12 Organometallic Chemistry Part (i) The Transition Elements By M. BOCHMANN Department of Chemical Sciences University of East Anglia. Norwich NR4 7TJ R. A. HEAD ICI New Science Group The Heath Runcorn Cheshire WA7 4QE M. D. JOHNSON Department of Chemistry University College London 20 Gordon Street London WCl H OAJ 1 Introduction 1983 was a year of consolidation in most areas of organotransition-metal chemistry a highlight being the surge of development in intermolecular activation of carbon- hydrogen bonds of both aliphatic and aromatic hydrocarbons. A great many reviews of interest to organic chemists have appeared notably on asymmetric hydrogena- tion,' rhodium-catalysed enantioselective hydrogenation,2 hydrogenation of carbon monoxide to ethanol and ethylene glycol,3 catalytic hydrocarbonylation of alcohols co-ordination chemistry of ylide~,~ 7r-complexes of cobalt( I) transition-metal nitrosyls in organic ~ynthesis,~ transition-metal formyl complexes,' electron-rich half-sandwich complexes,' homolytic displacement of transition-metal complexes from carbon," homolytic and radical pathways in the reactions of organo-chromium(111) compounds,' 'titanium-induced dicarbonyl coupling reactions,12 the organometallic complexes of titanium and zirconium as selective nucleophilic reagents in organic ~ynthesis,'~ and the organic chemistry of gold,14 as well as two reviews on the role of metal clusters in In addition the main papers presented at the Conference on Organometallic Chemistry Directed Towards Organic Synthesis held in Dijon in September have already been published." A review on ' W.S. Kennedy Acc. Chem. Rex 1983 16 106. H. Brunner Angew. Chem. int. Ed. Engl. 1983 22 897. D. B. Dornbek Adv. Cutul. 1983 32 326. M. E. Fakley and R. A. Head Appl. Catal. 1983 5 3. W. C. Kaske Coord. Chem. Rev. 1983 48 1. T. Funabiki Rev. Inorg. Chem. 1982 4 329, 'K. K. Pandey Coord. Chem. Rev, 1983 51 69. J. A. Gladysz Adv. Organomet. Chem. 1983 20 1. H. Werner Angew. Chem. In?. Ed. Engl. 1983 22 927. I" M. D. Johnson Acc. Chem. Rex 1983 16 343. I' J. H. Espenson hog. inorg. Chem. 1983 30,189. I' J. E. McMurry Acc. Chem. Rex 1983 16 405. I3 B. Weidmann and D. Seebach Angew. Chem. Int. Ed. Engl. 1983 22 31. G. K. Anderson Adv. Organomet. Chem.1982 20,40. Is R. D. Adam Acc. Chem. Rex 1983 16 67. l6 E. L. Muetterties and M. J. Krause Angew. Chem. in?. Ed. Engl. 1983 22 135. Pure Appl. Chem. 1983 55 1669. 273 274 M. Bochmann R. A. Head and M. D. Johnson the reactions of heteroaromatic transition-metal complexes with base clearly confirms that extensive earlier descriptions of covalent hydration of N-heteroaromatic ligancls has little foundation.18 2 Intermolecular Activation of Aliphatic and Aromatic Hydrocarbons The early observations of low-temperature homogeneous activation of alkenes and arenes by transition-metal complexes were made in 1965. Subsequently many examples of intramolecular activation e.g. cyclometalation have been described. In the past two years however clear examples of intermolecular activation including catalytic processes have been reported and our understanding of the nature of the reactive intermediates has grown.The dihydro-complex (1 ; R1= Ph)19 extrudes dihydrogen on irradiation in ben- zene and reacts with the solvent to give (2; R1= R2 = Ph). The corresponding complex (1; R' = Me)** reacts not only with benzene but also with cyclohexane and with neopentane to give (2; R'= Me R2 = cyclohexyl or neopentyl). Although the corresponding thermal process is also possible the thermal reductive elimination of the alkane from (2; R1= Me R2 = Alkyl) becomes dominant at higher tem- peratures. Competition experiments show that the relative reactivities of the organic substrates towards the transient intermediate formed from (I ; R' = Me) expressed as reactivity per available C-H bond are benzene (4) > cyclopropane (2.65) > cyclopentane (0.96) > neopentane (0.57) > cyclohexane (0.5) > cyclodecane (0.27) > cyclo-octane (0.04).(T-c :) 5(7-C M~,)wPR:)R~ M~,)I~H,(PR (1) (2) Phosphine ligands are by no means an absolute requirement for C- H activation. The related carbonyl complex (3)2' on irradiation with cyclohexane benzene per- deuteriobenzene or even methane in perfluorohexaneZ2 gives the alkyl or aryl complex (4) (R = neopentyl cyclohexyl phenyl C6D5 or methyl). These species are stable only at low temperatures but can be converted by reaction with carbon tetrachloride or N-bromosuccinimide into the corresponding more stable chloro- or bromo-alkyl complexes (5).The relative rates of reaction per available C-H bond observed using (3) are benzene (5) > neopentane (1.25) > cyclohexane (1). It is suggested that the pentamethylcyclopentadienyl ligand not only increases the electron density on the metal but also reduces the possibility of intramolecular C -H activation. The relative reactivity of the co-ordinatively unsaturated intermedi- ate is thus (q-C5Me5)Ir(PMe3)> (q-C,Me,)Ir(CO) > (q-C5H5)Ir(CO). However overall reactivity is likely to depend not only on the rate of reaction of the unsaturated complex but also on its rate of formation and the reverse. hv RH CHBr (~-C,Me,)1r(C0)~ (T-C,M~,)I~H(CO)R (7)-C,Me,)IrX(CO)R (3) (4) (5) X = Br or C1 l8 N. Serpone G.Ponterini M. A. Jamieson F. Bolletta and M. Maestro Coord. Chem. Rev. 1983,50,209. '' A. H.Janowicz and R. G. Bergman J. Am. Chem. SOC.,1982 104 352. 