Study of bite angle effects in hydroformylation

Hydroformylation of alkenes to form aldehydes is one of the most important homogeneously catalyzed industrial processes. Products such as n-butyraldehyde are important in the manufacture of plasticizer alcohols, as well as for alcohol solvents such as n-butanol. Typically, the linear (n) aldehyde products are more valuable than the branched (i) products. There are a number of catalytic systems responsible for this reaction. Unfortunately, these systems are not yet fully understood. Significant progress has been made, both experimentally and theoretically, on the study of such systems. In one important system, the catalyst is a rhodium metal atom bound to two phosphines and one carbonyl. More recent versions utilize a chelating bis-phosphine ligand. It has often been found that tying ligands together improves catalyst stability. The resultant bidentate ligands possess certain bite angles. When an α-olefin is used as feedstock, the product may be either a linear (n) or a branched (i) aldehyde. The groups of Casey and van Leeuwen found that these bite angles affect the activity and the regioselectivity of the catalyst. They proposed that the regioselectivity is determined by relative stabilities of the transition states of alkene insertion. Alkene coordination precedes a five-coordinate trigonal bipyramidal intermediate. In that intermediate, the bidentate ligand may be coordinated in either an equatorial-equatorial (ee) or an equatorial-axial (ea) fashion. Cundari's group proposed that those two intermediates may form three possible transition states, each of which shows a slightly different insertion barrier. In this study, we examine those three pathways using catalysts that contain bidentate ligands. The work specifically addresses the effect of the bite angle on transition state stabilities. Experimental data are interpreted in light of these results.