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.