Sterically Demanding, Sulfonated, Triarylphosphines: Application to Palladium-Catalyzed Cross-Coupling, Steric and Electronic Properties, and Coordination Chemistry

Tri(2,4-dimethyl-5-sulfonatophenyl)phosphine trisodium (TXPTS·Na₃) and tri(4-methoxy-2-methyl-5-sulfonatophenyl)phosphine trisodium (TMAPTS·Na₃) both provide more active catalysts for Suzuki and Sonogashira couplings of aryl bromides in aqueous solvents than tri(3-sulfonatophenyl)phosphine trisodium (TPPTS·Na₃). In the Heck coupling, TXPTS·Na₃ provides the most effective catalyst system. Cone angles determined from DFT-optimized structures show that both TXPTS·Na₃ (206°) and TMAPTS·Na₃ (208°) are significantly larger than TPPTS·Na₃ (165°). The identity of the counterion had a significant effect on the calculated cone angles for these ligands. The electronic properties of these ligands determined by the CO stretching frequencies of trans-RhL₂(Cl)CO complexes were identical, although calculated electronic parameters suggest subtle differences between these ligands. Similar to TPPTS·Na₃, both TXPTS·Na₃ and TMAPTS·Na₃ react with Pd(OAc)₂ in aqueous solvents to give L_nPd⁰ complexes and the corresponding phosphine oxide. The reduction of palladium(II) by TXPTS·Na₃ is significantly slower than is seen with TMAPTS·Na₃ or TPPTS·Na₃ at room temperature. Evidence of palladacycle complexes derived from TXPTS·Na₃ and TMAPTS·Na₃ by activation of an ortho-methyl substituent was also observed in ligand coordination studies and under catalytic reaction conditions.