DENSITY-FUNCTIONAL THEORY STUDY OF A LEWIS-ACID CATALYZED DIELS-ALDERREACTION - THE BUTADIENE PLUS ACROLEIN PARADIGM

The four transition structures (TS's) of the reaction between butadien e and acrolein, both uncatalyzed and catalyzed by BF3, have been theor etically studied taking into account electron correlation effects by m eans of DFT (B3LYP) calculations. In both the uncatalyzed and catalyze d reactions, the endo s-cis is the most stable of the four possible tr ansition structures. In the case of the catalyzed reaction, the inclus ion of electron correlation in the search for this transition structur e indicates the classical [4+2] reaction path, instead of that corresp onding to a [2+4] inverse electron, demands hetero-Diels-Alder reactio n, as happens when the Hartree-Fock level of theory is used. The B3LYP /6-31G(d) calculations lead to activation energy values close to those experimentally found. The origin of the endo/exo selectivity, both in the presence and in the absence of the catalyst, is discussed.