Metalation of Trimethylphosphine−Borane. Unexpected Reactivity of a Simple Phosphine−Borane-Stabilized Carbanion toward Siloxanes

The reaction between trimethylphosphine−borane (3) and 1 equiv of MeNa in diethyl ether gives the
polymeric complex [{Me2P(BH3)CH2}Na(THF)]∞ (5a) in good yield after recrystallization from THF. In
contrast, the reaction between 3 and 1 equiv of n-BuLi in diethyl ether gives the unusual complex
{Me2P(BH3)CHSiMe2OLi}4Li4(Et2O)2.75(THF)1.25 (6), which crystallizes as a cluster containing
[Me2P(BH3)CHSiMe2O]2- siloxy-functionalized carbanions formed via the reaction between the α-lithiated
intermediate {Me2P(BH3)CH2}Li (4) and silicone grease. Although the intermediate 4 could not be isolated,
reactions between in situ-generated 4 and sources of Me2SiO (Dow Corning silicone grease, silicone oil,
hexamethylcyclotrisiloxane (8)) proceed cleanly to give good yields of 6; no reaction was observed between
4 and hexamethyldisiloxane. Reactions between 4 and 8 generate half an equivalent of 3 per equivalent
of 4 consumed, and the cluster 6 reacts with excess siloxane to give 3 and unidentified products that
contain silicon, but not phosphorus; thus, the optimum ratio of 4 to 8 in these reactions is 6:1. Variable-temperature 1H, 7Li, 11B{1H}, and 31P{1H} NMR spectroscopy suggests that 6 is subject to dynamic
equilibria in solution that involve deaggregation of the cluster.