An NMR study on the mechanism of ethene hydromethoxycarbonylation catalyzed by cationic Pd(II)–PPh3 complexes

Abstract The reactivity of cis -[Pd(H 2 O) 2 (PPh 3 ) 2 ](TsO) 2 .2(H 2 O) ( I ,H 2 O), trans -[Pd(COEt)(TsO)(PPh 3 ) 2 ] ( II ) and trans -[Pd(COOMe)(TsO)(PPh 3 ) 2 ] ( III ) has been studied by 1 H and 31 P{ 1 H} NMR spectroscopy under conditions that mime the catalytic ethene hydromethoxycarbonylation (EHMC), i.e. in the presence of PPh 3 , H 2 O and TsOH. ( I ,H 2 O), in the presence of two equivalents of PPh 3 , reacts with MeOH and CO (0.3 MPa) at 193 K to give [Pd(COOMe)(TsO)(PPh 3 ) 3 ] ( III ′), which reacts with H 2 O in the presence of TsOH at 293 K to generate [PdH(PPh 3 ) 3 ](TsO) ( IV ) quantitatively. This hydride inserts ethene (0.3 MPa, 293 K) to give trans -[Pd(Et)(TsO)(PPh 3 ) 2 ] ( V ), which reacts with CO (0.3 MPa, 223 K) giving [Pd(COEt)(PPh 3 ) 3 ](TsO) ( II )′ and initiates the catalytic EHMC at 293 K. II , in combination with PPh 3 and TsOH, reacts at 293 K with MeOH with quantitative formation of methyl propanoate (MP) and IV and promotes the catalysis starting from this temperature, under 0.6 MPa of CO/ethene (1/1) when the ratio PPh 3 /TsOH/ II is 2/6/1; upon increasing the PPh 3 / II ratio, the catalytic activity passes through a maximum when the ratio is 4/1, even though it initiates at a higher temperature. In the absence of added ligand, MP is formed in a stoichiometric amount, catalysis is not observed and decomposition to Pd metal occurs. Therefore, PPh 3 is essential in order to stabilize hydride IV , though an excess of ligand is detrimental. III does not insert ethene even at 343 K, a temperature well above that at which catalysis is observed. All these experimental evidences support the Pd–H cycle.