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-pentadienylid ene]amine and (2Z)-2,4-pentadienal to 1,3-cyclohexadiene, 1,2-dihydropyridi ne, and 2H-pyran, respectively, was conducted. The study included a conform ational analysis of the reactants, locating transition structures and optim izing the closed products. The energy, geometry, and vibrational frequencie s of all structures were calculated at the MP2/6-31G** and B3LYP/6-31G** le vels. Also, electron correlation was improved by single-point calculations using the QCISD(T) and MP4SDTQ methods. The results were consistent with ex perimental values and accurately reproduced the decrease in activation ener gy of the heterosubstituted derivatives relative to the parent compound. Co mplementary natural bond orbital (NBO) computations helped to interpret suc h 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 elect rocyclization reactions studied, the results reveal a clear-cut trend to in stabilization in the closed forms when the terminal methylene groups are re placed with the heteroatoms. Thus, the process is clearly exothermic for he xatriene, much less so for the pentadienimine, and even slightly endothermi c for the pentadienal.