Awakening a Molecular Mummy: The Inter-and Intramolecular Photochemistry of Pyromellitic Diimides with Alkyl Carboxylates

TL;DR

This paper explores how pyromellitic diimides react with alkyl carboxylates through electron transfer processes, leading to various radical products and structural changes.

Contribution

The study reveals new insights into the photochemistry of pyromellitic diimides, including their radical anion properties and degradation pathways.

Findings

Pyromellitic diimides oxidize alkyl carboxylates via intermolecular electron transfer, forming benzylation products.

Intramolecular electron transfer leads to cyclization products like 10 from GABA substrate 9c.

The green pyromellitimide radical anion is stable in the dark but degrades under photochemical conditions.

Abstract

Pyromellitic acid diimides are not as chemically unreactive as conjecturable (and presupposed) from their numerous applications as electron acceptor units or electron carriers in molecular donor–acceptor dyads or triads. Similar to the corresponding phthalimides, electronically excited pyromellitic diimides oxidize alkyl carboxylates in aqueous solution via intermolecular electron transfer (PET) processes, which eventually results in radical–radical combination products, e.g., the benzylation product 6 from N,N′-dimethyl pyromellitic diimide 5. The analogous product 7 was formed with pivalic acid as tert-butyl radical source. One additional product 8 was isolated from alkylation/dearomatization and multiple radical additions, respectively, after prolonged irradiation. In intramolecular versions, from N-carboxyalkylated pyromellitic diimides 9a–e (C1 to C5-spaced), degradation processes were detected, e.g., the cyclization products 10 from the GABA substrate 9c. In sharp contrast to phthalimide photochemistry, the green pyromellitic diimide radical anion was detected here by UV-vis absorption (λabs = 720 nm), EPR (from 9d), and NMR spectroscopy for several intramolecular electron transfer examples. Only the yellow 1,4-quinodial structure is formed from intermolecular PET, which was deduced from the absorption spectra (λabs = 440 nm) and the subsequent chemistry. The pyromellitimide radical anion lives for hours at room temperature in the dark, but is further degraded under photochemical reaction conditions.

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