8 Aliphatic Compounds Part (i)Hydrocarbons By D. F. EWING Deparfment of Chemistry The University of Hull Hull HU6 7RX 1 Alkanes Although of limited interest it can often be quite difficult to make very long chain alkanes or derivatives when a specific length is required. A convenient procedure has been described' recently which starts from the bromoaldehyde (I) readily prepared by standard methods from cyclododecanone. Reaction of (1)with a Wittig reagent made from the acetal of (1)gave a 24-carbon bromoacetal(2) and repetition of this coupling step was carried out to give finally a bromoacetal containing 192 carbon atoms. The residual aldehyde function could be removed at any stage by treatment with a simple Wittig reagent and hydrogenolysis followed by hydrogena- tion was carried out to give a 104-carbon alkane.Extrusion of sulphur from sulphides or sulphones may be employed as a route to hydrocarbons. The usefulness of this approach can be enlarged by analogous reactions of selenides.2 Although the products of flash pyrolysis of selenides are dependent on the character of the bonding involving the selenium atom particularly useful starting materials are phenyl benzyl selenides which pyrolyse to give bibenzyls ArCH2CH2Ar (and diphenyldiselenide) in 65 to 90% yield. In some cases this reaction is cleaner than the corresponding reaction of sulphides or sulphones. Radical substitution reactions of alkanes are little studied now but one interesting report3 considers the reactive compound C1SO2NCO. Several possible products could be envisaged in this reaction but substitition by chlorine radical is the predominant step alkylisocyanates and alkanesulphonylchlorides being formed only to a very small extent.This reaction proceeds by either thermal or photo- chemical initiation and the hydrogen abstracting radical is probably NCO. 2 Alkenes Synthesis.-From Alkynes. The usefulness of the intermediates obtained by the addition of organometallic reagents to alkynes is legion and a very timely review * 0. I. Paynter D.J. Simmonds and M. C. Whiting J. Chem. Soc. Chem. Commun. 1982 1145. H. Higuchi T. Otsubo F. Ogura H. Yamaguchi Y.Sakata and S. Misumi Bull. Chem. SOC.Jpn. 1982,55 182. M. W. Mosher J. Org. Chem.. 1982,47 1875. 136 D.F.Ewing of this area has a~peared.~ This valuable account surveys carbometallation reactions of alkynes involving Li Zn Mg B Al and Cu as the 'metal' and highlights the activity in this field directed to the formation of specifically substituted alkenes particularly in natural product chemistry. Brown and co-workers5-' have developed several new variations of the hydro- boration of alkynes which significantly extend the usefulness of this reaction. The bromoborane complex R'BrBH-SMe is much more effective than a dialkylborane in the reaction with alkynes. Not only is there no competition from dihydroboration but the vinylborane (3) produced contains only one boroalkyl group which is subsequently transferred stereospecifically to the vinyl group [Scheme 1;reaction (a)].Thus no redundant boroalkylation is required in the formation of the reagent. R'BBr R'BOMe ii +L R' (a) R' R (3) R'BBr R' +p-b (b) )-7 Br R' (OMe)*B R' (4) R' % R' TxBR ' 7 Reagents i NaOMe -25 "C;ii NaOMe-I, 0 "C;iii H' heat Scheme 1 The only complication is formation of up to 15% of the corresponding vinyl iodide but this is less of a problem with bulky R' groups. This one-pot stereospecific synthesis of cis-alkenes has yields of 60-70%. If the hydroboration with R'BrBH-SMe is carried out on a bromoalkyne a bromovinylborane (4) is formed which can be stereospecifically converted to a trans-alkene [Scheme 1;reaction (b)]. Another similar route to trans-alkenes involves' formation of a thexyl vinyl- borane [5,Tx =Me2HCC(Me2)-] which shows the same specificity in the deboron- ation step.The starting point for this route is the versatile thexylchloroborane dime t h ylsulp hide complex TxClB H.Me2S obtained from 2,3-dime th ylbut -2-ene and BH2Cl.Me2S. This species affords an unusually stable alkyl derivative TxClBR' J. F. Normant and A. Alexakis Synthesis 1981,841. H. C. Brown and D. Basavaiah J. Org. Chem. 1982,47,3806. H. C. Brown D. Basavaiah and S. U. Kulkarni J. Org. Chem. 1982 47,3808. 'H. C. Brown H. D. Lee and S. U. Kulkarni Synthesis 1982,173. 137 Aliphatic Compounds -Part (i) Hydrocarbons when reacted with an alkene. Low-temperature hydridation regenerates the borane TxBHR' which is reacted with an alkyne to give (5).It is the high selectivity for the transfer of a primary alkyl group in (5) which provides this route for the formation of trans-alkenes under very mild conditions. The reaction appears to be general tolerating many common functional groups. The stereospecificity of the alkyl-group transfer in the deboronation of vinylboranes is responsible for a con- venient route to trisubstituted alkenes.8 Previous reports have described routes to 2-and E-1-halo-1-alkenylsilanes. It has now been established' that a generally reliable desilylation procedure is treat- ment with sodium methoxide in methanol. The 1-haloalkenes are obtained in 70 to 90% yield with no loss in stereochemical purity. Substitution of the Me& group by another halogen atom can be carried out by first performing an addition step trans -halogenation followed by base-induced desilico-halogenation.lo This pro- cedure gives E-1-bromo- 1-chloro- 1-alkenes. The isomeric 2-compounds were prepared via a hydroboration adduct of a 1-haloalkyne. Treatment of this borane directly with Br followed by NaOMe was not very effective in this case unless the borane was first converted to a borinic ester by oxidation with trimethylamine oxide. Synthesis of Alkenes by Elimination. A useful alternative" to the conversion of some primary alcohols to a selenate ester followed by oxidative elimination is to convert the alcohol to the iodide (via the tosylate) then induce elimination by in situ reaction with 1,5-diazabicyclo[4.3.0]non-5-ene or the analogous 1,8- diazabicyclo[5.4.0]undec-7-ene.These widely used reagents do not normally induce elimination in primary halides or sulphonates and only work with iodides if the @-carbon is disubstituted.p-Trimethylsilylethyl sulphoxides undergo elimination of the silylsulphonate rather faster than sulphenic acid is eliminated in the absence of a trimethylsilyl group.12 However this promoting effect is restricted to @-substituted systems since hydrogen is preferentially eliminated when both H and SiMe are present on the @-carbon. It is often the case that working on the synthesis of complex natural products provides a very severe test of currently available procedures. One example of a milder modification of an existing reaction which has been developed to meet the demands of natural product work is the conversion of 1,2-diols to 01efins.l~ The diol is first converted to a thionocarbonate by treatment with a slight excess of thiophosgene in presence of excess 4-dimethylamino-pyridine for one hour at 0 "C.Tertiary alcohols do not react. Elimination of the thionocarbonato group is promoted by reaction with 1,3-dimethyl-2-phenyl-l,3,3-diazaphospholidine. Yields are good (70 to go0/,) but perhaps not good enough to justify the use of these unusual reagents especially when another novel approach14 to this transforma- tion is considered. Treatment of a cis-or trans-diol with sodium iodide and CISiMe at room temperature is reported to effect elimination to the olefin in 80-90% yield for several steroidal compounds. H. C.Brown D. Basavaiah and S. U. Kulkarni J. Org. Chem. 1982 47 171. H. P. On W. Lewis and G. Zweifel Synthesis. 