Sulfur-Boron Lewis Pair-Catalyzed Hydrohalogenation of Alkynes via Formal Hydrogen Atom Transfer.

The catalytic synthesis of both (E)- and (Z)-haloalkenes from internal aryl alkyl alkynes via a sulfur-boron Lewis pair has been developed. The catalytically active Lewis pair complex is generated in situ from 2-naphthalenethiol and B(C6F5)3, enabling the selective synthesis of chloro-, bromo-, and iodoalkenes with exclusive Markovnikov regioselectivity. Both (E)- and (Z)-haloalkenes were obtained in good yields with high selectivities by simply controlling the reaction temperature. Experimental mechanistic studies support the intermediacy of vinyl radicals generated through an overall formal hydrogen atom transfer (HAT) process. Computational investigations further support a stepwise proton transfer-electron transfer (PT-ET) mechanism rather than a concerted HAT event. The origin of the stereodivergence is also elucidated by combining experimental observations with theoretical calculations. This work demonstrates a new strategy for vinyl radical generation through sulfur-boron Lewis pair catalysis and suggests that formal HAT reactivity can emerge from a stepwise PT-ET mechanism in main-group catalysis.