Study of the Electrocyclization of (Z)-Hexa-1,3,5-triene and Its Heterosubstituted Analogues Based on Ab Initio and DFT Calculations.

A comprehensive theoretical study of the electrocyclization of (Z)-hexa-1,3,5-triene and its heterosubstituted analogues (E,Z)-[(2Z)-2,4-pentadienylidene]amine and (2Z)-2,4-pentadienal to 1,3-cyclohexadiene, 1,2-dihydropyridine, and 2H-pyran, respectively, was conducted. The study included a conformational analysis of the reactants, locating transition structures and optimizing the closed products. The energy, geometry, and vibrational frequencies of all structures were calculated at the MP2/6-31G and B3LYP/6-31G levels. Also, electron correlation was improved by single-point calculations using the QCISD(T) and MP4SDTQ methods. The results were consistent with experimental values and accurately reproduced the decrease in activation energy of the heterosubstituted derivatives relative to the parent compound. Complementary natural bond orbital (NBO) computations helped to interpret such a decrease. The involvement of a lone pair of the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the cycle. In regard to the enthalpies of the electrocyclization reactions studied, the results reveal a clear-cut trend to instabilization in the closed forms when the terminal methylene groups are replaced with the heteroatoms. Thus, the process is clearly exothermic for hexatriene, much less so for the pentadienimine, and even slightly endothermic for the pentadienal.