Metal-Free Electrophilic Borylative Cyclizations of Alkynes.

Metal-free borylative cyclizations of alkynes have emerged as a powerful and versatile strategy for the construction of boron-containing cyclic frameworks. By exploiting the electrophilic activation of alkynes with boron Lewis acids, these transformations enable intramolecular nucleophilic attack rendering the simultaneous formation of CB and CC or CX bonds under mild, transition-metal-free conditions. While early examples relied on B(C₆F₅)₃ and delivered products of limited synthetic utility, recent developments based primarily on ClBcat and BCl₃ have greatly expanded the scope and practical relevance of these reactions. A wide range of heteroatom- and carbon-based nucleophiles can be engaged, providing access to diverse borylated hetero- and carbocycles, typically isolated as versatile boronate esters. This review summarizes recent advances in this rapidly developing field and reveals future opportunities for expanding molecular diversity through rational substrate design.