20 A. H. Janowicz and R. G. Bergrnan J. Am. Chern. SOC.,1983 105 3929. 2' J. K. Hoyano and W. A. G. Graham J. Am. Chem. SOC.,1982 104 3723. 22 J. K. Hoyano A. D. McMaster and W. A. G. Graham J. Am. Chem. Sor. 1983 105 7190. Organometallic Chemistry -Part (i) The Transition Elements 275 The corresponding rhodiurn(~rr) complex (6a)23 gives the aryl complex (7a) on reaction with PhMgBr in THF at -40°C. Reaction of (7a) with Li[HBBuS3] gives the hydridoaryl complex (7b) which undergoes arene exchange on heating with other aromatic compounds. On heating with toluene for example a mixture of meta- and para-substituted isomers (8a) and (8b) is formed the former under kinetic control the mixture under thermodynamic control.It is suggested that the rearrange- ment of (8a) into (8b) takes place without dissociation of the arene i.e. viu a q-arenerhodium(1) complex (9),or possibly via a dihydro-q2-benzynerhodium(~rr) complex (1 0). hu RH CHBr (v-C5Me5)RhX2(PMed -(v-C5Me5)RhX(PMe3)Phor~~1; (v-CSMeS)RhH(PMe3)Ar (6) a; X = C1 (7) a; X = Br (8) a; Ar = 4-MeC,H4 b;X=H b;X=H b; Ar = 3-MeC6H The thermal elimination of methane from the corresponding methyl complex (q-C5Me5)Rh(PMe3)(H)(Me)24 at -17 "C in deuteriobenzene is first order with a rate constant k = 6.5 x s-'. Irradiation of the dihydride (6b) in liquid propane at -55 "C gives exclusively the primary n-propylrhodium complex (q-C5Me5)-Rh( PMe,)( H)(Pr") which regenerates propane at -15 "C but which can be converted into the more stable bromoalkyl complex (q-C5Me5)Rh(PMe3)(Br)( Pr") on treatment with CHBr,.T2-Arene complexes (12) have been characterized in the low-temperature reaction of the arylrhenium complexes (1 la-c) with HBF4.Et20 in CH2C12.2' In the case of (12; Ar = Ph) the singlet C6H6 resonance at 87.23 in the 'H n.m.r. spectrum is unaffected by addition of free benzene which gives a separate resonance at 87.33; in the protonation of the 0- m- and p-tolyl complexes (1 Ib) the same q2-toluene cation (1 2; R = Me) is formed. Protonation of (1 lc) is more difficult and requires fluorosulphonic acid at -78 "C. In all cases deprotonation by bases such as triethyl- amine regenerates the q' -aryl complex (1 1).Similar stereochemically non-rigid q2-arene complexes (12; R = CHPh2)and (12; R = cycloheptatriene) are formed26 in the reaction of trityl or tropylium hexafluorophosphate with (7-C,H,)Re(NO) (C0)H in CH2CI2 at -78 "C. The temperature dependence of the 'H n.m.r. spectrum between -40 and -70 "C indicates that the rhenium moves around the six-membered ring of the former complex via a series of q'-arenium structures (13) corresponding to the Wheland intermediates long postulated in aromatic substitution reactions. 23 W. D. Jones and F. J. Feher J. Am. Chem. SOC. 1982 104,4240. 24 W. D. Jones and F. J. Feher Organomefallics 1983 2 562. 25 J. R. Sweet and W. A. G. Graham J.Am. Chem. Soc. 1983,. 105 305. 26 J. R. Sweet and W. A. G. Graham Organometallics 1983 2 135. M. Bochmann R. A. Head and M. D. Johnson Another q2-arene complex (15) has been identified in the reaction of the fac- complex (14) with anthra~ene.~' The rate of disappearance of (14) to (15) and to associated reduction products is independent of the concentrations of anthracene and dihydrogen indicating that the co-ordinatively unsaturated complex [RuH(Ph,P),]-is a common reactive intermediate. The trihydride IrH,(CO)(dppe) also loses dihydrogen both on heating and on irradiation yielding IrD,(CO)(dppe) in the presence of D2 and IrH(CO),(dppe) in the presence of CO. H/D Exchange takes place in the presence of C6D6 and when CO is also present benzaldehyde is also formed.28 (14) P = PPh (15) The analogy with electrophilic substitution is more evident with some complexes than with others.For example the square-planar octaethylporphyrinrhodium(1) cation prepared in situ from the corresponding chloro-complex and silver ion reacts with aromatic hydrocarbons predominantly at the para-position. The selectivity determined by competition methods anisole (1) > toluene (0.15) > benzene (0.14; statistically corrected) > chlorobenzene (0.002) is in the direction expected for an