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 (106degrees < beta(n) < 131degrees) 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/H-2) = 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/H-2) = 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 125degrees 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 beta-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 beta-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.