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

The rates of syn − anti isomerization in a series of cationic π-allylpalladium complexes [Pd(η 3 -Me 2 CCHCHD)(bisphosphine)]BAr F 4 [Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ] were measured by 1 H NMR spin-saturation transfer techniques. For the complexes with α,ω-bis(diphenylphosphino)alkanes (Ph 2 P(CH 2 ) n PPh 2, 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(η 5 -C 5 H 4 PAr 2 ) 2, Ar = 4-CF 3 C 6 H 4, Ph, 4-MeOC 6 H 4 ), it was fastest with the electron-withdrawing 4-CF 3 C 6 H 4 group and slowest with the electron-releasing 4-MeOC 6 H 4 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.