Metal-Free Catalytic Hydroboration of Unsaturated Compounds: A Greener Strategy for the Synthesis of Organoboranes

This review discusses the immense progress made over recent years in metal-free catalytic hydroboration. Reactions with several unsaturated functional groups, such as alkynes, alkenes, carbonyl compounds, carbon dioxide, carbodiimides, imines, allenes, pyridines, carboxylic acid derivatives etc., have been explored. Although there are significant developments in hydroboration reactions using metal-based catalysts, metal-free catalytic strategies are more desirable from a sustainability perspective. Considerable effort has been made by various research groups on metal-free catalysts in the hydroboration of unsaturated compounds. Lewis acid, Bronsted acid, Lewis acid-base pairs, and frustrated Lewis pairs were involved as metal-free sources to catalyze hydroboration reactions in a sustainable manner. Detailed mechanisms of the catalytic pathways of metal-free hydroboration are also discussed in this review. This discussion will provide an overview of the extent of work carried out as well as indicate scope for future research.