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.