Saturated and Unsaturated Hydrocarbons

BY R.P.C. COUSINS

1 Saturated Hydrocarbons

The development of radical deoxygenation procedures for derivatives of alcohols, using tin and silicon hydrides, has continued. The elimination of tin-based reagents has been seen as an objective in this work, because of the associated toxicity and environmental hazards. Triethylsilane in the presence of benzoyl peroxide is a satisfactory alternative which deoxygenates suitably functionalised primary and secondary alcohols in high yields. Dimethyl or diethyl phosphite has also been used to replace tributyl tin hydride, but also requires benzoyl peroxide as an initiator.

The ability of tris(trimethylsilyl)silane to reduce a range of organic halides, selenides, xanthates and isocyanides to the corresponding hydrocarbons has been highlighted, whilst similar reductions using dimethyl phosphite have also been published. The electroreductions of organic halides to hydrocarbons in the presence of a catalytic amount of SmCl3 proceed in good yields.

Catalytic hydrogenations of unfunctionalised olefins using chiral organolanthanide complexes have been shown to proceed with enantiomeric excesses of up to 96%; however, organoyttrium complexes enable catalytic hydrogenations of terminal olefins in the presence of substituted double bonds (Scheme 1). Sodium or lithium hydrogen selenide has been used for the selective reduction of the olefinic linkage of α,β-unsaturated carbonyl compounds in the presence of a terminal double bond (Scheme 2).

A zirconium-catalysed ethylmagnesation reaction with alkenes has enabled regiocontrolled double alkylation or hydroxyalkylation to be achieved in good yield. (Scheme 3).

The trapping of a carbon radical generated during a radical-based decarboxylative procedure with vinylphenylsulphone leads to an adduct (1), which can then be reduced to give the homologue (2) or alkylated before reduction to afford the hydrocarbon derivative (3) has been described (Scheme 4).

Tertiary, secondary, and benzylic alcohols can be alkylated reductively using trialkylboron and trifluoromethanesulphonic acid.

2 Olefinic Hydrocarbons

Dixanthantes of vic-diols can be easily deoxygenated to the corresponding olefins using triethylsilane and benzoyl peroxide in a radical reaction, both in cyclic and acyclic systems. This transformation can also be accomplished with diphenylsilane and AIBN. The sulphone adduct (1) produced during a radical decarboxylation, was smoothly converted via the vinyl sulphone (4) with sodium telluride into the corresponding terminal olefin; the procedure constitutes an overall methylenation of the acid (Scheme 4).

Dicyclopentadienyldimethyltitanium, a reportedly more stable version of the Tebbe reagent, allows direct methylenation of aldonolactones in good yields. Comparison of the Tebbe and Wittig reagents for ketone methylenation has indicated better product yields with the Tebbe reagent, particularly when using hindered ketone substrates. It has also been noted that the non-basic medium of the Tebbe reagents prevents racemisation of sensitive chiral centres. Benzylidenations of carbonyl compounds using dibenzyltitanocene leading to moderate stereoselectivities have been reported (Scheme 5).

The zinc-zirconium heterobimetallic reagents (5), which can be prepared readily by hydrozirconation of alkenylzinc halides, react with aldehydes to produce E-disubstituted olefins with high stereoselectivity whereas ketones lead to a mixture of stereoisomers (Scheme 6). Cyclic α-iodoenones and triflyloxyenones react smoothly with alkenylzinc derivatives in the presence of a catalytic amount of a palladium-phosphine complex to afford α-alkenylenones. Conjugate reduction of the adducts then leads to the corresponding α-alkenyl ketones with retention of stereochemistry (Scheme 7). The 1,2-metallate rearrangement of lithiodihydrofuran and dihydropyran systems using Grignard reagents in place of organolithium reagents has been accomplished with copper (I) catalysis leading to γ- and δ-alkenols respectively with high stereoselectivity (Scheme 8). A tandem ring opening-elimination reaction of erythro- or threo-secondary tetrahydrofurfurylic acetates with TMSCl/NaI affords pure Z- or E-γ-iodoalkenes, respectively (Scheme 9).

A one-step, palladium-promoted intermolecular coupling reaction involving three different alkenes and enabling a quick entry into the prostaglandins from the readily available chiral alcohol (6) has now been extended to a number of analogues (Scheme 10).

Indium trialkyls have been used in cross coupling reactions with chloroalkenes and allylic chlorides to afford alkylated systems in good yields. It was noted in these studies that all three alkyl groups in the indium reagent were transferred, rather than one group as in the case of aluminium trialkyls; however, only moderate stereocontrol was achieved. Tetraorganoindates, which are prepared by the addition of organolithium reagents to trialkylindium, have been shown to react with allyl bromides in the α-position with retention of stereochemistry at room temperature.

A simple method for dehydrating tertiary alcohols to the corresponding thermodynamically most stable alkene has been described which uses boron trifluoride etherate in methylene chloride at 25°C (Scheme 11).

The stereocontrolled reduction of mono- and disubstituted alkenyl halides with tributyltin hydride in the presence of a catalytic amount of a Pd(O) complex has been developed and the method works efficiently for iodides at 25°C, but for bromides a temperature of 75°C was required (Scheme 12). The reductions of monosubstituted allyl chlorides using DIBAL, in the presence of a Pd(O) catalyst, are complete at low temperatures without competitive isomerisation (Scheme 13). The conversion of vicinal dibromides to alkenes can be achieved using aluminium foil in the presence of catalytic dicyclopentadienyltitanium chloride in high yield at room temperature.

Epoxides are deoxygenated to the corresponding alkenes through the use of a complex prepared from VCl3(THF)3 and zinc; however, the reaction with acyclic cis-epoxides was shown to proceed with partial inversion of...