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.
