Magnesium-Mediated Nucleophilic Borylation of Carbonyl Electrophiles

The diboranate derivative, [HC{(Me)CNDipp}2Mg{pinBB(n-Bu)pin}], resulting from treatment of a β-diketiminato magnesium n-butyl derivative with the commercially available diborane, B2pin2, reacts as a source of the [Bpin] nucleophile with ketones and organic isocyanates. Reactions with benzophenone and 9-fluorenone afford products, which may be rationalized as enolate species resulting from kinetically controlled dearomatization rather than the thermodynamically preferred C-borylation of the electrophilic C═O unit provided by a previously described magnesium complex bearing a terminal [Bpin] nucleophile. The enolate derivatives react readily with further equivalents of the ketones to provide C–C coupling and tetra-alkoxyborate products via aldol condensation processes. In contrast to this divergent behavior, isocyanates with [HC{(Me)CNDipp}2Mg{pinBB(n-Bu)pin}] react like the terminal boryl species to yield the anticipated C-boryl amidate products. Although we have yet to identify the origin of this contrasting reactivity, these results indicate the potential of these systems, which are ostensibly both sources of identical [Bpin] nucleophiles, to provide divergent and highly selective access to kinetic or thermodynamic reaction products.