Reactivity of the triple ion and separated ion pair of tris(trimethylsilyl)methyllithium with aldehydes: A RINMR study

Low-temperature rapid-injection NMR (RINMR) experiments were performed on tris(trimethylsilyl) methyllithium. In THF/Me2O solutions, the separated ion (IS) reacted faster than can be measured at -130 degrees C with Mel and substituted benzaldehydes (k >= 2s(-1) whereas the contact ion (1C) dissociated to IS before reacting. Unexpectedly, the triple ion reacted faster with electron-rich benzaldehydes relative to electron-deficient ones. The addition of HMPA had no effect on the rate of reaction of the triple ion with p-diethylaminobenzaldehyde, and the immediate product of the reaction was the HMPA-solvated separated ion IS, with the Peterson product forming only slowly. Thus, the aldehyde is catalyzing the dissociation of the triple ion. HMPA greatly decelerated the reaction of IS (< 10(-10)), providing an estimate of the Lewis acid activating effect of a THF-solvated lithium cation in an organolithium addition to an aldehyde.