Ring strain release and pseudo anti-aromaticities control photochemical reactivities in photoclick reactions of solvated cyclopropenones

Abstract

Gas-evolving photochemical reactions use mild conditions to access strained organic compounds irreversibly. Cyclopropenones are a class of pseudo-aromatic light-responsive molecules used in bioorthogonal photoclick reactions; their excited-state decarbonylation reaction mechanisms are misunderstood due to their ultrafast (<100 femtosecond) lifetimes. We have combined state-of-the-art multiconfigurational quantum mechanical (QM) calculations and non-adiabatic molecular dynamics (NAMD) simulations to uncover the excited-state mechanism of cyclopropene and a photoprotected cyclooctyne-(COT)-precursor in gaseous and explicit aqueous environments. We explore the role of H-bonding with unprecedented QM/QM NAMD simulations (CAS-chromophore and HF-solvent) for the aqueous decarbonylation reaction. The cyclopropenones pass through asynchronous conical intersections but have dynamically concerted photodecarbonylation mechanisms. The cyclopropenones break planarity to relieve the S1 pseudo anti-aromaticity upon photoexcitation. The substantial structural distortions relieve pseudo anti-aromaticity towards non-aromatic structures and release inherent ring strain. After crossing to S0-state, the cyclopropenones maximize pseudo-aromaticities by reverting to the reactant or the second σCC bond breaks to form CO and an alkyne. The COT-precursor has a higher quantum yield of (53%) than cyclopropenone (28%) because these trajectories prefer to directly break a σCC bond to avoid the strained trans-cyclooctene S1-minimum geometry. Our QM/QM simulations show an increased quantum yield (58%) for the systems studied here. Favorable ground-state hydrogen-bonding interactions become repulsive in the excited state due to developing positive charges on oxygen in the S1-state, resulting from the nO → 𝜋* excitation.

Toward a Topological Classification of Nonadiabaticity in Chemical Reactions

“…Such a case is realized in the photochemical decarbonylation of cyclopropenones. 58 There are two proposed mechanisms for this reaction, one stepwise and one concerted. On the PES, the stepwise mechanism corresponds to two paths related by a mirror symmetry, each of which supports a minimum energy CI (MECI).…”