Mechanism of Nucleophilic Attack on 1- and 2-Bromo(π-allyl)palladium Complexes1

It has been demonstrated that attack of sodium phenoxide nucleophile on the π-allyl Pd complexes generated from both 2,3- ( 1 ) and 1,3-dibromo-1-propene ( 15 ) is strongly regioselective for the central carbon, producing 2-bromo- and 3-bromo-3-phenoxy-1-palladacyclobutane, respectively. Then, via different mechanisms, both intermediates converge to produce 1,2-diphenoxy-2-propene ( 2a ) as the final product in good yield. Through the use of extensive deuterium labeling studies, a multistep mechanism for the formation of 2a from 15 has been delineated. Phenol derivatives substituted with electron-donating groups attack the central carbon of the π-allyl Pd complexes derived from both 1 and 15 . Electron-withdrawing groups on phenol shift regioselectivity for attack on the terminal carbon, kinetically. The regioselectivity of phenoxide attack is sharply inverted to the terminal carbon for both 1 and 15 when the ligand is changed from the π-acceptor ligand triphenylphosphine to tetramethylethylenediamine. The use of catechol, now possessing two nucleophilic sites, has extended this methodology to the production of a substituted 1,4-dioxane product that represents a new ring annulation approach for the production of such compounds.