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.