Hydroformylation
Chelating diphosphines with large P-M-P bite angles near 120 degrees are interesting in the development of catalysts with controlled local geometry. In rhodium catalyzed hydroformylation, the key HRh(CO)2L2 catalysts are known to be mixtures of diequatorial diphosphine and equatorial-apical diphosphine complexes. We set out to test the hypothesis that very different regioselectivities of aldehyde formation might be obtained from diequatorial diphosphine rhodium complexes and from equatorial-apical diphosphine rhodium complexes. We found a strong correlation between regioselectivity for n-aldehyde formation and natural bite angle. Chelates like BISBI with wide natural bite angles gave a much higher % n-aldehyde than diphosphines with narrow bite angles. Attempts to explain this selectivity by steric effects alone have failed and the role of electronic effects of chelating ligands is being investigated.

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