| Abstract: | The mechanism of the Barton-McCombie reaction was studied by low temperature 119Sn NMR showing that tin radicals attack the thiocarbonyl sulfur instead of the sulfide sulfur in the deoxygenation of S-methyl xanthates. Various secondary and primary alcohols can be deoxygenated via their xanthates or thionocarbonates using either Et3B/O2 at room temperature, or AIBN in refluxing toluene. This procedure was applied in the deoxygenation of primary neopentyl alcohols, the most difficult example of the deoxygenation, giving high yields of the corresponding deoxy products. Low temperature 29Ni NMR studies show that the deoxygenation reaction with diphenylsilane follows the same pathway found for the tri-n-butyltin hydride reactions. Bis-xanthates, formed from vic-diols, can easily be transformed in high yields to the corresponding olefins with diphenylsilane in the presence of AIBN or benzoyl peroxide. This system can also be used in dehalogenation of alkyl halides. Phenylsilane and triphenylsilane are also good hydrogen sources in these radical reactions. Triethylsilane, which has strong Si-H bond, can be the hydrogen source in radical reactions provided that effective initiation is maintained by the portionwise addition of benzoyl peroxide. Yields of the deoxygenation of various primary and secondary alcohols are in the range of 82-100%, and the reaction conditions are well tolerated by various natural products. The same method can also be used to transform bis-xanthates into olefins. Dialkyl phosphites were also used as hydrogen sources in radical reduction reactions. These reactions, when initiated with benzoyl peroxide in refluxing dioxane or toluene, furnished the reduced products normally in higher than 90% yield. Dialkyl phosphites were also applicable for reduction of alkyl halides and phenylselenides. A series of alcohol thiocarbonyl derivatives was deoxygenated with the salt of hypophosphorous acid and a tertiary nitrogen base (e.g., triethylamine). The method was applicable to primary, secondary and tertiary alcohols. Alkyl halides also furnished the corresponding hydrocarbons in high yield. |
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