Lewis Acid-Mediated β-Hydride Abstraction Reactions of Divalent M(C(SiHMe2)3)2THF2 (M = Ca, Yb)

The divalent calcium and ytterbium compounds M(C(SiHMe2)(3))(2)THF2 contain beta-agostic SiH groups, as determined by spectroscopy and crystallography. Upon thermolysis, HC(SiHMe2)(3) is formed. However, the SiH groups are hydridic. The compounds M(C(SiHMe2)(3))(2)THF2 react with 1 and 2 equiv of the Lewis acid B(C6F5)(3) to form MC(SiHMe2)(3)HB(C6F5)(3))THF2 and M(HB(C6F5)(3))(2)THF2, respectively. These species contain the anion [HB(C6F5)(3)](-) from hydride abstraction rather than [(Me2HSi)(3)CB(C6F5)(3)](-) from alkyl abstraction. The 1,3-disitacyclobutane byproduct initially suggested beta-elimination [as the dimer of the silene Me2Si=C(SiHMe2)(2)], but the other products and reaction stoichiometry rule out that pathway. Additionally, Yb(C(SiHMe2)(3))(2)THF2 and the weak Lewis acid BPh3 react rapidly and also give the H-abstracted products. Despite the strong hydridic character of the SiH groups and the low-coordinate, Lewis acidic metal center in M(C(SiHMe2)(3)THF2 compounds, 8-elimination is not an observed reaction pathway.