Acyl radicals formation

The interaction of acrolein with a catalyst was investigated using a DPPH method (27), in which an acyl radical formed by the initiation was trapped by DPPH, and the rate of consumption of DPPH, corresponding to the rate of formation of the radical, was determined by visible light spectra. The rate equation for acyl radical formation is... 

When the irradiation was carried out with light including shorter wavelengths, the carbonyl groups also contributed to radical formation by the Norrish type-I reaction (75). This was confirmed by acyl radical formation during the warming-up process as in the case of polyethylene... 

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

Effects that are invisible and unimportant for conformational equilibria can play key roles in reactivity. For example, the large n - 0, jj interaction in aldehydes evolves, upon the C-Fl bond scission, into a 2c,3e-bond in acyl radical. The latter effect manifests itself as the source of dramatic weakening of the aldehyde C-H bond dissociation energy ( 88 kcal/mol) - much smaller than the BDE for C-Fl bond in ethene ( 111 kcal/ mol). The difference is especially striking since both carbon atoms are sp hybridized and expected to have relatively strong C-H bonds. However, the C(0)-H bond in aldehydes is even weaker than a typical C-H bond in allcanes. This structural feature and resulting ease of acyl radical formation has important consequences for the stability and reactivity of aldehydes under radical conditions (Figure 6.38). 

A possible mechanism for the formation of the furanones 6 and 7 is illustrated in Scheme 2. The initial alkoxy radical generated from the alcohol 5 and lead tetraacetate (LTA) undergoes /3-scission to produce the acyl radical intermediate 9. Subsequent cyclization to 10 proceeds through attack of the radical at the carbonyl oxygen. The resulting Pb(IV) intermediate 11 finally collapses via the reductive... 

Radical-based carbonylation procedures can be advantageously mediated by (TMSlsSiH. Examples of three-component coupling reactions are given in Reactions (74) and (75). The cascade proceeds by the addition of an alkyl or vinyl radical onto carbon monoxide with formation of an acyl radical intermediate, which can further react with electron-deficient olefins to lead to the polyfunctionalized compounds. ... 

If one takes into account not only the initial slope of the curves but also the part played by the formation of isobutyrate it can be seen that the amount of reaction products formed is almost equivalent to the loss of DiPK. In this case the formation of isobutyric acid represents the most important difference compared with irradiation without additive. It shows that in the presence of nitroxide the acyl radical may not only be captured by oxygen but can also react further as acyl-peroxy radical, without losing its carbonyl group in the process. 

The observed formation of isobutyrate (Figs. 5 and 6) would appear to be one of the possible reasons for the slow decrease in the nitroxide concentration. The formation of isobutyrate can be seen as a reaction competing with the capture of the acyl radicals by oxygen. The absence of isopropyl ether in the reaction mixture is explained by its immediate cleavage - following its formation analogous to isobutyrate - to nitroxide by oxygen-centered radicals (mainly acyl peroxy radicals). 

The unexpected formation of cyclopenta[b]indole 3-339 and cyclohepta[b]indole derivatives has been observed by Bennasar and coworkers when a mixture of 2-in-dolylselenoester 3-333 and different alkene acceptors (e. g., 3-335) was subjected to nonreductive radical conditions (hexabutylditin, benzene, irradiation or TTMSS, AIBN) [132]. The process can be explained by considering the initial formation of acyl radical 3-334, which carries out an intermolecular radical addition onto the alkene 3-335, generating intermediate 3-336 (Scheme 3.81). Subsequent 5-erafo-trig cyclization leads to the formation of indoline radical 3-337, which finally is oxidized via an unknown mechanism (the involvement of AIBN with 3-338 as intermediate is proposed) to give the indole derivative 3-339. 

Attack of molecular oxygen on this carbanion should yield acyl radicals which could react with loss of carbon monoxide, yielding the respective radicals, or with the formation of acyl peroxide radicals ... 

If we make the assumption that the reverse of reaction 15.5 is diffusion-controlled and assume that the activation enthalpy for the acyl radicals recombination is 8 kJ mol-1, the enthalpy of reaction 15.5 will be equal to (121 - 8) = 113 kJ mol-1. This conclusion helps us derive other useful data. Assuming that the thermal correction to 298.15 K is small and that the solvation enthalpies of the peroxide and the acyl radicals approximately cancel, we can accept that the enthalpy of reaction 15.5 in the gas phase is equal to 113 kJ mol-1 with an estimated uncertainty of, say, 15 kJ mol-1. Therefore, as the standard enthalpy of formation of gaseous PhC(0)00(0)CPh is available (-271.7 5.2 kJ mol-1 [59]), we can derive the standard enthalpy of formation of the acyl radical Af//°[PhC(0)0, g] -79 8 kJ mol-1. This value can finally be used, together with the standard enthalpy of formation of benzoic acid in the gas phase (-294.0 2.2 kJ mol-1 [59]), to obtain the O-H bond dissociation enthalpy in PhC(0)0H DH° [PhC(0)0-H] = 433 8 kJ mol-1. 

