions result in appreciable shortening of the Tc7Tc dis- tances compared to analogous distances in compact metallic Tc.1, 2, 51 The compounds studied 2, 51 can be divided into two structural types, viz., dinuclear complexes (clusters) with multiple M7Mbonds and polynuclear clusters.Dinuclear d 4 ± d4 complexes. Classical quadruple M7M bonds formed by s(4dz2), 2p(4dxz, 4dyz) and d(4dxy) orbitals of the M7M bonds (see Fig. 4) are characteristic of these com- pounds.1, 2, 51 These complexes have an eclipsed conformation of ligands (the idealised D4h symmetry, see Fig. 1 a,b) and Tc7Tc distances in the range from 2.16 to 2.19 A.2 All compounds are characterised by the absence of unpaired electrons, therefore their magnetic susceptibility is temperature-independent (see Table 8). Dinuclear d 4 ± d5 complexes are formed as a result of one- electron reduction of the compounds belonging to the preceding group and also have an eclipsed conformation of ligands 1, 2, 51 (see Fig. 1 a,b). As could be expected from Fig. 4, one unpaired electron is shared by two Tc atoms that form an M7M bond of high order (the Tc7Tc distances in d 4 ± d 5 complexes lie within the limits 2.10 ± 2.15 A)1, 2, 51 in the molecules of these com- pounds. It has been shown 3, 9, 52 ± 54 that this electron occupies the b1u HOMO (the d*M7Morbital) formed mainly by the 4dxy 8 10 12 14 Species Symmetry b1u [TcO4]7 Td [TcOCl4]7 C2u [TcCl6]27 Oh [TcI6]27 Oh [Tc2Cl8]37 D4h [Tc2Cl8]27 D4h [Tc8Cl12]27 D2h [Tc6Cl11]27 Oh!C1 [Tc2Cl6]27 D4d [Tc2(AcO)4Cl2]7 D4h ag d* p p p d d* dd b3u a2 d* e3 p* e2 d e2 d* e1 p a1 d eu b2g eg b2u b1g b2u b2u b1g b1g eg eg eg eg a2u a2u b1u b1u b2g b2g eu eu a1g a1g d'* p* s* d* d p s t2g a1 a1 b1 b2 t2g eg a2 E /eV Figure 4.Results of extended HuÈ ckel (EH) calculations of the electronic structure of technetium complexes.2, 34 The presence of electrons in theHOMO is depicted schematically by small double vertical lines. } Unpublished results by K E German (Institute of Physical Chemistry, Russian Academy of Sciences). 890 S V Kryutchkovorbitals of two technetium atoms.Thus, theM7Mbonds in these complexes are of formal order 3.5. Let us present the essentials of this proof, since they are of importance for considering the electronic structure of the compounds mentioned.2 EPR signals of all compounds belonging to this type are observed both in the crystalline state and in solutions. Typical EPR spectra of K3[Tc2Cl8] . 2H2O single crystal and K3[Tc2Cl8] . 2H2O solution in hydrochloric acid are shown in Figs 5 and 6, respectively. The EPR spectra of polycrystalline specimens are, as a rule, structureless lines with half-width *500 �º and effective g-factor geff=2.090.02. Clearly defined fine structure is observed in the single-crystal spectra, though their parameters are similar to those listed above.Well-resolved spectra of solutions of these complexes can hardly be obtained because of the impossibility of achieving high technetium concentrations; where it is possible, the spectra are similar to that shown in Fig. 6.4 EPR spectra of glassy frozen solutions of d 4 ¡À d 5 complexes have also been studied.9, 52 ¡À 54 It has been established that all dinuclear d 4 ¡À d 5 clusters studied have similar spectral patterns with the parameters coinciding within the limits of experimental error: gk=1.850.03, g\=2.130.03, Ak=7(1885)61074 cm71, A\=7(755)61074 cm71 and P=(51)61074 cm71.The spectra display strong anisotropy: lines of parallel orientation are observed in the region from 1600 to 5550 �º, while those of perpendicular orientation and, probably, peaks of additional absorption are observed in the region from 2400 to 3900 �º.From the data in Table 8 it can be seen that despite the chain structure of some of the compounds in question {e.g., [Tc2(A- cO)4]X (X=Cl, Br)}, intermolecular exchange interactions have no effect on their magnetic properties (the meff values are close to the spin-only value corresponding to one unpaired electron and are temperature-independent).2, 22 ¡À 24, 51 Thus, even qualitative analysis of the data on the magnetic susceptibility of d4 ¡À d 5 complexes and their EPR spectra suggests that the only unpaired electron does occupy the d*(b1u) MO,4 since only in this case can one expect that magnetic parameters will exhibit low sensitivity to the chemical nature of ligands.Accord- ing to the results of SCF-Xa-SW calculations of the electronic structure of the [Tc2Cl8]37 ion,9 the d* MO is the only MO of all unoccupied MOs to which the electrons of ligand atoms make no appreciable contribution (see Fig. 4). Amore reliable proof of this fact is known that is independent of the results of theoretical calculations of the electronic structure.3, 52, 53 Thus, the possible ground electronic states in the molecules of d 4 ¡À d 5 complexes and values of g-factors predicted by the theory of EPR spectra (the spin Hamiltonian method 55, 56) have been considered.All possible `reasonable' energy permutations of theMOlevels were taken into account when analysing the experimental values of g-factors (see Fig. 4). From the data obtained it follows that qualitative agree- ment between theory and experiment can be achieved only if the b1u MO is the highest occupied molecular orbital. Dinuclear d 5 ¡À d 5 complexes are characterised by extremely short interatomic Tc7Tc distances (*2.04 A), a chain structure, and idealised D4d symmetry of the ligand environment (staggered conformation, see Fig. 1 c).57, 58 The molecular and electronic structure of these compounds has been considered in detail.2 According to calculations 2, 18 (see Fig. 4), the e2 orbital formed by `cross' overlap of the 4dxy and 4dx2 ¡¦ y2 orbitals of two Tc atoms in the molecules of these compounds is the HOMO; formally, it is a low-energy d bonding orbital. Considerable distortion of the idealised D4d symmetry in the complexes results in the removal of degeneracy of the e2 level and in partial increase in its d bonding nature.Thus, the formation of formally quintuple M7M bonds in these compounds was first observed 57 in contrast to, e.g., analogous d 5 ¡À d 5 osmium complexes with triple M7M bonds.58, 59 Cotton et al.58 interpreted the M7M bonds in d 5 ¡À d 5 technetium complexes as triple ones and their shortening as the result of changes in the diffuseness of the s, p and d components of these bonds due to changes in the formal techne- tium oxidation staom the data in Fig. 4 it follows that the [Tc2X6]2n7 n ions must be spin-paired (see Table 8). However, weak EPR signals of polycrystalline specimens of dinuclear d 5 ¡À d 5 technetium com- plexes are observed in some instances. The parameters of these signals indicate that the specimens contain small amounts (<5%) of impurities (related paramagnetic d 4 ¡À d 5 complexes).Polynuclear complexes (clusters). Most of these compounds have an island structure 51 (see Fig. 1 d, e, f), except for two octanuclear [Tc8Br13]n7 clusters (n=0, 1) with a chain struc- ture.60, 61 This would seemingly simplify the interpretation of the magnetic properties of this class of compounds, since in most cases there is no need to take into account intermolecular exchange interactions. However, the interpretation of the results of magne- tochemical experiments on polynuclear technetium complexes appeared to be the most complex.This is first of all due to the complexity of the system of M7M bonds and high molecular weights of the compounds in question (and, hence, to the small contribution of spin-only paramagnetism to the magnetic suscept- ibility and to commensurable contributions of the Langevin diamagnetism and the Van Vleck paramagnetism), large relative 2000 3000 4000 5000 H /�º a b c d e DPPH Figure 5.EPR spectra:34 (a) K3[Tc2Cl8] . 2H2O at 78 K (the z crystal axis is perpendicular to the direction of the magnetic filed strength); (b, c) (Me4N)3{[Tc6(m-Cl)6Cl6]Cl2} at 293 and 78 K; (d, e) (Me4N)2..[Tc6(m-Cl)6Cl6] at 78 and 293 K, respectively; DPPH is diphenylpicryl- hydrazide. DPPH 200 �º H Figure 6. Typical EPR spectrum of Tc5�¢ 2 complexes in solutions of HCl at 293 K {[Tc2Cl8]37 in 4.0 M HCl}.4 Physicochemical properties of technetium acido clusters 891error of magnetic susceptibility measurements in the vicinity of zero values, strong effects of even small amounts of low-molec- ular-weight paramagnetic impurities as well as to a considerable uncertainty in the values of diamagnetic corrections for this class of compounds.In connection with the foregoing, the results of magnetochemical experiments were interpreted using EPR spec- troscopy (as a tool for qualitative and semiquantitative analysis) and the results of theoretical calculations.Trigonal-prismatic clusters. The following technetium com- pounds with this structure are known: (Et4N)2. .