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

Abstract

The chemistry of η³-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(η³-1,3-R′₂-C₃H₃){κ²-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(η³-1,3-Ph₂-C₃H₃){κ²-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 η¹-intermediate formed by the opening of the η³-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.