Cyclization of an α, β-Unsaturated hydrazone catalyzed by a BINOL-phosphoric acid: Pericyclic or not?

Density functional theory is used to study the mechanism of the title reaction, one of the first catalytic asymmetric 6π-electrocyclizations observed experimentally. The benzylideneacetone-derived phenyl hydrazone is chosen as model substrate for the cyclization reaction, both in the protonated (A) and unprotonated (B) form, while the isoelectronic carbon analogue, 1,5-diphenylpentadienyl anion (C), serves as a reference for comparisons. The barrier to cyclization is computed to be more than 15 kcal/mol lower in A compared with B, in line with the observed acid catalysis. The relevant transition states to cyclization are characterized for A and C using orbital inspection, natural bond orbital analysis, nucleus independent chemical shifts, and stereochemical indicators. The cyclization of C is confirmed to be pericyclic, while that of A can be described as pseudopericyclic ring closure involving an intramolecular nucleophilic addition.