Saturated and Unsaturated Hydrocarbons

BY A.R. HOWELL AND S.P. KEELING

1 Saturated Hydrocarbons

New approaches to effect deoxygenations of alcohols and their derivatives, as well as improvements on existing protocols, have been disclosed. Thus, β-cyclodextrin promotes the hydrogenolysis of allylic alcohols to olefins using hydrogen and hydridopentacyanocobalt, which is generated in situ. The principal products are trans-internal olefins, but tertiary allylic alcohols and those possessing a trisubstituted double bond do not react. Allylic alcohols, as well as saturated and benzylic alcohols, can be deoxygenated with the tungsten (II) complex, WCl2(PMePh2)4. Reaction rates vary widely (<1min. - 3 weeks), and double bond migration is sometimes observed. Thiols are also reduced by the complex. Electron rich benzylic alcohols can be reduced to the corresponding hydrocarbons with sodium borohydride trifluoroacetic acid (TFA). It is crucial for good yields that the TFA is added slowly to a mixture of the substrate and sodium borohydride in tetrahydrofuran. Benzylic alcohols can also be deoxygenated by radical-induced reactions with lithium aluminium hydride. tris(Trimethylsilyl)silane, diphenylsilane, and triethylsilane have been reported as comparable, or superior, alternatives to tributyltin hydride in the Barton-McCombie reaction. In an interesting extension of Barton's observation of varying rates of Bu3SnH induced reduction between particular thiocarbonate ester derivatives of primary and secondary alcohols, Sekine and Nakanishi have shown that 3',5'-dioxynucleosides can be converted into their 3'-deoxy derivatives by selective reduction of the corresponding bis-phenoxythiocarbonyl derivative (1) (Scheme 1). Benzylic and allylic alcohols can be hydrogenolysed as their acrylate or cinnamate derivatives by the action of triethylsilane in the presence of Wilkinson's catalyst.

Chloro-, bromo- and iodoaromatic compounds are dehalogenated reductively in the presence of KOH/polyethylene glycol (400) in boiling xylene. Alkyl bromides and iodides, as well as alkyl isocyanides, selenides and xanthates, can be reduced in good to excellent yields with either tris(methylthio)silane or tris(isopropylthio)silane. A combination of a catalytic quantity of iodide and phosphorus acid in refluxing acetonitrile has been shown to dehalogenate a variety of 2-chloro- and 2-bromocarbonyl compounds in moderate to excellent yields (74-87%). α-Bromoketones can also be dehalogenated cleanly by the chemoselective action of di-n-butyltin clihydride.

Water soluble alkenes are converted in high yields into alkanes upon treatment with palladium (II) acetate and triethoxysilane. Alkynes can be selectively transformed into either alkanes or alkenes (predominantly Z), depending on the number of equivalents of silane. Selective platinum-catalysed hydrogenations of olefins in the presence of terminal alkynes are possible when the alkyne is silyl-protected. The selectivity seems to be largely based on steric effects, since alkyne reduction does occur once terminal olefin reductions are complete, and disubstituted olefins require bulkier silyl groups on the alkyne moiety. β-Cyclodextrin and polyethylene glycol (400) are useful phase-transfer agents for the hydridopentacyanocobaltate anion-catalysed hydrogenation of conjugated dienes to monoolefins. The apparent regioselectivity of this cerium or lanthanide promoted reaction varies with the substitution pattern on the diene.

Two potentially useful reductive deoxygenations of ketones have been disclosed. In one procedure, a tandem denitration-deoxygenation of α-nitroketones is effected in good to excellent yields via lithium aluminium hydride reduction of the corresponding (p-tolylsulfonyl)hydrazone derivatives. Although perhaps not widely applicable, the useful conversion of acyl N-protected pyrroles to alkyl pyrroles is accomplished using a borane-tert-butylamine complex in the presence of aluminium chloride.

Alkyl iodides undergo coupling reactions with the nickelacycle (2) in the presence of anhydrous manganese (II) iodide, resulting in the formation of β-substituted propionic acids in good yields (Scheme 2). Another interesting coupling process involves the homo-coupling of alkyl halides, in the presence of an activated form of zerovalent copper. Allyl and benzyl halides and, also, primary alkyl iodides undergo homocoupling in high yields. However, secondary and tertiary iodides and bromides give only moderate to low yields, where eliminations and reductive dehalogenations predominate. Cyclisations of α,ω-dihaloalkanes have also been found to be feasible. Moderate to high yields result for smaller rings (3 membered rings best) with substantially reduced yields with increasing chain lengths.

2 Olefinic Hydrocarbons

Three new approaches to the cis-selective semihydrogenation of alkynes have been reported. Thus, terminal alkynes are reduced at room temperature by the stable, readily prepared copper (I) hydride reagent, [(Ph3P)CuH]6; elevated temperatures are required for unactivated, internal alkynes. Regio- as well as stereospecific cis-reductions of a wide variety of acetylenic derivatives can be achieved in absolute ethanol with zinc powder activated with 1,2-dibromoethane or zinc powder treated with dibromoethane, followed by lithium bromocuprate. Greater selectivity is realised under the first set of conditions, and both methods have the advantage over poisoned palladium catalysts of experimental simplicity, as well as shortened reaction times. Low-valent Group V metal reagents, prepared from NbCl5 or TaCl5 and zinc in a mixed solvent system of dimethoxyethane (or THF) and benzene, also yield Z-alkenes from alkynes.

Olefins undergo smooth bond migration to their thermodynamically more stable form on treatment with easily prepared potassium fluoride impregnated alumina. In the examples cited, terminal alkenes were isomerised to internal alkenes, and exocyclic double bonds to endocyclic ones.

An interesting communication has shown that 1-phenylethanols dehydrate more efficiently in the solid state than in solution. The reactions proceed at room temperature in excellent yield (≥ 97%) in the presence of either gaseous HCl (reaction conducted in a dessicator) or Cl3CCO2H, the latter giving complete conversion in less than 5...