We next performed the radical group transfer reaction in deuterated solvents in a gas-tight NMR tube, to enable direct monitoring of conversion by H NMR spectroscopy. The reaction was performed in the presence and absence of methanol-d4 (Figure and 1b, respectively). As expected, due to the high reactivity of silylium ions towards nucleophiles, when methanol was present in the reaction mixture we observed rapid formation of product 5a with no detectable intermediates (Figure ). In pure acetonitrile, however, the reaction proceeded via an intermediate that could be clearly observed at the start of the reaction. This intermediate was gradually converted into the dimeric product 12a as the reaction progressed. Lewis bases, such as MeCN or Bu3N, are known to stabilize silylium ions by coordination. Thus, we postulate that the observed intermediate is the solvent-stabilized silylium ion, which is subsequently captured by trace amounts of water to produce the dimeric product 12a.