Tautomerizations and other structural isomerizations

During the period covered by this review, several relevant reports dealing with photo-tautomerizations and other types of structural isomerization processes induced by UV/visible light were published. Some of these 

Broadband irradiation resulted in the prompt completion of the reaction (fast disappearance of the IR spectrum due to -p-ionone vwis noticed) and the sole observation of Z-retro-y-ionone. This result implies that, under broadband UV-irradiation conditions, the Z-p-ionone is unstable, very likely decaying to additional conformers of Z-retro-y-ionone, as reflected in the broader bands due to this photoproduct observed in the IR spectra of the broadband-irradiated matrix. 

Mateo-Marti and Pradier investigated the chemical modifications occurring within a tripeptide molecule (Glu-Pro-Glu IGF), when exposed to the influence of UV irradiation. The authors described the successful inclusion of the IGF molecule in an argon matrix, followed by the in situ UV irradiation of the sample and subsequent characterization of the compound photo-reactivity. The studies have been performed by combining, Fourier-transform reflexion absorption spectroscopy (FT-IRRAS) and X-ray photoelectron spectroscopy (XPS). The comparison of the IR spectra of the isolated oligopeptide in an argon matrix obtained before and after UV irradiation, revealed significant differences that could be associated either to a partial deprotonation of the molecule or to a tautomeric conversion of amide bonds to imide ones. XPS analyses undoubtedly confirmed the second hypothesis (amido-imido tautomerism Fig. 22). 

23 Observed photoisomerization of matrix-isolated saccharin. Reprinted with permission from L. Duarte, t Reva, M. L S. Cristiano and R. Fausto, J. Org. Chem., 2013, 78, 3271. Copyright (2013) American Chemical Society. 

Ivanov et al. investigated the tautomerism of 5-bromouracil, finding that the diketo tautomeric form dominates the spectra obtained for the compound isolated in low temperature neon, argon and kiyton matrices (Fig. 26). 

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