Neoteric solvents for exploratory catalysis: hydrophosphination catalysis with CHEM21 solvents†

Abstract

Exploratory catalytic hydrophosphination studies continue to be in toxic or environmentally harmful solvents, missing an opportunity for improved sustainability and safety. A comparative analysis of hydrophosphination catalysis using the three major categories of substrates, styrene, Michael acceptors, and unactivated alkenes, has been undertaken to assess a transition to green solvents. The compound selected, Cu(acac)₂, has been identified as a highly active and most general precatalyst for hydrophosphination with known mechanistic divergence based on substrate. Additionally, three group 1 alkoxides (LiOEt, NaOEt, KOEt) have been shown to be competent hydrophosphination catalysts for these categories of alkenes; under these conditions substantially lower loadings were realized compared to prior studies with group 1 metals. Eight solvents were investigated from categories outlined in the CHEM21 guide, and seven were highly effective for most reactions, regardless of catalysts or mechanism. These results demonstrate a straightforward path to improving the sustainability of future studies in this and related catalytic reactions through bioavailable solvents, heretofore unknown in hydrophosphination catalysis. Other key findings include the improved utilization of more sustainable and low toxicity group 1 catalysts in this reaction with greater conversion (i.e., reduced waste) as well as highlighting potential pitfalls of reactions involving phosphine substrates in bioavailable solvents.