Rate constants hydrogen abstraction, laser flash photolysis

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

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