Theoretical study on the stereochemistry of intramolecular hetero Diels-Alder cycloaddition reactions of azoalkenes

The molecular mechanism for the intramolecular hetero Diels-Alder cycloaddition reactions of azoalkenes has been studied by using quantum mechanical calculations at the density functional theory (DFT) Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar (B3LYP) and second-order Moller-Plesset (MP2) theory levels with the 6-31G* basis set. The molecular mechanism corresponds to a concerted mechanism with a slightly asynchronous bond-formation process where the formed internal bond (N4-C5) obtained with DFr is longer than the peripheral bond (C1-C6) formed in the transition state. Otherwise, the (C1-C6) bond obtained with MP2 is longer than the (N4-C5) bond formed in the transition state. Solvent (dichloromethane) effects have been considered with the polarizable continuum model (PCM). The MP2 calculations predicted an endo kinetic preference, whereas the DFr ones rendered endo/exo transition states that are practically isoenergetic. According to our analyses, the dispersion interaction (which stabilizes the endo orientation through pi-pi secondary contacts) and the solvent effects (which favor the exo addition) are the factors governing the stereoselectivity of the process. (C) 2003 Wiley Periodicals, Inc.