DFT Study of Pd(PMe3)/NMe3-Catalyzed Butadiene Telomerization of Methanol

A detailed DFT study of Pd(PMe3)/NMe3-catalyzed butadiene telomerization of methanol predicts that the rate- and selectivity-determining step of the catalytic cycle is the external nucleophilic attack of methoxide on the π-allyl-Pd fragment of the cationic (PMe3)Pd(η3:η2-octadienyl)+ (3a). A crucial factor affecting the regio- and chemoselectivity appears to be the equilibrium between the chelated 3a and the dechelated species (PMe3)Pd(η3-octadienyl)(L′)+ (L′ = butadiene 3x/3y, or PMe3 3p): 3a is predicted to convert highly selectively to the linear telomer 8-methoxy-1,6-octadiene, whereas dechelated 3x/3y and 3p should be the major sources of both the branched telomer 3-methoxy-1,7-octadiene and byproduct 1,3,7-octatriene (via a novel β-hydrogen elimination mechanism). Insights into base effects were gained by comparing LiOMe and NMe3 as cocatalysts.