electrophilic substitution reaction but is lower than is observed in nitration or bromination. Using the substituent constant u+the reaction has a p-value of ca. -2.5; the more negative value quoted by the authors29 appears to be from use of the less appropriate U-values.The analogy with electrophilic substitution is clearly much less in the case of the complexes (1) and (3) and is certainly not exclusive because the reaction of cyclopentane with the aquated chromium(I1) ion in the presence of hydrogen peroxide takes place by (i) reaction of chromous ion with hydrogen peroxide (ii) reaction of a hydroxyl radical with cyclopentane and (iii) capture of a cyclopentyl radical by chromous ion to give the penta-aquacyclopen- tylchromium(r1r) ion.,' Selectivity in the case of (r)-C6H,)RuH2(Pri3P) (16),' is dominated by the fact that on irradiation in cyclohexane at room temperature reaction does not appear to take place with the solvent. Instead an intramolecular cyclometallation takes 27 R.Wilczynski W. A. Fordyce and J. Halpern J. Am. Chem. SOC.,1983 105 2066. 28 B. J. Fisher and R. Eisenberg Organornefallics 1983 2 764. 29 Y. Aoyama T. Yoshida K. Sakurai and H. Ogoshi J. Chem. SOC.,Chern. Cornrnun. 1983 478. 30 J. H. Espenson P. Connolly D. Meyerstein and H. Cohen Inorg. Chem. 1983 22 1009. 3' H. Kletzin and H. Werner Angew. Chem. Inr. Ed. Engl. 1983 22 873. Organometallic Chemistry-Part (i) The Transition Elements place through activation of one of the methyl groups of the tri-isopropylphosphine. The four-membered metallocycle (1 7) does react with benzene (and perdeuterioben- zene) at room temperature to give the hydridoarylruthenium complex (18). Arene exchange takes place when (I 8) is heated with other hydrocarbons such as toluene.It is suggested that the apparent absence of reaction with alkanes is a result of the ready reductive elimination of the hydridoalkyl complexes. The corresponding aryl complex in which the benzene ligand is replaced by the hexamethylbenzene ligand does not undergo exchange with other arenes. (16) [Ru] = (q-C,H,)Ru(PPr;) A (18) Me H (17) Two studies reveal the potential of lanthanide complexes in C-H activation. Thus complex (19) (R = H or Me) reacts thermally with pyridine and with tetramethylsilane in hydrocarbon solvents to give isolable products of C-H activa- tion (20) and (21).32 The reaction of (20; R = Me) with '3C-labelled methane in [2HI,]cyclohexane at 70 "Cresults in the exchange of labelled and unlabelled methyl groups in the complex.33 Kinetic studies indicate that the exchange takes place by simultaneous unimolecular and bimolecular paths the former via a cyclometallated intermediate (22) and the latter by a concerted reaction through the four-centre transition state (23).It is also significant that the complex (19; R = Me) exists in the solid state as an unsymmetrical dimer. (T-C,Me,)&uR (q-CSMe,)2LuCH,SiMe3 + RH "CH (21) (~-CSMes),L~'3CH, + CH 32 P. L. Watson J. Chem. SOC.,Chem. Commun. 1983 276. 33 P. L. Watson J. Am. Chem. SOC..1983 105 6491. M. Bochmann R. A. Head and M. D. Johnson The dehydrogenation of cycloalkanes by the complex ReH,(R,P) (24) in the presence of 3,3-dimethylbutene as a hydrogen acceptor was described in 1982.34 With cyclopentane the product was a cyclopentadienylrhenium complex but with higher cycloalkanes the un-co-ordinated cycloalkene was obtained.The dehydrogen- ation is catalytic under mild conditions; thus 3 mM catalyst (24; R = 4-FC,H4) and dimethylbutene (50 mM) in cyclo-octane as solvent gives 5.3 mM cyclo-octene within 10 min at 30 "C and 30 mM cyclo-octene within 10 min at 80 0C.35 Methylcyclohexane gives 29% 3-methylcyclohexene 65% 4-methylcyclohexene and 6% of the exocyclic olefin. The activity of the catalyst is less with (24; R = Ph) or (24; R = 4-MeC,H4). The complex [IrH2S2L2]+ (S = water or acetone L = Ph3P) also dehydrogenates cyclopentane and cyclopentene to give cyclopentadienyliridium complexes; they dehydrogenate cyclo-octane and cyclo-octene to give cyclo-octadieneiridium com- p~exes.~~ Two examples of intramolecular C-H activation are of interest.Whereas catalytic deuterium/ hydrogen exchange takes place at the a-carbon of alkylamido-complexes of zirconium hafnium and niobium the corresponding exchange in alcohols cata- lysed by alkoxy-complexes of the same metals takes place exclusively at the p-carbon.37 The cyclometallation process also serves as the basis of the catalytic aminomethylation of terminal olefins. In an unrelated process substituted cyclopen- tanes can be formed enantioselectively through activation of a remote carbon chain as in Scheme 1.38 The enantioselectivity is achieved through suitable adaptation of the substrate (25) with a removable second chiral centre on the ester group thus allowing the intermediate to cyclize diastereoselectively through a highly ordered transition state derived from the intermediate (26).Scheme 1 The details of the temperature dependence of the I3C and 'Hn.m.r. spectra of tricarbonylcyclohexadienylmanganese(28 ; X = H)demonstrated most elegantly and persuasively the existence of intramolecular C-H-Mn interactions and the associated dynamic eq~ilibria.