Synthesis and Study of a Dialkylbiaryl Phosphine Ligand; Lessons for Rational Ligand Design

Abstract

The rational design and synthesis of a novel dialkylbiarylphosphine ligand, 2′-(dimethylphosphine)-2,6dimethoxy-1,1′-biphenyl (MeSPhos), for palladium-catalyzed C−N cross-coupling reactions is described. Based on previous results, it was hypothesized that a ligand with electronic properties similar to (2-biphenyl)dimethylphosphine (MeJPhos) but with greater steric bulk would allow the cross-coupling of previously inaccessible deactivated aryl chlorides. As predicted, MeSPhos exhibited similar electronic properties to MeJPhos. However, MeSPhos surprisingly showed a significantly smaller steric profile than MeJPhos. In comparison to the widely used CySPhos (2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl), MeSPhos promoted the oxidative addition of highly deactived aryl chlorides for which CySPhos was ineffective, but significantly decreased the rate of reductive elimination. The kinetics of cross-coupling reactions showed that the altered steric and electronic parameters of MeSPhos had a significant impact on the rate of cross-coupling, and the decreased steric bulk had a profound deleterious impact on the catalyst stability. With regard to this latter point, only the most activated aryl chlorides reacted at a sufficient rate to overcome the rate of catalyst decomposition. These results indicate that the relationship between the electron-donating ability of the phosphine ligand and the rate of oxidative addition is complex, and they also illustrate that increasing substitution on the biphenyl structure does not necessarily increase the steric bulk of the ligand.