{[Tc6Br6(m-Br)6]Br2}, (Me4N)3{[Tc6X6(m-X)6]X2} (X=Cl, Br), (Me4N)2[Tc6Cl6(m-Cl)6], (Bun4 N)3{[Tc6Cl6(m-Cl)6]Cl2} and (FeCp2)3{[Tc6Cl6(m-Cl)6]Cl2}. The metal core of these clusters has the shape of a trigonal prism with an equilateral triangle as the base (see Fig. 1 d).The lateral edges of the prism correspond to the M¡ÀM bonds of formal order 3.0 ¡À 4.0 (2.16 ¡À 2.21 A), while the edges of the prism base correspond to those of formal order 0.5 ¡À 1.0 (2.55 ¡À 2.75 A). These clusters have the idealised D3h symmetry. Generally, there is no agreement between the calculated values 2, 54 and the experimental results (see Table 8).In fact, according to extended Hu�� ckel (EH) calculations, the {[Tc6Br6(m-Br)6]Br2}27 and [Tc6Cl6(m-Cl)6]27 ions are in a spin- paired ground electronic state; however, experimental studies show that there is *0.1 and *0.2 unpaired electron per each cluster molecule, respectively, and that an EPR signal with geff=1.97 is observed (Fig. 5 d, e) for a polycrystalline specimen of the latter compound. It should be noted that a weak temper- ature-dependent paramagnetic component of the total magnetic susceptibility of these clusters can hardly be associated with the presence of mechanical impurities of other paramagnetic poly- nuclear clusters, since according to X-ray phase analysis data obtained with an accuracy of 5% to 10% (sensitivity of the method), both compounds contain no impurities. Thus, this paramagnetic component can be due to either the nature of the compounds under consideration or the presence of a very small amount of paramagnetic mono- or di-nuclear technetium com- plexes.However, the fact that EPR signals with such parameters as those of the spectra shown in Fig. 5 d, e could not be observed for any of the paramagnetic impurities of mono- or di-nuclear technetium complexes possible in this case is against the latter assumption. The interpretation of magnetochemical data for the com- pound with ferrocenium cations appeared to be all the more complex.29 In fact, strong paramagnetism (*5 unpaired electrons per molecule) of this compound (see Table 8), the lack of similarity between its EPR spectrum (broad spectra with geff *2) and spectra of compounds with similar anions and different tetraalkylammonium cations close in their size (Figs 7, 8; cf.Fig. 5), temperature-independent behaviour (down to *4 K) of all magnetic parameters as well as physicochemical properties of other technetium and rhenium clusters with ferrocenium cations indicate altogether that their electronic structure is unusual and complex.In this connection, the magnetic properties of all compounds with ferrocenium cations will be considered in Section VI. To a first approximation, the paramagnetism of {[Tc6X6(m-X)6]X2}37 ion corresponding to one unpaired electron per cluster (except for ferrocenium-containing compounds) is well described by theory 2, 34 (see Table 8, Fig. 4). However, no adequate description of EPR spectral parameters of these com- pounds (gk&2.00, g\&1.63) can be obtained using the EH calculations. In fact, calculations of the electronic structure of trigonal-prismatic technetium clusters 34, 62,63 suggest that the HOMO in these clusters can be either the a02 (dM¡ÀM) MO or the a00 2 (pM¡¦M) MO.However, it has been reported 64 that none of these variants of the electronic structure as well as all other possible variants based on the energy permutations of the MOs in the HOMO¡ÀLUMO region can explain the values of g-factors observed experimentally (the analysis of g-factors was performed analogously to that for d 4 ¡À d 5 complexes).Thus, magneto- chemical experiments show that further refinement of theoretical MO schemes is required. Such refinements have been carried out and reported.18, 34, 64 According to the results obtained, the experimental magnetic and structural properties of trigonal-prismatic clusters are not contra- dictory to the idea that there is anMOwith a 01 symmetry between the HOMO and the LUMO of these clusters.This orbital, composed mainly of diffuse 5s (5p) orbitals of the Tc atoms, is a 2sg bonding orbital in the direction of both vertical and horizon- talM7Mbonds in the metal core of the clusters (see Fig. 1 d). In other words, it seems likely that EH calculations 34, 62, 63 over- estimate the interaction of d* and p* MOs of the dimers in horizontal directions and underestimate the extent of participa- tion of the 5s and 5pz AOs in the formation of multiple M7M bonds.In conclusion of this Section let us consider the interpretation of magnetic and other physicochemical properties of certain trigonal-prismatic technetium clusters. For instance, the analysis of the magnetic susceptibility of the cluster (Me4N)2. 2000 3000 4000 5000 H /�º a b c d DPPH Figure 7.EPR spectra:4, 18, 34 (a, b) (Me4N)3{[Tc6(m-Br)6Br6]Br2} at 293 and 78 K, respectively; (c, d) (Bun4 N)2[Tc6(m3-Br)5Br6] at 293 and 78 K, respectively. a b c d e f DPPH 1000 2000 3000 4000 5000 H /�º Figure 8. EPR spectra:28, 29 (a, b) (Bun4 N)3{[Tc6(m-Cl)6Cl2]Cl2} at 300 and 78 K, respectively; (c ¡À e) (FeCp2)3{[Tc6(m-Cl)6Cl2]Cl2} at 300, 78 and 4 K, respectively; and (f) (FeCp2)2[Re2Br8] at 300 and 78 K, respectively. 892 S V Kryutchkov.[Tc6(m-Cl)6Cl6] (see Table 8) shows that it obeys the Curie ± Weiss law; a weak signal with a relative intensity of 0.2 of that of the [Tc6X14]37 signal and geff & 1.97 is observed in the EPR spectra of [Tc6(m-Cl)6Cl6]27, which means that the compound under consideration is paramagnetic.34 Since the [Tc6(m-Cl)6Cl6]27 ion has an even number of `metallic' electrons proper- ties can be manifested if the ground electron state is a triplet.Weak EPR signals and small meff values as compared to those expected from the theory for the triplet state suggest that only a part of [Tc6(m-Cl)6Cl6]27 clusters is paramagnetic in real crystals. Assum- ing that the relative intensity of the EPR signal (0.2) corresponds to the portion of triplet paramagnetic clusters, one can evaluate the magnetic moment of the hypothetical paramagnetic cluster using the formula m2=m2 eff/0.2 (see Ref. 33); the result obtained (m=2.77 mB) is rather close to the theoretical magnetic moment of a triplet species with geff=1.97. Attempts to explain experimen- tally observed temperature dependences of the magnetic suscept- ibility and intensity of the EPR signal by the influence of the Boltzmann population of the triplet excited state, intermolecular exchange interactions or splitting of the triplet state levels in the crystal field have failed.The simultaneous presence of singlet and triplet [Tc6(m-Cl)6Cl6]27 clusters in crystals can be explained by sensitiv- ity of the multiplicity of the ground state of the cluster to slight changes in its structure. In real crystals, a transition from the singlet ground state of the cluster to its triplet state can occur due to the presence of defects (vacancies, dislocations, disorder in the ligand environment, etc.) or, which is more probable in our opinion,64 due to fast kinetic S ± T exchange with involvement of delocalised MOs formed mainly by the 5s (5p) orbitals of the Tc atoms.The latter assumption is also supported by the long-wave IR spectra of trigonal-prismatic technetium clusters (Fig. 3) and unusual physicochemical properties of clusters with ferrocenium cations. In fact, the properties of the last-mentioned compounds taken altogether can be explained 28, 29 as a result of the formation of conduction bands and, perhaps, heavy-fermionic bands formed involving delocalised 5s (5p) orbitals of the Tc atoms and 4s orbitals of the Fe atoms of ferrocenium cations.The values of the g-factor in the EPR spectra of these compounds, close to that of the free electron, metallic type of the temperature dependence of conductivity, and unusual temperature dependence of the MoÈ ssbauer and X-ray photoelectron spectra are evidence for the formation of such bands.A more detailed theoretical interpreta- tion of these properties will be given in Section VI of this review. Tetragonal-prismatic clusters. This group comprises the fol- lowing compounds: {[Tc8(m-Br)8Br4]Br} . 2H2O, [H(H2O)2]. .[Tc8(m-Br)8Br4]Br}, [H(H2O)2]2{[Tc8(m-Br)8Br4]Br2}, (Bun4 N)2..{[Tc8(m-Br,I)8(Br,I)4]I2}, (FeCp2)2{[Tc8(m-Br)8Br4]Br2}. Clusters belonging to this type are close in structure to the trigonal- prismatic ones. Their major distinction lies in the fact that in this case the metal core of the cluster is a prism with a rhombus in its base (Fig. 1 e). According to the conventional count rule of valence electrons in clusters with acido ligands,65, 66 the first of the above compounds must have 43, while each of the others must have 44 `metallic' electrons. The calculated electronic structure of a cluster with 44 valence electrons is shown in Fig. 4.34, 62 As can be seen, these compounds must be spin-paired, whereas the [Tc8Br13] . 2H2O cluster must possess a trivial paramagnetism with meff corresponding to one unpaired electron.Indeed, the experimental data (see Table 8) show that only the compound [Tc8Br13] . 2H2O has unpaired electrons; other clusters are charac- terised by TIP with large w0M values due to the high molecular weights of the clusters and their structural peculiarities [w0M (TIP)=(570150)6106 CGSE units). Magnetic properties of the ferrocenium-containing bromide octanuclear cluster are of particular interest.From the data in Table 8 it can be seen that paramagnetism of this compound obeys the Curie ± Weiss law and corresponds to about two unpaired electrons per ferrocenium cation. No EPR signals of this compound are observed either at room temperature or at a liquid nitrogen temperature. Comparison of experimental results with the magnetic properties of ferrocenium-containing trigonal- prismatic chloride clusters considered above and (FeC- p2)2[Re2Br8] (this compound possesses trivial paramagnetism corresponding to about two unpaired electrons per formula unit and signals with geff=2.003 are observed in its EPR spectra 28, 29) shows that: (1) the EPR signals of all compounds in question are not due to localisation of the unpaired electrons on ferrocenium cations (this is also confirmed by the absence of EPR spectra of ferrocenium chloride and other ferrocenium compounds under the same conditions); (2) the EPR signals of the above clusters cannot also be due to localisation of the unpaired electrons on the cluster fragments, since in this case the recorded EPR spectra would be similar to those of the clusters with tetraalkylammonium cations; and (3) the paramagnetism of ferrocenium cations can affect the overall static magnetic susceptibility.Thus, it is most likely that all experimental facts as well as other unusual proper- ties of the compounds under study,28, 29 are due to fast dynamic electron transfer from the anion to the cation over distances 5 to 6 A in a band formed by the overlap of upper ns (np) AOs of the cluster anions and ferrocenium cations (see also Section VI).Octahedral clusters. This group comprises the following com- pounds: (Bu4N)2[Tc6(m3-Br)5Br6], [H3O(H2O)3]2. .