Nitrenes methylnitrene

The majority of recent research in the photochemistry of azides and the generation of nitrenes has involved aryl rather than alkyl azides. Amongst the latter, however, is an interesting study of the photodissociation of methyl azide at wavelengths from 292 to 325 nm. The photodissociation dynamics of methyl azide are apparently complex, with the predominant pathway producing CH2=NH by concerted 1,2-H shift and N2 extrusion. Triplet methylnitrene (CH3N) was also observed by emission spectroscopy, but seems to arise by a minor, spin-forbidden pathway. 

Easy thermal cleavage of the N-N bond in pyridine N-methylimines66 has precluded isolation of such compounds. The other cleavage product, presumably methylnitrene, has not been detected or trapped, but phenyl-nitrene was detected in the thermal cleavage of the N-N bond of 1,2,3-triazole N-phenylimines.106... 

Thus, in a period of only a few years (1980 to 1987), methylnitrene in its triplet ground state had gone from the tennons category to what can be considered the best characterized of all known organic nitrenes. Nevertheless, the question of whether singlet methylnitrene 14 is an intermediate in the thermal and photochemical decomposition of 13 remained nnsolved. 

A very shallow minimum for singlet methylnitrene in the A state was found, and its rearrangement to 15 was studied using the CAS(10,8)/MP2 procedure. It was possible to localize a transition state of Cj symmetry only 1.4 kcal/mol above the minimum of singlet nitrene. 

If a free singlet nitrene is an intermediate, produced upon decomposition of a vinylazide, then the activation energy of vinyl nitrene rearrangement to azirine must be very small. The situation is similar to the case of methylnitrene, produced upon photolysis of methyl azide (see Section 5.3). 

Methylnitrene Formation. Pyrolysis of Mc3SiN(Me)-OSiMes at 150 °C gives methylnitrene (MeN ) through a-deoxysilylation. The nitrene intermediate can be trapped by (MeO)2MeSiH to give the insertion product (MeO)2MeSiNHMe. 

Since there is no evidence of nitrene intermediates in the solution photochemistry of alkyl azides, attempts were made to detect triplet alkylnitrenes in matrix photochemical experiments. Photolysis of CH3N3 or CD3N3 at cryogenic temperatures in Ar, N2 and CO2 matrices fails to produce an IR spectrum attributable to triplet methylnitrene. The IR spectrum of imine CH2=NH (or CD2=ND) is observed instead. 

The situation with vinylnitrenes is analogous to methylnitrene and it is not clear if either of these singlet nitrenes are true reactive intermediates with finite lifetimes. Quantum chemical calculations can help to explain the properties of vinylnitrenes. All calculations reported in the literature have been concerned with only the simplest vinyl azide and vinylnitrene, CH2=CH-N3 and CH2=CH-N (29a), respectively. Early theoretical studies on the vinyl azide to azirine transformation employed semi-empirical calcula-tions, or ab initio calculations performed at relatively low levels of theory, and have focused only on the closed-shell singlet excited state ( A ) of vinylnitrene. We will discuss only the latest calculations. ... 

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