1981,999. lo R. P. Fisher H. P. On J. T. Snow and G. Zweifel Synthesis 1982. 127. S. Wolff M. E. Huecas and W. C. Agosta J. Org. Chem. 1982,47,4358. I. Fleming J. Goldhill and D. A. Perry J. Chem. SOC.,Perkin Trans. 1 1982 1563. l3 E. J. Corey and P. B. Hopkins TefruhedronLett. 1982.23 1979. l4 N. C. Barua and R. P. Sharma. Tetrahedron Lett. 1982,23,1365. 138 D. F.Ewing The direct transformation of a carbonyl compound into a vinyl derivative (Scheme 2)15 may prove valuable in the synthesis of natural products. An effective 6-trimethylsilylanion is provided by the new reagent Me3SiCH2CH2Li (or the analogous Grignard reagent). Attack on a carbonyl group gives the expected alcohol in 65 to 90% yield.In most cases this alcohol can be quantitatively converted to the required olefin. R' R' OH R' \CEO A \c/ -% \CHCH=CH2 R2/ R2/ \CH2CH2SiMe3 R2/ Reagents i Me3SiCH2CH,Li -78 "C; ii BF,.MeCOOH 25 "C Scheme 2 Nitroalkenes can be prepared by introducing a double bond into a nitroalkane via the phenyl selenide (cf. Annu. Rep. Prog. Chem. Sect. B 1981,78 149). An alternative approach is to introduce the nitro group directly.16 Formation of an adduct between an alkene and phenylselenyl bromide gives a 2-bromoalkyl phenyl selenide which reacts with silver nitrite in presence of mercuric chloride to give a nitroalkyl derivative. The nitroalkene is obtained in high yield by oxidative elimina- tion.A by-product produced at the addition stage is the analogous alcohol presum- ably arising from reaction of the nitrite nucleophile through an oxygen atom. Synthesis by other Methods. A preliminary rep~rt'~ gives details of a method for a direct alkylidenation reaction (Scheme 3) using a 1,l-dimetalloalkane. Little stereochemical control appears to operate. R2 R'CECH & R'CH=CHAI(Bu')2 A R*CH*CH=C/ 'R3 Reagents i (Bu'),AlH; ii R2R3C0 Scheme 3 Substitution of an alkylthio group on an olefin by alkyl or aryl functions under catalysis by low-valent nickel species has been studied by several workers recently. This reaction has now been extended" to replacement of an alkylthio group by hydrogen using a secondary Grignard reagent to achieve reduction.The competition between reduction and alkylation is controlled by the nature of the ligands on the nickel catalyst. Sequential replacement of bromo and alkylthio groups can be used to give E-dialkyl- and E-diarylolefins with total stereoselectivity whereas the corresponding 2-olefins undergo isomerization to a small extent (3% to lo%)." Is S. R. Wilson and A. S. Hedrinsky J. Org. Chem. 1982.47 1983. l6 T. Hayama S. Tomoda Y. Takeuchi and Y. Nomura Tetrahedron Lett. 1982.23.4733; Chem. Lett. 1982,1109. T. Yoshida Chern. Lett. 1982 429. E. Wenkert and T. W. Ferreira I. Chem. SOC.,Chem. Commun. 1982,840. l9 V. Fiandanese G. Marchese F. Naso and L. Ronzini J. Chem. SOC.,Chem. Commun. 1982,647. Aliphatic Compounds -Part (i) Hydrocarbons 139 Reactions of Alkenes.