Magnesium Catalysis for the Hydroboration of Carbodiimides

A beta-diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}(2)}MgnBu] (Dipp=2,6-iPr(2)C(6)H(3)), was shown to be an effective pre-catalyst for the first reported catalytic hydroboration of alkyl-and aryl-substituted carbodiimides with pinacol borane (HBpin). The catalytic reactions proceed under mild conditions to afford the corresponding N-borylated formamidine compounds in good yields. The reactions were observed to proceed through the intermediacy of magnesium amidinate and formamidinatoborate intermediates and an example of one of these latter species has been structurally characterised by an X-ray diffraction analysis. Crucially, no formation of the N-boryl formamidine products was observed in the absence of additional equivalents of the carbodiimide and HBpin substrates. This observation, supported by the evolution of a sigmoidal kinetic profile for the hydroboration of dicyclohexylcarbodiimide, has been rationalised as the consequence of an allosteric effect of the pinacol borane and carbodiimide on the magnesium formamidinatoborate intermediates.