Three Component Thio‐ and Carboboration of Alkynes: A Modular Route to Functionalised Bicyclic Boronates

The three-component elemento-boration of alkynes using a borane and a nucleophile is a highly efficient method to generate complex alkenyl-boranes. Benzoxaborinines are a class of alkenyl boranes of considerable importance, including in the fight against antibiotic resistance. However, a three-component elemento-boration process to form functionalised benzoxaborinines was an unmet challenge before this work. Herein, we report operationally simple three-component thio- and carbo-boration reactions to form functionalised benzoxaborinines using commercially available reagents. The processes also were applicable to form functionalised benzazaborinines, which are of interest as naphthalene bioisosteres. The nucleophile scope included thioethers, thiols, and (hetero)arenes. In contrast, when using amine nucleophiles, alkyne hydroamination occurred to form boranils. Mechanistic studies revealed a disparity between thioboration using thioethers and using thiols. While thioethers are effective nucleophiles in their own right, when using thiols the in situ formation of tri-thioboranes ((RS)3B) proceeded prior to thioboration, with thioborate anions ([(RS)4B)]−) calculated to be the key nucleophile. Note, combining Et2O∙BF3/RSH and a hindered base is attractive as a simple route to form tri-thioboranes in situ. Overall, this work is a notable addition to the toolbox for making functionalised bicyclic boronates, while demonstrating that the ubiquitous borane Et2O·BF3 can still be used to discover new borylation processes.