-There has been much interest in the addition reactions of organoselenium compounds in the last few years and the results of some very detailed studies are now being published.Investigation of the addition of ben- zeneselenenylchloride to 1,l -disubstituted olefins reveals that careful control of the reaction conditions can give rise to very high regiospecificity.20 The most crucial parameter is temperature. At -70°C chloride ion attack is controlled by steric factors and the resulting product always has anti-Markovnikov regiochemistry. Isomerization occurs at higher temperature. Peracid oxidative elimination shows poor specificity. Another elegant study21 of addition to allylic alcohols shows that the hydroxy group directs the selenenium ion to the syn face of the double bond and that under kinetic control the chloride ion favours axial attack.A series of rules are proposed which allow prediction of the course of this reaction. Two novel selenylation reactions have been reported. Phenylselenocyanate adds to olefins with complete stereospecificity and in high yield (70-95%)22 in presence of a Lewis acid catalyst (e.g. AlC& or BF3).With unsymmetrical alkenes a mixture of regioisomers was obtained but perhaps better control of the regiochemistry can be achieved. The reaction shown in Scheme 4 is brought about by benzeneselenenyl- iodide generated in sit^.^^ The chloride and bromide produce no products of this type which are thought to arise from formation of an episelenonium ion at one double bond which is then attacked by the second double bond.Reagent i PhSeSePh-I,-MeCN reflux Scheme 4 Diethylphosphoramidate is sufficiently nucleophilic to react with an alkene in presence of mercury(@ nitrate.24 The amidomercuration adduct is formed with high regioselectivity (Markovnikov) but in variable yield. Demercuration is achieved in situ with NaBH4 and subsequent hydrolysis gives an amine in 60 to 85% yield overall an economical amination reaction at least for reactive alkenes. In 1959 Reppe discovered that amines could be formed in low yield by reaction of an alkene with water carbon monoxide and a primary or secondary amine. This reaction has now been explored in more detail2' and it is found that with a suitable rhodium catalyst cyclohexene can be converted to amines of the type C6HI1CH2NR2 in up to 90% yield.A wide variety of nitrogen sources can be used and other types of alkene also react readily. This procedure clearly has considerable potential for one-step synthesis of amines. Oxidation of alkenes by thallium(II1) salts is a useful synthetic procedure and the sensitivity of the reaction to the nature of the anion has been exploited26 to 2o P.-T. Ho and R. J. Kolt Can. J. Chem. 1982,60,663. D. Liotta G. Zima and M. Saindane J. Org. Chem. 1982 47 1258. 22 S. Tomora Y. Takeuchi and Y. Nomura J. Chem. SOC.,Chem. Commun. 1982,871. 23 A. Toshimitsu S. Uemura and M. Okano J. Chem. SOC..Chem. Commun. 1982,87. 24 A. Koziara B. Olejniczak K. Osowska and A.Zwierak Synthesis. 1982,918. 25 F. Jachimowicz and J. W. Raksis J. Org. Chem. 1982,47,445. 26 A. J. Bloodworth and D. J. Lapham. J. Chem. SOC.,Perkin Trans. 1,1981,3265. 140 D. F. Ewing control the type of product (Scheme 5). Thallium acetate forms a stable adduct (6) whereas with the trifluoroacetate salt spontaneous oxidative dethalliation occurs readily to give (7) and (8).Regioselectivity in the oxidation of internal olefins is BuCH=CH2 i,BuCHCH2Tl(OCOMe)2 BuCHCHzBr I I iii OMe OMe (6) c BuCHCH2Tl(0COCF3) 4 BuCHCH20Me BuCHCHzOH +I [ &Me 1 OMe OMe (7) (8) Reagents i TI(OCOMe),-MeOH; ii KBr-crown ether-MeOH; iii TI(OCOCF,),-MeOH Scheme 5 difficult to achieve unless a carbonyl group is present when 1,3-diketones are formed preferentially in presence of PdC12/CuC1.The same catalyst also promotes oxidation of ally1 ethers or esters to the corresponding P-alkoxy or @-acetoxy The yields are not very high (40% to 75%) but the specificity is complete probably indicating co-ordination of the allylic oxygen to palladium in the active complex. Similar influence is observed in the reduction of unsaturated amides R2C=CHCONR1 by magnesium in methanol since unconjugated amides do not react.” Such a,@-unsaturated amides also co-ordinate with lithium directing lithiation with sec-butyl lithium to the p’-p~sition.