Carbon−Silicon Hyperconjugation and Strain-Enhanced Hyperconjugation: Structures of N-Methyl 2- and 4-tert-Butyldimethylsilylmethyl Pyridinium Cations

NBO analysis of the interaction of trialkylsilylmethyl substituents with the electron-deficient pyridinium cation suggests that hyperconjugation is strongest at the 2-position. Crystal structures of 2- and 4-tert-butyldimethylsilylmethyl-substituted N-methyl pyridinium cations 2d and 3d however do not reveal any significant structural differences arising from differences in hyperconjugation at the 2- and 4-positions. Previously observed differences in the triisopropylsilyl derivatives 2c and 3c are proposed to arise as a result of strain-enhanced hyperconjugation.