Figure 7. Relationship of oxidation and degree of polyunsaturation. Polyunsaturation is measured as the methylene bridge index (MBI), which is a more precise measure of extent of unsaturation and oxidizability than the double bond index. It is the mean number of 6is-allylic methylene bridge positions per fatty acid (or fatty acyl chain) in a lipid ensemble. The rate of lipid radical formation measures formation of an oxidative product, while O2 consumption (% O2 lost per sec) is a measure of utilization of a reactant. (Drawn using our data abstracted from Wagner, B.A., Buettner, G.R., and Bums, C.P. 1994, Biochemistry 33 4449-4453).

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

TMS)3SiH has also been used as the mediator of C—C bond formation between an acyl radical and an a, p-unsaturated lactam ester (Reaction 7.9). The resulting ketone can be envisaged as potentially useful for the synthesis of 2-acylindole alkaloids [17]. Here, the effects of both H-donating ability and steric hindrance given by the silicon hydride can be seen. 

Reaction (7.63) shows an example of C—S bond formation [73,74]. In fact, the aryl radical formed by iodine abstraction by (TMS)3Si radical rearranged by substitution to the sulfur atom, with expulsion of the acyl radical and concomitant formation of dihydrobenzothiophene (60). This procedure... 

Further evidence for the generation of acyl radicals is the formation of benzaldehyde on photolysis of benzoyl-1-naphthyl telluride in the presence of thiophenol. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Two steps must be considered in the mechanism of homolytic acylation, in addition to the formation of the acyl radical. The first fits in with the generally accepted mechanism of homolytic aromatic substitution, that is, the addition of the acyl radical to the aromatic nucleus to give an adduct in which the unpaired electron is delocalized over the residual heteroaromatic system (u-complex 6). 

The formation of carbon monoxide and carbon dioxide during the oxidation of acrolein is shown in Figure 5. The facts that the decomposition of peracrylic acid produced not carbon monoxide but carbon dioxide (28) and that in the initial state of the oxidation of acrolein only carbon monoxide is formed, indicate that the evolution of carbon monoxide during oxidation may be ascribed to the decomposition of the acyl radical. 

Some homolytic fragmentation reactions are driven by formation of small, stable molecules. Alkyl acyloxyl radicals (RCOp decarboxylate rapidly (fe > 1 x 10 s ) to give alkyl radicals, and even aryl acyloxyl radicals (ArCOp decarboxylate to aryl radicals with rate constants in the 10 s range." Azo radicals produced in the homolysis of azo initiators eliminate nitrogen rapidly. Elimination of carbon monoxide from acyl radicals occurs but is slow enough (fe 10" -10 such that the acyl radical can be trapped in a bimolecular process,... 

Figure 20.5. A graphical representation of the time evolution of transients for the Norrish type-I a-cleavage 43 and 46 amu fragments from acetone and from acetone-de- The representative sets of data points ( for 43 amu, for 46 amu fragments) are modeled with simple buildup and decay response functions, I(t) = 4[exp(—t/t2) — exp(—f/x])] the time constants of buildup and decay are Ti and T2, respectively. A modest isotope effect on the characteristic time for formation of these acyl radicals (60 and 80 fs, respectively) and a more prominent —CH3/—CD3 effect on decays through loss of CO (420 and 670 fs, respectively) were recorded. ...

In 02-saturated solutions, a major portion of acyl radicals (60-80%) react with O2 as in Eq. (22), the rest undergoing decarbonylation and finally formation of tert-butylperoxyl radicals, Eqs. (20) and (23). The source of Craq02 +, which is clearly an intermediate on the basis of the effects of methanol and Mn2+, is reaction 24. As discussed in greater detail below, Craq02 + is produced by disproportionation of the initially generated Craq(V). 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

We attribute this new absorbance to the formation of aldehyde groups on the polymer chain by hydrogen abstraction by the acyl radical formed in the primary photolysis of PS-tBVK (Equation 3). 

Acylation (Acidylation). A reaction leading to the formation of an org compd oontg one or several acyl radicals, RGO-Refs l)Lassar-Cohti, Arbeitsmethoden... 

The acyl-alkv biradical obtained by ring-opening of a cyclic ketone is able lo undergo intramolecular disproportionation in one of two ways. A hydrogen atom may be transferred to the acyl radical from the position adjacent to the alkyl group, and this produces an unsaturated aldehyde (4.21). Alternatively, a hydrogen may be transferred to the alkyl radical from the position adjacent to the acyl group, and this results in the formation of a ketene (4.22). Many ketenes are labile, and the use of a nucleophilic solvent or addend. 

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