ToMMgMe (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) is a precatalyst for addition reactions of pinacolborane (HBpin) and esters (RCO2R') resulting in C-O cleavage to give RCH2OBpin and R'OBpin. A number of functional groups are compatible with the catalyst system, including nitro, nitrile, halide, and thiophene. However, aldehydes and ketones are readily hydroborated under the catalytic conditions. The rate law for ester hydroboration, measured for ethyl acetate and isobutyl isobutyrate as -d[ester]/dt = k'[ToMMgMe]1[ester]1/2, rules out both ester/catalyst interactions (i.e. insertion) and HBpin/catalyst reactions (s-bond metathesis) as the turnover-limiting steps in the catalytic cycle. Interestingly, the rate law for ToMMg-catalyzed addition of isobutyraldehyde and HBpin reveals that neither insertion nor s-bond metathesis are rate-limiting. Instead, the turnover-limiting step is unimolecular and involves conversion of a species containing all three reactive components, namely ToMMgX, RCHO, and HBpin. This turnover-limiting step characterizes both aldehyde and ester hydroboration reactions.

Magnesium(II)-catalyzed hydroboration of carbonyl compounds

247th National Meeting (2014)

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