Steric vis-à-vis electronic influence of phosphines on biaryl motif: Ligand design for coupling reactions with Chloroarenes

• A set of 1-aryl-pyrazole/pyrrole derived phosphine ligands was synthesized.
• The Pd-L complexes were characterized by NMR and X-ray techniques.
• The issue of hemilabile participation has been discussed.

In order to assess relative contribution of steric factors and electron-richness of phosphine ligands on biaryl-type scaffolds, a set of 1-aryl-pyrazole/pyrrole derived ligands L1–L6 featuring either aryldicyclohexylphosphino or aryldiphenylphosphino donor group was synthesized. A bidentate coordination mode of ligands L1 or L2 was evident from a representative crystal structure that implied a possible hemilabile participation to facilitate catalytic steps. With N-arylpyrroles (L3–L5), where the second nitrogen donor on the heterocycle is absent, coupling reactions of unactivated chloroarenes still proceeded with comparable efficiency. Thus, suitably endowed triarylphosphines were found to be as efficient as more expensive aryldialkylphosphine analogs in reactions with chloroarenes, extending the scope of ligand design.

2-Dicyclohexylphosphino-1-aryl pyrazole (L1 and L2) and 2-dicyclohexyl- or 2-diphenylphosphino- N-phenylpyrroles (L3–L6) were synthesized. A comparative study of steric and electronic effects of these ligands (L1–L6) was undertaken with reference to palladium-catalyzed Suzuki–Miyaura coupling and Hiyama coupling reactions of chloroarenes. The issue of hemilabile participation has also been discussed.Figure optionsDownload as PowerPoint slide