Tetrazole systems

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. 

The reaction of the -C(Hal)=N-function with azide ion or hydrazoic acid is known to give the tetrazole system. As part of a mechanistic study of the one-pot synthesis of an azadibenzoporphyrine in 84% isolated yield from reaction of a 1-bromobenzopyrromethene hydrobromide 74 with sodium azide at 140 °C, 74 was treated with azide at lower temperature (60 °C) in an attempt to isolate the proposed azide mechanistic intermediate 75 however, the fused tetrazole 76 was isolated in 47% yield (identified by X-ray analysis) (Equation 4) <1999MI530>. Upon heating a dimethyl formamide (DMF) solution of tetrazole 76 to 140°C for 1 h, the desired porphyrin was indeed obtained in 14% yield, consistent with the temperature-dependent equilibrium between tetrazole and azide that has been observed with some fused tetrazoles. 

Proton and earbon NMR studies of interannular conjugation effects in aryl-substituted tetrazoles have been discussed <84CHEC-I(5)791>. The use of NMR spectroseopy to study tautomerism in tetrazole systems is eovered in Seetion 4.17.4. 

Several classes are known. Dimroth-type rearrangements occur by ring opening and reclosure so that one ring atom changes places with an exocyclic atom. The rearrangement of 5-phenylaminothiatriazole to l-phenyl-5-mercaptotetrazole in basic solution is reversible (Scheme 74). As the anion, the tetrazole system is the more stable whereas as the neutral species, the thiatriazole is more stable... 

Compounds of type 106 are readily available by intramolecular dehydrogenation of appropriate triazenopyrazoles 105 (Scheme 61) <1987CB1375>. A general approach to the fused tetrazole system is provided via intermolecular [3 + 2] cycloadditions of the azide group with the activated nitrile functionality present in cyanamides, thiocyanates, and cyanates as illustrated in Schemes 6264 . 

A wide range of stable metallic and molecular complexes of tetrazole systems, e.g. (3), has been described <77AHC(21)323) and many crystal structures have been determined for these complexes. The tetrazole ring is invariably planar with bond lengths characteristic of an aromatic system. Tetrazolate anion salts (4) are stable aromatic systems and they have been used widely as nucleophiles in synthetic reactions. 

Treatment of azide 130 with boiling water affords the urea derivative 152 in which the tetrazole system has been formed (Scheme 57) (Section IV,A,3,b) (80JOC1662)... 

Treatment of unsubstituted amidrazones with nitrous acid also provides a general route to S-substituted tetrazoles.1,288 This reaction has recently been used to obtain the ditetrazolylalkanes (100).289 Other general synthetic routes to the 5-substituted tetrazole system, which have been known for many years, have been thoroughly reviewed by Benson.1,2... 

A mixture of methyl-derivatives, i.e. (724) and (725), of the novel 1,2,4-triazolo[2,3-d]tetrazole system has been obtained by methylation of the azido-triazole anion (723). ... 

A comparison of the substituent effects in monosubstituted tetrazole systems and benzene derivatives has been presented (2011T(67)6316). For this purpose the pEDA index (2009JPOC(22)769), defined as the sum of the occupation of 2p orbitals at aU atoms of the ring minus 6, has been used as a substituent descriptor. In aU three cases (substituted benzenes, IH- and 2H-tetrazole derivatives) the pEDA index, which ilLustrates the Tc-electron transfer from the substituent to the ring or vice versa, is weU correlated with constants. A more detailed analysis revealed that the dependence of 2pj, occupancies at the carbon atoms of benzene in either ortho- or para-positions on pEDA values are represented by linear trends with cc= 0.971 and 0.968, respectively. In contrast, the same correlation for the carbon atom in the meta-position is worse cc= —0.791) and with a small... 

---