[Tc6(m3-Br)5Br6] and [H3O(H2O)3]2[Tc6(m3-Br)5Br6] . 4H2O (see Table 8), which are characterised by octahedral metal cores (see Fig. 1 f). However, in contrast to the classical [M6X8]4+ clusters (M=Mo, W; X=Cl, Br, I),66 three of the eight positions of the bridging halide ligands in the technetium clusters remain statisti- cally unoccupied. This effect of the distortion of octahedral clusters leads to their stabilisation despite the presence of nine `excess' metallic electrons as compared to the stable electronic configuration of the [M6X8]4+ clusters.The above technetium clusters each contain 33 valence electrons. Their electronic struc- tures are depicted schematically in Fig. 4. It can be seen that these clusters possess trivial paramagnetism with meff corresponding to one unpaired electron, which is confirmed by the data in Table 8. A typical EPR spectrum of one of the compounds in question shown in Fig. 7 is characterised by hyperfine structure and geff&2. Thus, the magnetochemical studies 18, 34 have shown that it is of crucial importance to know experimental magnetic properties of the maximum possible number of technetium compounds. Magnetochemistry of di- and polynuclear technetium clusters is of particular interest, since the magnetochemical studies of these compounds in combination with other methods of physicochem- ical analysis and theoretical calculations make it possible, first, to identify the above compounds and reveal the presence of impur- ities and, second, to study the electronic structure of these clusters that often have no close analogues.It should be noted that in some instances magnetochemical studies in combination with X-ray studies can provide valuable information on the composition and the formal oxidation state of technetium.For example, in such a way the formal charges of cluster anions in the compounds {[Tc8Br4(m-Br)]8Br} . 2H2O, [H(H2O)2]{[Tc8Br4(m-Br)8]Br}, [H3O(H2O)3]2[Tc6(m3-Br)5Br6] and [H3 O(H2O)3]2[Tc6(m3-Br)5Br6] . .4H2O9, 61, 67, 68 have been determined. In fact, it was impossible to locate hydrogen atoms in the structures of these clusters using the results of X-ray studies only; however, magnetic measurements made it possible to determine the number of unpaired electrons in the anions and, hence, the number of `metallic' electrons, the oxidation state of technetium and thereby the anionic charges in the clusters.Physicochemical properties of technetium acido clusters 893IV.X-Ray photoelectron study of the structure of technetium complexes 1. Binding energies of the core electrons of technetium atoms Table 9 presents the binding energies of the core electrons (Eb) in technetium clusters and some mononuclear complexes. It is note- worthy that the lines corresponding to Eb (Tc 3d5/2) are singlets in all compounds; judging by the half-widths of the lines (l1/ 242.0 eV), it is impossible to assign them to two or more chemically or structurally nonequivalent types of the Tc atoms. This conclusion is of particular importance for clusters with fractional oxidation state of technetium and [Tc2X6]27 anions Table 9.Binding energies of the core electrons (Eb/eV) of Tc and halogen atoms in the X-ray photoelectron spectra of technetium compounds and the effective charges on Tc atoms calculated by the Pauling electronegativities (EN) and EH methods.70 ± 74 Compound Tc Eb(Tc3d5/2) l1/2 Eb(Cl2s1/2) ZEN ZEH oxidation /eV [Eb(Br3p3/2)] (Z0EN) (Z0EH) state {Eb(I4d5/2)} K[TcO4] (9) +7.0 259.0 1.8 7 1.60 (1.60) 3.854 (3.92) NH4[TcO4] (10) +7.0 258.9 1.5 7 (1.57) (3.85) (FeCp2) [TcO4] (100) +7.0 258.9 1.5 7 (1.57) (3.85) Na[TcO4] .nH2O (11) +7.0 258.9 1.5 7 (1.57) (3.85) Ni[TcO4]2 . nH2O (12) +7.0 259.1 1.5 7 (1.63) (4.00) Co[TcO4]2 . nH2O (13) +7.0 259.2 1.7 7 (1.66) (4.08) Ca[TcO4]2 . nH2O (14) +7.0 259.2 1.6 7 (1.66) (4.08) Ba[TcO4]2 . nH2O (15) +7.0 259.0 1.8 7 (1.60) (3.92) (Ph4As)[TcO4] (16) +7.0 258.4 1.3 7 (1.41) (3.46) (Bun4 N)[TcO4] (17) +7.0 258.7 1.7 7 (1.49) (3.70) Tc2S7 (18) +7.0 254.7 1.4 7 0.31 (0.22) (0.65) K2[TcOCl5] (19) +5.0 257.6 1.6 269.0 1.00 7 (Bun4 N)[TcOCl4] (20) +5.0 257.4 1.5 268.9 1.05 2.312 (2.40) (Bun4 N)[TcOBr4] (21) +5.0 256.5 1.6 [181.4] 0.85 (2.00) K3[Tc2Cl8O2] (22) +4.5 256.8 1.4 268.9 1.08 (0.85) 7 Tc2O5 (23) +5.0 256.6 1.8 7 1.72 (0.80) 7 Tc2O5 .nH2O (24) +5.0 256.0 1.7 7 1.72 (0.65) 7 TcO2 (25) +4.0 255.2 1.8 7 1.52 (0.38) (0.9) TcO2 .nH2O (26) +4.0 255.4 2.0 7 1.52 (0.43) (0.9) K2[TcCl6] (27) +4.0 257.0 1.3 269.2 0.86 1.793 (1.793) (FeCp2)4[H3O(H2O)4]2[TcCl6]3 (270) +4.0 257.0 1.5 269.1 0.86 1.793 K2[TcBr6] (28) +4.0 256.2 1.6 [181.4] 0.69 (1.60) (NH4)2[TcBr6] (29) +4.0 256.2 1.5 [181.4] 0.69 (1.60) [TcPy2Cl4] (30) +4.0 256.8 1.6 268.8 0.83 7 K2[TcI6] (31) +4.0 255.6 1.6 {48.0} 0.46 0.934 (1.20) (NH4)2[TcI6] (32) +4.0 255.5 1.5 {48.0} 0.46 0.934 (1.20) (Bun4 N)2[Tc2Cl8] (33) +3.0 255.8 1.3 268.5 0.88 (0.54) 1.445 (1.445) [Tc2(AcO)4Cl] (34) +2.5 255.8 1.8 268.3 0.91 (0.54) 1.206 (1.30) K[Tc2(AcO)4Cl2] (35) +2.5 255.8 1.6 268.2 0.92 (0.54) 7 [Tc2(AcO)4Br] (36) +2.5 255.7 1.6 [181.4] 0.87 (0.49) 7 K3[Tc2Cl8] .nH2O (37) +2.5 255.5 1.8 269.0 0.80 (0.40) 1.065 (1.065) Tc4O5 .nH2O (38) +2.5 255.9 1.8 7 1.25 (0.58) 7 K2[Tc2Cl6] (39) +2.0 254.9 1.6 268.9 0.73 (0.30) 0.704 (0.704) K2[Tc2Br6] . nH2O (40) +2.0 254.6 1.7 [181.4] 0.62 (0.20) 7 (Et4N)2{[Tc6(m-Br)6Br6]Br2} (41) +2.0 254.7 1.2 [181.1], [182.6] 0.60 (0.22) (0.55) (Me4N)3{[Tc6(m-Br)6Br6]Br2} (42) +1.83(3) 254.8 1.2 [181.1], [182.5] 0.60 (0.25) (0.65) (Me4N)3{[Tc6(m-Cl)6Cl6]Cl2} (43) +1.83(3) 255.0 1.3 268.8, 270.6 0.70 (0.32) (0.76) (FeCp2)3{[Tc6(m-Cl)6Cl6]Cl2} (430) +1.83(3) 255.0 1.3 268.8, 270.6 0.70 (0.32) (0.76) (Bun4 N)3{[Tc6(m-Cl)6Cl6]Cl2} (4300) +1.83(3) 255.0 (1.4) 268.8, 270.6 0.70 (0.32) (0.76) (Me4N)2[Tc6(m-Cl)6Cl6] (44) +1.66(6) 255.3 (1.4) 268.9, 270.7 0.70 (0.42) 0.597 (0.597) {[Tc8(m-Br)8Br4]Br} .2H2O (1) +1.625 254.7 (1.2) [181.1], [182.6] 0.60 (0.22) (0.55) [H(H2O)2]2{[Tc8(m-Br)8Br4]Br2} (3) +1.5 254.7 (1.4) [181.0], [182.5] 0.55 (0.42) (0.55) (FeCp2)2{[Tc8(m-Br)8Br4]Br2} (30) +1.5 255.2 (1.4) [181.0], [182.5] 0.70 (0.42) (0.60) [H3O(H2O)3]2 [Tc6(m3-Br)5Br6] .nH2O (6) +1.5 255.3 (1.8) [181.1], [182.5] 0.55 (0.42) (0.90) (Bun4 N)2{[Tc8(m-X)8X4]I2} (45) (X=0.5 Br+0.5 I) +1.5 254.6 (1.5) [181.0], [182.5] 0.52 (0.17) 7 {48.4}, {49.8} {[Tc8(m-I)8I4]I} .nH2O (46) +1.625 254.5 (1.3) {48.4}, {49.8} 0.49 (0.17) 7 (FeCp2)3{[Tc6(m-I)6I6]I2} (460) +1.5 254.4 (1.4) {48.3}, {49.6} 0.49 (0.17) 7 TcC (47) 0 253.9 (1.5) 7 0 (0) (0) Tc (metal) (48) 0 253.9 (1.0) 7 0 (0) 0 (0) Notes. The binding energies are given relative to a peak from adsorbed vapours of diffusion oil [Eb(C1s)=284.3 eV]. The errors of the binding energy determination are0.2 eV, those of half-widths l1/2 are0.1 eV.The binding energies of bridging atoms are underlined. TheZ0EN values were determined for compounds withM7Mbonds, pertechnetates and binary compounds only from the correlation for mononuclear complexes (curve a in Fig. 9). The Z0EH values were determined only for those compounds for which the experimental chemical shifts in X-ray emission spectra 75 ± 77 and X-ray photoelectron spectra 70 ± 74 are known. 894 S V Kryutchkov(see Table 9) in which the Tc atoms formally either have different oxidation state or occupy strongly nonequivalent positions in the structure. Thus, equalisation of the electron density on technetium atoms occurs in all complexes. For most clusters such an equal- isation of the electron density due to the formation of multiple M7M bonds could be assumed on the basis of their struc- tural,1, 2, 19, 31, 51, 67 magnetic,18, 34 spectroscopic 8, 52, 53, 54 and other physicochemical properties; however, in the case of [Tc2X6]27 compounds and octanuclear clusters a conclusion that the Tc atoms are structurally nonequivalent would be the most probable.19, 57, 67, 69 However, the half-widths of the Tc 3d5/2 lines in the X-ray photoelectron spectra of these compounds are even smaller as compared to half-widths of the same lines in the spectra of other technetium complexes including mononuclear ones.70 ± 74 This indicates strong delocalisation of the electron density in such clusters.It is known that the chemical shifts in X-ray emission and X-ray photoelectron spectra often correlate with the effective charge on the atom considered.75�78 The method based on Pauling electronegativities (EN) 78 ± 80 is one of the simplest methods used in the effective charge calculations. As a rule, the effective charges (ZEN) calculated using this method correlate almost linearly with binding energies.78 ± 80 However, the EN method makes it possible to calculate the ZEN values determined only by conventional valence bonds.If an additional bonding occurs in a compound (e.g., due to hydrogen or p-donor bonds, trans-effect, dative bonds, multiple homopolar bonds, surface effects, etc.), then the ZEN values can differ substantially from the experimental effective charges; however, this difference itself will be indicative of such additional effects.79, 80 It should