Acyl radicals, fragmentation

In homogeneous solvent, the initial cleavage of the phenyl esters produces phenoxyl and acyl radical fragments which recombine in the solvent cage to give a mixture of o- and p-hydroxyketones and phenol as result of radical escape (Scheme 16). The overall quantum yield of the reaction and the product distribution is highly dependent on the H-bonding ability and polarity of the solvent [266]. 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

As data for the rates of spin-trapping reactions are accumulated, so it becomes possible to use the competition experiment in reverse , i.e. to determine rates of rearrangement, fragmentation, atom transfer, etc. which can compete with spin trapping. An attempt to estimate rates of decarbonylation of acyl radicals depended on this approach (Perkins and Roberts, 1973). Although the results obtained were intuitively reasonable, they depended on the assumption that the rate of scavenging of acyl radicals by MNP would be no different from that measured for the butoxycarbonyl radical. This still awaits experimental verification. Another application, reported recently, was to the rates of rearrangement (23) of a series of (o-(alkoxycarbonyl)-alkyl radicals... 

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. ...

The fate of the free acyl radical 68 and radical 74 is not known. Most probably it is a constituent of polymer deposits on the wall of the irradiation vessel which hitherto have not been identified more definitely.29 Moreover, the identification of methane and carbon monoxide among the gaseous products of the photolysis of 4-methylphenyl acetate (55) provides evidence for the existence of the acetyl fragment. This intermediate is expected to decarbonylate to give carbon monoxide and a methyl radical, which in turn abstracts hydrogen from the solvent.34... 

Thioesters, on the other hand, appear to undergo a-fragmentation to yield acyl radicals and thiolates (equation 5)40. 

Being primary, stable, intermediate oxidation products and owing to the simplicity of their dissociation to free radicals, peroxide compounds HOOH, ROOH, ROOR (where R is alkyl, acyl, arylacyl fragments or their derivatives) possess favorable initiation properties in chain oxidation and polymerization reactions. 

Decarbonylation of acyl radicals is another kind of fragmentation reaction ... 

In view of their importance in enantioselective reactions, many bornane sultams have been synthesized and characterized by MS. Their El mass spectra show molecular ions with low abundance or are absent. N-Substituted bornane sultams generally show the typical fragmentations of the substituents <2000THS(4)405>. 1-Alkyl-7-nitro-2,1-benzisothiazole 2,2-dioxides show elimination of an acyl radical from their molecular ions <1997EJS55>. 

CO can be lost from an acyl radical by a fragmentation reaction. In this case, the bond that breaks is directly attached (a) to the radical center. 

A free radical can add to CO or an isocyanide (RNC) in the course of a free-radical cyclization reaction, too, to give an acyl radical (RC=0) or an iminyl radical (RC=NR), either of which can undergo further reactions. In the following example, an alkyl radical adds to the terminal C of f-BuNC to give an iminyl radical. The iminyl radical then fragments to give t-Bu- and an alkyl cyanide N=CR. In a different substrate, the iminyl radical may undergo an addition or an abstraction reaction instead. 

The driving force for a number of fragmentation reactions, which are often endothermic, is an increase in entropy. An example of such transformations is the decarbonylation reaction of acyl radicals [RC(=0) ], which is generally endothermic, but can proceed with reasonable rates (104-107s 1 at 25 °C). In addition to these reactions being exothermic, they are also favoured by entropy and both of these factors contribute to give a fast and irreversible decarboxylation reaction. 

Small molecule-small molecule reactions. These reactions are almost non-existent in the mechanism except for the addition of oxygen to small radical fragments such as the acyl radical from type I cleavage of methyl ketones. The relatively high solubility of oxygen and the mobility of these small molecules in a medium that is essentially equivalent to a very viscous liquid in local regions, minimizes any special "polymer effect". The other stable small molecule product is water which may have effects on electrical properties but does not participate in the photooxidation sequence per se. 

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