β-Silyl Effects on the Stabilities of Carbanions and Carbon-Centered Radicals Derived from Ethyl Thionacetates, Diethyl Malonates, and Ethyl Acetoacetates

The effects of an α-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 α-MeCH 2 group by less than 1 kcal/mol, pointing to little or no stabilizing effect of a β-Me 3 Si group on carbanion stabilities. On the other hand, substitution of an α-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)COCH 3 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 β-Me 3 Si groups, relative to those of β-Me groups, on the corresponding carbon radicals. The BDE value of the acidic C−H bond in Me 3 CCH 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 β- 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 β- tert -butyl group in 9-neopentylfluorene, relative to that in fluorene, which is attributed to the relief of steric strain in forming the radical.