Diastereomerism in palladium(II) allyl complexes of P,P-, P,S- and S,S-donor ligands, Ph 2P(E)N(R)P(E′)Ph 2 [R=CHMe 2 or ( S)-*CHMePh; E=E′=lone pair or S]: solution behaviour, X-ray crystal structure and catalytic allylic alkylation reactions

Abstract

The reactions of achiral homodonor diphosphazane ligands, Ph 2P(E)N(CHMe 2)P(E′)Ph 2 [E=E′=lone pair ( 1) or S ( 2)] with the chloro bridged palladium dimers, [Pd(η 3-1,3-R′-R″C 3H 3)(μ-Cl)] 2 (R′, R″=H, Me or Ph) in the presence of NH 4PF 6 give cationic η 3-allyl palladium complexes, [Pd(η 3-1,3-R′-R″C 3H 3)(L-L′)]. Each of these complexes exists as single species in solution except the 1,3-dimethyl allyl complex, which exists as two isomers ( syn/ syn- and syn/ anti-) in solution. Analogous allyl palladium complexes derived from the chiral homodonor diphosphazane ligand, Ph 2PN(( S)-*CHMePh)PPh 2 ( 3) exist as a single species when the allyl moiety is symmetrical (R′=R″=H or Ph) but a 1:1 mixture of two different face-coordinated diastereomers if the allyl moiety is unsymmetrically substituted (R′≠R″). The 1,3-dimethyl–allyl complex, [Pd(η 3-1,3-Me 2C 3H 3){Ph 2PN(( S)-*CHMePh)PPh 2- k 2 P, P}]PF 6 ( 20) exists as a mixture of two isomers. The major isomer ( 20a) has the syn/ syn-allylic arrangement while the minor isomer ( 20b) has the syn/ anti configuration. These isomers ( 20a and 20b) equilibrate in solution via a syn– anti isomerisation pathway. Achiral and chiral heterodonor monosulphide ligands, Ph 2P(S)N(R)PPh 2 [R=CHMe 2 ( 4), ( S)-*CHMePh( 5)] give rise to allyl palladium complexes which exist as several isomers in solution. The presence of a chiral centre in the ligand 5 enables the observation of two NMR distinguishable face-coordinated diastereomers for its allyl complexes. However, the solid state structure of [Pd(η 3-C 3H 5){Ph 2P(S)N(( S)-*CHMePh)PPh 2- k 2 P, S}]PF 6 ( 27) shows the presence of only one diastereomer. Two-dimensional (2D) phase sensitive 1H 1H NOESY and ROESY measurements indicate that the monosulphide complexes undergo exchange by the opening of the η 3-allyl group selectively at the trans position with respect to the greater π-acceptor phosphorus centre to generate a η 1-bonded intermediate. Preliminary studies on the use of the ligands 1– 5 for catalytic allylic alkylation reactions of an unsymmetrically substituted substrate, ( E)-1-phenyl-2-propenyl-acetate are reported.