Catalytic hydroboration of aldehydes with electron-rich alkoxysilylenes: from mechanistic insights to highly active catalysts.

Main group metal complexes receive increasing interest due to their potential technical application in catalytic reactions. Here we report the synthesis and characterisation of a series of alkoxysilylenes L1(Cl)GaCH(R)OSiL2 (L1 = HC[C(Me)NDipp]2, Dipp = 2,6- i Pr2C6H3, L2 = PhC(N t Bu)2; R = Et 3, Ph 4, 3-Br-C6H45, 4-hexyl-C6H46), whose steric and electronic properties were systematically varied, and their high activity in the catalytic hydroboration of aldehydes. This comparative study showed excellent TOF values of up to 98 000 h-1 at 25 °C with a very low catalyst loading of only 0.001 mol%. Mechanistic studies verified the catalytic reaction mechanism, which was further proven by the isolation and structural characterisation (sc-XRD) of oxasilirane L2Si[OC(H)C10H15]OC(H)C6H6Ga(Cl)L1 (7), the key reaction intermediate formed via [2 + 1] cycloaddition of alkoxysilylene 4 with 1-adamantanecarboxaldehyde at -30 °C. Quantum chemical calculations proved that the catalytic activity of the alkoxysilylenes L1(Cl)GaCH(R)OSiL2 is strongly affected by the steric demand of the alkoxy unit (R). In addition, the electronic nature of the aldehyde was found to control the rate-limiting step.