Azides, thioacyl

Thiatriazoles generally decompose upon mild heating into nitrogen, sulfur, and an organic fragment (Scheme 2). Considerable attention has been given to this reaction because of the synthetic possibilities and because of the possible involvement of intermediates such as thioacyl azides (24), thioacyl nitrenes (25), thiazirines (e.g. (34)) and nitrogen sulfide (29). 

Jensen et. al. pointed out that if the suggested mechanism is correct the thioacyl azide (24 X = OR) must decompose rapidly because the characteristic azide band at approximately 2130 cm was not observed in the infrared spectrum of a decomposing thiatriazole <67ACS2792>. 

Pilgram et al. tried to intercept the proposed thioacyl azide with trivalent phosphorous compounds and they isolated 2,2-dihydro-l,3,4,5,2-thiatriazaphosphorines <71JHC951> but it is not possible to distinguish between a direct attack on the heterocyclic ring and attack on the thioacyl azide formed by ring opening. 

However, all attempts to demonstrate the formation of dinitrogen sulfide (29) by mass spectrometry in gas-phase pyrolysis failed and since dinitrogen sulfide (erroneously) had been reported <75CPL(33)393> to be a Stable compound at RT Equation (1) was excluded <78JOC48l6>. Holm et al. stated that the thioacyl azide may exist as two conformers E (31) and Z (32)) (Scheme 3) <78JOC48i6> where the rotation around the C—N bond is believed to be restricted because of a partial doublebond character. 

As an overall conclusion two routes seem to operate in the thermal degradation of 1,2,3,4-thiatriazoles a major route leading to nitrile (28) and dinitrogen sulfide (29) (Equation (1)), and another leading to isothiocyanate (27) most likely via thioacyl azide (24) from the Z-form (32) and presumably accompanied by formation of nitrile (28) from the -form (31). 

In this laboratory a number of experiments have been conducted to trap or obtain evidence for the hitherto unknown thioacyl azides, thioacyl nitrenes, and thiazirines during thermally19 or photochemically... 

Had the phosphazide 5 been formed in the reaction of the thiatriazoles, then loss of nitrogen to form compound 6 would be the logical consequence. The authors therefore favor the direct insertion route without intervention of the (unknown) thioacyl azide, but no decisive choice can be made between alternative reaction mechanisms, since, if a compound 5 is actually formed from an open-chain thioacyl azide, it may rapidly cyclize to form the thermodynamically stable 4. 

The structure is in part inferred from the observation that in boiling tetrahydrofuran the substance loses nitrogen and sulfur and gives pentyl cyanide. The absence of absorption in the neighbourhood of 2150 cm-1 indicative of an azide group excludes the thioacyl azide structure.17... 

In some cases at higher decomposition temperatures (above 100 °C), isothiocyanates 11 may be formed after concerted Curtius-type decomposition/rearrangement of a (Z)-thioacyl azide 10-Z, while the corresponding ( )-isomer would give the nitrile 12 in a Grob-like fragmentation. The nitrenes are formed after nitrogen extrusion from the thioacyl azides, which are the open-chain isomers of the 1,2,3,4-thiatriazoles (Scheme 3). 

A series of MO calculations has been performed on the structure, formation and nucleophilic reactivity of 1,2,3,4-thiatriazoles (1) (76AHC(20)145). However, these calculations have been criticized (see Section 4.28.2.2.3 for a detailed discussion). MO calculations have been carried out for the hypothetical ring closure of thioformyl azide 5-oxide (8 H substituted for Ph) to thiatriazole 5-oxide. It is suggested that the variable chemical behavior of thioacyl azides and their 5-oxides is due to disruption of the aromatic character of the thiatriazole 5-oxide (77T2231). 

Smith and Kenny attempted the preparation of thioacyl azides C—CS—Ng (as opposed to the type C—NH—CS—Ng dealt with above). They obtained only thiatriazoles again, as shown by the absence of an azide band in the i.r. spectra of their products. Thermolysis of their thiatriazoles gave no indication of an intermediate... 

Aryloxy- and alkyloxy-l,2,3,4-thiatriazoles have been prepared from azide ion and aryl chlorothionofbrmates " or alkyl chloro-thionoformates , respectively. Alkyloxythiatriazoles are also made from alkyl thionocarbazates and nitrous acid ° °. The subject has been reviewed by Martin . The 5-aryloxy- or 5-alkyloxy-l,2,3,4-thiatriazoles decompose thermolytically in a manner analogous to the decomposition of the other triazoles mentioned above they give nitrogen, sulphur and cyanates. This is the most used route to alkyl and aryl cyanates, R—OGN. The mechanism of this decomposition could be concerted, or it could involve a thioacyl azide ... 

The only data on hand to distinguish between the two mechanisms are the kinetic studies of Jensen on a variety of 5-alkoxy- and on 5-phenoxy-l,2,3,4-thiatriazoles. The energies of activation and the activation entropies are nearly the same for all compounds studied = 24-6 kcal mole" and = 4-4 e.u. Jensen interprets this as indicating a stepwise mechanism, in which thioacyl azide is formed in... 

Thioacyl azides or thiatriazoles or both might well be intermediates in reactions of fluorinated thfocarboxylic acids with sodium azide The photolysis of thiatriazoles leads to some rearrangement, forming iso thiocyanates , perhaps by way of the thioacyl azide. 

Imidoyl azides, RC(=NX)— Ns, display ring-chain tautomerism, as do die thioacyl azides, RCS—N3, discussed in the preceding section. Again, the cyclic valence tautomer is most often the more stable form, but in contrast to thioacyl azides, many imidoyl azides are known and are indefinitely stable at room temperature. 

As demonstrated above, azidoformates, which, like the very similar carbamoyl azides (30 equation 33),"9, 20, 50 pjay an important role in peptide chemistry and as nitrene precursors," can be generated in the usual way. TTieir thermal stability is generally higher than that of alkanoyl and aroyl azides. The tendency of acyl azides to form 1,2,3,4-oxatriazoles in a cyclization process is extremely low. In contrast to this thioacyl azides, which are only generated on treatment of thioxohydrazides with nitrous acid, seem to be only very short-lived intermediates, which immediately form 1,2,3,4-thiatriazoles as stable end products. 

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