Ketenimine mechanism

Some of the complications associated with the use of AIBN may be avoided by use of alternative azo-initiators. Azobisfmelhyl isobutyrale) (AIBMe) has a decomposition rate only slightly less than AIBN and has been promoted for use in laboratory studies of polymerization85 because the kinetics and mechanism of its decomposition kinetics are not complicated by ketenimine formation. 

A similar mechanism may be invoked to explain the reaction of 47 with isocyanides in which ketenimine complexes are formed (39) ... 

Kurtz and Sheehter48 have observed similarly that irradiation of 3,4,5-triphenylisoxazole gives AT-phenylbenzoylphenylketenimine, 3-benzoyl-2,3-diphenyl-l-azirine, and 1,4,5-triphenyloxazole. The formation of the azirine and ketenimine from this reaction and from the decomposition of vinyl azides would suggest a similarity in mechanism. 

While a polar mechanism has been suggested for the formation of ketenimines by attack of triphenylphosphine or triethyl phosphite on the nitrogen of chlorodiphenylacetonitrile (36), ketenimines form from 1-cyanoalkyl radicals as well (32, 50). 

Hydrostannation of highly polarized a,)8-unsaturated nitriles leads to the formation of N-stannylketenimines 31, 33) in what is considered to be a polar 1,4-addition. Thus, polar mechanisms are ascribed to reactions leading to either 1-cyanoalkyl or ketenimine adducts. 

Due to the matrix isolation experiments (Section VIII,B), Chapman reinterpreted all the above photolysis experiments, replacing the bicyclic azirine (162) by the ketenimine (183).49 The new general mechanism, replacing... 

Upon UV irradiation, ethyl 5-amino-l-phenyl-l,2,3-triazole-4-carboxylate (40) does not expel N2 to give ketenimines or aziridines. Instead, in the presence of triphenylene as photosensitizer, 40 is smoothly isomerized, as a first example of a photo-Dimroth rearrangement,163 to ethyl 5-anilino-1,2,3-triazole-4-carboxylate (298).164 The mechanism is explained by a radical rearrangement, e.g., 2971-297III (cf. Scheme 81). This isomerization type was observed also with 5-amino-1,4-diphenyl-1,2,3-triazoles.165... 

Qualitative relative reactivity data indicate that metal alkyls with alkyl groups that form the best radicals are the most reactive, suggesting a free-radical mechanism . These reactions give the 1,2-addition products as well as the 1,4-keteniminate isomers. 

Photocycloaddition of ketenimines to ketones yields oxetane derivatives. The kinetics and mechanism have been studied for alkyl- and phenyl-substituted ketenimines reacting with benzophenone and fluorenone . Some relative rates (from quantum yields, after correction for light absorption by ketenimines) are given below. 

The 4-benzoyl-2,3-thiophenedione (260) undergoes thermal or photochemical cycloaddition with diphenylketene to give the spirothiete (261) and the thiophen-2-one (262) <91T3045>. The probable mechanism is shown in Scheme 49. With ketenimines, however, the reaction takes a different course, the initial step being a [4 - - 2] cycloaddition (Scheme 49) <87H(26)625>. The product is either (263) or (264) depending on the substituent R. 

The formation of 1,3-oxathiolane with chloral and 1,3-dithio-lanes with cycloaliphatic thioketones occurs regioselectively to yield the sterically less hindered products. On the other hand, aromatic thioketones, e.g., thiobenzophenone or 9/ fluorene-9-thione, intercept 8 to give comparable amounts of both regio-isomeric adducts. Usually, stereoisomeric dipolarophiles such as fumaronitrile and maleonitrile as well as dimethyl fumarate and maleate form 18 in a stereoselective manner. However, in the case of extremely electron-poor dipolarophiles, e.g., dimethyl 1,2-dicyanofumarate or ( )-l,2-bis(trifluoromethyl)ethylene-l,2-dicarbonitrile, non-stereospecific formations of the corresponding tetrahydrothiophenes are described. i- s This result is interpreted in terms of a stepwise reaction mechanism with a zwitterion as the key intermediate. Alternatively, this intermediate can cyclize to form seven-membered ketenimines of type 20. With R = Cp3, this product can be isolated in a crystalline form, whereas in the case of R = CN, stable lactam 21 is obtained only after addition of water (eq 10). 

AMI studies of the 2 - - 2-cycloaddition of ketenimines with imines show that the reaction involves a stepwise mechanism via an intermediate and two transition states. A computational investigation of the thermal 2 - - 2-cycloaddition between imines and ketiminium cations finds that the reaction takes place via a stepwise mechanism.The reaction of benzyne with Schiff bases, diimines, yields l,4-bis(2-substituted acridin-10-yl)benzene via an initial double 2 - - 2-cycloaddition. 

LFP data obtained at room temperature " demonstrate that, in agreement with previous flash photolysis studies,carbazole 66a is mainly formed on the millisecond time scale in pentane. Moreover, the characteristic ketenimine IR band was detected at 1868 cm in CDjCN. This band appeared faster than the time resolution of the apparatus (-100ns). The decay of this band was accompanied by the appearance of the carbazole band at 1241cm with a rate constant of 1.0 0.2 x lO s , which is close to the value measured by Sundberg et The TRIR experiments thus demonstrate,that ketenimine 70a does indeed serve as a source of 66a on the longer time scale, via the mechanism shown in Scheme 11.38. 

Alper and coworkers successfully exploited the cydocarbonylation reactions of o-iodophenols 1, 2-hydroxy-3-iodopyridine 2, and o-iodoanilines 3 with heterocumu-lenes, giving rise to benzo(e]-l,3-oxazin-4-one 4, pyrido[3,2-e]-l,3-oxazin-4-one 5, 4 (3H)-quinazolinone 6, and 4(3H)-quinazolindione 7 derivatives (Scheme 13.2) (21,22]. The reactions proceed in the presence of equimolar amount of palladium acetate and a bidentate phosphine ligand (dppb or dppf) and a base (i-Pr2NEt or K2CO3), in benzene or THF at 80-100 °C under a pressure of 20 atm of carbon monoxide. Carbodumides, isocyanate, and ketenimines were used as cumulenes, and good regioselectivities were achieved with unsymmetrical reactants. The reaction mechanism is beheved to involve in situ formation of a carbamate ester or urea-type intermediate followed by palladium-catalyzed carbonylation and cyclization to yield the products. 

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