A theoretical study on intramolecular anion radical [2 + 2] cyclobutanation of bis(enones): Dramatic effects of the electron-deficient enone partners on the yielding of cyclobutane ring systems

DFT (U)B3LYP/6-311+G(d, p) calculations have been carried out to investigate the effects of substituent groups on the intramolecular [2 + 2] cyclobutanation reactions of the anion radical bis(enones). For the first time, we obtained the cyclobutane products containing non-aromatic substituent groups, thereby correcting the preconcept that no cyclobutanation reaction can occur in the absence of an aromatic group. All computational results confirm that the reactions proceed via a stepwise fashion. Furthermore, another new mechanistic aspect of the reaction is clarified: the first cyclization step requires a negative reaction center, while the second step prefers a neutral one; an electron-deficient substituent is not necessary in the first cyclization step but it is an indispensable element in the second step.