The Heck reaction has become an indispensable tool for the formation of aryl (or vinyl) -alkene C-C bonds, that often might otherwise be difficult to assemble. The Heck reaction can give rise to linear (β) and branched (α) isomers (Figure ) where the regioselectivity for intermolecular Heck reactions is governed by the catalyst, reagents, and the nature of aryl and alkene substituents X and R. The rule of thumb for the effect of R on the regioselectivity is that electron withdrawing groups (EWGs) tend to give the β isomer while electron donating groups (EDGs) tend to give mixtures. Certain combinations of X, R, catalyst, and reagents tend to favor the α isomer, by favoring a different ("cationic") reaction pathway than the more conventional ("neutral") pathway. However, there are many exceptions to these rules of as we discuss below and some have even argued that "we cannot make any predictions about the regiochemistry of arylation ... and temptations to find simple rules should be discarded." There is thus a great deal of interest in gaining a better understanding of the factors that govern the regioselectivity both theoretically and experimentally. and).