RADICAL CHAIN REDUCTION OF ALKYL-HALIDES, DIALKYL SULFIDES AND O-ALKYL S-METHYL DITHIOCARBONATES TO ALKANES BY TRIALKYLSILANES

• Published in: Journal of the Chemical Society, Perkin Transactions 1 no. 1; pp. 103 - 112
• Main Authors: COLE, SJ, KIRWAN, JN, ROBERTS, BP, WILLIS, CR
• Format: Journal Article
• Language: English
• Published: CAMBRIDGE Royal Soc Chemistry 01.01.1991; Royal Society of Chemistry
• Subjects: Chemistry; Chemistry, Organic; Exact sciences and technology; Free radicals chemistry; Organic chemistry; Physical Sciences; Reactivity and mechanisms; Science & Technology; SECONDARY ALCOHOLS; UNSATURATED-COMPOUNDS; ORGANIC-SYNTHESIS; TRIETHYLSILYL RADICALS; HYDROGEN-ATOM TRANSFER; DEOXYGENATION; ELECTRON-PARAMAGNETIC RESONANCE; ABSOLUTE RATE CONSTANTS; TRIBUTYLGERMANIUM HYDRIDE; ACTIVE OLEFINS; Chemical reduction; Chemical reactivity; Free radical reaction; Radical mechanism; Organic sulfide; Halogen Organic compounds
• ISSN: 0300-922X; 2050-8255; 1364-5463
• DOI: 10.1039/p19910000103

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.