Palladium(II)-catalyzed cycloisomerization of substituted 1,5-hexadienes: a combined experimental and computational study on an open and an interrupted hydropalladation/carbopalladation/β-hydride elimination (HCHe) catalytic cycle.

The Pd(II)-catalyzed cycloisomerization of 3-alkoxycarbonyl-3-hydroxy-substituted 1,5-hexadienes has been studied experimentally and computationally. Experimentally, the reaction is characterized by a rapid room temperature formation of monomeric as well as dimeric cycloisomerization products using the commercially available precatalyst (CH(3)CN)(4)Pd(2). In situ NMR measurements indicate the initial kinetic advantage of the desired cycloisomerization pathway to methylene cyclopentanes; however, double bond isomerization, elimination, and dimer formation are competitive undesired pathways. Evaluation of the obtained product structures by NMR spectroscopy and X-ray crystallography indicates that the sole determinant for the monomer/dimer ratio is the regioselectivity of the initial hydropalladation in favor of the allylic (monomer formation) or the homoallylic double bond (dimer formation). In order to account for the experimental results, we propose the coexistence of two product-forming catalytic cycles, an open, monomer generating, as well as an interrupted and redirected, dimer generating, hydropalladation/carbopalladation/β-hydride elimination (HCHe) process. Results from computational studies of the proposed competing catalytic cycles are supportive to our mechanistic hypothesis and pinpoint the pivotal importance of Pd(II)-hydroxo-chelate complexes for the reactivity-stability interplay of on- and off-pathway intermediates.