8 Aliphatic Compounds Part (i) Hydrocarbons By D. F. EWlNG Department of Chemistry The University of Hull Hull HU6 7RX 1 Alkanes Although the reduction of primary and secondary alkyl halides to the corresponding hydrocarbon can be readily achieved by a variety of metal hydrides the analogous reduction of tertiary compounds has often proved troublesome. The lithium ate- complex (l),derived from 9-butyl-9-borabicyclo[3.3.llnonane and butyl-lithium is capable of selective removal of the halogen atom from tertiary alkyl benzyl and allylic halides in excellent yields (60-100%).' There is a very marked reduction in the reactivity of (1)towards secondary and primary alkyl halides and this high selectivity when combined with the mild reaction conditions for tertiary systems (3 hours at 20 "C) makes (1)a reagent of some utility.Benzylic and allylic halides are also easily reduced by (l),in keeping with a mechanism involving formation of a carbo-cation. (Alky1azo)diphenylmethanols (2) are a good source of alkyl radicals and hence can be used to hydroalkylate alkenes such as norbornene acrylonitrile and crotonaldehyde.* The regiochemistry is consistent with initial attack by alkyl radical (primary or secondary) e.g. H,C=CHCN gives RCHzCH2CN (R = Me Et or Pri) but the yields are not high enough (35% for hydroethylation) for this to be a useful method of generating alkanes except in special cases. Another intriguing report3 concerns the reaction of alcohols with iodine and hydrogen at 7MPa and 300"C. Under these conditions the corresponding iodide is formed initially and subsequent hydrogenolysis gives the corresponding alkane or a rearranged product.For example n-butanol gives n-butane (%YO) isobutane (20%) and propane H. Toi Y. Yamamoto A. Sonoda and S.-I. Murahashi Tetrahedron 1981,37 2261. * D. W. K. Yeung and J. Warkentin Can. J. Chem. 1980 58,2386. W. W. Paudler and T. E. Walton I. Org. Chem. 1981,46,4306. 147 148 D. F. Ewing (15%). It is noteworthy that this reduction by molecular hydrogen does not require a noble-metal catalyst and further exploration of this reaction may lead to inexpensive procedures that are of synthetic value. 2 Alkenes Synthesis.-From Alkynes. Specifically modified palladium catalysts have been developed4 which permit rapid stereosFlective transfer reduction of alkynes to alkenes.The best hydrogen donor is sodium phosphinate which is insoluble in organic solvents; hence benzyltriethylammonium chloride was used as a phase- transfer catalyst. Palladium metal is too reactive but suitable poisoning of Pd on charcoal could be achieved by precipitation of mercury or lead (by the reduction of an aqueous solution of an appropriate salt with NaBH,). Catalysts prepared in this fashion can be stored indefinitely. Yields of 70-97% have been achieved for the reduction of alkynes such as R'C=CR2(R' = H R2 = Ph cyclohexenyl or 4-N02C6H4COOCH,; R' = R2 = Ph; R' = Me R2 = Ph). A nitro-group is reduced to an amino-group but other functional groups are unaffected.The addition of vinyl cuprates to @-unsaturated sulphones offers promise as a novel stereoselective route to 2-olefins.' Vinyl cuprates are generated from alkyl cuprates and acetylene in the usual way (Scheme 1)and the addition to the alkenyl sulphone proceeds without loss of the 2 stereochemistry of the vinyl group. Mild reductive desulphonation affords the 2-alkene (3). The coupling of vinyl cuprates with aryl iodides has also been achieved6 efficiently using a palladium complex as catalyst (Scheme 1).Both phenyl- and thienyl-ethenes can be prepared in 65-80% yield. (R\_j];uLi + R' \=/CR2R3CH,S0 Ph RiCuLi + 2HCzzCH --) R'9Y iii 1 X Reagents i PhS0,CH=CR2R3; ii H'; iii 6% Na-Hg at 25 OC; iv ZnBr, THF 5% [Pd(PPh,),] IPhX Scheme 1 Carbometallation reactions of propargyl and homopropargyl derivatives usually display the opposite regioselectivity to that found for simple alkynes or they are non-selective.In contrast the recently developed zirconium-catalysed carboalumi- nation of alkynes shows' an unexpected regioselectivity with propargyl and homopropargyl derivatives (4; n = 1 or 2) that contain OH OSiMe2But,SPh or R. A. W. Johnstone and A. H. Wilby Tetrahedron 1981,37 3667. ' G. De Chirico V. Fiandanese G. Marchese F. Naso and 0.Sciacovelli J. Chem. SOC.,Chem. Commun. 1981,523. N. Jabri A.Alexakis and J. F. Normant TetrahedronLett. 1981 22 3851. C. L.Rand D. E. Van Horn M. W. Moore and E. Negishi J. Org. Chem. 1981 46,4093. Aliphatic Compounds -Part (i) Hydrocarbons iodo groups.The regioselectivity is very high (over 98% in many cases) and in all cases it results in alkylation at the substituted carbon (Scheme 2). The intermediate alane can be readily converted into a wide variety of synthons by established methods with overall yields in the range 60-90%. A x(cH2)nt=(" X(CH,),C CH Me AIMe -!L x(cH2)n>_(H z Me (4) X = OH OSiMe2Bu',SPh or I n = lor2 Reagents i Me,Al Cl,ZrCp, at 20 "C;ii H20 (Z = H) or I (Z = I) Scheme 2 Synthesisof AikenesfromCarbonyl Compounds. Olefination of carbonyl compounds by using stabilized ylides is not always fully stereospecific and the separation of the isomeric olefins may be difficult. Using Ph2P0 as the stabilizing group has the advantage that the reaction with aldehydes stops at the alcohol stage.' Isolation and purification of the alcohol permits subsequent conversion into a pure Z-alkene.The reaction of the ylide with an ester affords the P-keto-phosphinate which is readily reduced (by NaBHJ to the other diastereoisomeric alcohol; this in turn affords the pure E-olefin.