Hydrophosphinylation of Alkynes via Neutral Magnesium Complexes: Evidence for Ligand Dependency in Structure-Activity Relationships

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The pursuit of practical, straightforward, and sustainable methods for forming carbon-phosphorus bonds is crucial in both academia and industry. Traditional synthetic methods often rely on hazardous, halogenated precursors through salt-metathesis routes. In this study, we have synthesized and characterized magnesium complexes [L(Mg-nBu)2] (L = bis(diiminate); nBu = n-butyl) 1 and 2. Complex 1 effectively catalyzes the hydrophosphinylation of alkynes resulting stepwise hydrophosphinylated products namely monophosphinylated vinyledene- and 1,2-diphosphinylated alkanes. While doubly addition products with the alkynes are predominant, this catalytic reaction produces anti-Markovnikov products with inactivated alkenes, whereas activated alkenes giving rise to conjugated products. This transformation showcases an excellent atom economy, broad functional group tolerance and gram scale synthesis for organophosphorus compounds. Through controlled experiments, kinetic studies, and density functional theoretical calculations, we elucidated the reaction mechanism, identifying the active catalytic species and revealing a stepwise hydrophosphinylation process of alkynes. Although complex 1 showed its potential in the hydrophosphinylation of alkynes, complexes 2 and 3 produced a lower yield of hydrophosphinylated products, indicating the role of ligand (spacer) in this catalytic transformation. This work is the first to demonstrate that a neutral magnesium complex can independently catalyse the hydrophosphinylation of alkynes and offers opportunities for the hydrophosphinylation of other compounds catalyzed by main-group metal complexes.