Stereoelectronic Effect of the Trimethylsilyl Substituent upon C‒O Bond Lengths at the β Position: Some Structural Studies

Abstract

Results of low-temperature (130 K) crystal structure analyses for seven β-trimethylsilyl-substituted cyclohexylnitrobenzoate esters are reported. For those molecules (three) with the Si-C and C-O bonds antiperiplanar the C-O bond lengths are increased by 0.014 Å av (Δ/σ min = 2.9) compared with that in the silicon-free analogue. For those molecules (four) with the Si-C and C-O bonds gauche no such systematic lengthening of the C-O bonds is observed. The result is in qualitative agreement with that [Δl ∝ cos2 (Si-C-C-O)] predicted from semiempirical MO calculations on a simple model complex and is attributed to the effects of interactions between the Si-C σ and C-O σ* orbitals. It is suggeste..