Effects of Bidentate Phosphine Ligands onsyn−antiIsomerization in π-Allylpalladium Complexes

Abstract

The rates of syn−anti isomerization in a series of cationic π-allylpalladium complexes [Pd(η3-Me2CCHCHD)(bisphosphine)]BArF4 [ArF = 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.