* Aromatic aldehydes are not very reactive toward phos- phonate-stabilized anions and the high reaction temperatures that are necessary result in indifferent yields. However addition of a catalytic amount of a crown ether so greatly enhances the nucleophilicity of the carbanion that stilbenes can be obtained in nearly quantitative yields corresponding to a rate increase of several hundred-fold.g A 'one-pot' conversion of esters or lactones (e.g. R'C02Me) into the corresponding olefin (R'CH=CR2R3) has been described in some studies of natural products." The carbonyl species is reduced to the hemiacetal with Bui2A1H at -65 "Cbefore its reaction with a conventional Wittig reagent.An alternative to the Wittig procedure for the formation of substituted vinyl bromides involves the reaction of carbonyl compounds with dibromomethyl- lithium," followed by reductive elimination of HOBr. Yields are good (70-95%) but stereocontrol is poor with only a slight preference for the E-form. (Trimethyl- silylacety1)trimethylsilane (5) contains both a-and 0-ketosilane structures and is the starting point for a novel synthetic route to functionalized trisubstituted olefins (Scheme 3).'* Groups R' and R2 are introduced via successive alkylation of the lithium enolate with an alkyl halide (R' = Me Et allyl or benzyl) and an aldehyde (R2 = alkyl alkenyl or alkynyl).Step v proceeds spontaneously to afford essentially pure E-olefin with overall yields of 72-92% Synthesis of Alkenes by Elimination. There has been much interest in recent years in the formation of olefins by the oxidative syn-elimination of PhSeOH from alkyl A. D. Buss and S. Warren J. Chem. SOC.,Chem. Commun. 1981 100. R. Baker and R. J. Sims Synthesis 1981 117. lo W. Boland P. Ney and L. Jaenicke Synthesis 1980 1015. l1 D. R. Williams K. Nishitani W. Bennett and S. Y. Sit Tetrahedron Lett. 1981 22 3745. l2 J. A. Miller and G. Zweifel J. Am. Chem. SOC.,1981 103 6217. 150 D. F. Ewing Me3Si OLi 1 \/ Me3SiCH2C//O -b (5) \ SiMe3 H/c=c \SiMe3 Reagents i Lithium di-isopropylamide (LDA) THF at -78 "C; ii alkyl halide R'X at -25 "C in THF; iii LDA at 0-5 "C; iv R'CHO at -78 "C; v at -78 'C; vi H,O, OH-Scheme 3 phenyl selenides and this reaction is rep~rted'~ to afford nitroalkenes in 55-85% yield (Scheme 4).The nitro-selenides (6) are somewhat unstable and are oxidized without isolation. R'CH2CHR2I SePhIR'CH2CR2 A R'CH2CR2 -%R'CH=CR2 II I I NO2 NO; NO2 NO2 Reagents i Bu"Li THF at 0 "C; ii PhSeBr; iii 35% H,O Scheme 4 A new reagent for the 'reductive' elimination of -hydroxylated phenylselenides phenylsulphides and iodides is Me3SiC1 and NaI in acetonitrile.'* The exact mechanism is unknown but yields of 70-90% were achieved without loss of stereochemistry.In cases where low yields are found with phenyl selenoxide eliminations the analogous pyridylselenium derivative may be more effective." The PySe moiety is a better leaving group but can be readily introduced by nucleophilic substitution with sodium pyridine selenate. An efficient method of generating double-bonds by the elimination of telluroxide has not been found to date. This difficulty can be circumvented'6 by treatment of the telluride with chloramine-T in refluxing THF to afford olefins in 66-93% yield (for C8 to C13 terminal olefins). Presumably the intermediate tellurium(v) species RCH2CH2Te(Ph)=NTosyl is formed and then eliminated as the N-tosyltel- l3 T. Sakakibara I. Takai E. Ohara and R. Sudoh J. Chem. Soc.Chem. Commun. 1981,261. '' D. L. J. Clive and V. N. KBB,J. Org. Chem. 1981 46 231. '' A. Toshimitsu H. Owada S. Uemura and M. Okano Tetrahedron Lett. 1980 21 5037. l6 T. Otsubo F. Ogura H. Yarnaguchi H. Higuchi Y. Sakata and S.Misumi Chem. Lett. 1981,447. Aliphatic Compounds -Part (i) Hydrocarbons luramide. Anodic oxidation of carboxylic acids that have a p -trimethylsilyl group gives exclusively the terminal olefin (7) (Scheme 5).17 These acids are conveniently prepared by a malonic acid synthesis with Me,SiCH2Cl and RC1. A 'one-pot' conversion of a-chlorocarbonyl compounds R'R2CClCOX (X = H alkyl C1 or alkoxy) into the alkenes R'R2C=CR3 by reaction with a Grignard reagent has been described.18 RCHCH2SiMe3 aRCH=CH2 I C02Na Reagents i RCH,CH,NH,; ii triphenylpyridine at 150"C Scheme 5 A milder alternative to the Hofmann elimination of quaternary amines is shown in Scheme 5.The pyrylium salt (8)reacts readily" with primary amines to give the corresponding pyridinium species which undergoes elimination at 150"C in the presence of a non-nucleophilic base. Yields are good (80-97% for C5to C12alkenes) but the product is not pure terminal alkene some 25% being the isomeric 2-enes. Synthesis of Alkenes by Alkylation of Vinyl Compounds. A drawback to the use of organoboranes for the alkylation of a lithium vinyl derivative is the fact that only one of the alkyl groups of the borane is utilized. The 'throwaway' group i.e. 9-borabicyclo[3.3. llnonane which was developed earlier is readily alkylated on boron and it shows no tendency to compete with the alkyl group in transfer to a vinyl compound in a normal coupling reaction.20 Yields are in the range 40-80%.Alkenyl-zirconium complexes have been usedz1 to alkenylate steroids at C-18 or at C-20. The general synthetic utility of this reaction has not been explored but since it invokes attack at palladium in a r-ally1 complex it may have wider application. Reactions of A1kenes.-A number of interesting addition reactions of alkenes have been studied (Scheme 6). Methanethiolsulphinate MeS(O)SMe reacts with trifluoracetic anhydride at -10 "C in carbon tetrachloride to afford an unstable sulphenyl ester (reagent i) which adds to simple olefins in a regiospecific (Markownikoff) and stereospecific (trans) manner [Scheme 6; reaction (a)]." l7 T.Shond H. Ohmizu and N. Kise Chem. Lett. 1980 1517. J. Barluenga M. Yus J. M. Concellon and P. Bernad J. Org. Chem. 1981,46,2721. l9 A. R. Katritzky and A. M. El-Mowafy J. Chem. SOC.,Chem. Commun. 1981 96. 2o A.B.Levy R. Angelastro and E. R. Marinelli Synthesis 1980 945. 21 J. S.Temple and J. Schwartz J. Am. Chem. SOC.,1980,102 7381. 22 T.Morishita N. Furukawa and S. Oae Tetrahedron 1981,37 2539. 