Decomposition pathways, possible

Fig. 3.3.4 Reaction mechanism of the coelenterazine bioluminescence showing two possible routes of peroxide decomposition, the dioxetanone pathway (upper route) and linear decomposition pathway (lower route). The Oplopborus bioluminescence takes place via the dioxetanone pathway. The light emitter is considered to be the amide-anion of coelenteramide (see Section 5.4).

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

There has been little insight into potential decomposition pathways for the Ni(II) system due to sparse experimental evidence. Polymerization results with catalysts bearing different alkyl and fluorinated substituents have suggested that a C-H activation process analogous to that occuring with the Pd(II) catalysts is unlikely with Ni(TT) [28], Instead, side reactions between Ni and the aluminum coactivator, present as it is in such large excess, have been implicated. The formation of nickel dialkyl species and their subsequent reductive elimination to Ni(0) is one possible deactivation mechanism [68]. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

Fig. 3.39. One possible decomposition pathway for cationic iron(IV) carbene complexes.

Turner CE, El-Sohly MA, Constituents of Cannabis sativa L. XVI. Possible decomposition pathway of A -tetrahydrocannabinol to atnn moX, J Pleterocycl Chem 16 1667-1668, 1979. 

A subsequent study examined phenylperoxy radical in greater detail. Fadden et identified five possible unimolecular decomposition pathways for phenylperoxy radical (Fig. 10) via oxygen atom loss to form phenoxy radical (Fig. 10, route A), via a dioxiranyl radical species (Fig. 10, route B), via a dioxetanyl radical... 

The overall pathways of benzene oxidation and the decompositions of possible intermediates have been well characterized via theoretical methods. Thus far, we have discussed these species mainly in the context of their oxidation mechanisms, but phenylperoxy and phenoxy radicals have also been investigated as individual experimental targets. 

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

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