A Boron-Boron Double Transborylation Strategy for the Synthesis of gem-Diborylalkanes

Olefin hydroboration reactions provide efficient access to synthetically versatile and easily handled organoboronic esters. In this study, we demonstrate that the commercially available organoborane reagent 9-borabicyclo[3.3.1]nonane (H-B- 9-BBN) can serve as a catalyst for the sequential double hydroboration of alkynes using pinacolborane (HBpin). This strategy, which is effective for a wide range of terminal alkynes, is predicated upon a key C(sp(3))-B/B-H transborylation reaction. Transition-state thermodynamic parameters and 10-boron-isotopic labeling experiments are indicative of a sigma-bond metathesis exchange pathway.