Unexpected Activation of Ethers and Polysiloxanes by a Phosphine-Borane-Stabilized 1,3-Dicarbanion/Butyllithium Couple

Abstract

The reaction between the tertiary phosphine-borane PhP(BH3)(CH2Ph)2 (6) and 1 equiv of nBuLi, PhCH2Na, or PhCH2K gives the monodeprotonation products [PhP(BH3)(CHPh)(CH2Ph)]MLn (MLn = Li(OEt2)0.25 (7), Na(THF)0.5 (8), K(OEt2)0.5 (9)). Similarly, the reaction between 6 and 2 equiv of either PhCH2Na or PhCH2K gives the 1,3-dicarbanion complexes [PhP(BH3)(CHPh)2][MLn]2 (MLn = Na(THF) (10), K(THF)0.25 (11)). However, the reaction between 6 and 2 equiv of nBuLi in Et2O, THF, or nBu2O gives a mixture of mono- and dimetalated products. Unexpectedly, the reaction between 6 and 3 equiv of nBuLi in diethyl ether in the presence of silicone grease gives the trinuclear cluster [PhP(BH3)(CHPh)2]Li3(OSiMe2CH2CH2CH2CH3)(OEt2)3 (12); this compound may be isolated in essentially quantitative yield from the reaction between 6, hexamethyltrisiloxane, and nBuLi in a 3:1:9 ratio. Similar tri-lithium complexes are isolated from the reaction between 6 and 3 equiv of nBuLi in Et2O or tBuOMe, which yields [PhP(BH3)(CHPh)2]Li3(OEt)(OEt2)3 (13) and [PhP(BH3)(CHPh)2]Li3(OMe)(OtBuMe)3 (14), respectively. The reaction between 6, 2 equiv of nBuLi, and 1 equiv of in situ generated LiOEt or LiOiPr gave the tri-lithium clusters 13 and [PhP(BH3)(CHPh)2]Li3(OiPr)(OEt2)3 (16), respectively.