Magnesium-catalyzed hydrosilylation of α,β-unsaturated esters

To(M)MgHB(C(6)F(5))(3) (1, To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of To(M)MgMe, PhSiH(3), and B(C(6)F(5))(3). Unlike the transient To(M)MgH formed from the reaction of To(M)MgMe and PhSiH(3), the borate adduct 1 persists in solution and in the solid state. Crystallographic characterization reveals tripodal coordination of the HB(C(6)F(5))(3) moiety to the six-coordinate magnesium center with a ∠Mg–H–B of 141(3)°. The pathway for formation of 1 is proposed to involve the reaction of To(M)MgMe and a PhSiH(3)/B(C(6)F(5))(3) adduct because the other possible intermediates, To(M)MgH and To(M)MgMeB(C(6)F(5))(3), react to give an intractable black solid and To(M)MgC(6)F(5), respectively. Under catalytic conditions, silyl ketene acetals are isolated in high yield from the addition of hydrosilanes to α,β-unsaturated esters with 1 as the catalyst.