Influence of the Bite Angle on the Hydroformylation of Internal Olefins to Linear Aldehydes

A novel series of ligands (1−10) that induce wide bite angles (106° < βn < 131°) has been synthesized. Compared to the xantphos series (e.g., 13 versus 5), the introduction of the phosphacyclic moiety results in ligands with a slightly larger bite angle. High-pressure IR and high-pressure NMR studies of the (diphosphine)RhH(CO)2 complexes show that most ligands (3−7) adopt a bis-equatorial binding mode exclusively in the trigonal bipyramidal rhodium complex. Subtle changes in ligand structure have a large impact on activity and selectivity in the hydroformylation of 1-octene and trans-2-octene. Rates up to 3275 (mol aldehyde)(mol Rh)-1 h-1 (p(CO/H2) = 20 bar, T = 353 K, [Rh] = 1 mM, [1-octene] = 637 mM) and regioselectivities > 99% toward the linear product were obtained when 1-octene was used as substrate. For trans-2-octene rates up to 250 (mol aldehyde)(mol Rh)-1 h-1 (p(CO/H2) = 3.6 bar, T = 393 K, [Rh] = 1 mM, [trans-2-octene] = 640 mM) and high regioselectivities up to 96% toward the linear product, which are unprecedented, were obtained. A correlation between the selectivity for the hydroformylation of 1-octene and trans-2-octene has been observed, suggesting that the selectivity-determining step remains unchanged between terminal and internal olefins. Ligands with a larger bite angle lead to more selective systems, but above 125° the regioselectivity drops. Furthermore it is no longer the selective formation of linear alkyl species that determines the high regioselectivities. Instead the differences in rate of β-hydrogen elimination from the branched alkyl intermediate and the linear alkyl intermediate versus CO insertion determine the regioselectivity. For both substrates a decrease in activity with an increase in bite angle is observed. It is suggested that the aforementioned rates of β-hydrogen elimination versus CO insertion must play a crucial role in this bite angle effect on activity, because previous studies have shown that an increase in bite angle leads to an increase in activity.