Xantphite: A new family of ligands for catalysis. Applications in the hydroformylation of alkenes

The novel bulky diphosphite (P boolean AND P) ligands (3 and 4) based on th e 2,7,9,9-tetramethyl-9H-xanthene-4,5-diol (2) backbone were investigated i n the Rh-catalyzed hydroformylation of oct-1-ene, styrene, and (E)-oct-2-en e. These diphosphites gave rise to very active and selective catalysts for the hydroformylation of oct-1-ene to nonanal with average rates > 10000 (mo l aldehyde)(mol Rh)(-1)h(-1) (P(CO/H-2) = 20 bar, T= 80 degrees, [Rh] = 1 m M) and maximum selectivities of 79% for the linear product. Relatively high selectivities towards the linear aldehyde (up to 70%, linear/branched up t o 2.3) but very high activities (up to 39000 (mol aldehyde)(mol Rh)(-1)h(-1 )) were observed for the hydroformylation of styrene in the presence of the se bidentate ligands (P(CO/ H-2) = 2 - 10 bar, T= 120 degrees, [Rh] = 0.2 m M). Remarkable activities (up to 980 (mol aldehyde)(mol Rh)(-1)h(-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/H-2) = 2 bar, T= 120 degrees, [Rh] = 1 mm). A detailed study of the solution str ucture of the catalyst under catalytic conditions was performed by NMR and high-pressure FT-IR. The spectroscopic data revealed that under hydroformyl ation conditions, the bidentate ligands rapidly formed stable, well-defined catalysts with the structure [RhH(CO)(2)(P boolean AND P)]. All the ligand s showed a preference for an equatorial-apical (ea) coordination mode in th e trigonal bipyramidal Rh-complexes, indicating that a bisequatorial (ee) c oordination is not a prerequisite for highly selective catalysts.