Isomerizing Methoxycarbonylation of Alkenes to Esters Using a Bis(phosphorinone)xylene Palladium Catalyst

The synthesis and characterization of bulky diphosphine 1,2-bis­(4-phosphorinone)­xylene, BPX, and its palladium complexes [(BPX)­PdCl2] and [(BPX)­Pd­(O2CCF3)2] are described. BPX was evaluated as a ligand in Pd-catalyzed isomerizing methoxycarbonylation. A broad range of alkenes, including terminal, internal, branched, and functionalized alkenes, can be converted to esters with activities and selectivities matching or surpassing the performance of the state-of-the-art palladium bis­(di­(tert-butyl)­phosphino-o-xylene (Pd-DTBPX) catalyst. A molecular structure of the precatalyst [(BPX)­Pd­(O2CCF3)2] was obtained showing a square planar geometry and a bite angle of 100.11(3)°. Rhodium carbonyl complexes [(BPX)­Rh­(CO)­Cl] and [(DTBPX)­Rh­(CO)­Cl] were synthesized to compare the relative electronic parameters, revealing a ν­(CO) of 1956.8 and 1948.3 cm–1, respectively, suggesting a reduced ability of BPX to donate electron density to the metal relative to DTBPX. Competitive protonation experiments between BPX and DTBPX in the presence of CH3SO3H exclusively produce [DTBPX(H)2]2+, providing additional evidence that BPX is a much weaker base than DTBPX. This could be due to either the effect of the electron-withdrawing ketone group in the phosphorinone ring or the compression of the C–P–C bond angle induced by the ring structure. The 31P NMR (CDCl3) chemical shift of BPX is 5.6 ppm, upfield of DTBPX at 27.6 ppm. This anomalous result is attributed to a strong gamma substituent effect of CO in the BPX ligand. The improved activity of Pd-BPX, relative to Pd-DTBPX, could be attributed to a more electrophilic PdII center, which could accelerate the rate-determining methanolysis step.