be noted 79, 80 that effective charges determined experimentally by different physicochemical or theoretical methods can differ con- siderably for the same compound, since each method is charac- terised by specific `sensitivity' to an effective space region around the atom in question.Using the EN calculations carried out in the framework of a unified approach,70, 74 it has been possible to reveal the reasons for changes in the binding energies of the core electrons of the Tc atoms; this is necessary for determining the oxidation state of technetium in newly synthesised compounds.In other words, this method allows one to understand specific features of the structure of technetium compounds with M7M bonds.It should be noted that, although EN calculations are approximate and arbitrary, this method has some advantages over the traditionally used purely qualitative description of changes in the binding energies of the core electrons depending on the oxidation state of the atom and on the electronegativity of the ligands surrounding this atom, since it relates these two parameters on which the binding energy depends in most instances.Of course, this method lacks sufficient physical grounds characteristic of more rigorous computational methods such as, e.g., theMOLCAO method. However, as a rule, EN calculations make it possible to obtain almost linear correla- tions between the effective charges 78 ± 80 and the binding energies, especially, for compounds of similar composition and structure. The ZEN values were calculated using the following electro- negativities (w) of technetium obtained by interpolating the w values for Re, Mo, Mn, and Cr in different oxidation states:79, 80 w(TcI)=1.10, w(TcII)=1.40, w(TcIII)=1.60, w(TcIV)=1.80, w(TcV)=1.95, TcVI)=2.10 and w(TcVII)=2.25.The ZEN cal- culations were performed 70 ± 74 according to the Batsanov proce- dure 79, 80 assuming the absence of M7M bonds and other peculiarities of the molecular, crystal and electronic structure capable of affecting the w(Tc) value.Thus, it has been assumed a priori that in the case of systematic deviations of ZEN values from the experimental effective charges (Z0EN) for the clusters (see Table 9) these deviations must be due to a specific effect of the M7M bonds, since the nature of the M7X bonds in mono-, di- and poly-nuclear clusters remains unchanged in most instances, except for theM7m-X andM7Xax bonds.The dependence of the DEb (Tc 3d5/2) values on ZEN for technetium complexes is shown in Fig. 9. It can be seen that all experimental points fall on two curves. The curve a closely resembles an almost straight line and describes the dependence DEb (Tc 3d5/2)=j(ZEN) for compounds containing no Tc7Tc bonds.The points that obey the dependence DEb (T- c 3d5/2)=j0(Z0EN) and correspond to the technetium compounds with strong Tc7Tc bonds and to binary compounds fall on curve b. The curve b in Fig. 9 is much below the curve a. This is a direct consequence of neglecting the effect of homopolar multiple Tc7Tc bonds on the electronegativity of Tc atoms, which leads to overestimation of the ZEN values as compared to the `true' effective charges (Z0EN) obtained within the framework of the EN model. Thus, the Z0EN values for technetium clusters can be determined either by introducing some corrections to the techne- tium electronegativity or from experimental DEb (Tc 3d5/2) values using the correlation curve for mononuclear complexes (see Fig. 9, curve a). Comparison of the curves a and b in Fig. 9 as well as comparison of ZEN and Z0EN values (see Table 9) shows that the formation of the metal ± metal bonds in technetium clusters leads to a decrease in Zeff on the metal atoms. It should be noted that no analogous decrease in Zeff is observed upon the formation of M7Mbonds in clusters of other transition d elements with similar structures and properties.At first glance, it may appear that this technetium `anomaly' is only a consequence of inadequacy of the procedure for calculating the ZEN values. However, this is not the case. To make sure that Zeff values decrease as the M7M bonds are formed in the case of technetium, it is sufficient to compare the experimental values of changes in the binding energies of the core electrons (DEb ) for some transition d elements (Table 10); for molybdenum, the opposite is observed (Zeff increase as theM7M bonds are formed), while the Zeff values remain unchanged for rhenium and most of other elements.In fact, from the data in Table 10 and Refs 19, 51, 70 ± 74, 78 and 81 ± 85 it follows that the mean change in the DEb value corresponing to a change of unity in the formal oxidation state of a metalM(M=Tc, Re, and Mo) in complexes with similar stoichiometry and structures is *0.7 eV.Analogous changes in DEb on going from mononuclear d 3 complexes to dinuclear d 4 ± d 4 ones are +1.2 eV for Tc, +0.7 eV for Re and 73.3 eV for Mo. Data obtained by X-ray emission spectroscopy 86 ± 88 allowed one to draw analogous con- clusions that Zeff values increase in the case of the Mo7Mo bonds. 5.0 4.0 3.0 2.0 1.0 0 0.5 1.0 1.5 ZEN DEb(Tc3d5/2) /eV a b 47 48 18 46 32 1,3 41 40 42 45 6 13, 14 12 9, 15 10, 11 17 16 38 23 24 26 25 31 37 39 44 43 36 33 34 35 28, 29 21 30 27 19 20 22 Figure 9. Dependence of the chemical shifts of binding energies of the core electrons [DEb (Tc 3d5/2)] in technetium compounds on the effective charge on the Tc atoms (ZEN) calculated by the Pauling electronegativities method (the numbering of points corresponds to that of the compounds in Table 9): (a) correlation for mononuclear complexes (filled circles); (b) correlation for compounds with M7M bonds, di- and polynuclear clusters with acido ligands (open circles) and for binary compounds (sulfides and oxides, open triangles).72 For visual aids, correlation lines are drawn.Physicochemical properties of technetium acido clusters 895It is clear that in the general case the effective atomic charge must depend on the formal order of the bond between this atom and another atom, since a redistribution of the electron density of valence electrons mostly occurs as the bond order increases.This appreciably changes the conditions of shielding of the positive nuclear charge by outer electrons. Simultaneously, when two atoms approach each other as a result of the increase in the order of the interatomic bond, their inner atomic electron shells must undergo an increasing repulsion, which brings about the opposite result, viz., a decrease in the extent of shielding of the positive nuclear charge due to the formation of the multiple bond.Since each physical method is characterised by specific `sensitivity' to an effective space region around the atom M, the net change in Zeff will strongly depend on many factors and can be either positive or negative, or be equal to zero.A theoretical explanation for the effect of a decrease in Zeff on the Tc atoms in the formation of M7M bonds consists in the following:64 in contrast to clusters of other d elements, it is highly probable that the outer 5s (5p) AOs of technetium clusters partic- ipate in the formation of `metallic' bonding MOs. Thus, it has been shown experimentally 64 and theoretically 89 that the a 01 MO composed mainly of 5s (5p) orbitals of six Tc atoms represents the HOMO in the trigonal-prismatic technetium clusters [Tc6X14]37 (X=Cl and Br) (see also Section III).Since these outer AOs possess an increased capability of shielding the positive nuclear charge, their participation in the formation of the M7M bonds results in a decrease in Zeff. It should be noted that an analogous conclusion that Zeff values of the Tc atoms are decreased upon the formation of M7M bonds was drawn on the basis of analysis of changes in the chemical shifts in theKa1 X-ray emission spectra of technetium compounds.76, 77 The effective charge on the Tc atoms have also been evaluated using extended HuÈ ckel calculations.71 ± 73 This semiempirical MO method allows one to take into account the effect ofM7Mbonds.Therefore, the effective charge obtained by EH calculations (ZEH) can correlate directly with the chemical shifts in the spectra of Tc compounds without introduction of corrections for the M7M bonds. However, a strong parametrisation effect on the results of calculations hampers the use of EH calculations. To diminish this effect, only the results of calculations for the chloride technetium complexes ([TcCl6]27, [Tc2Cl8]27, [Tc2Cl8]37, [Tc2Cl6]27, and [Tc6Cl14]37) were used. The more detailed consideration of the general procedure for calculations and parametrisation, and the procedure for determination of the effective charge (Z0EH) using EH calculations and the data of X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPES) simulta- neously is given in Section V.These values are listed in Table 9. The dependence of DEb (Tc 3d5/2) on ZEH is shown in Fig. 10. Comparison of the data in Figs. 9 and 10 and the Z0EN and Z0EH values shows that Zeff calculated by the EN and EH methods differ considerably; however, they are related by an unambiguous monotonic dependence. In addition, common to both mononu- clear complexes and clusters is the observed dependence of the DEb (Tc 3d5/2) values on ZEH rather than on ZEN.This is under- standable since the EH calculations (in contrast to the EN calculations) take into account electronic effects due to the formation of theM7Mbonds. 