~’Isomerization usually occurs with acyclic alkenes resulting in formation of two substitution products -CHX -C( CONR,) =CH -and -CH=C( CONR,) -CHX- . Alkenes are normally inert to LiBh but in presence of an ester rapid hydrobor- ation occurs at 25 “C to give a dialkylb~rinate.~’ It is also observed that the presence of the alkene has a rate-enhancing effect on the reduction of the carbonyl group indicating that a.coupled reaction is involved (Scheme 6).A second hydroboration step follows or even a third at high concentrations of alkene. This interesting observation may point to other areas where coupled reactions can occur. OR’ OR OR’ I / RC/ R-CO=O--Li RCH 4 $ -* ‘OLi LiBH4 k R2CH2CH2BH2 R,CH=CH~ H’A‘H RZCH=CH2 Scheme 6 27 J. Tsuji H. Nagashima and K. Hori Tetrahedron Lett. 1982,23,2679. 28 R. Brettle and S. M. Shibib J. Chem. SOC.,Perkin Truns. 1 1981 2914. 29 D. J. Kempf K. D. Wilson and P.Beak J. Org. Chem. 1982,47 1612. 30 H. C. Brown and S. Narasimhan Orgunometuffics,1982 1 763. Aliphatic Compounds -Part (i) Hydrocarbons 141 A clever method for cis-hydroxylation of olefins has been presented by Corey and Das31 The bromohydrin is formed first and then esterified with excess cyanoacetic acid (Scheme 7).The key step is enolization of the resulting P-ketonitrile which facilitates an internal nucleophilic displacement. This procedure will be of value for the mildness of the conditions used and the resulting stereospecificity. liii 0 1\=CHCN 0 li" VI YOH YOH *OCOCH,CN *OH Reagents i NBS 0 "C; ii CNCH,COOH-TosC1; iii;NaH 0 "C; iv H'; v K2C03 Scheme 7 Oxidation without chemical oxidant (and hence without contaminating reduced species) is an attractive proposition and just such a possibility has been described3' for the epoxidation of olefins.The oxygen is provided by water and the reaction is carried out in an electrochemical cell in presence of a anion-exchange resin containing quaternary ammonium groups. The best resin anion is bromide and the amount of water in the solvent (dimethylformamide-benzene) is critical. Several facets of this reaction would repay further investigation since its scope is fairly wide (groups such as C1 COOR CONEt are tolerated) and the resin beads can be reused several times at least. Another reaction involving a heterogeneous phase is that shown in Scheme 8.This simple but efficient transformation uses t-butyl hypo- chlorite in presence of silica gel.33 The relative proportions of monochloro (9) and CI (9) Reagent i Me,COCl/SiO Scheme 8 31 E.J. Corey and J. Das Tetrahedron Lett. 1982,23,4217. " J. Yoshida J. Hashimoto and N. Kawabata J. Org. Chem. 1982 47 3575. 33 W. Sato N. Ikeda and H. Yamamoto Chern. Lett. 1982 141. 142 D. F. Ewing dichloro (10) products can be varied by choice of solvent e.g. 5% (9) and 69% (10) in ether compared with 73% (9) and 5% (10) in hexane. 3 Polyenes Synthesis.-The bistrimethylsilyl species (11)has been shown to react with carbonyl compounds to give dienes directly but in low yield and with poor stereoselectivity. However in the presence of MgBrz the intermediate alcohols (Scheme 9) are isolable Lit .-* -.Me,%-SiMe (11) li RhSiMe 5R-SiMe SiMe Yh R *SiMe Reagents i 2MgBr -78 "C RCHO; ii H,SO,-THF; iii THF heat Scheme 9 in over 80% yield with excellent stereo~electivity.