Curtius-type rearrangements

N-benzylaniline with phosgene, and then with sodium azide to produce carbonyl azide 52. On heating, nitrogen is evolved and a separable mixture of nitrene insertion product 53 and the desired ketoindazole 54 results. The latter reaction appears to be a Curtius-type rearrangement to produce an N-isocyanate (54a), which then cyclizes. Alkylation of the enol of 54... 

When mesitylene-2-sulphonyl azide (3) is heated to 150 °C in n-dodecane, a Curtius-type rearrangement of the nitrene (4) occurs as discussed in Section 2.1 i to give 2,4,6-trimethylaniline and the hexa-methylazobenzene 14>. A similar result has now been observed by a careful analysis of the thermolysis products of durene-3-sulphonyl azide in w-dodecane at 150 °C. The amine is definitely formed but the azo-compound could barely be detected 13>. 

Photolysis of pentacoordinate pentavalent phosphorus azides leads to a Curtius-type reaction36,66. Further investigations into the photolytic behaviour of 25-phosphorus azide derivatives showed that three reaction types can be observed, depending on the nature of the substituents (a) Curtius-type rearrangement (b) tautomeric equilibrium between cyclic and open azides and (c) hydrogen abstraction reaction66. 

Photolysis of 5,5-dimethyl-3-azido-2-cyclohexene-l-one gave only the azepine (52).20 This product probably arises by a Curtius-type rearrangement to give a cyclic ketenimine which is hydrolyzed to the product 52. 

The thermal decomposition of terminal vinyl azides was originally believed to give only nitriles or, in some cases, indoles. The presence of 3-monosubstituted 1-azirines, however, has been inferred in the photolytic decomposition of some terminal azides.22,29 30 The nitrile is thought to arise in a similar manner to the ketenimine by an analogous Curtius-type rearrangement. The ketenimine (53) derived from terminal azides is unstable and rearranges to the nitrile (54). 

Nguyen Minh, T. Mechanism of the Curtius-type rearrangement in the boron series. An ab initio study of the boryinitrene (H2B-N)-iminoborane (HB=NH) isomerization. J. Chem. Soc., Chem. Commun. 1987, 342-344. 

Nguyen Minh, T., Fitzpatrick, N. J. Intermediacy of nitrene in the Curtius-type rearrangement of phosphinic azides. Insights from ab initio study of the H2P( 0)N. dbiharw. HP( 0) NH interconversion. Polyhedron 1988, 7, 223-227. 

Even if no nitrenes are produced as intermediates, the isocyanates formed in Curtius-type rearrangements are of value for the synthesis of heterocyclic compounds. For example, transient isocyanates cyclize unto azine rings, giving imidazoazines (Scheme 21).161 N,N-Diphenylcarbamoyl azide rearranges thermally to 1-phenylindazolone (Eq. 35) via the elusive diphenyla-mino isocyanate.162 Further examples have been summarized by Reichen.163... 

A novel synthesis of 4-ethyl-2//-l,2,4-benzothiadiazin-3(4//)-one (271 60%), which employs a photoinduced Curtius-type rearrangement of 2-azido-3-ethylbenzothiazolium tetrafluoroborate (270) in 0.1 M tetrafluoroboric acid, has been reported <87HCA2(M5>. The by-products are 2-amino-3-ethylbenzothiazolium tetrafluoroborate (272 12 /o) and 3-ethylbenzothiazol-2(3//)-one (273 4.4%) (Scheme 49). An acidic medium appears to be necessary for successful ring expansion since photolysis in methanol solution yields only the benzothiazolone (273) and the 2-amino derivative (272) in 8% and 89% yield, respectively. 

This can be thought to arise by the trapping of the N-sulphonylaniline by solvent methanol (equation 80). Thus, arenesulphonyl azides are able to undergo Curtius-type rearrangements. 

Diphenylphosphoryl azide (DPPA) and triethylamine were formerly employed by Yamada and co-workers in a simple one-pot synthesis of urethanes from carboxylic adds [250]. This procedure involved treatment of a carboxylic acid with triethylamine to produce the triethylammonium carboxylate salt, followed by heating in the presence of DPPA to yield the isocyanate via a Curtius-type rearrangement of the acyl azide. An alcohol was then introduced, functionalizing the isocyanate in situ to yield the urethane. 

In an early experiment, Abramovitch and Breslow observed the formation of traces of alkyl azides during Curtius-type rearrangement of sulfonyl azides. This formation was rationalized by a direct reaction between alkyl radicals and sulfonyl azides. Roberts examined the reaction of aryl and alkyl sulfonyl azides with allylstarmanes in order to... 

Iminophosphonium Salt Formation. Irradiation of azi-dotris(dimethylamino)phosphonium hexafluorophosphate in acetonitrile at 254 nm using a Rayonnet photochemical reactor for 15 h at 25 °C gives the iminophosphonium salt with release of nitrogen (eq 3). This is the first example reported of a Curtius-type rearrangement involving a charged atom. ... 

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