Hydrophosphanylation of Alkynes via Magnesium Complexes: Evidence for Ligand Dependency in Structure–Activity Relationships

Pursuing practical, straightforward, and sustainable methods for forming carbon-phosphorus bonds is crucial in academia and industry. In this study, we showed that bis(diiminate)-based magnesium complexes [L(Mg-nBu)₂] (nBu = n-butyl) could effectively catalyze the hydrophosphanylation of alkynes, resulting in monophosphanylated vinyledene- and 1,2-diphosphanylated alkanes in a stepwise manner. This transformation showcases an excellent atom economy, broad functional group tolerance, and gram-scale synthesis for organophosphorus compounds. Through controlled experiments and with the support of DFT calculations, we elucidated the reaction mechanism, identifying the active catalytic species and revealing a stepwise hydrophosphanylation process of alkynes. Although complex Mg-1 showed its potential in this transformation, complexes Mg-2 and Mg-3, having ethyl and phenyl spacers, produced a lower yield of hydrophosphanylated products, indicating the role of ligand (spacer) in this catalytic reaction. Further, the activity of Mg-1 was compared with a monomeric magnesium complex, Mg-4, and it was found that the performance of the Mg-4 in alkyne hydrophosphanylation is quite lower than the results obtained by using Mg-1. This work demonstrated that a dimeric magnesium complex with a suitable spacer can enhance the catalytic activity manyfolds in the hydrophosphanylation of alkynes.