posted on 2021-11-05, 01:03authored byHou-ji Cao, Meng Chen, Fangxiang Sun, Yue Zhao, Changsheng Lu, Xiaolei Zhang, Zhuangzhi Shi, Hong Yan
The
development of synthetic methods for selective multiple functionalization
of B–H bonds offers great opportunity in expanding the application
potentials of carboranes. Herein, regioselective poly-arylation of o-, m-, and p-carboranes
has been developed by the imine-directed, Pd-catalyzed iterative B–H
activation. The isolation of key palladium intermediates in the early
stage and late stage of B–H palladation validates the distinct
chelation-assisted modes at the PdII center during the
iterative activation pathways. In particular, a scarce three-coordinated,
T-shaped palladium complex has been isolated and proven to be effective
for subsequent arylation reaction. Stoichiometric control on the catalytic
penta-arylation of p-carborane led to the exhaustive
isolation of mono-, di-, tri-, tetra-, and penta-arylated products,
which provides a rationale that the 5-fold B–H activation proceeds
in the sequence from vertex B(2) to B(4), then to B(5), and finally
to B(3) and B(6). It is the steric factor of the introduced aryl groups
that affects the performance of metal on the vertex selectivity and
chelation mode. A series of cage B(3,4,5,6)-tetra-arylated o-carboranes and B(2,3,4,5,6)-penta-arylated m- and p-carboranes were synthesized in good to excellent
yields as an unreported type of molecular propellers. The synthetic
utility of this poly-arylation method has also been illustrated.