Electronic and steric ligand effects on the activity and regiochemistry in the Heck reaction

The cationic phenylpalladium(II)diimine catalyst has been systematically substituted, both in a symmetrical and in an unsymmetrical manner, using Me, t-Bu, OMe, and F groups. The effects of these substitutions on the insertion aptitude and regioselectivity of propene have been investigated using DFT calculations. For symmetrical substitutions, a correlation has been found between the stability of the catalyst HOMO and the insertion barrier: a stable HOMO leading to a comparatively low barrier of insertion. In the case of unsymmetrical substitutions of F and OMe, trans-influences led to relatively large differences in insertion aptitudes. Steric effects were notable when t-Bu replaced hydrogen at the nitrogen positions, primarily through the interaction between t-Bu and alkene methyl groups. Insertion barriers ranged from 9.3 to 13.7 kcal/mol. There was a general preference for 1,2-insertions. The regioselectivity, DeltaDeltaE = E*(2,1) - E*(1,2), was in most cases modest (<1 kcal/mol). None of the investigated substitution patterns led to 2,1-insertion and only the unsymmetrical positioning of t-Bu improved the 1,2-insertion preference significantly.