2. Binding energies of the core electrons of halogen atoms Table 11 lists the binding energies of the core electrons of the halogen atoms [Eb (X)] in clusters of transition d elements.As can be seen, the Eb (X) values vary depending on the nature of the halogen ± metal bonds. For technetium clusters, the smallest Eb (X) values have been found for axial halogen atoms in carboxylate dinuclear complexes with the `lantern'-type structure (see Table 9).Since the Eb(Xax) values are close to analogous values for pure ionic compounds,78 it can be assumed that all Tc7Xax bonds are ionic. It should also be noted that these types of bonds of halogen atoms are much weaker due to the trans-effect of multiple metal ± metal bonds. Previously,78, 84, 85 an analogous phenomenon has been observed experimentally for dinuclear Re and Rh complexes.Table 10. Chemical shifts of binding energies (DEb) of the core electrons of transition metals in their clusters relative to those of pure metals (averaged values, according to the reported data 19, 51, 70 ± 74, 78, 81 ± 85). Cluster DEb / eV Cluster DEb / eV [Re2Cl8]27 3.0 [Ta6Cl12]2+ 2.6 (ReCl3)n 2.7 [Ta6Cl12]4+ 2.6 (ReCl2)n 2.0 [Mo2Cl8]47 3.6 [Nb6Cl12]2+ 2.6 (MoCl2)n 3.3 [Nb6Cl14]2+ 2.6 [Rh2(AcO)4] 2.4 [Tc6(m-Cl)6Cl6]n7 1.0 [L2Pd(AcO)2]2 2.7 [Tc2Cl8]27 1.9 [Cr2Cl9]37 2.6 [Tc2Cl6]27 1.1 0 1.0 2.0 3.0 4.0 5.0 1.0 2.0 3.0 4.0 5.0 ZEH 20 27 31 34 33 37 3 44 9 DEb(Tc3d5/2) /eV Figure 10.Dependence of the chemical shifts of binding energies of the core electrons DEb (Tc 3d5/2) in technetium compounds on the effective charge on the Tc atoms (ZEH) calculated by the EH method (the numbering of points corresponds to that of the compounds in Table 9).72 Table 11.Binding energies (Eb / eV)a of the core electrons of halogen atoms in clusters of transition d elements (averaged values, according to the reported data 19, 51, 70 ± 72, 78, 81 ± 85). Clus- Eb(Cl3p3/2) Eb(Br3d5/2) Eb(I4d5/2) ter- forming ax eq brid- ax eq brid- ax eq brid- metal ge ge ge Tc 197.2 198.0 199.5 67.7 67.7 69.2 7 48.4 49.8 Re 197.5 198.5 199.7 68.1 68.8 68.8 48.5 49.2 7 Mo 197.3 197.8 199.8 7 67.7 68.8 7 48.5 49.7 Rh 197.4 198.1 7 68.2 68.7 7 7 7 7 Pt 197.5 198.2 7 68.2 68.9 7 48.6 7 7 Nb 7 197.0 198.7 7 67.4 7 7 7 7 Ta 7 197.0 199.1 7 68.0 7 7 7 7 Kb 197.5 7 7 67.8 7 7 7 7 7 Note.The following notations are used: ax is axial, eq is terminal (equatorial), and br is bridging.a The error of binding energy determi- nation is 0.4 eV. b Potassium halides. The binding energies are given relative to the C1s line (284.3 eV). 896 S V KryutchkovThe Eb (X) values for the equatorial (terminal) halogen atoms in the technetium clusters correspond to the binding energies of the core electrons of the terminal halogen atoms in mononuclear halide complexes (see Table 9).The Tc7Xeq bond lengths are also close to those of the Tc7X bonds in halide TcV and TcIV complexes.1, 2, 31, 90, 91 The Eb (X) values for these types of halo- gens indicate weak ionicity of the Tc7Xeq bonds.78 The largest Eb (X) values are observed for the bridging halogen atoms in polynuclear clusters (see Table 9); they are close to the Eb (X) for the halogen elements,78 which indicates that the Tc7(m-X) bonds are covalent.As can be seen from the data presented, the structure and composition of technetium complexes can be qualitatively and semiquantitatively judged from the Eb (X) values of the halogen atoms. Indeed, the structures of polynuclear technetium clusters contain all three types of halogen atoms, whereas two lines corresponding to Eb (X) of the terminal and bridging halogen atoms with relative intensities proportional to the stoichiometric amount of these halogen atoms in the cluster are observed in the X-ray photoelectron spectra of these compounds (Fig. 11). It is likely that the peaks corresponding to axial halogen atoms or halide ions are not observed in the spectra due to their low relative intensity.The `sensitivity' of the chemical shifts in the X-ray photo- electron spectra to the coordination type of halide ligands can serve as a convenient tool for tentative analysis of the structure. In some instances, the data obtained by X-ray photoelectron spec- troscopy significantly complement the information obtained by X-ray analysis and chemical analysis. Thus, an attempt to establish unambiguously the exact stoichiometric composition of a bromide ± iodide octanuclear technetium cluster using X-ray analysis 92 and chemical analysis has failed.Only additionally taking the X-ray photoelectron spectra has made it possible to establish the stoichiometry, composition and structure of the cluster (Bun4 N)2[Tc8(m-Br)4(m-I)4Br2I2]I2 (see Table 9). 3. Binding energy X-ray photoelectron spectra The binding energy X-ray photoelectron spectra of a number of technetium compounds have been considered.71 ± 73 The experi- mental spectra have been compared with the results of theoretical SCF-Xa-SW calculations for K2TcCl6 93 and K3Tc2Cl8 . 2H2O.9 It has been shown that at a qualitative level the calculations represent satisfactorily the fine structure of X-ray photoelectron spectra in most cases.However, the spectra of solids are not always well resolved due to line broadening. The approximate character of the theoretical calculations makes it possible, as a rule, to determine only the order in which the MOs are arranged on the energy scale rather than the exact absolute values of their vertical ionisation potentials. Despite these drawbacks the follow- ing qualitative conclusions were drawn:71 ± 73 1.The high-lying MOs (dM7M and d*M7M orbitals according to the calculations and EPR spectroscopy data 9, 52, 53, 94) appear to be separated in energy from other MOs on going from [TcCl6]27 to [Tc2Cl8]37. 2. Replacement of Cl atoms by m-bridging acetate ions results in a higher degree of mixing in energy between the p-components of theM7M and M7L bonds; however, the dM7M and d*M7M components remain (though to a lesser extent) separated from other MOs. This is probably due to the formation of cyclic closed p-systems of the type 3.Replacement of the axial ligands in the {[Tc2(A- cO)4]Cln}(n71)7 (n=1 or 2) anion by Br7 does not result in considerable changes in the patterns of the binding energy spectra, which indicates a small contribution of AOs of the axial ligands to the dM7M, pM7M and pM7L MOs, since the axial ligands form mainly ionic bonds with the Tc atoms and only due to the trans- effect 78, 84, 85 do they somewhat weaken the s component of the M7Mbonds, which is hardly observed in this spectral region. 4. There is even greater separation in energy of the d component of the M7M bond from other types of MOs in the complex K2[Tc2Cl6], which is due to specific features of the bond formation in d 5 ± d 5 complexes with an idealised D4d symmetry.According to the data reported in Ref. 57, the formation of d bonds in K2[Tc2Cl6] occurs due to `cross' overlap of the 4dxy and 4dx2 ¡ y2 orbitals of the Tc atoms.This results in considerable decrease in the vertical ionisation potential of d MO, since the 4dx2 ¡ y2 AOs of technetium participate mainly in the formation of the s bonds with ligands. It should be noted that this energy separation of the s MO is not observed in the complex K2[Tc2Br6],18 since the Br7 ions, being ligands of weaker field and lower electronegativity as compared to the Cl7 ions, do not cause such a substantial decrease in the vertical ionisation potential of d MOs. 5. The formation of high-lying MOs in polynuclear clusters occurs mainly involving d MOs and p MOs of dinuclear clus- ters,2, 51, 62, 63, 64 which results in an increase in their vertical C O O Tc Tc . 204 202 200 198 196 1 2 Cl2p3/2 Cl2p1/2 184 182 180 178 3 4 Br 2p3/2 186 Eb /eV 623 619 615 5 I3d3/2 c a b Figure 11.Binding energy X-ray photoelectron spectra of the core electrons of the halogen atoms in polynuclear (1, 3, 5) and mononuclear (2, 4) chloride (a), bromide (b) and iodide (c) complexes: (1) (Me4N)2[Tc6Cl12]; (2) K2[TcOCl5]; (3) [Tc8Br13] . 2H2O; (4) (NH4)2[TcBr6]; and (5) [Tc8I13] .2H2O. Dashed lines represent the deconvolution of experimental spectra by the least squares method assuming a Lorentzian shape of single peaks.Physicochemical properties of technetium acido clusters 897ionisation potentials. Therefore, in most cases no low-energy lines corresponding to separated high-lying MOs are observed in the X-ray photoelectron spectra of polynuclear clusters and lines corresponding to all components of theM7Mbonds are strongly mixed in energy.The exception is the complex (Me4N)2[Tc6Cl12] and clusters with ferrocenium cations (peaks near the Fermi level are observed in their X-ray photoelectron spectra).29 This is probably due to the formation of a conduction band in these compounds involving high-lying 5s (5p) orbitals of the Tc atoms. V. Chemical shifts in X-ray Ka emission spectra of technetium compounds In the preceding Section we have considered the X-ray photo- electron spectra of technetium compounds 70 ± 74 and demon- strated their efficiency for studying the electronic structure of clusters.Here, the results of joint XES and XPES studies of the electronic structure of technetium compounds are presented and the experimental results obtained are compared with those of semiempirical EH calculations.Table 12 presents the experimental and theoretical results.75 ± 77 The interdependence of the experimental chemical Table 12. Experimental values of the chemical shifts in the X-ray emission spectra (dEKa1) and X-ray photoelectron spectra (DE3d5/2) spectra of technetium compounds (with respect to metallic Tc) and the calculated effective partial charges on Tc atoms.77 Compound Tc oxid- dEKa1 DE3d5/2 ZEH Q5s Q5p Q4d qs+d qs qd ation state / meV / eV (FeCp2)[TcO4] (100) 7.0 7233(8) 5.0(2) 3.85 1.00 70.1 2.90 1.73(5) 0.28 1.45 K[TcO4] (9) 7.0 7234(4) 