Concerted decomposition

Multi functional initiators where the radical generating functions are in appropriate proximity may decompose in a concerted manner or in a way such that the intermediate species can neither be observed nor isolated. Examples of such behavior are peroxyoxalate esters (see 3.3.2.3.1) and a-hydroperoxy diazenes (e.g. 31), derived peroxyesters (65)2S3 2S4 and bis- and multi-diazenes such as 66.233,250 

It has been reported that the a-hydroperoxy diazenes may undergo induced 

Initiators where the radical generating functions are sufficiently remote from each other break-down in a non-concerted fashion. Examples include the azoperoxide (68) and the bis-diazene (67).261 Their chemistry is often understandable in terms of the chemistry of analogous monofunctional initiators.260 This class also includes the dialkyl peroxyketaLs (see 3.3.2.4) and hydroperoxyketals (see 3.3.2.5). 

The use of initiators such as 68 has been promoted for achieving higher molecular weights or higher conversions in conventional polymerization and for the production of block and graft copolymers. The use and applications of multifunctional initiators in the synthesis of block and graft copolymers is briefly described in Section 7.6.1. 

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One is the concerted decomposition of a dioxetanone structure that is proposed for the chemiluminescence and bioluminescence of both firefly luciferin (Hopkins et al., 1967 McCapra et al., 1968 Shimomura et al., 1977) and Cypridina luciferin (McCapra and Chang, 1967 Shimomura and Johnson, 1971). The other is the linear decomposition mechanism that has been proposed for the bioluminescence reaction of fireflies by DeLuca and Dempsey (1970), but not substantiated. In the case of the Oplopborus bioluminescence, investigation of the reaction pathway by 180-labeling experiments has shown that one O atom of the product CO2 derives from molecular oxygen, indicating that the dioxetanone pathway takes place in this bioluminescence system as well (Shimomura et al., 1978). It appears that the involvement of a dioxetane intermediate is quite widespread in bioluminescence. 

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

Decarboxylative condensations of this type are sometimes carried out in pyridine, which cannot form an imine intermediate, but has been shown to catalyze the decarboxylation of arylidene malonic acids.215 The decarboxylation occurs by concerted decomposition of the adduct of pyridine to the a, 3-unsaturated diacid. 

Figure 8 Chemiluminescent mechanism for 1,2-dioxetanes (A) a concerted decomposition process (B) a two-step biradical process.

Nangia and Benson [185] noted that such one decomposition act should have high activation energy (more than 18 kJ mol ) and does not occur rapidly. The preexponential factor A of concerted decomposition should be less than value 1013 s 1 [186], Consequently such one-step decay of tetroxide should occur slowly. 

The model of interacting oscillators was developed to describe the concerted decomposition of a molecule [10]. The decomposing molecule is treated as a collection of oscillators. The reaction of concerted decomposition is described as the transition of the system of oscillators from the thermal vibration with amplitude d to the vibration with critical amplitude d. If n bonds participate in the concerted decomposition and the activated energy is equal to En the rate constant of concerted decomposition kn depends on n and En according to the equation ... 

Two mechanisms of azo-compound decomposition were discussed intensively in the literature concerted decomposition with simultaneous dissociation of two C—N bonds... 

This equation helps to distinguish between the concerted and the nonconcerted decomposition of the initiator (for concerted decomposition, kfl = 0). 

Malonic acid and cyanoacetic acid can also be used as the potential nucleophiles. The mechanism of the addition step is likely to involve iminium ions when secondary amines are used as catalysts. With malonic acid or cyanoacetic acid as reactant, the products usually undergo decarboxylation. This may occur as a concerted decomposition of the adduct.116... 

From my estimates on the thermodynamic properties of peroxy and polyoxide molecules and radicals, we can estimate that the bond dissociation energy of the tetroxide is about 5 kcal. Thus, at room temperature, or even at dry ice temperature, the tetroxide is extremely unstable and should redissociate into the more stable (from a thermodynamic point of view) peroxy radicals. The competing step would be a concerted decomposition into an RO and an R03 (Step 14) radical, which would be uphill by 20 kcal., or else a concerted decomposition into 2 RO radicals and 02 (Step 14 ). The latter is almost thermoneutral. If we take the current data at face value, it provides, from the reported activation energy at least, strong evidence that the propagating interaction of two alkylperoxy radicals proceeds in a concerted fashion. 

Goldstein and co-workers have pointed out that this oxygen exchange can take place at least partly through nonradical rearrangements 9.25 or 9.26,82 and isotope effects in acetyl peroxide decomposition are consistent with significant concerted decomposition.63... 

Equation 9.29) and later the second (Equation 9.30). One test is to measure the effect on decomposition rate of changing one of the R groups. If R—N=N—R and R—N=N—R, where R - is a more highly stabilized radical than R, are compared, a lower rate should be found for the unsymmetrical compound in a concerted decomposition in a stepwise one the better radical R- should come off first in either compound, and the rate should be unaffected. The results support the concerted process (Equation 9.28) for the symmetrical azo compounds with R- a well-stabilized radical such as < 2CCN, 2CH, and so forth, but leave... 

We have excluded pathways which might involve concerted decomposition of dioxyphosphoranes to cyclic ethers with retention of stereochemistry at least for symmetrical 1,2-diols by examining the reaction of d, l-2,3-butanediol with DTPP. The C NMR spectrum of the reaction mixture is consistent only with the cis epoxide exhibiting resonances at 6 12.9 and 52.4 ppm. 

For the concerted decomposition of N-nitroso compounds in (58), we require only that the orbitals labeled and three-membered ring and forbidden for the five-membered ring. The elimination of dinitrogen oxide from N-nitrosoaziridines is syn stereospecific (Clark and... 

Concerted Decomposition. Both unimolecular and bimolecular reactions involving only molecular species (no free radicals or ions) may play a role in certain aspects of coal conversion. In some types of concerted reactions, however, such as reactions involving C-C or C-0 bond breaking, the significance of such reactions appears to be drastically limited by the lack of suitable molecular structures. 

Klein and Virk (22) have pointed out that a key concerted decomposition reaction may involve adjacent (ortho) hydroxyl and ether structures on an aromatic ring, for example,... 

The Curtius rearrangement also occurs in the direct photolysis of acylazides, but additional products have been isolated and have been shown to arise from typical nitrene reactions . For example, Horner et have trapped benzoyl nitrene with a variety of scavengers. Similarly to the Wolff rearrangement (vide infra) it has been shown ° that the Curtius rearrangement occius via a singlet intermediate this intermediate can be a nitrene or an excited azide molecule in which a concerted decomposition leads to isocyanate derivatives. This reaction could occur in competition with nitrene formation or, alternatively, originate from a vibrationally-excited ground state azide molecule. 

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