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

The chemistry of 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(3-1,3-R2-C3H3){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(3-1,3-Ph2-C3H3){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 1H–1H 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 1-intermediate formed by the opening of the 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.