5.1(2) 3.854 a 0.779 a 70.219 a 3.294 a 1.73(3) 0.28 1.45 3.92 1.0 70.1 3.0 (Ph4As)[TcO4] (16) 7.0 7227(8) 4.5(2) 3.46 0.9 0 2.55 1.69(5) 0.26 1.43 Na[TcO4] .nH2O (11) 7.0 7218(3) 7 3.85 7 7 7 1.63(2) 0.28 1.35 (Bun4 N)[TcO4] (17) 7.0 7198(4) 4.8(2) 3.70 0.97 70.1 2.78 1.51(2) 0.27 1.24 Tc2S7 (18) 7.0 772(50) 0.8(2) 0.65 0.45 70.2 0.40 0.7(4) 0.19 0.51 (Bun4 N)[TcOCl4] (20) 5.0 791(20) 3.5(2) 2.312 a 0.734 a 70.237 a 1.815 a 0.82(13) 0.19 0.63 2.40 0.81 70.25 1.84 (Bun4 N)[TcOBr4] (21) 5.0 747(7) 2.6(2) 2.0 0.71 70.2 1.29 0.54(5) 0.19 0.35 [TcO2Py4Cl] .2H2O (49) 5.0 7154(3) 7 7 7 7 7 1.23(2) 0.24 0.99 Tc2O5 (23) 5.0 7104(5) 7 7 7 7 7 0.91(3) 0.20 0.71 TcO2 (25) 4.0 789(2) 1.3(2) 0.90 0.5 70.25 0.65 0.81(2) 0.19 0.62 TcS2 (50) 4.0 740(3) 7 7 7 7 7 0.50(2) 0.18 0.32 K2[TcF6] (51) 4.0 7133(30) 7 2.6 7 7 7 1.09(19) 0.22 0.87 K2[TcCl6] (27) b 4.0 751(6) 3.1(2) 1.793 a 0.723 a 70.283 a 1.353 a 0.57(4) 0.19 0.38 (Me4N)2[TcCl6] (52) 4.0 736(5) 7 1.45 7 7 7 0.47(3) 0.18 0.29 (Et4N)2[TcCl6] (53) 4.0 739(3) 7 1.50 7 7 7 0.49(2) 0.18 0.31 (Bun4 N)2[TcCl6] (54) 4.0 736(5) 7 1.45 7 7 7 0.47(3) 0.18 0.29 K2[TcBr6] (28) 4.0 714(5) 2.3(2) 1.60 0.84 70.2 0.96 0.33(3) 0.18 0.15 (NH4)2[TcBr6] (29) 4.0 6(10) 7 7 7 7 7 0.20(3) 0.18 0.02 (Me4N)2[TcBr6] (55) 4.0 21(4) 7 7 7 7 7 0.11(3) 0.18 70.07 K2[TcI6] (31) 4.0 23(4) 1.7(2) 0.934 a 0.560 a 70.450 a 0.824 a 0.09(9) 0.18 70.09 1.2 0.67 70.2 0.73 (Et4N)2[TcI6] (56) 4.0 24(6) 7 7 7 7 7 0.08(6) 0.18 70.10 (Bun4 N)2[Tc2Cl8] (57) b 3.0 752(5) 1.9(2) 1.445 a 0.648 a 70.235 a 1.032 a 0.57(4) 0.19 0.38 (Bun4 N)2[Tc2Br8] (58) 3.0 748(10) 7 7 7 7 7 0.55(7) 0.18 0.33 K2[Tc2(SO4)4] .2H2O (59) 3.0 742(3) 7 7 7 7 7 0.51(2) 0.18 0.33 [Tc2(AcO)4Cl2] (60) 3.0 742(6) 7 7 7 7 7 0.51(4) 0.18 0.33 [Tc2(AcO)4Cl] (34) 2.5 717(4) 1.9(2) 1.206 a 0.657 a 70.222 a 0.771 a 0.35(3) 0.18 0.17 1.30 0.65 70.25 0.90 K3[Tc2Cl8] .nH2O (37) b 2.5 19(8) 1.6(2) 1.065 a 0.647 a 70.233 a 0.651 a 0.12(5) 0.18 70.06 Py3[Tc2Cl8] (61) 2.5 34(10) 7 0.80 7 7 7 0.02(7) 0.18 70.16 Tc4O5 (380) 2.5 716(16) 2.0(2) 1.40 0.66 70.2 0.94 0.34(10) 0.18 0.16 K2[Tc2Cl6] (39) b 2.0 60(3) 1.0(2) 0.704 a 0.647 a 70.209 a 0.266 a 70.14(2) 0.20 70.34 (Et4N)2{[Tc6(m-Br)6Br6]Br2} (41) 2.0 40(4) 0.8(2) 0.55 0.62 70.31 0.24 70.02(2) 0.18 70.20 (Me4N)3{[Tc6(m-Br)6Br6]Br2} (42) 1.83(3) 42(4) 0.9(2) 0.65 0.63 70.27 0.29 70.03(2) 0.18 70.21 (Me4N)3{[Tc6(m-Cl)6Cl6]Cl2} (43) 1.83(3) 42(3) 0.5(2) 0.35 0.61 70.40 0.14 70.03(2) 0.18 70.21 (Me4N)2[Tc6(m-Cl)6Cl6] (44) b 1.66(6) 0(8) 0.07(2) 0.597 a 0.555 a 70.229 a 0.271 a 0.24(5) 0.18 0.06 {[Tc8(m-Br)8Br4]Br} . 2H2O (1) 1.625 49(3) 0.8(2) 0.55 0.63 70.3 0.22 70.07(5) 0.19 70.26 [H(H2O)2]2{[Tc8(m-Br)8Br4]Br2} (3) 1.5 43(6) 0.8(2) 0.55 0.62 70.3 0.23 70.04(2) 0.18 70.22 [H3O(H2O)3]2[Tc6(m3-Br)5Br6] (50) 1.5 0(2) 1.4(2) 1.0 0.61 0.20 0.59 0.24(1) 0.18 0.06 [H3O(H2O)3]2[Tc6(m3-Br)5Br6] . nH2O (6) 1.5 42(6) 7 7 7 7 7 70.03(2) 0.18 70.21 Tc(CO)5Cl (62) 1.0 746(12) 7 7 7 7 7 0.54(7) 0.18 0.36 Tc(CO)5Br (63) 1.0 750(12) 7 7 7 7 7 0.56(7) 0.18 0.38 TcC (47) 0 714(5) 0.02(2) 0 0.18 0 0.18 0.33 0.18 0.15 a Results of EH calculations.b These compounds were used to calculate Cnl and dnl in Eqn (1). The partial charges Qnl and ql were determined from Eqns (1) and (2) using the ZEH effective charges and the Hartree ± Fock approximation, respectively (see text). 898 S V Kryutchkovshifts in the X-ray emission (dE) and X-ray photoelectron (DE) spectra is shown in Fig. 12. According to the effective ion model,98 the dE and DE values depend on the effective charge of the central atom (Q) as follows: dE=C5sQ5s+C4dQ4d+Q5s(Q5s71)d5s+ (1) +Q4d (Q4d71)d4d+Q5sQ4d d5s,4d ; DE=Q5sDE5s+Q4dDE4d+Q5s(Q5s71)D5s+ (2) +Q4d (Q4d71)D4d+Q5sQ4dD5s,4d+jM, where Qnl is the partial charge remaining on an atom after the nl-electron has been involved in the formation of a bond, Cnl (DEnl) is the line shift in the X-ray emission spectrum (or in the X-ray photoelectron spectrum) after abstraction of the nl- electron; d and D are the respective corrections, which correspond to the changes in the electrostatic energy of electrons due to the emission of an X-ray quantum and jM is the Madelung constant.Since for transition d elements Q=Qns+Q(n71)d+Qnp and Qnp is small and remains virtually constant on going from one compound to another (see, e.g., the data of EH calculations for Tc compounds in Table 2), it can be neglected. Then, in the general case, dE and DE can be represented as functions ofQ5s+Q4d.The experimental dependences of dE and DE on Q5s+Q4d (obtained from EH calculations) have been reported.77 It has been shown that the dependence DE=f(Q5s+Q4d) is close to a linear one, whereas the dependence dE=j(Q5s+Q4d) is less unambiguous. The values of the coefficients C5s, C4d, d5s, d4s and d5s,4d were determined after substituting the Q5s+Q4d values taken from EH calculations into Eqn (1) and solving the system of five linear equations with five unknowns.77 To reduce systematic errors due to inaccuracy in the parametrisation, the data calculated only for the chloride compounds [TcCl6]27, [Tc2Cl8]37, [Tc2Cl8]27, [Tc2Cl6]27 and [Tc6Cl12]27 were used.77 For comparison, analo- gous coefficients 77 calculated by the Hartree ± Fock SCF method for the isolated Tc atoms in the approximation of two alternative configurations (4d 65s 1 and 4d 5s 2) have also been used. It has been shown that, at least except for sign, the values of all the coefficients in question (C5s, C4d, d5s, d4d and d5s,4d) are close.This indicates indirectly that EH calculations provide a good basis for theoretical description of dE at qualitative and semiquantitative levels.A one-to-one correspondence between the DE and Q5s+Q4d values has been established by extrapolating the DE=f(Q5s+Q4d) dependence to all other technetium com- pounds for which the X-ray emission and X-ray photoelectron spectra have been measured.77 At this point, it has been assumed that jM is independent of Q5s+Q4d or varies monotonically as the latter parameter varies; this has been strictly substantiated only for compounds with similar structure and composition.Then, substituting the Q5s+Q4d value found into Eqn (1) and using the coefficients of this equation calculated in the EH approximation, one could calculate the Q5s, Q4d, and Q4p partial charges (see Table 12). To pass to another, more physically well- founded scale of effective charges, the values of coefficients of Eqn (1) calculated by the Hartree ± Fock method were used assuming that Zeff of technetium (Zeff=qs+qd) in KTcO4 coincides with the effective charge of ruthenium in RuO4 and is equal to 1.71.96, 97 From the fact that the metal atoms in both d 0 complexes have the same oxygen environment and that the electronegativities of the Tc and Ru atoms are almost equal it follows that these charges are approximately equal.79,80 Having calculated the partial charges qs and qd on technetium atoms in KTcO4 from Eqn (1) and assuming that the found value of the ratioQ5s / qs (3.57) remains almost constant, it was also possible to calculate77 the qs and qd values for all other technetium com- pounds for which EH data are available (see Table 12).These calculations have shown that all the experimental results can be described by the following equation: dE=(37.52.1)7(156.02.9) (qs+qd). (3) Since the correlation coefficient of the linear dependence (3) for nine experimental points is 0.999, it was assumed that dE for all other compounds is also described by this equation.Simultaneous solution of Eqns (1) and (3) performed under this assumption allowed one to find qs and qd values for all other technetium compounds. The results obtained 77 are listed in Table 12. The effective charges on the Tc atoms calculated using the two differ- ent procedures differ considerably in magnitude. However, all main tendencies and regularities of changes in Zeff on going from one compound to another are the same for both scales of effective charges:75 ± 77 1.The Q5p values and their changes for all compounds are small, which indicates that the extent of participation of the 5pAO in the formation of chemical bonds is small. The Q5s (qs) and Q4d (qd) values decrease regularly as the formal oxidation state of technetium decreases, which points to increasing covalency of chemical bonds in this series of compounds. 2. A tendency for Zeff to increase with increase in the Tc oxidation state, total electronegativity of ligands in the series F>O>Cl>Br>S&I and the polarising capacity of cations Zcat/Rcat is observed for compounds containing noM7Mbonds. 