~~ With aliphatic aldehydes in particular B(OMe) is even better than MgBrz and the lE,3E-butadienes are obtained in ca. 90% yield. Ally1 sulphides R'R2C=CR3CH2X (X= Spy) or sul-phoxides (X = S(O)Ph) react via the corresponding carbanion with Bu3SnCH31 at -78 "Cto form (12) which spontaneously eliminates to give dienes in 50 to 90% yield.3' R' R3 M ~2 CHCH,SnBu, I X (12) Direct coupling of lithium vinyl cuprates (Vi)2CuLi with vinyl iodides in presence of ZnBr (cf. Annu. Rep. Prog. Chem. Sect. B 1981 78 156) is an inefficient procedure since only one of the vinyl groups (Vij is transferred.However vinylmag- nesium compounds (Vi)Cu MgX2 react with vinyliodide in presence of Pd(PPh& to produce the diene with complete stereoselectivity albeit in rather variable yield (55 to 70O/0),~~The addition of MgBrz to a solution of a lithium cuprate generates 34 T. H. Chan and J. S. Li J. Chem. Soc. Chem. Commun. 1982,909. 35 M. Ochiai S. Tada K. Sumi and E. Fujita Tetrahedron Lerr. 1982,23,2205. 36 N. Jabri A. Alexakis and J. F. Normant Tetrahedron Lert. 1982 23 1589. Aliphatic Compounds -Part (i) Hydrocarbons the magnesium cuprate which reacts smoothly with two equivalents of vinyl iodide in presence of ZnBr,. Dienoic esters with a conjugated E,Z geometry are important as aroma agents and as insect pheremones and an interesting application of heterogeneous catalysis with alumina is reported37 to give such compounds in high yields.The key step is the conversion of (13)to (14) by A1203. Complexes of the type Ti(Cp),R (R = alkyl alkenyl allyl) cause convergent double bond shifts in 1,5-dienes such that conju- gated 2,4-dienes are formed.38 Unfortunately the stereochemical control is poor since a mixture of (E,E)-,(E,Z)-,and (2,Z)-isomers are obtained. R'CH=C=CHCH,COOR~ (13) (14) One structure which occurs commonly in terpenoid compounds is the 1,5-enyne moiety and two groups have reported on routes to such compounds (Scheme 10). R2 R'C=CCH2CH2CH=C / LiCrCCH2CI . R~CECCH~BR \L ii (16) 'R3 (15) R' ICHZ=C=CCH2CH=C R2 / 'R3 (17) Reagents i BR, -78 "C; ii LiOMe-CuI -78 "C Scheme 10 Alkyne boranes (15) couple with allylic bromides in presence of cuprous iodide almost entirely at the a-position to give the required enynes (16) in 70% yie1d.j' Only small amounts of the allenic isomers are formed.Other workers4' report exactly the opposite when the coupling reaction is carried out in the absence of CuI. Attack occurs cleanly at the y-position to give the allenic isomers (17). These borates (or corresponding alanates) also react at the y-position with COz to give an allenic acid and with a ketone to give an allenic alcohol. Nickel(I1) acetylacetonate has been found4' to be an efficient catalyst for the dimerization of the anions of aryl allyl sulphones R2C=CHCHLiS02Ph to form trienes of the type R2C=CHCH=CHCH=CR2 in good yield (ca.80%) but with no stereochemical control. The mechanism may involve oxidative dimeriz- ation of the sulphone with subsequent desulphonylation. Another interesting appli- 37 S. Tsuboi T. Masuda and A. Takeda J. Org. Chem. 1982.47,4478. 38 K. Mach F. Turecek H. Antropiusova L. Petrusova and V. Hanus Synthesis 1982. 53. 39 S. Hara Y. Satoh and A. Suzuki Chem. Lett.. 1982 1289. 40 N. R. Pearson G. Hahn and G. Zweifel J. Org. Chem. 1982,47,3364. 41 M. Julia and J. N. Verpreux Tetrahedron Lett. 1982 23 2457. 