| Abstract
| - A novel series of phenoxaphosphino- and dibenzophosphole-modified xanthene-type ligands that have large natural bite angles have been prepared and successfully applied in the rhodium-catalyzed hydroformylation of terminal and internal olefins. The effect of natural bite angle on hydroformylation activity and selectivity for internal olefins is described.
- A novel series of ligands (1−10) that induce wide bite angles (106° < βn< 131°) has beensynthesized. Compared to the xantphos series (e.g., 13 versus 5), the introduction of thephosphacyclic moiety results in ligands with a slightly larger bite angle. High-pressure IRand high-pressure NMR studies of the (diphosphine)RhH(CO)2 complexes show that mostligands (3−7) adopt a bis-equatorial binding mode exclusively in the trigonal bipyramidalrhodium complex. Subtle changes in ligand structure have a large impact on activity andselectivity in the hydroformylation of 1-octene and trans-2-octene. Rates up to 3275 (molaldehyde)(mol Rh)-1 h-1 (p(CO/H2) = 20 bar, T = 353 K, [Rh] = 1 mM, [1-octene] = 637mM) and regioselectivities > 99% toward the linear product were obtained when 1-octenewas 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 regioselectivitiesup to 96% toward the linear product, which are unprecedented, were obtained. A correlationbetween the selectivity for the hydroformylation of 1-octene and trans-2-octene has beenobserved, suggesting that the selectivity-determining step remains unchanged betweenterminal 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 formationof linear alkyl species that determines the high regioselectivities. Instead the differences inrate of β-hydrogen elimination from the branched alkyl intermediate and the linear alkylintermediate versus CO insertion determine the regioselectivity. For both substrates adecrease in activity with an increase in bite angle is observed. It is suggested that theaforementioned rates of β-hydrogen elimination versus CO insertion must play a crucialrole in this bite angle effect on activity, because previous studies have shown that an increasein bite angle leads to an increase in activity.
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