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This study addressed the mechanism of the recently reported Lewis acid-catalyzed manifold for additions of allylboronates to aldehydes. A series of control experiments using various reagents and conditions, and kinetic studies by low-temperature NMR spectroscopy, were performed to investigate the origin of the activation provided by the best catalyst, Sc(OTf)3. The results obtained are strongly supportive of a mechanism involving electrophilic boron activation by coordination of the metal ion to one of the boronate oxygens in a closed bimolecular transition state.
Rauniyar et al. (Tue,) studied this question.