Expedient access to bora-butenolide bioisosteres by counteranion-mediated trans-hydroboration of alkynes.

The hydroboration of alkynes is a textbook example of a syn-selective concerted addition reaction, while trans-selective additions of borane to alkynes remain to be developed. We herein report a transition metal-free anti-addition of pinacolborane to alkynes, facilitated by the counteranion effect. This work further develops Chan alkyne reduction by utilizing the borane instead of aluminohydride reagents, enabling the facile synthesis of valuable five-membered boracycles that constitute isosteric alternatives to bioactive butenolides and a versatile platform for abundant downstream transformations. The practical method is distinguished by excellent regioselectivity, a broad substrate scope, and high compatibility with a variety of functional groups. The exploration of trans-selective patterns affords not only a stereo-complementary approach to traditional organic synthesis, but also mandates a new perspective on the noncanonical trans-hydroboration mechanism. A combination of control experiments and computational studies at the DFT level of theory reveal the previously unrecognized role of the HMDS counteranion in a stepwise intermolecular hydrogen transfer process.