Diastereomerism in palladium(II) allyl complexes of P,P-, P,S- and S,S-donor ligands, Ph2P(E)N(R)P(E′)Ph2 [R=CHMe2 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, Ph2P(E)N(CHMe2)P(E′)Ph2 [E=E′=lone pair (1) or S (2)] with the chloro bridged palladium dimers, [Pd(η3-1,3-R′-R″C3H3)(μ-Cl)]2 (R′, R″=H, Me or Ph) in the presence of NH4PF6 give cationic η3-allyl palladium complexes, [Pd(η3-1,3-R′-R″C3H3)(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, Ph2PN((S)-*CHMePh)PPh2 (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(η3-1,3-Me2C3H3){Ph2PN((S)-*CHMePh)PPh2-k2P,P}]PF6 (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, Ph2P(S)N(R)PPh2 [R=CHMe2 (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(η3-C3H5){Ph2P(S)N((S)-*CHMePh)PPh2-k2P,S}]PF6 (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 η3-allyl group selectively at the trans position with respect to the greater π-acceptor phosphorus centre to generate a η1-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. The reactions of achiral homodonor diphosphazane ligands, Ph2P(E)N(CHMe2)P(E′)Ph2 [E=E′=lone pair (1) or S (2)] with the chloro bridged palladium dimers, [Pd(η3-1,3-R′-R″C3H3)(μ-Cl)]2 (R′, R″=H, Me or Ph) in the presence of NH4PF6 give cationic η3-allyl palladium complexes, [Pd(η3-1,3-R′-R″C3H3)(L-L′)].