Concerted radical decomposition

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. 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

The decomposition of the peroxyketals (53) follows a stepwise, rather than a concerted mechanism. Initial homolysis of one of the 0-0 bonds gives an aikoxy radical and an a-peroxyalkoxy radical (Scheme 3.36).306"08"210 This latter species decomposes by P-scission with loss of either a peroxy radical to form a ketone as byproduct or an alkyl radical to form a peroxyester intermediate. The peroxyester formed may also decompose to radicals under the reaction conditions. Thus, four radicals may be derived from the one initiator molecule. 

Predictive equations for the rates of decomposition of four families of free radical initiators are established in this research. The four initiator families, each treated separately, are irons-symmetric bisalkyl diazenes (reaction 1), trans-phenyl, alkyl diazenes (reaction 2), tert-butyl peresters (reaction 3) and hydrocarbons (reaction 4). The probable rate determining steps of these reactions are given below. For the decomposition of peresters, R is chosen so that the concerted mechanism of decomposition operates for all the members of the family (see below)... 

In the case of reaction 3, entries 1 and 2, that is, iert-butyl peracetate and (ert-butyl perpropionate, almost certainly decompose by a stepwise mechanism, rather than the concerted mechanism assumed for reaction 3. Entry 3, tert-butyl perisobutyrate, probably forms the least stable R radical by the perester decomposition mechanism which is still mostly concerted in nature (36). 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

Although the concerted mechanism described in the preceding paragraph is available only to those azo compounds with appropriate orbital arrangements, the nonconcerted mechanism occurs at low enough temperatures to be synthetically useful. The elimination can also be carried out photochemically. These reactions presumably occur by stepwise elimination of nitrogen, and the ease of decomposition depends on the stability of the radical R ... 

The mode of fission of some azo compounds into alkyl radicals and nitrogen has been studied by Pryor and Smith<8) using the following postulates (1) A molecule that decomposes by a concerted scission of both C—N bonds will not undergo cage return and will have a rate constant independent of viscosity (2) a molecule that decomposes by a stepwise scission of the C—N bonds can undergo cage recombination and the rate constant for decomposition will decrease with solvent viscosity increase provided that the lifetime of the radicals produced by the initial homolysis is of the same order... 

Hydrogen peroxide was identified as the product of secondary peroxyl radical disproportionation [187-192], It cannot be explained by the concerted mechanism of tetroxide decomposition. 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

Warbentin (19) has indicated how the radical thermochemistry involved can assist in assigning a mechanism for the thermal decomposition of phenyl oxalate. Exclusive initial fission of either the one C—C or the two C—O bonds would lead, via PhOCO or OCCO intermediates, to high C02 or high CO yields. In experiments lasting about 75 hours at 500°K. in diphenyl ether, comparable amounts of CO (13%) and C02 (9% ) are formed. The following steps, possibly concerted ... 

Theoretical studies suggest reaction (71) occurs by a concerted process in which the H from H02 is transferred simultaneously to the oxygen of C=0, while the terminal oxygen of H02 adds to the carbon (Evleth et al., 1993). The forward rate constant, k7l, is 7.9 X 10 14 cm3 molecule-1 s-1, leading to a lifetime of HCHO with respect to H02 of only 7 h at [H02] = 5 X 10x radicals cm-3. However, the reverse decomposition is also fast, k 7i = 150 s-1 at 298 K. If the rate constant for the reaction of the peroxy radical formed in (71) with NO is 8 X 10-12 cm3 molecule-1 s-1,... 

Ethyl peracetate was the first ester of a peroxy acid, and was characterized by Baeyer and Villiger in 1901. Kinetic studies of perester decomposition were reported by Blomquist and Ferris in 1951, and in 1958 Bartlett and Hiatt proposed that concerted multiple bond scission of peresters could occur when stabilized radicals were formed (equation 46). As noted below (equation 57), polar effects in perester decomposition are also significant. 

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