Y Variable Metal Chelation Modes and Activation Sequence in Pd-Catalyzed B-H Poly-arylation of Carboranes

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 pentaarylation 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)-tetraarylated 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.