Resolving a Sequential Post-Transition-State Bifurcation Mechanism in the Dimerization of 2,3-Diazacyclopentadienone

Bifurcating reactions yield multiple products from a single transition state (TS) without intervening minima, rendering product selectivity a formidable challenge, which is governed by dynamic effects rather than TS energetics. Herein, we report a rare instance of product selectivity in a bifurcating dimerization pathway of 2,3-diazacyclopentadienone. The reaction proceeds through two sequential unsymmetric bifurcations on the potential energy surface via a stepwise mechanism involving an intermediate, ultimately yielding a selectively dimerized product. The adduct formed via the N═N moiety acting as the dienophile is kinetically favored and proceeds through a closed-shell TS, while the thermodynamically controlled adduct where the C═C moiety serves as the dienophile is accessed through a singlet biradicaloid TS. The work further rationalizes the intricate interplay of the electronic structure and behavior that modulates electron flow and drives the dimerization that ultimately originates unprecedented product selectivity in a complex bifurcating landscape.