Triarylborane-Catalyzed Homogeneous Molecular Transformations Using Organic Reductants

Triarylboranes have emerged as powerful main-group catalysts for the reduction of organic molecules using a variety of reductants, including molecular hydrogen (H2), hydrosilanes, Hantzsch esters, and ammonia borane. This development has been largely enabled by the extensive structural diversification of triarylboranes, which allows for systematic tuning of the Lewis acidity and steric environment at the boron center. Since the seminal review of this area more than a decade ago, the field has undergone remarkable growth, prompting this comprehensive account covering three key directions: (1) triarylboranes that, unlike conventional B(C6F5)3, remain catalytically active in the presence of Lewis bases such as H2O, CO2, and CO, enabling the use of pure or crude H2; (2) diverse organic reductants, including ammonia borane, Hantzsch esters, cyclohexadienes, and other hydride donors; and (3) mechanistic insights into reductant activation and stereocontrol. Covering advances from 2016 to 2025, this review discusses innovations in borane design, substrate scope, functional-group tolerance, and mechanism across each reductant class. By unifying these related yet independent developments, it aims to provide synthetic chemists and theoreticians with timely insights into triarylborane-catalyzed homogeneous reductions, highlighting main-group catalysts as sustainable, practical alternatives to precious-transition-metal systems.