Photochemistry of 1- and 2-Methyl-5-aminotetrazoles: Structural Effects on Reaction Pathways

Abstract

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.

Insights into the photochemistry of 5-aminotetrazole derivatives with applications in coordination chemistry. Effect of the saccharyl moiety on the photostability

“…Nevertheless, under the experimental conditions used, the antiaromatic three membered ring 7 could indeed be generated as the sole photoproduct and was found to be photostable. This result is even more remarkable because, contrarily to what is observed in this case, the diazirene derivative of 2-methyl-5-aminotetrazole was found to react upon irradiation at 325 nm [33]. On the other hand, the observed photostability of the antiaromatic three membered diazirene 7 follows the trend observed for a previously studied S-linked tetrazole-saccharinate [10], and appears to be a common phenomenon on this type of conjugates, probably induced by the stabilizing effect of the electron-withdrawing saccharyl moiety.…”

“…From the very first studies [32], it was postulated that the parent tetrazole in its gas phase most stable isomeric 2H-form, as well as 2-, 5-and 2,5-substituted tetrazoles, undergo fragmentation through formation of a nitrile imine intermediate that cyclizes to a 1H-diazirene, then leading to a final carbodiimide through rearrangement. However, this pattern of reactivity was demonstrated experimentally only very recently, during studies on the photochemistry of tetrazoles under low-temperature matrix isolation conditions [5,13,15,21,33,35]. The observed outline of photo-fragmentation of 4 upon irradiation at λ = 250 nm follows this general pattern, which, as also found in other cases [10,33], is accompanied by an additional reaction path involving as reactant the 1H-diazirene (see Scheme 3).…”

“…Subsequent irradiation of the same matrix at λ = 250 nm resulted in a fast consumption of both the reactant 4 (see absorption at 1662 cm −1 ) and the diazirene 7 while, simultaneously, several new bands (2263, 2138, 2118, 2050 cm −1 ), due to other photoproduced species, appeared in the spectrum. The identification of the photoproducts corresponding to these characteristic absorption bands could be easily achieved by comparison with the reported results obtained for the photolysis of an S-linked tetrazole-saccharinate and the parent 2-methyl-5-aminotetrazole [10,33]. The band at 2138 cm −1 was ascribed to the νNCN antisymmetric stretching of carbodiimide 8 (see Scheme 3), the band at 2263 cm −1 was ascribed to the νC≡N stretching of nitrile 9 and the band at 2050 cm −1 was ascribed to the νCN stretching of CNH 12.…”

“…of irradiation, suggesting that the efficiency of the reaction is rather low at this wavelength, it should be noticed that the capture of antiaromatic (i.e. 4π systems) structures such as 7 proved to be quite challenging, and has only been observed in rare cases, mostly upon isolation in cryogenic matrices [10,33,[35][36][37]. Nevertheless, under the experimental conditions used, the antiaromatic three membered ring 7 could indeed be generated as the sole photoproduct and was found to be photostable.…”

New endoperoxides highly active in vivo and in vitro against artemisinin-resistant Plasmodium falciparum

“…An additional interest of this substitution pattern resides in the potential of 2,5-disubstituted tetrazoles to act as probes. It was demonstrated that, upon narrow-band UV-induced irradiation, 2,5-dissubstituted tetrazoles generate nitrile imine dipolar species in situ, via extrusion of N 2 , which may then react with an alkene dipolarophile through a 1,3-dipolar cycloaddition reaction [ 49 , 50 ]. Thus, the trioxolane–tetrazole conjugates LC131 and LC136 could also be developed as tags for mechanistic studies.…”