title: Cyclization of an α,β-unsaturated hydrazone catalyzed by a BINOL-phosphoric acid: Pericyclic or not?
abstract: 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.
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470, Mülheim an der Ruhr, Germany.
authors.forename: Berit
authors.initials: B
authors.lastname: Heggen
authors.affiliation: Centre for Excellence in Basic Sciences Health Centre, University of Mumbai, Vidhyanagaria Campus, Mumbai, 400098, India.
authors.forename: Mahendra
authors.initials: M
authors.lastname: Patil
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470, Mülheim an der Ruhr, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Thiel
keywords: density functional theory; electrocyclization; nucleus independent chemical shift; pseudopericyclic; transition state