152 D. F. Ewing Products from a variety of sulphenyl trifluoroacetates and olefins were investigated yields being in the range 60-90%. An interesting route to vinyl sulphones (9) is presented in Scheme 6 [reaction (b)]. Addition of selenosulphonates (PhSeS0,Ar) to olefins in the presence of BF etherate proceeds with trans-stereochemistry and in most cases with exclusively Markownikoff ~rientation;'~ selenoxide elimination gives the sulphone in good yield.If the reactants are heated in the absence of a Lewis base free-radical addition occurs to give the anti-Markownikoff product. Two methods of introducing an amino-group into a double-bond are shown in reactions (c) and (d) in Scheme6. The amidation reactionz4 is an example of nucleophilic addition by the widely applied mercuriation-demercuriation pro-cedure. Simultaneous introduction of bromo- and amino-groups into an alkyl residue is conveniently achieved" by the reaction of an alkene with NN-dibromophosphoramidic acid followed by reduction of the N-brominated intermediate and hydrolysis to the amine salt (10).The regio-control is anti-Markownikoff ; this is consistent with a radical-chain mechanism for this reaction which offers a convenient route to aziridines. OCOCF3 IR'CH-CH~ SO2Ph R' AH-CHZSePh -% PhS02CR1=CH2 (9) R'CH-CH2HgN03INHCOR' INHCOR~ h R'CH-CH3 R'CH=CH2 (c) R'CH-CH,NHP(OEt),I1 0 IBr % R'CH-CHzkH3 I Br (10) C1- R'CH2-CH2BR22 -% R'CH2CH2Br Reagents i MeSOCOCF,; ii PhSeSO'Ar BF * Et20;iii 3-C1C,H4COOOH; iv R'CONH, Hg(NO,),; v NaBH,; vi NN-dibromophosphoramidicacid NaHSO,; vii HCl in benzene; viii R2 BH; ix Br Scheme 6 A facile conversion of terminal alkenes into the corresponding. primary alkyl bromide is possible via an organoborane reaction (e) as shown in Scheme6.26 This procedure is ideal for functionally substituted alkyl bromides for which substitu- tion reactions would be ruled out.The oxidation of an olefin to an a-glycol or an a-ketol by permanganate is a very old and much studied reaction but the precise relationship between the two mechanistic pathways has remained obscure. It has now been established that for reactions starting with simple alkenes the cyclic hypomanganate ester (11) (see Scheme 7) can be hydrolysed to the glycol at pH 9 but in neutral media this Mn" 23 T. G. Back and S. Collins J. Org. Chem. 1981,46 3249. 24 J. Barluenga C. JimCnez C. Nhjera and M. Yus J. Chem. SOC.,Chem. Commun. 1981,670. 2s S. Zawadzki and A. Zwierzak Tetrahedron 1981,37,2675. 26 G.W. Kabalka K. A. R. Sastry H. C. Hsu and M. D. Hylarides J. Org. Chem. 1981 46 3113.Aliphatic Compounds -Part (i) Hydrocarbons I I CHOH CHOH CHOH Scheme 7 ester is rapidly transformed into the MnV1 ester (12) by oxidation with excess permanganate. Careful studies with deuteriated ethene indicated that (12) is conver- ted into a Mn'" ester of the ketol by oxidative decomposition; the free ketol is liberated by subsequent hydroly~is.~' These studies will assist in the selection of conditions for optimum discrimination between the two types of products in the case of more complex substituted alkenes. A very efficient catalytic procedure has been described2* for the oxidation of terminal olefins to methyl ketones. The addition of hydrogen peroxide to the solution of the olefin in Bu'OH in the presence of a catalytic amount of palladium acetate results in 90% conversion into ketones with a 75-80% selectivity for the 2-keto-derivative.There appears to be negligible oxidation at C-1 and only minor oxidation at other sites. A further very extensive study has appeared29 of the Pdo-catalysed reaction of vinyl bromides with cup-unsaturated compounds in the presence of an amine (Scheme 8). The active catalyst was not characterized but is likely to be a bisphos- phine palladium(0) complex which reacts initially with the vinyl halide. In the presence of a secondary amine acrolein or methacrolein acetal reacts with the palladium vinyl species to give [reaction (a)] either a diene acetal (13) or the amino-acetal (14) the relative proportions varying in a complex way with the structure of the alkene.With cup-unsaturated acids (or esters amides or nitriles) the dienoic acid is formed in good yield [reaction (b)]. This procedure obviously has wide synthetic scope for conversion of alkenes into functionalized dienes although the rate of reaction and the yields of products depend upon the nature of the substituents on the alkene (H or alk~l).~~ The reaction of the complex [Zr(Cp),(isoprene)] with alk-1 -enes and alk-2-enes results exclusively in the coupling of C-4 of the isoprene moiety with C-2 of the alkene.30 This regiospecificity is 98-99% with respect to both alkene and diene. In the analogous reaction with alkynes the high specificity for C-4 of isoprene is maintained but for instance in pent-2-yne coupling occurs equally readily at C-2 and C-3.Aryl-acetylenes exhibit different behaviour (vide infru). '' S. Wolfe C. F. Ingold and R. U. Lemieux J. Am. Chem. Soc. 1981,103,938. 28 M. Roussel and H. Minoun J. Org. Chem. 1980,45,5387. *' B. A.Patel J.-I. I. Kim D. B. Bender L.-C. Kao and R. F. Heck J. Org. Chem. 1981 46 1061; J.-I. I. Kim B. A. Patel and R. F. Heck ibid. p. 1067. 30 H. Yasuda Y. Kajihara K. Nagasuna K. Mashima and A. Nakamura Chem. Letr. 1981 719. 154 D. F.Ewing R' + R:NH -& R,< R2 Br CH(OMe)2 CH(OMe)2 (13) + R;NCR1 R2CR3=CHCHzCH(OMe)2 (14) CO H X = COZH COZMe CONHz or CN Reagents i Pd(OAc), tri-o-tolylphosphine Scheme 8 The catalyst WC1,/SnMe4/EtOAc has been reported31 to be effective in the metathesis of a-olefins to long-chain internal olefins (Scheme 9).Although both geometric isomers are formed and the yields are somewhat variable (30-50% of pure trans-isomer was isolated) the product is largely free from contamination by the next lower homologue. Hydrozirconation with [(Cp)ZrHCl] {generated in situ from [(Cp),ZrClz]) results in moving the functionality to the end of the chain and appropriate treatment of the alkyl-zirconium species can lead to halides alcohols aldehydes or carboxylic acids. Alkyl iodides that contain up to 42 carbon atoms may be prepared31 by this method. Isomerization of olefins using CnHZn+l CH=CHz A CnHZn+l CEI=CHCnHzn+1 )i CZnH4n+l CHZCHZZr(Cp)ZCl w CZnH4n+1CHZCHZI CZ~H~~+ICHZCH~OH Reagents i WCl, SnMe, EtOAc; ii [(Cp),ZrCl,] NaAIH,(OCH,CH,OMe),; iii I,; iv 0 Scheme 9 [RUC~~(PP~~)~] as a homogeneous catalyst has been known for some time.This catalyst has now been incorporated into a polystyryldiphenylphosphineresin and the resulting supported heterogeneous catalyst can be used with very little loss of catalytic activity for hundreds of cycles of the isomerization of 3-phenylpropene to cis-and trans- 1-phenylpr~pene.