Heterocumulenes ketenimines

Far more interesting are the elimination sequences which lead to the generation of reactive heterocumulenes (ketenimines, carbodiimides, isocyanates, isothiocyanates) or 1,3-dipoles (nitrile oxides, nitrile imides, nitrile... 

The 27T-electrons of the carbon-nitrogen double bond of 1-azirines can participate in thermal symmetry-allowed [4 + 2] cycloadditions with a variety of substrates such as cyclo-pentadienones, isobenzofurans, triazines and tetrazines 71AHC(13)45). Cycloadditions also occur with heterocumulenes such as ketenes, ketenimines, isocyanates and carbon disulfide. It is also possible for the 27r-electrons of 1-azirines to participate in ene reactions 73HCA1351). 

Diazo(trimethylsilyl)methyl lithium (3) was found to be the reagent of choice for the synthesis of azoles from heterocumulenes (Scheme 8.43). The reaction is typically carried out in ether at 0-20 °C. Thus, alkyl- (or aryl-)substituted keteni-mines are transformed into 1,2,3-triazoles 188 (246), while C-acceptor-substituted ketenimines yield either 4-aminopyrazoles 189 or 1,2,3-triazoles, depending on the substituents (247). Isocyanates are converted into 5-hydroxy-1,2,3-triazoles 190 (248). Reaction of 3 with isothiocyanates are strongly solvent dependent. 

N-Vinyl heterocumulenes represent a new, highly reactive 2-azadiene species, which react, in general, with electron-rich alkenes and alkynes. Accordingly, we think it is of interest to complement the utility of electron-poor 2-azadienes in [4 + 2] cycloadditions by showing some examples involving A-vinyl isocyanates, -isothiocyanates, -carbodiimides, and -ketenimines. 

In the absence of dipolarophiles the intermediate loses sulfur to give the carbodiimide however, the intermediate may be trapped with a number of alkenes, heterocumulenes and other reagents such as ynamines, ketenes, isocyanates, isothiocyanates, carbodiimides, ketenimines, sulfonylimines, imines, nitriles, thiocarbonyl compounds, Wittig reagents and the already mentioned enamines (80AG277). Contemporary knowledge of the chemistry of these sulfonyliminothiatriazolines is mainly due to the meticulous work of L abbe and his coworkers (80AG277). 

In addition to azomethines and oximes, various heterocumulenes, such as ketenimines, isocyanates and isothiocyanates, can be used to synthesize imidoyl chlorides. For example, addition of chlorine to ketenimines LXIX affords a-chloroimidoyl chlorides LXX... 

A number of reports concern the attack of nucleophiles on perfluoro-olefins to give cumulenes or heterocumulenes, which in turn imdergo nucleophilic attack to give heterocyclic compounds with perfluoroalkyi substituents. o-Phenylenedi-amine reacts with hexafluoropropene to give 2-(o j8j8>tetrafluoroethyl)benzimidazole (301) ketenimine intermediates have also been utilized in the formation of quinolines (302) (see p. lll) 89.59o j,y intramolecular cyclization, and (303) (see p. 112) by intermolecular cyclization, naphthyridines (304), pyridopyrimidines (305), and isoquinolines (306). The pyran derivative (307) is formed via isomerization of an allene intermediate (see p. 100). 3-Fluoro-2-isopropyl-l,2,4-thiadiazolin-... 

When C=N groups are part of a heterocumulene system, as in ketenimines and carbodi-imides, they readily undergo [2+2] cycloaddition reactions with isocyanates. For example, C,C-diphenyl-A -p-tolylketenimine 138 reacts with phenyl isocyanate to give the four-membered ring cycloadduct 139 in 83 % yield. The cycloaddition proceeds exclusively across the C=C bond of the ketene imine. ... 

However, instead of the o-quinoid ketenimines 89 and 91, which are the expected primary ring-opening products of azirines 83 and 84, respectively, the novel bond-shift isomers 87 and 88 were observed, which may be formally regarded as cyclic nilrile ylides. The existence of such yhdic heterocumulenes had been predicted previously. ... 

The aza-Wittig reaction of phosphazenes with heterocumulenes such as isocyanates (X = NR) and ketenes (X = CR2) has been extensively used for the formation of carbodi-imides and ketenimines, respectively (Scheme 15.9). 

---