~~" This work has been extended:39bic (a) using 2H 34 D. Baudry M. Ephritikhine and H. Felkin J. Chem. Soc. Chem. Commun. 1982 606. 35 D. Baudry M. Ephritikhine H. Felkin and R. Holmes-Smith J. Chem. Sac. Chem Commun.,1983,788. 36 R. H. Crabtree M. F. Melleas and J. M. Mihelcic J. Am. Chem. SOC.,1982 104 107.31 W. A. Nugent D. M. Ovenall and S. J. Holmes Organometallics 1983 2 161. 38 D. F. Taber and K. Raman J. Am. Chem. SOC.,1983 105 5935. 39 M. Brookhart W. Lamanna and M. B. Humphrey J. Am. Chem. SOC.,1982 104 2117; M. Brookhart W. Lamanna and A. R. Pinhas Organometallics 1983. 2.638; M. Brookhart and A. Lukacs ibid. p. 649. Organometallic Chemistry -Part (i) The Transition Elements n.m.r. spectroscopy to show that deuteriation of the hexadienemanganate(1) complex (27) takes place with endo-stereospecificity to give (28; X = D); (b) that the bridging hydrogen is acidic; (c) that the methylation of (27) with for example methyl iodide gives two isomers (30a) and (30b) via an intermediate (29) in which the methyl group is bonded to the metal; and (d) that deprotonation of (30a) and of (30b) leads to the same diene complex (31) which (e) on further methylation in the presence of carbon monoxide gives two products (33) and (34) the former being derived from the complex (32) which is unable to manifest the appropriate C-H-Mn bridging (Scheme 2).Several dynamic processes have been studied with these and related more highly substituted complexes. The cyclic dienes can be liberated from the dienyl complexes such as (27) and (31) by reaction with molecular oxygen and the cycloalkenes can be released from the enyl complexes such as (28) (30) and (34) by reaction with LiBEt,H under 1 atm CO. H (32) (34) Scheme 2 3 Rearrangements Related to Coenzyme BI2 Chemistry Three organometallic rearrangements related to bio-organic rearrangements cata- lysed by coenzyme BI2have been described two of which relate to the diodehydrase conversion of diols into aldehydes.Three diastereoisomeric 6,7-dihydroxycycloun- decyl iodides (35) have been synthesized and converted into cycloundecanone by reaction with borohydride ion in the presence of catalytic amounts of cobalt chelates and even of cobalt(r1) chloride (Scheme 3).40 The maximum yield of cycloundecanone P. Muller and J. Retey J. Chem. Soc. Chem. Comrnun. 1983. 1342. M. Bochmann R. A. Head and M.D. Johnson “-0 Om -HO (38) “ 0 Om Reagent iBH; trace.(CO)I,[(CO) = 2,3,9,10-tetramethyl-1,4,8,1 l-ol-I-tetra-azaundeca-l,3,8,lO-tetraen-l olatocobalt] Scheme 3 (75%) was obtained using the chelate 2,3,9,lO-tetramethyl- 1,4,8,1 l-tetra-azaundeca- 1,3,8,lO-tetraen-ll-ol-olato (36) the yield being almost independent of the concentra- tion of the organic substrate but being decreased by high concentrations of the cobalt complex.It is proposed that the cyclic ketone is formed by rearrangement of the conjugate base of the 1,2-dihydroxycycloundecylradical (38) which is formed by a transannular hydrogen atom transfer from the first-formed 6,7-dihydroxycyc- loundecyl radical (37) rather than by a cobalt-mediated process. Indeed it is also suggested that the organocobalt complex (39) formed by capture of cobalt(r1) by (38) is the precursor of 2-hydroxyundecanone which is formed as a by-product. Similar conclusions are drawn from an extremely thorough study of the kinetics and products of decomposition of the previously unknown 1;2-dihydroxyethyl- (40) and protected 1,2-dihydroxyethyl-cobalt(r1~)complexes [also containing the chelate ligand (36)] in alkaline methanol (Scheme 4).41 High (>95%) yields of acetaldehyde are obtained through the formation of the 1,2-dihydroxyethyl radical which is 4’ R.G. Finke W. P. McKenna D. A. Schiraldi B. L. Smith and C. Pierpont J. Am. Chern. SOC.,1983 105 7592 R. G. Finke and D. A. Schiraldi ihid. p. 7605. Organometallic Chemistry-Part (i) The Transition Elements 28 1 (Co)CH(OH)CH,(OH) (CO")+ .CH(OH)CH20H 2.CH(O-)CH,OH -+ (40) 1ii (Co)CH,CHO ~t+ MeCHO (Co) as in Scheme 3 above. Reagents i -OMe:ii A hydrogen atom source probably solvent methanol.Scheme 4 substantially more acidic than the parent ligand and rearranges through its conjugate base directly without cobalt mediation to acetaldehyde. It is clearly demonstrated that the formylmethyl complex (41) is not an intermediate on the reaction path leading to aldehyde. In contrast Golding4* has