DIPHOSPHINES WITH NATURAL BITE ANGLES NEAR 120-DEGREES INCREASE SELECTIVITY FOR NORMAL-ALDEHYDE FORMATION IN RHODIUM-CATALYZED HYDROFORMYLATION

The use of 2,2'-bis[(diphenylphosphino)methyl]-1,1'-biphenyl (BISBI, 1), trans-1,2-bis[(diphenylphosphino)methyl]cyclopropane (T-BDCP, 2), and other diphosphines with large natural bite angles as ligands in rhodium-catalyzed hydroformylation has been studied. The X-ray crystal structure of (BISBI)Rh(PPh3)(CO)H.CH2Cl2 (7.CH2Cl2) indicated a trigonal bipyramidal structure with the three phosphorus atoms in the equatorial plane. The P-Rh-P bite angle of the BISBI ligand in 7.CH2Cl2 of 124.8-degrees is much smaller than the 152-degrees P-Fe-P bite angle found in (BISBI)Fe(CO)3 and indicates that the BISBI ligand is rather flexible. NMR studies indicate that rapid exchange (DELTA-G(double dagger) = 15.5 kcal mol-1) occurs between the coordinated PPh3 of 7 and free PPh3. 7 reacted with CO to produce (BISBI)Rh(CO)2H (9), which was shown by IR and NMR studies to have a trigonal bipyramidal structure with BISBI in the equatorial plane and hydride in an apical position. The solution structures of the T-BDCP complexes (T-BDCP)Rh(PPh3)(CO)H (13) and (T-BDCP)Rh(CO)2H (14) were shown by spectroscopy to be similar to the related BISBI compounds. A correlation between the size of the natural bite angle of chelating diphosphines and the regioselectivity for formation of straight-chain aldehydes in the rhodium-catalyzed hydroformylation of 1-hexene was observed.