By Nitrene Rearrangement

The Tiemann rearrangement of amidoximes affords carbodiimides through a nidene intermediate. For example, reaction of the shown amidoxime 98 with benzenesulfonyl chloride in pyridine affords N-phenyl-N -cyclohexylcarbodiimide 99 in 64 % yield.  

DiaryIcarbodiimides are similarly obtained from amidoximes andPOCls in pyridine.The yields range from 51-74 %. Cyclic carbodiimides are also synthesized using the Tiemann rearrangement (see Section 11.2.3). 

The reaction of the tetrazolium salt 100 in the presence of triethylamine to give diethyl-carbodiimide 101 also involves anitrene intermediate. Mono aryl substituted tetrazolium salts react similarly to afford N-aryl-N alkylcarbodiimides.  

Likewise, oxadiazolium salts 102 react with triethylamine to give carbodiimides 103 (R = n-Bu, cyclohexyl, Ph).  

The thermolysis of 2-methyl-5-phenyl-l,3,4-oxadiazol-2-(3H)-one 106 produces a nitrile imine intermediate 105 which rearranges to the carbodiimide 107. The same carbodi-imide is obtained in the thermolysis of 2-methyl-5-phenyltetrazole 104. 

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Carbenes 210 generated from the triazoloquinoline 209 by FVT rearrange into a seven-membered ring ketenimine 211, similar to carbodiimide 208. The ketenimine similarly rearranges to 1-naphthylnitrene 212 and nitrene derivatives 213, 214, 215, and 216 (Scheme 36) <2004JOC2033>. 

H)-Oxazolones are formed by the spontaneous cyclization of /3-oxo isocyanates (equation 134). Similarly, o-hydroxyphenyl isocyanate, produced by the Curtius rearrangement of the azide of salicylic acid or by the action of sodium hypochlorite on salicylamide, forms benzoxazolone (equation 135). An analogous reaction is the formation of IV-phenyl-benzoxazolone by the action of thionyl chloride on the hydroxamic acid shown in equation (136) (78TL2325). Pyrolysis of aryl azidoformates affords benzoxazolones by nitrene insertion (equation 137) (81CC241). 

Overall, then, the Curtius rearrangement converts an acid chloride to an amine with loss of a car- tk>n atom—very useful. Also useful is the related Hofmann rearrangement, which turns an amide ito an amine with loss of a carbon atom. This time we start with a primary amide and make a trene by treatment with base and bromine. Notice how close this nitrene-forming reaction is to the tarbene-forming reactions we talked about on p. 1072. The nitrene rearranges just as in the Curtius reaction, giving an isocyanate that can be hydrolysed to the amine. 

Although this chapter is primarily concerned with the functionalization of unactivated spi C—bonds by nitrene insertion, other relevant aspects of nitrene chemistry are included. Thus the brief section on ni> trene reactivity highlights the rearrangement reactions which often compete with C—H insertion, and the final section covers insertion into sp C—H bonds, since many of these reactions have found wide use in recent years in the synthesis of natural products. 

Decomposition of sulphonyl a zides in aromatic solvents may lead to aromatic substitution , which is thought to involve the addition of the singlet nitrene to the aromatic nucleus, followed by further rearrangement to give productsIn aromatic solvents, the yields of unsubstituted sulphonamides (hydrogen abstraction) are better than in aliphatic hydrocarbons Due to the absence of biaryls in... 

The D-hex-2-ulopyranosyl azides (327) lose nitrogen on photolysis and are converted to a mixture of two labile imidates. The intermediate nitrenes rearrange by either C2-C3 cleavage (major pathway) or C1-C2 cleavage (minor pathway) followed by carbon-nitrogen bond formation to yield the imidates (328) and (329) respectively, irrespective of the anomeric configuration of the azides studied. The E,E, E,Z- and Z,Z-isomers of 2,6-di(4 -azidobenzylidene)cyclohexanone do not interconvert on irradiation either in the crystalline state or when adsorbed on silica gel but decompose to yield nitrene-derived products. The main decomposition products of the azides of terephthalic and isophthalic acids are reported... 

AI mixed with the amides of isoxazole-5-carboxylic acids, the mixture being difficult to separate (75ACS(B)65). Such problems are typical in the preparation of many other amino-substituted heterocycles by these nitrene rearrangements (Scheme 73). 

Flash thermolysis of 3-aryl[l,2,3]triazolo[l,5-a]pyridines (277) under mild conditions (380-500°C, 10-3 torr) affords carbazoles (279, 280) in nearly quantitative yields.232 The triazoles exist as the valence tautomeric diazo compounds 278 in the gas phase.189 The substitution patterns in the products (279 and 280) demonstrate that the reactions take place exclusively by a carbene-nitrene rearrangement in which the pyridylcarbenes insert into the 2,3-bond in pyridine (Scheme 52). 

Apparently, 139 is not formed by thermal rearrangement of the anthranil (see Section III,C,5,a) but directly from the azide, presumably by singlet nitrene attack at the pyridine ring nitrogen. Zwitterion 139 becomes the major product when the decomposition is carried out at higher temperature (215°C). Analogous by-products have also been noted in other azide decompositions. For example, low yields of 3-aryl-4-quinolones accompany 3-(j3-styryl)anthranil formation189 (see also Section III,C,5,a). 

V-ethoxycarbonyl nitrene to an intermediate azepine (33a) (itself formed by nitrene addition to 1.4-di-f-butyl-benzene) gives a 1,2-adduct (34a) which is in equilibrium with the Cope rearrangement product (34b) at 130C. The adducts (34) rearrange under the reaction conditions to give the 1,4-adducts (35a) and (35b). 43... 

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