~' Similar properties are observed for analogous supported iridium and rhodium catalysts. The conversion of alkenes into cyclopropanes continues to be of widespread interest and several new synthetic methods have been described. Dibromomalonic ester condenses with olefins in the presence of copper powder providing a novel '' T. Gibson and L.Tulich J. Org. Chem. 1981,46 1821. 32 A. Zoran Y. Sasson and J. Blum J. Org. Chem. 1981 46 255. Aliphatic Compounds -Part (i) Hydrocarbons 155 route to derivatives of gem -dialkoxycarbonylcyclopropane.33 The reaction is appli- cable to a wide range of olefins and proceeds smoothly at moderate temperature to give fairly good yields particularly in polar solvents. It is unlikely that free carbene [i.e.:C(CO,Et),] is involved since no evidence of C-H insertion was found. Radical attack by CBr(CO,Et) followed by elimination of Br is consistent with the complete lack of stereospecificityin the reaction with P-methylstyrenes. In a later the same workers have investigated the use of cuprous bromide as an alternative to copper and have extended the reaction to the use of dibromomalononitrile.Styrenes lead to higher yields of cyclopropane derivatives than do alkenes but the same lack of stereospecificity is ~bserved.'~ The use of di-iodomethane for the photocyclopropanization of olefins has been studied in detail.35 It has been found to be a synthetically useful procedure for about twenty alkenes and cycloalkenes and shows significantly less sensitivity to steric effects than does the traditional Simmons-Smith method. This is evident in the increasing relative rate for increased substitution about the double-bond. Although this is a photochemical reaction the methylene-transfer species is thought to be the a-iodo-cation CH21+. N,=CHCO,Et (15) Diazoacetates (15) have been used with rhodium and copper catalysts to effect the cyclopropanization of olefins presumably via a metal-carbene complex.However the synthetic utility of this reaction is limited by extensive competition from attack of the carbene on the diazo-compound and it has been usual to have the olefin reactant in large excess. It has now been that careful control of the rate of addition of the diazo reactant (i.e.low rates for low catalyst concentration and a reduction in the rate as the reaction proceeds) leads to yields of product as high as 90%. Of the two rhodium catalysts R~,(OAC)~ and Rh,(CO),, the acetate is the more effective although there is little difference in the control of the stereochemistry of the product. Both cationic and neutral ferrate complexes have been utilized in the alkylidation of alkenes (Scheme 10).The highly electrophilic carbene complex (16)reacts rapidly Alkene + [Cp(CO)#eCHPh]PF6 (16) Alkene + Cp(CO)2FeCHSPh Me Reagents i at -78 "C;ii FSO,Me at 25 "C Scheme 10 33 N.Kawabata and M. Tanimoto Tetrahedron 1980 36 3517. 34 N. Kawabata S. Yano J. Hashimoto and J. Yoshida Bull. Chem. SOC.Jpn. 1981,54,2539. 35 P. J. Kropp N. J. Pienta J. A. Sawyer and R. P. Polniaszek Tetrahedron 1981,37 3229. 36 M. P. Doyle D. Van Leusen and W. H. Tamblyn Synthesis 1981 787. 156 D. F. Ewing with unactivated alkenes at -78 "C[reaction (a)] to effect efficient transfer of the benzylidene ligand,37 giving phenylcyclopropanes (18;X = Ph). This reaction has two notable features. It is highly stereoselective in favour of the less stable isomer and is almost insensitive to steric effects with tetrasubstituted alkenes.In the presence of FSO,Me the sulphide complex (17) is converted into the analogous methylsulphonlum species which reacts with alkenes [reaction (b)] to afford methyl- cyclopropanes (18; X = Me).,' The active species is probably the ethylidene com- plex cation analogous to that in (16) but no evidence of the corresponding ethene complex was found. Ene-complexes can be formed from ylidene-complexes by proton shift.38 This may prove to be a general route for the alkylidation of alkenes. 3 Dienes Synthesis.-Coupling of alkenyl cuprates with alkenyl halides is not usually very successful because even with the available catalysts the required reaction tem- perature is high enough to result in serious decomposition of the cuprate to give the symmetrical diene (19) (see Scheme 11).It has now been discovered39 that -& R'>-\ + "i= R2 ZnBr R2 CuLiBr (21) ii 1 1 R' R' RZ)y2 RZQ' R' (19) Reagents i ZnBr,; ii Pd 4PPh3 R3R4C=CHI Scheme 11 the addition of one equivalent of zinc bromide raises the yield of the required diene (20) to over 90%. Furthermore the stereochemical purity is extremely high [typi- cally 99% of (20; R' and R3 are trans) and 1% of (20; R' and R3 are cis)]. These interesting results are due to the formation of the relatively stable alkenylzinc species (21).In cross-coupling reactions with alkenyl bromides an alternative to alkenyl cuprates is an alk-1-enylborane (22) (see Scheme 12) and it has now been shown4' that 2-alkenylboranes (22; R' = H R2= alkyl or siamyl) give rise to 2,E-or 2,Z-dienes of very high isomeric purity.This reaction complements earlier work with E-alkenylboranes which provide a route to E,E- and E,Z-dienes. Although yields are not always very high all four isomeric dienes can be obtained via organoboranes. 37 M. Brookhart M. B. Humphrey H. J. Kratzer and G. 0.Nelson J. Am.Chem. SOC.,1980,102,7802. 38 K. A. M. Kremer P. Helquist and R. C. Kerber J. Am. Chem. Soc. 1981,103 1862. 39 N. Jabri A. Alexakis and J. F. Normant Tetrahedron Lett. 1981 22 959. 