Saturated and Unsaturated Hydrocarbons

BY K. COOPER

1 Saturated Hydrocarbons

Although the reduction of halides with lithium aluminium hydride has been known ever since the discovery of the latter, reductions can be slow and the yields can be poor, especially with aromatic halides. Two improved methods of using the reagent have been reported in 1982. By using a clear solution of lithium aluminium hydride in THF, the yields are greatly improved, and by sonicating the reaction mixture, aromatic bromides or iodides are reduced quickly, in excellent yields (70 — 99%).

The radical chain reduction of thioformates, which are generated from tertiary alcohols, with tri-n-butyltin hydride has been reported by Barton and co-workers, and the mildness of tri-n-butyltin hydride as a reducing agent has been nicely demonstrated by the conversion of the bromo-endoperoxide (1) into the endoperoxide (2). The known reduction of nitro-compounds to alkanes, using tri-n-butyltin hydride, has been utilized in the synthesis of the phenylalkyl sulphoxide (6). Thus, Michael addition of the nitroalkane (3) to the αβ-unsaturated sulphoxide (4) furnishes the intermediate nitro-sulphoxide (5), which cleanly gives the sulphoxide (6) in high overall yield ( > 60%).

The combination of tetrakis(triphenylphosphine)palladium and lithium triethylborohydride is an effective reagent for the removal of allylic ethers, sulphides, sulphoxides, sulphones, and silyl ethers, and the stereochemical integrity of the double-bond is maintained. A preliminary report has appeared on the use of tetra-alkylammonium graphite lamellar compounds as electron reservoirs, and thus as reagents for reductive dehalogenation. The graphite compound is built up by electrochemical means, and on completion of the reduction it may be filtered off and re-used. Enol triflates, which are readily prepared from ketones, undergo rapid hydrogenolysis to give the corresponding alkanes in high yield (65 — 90%), and the transfer hydrogenation of aromatic nitriles (using ammonium formate as the hydrogen source, with palladium on charcoal as the catalyst) gives the corresponding methyl compounds (Scheme 1). A range of additional functionality is tolerated in the aromatic ring, but the reaction fails for aliphatic nitrites.

The generation of uranium trichloride in situ, by the reaction of uranium tetrachloride with lithium hydride or lithium aluminium hydride, catalyses the reduction of alkenes by lithium hydride or lithium aluminium hydride, but the reaction is at present limited to simple olefins. Nakao has reported that the supported catalyst that is made by immobilizing colloidal nickel boride on magnesium hydroxide exhibits much higher activity for hydrogenation of olefins than the previously reported sol-type catalyst. The isoprenoid alkane (8) appears in a variety of marine surface sediments, and its structure has been confirmed by total synthesis, employing the straightforward hydrogenation of the olefin (7) (Scheme 2). Hydrogenation of allylic and homoallylic alcohols with asymmetric induction can be achieved when chelate bis-phosphine rhodium complexes are used (Scheme 3). The asymmetry can be rationalized by using a model where the non-bonded interactions that are experienced by the methyl group in the transition-state are minimized. The combination of hydroxylamine and ethyl acetate generates di-imide in a straightforward fashion, and affords a new method of performing reductions with di-imide.

Alkyl methyl ketones are alkylated reductively with optically active isobutyl-aluminium dichloride in benzene to give optically active phenylalkanes, although the enantiomeric excesses are low (Scheme 4); the reductive vinylation of carbonyl compounds has been accomplished by addition of β-trimethylsilyl-ethyl-lithium to the carbonyl compound (9) followed by protodesilylation of the intermediate (10). The palladium-promoted alkylation of some alkenes, using a chiral ligand and a racemic nucleophile or a chiral sulphoxide, leads to alkanes after reductive cleavage of the intermediate σ-complexes, as outlined in Scheme 5. The chemical yields of products were poor (20 — 30%) and the optical purities were only moderate (up to 40%).

Diferrocenyl- and diaryl-carbinols (11) are reductively coupled when their corresponding lithio-derivatives are treated with titanium trichloride, giving high yields (70 — 80%) of the ethanes (12), whereas electroreductive dimerization of phenyl bromoacetic esters gives mixtures of meso- and (R,S)-succinates in moderate yield (50%) (Scheme 6). α, ω-Di-iodides (13) react with t-butyl-lithium at low temperature to give the cycloalkanes (14) in excellent yields (85 — 98%), and the reaction can be viewed as an initial metal–halogen exchange followed by a Wurtz coupling. Although the corresponding dibromides do not react cleanly, the first synthesis of [1,1,1]propellane (16) has been accomplished, by treatment of the dibromide (15) with t-butyl-lithium; the propellane is a fairly stable compound, with a t1/2 of 5 minutes at 114 °C.

2 Olefinic Hydrocarbons

Methylenetriphenylphosphorane has found wide use in the synthesis of terminal olefins but gives low yields when applied to hindered ketones. Corey and Kang have now shown that the phosphorane is deprotonated to give the lithio-derivative (17), which is a highly reactive ylide reagent, reacting with hindered ketones, epoxides, and aldehydes as outlined in Scheme 7. The Wittig reaction can be carried out by a solid-liquid transfer process, giving high yields of alkenes (70 — 95 %) (Scheme 8), and the anions of Wittig reagents (18) are trapped with methyl chloroformate to give the stable Wittig equivalents (19). By heating (19) in the presence of aldehydes, the corresponding olefins (20) are formed in high yield (70 — 80%); ylides or carbonyl compounds with α-hydrogen atoms, however, cannot be used in this method.

Phosphinothioic amides are highly effective reagents for the alkylidenation of ketones, and the method has been developed as a means of methylenation coupled with optical resolution. The reaction of the anion of the phosphinothioic amide (21) gives the diastereoisomeric mixture (22), which, after separation, can be converted into the optically active olefins (23). The method has been applied to the synthesis of the (+)- and (-)-iridoid monoterpene hop ether (24), and can be extended to alkylidenation with resolution.

The reaction of alkenyldi-isobutylalane with titanocene dichloride gives dimetalloalkanes...