Radical Chain Reduction Of Alkyl-Halides, Dialkyl Sulfides And O-Alkyl S-Methyl Dithiocarbonates To Alkanes By Trialkylsilanes

Saturated primary, secondary and tertiary alkyl halides RX (X = Cl, Br or I) are reduced to the corresponding alkanes RH in essentially quantitative yield by triethylsilane in refluxing hexane or cyclohexane in the presence of a suitable initiator and an alkanethiol catalyst. Reduction proceeds by a radical chain mechanism and the thiol acts as a polarity reversal catalyst which mediates hydrogen-atom transfer from the Si-H group of the silane to the alkyl radical R-center-dot. Triphenylsilanethiol and perfluorohexanesulphenyl chloride are also effective catalysts; the latter is probably reduced in situ to the corresponding fluorinated thiol. Other silanes R3SiH (R = Pr(n), Pr(i) or Ph) also bring about reduction. The silane-thiol couple therefore serves as a useful replacement for tributylstannane as a homolytic reducing agent for alkyl halides. Reduction of 6-bromohex-1-ene, to give a mixture of hex-1-ene and methylcyclopentane, is more sluggush than reduction of saturated halides and this is attributed to removal of the thiol catalyst by addition across the C = C bond. Ethyl 4-bromobutanoate is smoothyl reduced to ethyl butanoate without interference from the ester function. Dialkyl sulphides are reduced to alkanes by triethylsilane in a radical chain reaction, but the effect of added thiol depends on the nature of the S-alkyl groups in the sulphide. The trialkylsilane-thiol couple can also successfully replace trialkylstannane as the reducing agent in the Barton-McCombie deoxygenation of primary and secondary alcohols via their S-methyl dithiocarbonate (xanthate) esters. Good yields of deoxy compounds are obtained from octan-1-ol, octan-2-ol, octadecan-1-ol, 5-alpha-cholestan-3-beta-ol, cholesterol and 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose.