Methylated triazoles and tetrazoles

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

The nitrogen NMR shieldings in methylated triazoles and tetrazoles were computed at the GIAO/HF/6-31-I--I-G level and more recently using multireference... 

Introduction. Annular tautomers are prototropic tautomers in which the migrating proton is restricted to ring atoms, which for azoles are usually nitrogen atoms, e.g., 239 240 and 241 242. For unsubstituted pyrazole 239 (R = R2 = H) and imidazole 241 (R = H) the two tautomers are identical, but this does not apply to substituted derivatives. For triazoles and tetrazoles the unsubstituted rings can occur as two distinct tautomers. However, interconversion occurs readily and such tautomers cannot usually be separated. In rare examples where prototropic tautomers can be separated as discrete crystal forms they are referred to as desmotropes, e.g., isomers 147 and 148 (Section 2.4.3.4). Further examples of desmotropes are 3-methyl-4-nitropyrazole 239 (R1 = N02, R2 = Me) and 5-methyl-4-nitropyrazole 240 (R1 = N02, R2 = Me) <1994CC1143>. 

For both azole and benzazole rings the introduction of further heteroatoms into the ring affects the ease of quaternization. In series with the same number and orientation of heteroatoms, rate constants increase in the order X = 0requires stronger reagents and conditions methyl fluorosulfonate is sometimes used (78AHC(22)71). The 1-or 2-substituted 1,2,3-triazoles are difficult to alkylate, but methyl fluorosulfonate succeeds (7IACS2087). 

It should be noted in conclusion that l-amino 4-methyl-5-imino 4,5-dihydro-l//-tetrazole 306 obtained by elimination of I INj from l,5-diamino-4-methyltetrazolium salt (X = N3) 172 (cf. Section 6.07.5.2.1) at 160-185°C decomposes giving HCN, 1,2,4-triazole, and cross-linked products <2005THE168>. 

Other studies (see, e.g., ref. 187c and references cited therein) have revealed that, in systems containing only O and N as heteroatoms, the azide form 184 is usually the only isomer observable in solution or in the solid state, whatever the position of the heteroatoms. Recent work has shown that the tetrazole forms are predominant in systems containing only nitrogen as the heteroatom in two special cases. If a proton is removed from the azide (e.g., 305), the anion 306 cyclizes to the tetrazole 306b. The cyclized form is likewise formed from the anions of azido-indazoles, -imidazoles, and -benzimidazoles but cycliz-ation is only partial with s-triazoles and negligible with tetrazoles. It is possible to trap the cyclized anions (e.g., 306b) by methylation - (Section IV,C,2) and obtain stable pyrazolo[l,5-rf]tetrazoles, 307 and 308. 

There is a far from complete set of comparisons of relative reactivities" in nucleophilic displacement of halide. From a study of reactions of bromo-A-methyl-tetrazoles, -triazoles and -imidazoles with piperidine, it seems that the presence of two doubly bonded nitrogens is necessary to overcome electron release from the singly bonded nitrogen to approach the reactivity of 2-bromopyridine. ... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Acid hydrolysis of 3-methyl-6-phenyl-l,2,4-triazine 4-oxide (827) yields 4-phenyl-1,2,3-triazole through an acyclic intermediate (828) (Scheme 168) <89AHC(46)73>. 1,2,4-Triazine 2-oxides (829) undergo rearrangement in basic conditions (Equation (80)) to form 4-substituted 2//-1,2,3-triazoles <89AHC(46)73>. l-(2-Nitrophenyl)-5-aryltetrazoles (830) and l-aryl-5-(2-nitrophenyl)tetrazoles are converted into 2-arylbenzotriazoles (831) by refluxing in nitrobenzene (Equation (81)) <81AJC69l>. 

If this compd has an amino group in position 4 of the pyridine ring and a methyl group in position 1 of the tetrazole ring, then the name will be 4f-Aminopyrido-l 2. 4,5-( 1 -methyl-sym-triazole)... 

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