(Perfluoro)alkylsilyl-Substituted 2-[Bis(4-aryl)phosphino]pyridines: Synthesis and Comparison of Their Palladium Complexes in Methoxycarbonylation of Phenylacetylene in Regular Solvents and Supercritical CO₂

Abstract

The synthesis and characterization of three new 2-[bis{4-(bromo)phenyl}phosphino]-pyridine (8), 2-[bis{4-(trimethylsilyl)phenyl}phosphino]pyridine (9), and 2-[bis{4-((2-(perfluorohexyl)ethyl)dimethylsilyl)phenyl}phosphino]pyridine (10) ligands (L) is reported. The corresponding compounds trans-/cis-[PdCl 2 (L) 2 ] 11-13, trans-[PdCl(Me)(L) 2 ] 14-16, and maleic anhydride complexes [Pd(L) 2 (cyclo-(-CH=CHC(O)OC(O)-)] 17 and 18 were synthesized and characterized. In the methoxycarbonylation of phenylacetylene in methanol catalysts derived from [Pd(OAc) 2 ] as the palladium source with either 9 or 10 as ligand showed the same activity (TON = 4000, 50 min reaction time) and selectivity (about 98% for the branched product) as the catalyst obtained from [Pd(OAc) 2 ] and 2-[diphenylphosphino]-pyridine (2). Both 2 and 10 were also tested in this reaction using a 1:1 mixture of methanol and α,α',α--trifluorotoluene as solvent system: fluorous 10 generated a more active catalyst (TON = 3400 for 10 vs 3000 for 2). In supercritical CO 2 the use of 10 as ligand (with [Pd, (OAc) 2 ]) in this reaction gave a TON of 2000 (50 min reaction time) and a selectivity of >99% for the branched product. Nonfluorous [Pd(2)2(cyclo-(CH=CHC(O)OC(O)-)] gave a particularly active catalyst for the methoxycarbonylation of phenylacetylene (TON of about 8000, 50 min reaction time, TOF 5 0 of about 30 000 mol phenylacetylene per mol Pd per hour in methanol) with no loss in selectivity (98% selectivity toward the branched product).