synthesized several racemic ethoxycarbonyl-substituted but-3-enyl (42) and (43) and cyclopropylcarbinyl (44) and (45) cobaloximes which rearrange in the presence of trifluoroacetic acid to an equilibrium mixture containing predominantly (43) (Scheme 5). These rearrangements are not only much slower than were observed in the corresponding methyl-substituted complexes but the most stable species also differs confirming the rather special stability of the carbon-cobalt bond in alkoxycarbonylmethylcobalt(Irr) complexes.C0,Et + EtocoY EtOCO EtOCOfro) b-cco (44) (45) (Co) = Co(dmgH),Py Reagent i CF3CO2H 1.0 M. Scheme 5 4 Diels-Alder Reactions Diazadieneiron(0) complexes catalyse the Diels-Alder reaction between 1,3-dienes and disubstituted alkynes to give hexa- 1,4-dienes (46) and (47) at moderate conver- sions and high selectivities. The catalyst can be prepared in situ for example from Fe( acac),/2 diazadiene/6 EtMgBr. Diene dimerization which is the reaction product in the absence of alkynes is not observed.43 The intramolecular Diels-Alder reaction of (48) to give (49) is catalysed by the weakly Lewis-acidic complexes of MC1(q3-allyl)(CO),(MeCN) (M = Mo or W) preferably in alcoholic solvents -which stabilize the catalyst.# 42 B.T. Golding and S. Mwesigye-Kibende J. Chem. SOC.,Chem. Commun. 1983 1103. 43 H. tom Dieck and R. Diercks Angew. Chem. 1983,95 801. 44 M. S. Bailey B. J. Brisdon D. W. Brown and K. M. Stark Tetrahedron Lett. 1983 24 3037. M.Bochmann R A. Head and M.D. Johnson (48) (49) Vinyl-substituted chromium and tungsten Fischer-carbene complexes (50) react with dienes lo4 times faster than methyl acrylate the nearest analogue. The reaction is stereoselective; for example the chromium complex reacts with isoprene to give the cyclohexenes (5 1a and b) in a 92 :8 ratio. The reaction of (50)with cyclopentadiene is complete within 3 min at 25 "C and gives the endo-product in 94% selectivity.The M(CO)5fragment can be replaced by =0,H2,or =CH2 under mild condition^.^' 5 Cross-coupling Reactions The palladium or nickel-catalysed cross-coupling reactions of carbanions with aryl vinyl benzyl or ally1 halides have continued to be versatile methods for C-C bond formation and efforts have now been made to introduce building blocks carrying functional groups in this way. Enol ethers are metallated by t-butjl-lithium. Trans- metallation with ZnClz is necessary to achieve a reaction with aryl iodides and vinyl bromides and iodides in the presence of a palladium(0) catalyst and subsequent protonation converts the products into acyl compounds. Allenic ether anions react similarly; acidic work-up gives a$-unsaturated ketones in moderate to good yield (Scheme 6).46 OEt Reagents i Bu'Li; ii ZnCI,; iii RI-Pd; iv H+ Scheme 6 Alkenylmetal complexes containing a-alkylthio or a-trialkylsilyl substituents react with vinylic iodides to give hetero-substituted dienes (52) in good yield.Zinc is again preferred as electropositive metal though vinyldialkylaluminium compounds 45 W. D. Wulff and D. C. Young J. Am. Chem. Soc. 1983 105 6726. 46 C. E. Russel and L. S. Hegedus J. Am. Chem. Soc. 1983 105,943. Organometallic Chemistry -Part (i) The Transition Elements and trialkylborates are equally useful. However organometallics derived from Me,S Me,SO MeN02 and others do not react. The exact nature of the palladium(0) catalyst does not appear to be critical in these reactions.47 Z = OEt SEt or SiMe,; (52) M = ZnC1 AIB& or BR;Li+ High optical yields have been obtained in cross-coupling reactions of a11y1 phenyl ethers with Grignard reagents catalysed by nickel complexes with (-)-( S,S)-2,3 -bis(dipheny1phosphino)butane as the asymmetric chelating phosphine ligand.Com- plexes of structure (53) are postulated as intermediates. Reaction of (54) with EtMgBr gave (55) in 85% yield and 98% optical yield.48 Attempts have been made systematically to determine those reaction parameters responsible for chemical and optical yield in nickel-catalysed cross-coupling reac- tions. The coupling of aryl halides with s-butylmagnesium halides was used as a model reaction with NiC12L [L = asymmetric I ,2-bis(diphenylphosphino)ethane derivative].Solvent concentration relative reactant ratios and most important the nature of the halide in the Grignard reagent influence the optical yield. The best results were obtained with aryl and magnesium bromide (up to 50.7 e.e.).49 6 Titanium Alkylations with organotitanium reagents have attracted much interest recently," and have now been applied to the synthesis of (57) in a one-pot reaction from the ketone (56) using a 1 :1 mixture of MeTiC1 and Me,TiC12. Compound (57) is a precursor to the tetrahydrocannabinoid (58)? 47 E. Negishi and F. T. Luo J. Org. Chem. 1983 48 1560. 