40 N. Miyaura H. Suginome and A. Suzuki Tetrahedron Lett. 1981 22 127. Aliphatic Compounds -Part (i) Hydrocarbons R' R4 R' R2 BX R3 RS R2 (22) R3 RS Reagents i Pd(PPh,), 2EtONa Scheme 12 A convenient route to 2-alkyl-1,3-dienes involves the reaction of the correspond- ing dienyl Grignard reagent with an alkenyl iodide in the presence of transition-metal catalysts.Thus direct alkylation is effected at -30 "Cin THF in the presence of cuprous iodide and arylation occurs in the presence of [Pd(PPh3)4].41 Grignard alkylation (Scheme 13) has also been used as a route to allenes (23; R4 = H).42 R' R2*R4 CI \ R' R2 *R4 R3 (24) Reagents i PdC12 NeSO,(neophyl); ii dimethylglyoxime MeOH Scheme 13 Both propargylic and allenic halides are alkylated at the terminal carbon the reaction proceeding via the corresponding allenic palladium complex.The alkylated propargyl compound (24) is not formed to any significant extent. Similar preference for allene formation is shown in the vinylation of propargyl tosylates (25)(see Scheme 13) with a vinyl c~prate.~~ The vinyl group is introduced at the opposite end of the incipient allenic species to give (23; R4 = vinyl R' = R2= H). The synthesis of terminal conjugated dienes via a Pdo .rr-allylic complex has been extended to the formation of analogous p01yenes.~~ For example (2E,4E)-hexa-2,4- diene acetate was refluxed with palladium acetate in the presence of excess triphenylphosphine to give an 87% yield of hexa-1,3,5-triene which was at least 97% the E-isomer. (3E,5E)-Octa-1,3,5,7-tetraene was obtained in 48% yield in 41 S.Nunomoto Y. Kawakami and Y. Yamashita Bull. Chem. SOC. Jpn. 1981 54 2831. 42 T. Jeffery-Luong and G. Linstrumelle Tetrahedron Lett. 1980 21 5019. 43 R. Baudouy and J. GorC J. Chem. Res. 1981 (S),278; (M),3081. 44 K. Yamamoto S. Suzuki and J. Tsuji Bull Chem. SOC.Jpn. 1981,54 2541. 158 D. F. Ewing a similar fashion. Whilst the formal 1,w-elimination of acetic acid obviously occurs stereoselectively in these cases 1,2-elirnination from 1-vinylbut-3-enyl acetate gave an E/Z mixture of hexatriene~.~~ Reagents i T13+ MeOH Scheme 14 Reactions of Dienes.-The reaction of conjugated dienes such as buta- 1,3-diene 2,3-dimethylbuta-1,3-diene,and cyclohexa-l,3-diene with thallium(II1) acetate in acetic acid at 10-65 OC for 0.5 to 15 hours gives a mixture of the corresponding 1,2- and 1,4-diacetoxy-alkenes in 10-92Oh yield.In most cases 1,2-addition predominate^.^^ This reaction is assumed to proceed by initial acetoxythallation followed by sequential or synchronous dethallation and attack by an acetoxy-group. In contrast to the above reaction the major products in the oxidation of conjugated dienes by thallium(II1) nitrate are those resulting from vinyl migration (Scheme 14). For example 2,3-dimethylbutadiene is converted (in 35% yield) into 4-methylpen- tenone (26).46 With more complex dienes a variety of products are formid. Migration of the vinyl group probably proceeds via a cyclopropylium cation. In another study of the oxidation of butadiene it has been shown4' that tellurium oxide in acetic acid in the presence of a 3- to 5-fold excess of LiBr gives the 1,4-diacetoxy-derivative almost exclusively.This very high selectivity for 1,4- addition is unusual; other halide salts (e.g.NaCl and LiCl) were less selective. The formation of a six-membered ring via concerted cycloaddition reactions of butadiene is a well-established procedure. The generation of a cyclopentene ring is more unusual and often shows poor lack of stereo- and regio-control since it requires addition of a carbene followed by thermal rearrangement (cf. Scheme 15). It has now been dem~nstrated~~ that very high stereoselectivity is possible by using an alkoxy-carbene largely because the conditions required for the concerted [1,3] I R4 OCH2CH,CI Reagents i CHOCH,CI; ii Bu"Li (excess) THF at 50 "C,for 1-2 hours Scheme 15 45 S.Uemura H. Miyoshi A. Tabata and M. Okano Tetrahedron 1981 37 291. 46 M. Murakami and S. Nishida Chem. Lett. 1981,997. 4' S. Uemura S. Fukuzawa and M. Okano Tetrahedron Lett. 1981,22 5331. 48 R. L. Danheiser C. Martinez-Davila R. J. Auchus and J. T. Kadonaga J. Am. Chem. SOC. 1981 103,2443. Aliphatic Compounds -Part (i) Hydrocarbons sigmatropic shift are significantly milder than is usually the case (Scheme 15). Stereochemically cyclopentenols are accessible by this procedure in 45-80% overall yield. Another cycloaddition which has received attention is the reaction of dienes with aromatic sulphonyl azides although the intermediate cyclic addition products were not isolated.49 With non-conjugated dienes (e.g.hexa-1,5-diene) a sulphonimide is the initial product hydrolysis giving the corresponding unsaturated ketone (e.g. hex-5-en-2-one). More complex results are obtained with conjugated dienes since the initial product an enamine is hydrolysed by pathways correspond- ing to migration of both hydrogen and a vinyl group. Reactions of 7r-allylpalladium species that are derived from conjugated dienes [e.g. (27); see Scheme 161 generally lead to E-olefins as might be expected from thermodynamic considerations. However if the complex is treated with dimethyl- glyoxime in methanol/pyridine inversion of configuration around the allylic moiety occurs leading to 2-olefins of high stereochemical purity. This interesting conver- sion has been employed5’ to convert the complexes from anti-Markownikoff sulphonylpalladiation of dienes into 2-olefins as shown in Scheme 16.Both regio-isomers are formed and the stereochemistry of the products (27) and (28) is independent of the starting diene (cis-trans mixtures were usually used). R2 R3 RZ R3 R’ S0,Ne PdCI, (Ne = neophyl) Reagents i PdCI, NeSO,(neophyl); ii dimethylglyoxime MeOH Scheme 16 A two-volume work on ketenes allenes and related compounds has appeared” in the ‘Chemistry of Functional Groups’ series edited by Patai. It covers the fifteen years since the first volume on alkene chemistry appeared and will be an invaluable reference work in this field. Iodination of 1,l-dimethylallene in the presence of mercuric acetate in methanol is known to involve addition at the substituted double-bond to give a tertiary alcohol that is iodinated on the vinyl group i.e.Me2C(OH)CICH2. In the absence of metallic salts however the regioselectivity is reversed.’