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Conformational Analysis of Phosphinamine Ligands

Brown, John M.; Hulmes, David I.; Long, James M.; Valk, Jean Marc;Pearson, Stuart; Bayston, Daniel M.; Goeke, Andreas; Muir, Jayne M.; Alcock, Nathaniel W.

Dyson Perrins Laboratory, South Parks Rd., OXFORD OX1 3QY;

Department of Chemistry, University of Warwick, Coventry CV4 7AL (NWA)

During the course of our work on asymmetric catalysis by ligands of the 1-(phosphinoaryl)-isoquinoline class a number of X-ray structures have been obtained which give insights into the conformation of the chelate ring, and the sensitivity of its geometry to changes in the backbone structure. Further, comparisons may be made with both the biaryl diphosphines related to BINAP and other ligands such as the phosphinoaryl oxazolines, which have enjoyed wide application in catalysis

The basic ligand structure possesses a number of surprising features. The bite angle is quite small, and some strain is inherent in the chelate. It is manifested most obviously by a distortion of the bond between the isoquinoline nitrogen and the metal, whose vector is distorted out of the mean ring plane by up to 30_. Coordination of reactants and reagents in a catalytic cycle will induce steric interactions involving the P-aryl groups of the P-N ligand for substituents which are cis- to phosphorus. The effects for substituents cis- to nitrogen is more subtle but its effects are crucial in at least one case. The examination of molecular models based on the X-ray structures reported is a critical part of the development of designed catalysts in the future. On close analysis there are subtle differences between the QUINAP family and both of the alternatives described above, particularly in the detailed structure of the chelate ring. It may be possible to understand the subtle differences in their catalytic chemistry through this analysis.

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