Photochemical Generation of Benzoazetinone by UV Excitation of Matrix-Isolated Precursors: Isatin or Isatoic Anhydride.

Hanna Rostkowska,Leszek Lapinski,Maciej J Nowak

doi:10.1021/acs.jpca.0c02562

Hanna Rostkowska, Leszek Lapinski + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpca.0c02562

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Abstract

Benzoazetinone was photochemically generated by UV irradiation of isatin isolated in low-temperature Ar matrixes. Upon UV (λ = 278 nm) excitation of isatin, monomers of the compound underwent decarbonylation and the remaining part of the molecule adopted the benzoazetinone structure or the structure of its open-ring isomer α-iminoketene. The same products (benzoazetinone and α-iminoketene) were generated by UV (λ = 278 nm) induced decarboxylation of matrix-isolated monomers of isatoic anhydride. Photoproduced α-iminoketene appeared in the low-temperature matrixes as a mixture of syn and anti isomers. Photoproducts generated upon λ = 278 nm irradiation of matrix-isolated isatin were subsequently exposed to λ = 532 nm light. That irradiation resulted in the shift of the α-iminoketene–benzoazetinone population ratio in favor of the latter closed-ring structure. The next irradiation at 305 nm caused the shift of the α-iminoketene–benzoazetinone population ratio in the opposite direction, that is, in favor of the open-ring isomer. Neither benzoazetinone nor its α-iminoketene open-ring isomer was generated upon UV (λ = 278 nm) irradiation of phthalimide isolated in Ar matrixes. Instead, the UV-excited monomers of this compound underwent such phototransformations as oxo → hydroxy phototautomerism or degradation of the five-membered ring with release of HNCO and CO. The efficiency of these photoconversions was low.

Highlights

Azetinone molecules are interesting because of the partially antiaromatic character of their four-membered ring.[1,2] The saturated azetidinone moiety is a common structural fragment of natural semisynthetic and synthetic β-lactam anitibiotics.[3,4] It has been recognized as a building block for the synthesis of many biologically important compounds, such as amino acids, peptides, alkaloids, aminosugars, and others.[5]Azetinone itself is an extremely unstable compound, even at low temperature
We demonstrated that photodecarbonylation of isatin as well as photodecarboxylation of isatoic anhydride yielded benzoazetinone and its open-ring isomer α-iminoketene
Intense bands appearing in the 1860−1820 cm−1 wavenumber range are typical of the stretching vibrations of the carbonyl group attached to a four-membered ring.[7−9,20−22] benzoazetinone may be treated as one of the obvious candidates for the structure of the main photoproduct generated by photodecarbonylation of isatin or by photodecarboxylation of isatoic anhydride

Summary

Introduction

Azetinone molecules are interesting because of the partially antiaromatic character of their four-membered ring.[1,2] The saturated azetidinone moiety is a common structural fragment of natural semisynthetic and synthetic β-lactam anitibiotics.[3,4] It has been recognized as a building block for the synthesis of many biologically important compounds, such as amino acids, peptides, alkaloids, aminosugars, and others.[5]Azetinone itself is an extremely unstable compound, even at low temperature. We attempted to generate benzoazetinone by in situ UV excitation of matrix-isolated precursors: isatin, phthalimide, or isatoic anhydride (Scheme 1).

Results

Conclusion