Nitrenes fluorophenyl

Our prediction, that 2-fluorophenylnitrene (lOf) should preferentially cyclize away from the fluorine, was subsequently verified experimentally by Leyva and Sagredo.89 They photolyzed 2-fluorophenyl azide in diethylamine and isolated the trapping product expected from cyclization of the nitrene away from the fluoro substituent (Scheme 20).89... 

As mentioned previously, unique kinetic results were obtained upon LFP of o-fluorophenyl azide,in that the singlet nitrene decays faster than the ketenimine is formed. This finding requires the presence of an intermediate, presumably ben-zazirine 40, between the singlet nitrene and ketenimine 42. The data could be interpreted by assuming that azirine 40 reverts easily to singlet nitrene according to the scheme below.The equilibrium constant is equal to the ratio of [40]/[39s] and was deduced to be 0.5 with AG 350 caFmol. Younger and Bell have also reported a system in which a benzazirine and ketenimine interconvert. 

INTERCONVERSION OF SINGLET NITRENE AND AZIRINE IN THE CASE OF ORTHO-FLUOROPHENYL AZIDE EXPERIMENT AND THEORY... 

This hypothesis was supported by analysis of the transient spectrum obtained upon LFP of 2-fluorophenyl azide, which reveals the presence of triplet nitrene 20a despite the small ratio of kisc/koss- This is clearly evident in Fig. 16 (Insert Spectrum 1), which presents the spectrum of the products formed from the decay of singlet nitrene 16a at room temperature. This spectrum is the sum of the spectrum of triplet nitrene 20a (narrow band at 303 nm and weak absorption below 450 nm) and ketenimine 18a (broad band at 350 nm). This complicated spectrum can be eompared with the simpler spectrum of ketenimine 18b (Spectrum 2) and the spectrum of triplet nitrene 20a observed as a persistent species in a low-temperature matrix (Spectrum 3). It is clear that the yield of triplet nitrene 20a is significant at room temperature. However, if one postulates that azirine 17a does not inter-eonvert with singlet nitrene 16a (Scheme 6, -r). then the yield of... 

Levya, E. Sagredo, R. Photochanistiy of fluorophenyl azides in diethylamine. Nitrene reaction vCTsns ring expansion, Tetrahedron 1998,54, 7367-7374. 

However for a few ortfto-substituted phenylnitrenes (namely, orf/to-fluorophenyl-, ort/to-biphenyl- and 2,4,6-tri-ferf-butylphenylnitrene), the ring-opening reaction was found to be the rate-limiting step (Scheme 11.31). 54,198,213,214 jjj jjjg nitrenes,... 

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