3. A `loss of sensitivity' of Zeff to the electronegativity of ligands, the nature of cation and the formal oxidation state of technetium with increase in the number of electrons participating in the formation ofM7Mbonds is observed for compounds with M7M bonds.In some cases the Zeff value in technetium clusters changes upon insignificant change in the formal oxidation state of Tc or in the structure of the cluster (see, e.g., compounds 41 and 42, 43 and 44, 1 and 3, 50 and 500, and 25 and 30 in Table 12).The partial charges of ligands in technetium compounds (Table 13) have been determined from the analysis of qs+d values, comparison of these values with Zeff(Ru) 96, 97 and using the dependence of qs+d on ZEH for the ligands.77 The algebraic sums of charges on all atoms in Tc compounds have been calculated from the data in Tables 12 and 13.77 These 7200 7100 0 100 dEKa1 /meV DE3d5/2 /eV 1 3 4 47 18 25 57 21 44 37 43 1 42 50 41 39 31 28 27 20 9 16 100 17 380 34 2 Figure 12.Correlation between the chemical shifts in the Ka1 X-ray emission spectra (dEKa1) and X-ray photoelectron spectra (DE3d5/2) of technetium compounds 77 (the numbering of points corresponds to that of compounds in Table 12).The scatter of experimental data is shown by dashed lines. Physicochemical properties of technetium acido clusters 899sums differ considerably from zero for binary compounds, which is probably associated with great differences in the nature of ligand bonding with technetium (apparently, one cannot use the same qs+d value for the ligands in all these compounds). System- atic deviations of the sums of charges from zero are observed for compounds with M7M bonds (di- and polynuclear clusters).A priori this may be due either to the same reasons as for binary compounds or to systematic changes (as a rule, a decrease) in Zeff of the Tc atoms caused by the M7M bonds. The latter assump- tion seems to be more reasonable, since X-ray photoelectron spectra of technetium compounds indicate that Zeff of halide ligands of the same structural type are constant irrespective of the presence of M7M bonds in the compounds 67, 69, 81 (see Section IV).The averaged deviations of Sqs+d from zero calculated per two Tc atoms using the above procedure for di- and polynuclear clusters of different structural types are equal to70.22 (dinuclear d 4 ± d 4 clusters),70.44 (dinuclear d 4 ± d 5 clusters), 70.80 (dinu- clear d 5 ± d 5 clusters), 70.53 (trigonal-prismatic clusters) and 70.17 (tetragonal-prismatic clusters).77 As can be seen, the decrease in Zeff due to the formation of M7M bonds is a maximum in the case of dinuclear d 5 ± d 5 clusters.This correlates with the shortest M7M distance (2.04 A) in their molecules.57 The influence of relatively long M7M bonds (*2.6 A)2 on the Zeff value is minimal.It is likely that the effect of multiple M±M bonds in polynuclear prismatic clusters is more pronounced than, e.g., in d 4 ± d 4 complexes, owing to the additional contribution from several weaker M7M bonds. Thus, the results obtained 75 ± 77 confirm again that the for- mation of M7M bonds may cause changes in the effective charges on the metal atoms.3, 51, 70 ± 73, 87, 88 We believe that this phenomenon is due to the specific effect of the system of M7M bonds on the electronic structure of clusters.Therefore, the M7M bonds in technetium clusters (as well as in a number of clusters of other elements) can be considered as specific additional ligands affecting Zeff on the metal core atoms of the clusters.Later, another approach to the interpretation of the data on chemical shifts in the Ka1,2 and Kb1,3 lines in X-ray emission spectra of molybdenum, technetium and ruthenium compounds was proposed.75 ± 77 These results were generalised by Batrakov et al.98 who have analysed two independent sets of experimental parameters corresponding to dKa1,2 and dKb1,3 in contrast to the chemical shifts for technetium compounds that we have consid- ered previously.However, a considerable increase in the number of parameters directly determined in the experiments does not allow one to interpret the experimental results unambiguously. Thus, e.g., Batrakov et al.98 had to make some assumptions on the electron configuration of the reference compound (TcO2 in the ionic state 1+ or 3+ has been used as reference compound for technetium compounds).However, from the above discussion it follows that it is just technetium dioxide that can scarcely be used as reference compound, because some of its electronic properties make it different from most other technetium compounds (see also Section IV, Tables 9 and 12 and Figs 9 and 12).Moreover, our calculations performed using a representative random sample of experimental dKa1,2 and dKb1,3 values taken from Ref. 98 and the approach described above in this Section showed that the final conclusions do not differ fundamentally from those drawn in Ref. 77. In this connection it is not expedient to consider the data and, especially, the interpretation of results obtained by Batrakov et al.98 in detail.We will restrict ourselves to major conclusions only. The diagram of the chemical shifts of the centres of gravity of the TcKa1,2 and TcKb1,3 doublets for technetium compounds (see Ref. 98) is shown in Fig. 13. As can be seen, in the coordinate system used all experimental points fall on five curves correspond- ing to different types of technetium compounds, viz., oxides, iodides, bromides, chlorides and clusters (including metallic technetium).It should be noted that similar dependences have been obtained for technetium compounds 98 in the coordinates of partial charges Q4d and Q5s which were determined using the approach described above. It was confirmed that the formation of M7M bonds (M=Tc, Mo) results in changes in the effective charges on the atoms of these metals, which is also an important result of this study.VI. Compounds with ferrocenium cations Considering various methods of investigation of technetium clusters, we shall dwell on the 57Fe MoÈ ssbauer spectroscopy of ferrocenium-containing complexes. Table 14 presents the param- eters of the 57Fe MoÈ ssbauer spectra of compounds with ferroce- nium cations.As can be seen, the parameters of the MoÈ ssbauer spectra depend strongly on temperature and the chemical nature of the anion. At room temperature, doublet (singlet for two compounds) spectra are observed; the chemical shifts d in the Table 13. Partial charges on ligands and cations in technetium com- pounds.77 Ligand, Charge Ligand Charge Ligand Charge cation Inorganic +0.82 Clax 70.45 Breq 70.34 cation (clusters) Bulky +0.87 Cleq 70.36 Brbridge 70.13 organic Cleq 70.40 Iax 70.29 cation (clusters) Feq 70.46 Clbridge 70.16 S 70.22(10) Oeq 70.30(15) Brax 70.37 C 70.33 {oxides, [TcOX4]7} Oeq 70.64(1) Breq 70.32 CO 70.03 {[TcO4]7} Note.The last significant figures of the maximum deviations from average charge values are given in parentheses. 200 100 0 7100 dKa1,2 /meV dKb1,3 /meV [s(dKa1,2)=3 meV] [s(dKb1,3)=6 meV] 750 50 150 Bromides Clusters Chlorides Iodides D4d [Tc2Cl6]27 D4h [Tc2Br8]27 D3h [Tc6Cl12]27 Oxides 55 39 29 28 59 58 48 44 64 52 53 54 27 51 23 49 11 25 31 56 41 42 1 34 3 43 Figure 13. Diagram of the chemical shifts of the centres of gravity of the TcKa1,2 and TcKb1,3 doublets in the X-ray emission spectra for techne- tium compounds according to Batrakov et al.98 The numbering of points corresponds to those of compounds in Tables 9 and 12; the point No. 64 corresponds to (Me4N)2[TcI6]). 900 S V Kryutchkovspectra depend slightly on the nature of the anion and the quadrupole splitting D decreases for the anions in the series: [TcO4]7 > [Re2Br8]27 > [Tc6I14]37 > [PF6]27 > [TcCl6]27 > [Tc6Cl14]37& [Tc8Br14]27.Spectral line broadening is observed as the temperature decreases. Additional lines appear in the spectra of compounds with the [TcCl6]27, [Tc6Cl14]37 and [Tc8Br14]27 anions; the parameters of one of these lines are close to those of the Fe2+ signal in pure ferrocene 30 (see Table 14 and Fig. 14). For cluster anions, this is observed even at 78 K, whereas for ferrocenium hexachlorotechnetate(IV) this is observed only at 5 K (at 78 K, the spectrum of this compound is appreciably broadened and can be deconvoluted into two doublets with small D by the least squares method assuming a Lorentzian lineshape).The spectrum of ferrocenium pertechnetate at 5 K is a broad singlet; however, attempts at its deconvoluting into components failed.Physicochemical studies 28, 29 of compounds with ferrocenium cations (see also Sections II ± IV) allow one to draw the following principal conclusions: 1. All compounds possess anomalous peculiarities of elec- tronic structure due to the changes in their physicochemical properties with time and temperature. 2. Anomalous properties increase for the series of anions as follows: [TcO4]7&[Re2Br8]27&[Tc6I14]37<[TcCl6]27< [Tc6Cl14]37<[Tc8Br14]27.The following explanation of this anomaly was proposed.28, 29 It was shown64 that the probability of participation of outer ns (np) technetium (rhenium) AOs in the formation of coordinate bonds formed by this atom increases in the above series of anions (except for the iodide cluster).This is due to the fact that the 4d AOs become destabilised to a greater extent than diffuse 5s (5p) AOs capable of effective interaction at comparatively long dis- tances (> 2.5 A) as the formal oxidation state of technetium (the effective charge on its atoms) decreases. There are also low-lying MOs formed by diffuse 4s (4p) orbitals of Fe atoms in ferrocenium cations.99 An approach of Fe and Tc atoms within a distance of *5 A (it is this distance that is observed in ferrocenium com- plexes of technetium) must result in direct electron overlap of ns (np) orbitals of the Fe and Tc atoms and, hence, in the formation of conduction bands.Indeed, all compounds possess temperature-independent semiconductor-type conductivity. This indicates that the Fermi levels of the ferrocenium cations and technetium (rhenium) anions in these compounds are inside or near the sp conduction bands. This may result in tunnelling of the valence sp electrons between the anions and cations at frequency o.Such a dynamic electron transfer along the anion ± cation chains can be detected using only those physical methods that have a better time resolution than the average lifetime of the valence electron in the proximity of the atom under observation.In other words, these changes can be detected using photoelectron spectroscopy with a time resolution (t) of 10716 ± 10717 s, whereas only an averaged picture can be obtained using such integral methods as static magnetic susceptibility, heat capacity and conductivity measurements, X-ray