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.