Palladium(II) allyl complexes of chiral diphosphazane ligands: ambident coordination behaviour and stereodynamic studies in solution

Abstract

The chemistry of eta(3)-allyl palladium complexes of pyrazolyl substituted diphosphazane ligands, Ph2P(E) N(R) PPh( N2C3HMe2-3,5) [E = lone pair, R = CHMe2 (1); E = lone pair, R = (S)-* CHMePh (2); E = S, R = CHMe2 (3)] bearing a stereogenic phosphorus centre has been investigated and the complexes, [Pd(eta(3)-1,3-R'(2)-C3H3){kappa(2)-Ph2P(E) N( R) PPh(N2C3HMe2-3,5)}](PF6) [E = lone pair or sulfur; R = CHMe2 or (S)-* CHMePh; R' = H, Me or Ph; 4-13], have been isolated. Detailed NMR studies reveal that these complexes exist as a mixture of isomers in solution. The structures of four complexes have been determined by X-ray crystallography and only one isomer is observed in the solid state in each case. The allyl complexes formed by ligands 1 and 2 display a P, N-coordination mode except the 1,3-diphenyl allyl complex, [Pd(eta(3)-1,3-Ph-2-C3H3 {kappa(2)-Ph2PN(CHMe2)PPh(N2C3HMe2-3,5)}](PF6) (8), which shows the presence of both P, N- and P, P-coordinated isomers in solution with the former predominating. On the other hand, the complexes bearing the diphosphazane monosulfide ligand 3 display P,S-coordination. Two-dimensional phase sensitive H-1-H-1 NOESY and ROESY measurements indicate that several of the above allyl palladium complexes undergo syn-anti and/or cis-trans isomerization in solution through an eta(1)-intermediate formed by the opening of the eta(3)-allyl group selectively at the trans position with respect to the phosphorus centre. Preliminary investigations show that the diphosphazanes (1 and (SR)-2) function as efficient auxiliary ligands for catalytic allylic alkylation reactions but lead to only low levels of enantiomeric excess