Xantphite: A New Family of Ligands for Catalysis. Applications in the Hydroformylation of Alkenes

Abstract

The novel bulky diphosphite (P∩P) ligands (3 and 4) based on the 2,7,9,9-tetramethyl-9H-xanthene-4,5-diol (2) backbone were investigated in the Rh-catalyzed hydroformylation of oct-1-ene, styrene, and (E)-oct-2-ene. These diphosphites gave rise to very active and selective catalysts for the hydroformylation of oct-1-ene to nonanal with average rates>10000 (mol aldehyde)(mol Rh)−1h−1 (P(CO/H2)=20 bar, T=80°, [Rh]=1 mM) and maximum selectivities of 79% for the linear product. Relatively high selectivities towards the linear aldehyde (up to 70%, linear/branched up to 2.3) but very high activities (up to 39000 (mol aldehyde)(mol Rh)−1h−1) were observed for the hydroformylation of styrene in the presence of these bidentate ligands (P(CO/H2)=2 – 10 bar, T=120°, [Rh]=0.2 mM). Remarkable activities (up to 980 (mol aldehyde)(mol Rh)−1h−1) were achieved with these diphosphites for the hydroformylation of (E)-oct-2-ene with selectivities for the linear product of 74% (l/b up to 2.8, P(CO/H2)=2 bar, T=120°, [Rh]=1 mM). A detailed study of the solution structure of the catalyst under catalytic conditions was performed by NMR and high-pressure FT-IR. The spectroscopic data revealed that under hydroformylation conditions, the bidentate ligands rapidly formed stable, well-defined catalysts with the structure [RhH(CO)2(P∩P)]. All the ligands showed a preference for an equatorial-apical (ea) coordination mode in the trigonal bipyramidal Rh-complexes, indicating that a bis-equatorial (ee) coordination is not a prerequisite for highly selective catalysts.