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 involving, for example, the highly reactive phlorin leading to porphyrin. Here, we report our use of a molecular-strain engineering (MSE) strategy, involving the harnessing of intramolecular strain, to trap the native phlorin during porphyrin synthesis. By mechanically constraining its periphery, a phlorin, which is stable towards oxidation, has been captured as an isolable intermediate and fully characterized. This strained intermediate can be 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 the 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.

Mechanical trapping of the phlorin intermediate

ACS Spring 2022

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