Mechanical Trapping of the Phlorin Intermediate

Abstract

Trapping unstable intermediates for elucidating reaction mechanisms in chemistry presents a formidable challenge. There has long been a lack of direct evidence for key intermediates like the highly reactive phlorin leading to porphyrin. Here, we report a molecular-strain engineering (MSE) strategy that harnesses intramolecular strain to trap the native phlorin during porphyrin synthesis. By mechanically constraining its periphery, a phlorin stable towards oxidation was captured as an isolable intermediate and fully characterized. This strained intermediate was transformed quantitatively into the corresponding bow-shaped porphyrin upon photoirradiation. Theoretical calculations indicate that the constraint imposed on the phlorin enhances the activation energy to reach the transition state of its oxidative aromatization and destabilizes the resulting porphyrin, thus facilitating trapping of this highly reactive intermediate. This proof-of-concept demonstration establishes the potential of MSE to capture native intermediates using mechanical stress as an alternative to adding scavengers and opens up a convenient way for carrying out mechanistic investigations.