Ketenimine, decomposition

The byproducts of decomposition of certain dialkyldiazcncs can be a concern. Consider the case of AIBN decomposition (Scheme 3.13). The major byproduct is the ketenimine (lO).61 100"102 This compound is itself thermally labile and reverts to cyanoisopropyl radicals at a rate constant similar lo that for AIBN thermolysis.59,60 102 This complicates any analysis of the kinetics of initiation/2,60... 

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. 

Even though AIBN has a low transfer constant, the ketenimine formed by combination of cyanoisopropyl radicals (Scheme 3.13) is anticipated to be more susceptible to induced decomposition (Scheme 3.22).1Cb... 

Another example of competition between the photocycloaddition reaction and energy transfer is provided by the irradiation of carbonyl compounds in the presence of dimethyl-AL(2-cyanopropyl)ketenimine 71.96 Here, the energy transfer is indicated by decomposition of the ketenimine. The ratio of photocycloaddition to decomposition products is dependent upon the n,n triplet energy of the carbonyl compound. These experiments are summarized in Table IX. Unlike the previously mentioned case with norbomene, the threshold energy for the competition is very broad, although in about the same region. 

Flash thermolysis of 4-substituted isoxazol-5(4//)-ones can be used to generate alkynes, isocyanides, aminoisocyanides and ketenimines (77C258, 76HCA2615). Decomposition of the oxime (538) at an oven temperature of 450 °C produces C02, benzonitrile and fulminic acid (539 equation 5) (79AG(E)467). This method thus offers a safe alternative to the synthesis of fulminic acid from the explosive metal fulminates. 

The preceding scheme also shows how a controlled thermal decomposition of the metallated thioimidoesters (enaminates (4)) by in situ trapping by an organometallic of the ketenimines (5) formed by loss of MeSLi allows a very direct route to ketones [381, 382]. An example of this application is given over. 

Decomposition of 2,6-difluorophenyl azide by LFP (266 nm) generated a singlet nitrene which was detected by time-resolved IR spectroscopy (1404 cm-1).75 The nitrene could only be detected between 243 and 283 K. At 298 K, the nitrene decay products, a ketenimine (1576 cm-1) and a triplet nitrene (1444 cm-1), were observed. The IR assignments were consistent with DFT calculations and previous UV-visible detection results. 

The decomposition of a me-triazole would appear to offer a direct route to the 2-azirine ring system. However, photolysis of triazole (3) does not give the 2-azirine (4) but indole (5) and ketenimine (6).2... 

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). 

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. 

The products of nucleophilic trapping (47, 48), after decomposition of 46, were initially rationalized as arising from benzazirine 50. This explanation was generally accepted in subsequent studies " and supported by calculations,but in 1978, Chapman and LeRoux detected 1-aza-l,2,4,6-cycloheptatetraene (51) using matrix isolation techniques. The existence of the cyclic ketenimine 51, was confirmed by later spectroscopic studies in matrices and in solution. It was also established that ketenimine 51 is the species trapped by nucleophiles in solution to form azepines 47 and 48. ... 

In the decomposition of AIBN, besides the formation of the products (tetramethyl succinonitrile (TMSN), isobutyronitrile (IBN) and MAN) from the normal Carbon-Carbon reaction of the cyanoisopropyl radicals, a ketenimine (5) from a Carbon-Nitrogen reaction also occurs. This arises because one of a pair of cyanoisopropyl radicals can tautomerize to a keteniminyl radical which subsequently reacts with the second cyanoisopropyl radical to form the ketenimine. This reaction is thermally reversible and the ultimate products would be TMSN, IBN and any polymerized MAN. 

The decomposition of an unsymmetrical azonitrile (6) results in the formation of 2 ketenimines (7) and (8) in addition to the normal products (Scheme 3). 

The free-radicals that forni can recombine inside or outside the solvent cage, where the decompositions take place, to yield either tetramethylsuccinonitrile or a ketenimine. ... 

As stated earlier, not all free radicals that form, however, initiate polymerizations. Some are lost to side reactions. Thus, for instance, some free radicals that form can recombine inside or outside the solvent cage, where the decompositions take place, to yield either tetramethylsuccinonitrile or a ketenimine [4, 5] ... 

The retroprocess B— A represents another possibility for the heterolysis of C=C in ketenimines. The step B gives the obligatory decomposition of charges... 

In 1961, Smolinsky reported on vapor phase pyrolysis of a-azidostyrene (52, Scheme 5.8), which furnished 3-phenyl-2//-azirine as the main product and provided the first example of the synthesis of such strained heterocycles from vinyl azides. By analyzing the IR data of the pyrolysates produced from 52, it was shown one year later that N-phenylketenimine is formed as a side product. By utilizing IR and NMR spectroscopy at low temperature, Wentrup and coworkers have smdied recently the detailed structures of another azirine-ketenimine pair, generated by thermal or photochanical decomposition of an enazide. The transformation of vinyl azides into 2//-azirines is currently the most frequently used access to these heterocycles. The manifold chemistry of azirines was reviewed several times," " and most of the aspects of their synthesis from alkenyl azides are summarized in Chapter 6 (Gilchrist Alwes). Therefore, only some additional certain points are included here. 

Figure 6. [Fe][DPPH] —[Fe]o vs, [DPPH] from decomposition of optically active hyponitrite (-%-) (22) and ketenimine...

The mechanism of decomposition of AIBN and other azonitriles is complicated by the formation of ketenimines as unstable intermediates. In the presence of high concentrations of RAFT agents, the ketenimine is intercepted and converted to by-products, which reduce the initiator efficiency and may cause some retardation. 

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