144 D. F. Ewing cation42 of sulphone chemistry is shown in Scheme 11. The addition to (19) occurs exclusively across the 1,4 positions but not with total stereocontrol (two isomers in ratio 7 to 3).Alkenylcuprates are also effective in addition leading to a tetraene after sulphur extrusion. These polyenes [and the analogous isomers formed from (l8)l are valuable perfume additives. A to E,Z-1,Sdienes involving flash pyrolysis of a bicyclic sulphone is not likely to be of wide application. Reagents i Bu'OK-DMSO; ii Ally1 Br-DMSO; iii I,; iv R,CuLi-Et,O; v KOH-Bu'OH Scheme 11 Arylallenes are conveniently obtained by the reaction of an aryl halide with an allenyl-lithium derivative in presence of a palladium(0) complex.44 Only two examples are described but the yields are high (78% and 9OOh). Coupling with vinyl halides is also described. t -Butyl silver reacts with 2,4-diynylsulphinates such as HCsCCrCCR'R20S(0)Me to give alkylation at position 5 with resulting transmutation to 'a tetraene such as Me3CCH=C=C=C=CR'R2.The yields of these cumulene molecules are good (60 to 80%) but other alkyl silver derivatives are inactive as are also the analogous sulphonate substrate 4 Alkynes Synthesis.-A has appeared recently entitled 'Synthesis of Acetylenes Allenes and Cumulenes A Laboratory Manual'. This book is a superb collection of information concerning the practical preparation of the title compounds pre- sented in the style of Organic Syntheses. Over 200 procedures are described in detail and this is an indispensable (if expensive) volume for workers in this area. 42 F. Naf R. Decorzant and S. D. Escher Tetrahedron Left. 1982,23 5043. 43 J. 1. Cadogan C. M. Buchan I. Gosney B.J. Harnill and L. M. McLaughlin I. Chem. Soc. Chem. Commun. 1982,325. 44 T. Jeffrey-Luong and G. Linstrumelle Synthesis 1982 738. " E. A. Oostveen C. J. Elsevier J. Meijer and P. Vermeer J. Org. Chem. 1982,47,371. 46 L Synthesis of Acetylenes Allenes and Cumulenes A Laboratory Manual,' ed. L. Brandsrna and H. D. Verkruijsse Elsevier Scientific Publishing Company Amsterdam 1981. Aliphatic Compounds -Part (i) Hydrocarbons 145 The preparation of acetylenes with tertiary alkyl substituents cannot be achieved by the standard reaction of metal alkynide with the appropriate alkyl halides. However tertiary halides do react with silylalkynes in presence of AlCl to give addition followed by loss of silyl halide. Compounds such as di-( 1-adamanty1)- acetylene can be obtained4’ by this method.One drawback to the formation of alkynes by elimination of hydrogen halide is the need for a very strong base such as alkoxide or metal amide. Hence the discovery that elimination from the allylic alcohol ArOCH2CH=C(Br)CH20H only requires treatment with potassium car- bonate in refluxing methyl ethyl ketone is most intere~ting.~’ The essential presence of the allylic oxygen atom suggests that anchimeric assistance may be involved. It is not clear whether the presence of the aryloxy group is important. A thorough study has appeared49 of the reaction between (Me0)2P(0)CHN2 and aldehydes or ketones. The carbonyl group in R’R2C0 is thought to be converted to a diazoethene R’R2C=CN2 which eliminates nitrogen to give a carbene.Rearrangement affords an alkyne R1C=CR2. The metathesis of functionalized aryl acetylenes has been studied” using Mo(CO),-4-C1C6H4OH as a homogeneous catalyst with variable results. A rather better catalyst” is M00~(acac)~-AlEt,-PhOH which efficiently gives the statistical distribution of the product alkynes with negligible isomerization of the triple bond. Functional groups which are tolerated include double bond halogen and ester. Reactions.-Catalytic hydrogenation of the alkyne bond in RC6H4CECC6H4R over Raney Ni occurs readily when the pura-substituent is H Me or Et. However with R = Pr’ or But the triple bond is not reduced.” Calculation on model com- pounds indicate that the CEC moiety would lie at 0.1125 nm from the metal surface and hence this must represent a distance greater than the maximum for efficient hydrogenation.The fact that di-t-butylacetylene is readily hydrogenated suggests that the extended conjugated system in the diarylacetylenes may prevent bending of the molecule to place the triple bond closer to the surface. A novel addition reaction of alkynes is shown in Scheme 12. Arylselenosulphon- ates add to aryl and alkyl substituted acetylenes to afford P-(phenylse1eno)vinylsul-phones in 52 to 86% yield.’ The mechanism is thought to involve free radicals. Alkynylsulphones are readily formed by oxidative elimination. Further detailed studys4 of the stereochemical course of the acetoxymercuration of alkynes (Scheme 12) reveals that an antarafacial mode of addition operates for hex-3-yne to give the E-adduct in high yield.This was unequivocably established by X-ray structure analysis. In contrast stilbene reacts in a suprafacial mode to give the 2-adduct. The conventional procedures for hydration of a triple bond are relatively unselective between terminal and internal positions. The reagent PhHgOH has been found’’ to show a high reactivity with terminal triple bonds forming a phenylmercury 47 G. Capozzi G. Romeo and F. Marcuzzi J. Chem. SOC.,Chem. Commun. 1982,959. 48 P. F. Schuda and M. R. Heimann J. Org. Chem. 1982,47,2484. 49 J. C. Gilbert and U. Weerasooriya I. Org.Chem. 1982 47 1837. 50 D. Villemin and P. Cadiot Tetrahedron Lett. 1982,23 5139. ’’ M. Petit A. Mortreux and F. Petit J. Chem. SOC.,Chem. Commun.1982 1385. 52 G. Y. Han P. F. Han J. Perkins and H. C. McBay J. Org. Chem. 1981,46,4695. 53 T. Miura and M. Kobayashi J. Chem. SOC.,Chem. Commun. 1982,438. 54 R. D. Bach R. A Woodward T. J. Anderson and M. D. Glick J. Org. Chem. 1982,41 3707. 55 V. Janout and S. L. Regen J. Org. Chem. 1982 47,3331. 146 D. F. Ewing Reagents i ArS0,SePh; ii. excess 30% H202, 70 "C; iii Hg(OAc),-HOAc 25 "C Scheme 12 acetylide which is easily hydrolysed to give a methyl ketone. Internal triple bonds are totally unreactive to this reagent as are a wide range of other groups such as acetal thioacetal lactone alkene epoxide and secondary bromide. Apart from derivatives of titanocene very few titanium complexes are used in organic chemistry. The reagent Ti(OCHMe2)4 has been to generate propar- gylic titanium complexes (20)that show remarkable variations in the regiochemistry of the reaction with aldehydes (Scheme 13).For methyl acetylene complexes (20 R2= H) the electrophile attacks y-carbon exclusively to give an allenic alcohol but if R2 = Me in (20) the selectivity is completely reversed leading to formation of the acetylenic al~ohol.~' This (presumably) sterically controlled regioselectivity is better than that observed for propargylic salts with Li Mg or Zn species and may be of importance in the synthesis of useful alkynes such as those discussed above. R'C=CCHR2CHR30H + R'C=C=CHR~ I CHR30H Reagents i. Me,CLi 0 "C;ii Ti(OCHMe,), -78 "C; iii R'CHO Scheme 13 Two new reagents have been disc~vered~'.~~ which generate 1-alkynyl phenyl- selenides in high yield under mild conditions.Phenylselenocyanate reacts with terminal alkynes at room temperature in presence of CuCN and triethylamine but 56 M. Ishiguro N. Ikeda and H. Yamamoto J. Org. Chem. 1982,47,2225. '' T. Hayama S. Tomoda Y. Takeuchi and Y. Nomura Chem. Lett. 1982,1249. 58 S. Tomoda Y. Takeuchi and Y. Nomura Chem. Lett. 1982,252. Aliphatic Compounds -Part (i) Hydrocarbons even more convenient is the analogous reaction with benzeneselenenyl nitrite formed in situ from PhSeBr and AgN02. Brief mention is made of a hydroformyla- tion reaction of alkynes using Rh4(C0),* as catalyst.59 Although the conversion rate is high (ca. 100%)the yield of a,&unsaturated ketones is very variable.This reaction requires further development. *’ T.Mise P.Hong and H. Yamazaki Chem. Lett. 1982,401.