Nitrenes fluorinated

Our calculations showed that the first step, cyclization of the nitrene to an azabicyclo[4.1.0]heptatriene, is rate-determining. Our calculated barriers for cyclization of four fluorinated derivatives of lb are given in Table 6.87 The CASPT2/cc-pVDZ barrier of 13.4 kcal/mol for cyclization of 2,6-difluoro-phenylnitrene (10b) is 4.1 kcal/mol higher than the barrier computed for lb -+ 2b. In contrast, the calculated barriers to rearrangement of 3,5-difluoro-phenylnitrene (lOe) and 4-fluorophenylnitrene (10c) are very similar to that computed for unsubstituted phenylnitrene (lb). These computational results are consistent with the observed reluctance of pentafluorophenylnitrene (10a) and 2,6-difluorophenylnitrene (10b) to rearrange,48 1 81 83 and with the relative ease... 

For 2-fluorophenylnitrene (lOf), the barrier for cyclization at the fluorinated ortho carbon is computed to be ca. 3 kcal/mol higher than that for cyclization at the unfluorinated ortho carbon (Table 6).87 These results are in agreement with experimental observations that nitrene lOf rearranges rapidly to a ketenimine in solution810 and that 2,4-difluorophenylnitrene (lOd) undergoes ring expansion some 15 times faster than 2,6-difluorophenylnitrene (10b).81 c,d Both lOf and 2,4-difluorophenylnitrene (lOd) can cyclize at an unfluorinated ortho carbon but this is not possible for 2,6-difluorophenylnitrene (10b). 

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... 

It is presumed that the barrier-to-1,2-fluorine migration in the singlet nitrene substantially exceeds the analogous barrier-to-hydrogen shift. This difference allows a portion of the nascent singlet nitrene to relax to the lower energy triplet in the matrix. 

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

Placement of fluorine substituents at both ortho positions raises the barrier to cyclization by 3 kcal/mol, relative to the unsubstituted system. The work of Leyva and Sagredo" demonstrated, in fact, that cyclization of the singlet nitrene 39s proceeds away from the fluorine substituent. The steric argument predicts that a single o-fiuorine substituent will have little influence on the rate of conversion of 39s to 40, since cyclization occurs at the unsubstituted ortho-carbon."" ... 

Intersystem crossing rate constants of ortho- and meto-substituted singlet phenyl nitrenes are presented in Table 3. Mono- and di-ort/io-fluorine substituents have no influence on ISC rate constants. No effect with meta, metfl-difluoro substitution is observed either. Pentafluoro substitution has no efffect on kisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride. 

Curiously, the addition of a second orf/zo-fluorine substituent (i.e. in benzazirine 17d) raises the barrier to reversion to singlet nitrene 16d, relative to the mono orf/io-fluoro system (Fig. 14). This is partly due to steric hindrance by fluorine in the transition state for cyclization, but also to the stabilization of 17d by the fluorine attached directly to the azirine ring vide suprd)F The addition of the second fluorine substituent (benzazirine 17d) decreases the barrier to conversion of azirine 17d to ketenimine 18d slightly... 

Thermal decomposition of aryl azides under a nitrogen atmosphere leads to nitrenes which are able to substitute fluorine in polyfluorinated naphthalenes, albeit with a poor yield (4%). Since no substitution product was isolated when oxygen was present during this reaction, the intermediacy of a triplet nitrene was proposed, which is completely trapped by oxygen.207... 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

While this method has been used for a variety of compounds, replacement of F by N is very rare, although unusual compounds like the nitrenes OsF5(NC1) [83] or ReFs(NF) [84] have been isolated from such reactions. Even less favorable is fluorine replacement by Cl. Therefore this type of reaction is limited to very special but interesting cases [85,86] ... 

Morawietz, J. Sander, W. Photochemistry of fluorinated phenyl nitrenes matrix isolation of fluorinated azirines, J. Org. Chem. 1996, 61,4351-4354. 

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