Tetrazole-azide tautomerism

C. Ligands Susceptible to Ring-Chain and Azide-Tetrazole Tautomerism... 

The fused tetrazole 422 (Scheme 19) was originally assumed307" to be nitrated on the thiazole ring, but recent NMR studies have shown that a p-nitrophenyl derivative is formed.1836 Since 422 (and 424) display azide tetrazole tautomerism (Section IV,B,1), the exact structure of the reacting species is unknown, but the product exists predominantly as the azide. 

PMR spectra have frequently been used to solve structural problems including azide-tetrazole tautomerism (Section IV,B,l),185b, 186<1,187b,c> 307b, 308,382,383.435 the sjte 0f C-protonation (Section IV,C,l,a)281,393, 395 and TV-protonation (Section IV,C,l,b),268b,36z and the conformation of C-phenyl substituents.146,451 The low-field displacement of chemical shifts by the introduction of a COR group into the molecule has frequently been noted,1860, 195, 203, 364,365 and use has been made of this effect to study the site of N-acylation (Section IV,C,3). 

Hydrazono-4-oxo-4ff-pyrido[l,2-u]pyrimidines were transformed by sodium nitrite in hydrochloric acid at 0-5 C to the corresponding 2-azido compound (102).166-287 2-Azido-4-oxo-4//-pyrido[l,2-azide-tetrazole tautomerism.290... 

Azides and tetrazoles usually equilibrate slowly at room temperature and the position of the equilibrium, which varies with temperature and solvent, is also susceptible to steric and electronic effects, electron-withdrawing substituents favouring the azide tautomer. The azinyl-azide/tetrazole tautomeric equilibrium, which has been extensively investigated and reviewed [9, 10], also arises when the C = N double bond comprises part of a heterocyclic ring, as exemplified by 2-azidoadenosine (5a, R = J3-d-ribofuranosyl) synthesized for use, as the diphosphate, as a photoaffinity reagent for the ADP receptor [11]. The azide was found to equilibrate with one or both of the two possible tetrazoles (5b and 5c) with an azide. tetrazole ratio of 1 1 at pH 7.0, and in contrast to the azide, which was readily photolyzed at 270nm, the tetrazole tautomer was resistant to photolysis. Azides that exist predominantly as tetrazoles may thus exhibit different... 

Azide-tetrazole tautomerism was discussed in CHEC-I. When imidazolidines react with phenyl-isocyanate the product may be cyclic or acyclic depending on the substituents (see also Section... 

Azide-tetrazole tautomerism has been investigated by n.m.r. spectroscopy and dipole-moment measurements for the 2-azidothiazole system, (801) (802), and for 2-azidobenzothiazole, 2-azidobenzoxazole, and 3-azidoisoxazole. Chlorosulphonation of the tetrazolobenzothiazole (803) with chlorosulphonic acid yields 2-azido-6-chlorosulphonyl-benzothiazole. ... 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and hve-membered heterocyclic azides. Tire tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

Studies of presumed azido-tetrazole tautomerism for 3-azido-l,2,4-benzotria-zine 1-oxides 3 show that compounds 3 exist only in the form of azides, both in the crystalline state and in solution. Similar results were obtained in the studies of 3-azido-1,2,4-triazine 1-oxides 4, which have never been detected in the tetrazole form (69CB3818, 77JHC1221, 82JOC3886). 

Azido-tetrazole tautomerism is also observed for partially hydrogenated azoles and even for monocyclic tetrazoles. The latter (258, X = NR ) may exist in equilibrium with an acyclic imidoyl azide... 

Mass fragmentation of modified nucleobases of 6-methyl tetrazolo[l,5-c]pyrimidin-5(6fl)-one and its 7 and 8-methyl derivatives suggested the occurrence of both azide and tetrazole tautomeric forms of M (661 and 662). For the 8-halo derivatives, only the of the tetrazole form was proposed (930MS643) (Scheme 135). 

Tetrazoles tautomeric with azides and their (selective) photochemistry in matrix are a favorite topic of mechanistic investigation (see the chapter by Fausto in Vol. 39 of this series)." Recent reports concern tetrazolyl-1,2.4-triazoles, and a tetrazolylethanol." On the other hand, preliminary data about the photochemistry of the 2-azido analogue of 2 -deoxyuridine (that exists as the tetrazole tautomer) have been reported, showing the formation of the corresponding amine." Non nitrene paths can also be followed, as in the case of some phenyltetrazolethione that cleave forming an open-chain diradical that in turn loses a sulfur atom (Scheme 15). ... 

For some compounds NMR data were collected to determine the equilibrium constants for the tautomerism between the azide form such as for 30 and the tetrazole derivative 31 (Scheme 3). Similar investigations were conducted for the compounds 28 and 29 and the solvent effects and substituent effects are discussed <1996JHC747>. 

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