Electronic and Steric Ligand Effects on the Activity and Regiochemistry in the Heck Reaction

• Published in: Organometallics 2002-05, Vol.21 (11), p.2248-2253
• Main Authors: von Schenck, Henrik, Åkermark, Björn, Svensson, Mats
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om011095w

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, ΔΔE = E*(2,1) − E*(1,2), was in most cases modest (