For
the synthesis of vinyl boronate esters, the direct catalytic
H2-acceptorless dehydrogenative boration of alkenes is
one of the promising strategies. In this paper, the density functional
theory method was employed to investigate the reaction mechanism of
dehydrogenative boration and transfer boration of alkenes catalyzed
by a zirconium complex (Cp2ZrH2). There are
two possible pathways for this reaction: the alkene insertion followed
by the dehydrogenative boration (path A) and the alkene insertion
after the dehydrogenative boration (path B). The calculated results
showed that path A is more favorable than path B, and that the rate-determining
step is the C–B coupling step with an energy barrier of 18.7
kcal/mol. The reaction modes of the C–B coupling assisted dehydrogenative
boration and the alkene insertion were also discussed. These analyses
reveal a novel hydrogen release behavior in dehydrogenative boration
and the alkene insertion modes and sequences were proposed to be of
importance in the chemoselectivity of this reaction. In addition,
the X ligand effect (X = H, Cl) on the catalytic activity of the zirconium
complex was explored, indicating that the H ligand could enhance the
catalytic activity of the complex for styrene dehydrogenative boration.