* Hence with iodine in 49 R. A. Abramovitch M. Ortiz andS. P. McManus J. Org. Chem. 1981 46 330. ’ ” Y. Tamaru M. Kagotani R. Suzuki and Z. Yoshida J. Org. Chem.. 1981,46,3314. ’’ ‘The Chemistry of Ketenes Allenes and Related Compounds’ ed. S. Patai J. Wiley and Sons,New York 1980. 52 C. Georgoulis W. Srnadja and J. M. Valery Synthesis 1981 572. 160 D. F.Ewing chloroform followed by treatment with an alkoxide the isomeric primary alcohol Me2CCICH20His formed. An interesting class of compounds a@-unsaturatedhydroxy-ketones (29) has been identified53as the major product from the treatment of allenes with dicobalt octacarbonyl methyl iodide and carbon monoxide in the presence of a phase-transfer catalyst (cetyltrimethylammonium bromide) in a two-phase system (5M-NaOH/benzene).Carbonylation is presumed to occur by addition of acetylcobalt tetracarbonyl (generated in situ) to the allene followed by attack of hydroxide ion. Yields of (29) are low (23-43%) and a dienone (30)is a by-product in most cases. Reactions carried out in the absence of a phase-transfer catalyst gave only this latter type of product. IIR’R~C ,c C II C R3/ ‘H CH3 OH 0 OH I II I R‘R~C‘c/ C \c/ CR’R~ II II C0\ ~3’ ‘H H R3 R’R~C /c \c,~~i~2 C (29) (30) A very detailed study has been made54of the reaction of arenesulphenyl halides with allenes.Both steric and electronic effects are generally of minimal importance in the rate-determining step (SN2attack on sulphur to form an alkylidenethiiranium ion intermediate) but some steric influence is apparent in the final step (addition of halide ion) particularly if the arene moiety possesses two bulky ortho-groups. The analogous addition reaction with areneselenyl halides is more complex. Product distributions reflect changes in chemoselectivity and configurational selectivity in accord with two different types of steric interaction in the product-determining step. The most likely intermediate is thought to be an alkylidene-episelenurane which subsequently isomerizes to an alkylideneseleniranium ion but a complete understanding of all the subtleties of these addition reactions of allenes is not achieved.4 Alkynes Synthesis.-An improved synthesis of alkynes from vicinal dibromides has been reported,56 involving the use of a phase-transfer catalyst. Typically 0.1 mole of dibromide and 1mmole of tetraoctylammonium bromide in a hydrocarbon solvent is stirred with 0.25 mole of solid KOH for six hours at 90 “C to give the. alkyne in 85-95% yield. This catalytic method is simpler and cheaper than using potassium t-butoxide with 18-crown-6. A novel route to alkynes is shown in Scheme 17. The keten adduct of anthracene (31) is readily alkylated via its lithium enolate to give (32; R’= alkyl or benzyl). The dialkylated species (33)is accessible by two routes (see reagents in the Scheme) ’’ S.Gambarotta and H. Alper J. Org. Chem. 1981 46 2142. 54 D. G. Garratt and P. L. Beaulieu Can. J. Chem. 1980,58,2737. 55 D. G. Garratt P. L. Beaulieu V. M. Morisset and M. Ujjainwalla Can. J. Chem. 1980,58 2745. 56 E. V. Dehmlow and M. Lissel Tetrahedron 1981 37 1653. Aliphatic Compounds -Part (i) Hydrocarbons pi R’C~CR’ Reagents i Bun Li R’Br ;ii either R’MgX TsOH in benzene or tri-isopropylbenzenesulphonylhydrazide Bu“Li R’X; iii heat Scheme 17 and thermal elimination provides the alk~ne.~’ With yields in the range 60-loo% this reaction may be specially attractive in some cases. Several workers have investigated methods for the synthesis of aryl-acetylenes. The decarbonylation of diaryl-cyclopropenones has been noted in the past but this reaction has now been made into a general preparative method for diaryl-acetyl- enes.’* Thermal decomposition of the cyclopropenones in the presence of alumina gives the alkynes in high yield usually >go%.Six established methods for the generation of alkynes have been sur~eyed,~’ and the best methods can now be specified for ArCZCH and ArCECR depending on the nature of the group R and the type of substituent in the aryl ring. The extent to which these guidelines can be usefully applied to other systems (e.g. heteroaryl-alkynes) is uncertain. An attractive extension to the reaction involving coupling between a protected acetyl- ene and an aryl halide has been described.60 Treatment of trimethylsilylacetylene with a substituted aryl bromide in triethylamine in the presence of palladium acetate and triphenylphosphine affords the corresponding protected aryl-acetylene in good yield.The basic solvent acts as a scavenger for the hydrogen bromide that is generated. The trimethylsilyl protecting group is smoothly removed with potassium carbonate at ambient temperature. This mild procedure tolerates sensitive aryl substituents such as CHO NH2,and C02Me and hence usefully extends the range of accessible ethynylated benzene derivatives. The reaction of an aryl-lithium species with 1,l-difluoro-2,2-dichloroethene followed by treatment with two equivalents of n-butyl-lithium provides access to the corresponding ethynyl-arene (Scheme 18).This approach has now been applied successfully6’ to the synthesis of 2,5diethynylthiophen and 2,Sdiethynylfuran and related derivatives.A ‘one-pot’ procedure for the synthesis of 1,4-enynes has been ArLi ArCF=CC12 -% ArCECLi -% ArC=CH Reagents i F,C=CCI,; ii BunLi; iii H,O Scheme 18 57 B. Tarnchompoo Y. Thebtaranonth and S. Utamapanya Chem. Lett. 1981 1241. j8 D. H. Wadsworth and B. A. Donatelli Synthesis 1981 285. 59 D. Mesnard F. Bernadou and L. Miginiac J. Chem. Res. 1981 (S),270; (M),3216. 6o W. B. Austin N. Bilow W. J. Kelleghan and K. S. Y. Lau J. Org. Chem. 1981,46 2280. K. Okuhara Bull. Chem. SOC.Jpn. 1981 54 2045. 