Catalytic hydroboration of aldehydes and ketones with an electron-rich acyclic metallasilylene.

The application of main group metal complexes in catalytic reactions is of increasing interest. Here we show that the electron-rich, acyclic metallasilylene L'(Cl)GaSiL C (L' = HC[C(Me)NDipp] 2 , Dipp = 2,6- i Pr 2 C 6 H 3 ; L = PhC(N t Bu) 2 ) acts as a precatalyst in the hydroboration of aldehydes with HBPin. Mechanistic studies with iso-valeraldehyde show that silylene C first reacts with the aldehyde with [2 + 1] cycloaddition in an oxidative addition to the oxasilirane 1, followed by formation of the alkoxysilylene LSiOCH[Ga(Cl)L']CH 2 CHMe 2 (2), whose formation formally results from a reductive elimination reaction at the Si center. Alkoxysilylene 2 represents the active hydroboration catalyst and shows the highest catalytic activity with n -hexanal (reaction time: 40 min, yield: >99%, TOF = 150 h -1 ) at room temperature with a catalytic load of only 1 mol%. Furthermore, the hydroboration reaction catalysed by alkoxysilylene 2 is a living reaction with good chemoselectivity. Quantum chemical calculations not only provide mechanistic insights into the formation of alkoxysilylene 2 but also show that two completely different hydroboration mechanisms are possible.