It has been widely reported that
the transition-metal catalysts
can invert the chemoselectivity of hydroboration of multifunctional
substrates, but the fundamental reasons for this inversion have rarely
been studied. In this work, mechanistic details and chemoselectivity
of the hydroboration of various E–C (E = CH, CH2, O, N, etc.) multiple bonds has been
explored through density functional theory calculations, including
reactions using free HBCy2 or captured HBCy2 by an N,O-chelated Ir(I) complex which includes a phosphoamidate
group. The computational results demonstrate the formation of a 3-center
π complex before the 4-center transition state when using free
HBCy2. When the Ir(I) complex is added, the HBCy2 would be captured to form a metallaheterocyclic Shimoi-type intermediate
in which a δ-[M]···H–B agostic interaction
appears. Afterward, the E–C functional group
intersects this hemilabile interaction rather than the PO···B
coordination bond, and the following hydride transfer is found to
be rate-determining. The frontier molecular orbital analysis discloses
that the olefins that have typical delocalized π electrons tend
to give more effective overlap with the LUMO of free HBCy2, but the aromatic substituents could significantly reduce its reactivity
due to the enhanced electron delocalization. The polarized carbonyl
group facilitates σ coordination with the metal center, but
the vinyl group favors interaction through π coordination. The
superiority of σ coordination results in the priority of hydroboration
of aldehyde over the olefins when using the captured HBCy2. Thus, it is the conversion of the more favored bonding groups by
the transition metal that should be fundamentally responsible for
the chemoselectivity inversion.