Pd-catalyzed cycloisomerization to 1,2-dialkylidenecycloalkanes. 2. Alternative catalyst system

Article abstract of DOI:10.1021/ja00089a016

The mechanisms by which palladium complexes may catalyze the cycloisomerization of 1,6- and 1,7-enynes to dialkylidenecycloalkanes were probed by exploring a catalyst system different than a ligated palladium acetate which previously has proven to be successful. Although carboxylic acids showed no discernible interaction with palladium(0) complexes, this combination proved to be a powerful catalyst system to effect this cycloisomerization. The fact that the two catalyst systems do not have the same reactivity profile suggests this new catalyst system may operate by a different mechanism. Evidence supporting a pathway invoking formation of a hydridopalladium acetate followed by hydropalladation as initiation is presented. Steric and electronic effects direct the regioselectivity of the termination step to form either 1,3- or 1,4-diene products. The 1,3-diene products participate exceedingly well in Diels-Alder reactions, both inter- and intramolecularly. The presence of an oxygen substituent at the position allylic to the diene served as both a regiochemical control element for the palladium-catalyzed cycloisomerization and a diastereochemical control element for the Diels-Alder reaction. The net result of these two steps the first of which is a catalyzed isomerization and the second an addition, is a highly efficient approach to complex polycycles in terms of both selectivity and atom economy.