Ketenimines rearrangement

There is no unity of opinion in the literature concerning a classification, i.e, whether to call these transformations aza-Claisen or aza-Cope rearrangements. It is accepted that the term aza-Claisen should be reserved only for those processes in which a carbon atom in the allyl vinyl ether system has been replaced by nitrogen357. Three different types of aliphatic 3-aza-Cope reactions which were studied theoretically are the rearrangements of 3-aza-l,5-hexadienes (610, equation 262), 3-azonia-l,5-hexadienes (611, equation 263) and 3-aza-l,2,5-hexatrienes (612, equation 264) (the latter is a ketenimine rearrangement )357. 

Murugavel and Punniyamurthy developed a Cul-catalyzed one-pot synthesis of iminocoumarin aryl methyl ethers from ynal, phenol, and sulfonyl azide via a cascade [3-1-2]-cycloaddition, 1,3-pseudopericyclic ketenimine rearrangement, 1,4-conjugate addition, and aldol-type condensation at room temperature under air [195] (Scheme 8.120). 

An unusual domino process was observed by Biehl and coworkers [69] in the reaction of 2-bromo-l-naphthol 4-196 with arylacetonitriles in the presence of LDA or LiTMP by employing 3-thienylacetonitrile 4-197, the tetracyclic compound 4-200 was obtained in 57% yield (Scheme 4.43). The reaction probably includes the formation of an aryne and a ketenimine which undergo [2+2] cycloaddition to give 4-198, followed by rearrangement and allylic addition to the intermediately formed aryl cyano compound 4-199. 

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). 

Moreover, metalated 2-benzyl-4,5-dihydrooxazoles partially rearrange to ketenimines (10-15%), as is evident from their IR spectra (v = 2018 cm1), as well as trapping with chlorotrimethylsilane 2. 

Figure 11.5. The magnitude of ko s decreases with decreasing temperature until 170 K, whereupon it reaches a value of 3.2 x 10 s. Below this temperature, koBs remains constant. " The breakpoint in the Arrhenius plot is 180-200 K, which is in exactly the same temperature range in which the solution phase chemistry changes from the trapping of ketenimine 30 with diethylamine to the dimerization of 33t. Thus, the low-temperature data of Figure 11.5 were associated with k]sc, the rate constant for intersystem crossing of singlet to triplet phenylnitrene, and the high temperature data with k., the rate constant for rearrangement of 33t.

In the liquid phase, singlet phenylnitrene is rapidly relaxed by collision with solvent and cannot surmount the barrier to form cyanocyclopentadiene at ambient temperature. Under these conditions PN isomerizes over a small barrier to form cyclic ketenimine K. Later, computational work of Karney and Borden would show this to be a two-step process involving benzazirine BZ, the species trapped by ethanethiol (Scheme 2). In the liquid phase, PN prefers rearrangement to intersystem crossing (ISC) to the lower-energy triplet state at ambient temperature. Intersystem crossing is not an activated process and its rate is not expected to vary with temperature. The rate of... 

Azobisisobutyronitrile (Formula 496) undergoes rearrangement to the ketenimine (Formula 497) on irradiation in benzene (225). This is the choice method of preparation of Formula 497 (225). 

The intermediate Ni(CNBu )3 N2C(CN)2), formed by adding diazodi-cyanomethane to Ni(CNBu )4, rearranges, probably via a carbene insertion, to give the ketenimine complex Ni Bu N=C=C(CN)2 (CNBu )2. An X-ray structural determination of the product has been reported (436). 

Ultraviolet irradiation of mesityl azide 288 in the presence of tetracyanoethylene has resulted in the isolation of the intermediate azomethine ylide 289 (from trapping of the aryl nitrene) together with its rearrangement product, the spiroazepine 290 (Scheme 36) <1997JOC3055>. Photolysis (at 313 nm) at low temperature of 1- and 2-azido-naphthalenes in an Ar matrix provided access to the novel seven-membered cyclic ketenimines 291 and 292, respectively <2004JA237>. 

The sulfonamide (30) undergoes a clean, uncatalysed deep-seated rearrangement to sulfone (31) via a ketenimine intermediate, a rearrangement in which both the / -toluenesulfonyl (Ts) and p-methoxybenzyl (PMB) groups sequentially migrate from the nitrogen atom on to a neighbouring carbon at the /3-position.51... 

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>. 

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). 

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