Effects of bidentate phosphine ligands on syn - Anti isomerization in π-allylpalladium complexes

Article abstract of DOI:10.1021/om020680+

The rates of syn-anti isoinerization in a series of cationic π-allylpalladium complexes [Pd(η³-Me₂CCHCHD)(bisphosphine)]BAr^F₄ [Ar^F = 3,5-(CF₃)₂C₆H₃] were measured by ¹H NMR spin-saturation transfer techniques. For the complexes with α,ω-bis(diphenylphosphino)-alkanes (Ph₂P(CH₂)_nPPh₂, n = 2-4), a correlation between the bisphosphine's bite angles and the syn-anti isomerization rates was observed, the bisphosphine with smaller bite angle accelerating the syn-anti isomerization. The isomerization rate was also dependent on the electronic characteristics of the bisphosphine ligands. Of the substituted dppf ligands (Fe(η⁵-C₅H₄PAr₂)₂, Ar = 4-CF₃C₆H₄, Ph, 4-MeOC₆H₄), it was fastest with the electron-withdrawing 4-CF₃C₆H₄ group and slowest with the electron-releasing 4-MeOC₆H₄ group on the phosphorus atoms. These bisphosphines were used as supporting ligands for the palladium-catalyzed allylic alkylation of (Z)-cinnamyl acetate and (Z)-2-hexenyl acetate with the sodium salt of dimethyl methylmalonate. The alkylation products with E-geometry, which result from the isomerization of π-allylpalladium intermediates from anti to syn, were formed more with the bisphosphines of faster syn-anti isomerization.