Mechanistic Insights on a Metal‐Free Borylative Cyclization of Alkynes Using BCl₃: A Theoretical Investigation

Abstract

The metal-free reactions between alkynes and BCl 3 were investigated by meanso fD FT calculations, focusing on elucidating the detailed cyclizationm echanism and the different chemoselectivity between internal and terminal alkynes. The results showed that borylative cyclization of internal alkynes proceeded via kinetically favorable stepwise complexation, electrophilicd earomatization, hydrogen migration and rearomatization. In contrast, for terminal alkynes, the competitive chloroboration step, which followed the complexation step, was more feasible than electrophilic dearomatization, affording the thermodynamically stable addition products irreversibly.Borylative cyclizationr eactions, which construct (hetero)cyclic frameworks with concomitanti ncorporation of boryl substituents, afford structurally versatile and synthetically valuable intermediates. [1] Such reactions, in combination with the diverse downstream transformations of boryl functionalities, [2] provide powerful protocols for the construction and derivatization of ubiquitous (hetero)cyclic moieties. Thus, metal-catalyzed borylative cyclization reactions have attracted tremendous interest and gained great development in the last two decades. [1] Mechanistically,m etal complexes with boryl ligands are believed to playv ital roles in incorporating the boryl substituents. [3] In contrast, the borylative cyclization reactions without metal catalysts have been far less explored. Some achievements in this field began to appear in recent years. Among these, B(C 6 F 5 ) 3 has been documented to be competent both as ab oron source and reaction initiator,t hus affording borylated polycyclic structures. [4] More recently,t he Blum, [5] Ingleson [6] and Li [7] groups reported metal-free protocols for borylative cyclization.T he Ingleson group achieved ab orylative cyclization of internal alkynes with inexpensive BCl 3 under mild conditions, [6a] affording polycyclic frameworks containing aC (sp 2 )À BCl 2 moiety directly and economically.T hen, in situ alcoholysis Scheme1.The different chemoselectivity between internal and terminal alkynes.[a] Dr.