Silyl radical inversion

Rate constants for the reaction of thiyl radicals with the t-BuMePhSiH were also extracted from the kinetic analysis of the thiol-catalysed radical-chain racemization of enantiomerically pure (S)-isomer [34]. Scheme 3.2 shows the reaction mechanism that involves the rapid inversion of silyl radicals together with reactions of interest. The values in cyclohexane solvent at 60 °C are collected in the last column of Table 3.5. 

Since 19951, to the best of our knowledge, there have appeared few papers detailing structural investigations of silyl radicals. Of those few, Matsumoto and coworkers investigated the isomerization of silyl radicals derived from 9,10-di-terf-butyl-9,10-dihydro-9, 10-disilaanthracenes (2)6. Irradiation of a di-terf-butyl peroxide (DTBP)/pentane solution of either cis-2 or trans-2 affords the same 81% cist 19% trans mixture of 2. In the absence of DTBP and irradiation, solution NMR studies indicate that each isomer of 2 is unchanged in the —85 to 20 °C temperature range. The authors propose that the radicals 3 derived from 2 isomerize to each other via inversion of the radical centre (equation 1) followed by hydrogen abstraction from the parent compound 2 (an identity reaction). 

Concerning stereochemistry, reactions at silicon usually proceed with high stereoselectivity, with either retention or inversion of configuration, and only rarely with racemization. Moreover, asymmetric silyl radicals and silyl anions show significant optical stability. 

Interestingly, the pyramidal silyl radical exhibits significant configurational stability. Inversion of the pyramidal species is slow enough to allow reactions in which the configuration at silicon is retained. 

These experiments offer good chemical evidence in support of the pyramidal structure of the silyl radical and its relatively slow rate of inversion. However a marked destabilization was observed for the chiral disilanyl radical 6 (eq. [3]) (19). 

Lithium aluminium hydride reduction, with inversion of the chlorosilane formed yielded the hydrosilane ([a]D + 20°). Assuming a neat inversion in the reduction step, it appeared that the formation and trapping of the silyl radical occurred with 65% retention. 

Stereoselectivity of radical addition is not limited to sulfonyl radicals. The (rani-addition has also been observed for tin, bromine, chlorine, and silyl radicals. Varying degrees of selectivity has been observed for addition of carbon-centered radicals, depending on the substituents size and effect on the inversion barrier. Because the importance of negative hyperconjugation decreases for radicals in comparison to the anions, the barriers for inversion decrease in parallel. As a result, the selectivity can erode under conditions when trapping of the radical is slower than the equilibration, e.g. in the thiol-yne click reaction, which often provide a mixture of E and Z-vinyl sulfides. ... 

---