RESUMO
The reaction pathway of an enantioselective 5-endo-trig-type cyclization of 3-alkenoic acids catalyzed by a chiral palladium-spiro-bis(isoxazoline) complex, Pd-SPRIX, has been studied by density functional theory calculations. The most plausible pathway involves intramolecular nucleophilic attack of the carboxylate moiety on the C=C double bond activated by Pd-SPRIX and ß-H elimination from the resulting organopalladium intermediate. The enantioselectivity was determined in the cyclization step through the formation of a π-olefin complex, in which one of the two enantiofaces of the olefin moiety was selected. The ß-H elimination occurs via a seven-membered cyclic structure in which the acetate ligand plays a key role in lowering the activation barrier of the transition state. In the elimination step, the SPRIX ligand was found to behave as a monodentate ligand due to the hemilability of one of the isoxazoline units thereby facilitating the elimination. Natural population analysis of this pathway showed that the more weakly electron-donating SPRIX ligand, compared with the bis(oxazoline) ligand, BOX, facilitated the formation of the π-olefin complex intermediate, leading to a smaller overall activation energy and a higher reactivity of the Pd-SPRIX catalyst.