Abstract:Tetrazoles remain a challenge to photochemists. Photolysis leads to cleavage of the tetrazolyl ring, may involve various photodegradation pathways and may produce a diversity of photoproducts, depending on the structure and conformational flexibility of the substituents and the possibility of tautomerism. If the photochemistry of tetrazoles is considered within the frame of synthetic applications the subject is even more challenging, since the ultimate goal is to achieve selectivity and high yield. In addition, the photoproducts must remain stable and allow isolation or trapping, in order to be used in other reactions. This review summarises the photochemical transformations of tetrazole derivatives that can be used as effective synthetic routes to other compounds.

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The influence of the position of the methyl substituent in 1- and 2-methyl-substituted 5-aminotetrazoles on the photochemistry of these molecules is evaluated. The two compounds were isolated in an argon matrix (15 K) and the matrix was subjected to in situ narrowband UV excitation at different wavelengths, which induce selectively photochemical transformations of different species (reactants and initially formed photoproducts). The progress of the reactions was followed by infrared spectroscopy, supported by quantum chemical calculations. It is shown that the photochemistries of the two isomers, 1-methyl-(1H)-tetrazole-5-amine (1a) and 2-methyl-(2H)-tetrazole-5-amine (1b), although resulting in a common intermediate diazirine 3, which undergoes subsequent photoconversion into 1-amino-3-methylcarbodiimide (HN-N═C═N-CH), show marked differences: formation of the amino cyanamide 4 (HN-N(CH)-C≡N) is only observed from the photocleavage of the isomer 1a, whereas formation of the nitrile imine 2 (HN-C═N═N-CH) is only obtained from photolysis of 1b. The exclusive formation of nitrile imine from the isomer 1b points to the possibility that only the 2H-tetrazoles forms can give a direct access to nitrile imines, while observation of the amino cyanamide 4 represents a novel reaction pathway in the photochemistry of tetrazoles and seems to be characteristic of 1H-tetrazoles. The structural and vibrational characterization of both reactants and photoproducts has been undertaken.

A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for conformational isomerization allow extensive conformational cooling during deposition. Deposition of the matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which bear different types of intramolecular H-bonds: 1H-I has an NH · · · O hydrogen bond, whereas 2H-I has an OH · · · N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and 2H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being photostable.