Experimental and Computational NMR Spectroscopic Investigation of Silyl-Substituted Carbocations

Organosilicon compounds are widely used in organic synthesis. The understanding of the structure and properties of the intermediates involved in their reactions is a prerequisite for further development and optimization of useful synthetic transformations involving silicon substituted compounds. Trialkylsilyl-substituted carbocations are particularly important as reactive intermediates in chemical reactions in polar media. Silicon substituted carbocations are however not generally accessible in superacid solutions. Due to the high affinity of silicon for fluorine and oxygen, the facile formation of five-coordinated Si-intermediates or transition states, and the polar nature of the carbon silicon bond (C(delta-)-Si(delta+)), the C-Si bond is prone to easy cleavage (1). Recently we have been successful in observing trialkylsilyl substituted carbocations as non-transient species in solution using suitable experimental methodology such as matrix co-condensation techniques and carefully controlled experimental conditions (2). A silyl substituent in the vicinity of a positively charged carbon interacts differently depending on the number of intervening bonds and the spatial arrangement (3). The effect of a silyl group directly attached to the C(+) carbon is called a-effect, a silicon two bonds away can give rise to a beta-effect and silyl substituents separated by three bonds from the formally positively charged carbon may exhibit a gamma-effect. A summary of our recent NMR spectroscopic investigations involving representative examples of alpha, beta- and gamma-silyl substituted carbocations together with parallel computational studies of their structures and NMR properties is presented.