analysis, etc.The results obtained by MoÈ ssbauer, EPR, and IR spectroscopy with a time resolution of 1077 s, *1079± 10710 s and 10711± 10713 s, respectively, will be intermediate due to line broadening in spectra caused by dynamic electron transfer and vibronic interaction. In the context of the proposed explanation of the properties of compounds with ferrocenium cations,28, 29 attention must be focused on the magnetic properties (see Sections III and IV) and changes in the photoelectron spectra of these compounds at low Table 14.Parameters of 57Fe MoÈ ssbauer spectra (d, D, G/mm s71) of compounds with ferrocenium cations.28, 29 Compound T /K d D G Notes (FeCp2)[TcO4] 300 0.65 0.74 0.54 doublet 300 a 0.61 0.70 0.60 " 78 0.75 0.65 0.69 " 5 0.73 0 1.68 broad singlet (FeCp2)4[H3O(H2O)4]2[TcCl6]3 300 0.66 0.14 0.36 doublet 300 a 0.62 0 0.60 singlet 78 0.74 (A) 0.18 0.38 two doublets in the ratio 78 0.66 (B) 0.55 0.37 A: B=7.6 : 1.0 5 0.80 (A) 0 0.53 singlet and doublet in the ratio 5 0.86 (B) 2.37 0.25 A: B=6.6 : 1.0 (FeCp2)3[Tc6Cl14] 300 0.66 0 0.50 singlet 78 0.65 (A) 0 0.60 singlet and two doublets in the ratio 78 0.80 (B) 2.30 0.40 A: B:C=2.3 : 1.0 : 1.0 78 0.70 (C) 0.30 0.50 (FeCp)2[Tc8Br14] 300 0.70 0 0.45 singlet 78 0.62 (A) 0 0.60 singlet and two doublets in the ratio 78 0.83 (B) 2.35 0.60 A: B:C=2.0 : 1.0 : 1.0 78 0.72 (C) 0.35 0.42 (FeCp2)2[Tc6I14] 300 0.68 0.44 0.47 doublet 300 b 0.68 0.46 0.48 " 78 0.78 0.46 0.53 " (FeCp2)2[Re2Br8] 300 0.62 0.68 0.59 " 300 b 0.62 0.71 0.67 " 78 0.72 0.64 0.65 " (FeCp2)2PF6 300 0.66 0.30 0.31 " 78 0.77 0.30 0.30 " FeCp2 300 0.60 2.37 0.24 " 78 0.76 2.38 0.30 " a The specimen was preliminarily frozen to 5 K.b The specimen was preliminarily frozen to 78 K. Physicochemical properties of technetium acido clusters 901temperatures. Thus, Table 9 presents the experimental parame- ters of X-ray photoelectron spectra of several technetium and rhenium compounds (see also Refs 28 and 29).From these data it follows that the X-ray photoelectron spectra of the compounds with ferrocenium cations recorded at room temperature for the first time do not differ fundamentally from those of other technetium and rhenium compounds. All major spectral regular- ities correspond to those described above (see Sections IV.1 ± IV.3).The only exception is the increased binding energy corre- sponding to the Tc 3d line in the case of (FeCp2)2[Tc8Br14]. However, anomalous electron emission is observed at a temper- ature of about 100Kinstead of conventional X-ray photoelectron spectra of the compounds with ferrocenium cations and [TcCl6]27, [Tc6Cl14]37 and [Tc8Br14]27 anions. No temperature dependence of spectra of other compounds has been found. The phenomenon of anomalous electron emission consists in the following.28, 29 The lines of the conventional X-ray photo- electron spectrum (Fig. 15) become diffuse and disappear as the temperature of the specimens decreases below a specific threshold value (*100 K). The spectrum of inelastic electrons observed simultaneously with the lines of the X-ray photoelectron spectrum loses its true shape and also disappears against the background of a group of very intense lines; the structure and number of these lines vary substantially on fast freezing of the specimen.The amplitudes of the most intense peaks are approximately 2 orders of magnitude larger than the intensities of the lines in the conven- tional X-ray photoelectron spectrum, which usually results in overloading of the detector even at the minimum power of the X- ray radiation source (100 W for an HP 5950A spectrometer).Further cooling of the specimens leads to substantial increase in the upper boundary kinetic energy (UBKE) of emitted electrons (from 10 to 51500 eV). The UBKE changes analogously with the increase in duration of exposure of the specimens in which approximate temperature equilibrium was reached to X-ray radiation.The UBKE decreases slowly until the anomalous emission ceases and the conventional X-ray photoelectron spec- trum recovers as the specimens undergo slow unfreezing. The qualitative and quantitative spectral pattern is reproduced within short time intervals (*5 min) under conditions of relative stabi- lisation of the specimen temperature.To a first approximation, the intensities of the peaks were found to be proportional to the power of the X-ray tube of the spectrometer. With an accuracy of *0.1 s, no time lag was found on switching off the X-ray tube. The kinetic energy of emitted electrons can exceed the energy of exciting X-ray quanta (1486.6 eV). The second peculiarity of the X-ray photoelectron spectra of compounds with ferrocenium cations is that the spectra recorded repeatedly at room temperature (after freezing of the specimens to 78 K followed by their unfreezing) can differ from those recorded initially at the same temperature.These differences include 28, 29 the following features: (1) the peaks corresponding to the core electrons of the Fe atoms correspond in the repeatedly recorded spectra to three nonequivalent states of these atoms {compounds with [Tc6Cl14]27 and [Tc8Br14]27 anions} and (2) the same holds for the peaks corresponding to the core electrons of the halogen atoms {the compound with the [TcCl6]27 anion}.These data could indicate that the compounds with ferrocenium cations are initially in the metastable state and are stabilised on freezing.However, the temperature dependences of the heat capacity and the magnetic susceptibility of the compounds studied show that no phase transitions occur in the temperature range from 300 to 100 K. Thus, the effects observed are due to pure electronic peculiarities of the structure of the compounds under consider- ation. It should be noted that the MoÈ ssbauer and IR spectra (see Sections II and VI, respectively) of the specimens that were preliminarily frozen repeatedly recorded at room temperature also differ somewhat from those recorded initially.} Phenomena similar to that observed 28, 29 are known: exoelec- tron emission (see, e.g., Ref. 100) and mechanoemission.101 It is likely that the charges accumulated in the specimen subjected to X-ray radiation are to a great extent responsible for the appear- } Changes in the specimens caused by freezing cannot be due to any chemical processes, since the specimens are stable towards water sorption on the surface; no changes in the chemical composition were found.28, 29 Intensity (rel.units) a b d e c 72 71 0 1V /mm s71 Figure 14. 57FeMoÈ ssbauer spectra of: (a) FeCp2 at 300 K; (b, c) (FeCp2)3. .{[Tc6Cl6(m-Cl)6]Cl2} at 300 and 80 K; (d, e) (FeCp2)2[PF6] at 300 and 80 K. 200 400 600 800 E /eV 5000 I /pulses a b c Figure 15. Dynamics of anomalous electron emission for a (FeCp2)2[Tc8Br14] specimen preliminarily cooled to*100 K after switch- ing on the X-ray radiation source (at a beam power of 200 W) during data collection (1000 eV/256 channels) in one scan (data collecting time of 210 s).Start of data collection after switching on the X-ray radiation source: (a) 0 min; (b) 4 min.; and (c) 8 min. (Here E is the kinetic energy of electrons).28, 29 902 S V Kryutchkovance of anomalous electron emission.78 The peculiarities of the low-temperature X-ray photoelectron spectra of some com- pounds with ferrocenium cations observed in the experiments may be due to the multiphoton photoeffect similar to the laser effect.28, 29 As has been mentioned in Section III, the magnetic properties of compounds with ferrocenium cations are also anomalous.In fact, as can be seen from the data in Table 8, in all cases there is more than one unpaired electron per ferrocenium cation.It is obvious that the effective number of unpaired electrons may be increased due to the partial unpairing of the occupied cationic and anionic MOs composed mainly of ns (np) AOs and forming the conduction bands. The effective number of unpaired electrons may also change appreciably due to the changes in the vibronic interaction caused by temperature changes. In this case the para- magnetism of the compound will be the sum of the contributions of d electrons of the cations and anions and s electrons of the conduction band.Because of the fast dynamic electron transfer (the s ± d ± s exchange), the lines in the EPR spectra of the compounds will be appreciably broadened or unobservable {as in the case of compound with the [Tc8Br14]27 anion}. The central, more narrow component of the EPR spectra will correspond mainly to d electrons of cations and anions, whereas the broader component will correspond to the electrons of the s conduction band.VII. Conclusion The experimental and theoretical substantiation of the participa- tion of the outer 5s (5pz) orbitals of the Tc atoms in the formation of bonding `metallic' MOs in technetium clusters with the formal oxidation state of Tc no more than 2.0 is presented in this review.On the one hand, this effect results in strengthening of theM7M bonding in technetium clusters containing a formally `excess' number of `metallic' electrons (i.e., the number exceeding that corresponding to the closed shell of the bonding MOs composed of 4d AOs). On the other hand, it enhances shielding of the positive nuclear charges (a decrease in Zeff) of the Tc atoms in technetium clusters, thus decreasing the Coulomb repulsion in the formation of M7M bonds and increasing the contribution of electron correlation effects to the ground electronic state of the system.Thus, it also leads to an increase in the cluster-forming properties of technetium. 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ISSN:0036-021X    

出版商:RSC    

年代:1998

数据来源: RSC