162 D. F. Ewing (H2C=C-CH2)3B + 3EtOCECH I R li (H2C=C -CH2 -C=CH)3B -L H,C=C -CH;? -C=CHAlEt I I I I R OEt R OEt (34) Reagents i at -70 "C; ii Et,AI; iii at 180 "C Scheme 19 described.62 The strategy is to obtain an alkoxy-alkene which will undergo 1,2- elimination to generate a triple bond and this is achieved as shown in Scheme 19.Although the (alkoxy-viny1)borane (34) is obtained quantitatively from the alkoxy- acetylene direct 1,2-elimination could not be effected with normal reagents (acids or base) but conversion into a vinylaluminium species in situ gives the desired lability and facile transformation into the enyne. Reactions of A1kynes.-A new catalyst for the hydrogenation of alkynes has been de~eloped.~~ Chloromethylated cross-linked polystyrene beads were treated with anthranilic acid and then palladium chloride to give a catalyst which converted phenylacetylene (30 p.s.i.g.H for 7.3 hours) into styrene 82% and ethylbenzene (14%). Several disubstituted alkynes were converted into cis-alkenes (60-90%) and alkanes (4-20%). This is clearly less selective than the Lindlar catalyst but it has the advantage of being stable in air and of remaining active indefinitely if stored at ambient temperature. The alkylation of terminal alkynes with organoaluminium compounds in the presence of zirconocene has been thought to proceed via Al-assisted carbozirconation. However it has now been unequivocably established64 that in the typical cases examined this reaction proceeds via Zr- assisted carboalumination. This improved understanding of the role of the zirconium species allows a reagent system to be selected which prevents the competing hydrometallation reaction (which requires an alkyl-zirconium species).For example Prn2A1C1/ZrC12Cp2 shows no ligand exchange and with hept-1-yne it affords 2-n-propylhept-1-ene (76%) and (E)dec-4-ene (21%) with negligible formation of hept-1 -ene. In contrast a trialkyl-aluminium does exchange with ZrC12Cp2 and that catalyst system would produce some hept-1-ene in the above reaction. The alkylation of but-2-yne with the diene complex [ZrCp,(isoprene)] is highly stereo- specific giving 99% alkylation at C-2 but approximately equal amounts of alkyl- ation at C-2 and C-3 occur with ~ent-2-yne.~' In both cases the isoprene unit couples only at C-4. Phenyl- and diphenyl-acetylene react differently giving the 62 Yu.N. Bubnov A. V. Tysban and B. M. Mikhailov Synthesis 1980,904 63 N. L. Holy and S. R. Shelton Tetrahedron 1981,37 25. 64 T. Yoshida and E.-I. Negishi J. Am. Chem. SOC., 1981 103,4985. Aliphatic Compounds -Part (i) Hydrocarbons 163 corresponding dihydro-dimers (1,4-diphenyl- and 1,2,3,4-tetraphenyl-buta-1,3-diene) without any coupling to isoprene. The addition of a carboxylic acid to a triple-bond catalysed by silver salts has been investigated6' for several compounds (35) (see Scheme20). For (35; R' R2 = CH20COMe CH2COMe or C02Me) yields are in the range 50-95% but aryl- and alkyl-acetylenes (35;R'R2 = Ph Et or H) give much poorer yields (O-28%). Silver carbonate was the best catalyst and stereospecificity was good in most cases the isomer ratios being between 4 1 and 9 :1.The most likely mechanism is initial formation of a silver r-complex followed by attack by the carboxylate anion. R'CGCR~ + R'C=CR~ -+R'C=CHR' '-,+ ,/' I (35) Ag OCOR~ Reagents i Ag salt; ii R'C0,H Scheme 20 Interest in metallation reactions of alkynes continues to increase and several interesting reports have appeared. Phenylselenol reacts slowly with alkynes at ambient temperature in the absence of a base to give the 2-vinylselenide but the stereospecificity is reduced at higher temperatures (Scheme 2 1).66Aryl-acetyl-enes react more rapidly to give the product that is expected from initial formation of the more stable carbanion but for alkyl-acetylenes the site of attack by PhSe- is governed by steric effects.Pure E-vinylselenides were obtained by hydrogenation of seleno-alkynes [reaction (b) in Scheme211. A different regio- and stereo- chemistry is found for the addition of benzeneselenyl chloride to propargyl alcohols (36) [reaction (c)] consistent with an initial step involving electrophilic The addition is invariably anti but the regioselectivity is structure-dependent. For alkynes with small substituents geminal to the hydroxy-group e.g. (36;R2 = R3 = Me Et Pr' or Ph) rapid formation of the thermodynamically favoured Markownikoff product occurs but for large substituents e.g. (36;R2 = R3 = Bu') the reaction is slow and leads to the anti-Markownikoff product. There are a number of complex subtleties to the steric control of this reaction and the product distribution may reflect the relative rates of addition and is~rnerization.~~ Hydromagnesiation of disubstituted acetylenes with Bu'MgBr in the presence of Cp2TiC12 has been shown68 to occur with high stereoselectivity to afford E-alkenyl- R'CZCR2 (5 R'CH=CR2SePh * R'CGCSePh (b) R' c1 R' SePh Reagents i PhSeH at 20 "C; ii LiAlH,; iii PhSeCl Scheme 21 65 Y.Ishino I. Nishiguchi S.Nakao and T. Hirashima Chem. Lett. 1981 641. 66 J. V. Comasseto J. T. B. Ferreira and N. Petragnani J. Organomet. Chem. 1981,216 287. 67 D. G. Garratt P. L. Beaulieu and V. M. Morisset Can. J. Chem. 1981 59 927. F. Sato H. Ishikawa and M. Sato Tetrahedron Lett. 1981 22 85. 164 D. F. Ewing magnesium bromides. The regioselectivity is also high for aryl-acetylenes the magnesium atom being attached to the carbon adjacent to the phenyl group.Not unexpectedly unsymmetrical alkynes show negligible regioselectivity. For silyl- acetylene magnesiation also occurs at the carbon that is a to the silyl group. Such regio-control is consistent with initial attack on the alkyne by a hydride species [probably (CP)~T~H] followed by transmetallation to magnesium. A full report has now appeared69 concerning the silylcupration of acetylenes using the reagent (Me,PhSi),CuLi.LiCN. At 0 "C (or even at -78 "C) terminal alkynes silylate selec- tively on the terminal carbon with the result that the final products are 2,2-disubstituted vinylsilanes. The reactions between the syn -adducts of various acety- lenes with the above reagent and electrophiles such as iodine acyl and alkyl halides enones and epoxides are described.Silylcupration of acetylenes is clearly a powerful