48 G. Consiglio F. Morandini and 0.piccolo J. Chem. SOC.,Chem. Commun. 1983 112. 49 G. Consiglio F. Morandini and 0. Piccolo Tetrahedron 1983 39 2699.50 For reviews see M. T. Reetz Top. Curr. Chern. 1982 106 1; and ref. 13. M. T. Reetz and J. Westerrnann J. Org. Chern. 1983. 48 254. M. Bochmann R. A. Head and M. D. Johnson (58) Titanium compounds react very much faster with aldehydes than with ketones.I3 The selective alkylation of keto-groups with lithium reagents in the presence of aldehydes can be achieved if the latter are protected by reaction with Ti(NEt,) at low temperatures (Scheme 7). For differentiation between ketones the more reactive Ti(NMe2) is more suitable ; treatment of cyclohexanone and heptan-4-one mixtures with the titanium compound followed by lithium or Grignard reagents gives (59) in 99% selectivity (Scheme 7). The compound Mn(NEt,) is also a very mild protecting agent.52 0 OTi(NR,) ~ ,k + Ti(NR2) R,"R RR R HO + PrJ,Pr 8 -ii iii 6+ Pr.TPr R (59) Reagents i -78 "C; ii Ti(NR2)4; iii LiR Scheme 7 The cyclic titanium alkyl complex (60) is known to cleave isobutene to give the unstable methylidene complex (61) which can be thought of as an analogue of a Wittig reagent.With acid chlorides however the enolate (62) a non-Wittig product is formed. There is no isomerization of the double bond. Protonation gives ketones in near-quantitative yields even with sterically hindered acid ~hlorides.~~ Cp,Ti 3< -Cp,Ti=CH + A 7 Miscellaneous An improved method for the synthesis of unsymmetrically substituted dibenzofurans (64) involves treatment of o-bromophenyl-phenyl ethers (63) (from substituted fluorobenzene and sodium o-bromophenolate) in N,N-dimethylacetamide with base in the presence of 0.1 equivalent of palladium a~etate.'~ 52 M.T. Reetz B. Wendworth and R. Peter J. Chem. SOC.,Chem. Commun. 1983 406. 53 J. R. Stille and R. H. Grubbs J. Am. Chem. SOC. 1983 105 1664. 54 D. E. Ames and A. Opalko Synthesis 1983 234. Organometallic Chemistry -Part (i) The Transition Elements 285 0 OTiCICp, 'dC1 '& + Cp,Ti=CH2-(63) (64) R = NO, CN H CH20H Me or C02H The activities and selectivities of rhodium acetate palladium acetate and copper trifluoromethanesulphonate as catalysts in the cyclopropanation of large numbers of dienes and trienes with diazoacetic ester were subject to a systematic Rhodium acetate gives the highest yields and works especially well with electron-rich double bonds for example in alkyl-disubstituted 2-olefins.Palladium acetate is less active and cyclopropanates preferentially terminal double bonds where steric hindrance is minimized. Copper is a better catalyst than palladium. The diqerent behaviour patterns are taken as an indication that a carbenoid mechanism operates for rhodium with no co-ordination of the olefin to the metal whereas olefin co- ordination is important with palladium. Rhodium is the best catalyst for the cyclo- propanation of 1,1-dichlorodienes e.g. (65). cl\ c1>-?=("' + Me (65) The transfer of co-ordinated nitrene to an olefin to give aziridines has been achieved with manganese c~mplexes.~~ Photolysis of (5,10,15,20-tetramesitylpor-phyrinato)manganese(rIr) azide gives the very stable Mn" nitride complex (66).Reaction with trifluoroacetic anhydride generates the intermediate (67); the now labilized nitrene is transferred to cyclo-octene in a stoicheiometric reaction to give (68) in near-quantitative yield. Treatment with base or extraction with dilute hydro- chloric acid converts (68) into the parent compound. Tetrahydrofurans react with HSiR and CO in the presence of CO~(CO)~ and under drastic conditions (140 "C/50 atm) to give ring-opened products such as R3SiO(CH2)4CH0 and R3SiO(CH2)4C(OSiR3)=CHOSiR3 depending on the 55 A. J. Anciaux A. Demonceau A. F. Noels R. Warin A. J. Hubert and P. Teyssie Tetrahedron 1983 39 2 169. 56 J. T. Groves and T.Takahashi J. Am. Chem. Soc. 1983 105 2073. M. Bochmann R. A. Head and M. D. Johnson ,COCF THF:HSiR ratio. It has now been found that a similar reductive ring-opening proceeds at room temperature under 1 atm CO to give (69). A variety of substituted tetrahydrofurans react similarly. It has been suggested that R,S~CO(CO)~ and R3SiO(CH2)4Co(C0)4 are intermediates.” HSiMe,Et (3 equiv.) R,SiO CO (1 atm) Co,(CO) OSiR (69) 8 Phase-transfer Catalysis Phase-transfer catalysis is that which is typically effected in an aqueous/organic two-phase system with an anion transfer agent such as a crown ether or a quaternary ammonium salt. The benefits of such reactions include both mild conditions and ease of product isolation.Free radicals have been proposed as intermediates in several transition-metal- catalysed phase-transfer reactions but now their formation is confirmed in the conversion of a-phenylethyl bromide exclusively into 2,3-diphenylbutane (70) under an atmosphere of C0.58 In CH,Cl as solvent both CO,(CO)~ and Pd(dba) (dba = dibenzylideneacetone) afford meso and racemic forms of the product in equal proportions which is satisfactorily explained only by the intermediary of a planar a-phenylethyl radical. CO-Pd(dba),-[PhCH,N(C,H,),]CI PhCH(Me)CPhH(Me) 2PhCH(Me)Br CHZCl2-SN-NaOH,20 h 25 “C ’ (70) A striking example of how a change in organic solvent can alter the course of a reaction is provided when the cobalt-catalysed carbonylation of a-phenylethyl chloride and bromide as described above in CH2Clz solution is carried out in highly polar solvents such as alcohols or ethers.59 Hydratropic acid (71) is isolated in over 90% yield after acidification of the aqueous phase and since it has the opposite absolute configuration to the starting benzyl halide it appears that the reaction proceeds by an SN2mechanism rather than a free-radical pathway.Double carbonylation is unexpectedly achieved using a two-phase system comprising t-amyl 57 T. Murai Y. Hatayama S. Murai and N. Sonoda Organometallics 1983 2 1883. V. Galamb and H. Alper J. Chem. SOC.,Chem. Commun. 1983 88. 59 F. Francalanci A. Gardano L. Abis T. Fiorani and M. Foa J. Organomeial. Chem. 1983. 243 87. Organometallic Chemistry-Part ( i) The Transition Elements Ph Ph Ph Me +02H Me 0 (72) alcohol and 20% aqueous NaOH where >80% a-keto-P-butyric acid (72) is recovered from the aqueous phase via the sodium salt.Carbonylation reactions involving aryl halides are generally considered difficult to perform with metals other than palladium but full details of the use of cobalt catalysts under phase-transfer conditions have now been published.60 In a mixture of benzene and 5M aqueous NaOH with tetrabutylammonium bromide as transfer agent various aryl chlorides and bromides were converted into their corresponding carboxylic acids (73) in high yield under very mild conditions (65 "C 1 atm CO). Interestingly the carbonylation only proceeds when the reaction is irradiated with visible light typically from a commercial 125 W sunlamp.CO (I atm)-Co,(CO),~Bu,N]CI ArBr # ArC02Na C6H,-5N-NaOH 14.5 h 65 "C hv ArBr = PhBr 2-or 4-BrC6H4Me 2-or 4-BrC6H40Me 4-BrC6H,COMe 4-BrC6H4No2 4-BrC&&I or 1-or 2-bromonaphthalene Benzolactams and lactones e.g. (74) are conveniently prepared from aryl halides bearing amino- or hydroxy-functionalized groups on a side chain a-to the halogen. While some details of the mechanism are unclear evidence suggests a SRN1 condensa-tion of [Co(CO),]- with the aryl halide to form ArCo(CO), from which the product is formed by CO insertion followed by nucleophilic attack of hydroxide ion on the aryl carbonyl carbon atom. &m +co+ Qo d (74) (92%) Bromostyrenes are smoothly carbonylated to acids (75) in good yields using mild conditions (50°C 1 atm CO) with either Pd(Ph3P) or Pd(dppe)2 as catalyst.The reactions exhibit high retention of stereochemistry ; thus E-bromostyrene affords only E-cinnamic acid in 9 1 YO yield.6' H CO-W(PPh,),-[PhCH,NEt,]Cl ArM ArHH b C,H,-SN-NaOH 17.5 h 50°C then H,O+ H Br H CO,H (El Ar = Ph p-ClC6H4 3,4-(MeO)2C,H J. J. Brunet C. Sidot and P. Caubere J. Org. Chem. 1983 48 1166. 6' V. Galamb and H. Alper Transition Met. Chem. 1983 8 271. 288 M. Bochmann R. A. Head and M. D. Johnson Vinylic dibromides readily prepared from aromatic aldehydes react under car- bonylation conditions with Pd(d~pe)~ as catalyst to give either substituted buta- 1,3- diynes or carboxylic acids depending upon the organic solvent used (Scheme 8).62 When benzene is used as organic phase and benzyltriethylammonium chloride as transfer agent reasonable isolated yields of the diyne (76) are obtained.However under the same conditions aliphatic vinylic dibromides are converted into their corresponding mono- and di-carboxylic dcids. By replacing benzene as solvent by t-amyl alcohol benzalmalonic acids (77) are obtained in very good yield (87-93'/0). ArCH=C(CO,H) &!-ArCH=CBr + CO -ArCrC-CECAr (77) (87-93%) (76) (2O-68%) Ar = Ph p-MeC,H, or p-MeOC,H Reagents i Pd(dppe)2-[PhCH2NEt,]cl-c6H6-5N-NaoH, 50-70 "C; ii Pd(dppe),-[PhCH2NEt3]CI-t-amyl alcohol-5N-NaOH 50-70 "C Scheme 8 Palladium-catalysed conjugate addition of 0-hydroxyarylmercury(r1) compounds to a,p-enones gives convenient starting materials from which to prepare 2-chromanols and 2-chr0menes.