Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

Abstract

Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.

Pyrolysis of hydrazine derivatives and related compounds with N N single bonds

“…It is likely that elimination of EtOH first generates an N-acylnitrile imine 150, which then cyclizes to the oxadiazole. This is a well-established type of reaction of N-acylnitrile imines [10,134].…”

“…The isocyanate-forming reactions are analogous to the Curtius rearrangement of acyl azides and probably does not involve free acylnitrenes [10]. M a n u s c r i p t 20 Aminimides derived from amidrazones are described in Section 13.…”

“…We will deal primarily with pyrolysis reactions of 71 hydrazine derivatives understood in the widest sense. This will include some triazenes, 72 M a n u s c r i p t 3 azimines, and 1,2,3-triazines inasmuch as N-N single bond cleavage is involved, but 73 fragmentation of the aromatic pyrazoles [8], triazoles [10] and tetrazoles [10] will be excluded, 74 as pyrolyses of these compounds have been described elsewhere. Likewise, azides are 75 excluded, as they have been covered recently [10].…”

“…This will include some triazenes, 72 M a n u s c r i p t 3 azimines, and 1,2,3-triazines inasmuch as N-N single bond cleavage is involved, but 73 fragmentation of the aromatic pyrazoles [8], triazoles [10] and tetrazoles [10] will be excluded, 74 as pyrolyses of these compounds have been described elsewhere. Likewise, azides are 75 excluded, as they have been covered recently [10]. The formation of diazoalkanes and hence 76 carbenes by thermolysis of salts of tosylhydrazones (Bamford-Stevens reaction) [11] will not 77 be covered in detail, as numerous reviews and books are available.…”

“…It will be mentioned 78 briefly in Section 6 together with other pyrolysis reactions of hydrazones. The highly 79 interesting thermal chemistry of nitrile imines RCNNR' will also not be covered here, as 80 recent reviews are available [10,12]. Nitrosamines R2N-NO and nitramines R2N-NO2 are 81 outside the scope of this review, but it is noted that the environmentally important assay of 82 nitrosamines is often performed by pyrolysis-GC-TEA, which involves thermal cleavage to 83 aminyl and NO radicals [13].…”