method for the synthesis of a wide range of vinylsilanes. In connection with some work on the synthesis of p-lactam antibiotics it has been suggested" that hydro- stannation of internal alkynes may offer a useful method for the regioselective formation of ketones (Scheme 22). The alkenyl-stannane (37) was the dominant isomer when R was a p-lactam group but selectivity is not high (2:l) and the extent to which the group R co-ordinates to tin and directs the addition is not established. If the methyl group is replaced by CHOHMe the regioselectivity is reversed owing to the co-ordinating effect of the OH group.A more general exploration of this reaction is required if its general usefulness is to be assessed. RCH2, \ RCH2CGCNie A C=CHMe -RCH2COCH2Me Bu3Sn' Reagents i 1.5 equivalents of Bu",SnH at 90 "C;ii 3-chloroperoxybenzoic acid; iii excess HCO,H at r.t.; iv LiOH in aqueous THF Scheme 22 Extensive use has been made of the reactions of propargylic halides esters and tosylates (38) with alkyl-copper species to give either alkylated acetylenes (by direct substitution) or allenes (by 1,3-substitution) (see Scheme 23). A very thorough study has now been made of the various factors that might influence the course of this reaction. The results show that a large steric effect at the acetylenic site [R' in (38)] or at the propargylic site [R2 in (38)] shifts the balance of the reaction to the other site to give (40) and (39) respectively; very large groups such as Me3C almost exclude substitution at a given site.Leaving-group effects were examined by varying R3in (38;R' = cyclohexyl,R2 = H). In terms of the pK of the conjugate acid R3H,no clear trends were apparent for R3 = OTos OAc and OC02Me. The acetate produced mainly the alcohol (41) in this case and in the case of several other variations in R' and R2. Products of the type (42) were rarely observed. The most dominant factor in determining the site selectivity was the nature of the organocopper reagent. The species Me2CuLi usually gave a mixture of (39) and (40)together with some (41) whereas MeCuMgBrI exhibited a high ratio of alkyne 69 I.Fleming T. W. Newton and F. Roessler J. Chem. SOC., Perkin Trans. 1 1981 2527. 'O A. Nishida M. Shibasaki and S. Ikegami Tetrahedron Lett. 1981,22 4819. Aliphatic Compounds -Part (i) Hydrocarbons to allene with no formation of (41). Virtually exclusive formation of allene was observed with MeCuLiBreMgBrI in THF suggesting that a distinctly different reactive species is involved in this case. A number of other less important facets of this reaction are covered in the 33 experiments reported in this work.” R’CzCCR2C5H11 + R1CrCCR2C5Hll+ R1C~CCR2C5H11 I I Me OH GR3\ I (39) (41) R’ R2 R’ R2 \ / \ / /c=c=c \ + /c=c=c \ Me C5Hll C5Hll (40) (42) Scheme 23 Olefination of the carbonyl group in aldehydes and ketones is achieved efficiently by their reaction with the ambident anion (44)that is derived by metallation of 1,3-bistrimethylsilylpropyne(43;R = Me) [reaction (a) in Scheme 24].72In the case of aldehydes stereo-control is achieved by increasing the size of the substituent on the silicon at the carbanionic site.For example the enynes from hexanal have a Z/E ratio of 3 :1 for R = Me but a ratio of 31 :1 is observed if R = But. A steric effect in the transition state is responsible for this enhancement of the proportion of the 2-isomer. Less obvious is the reason for a two-fold increase in the Z/E ratio when the carbanion (44)is effectively a Grignard reagent (M = MgBr) rather than a lithio-derivative (M = Li). A greater tendency to form (45)may lead to a cyclic transition state thus enhancing the steric interaction.Several reactions of propargyltrimethylsilane (46) have been explored73 [reaction (b) in Scheme 241. M’ RMe2SiCH -CrCSiMe3 (44) RMe2SiCH2C=CSiMe3 I* 1L 5R’CH=CHCZCSiMe (a) M (43) RMe2SiCH=C=C / \ SiMe3 (45) RC-C-SiMe3 __* RCX=C=CH2 (b) (46) (47) Reagents i Bu‘Li then MgBr, if required; ii R’CHO Scheme 24 71 T. L. Macdonald D. R. Reagan and R. S. Brinkmeyer J. Org. Chem. 1980,45,4740. ” Y.Yamakado M. Ishiguro N. Ikeda and H. Yamamoto J. Am. Chem. Soc. 1981,103,5568. 73 T.Floor and P. E. Paterson J. Org. Chem. 1980,45,5006. 166 D. F. Ewing Electrophilic reagents such as trifluoroacetic acid Br, I, and acetyl chloride/AlCl afford the corresponding 3-substituted allene.This is a convenient route to several compounds (47; X = H Br or I). Treatment of (46) with base gives a mixture of the products from 3-substitution i.e. the allene (47; X = H) and l-substitution i.e. the methylacetylene. Homolytic chlorination of six acetylenes with sulphuryl chloride in benzene gave mixtures of the corresponding E-and Z-dichloro-alkenes in good yield.74 The isomer ratio depends very markedly on the substituents on the alkyne the proportion of Z-isomer increasing with the size of these groups. An interesting route to naphthoquinones is shown in Scheme 25. A wide range of alkynes from electron-rich alkyl-acetylenes to electron-deficient alkynyl esters (48) Reagents i MeCN in a sealed tube at 100 "C Scheme 25 react with a cyclic iron complex [48; M = Fe(CO),] to result in the formation of a quinone ring in high yield (70-100%).75 The analogous cobalt complex [48; M = CoCl(PPh,),] requires the presence of AgBF to induce the formation of the quinone but is superior to the iron complex for sterically hindered alkynes.Extensions of this approach could lead to a fruitful development of reactions for ring formation by incorporation of a two-carbon acetylenic unit. Another potentially interesting development is the addition of a carbene to alkynes with the complex [Cp(CO),FeCHR]' PF6-. A simple alkyne but-2-yne shows a surprisingly high reactivity towards this reagent leading to the formation of dimethylphenyl-cyclopropenium ion (49). The chemistry of this reaction is largely unexplored as yet.37 Ph 74 S.Eumura C. Masaki A. Toshimitsu and S. Sawada Bull. Chem. SOC.Jpn. 1981 54,2843 '' L.S.Liebeskind S. L. Baysdon and M. S. South J. Am. Chem. Soc. 1980,102,7397.