Saturated and Unsaturated Hydrocarbons

BY A.R. HOWELL

1 Saturated Hydrocarbons

A variety of new methods for the deoxygenation of alcohols has appeared. Thus, treatment of diaryl methanols with a mixture of dichloromethylsilane and sodium iodide in either acetonitrile or a mixture of dichloromethane and acetone produces diarylmethanes rapidly and in high yields. In addition, diaryl or aryl alkyl carbinols are deoxygenated selectively in the presence of other reducible functional groups (such as hydroxyl or ester) by the action of boron trifluoride etherate and triethylsilane. Tertiary alcohols can be converted into the corresponding alkanes by a two step sequence involving heating a toluene solution of the alcohol with washed Raney nickel, which leads to an alkene/alkane mixture, followed by hydrogenation. Several functional groups, e.g. ethoxylethyl ethers, epoxides, and olefins, do not tolerate the reaction conditions.

Radical deoxygenations of secondary alcohols can be accomplished by the reaction of their dithiocarbonate derivatives with n-Bu3SnH-Et3B. The chemoselective reductive cleavage of allylic acetates of 1,2- and 2,3-unsaturated monosaccharides has been realised by a three component reducing system comprised of diphenylsilane, a soluble palladium(0) catalyst and catalytic amounts of zinc chloride. Hydride substitution proceeds with absolute inversion of configuration (Scheme 1).

Tris(trimethylsilyl)silane reduces alkyl and benzyl chlorides, bromides and iodides in a most effective manner. The method rivals tributyltin hydride in efficiency and is a superior reagent from ecological and practical perspectives. Reductive deselenisation can be performed rapidly, conveniently and in high yield with nickel boride, which is generated in situ by adding sodium borohydride to a tetrahydrofuran solution of nickel chloride hexahydrate.

Selective hydrogenations of carbon-carbon double bonds can be achieved by the simultaneous addition of the substrate alkene and trimethylsilyl chloride or water to Nickel Complex Reducing Agents (NiCRA). The less substituted double bond is preferentially reduced in dienes, and carbonyl, ester or acid moieties are untouched.

An interesting extension of the stereoselective reductions produced by Bakers yeast has been reported. Thus, (E)-2-methyl- and (E)-3-methyl-2,4-pentadien-1-ols are reduced to (S)-2-methyl- and (S)-3-methyl-4-penten-1-ols, respectively, which are useful precursors to bifunctional and enantiomerically pure C6-building blocks, as illustrated in Scheme 2.

2 Olefinic Hydrocarbons

Alkenes are produced in high yield from primary and secondary alcohols by their reaction with 1,1,1-trichloro-3,3,3-trifluoroacetone and a catalytic amount of para-toluenesulphonic acid. Dehydration of 2-octanol gives exclusively trans 2-octene.

Epoxides are converted cleanly to olefins by the action of magnesium reduced titanocene dichloride. trans-Epoxides lead exclusively to trans-alkenes, while cis-epoxides furnish predominantly cis-alkenes. The reduction of epoxides with concomitant alkylation has been accomplished by reaction with lithium tetraalkylcerate. Styrene oxide gives, chiefly, terminal olefins, while alkyl substituted ethylene oxide predominantly affords internal olefins (Scheme 3).

Olefins can be prepared from vicinal dibromides, using sodium O,O-diethylphosphite in the presence of catalytic tellurium. Unlike other reponed tellurium promoted debrominations, this method proceeds at room temperature. The reaction takes place with high anti-stereoselectivity.

Beckmann fragmentation, rather than simple rearrangement, has been observed for ketoximes having α-substitutents (Y) which can stabilise intermediary carbocations (Scheme 4). The control of the stereo- and regiochemistry of the resulting double bond has been difficult. For cyclic ketoximes a solution utilising silicon-directed Beckmann fragmentation has been reported. With a trimethylsilyl group on the β-carbon of the ketoxime, complete regio- and stereoselective double bond formation can be realised, as illustrated in Scheme 5. The methodology has been employed in the synthesis of pheromones.

The much studied Wittig reaction continues to receive attention. In an extension of studies on the semistabilised allylic phosphorus ylides, Tamura et al. have looked at the scope and limitations of these ylides. It was shown that sterically crowded β,γ-disubstituted allylic tributylphosphorus ylides afford E-olefins with high stereoselectivity (>92%). As the steric demand of the ylides decreased, bulky aldehydes were required for E-selectivity. Z-Selectivity resulted when allylic triphenylphosphorus ylides and tertiary aldehydes were employed. Benzylphosphonium ylides, which are also semi-stabilised, produce largely E-olefins upon reaction with aldehydes.

Optically active phosphonates have been utilised to improve the stereoselectivity of exocyclic double bond formation in prostacyclin analogues. The (-)-8-phenyl-menthyl phosphonoacetate (1a) improves the ratio of products (2) and (3) from 1:1 (no chiral auxiliary) to 86:14. The enantiomeric phosphonate (1b) gives the same products in an inverse ratio (15:85) (Scheme 6).

The Wittig-Horner reaction can be used to prepare α-labelled functional olefins (%D>95%). This is accomplished by running the reaction in the presence of a 6M K2CO3-deuterium oxide solution.

The electrolysis of a-substituted phosphonates on Pt or glassy carbon cathodes proceeds with cleavage of the activated C-H bond. The resulting carbanion then reacts further with carbonyl compounds to give olefins in a satisfactory yield.

A new method for carbon-carbon double bond formation, promoted by tri-n-butylphosphine and zinc powder, has been reported. Heating an equivalent amount of an aldehyde, bromoacetic ester, tri-n-butylphosphine and zinc powder (or a catalytic amount) at ~100°C results in exclusive formation of E-olefins in good yield. The procedure is much simpler than the associated Wittig reaction and requires no base or solvent.

Cohen has disclosed an improved, "one-pot" procedure for the preparation of alkylidene- and allylidenecyclopropanes from α-lithio(cyclopropyl)silanes (Scheme 7). The allylidene cyclopropanes...