Hydrogen abstraction reactions, reversibility

FIGURE 72. Calculated transition states at MP2 of the hydrogen abstraction reactions of trialkyl-stannane with hydrogen and alkyl radicals which are shown in Figure 71. Bond distances are in A, angles in deg. The calculated activation energies (kcal mol 1) at QCISD//MP2 (MP2 values in parentheses) refer to the forward reaction (A /i 1 i ) and the reverse reaction (A /i2 ), respectively. Reproduced by permission of The Royal Society of Chemistry from Reference 191... 

Cantrell, T. S., Photochemical reactions of benzoic acid cycloaddition, hydrogen abstraction and reverse type II elimination, /. Am. Chem. Soc., 95, 2714, 1973. 

To test this hypothesis, a-naphthaldehyde, which is apparently inert toward photoreduction, was irradiated in the presence of optically active 2-octanol. If a reversible hydrogen abstraction were to occur, a loss of optical activity in the 2-octanol should result. The results showed no loss in optical activity thus the question of a reversible reaction has been answered/33 ... 

We saw earlier that when benzophenone is photoreduced in the presence of optically active 2-butanol, the alcohol recovered from the reaction loses no optical activity/541 This was presented as evidence that there could be no appreciable reversibility of the initial hydrogen abstraction since this should lead to racemization of the unreacted alcohol. However, if one uses relabeled benzhydrol and examines the initially produced benzpinacol for the presence of the label, one finds that the product pinacol contains no 14C. This would indicate that there must be some type of rapid transfer of the hydrogen radical from the ketyl radical produced upon abstraction from benzhydrol,... 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

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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