REGIOSPECIFIC AND DIASTEREOSELECTIVE C-H AND C-SI BOND ACTIVATION OF OMEGA-SILYL-SUBSTITUTED ALKANE NITRILES BY BARE CO-PHASE( CATIONS IN THE GAS)

The unimolecular decompositions of 5-methyl-7-trimethylsilylheptanenit rile (1) and 6-methyl-8-trimethylsilyloctanenitrile (2) complexed to ' 'bare'' Co+ cations have been investigated by means of tandem mass spe ctrometry. The Co+-mediated bond activations display a high degree of selectivity in that the major (>90%) neutral product formed is trimeth ylsilane. Regio- and diastereospecific labeling experiments were condu cted to gain further insight into the mechanistic details, in particul ar, the unique regio- and stereoselectivity of the losses of trimethyl silane from 1/Co+ and 2/Co+, respectively. The reaction follows a clea n 1,2-elimination process, in which the building blocks of the neutral HSiMe(3) stem exclusively from activation of a C-H bond in the (omega - 1) position and the Si-C bond. The observed regiospecificity can on ly be accounted for by invoking a metal-ion-mediated cooperation of th e SiMe(3) and the CN groups in the course of the multistep reaction se quence. The examination of diastereospecifically labeled isotopologues of 1 and 2 reveals a remarkably high diastereoselectivity for the des ilylation, and the steric effects (SE) clearly exceed the kinetic isot ope effects (KIE) associated with C-H and C-D bond activation, respect ively; i.e., KIE = 1.6 +/- 0.1 and SE = 4.1 +/- 0.2 for 1/Co+, and KIE = 1.5 +/- 0.1 and SE = 3.0 +/- 0.2 for 2/Co+. These unprecedented exa mples for stereoselective C-Si bond activations by Co+ in the gas phas e can be rationalized in terms of steric interactions in the different conformations of the metallacyclic intermediates.