~~ The acid conditions employed for the addition have precluded the use of amino-functionalized arylmercurials but it has now been found that by using suitable protecting groups a wide range of 4-phenylbutan-2-ones (78) can be prepared under phase-transfer conditions (Scheme 9).64The 4-phenylbutan-2- ones (78) themselves are valuable synthetic intermediates from which to prepare dihydroquinolinium salts (79) quinolines (80) or 1,2,3,4-tetrahydroquinolines(8 1).Highly desirable reactions of C-H bonds in unactivated alkanes are now available using the very simple two-phase system comprising an organic solvent such as CH2C12 containing alkane and Mn(TPP)X as both catalyst and transfer agent together with a saturated aqueous solution of NaX (X = NO2 N, C1 Br or I; Scheme In the presence of an oxidizing agent such as iodosylbenzene only one of the alkane hydrogen atoms is replaced by X in what appears to be a free-radical process.Hydrocarbons shown to undergo the reaction include cyclohexane isobutane 2,3-dimethylbutane and t-butylbenzene where in all cases iodosylben- zene is the preferred oxidant. Although there are very few known homogeneous catalysts for the hydrogenation of arenes it has now been found that the simple ion-pair [(C,H,,),NMe]+ [RhC14]- is exceptionally efficient under very mild phase-transfer conditions (Scheme 1 1).66 Deuterium labelling experiments indicate that the water hydrogen atoms are not present in the saturated product.However the presence of water is essential as the catalyst is totally inactive in dry CH2C12. Partial hydrogenation also takes place with arenes and acetylenes ; for instance naphthalene affords tetralin (99'/0) and diphenylacetylene gives only cis-and trans-stilbene (ratio 78 :22). 62 V. Galamb M. Gopal and H. Alper Organometallics 1983 2 801. 63 S. Cacchi D. Misiti and G. Palmieri J. Org. Chern. 1982 47 2995. 64 S. Cacchi and G. Palmieri. Tetrahedron 1983 39 3373. 65 C. L. Hill J. A. Smegal and T. J. Henly J. Org. Chern. 1983 48 3277. 66 J. Blum I. Amer A. Zoran and Y. Sasson Tetrahedron Lett. 1983 24 4139. Organometallic Chemistry -Part (i) The Transition Elements HgCl li eR2 NHZ (78) (8693%) (79) (80) (81) For R' = H R2 = Me; Z = CO,CH,Ph; X = Me C1 HgCI or MeCO R' = H R2 = Et; Z = CO,CH,Ph;X = Me R' = Ph R2 = Me; Z = CO,CH,Ph;X = C1 I = C0,Et; X = Me or CI Z = COCF,;X = C1 R' = R2 = Ph; Z = CO2CH,Ph;X = Me Reagents i PdCI2-[Bu4N]CI-CH2C1,-3N-.HC1 25 "C 5-9 h; ii 37% HBr-AcOH or 1N-NaOH-EtOH then HBr(aq); iii HBr-MeNO, 40°C; iv Zn-37% HBr-AcOH 25 "C Scheme 9 55% 21 '/o (X = N3) Reagent i Mn(TPP)X-PhIO-CH,C12-sat.NaX(aq) 25 "C 12 h Scheme 10 X X 27-1 00% X = H Me Et F OH NMe, COMe or C02Me Reagent i RhCl3.3H,O-[(C,H ,,),NMe]CI-CH2C12-H20-H2 (I atrn) 30 "C 5 h Scheme 11 290 M. Bochmann R. A. Head and M. D. Johnson 9 Heterocyclic Chemistry Cyclic nitrones have proved to be valuable intermediates in the synthesis of many biologically active nitrogen heterocycles although they are invariably prepared by stoicheiometric reactions.The catalytic oxidation of N N-disubstituted hydroxyl- amines has been achieved using either palladium black or IUICI(PP~~)~ where the nitrone (82) is isolated in very high yield.67 Pd-H,O 80°C <'> <:> +H2 I I OH 0-(82) (57-80%) R = (CH2)2-,(CH,), or o-C,H,CH2 When the reaction is carried out in the presence of alkenes a 1,3-dipolar addition takes place isoxazoles (83) being obtained in excellent yield. The reaction proceeds with high regioselectivity ; thus with ethyl crotonate and N-hydroxypiperidine 2-methyl-3-ethoxycarbonylhexahydropyridinoisoxazole (83) ; R' = CO,Et R2 = Me) is isolated in 85% yield in entirely the trans,trans-stereochemistry (established by n.m.r.).The nature of the substituent on the alkene determines its ultimate position in the isoxazole. Electron-withdrawing groups give 3-substitution whereas electron- donating groups are found to give 2-substituted products. RZ (83) R' = C02Et R2 = H or Me R' = H R2 = Ph or OBu" Imidazoles substituted in 2,5-positions (84) are prepared under mild conditions from a-halogeno-oximes and amidines using Fe3(CO) as deoxygenating agent.68 Several iron catalysts were examined and the trinuclear cluster was found con- sistently to give good yields of imidazoles which appear to be formed by deoxygena- tion of intermediate oxadiazines (85). NH II R'-C-CH,X + R~-c-N-11 I NOH Me R' = Ph or Me X = C1; R2 = Ph R' = Ph p-MeC,H, p-BrC,H, or EtOCO; X = Br; R2 = Ph R' = Ph; X = Br; R2 = rn-MeC,H 67 S.Murahashi H. Mitsui T. Watanabe and S. Zenki Tetrahedron Lerr. 1983 24 1049. 6U S. Nakarishi. i. Nantaku and Y. Otsuji Chem. Lett.. 1983 341. Organometallic Chemistry -Part (i) The Transition Elements 29 1 Oxidation of butan- 1,4-diols with a combination of molecular bromine and nickel(I1) alkanoate affords the corresponding 8-butyrolactone (86) with a high degree of regio~electivity.~~ The solvent employed for the reaction has a remarkable influence on oxidation selectivity. Using trimethylacetonitrile or CH2C12 a ratio (86a):(86b) of 2 1 is observed but with acetonitrile-CH2C1 (20/80) of DMF the selectivity increases to 4 1 (R'= R2= Me).R' R' M. P. Doyle R.L.Dow,V. Bagheri and W. P. Patrie J. Org. Chern. 1983 48 476.