Nitrenes molecular rearrangements

Flash vacuum thermolysis of 6-chloro-3-diazoindazole (2b) resulted, upon loss of nitrogen, in the formation of carbene 102, which could intra-molecularly rearrange to the nitrene 103, or to the azabenzocyclopropene 104, or to azacycloheptatrienylidene 105 (Scheme 29). The only isolable product was 106, formed by dimerization of the nitrene 103 (78CB2258). 

We welcome one new author to the series Prof K. C. Westaway has reviewed Nucleophilic Aliphatic Substitution in place of Prof I. Lee, whose cameo contribution to ORM 2004 is gratefully acknowledged. The contribution of Prof K. Banert and Dr H. Hahn to that volume is also particularly appreciated since it enabled the now returning author of Molecular Rearrangements, Pt 1 to enjoy a productive maternity leave. Thanks are also due to outgoing author Prof D. M. Hodgson who, following his expert contributions over several years, felt able to leave the Carbenes and Nitrenes chapter in the capable hands of his co-authors. 

Eor a treatise on azides, which includes discussion of rearrangement reactions, see Scriven, E.F.V. Azides and Nitrenes, Academic Press, NY, 1984. For a review of rearrangements of alkyl and aryl azides, see Stevens, T.S. Watts, W.E. Selected Molecular Rearrangements, Van Nostrand-Reinhold, Princeton, NJ, 1973, pp. 45-52. For reviews of the formation of nitrenes from alkyl and aryl azides, see, in Lwowski, W. Nitrenes, Wiley, NY, 1970, the chapters by Lewis, E.D. Saunders, Jr., W.H. pp. 47-97, 47-78 and by Smith, P.A.S. pp. 99-162. 

Organoboranes react with ethyl 4-nitrobenzenesulphonyloxycarbamate under basic two-phase conditions in the presence of benzyltriethylammonium chloride or Aliquat to yield ethyl (V-alkylcarbamates [38]. The reaction probably proceeds via the initial formation of the nitrene, which reacts with the borane to form a B -N+ zwitterion. Subsequent rearrangement and solvolysis leads to the product. Aliquat is the better catalyst for the higher-molecular-weight boranes. 

This is called the Curtius rearrangement. There is no evidence for the existence of the free nitrene in this reaction, unlike in the Wolff rearrangement where there is some evidence for the existence of the free carbene, so probably the steps are concerted in this case. The driving force for this reaction is the production of molecular nitrogen. This provides so much energy, that it is possible to harness it to expand an aromatic ring. Indicate the pathway that is followed when an aryl azide is heated with phenylamine. 

However, on bending the carbene or nitrene 10° out of the molecular plane -as required for rearrangement — both methods give identical answers the carbene or nitrene lone-pair conjugates with the ring. The HOMO and LUMO orbitals in phenylnitrene are shown below. The more electrophilic the ring, the more pronounced... 

The primary fragmentation and rearrangement processes in the molecular ions of the iV-sulfonylimines are more numerous. Scheme 11 shows the primary fragmentations of 1,2,4-triazole 4-sulfonylimines.81 The favored reaction is the cleavage of the N-N bond, as is shown clearly by spectra at low electron energy. Cleavage of the N-S bond is much less frequent, giving rise to 92, which formally corresponds to an ionized 1,2,4-triazolium nitrene. 

---