Synthesis and characterisation of benzyl phosphino-thioether and -thiolato Pd(ii) complexes and their applications in Suzuki coupling reactions

Abstract

The synthesis of the benzyl phosphinothioether derivatives Ph₂PCH₂CH(Et)SR and their corresponding palladium complexes are reported, where R = CH₂Ph (3), R = CH₂-3,5-Me-C₆H₃ (4) and R = 1-CH₂C₁₀H₇ (5). Crystallographic data obtained for the complexes Pd(3)Cl₂ and Pd(4)Cl₂ show intra- and inter-molecular π–π interactions between the aromatic rings on the P and S substituents, and NOE experiments for Pd(4)Cl₂ show that these interactions persist in solution. The performance of the phosphinothioether palladium complexes in aryl–aryl cross-coupling reactions is compared with that of the corresponding complex of the parent phosphinothiolato ligand Ph₂PCH₂CH(Et)S⁻ (1). High turnover numbers up to 2000000 are reported for the coupling of bromobenzene, using the palladium dimer [Pd(1)I]₂ as the catalyst precursor. Kinetic studies show a linear dependence of the reaction on catalyst loading. The effect of other variables on the cross-coupling reaction, such as temperature, solvent and base, is also reported.