beta-Silyl effects on the stabilities of carbanions and carbon-centered radicals derived from ethyl thionacetates, diethyl malonates, and ethyl acetoacetates

The effects of an alpha-Me(3)SiCH(2) group on the equilibrium acidities in DMSO of the acidic C-H bonds in esters, including ethyl thionacetate, diethyl malonate, and ethyl acetoacetate, were found to differ from that of an alpha-MeCH(2) group by less than 1 kcal/mol, pointing to little or no stabilizing effect of a beta-Me(3)Si group on carbanion stabilities. On the other hand, substitution of an alpha-Me(3)SiCH(2) group for one of the acidic hydrogen atoms in these esters to give Me(3)SiCH(2)CH(2)C(=S)OEt, Me(3)SiCH(2)CH(CO(2)Et)(2), and Me(3)SiCH(2)CH(CO(2)Et)COCH3 was found to decrease the homolytic bond dissociation energies (BDEs) of the remaining acidic C-H bonds by 7.1, 8.2, and 7.1 kcal/mol, respectively, relative to those of the parent esters and by 2.7, 4.5, and 4.4 kcal/mol, respectively, relative to the BDEs of the acidic C-H bonds in the corresponding MeCH(2)-substituted esters. The differences in BDEs observed between the Me(3)SiCH(2)- and the MeCH(2)-substituted ester derivatives are indicative of the presence of 3-5 kcal/mol C-H bond stabilizing effects of beta-Me(3)Si groups, relative to those of beta-Me groups, on the corresponding carbon radicals. The BDE value of the acidic C-H bond in Me3(C)CH(2)CH(2)C(=S)OEt was found to be 86.4 kcal/mol, which represents a 0.6 kcal/mol increase in BDE relative to that of the acidic C-H bond in MeCH(2)CH(2)C(=S)OEt. This result demonstrates the absence of a stabilizing effect of a beta-tert-butyl group on an open-chain carbon-centered radical. This is contrary to an apparent 6.4 kcal/mol C-H bond stabilizing effect of the beta-tert-butyl group in 9-neopentylfluorene, relative to that in fluorene, which is attributed to the relief of steric strain in forming the radical.