Photolysis of carbonyl groups

In this case, the grafting reactions start with the photolysis of carbonyl groups. 

Acyl radicals may also arise in Norrish I photolysis of carbonyl groups containing polymers [14]. 

Polyolefins.— The mechanism(s) of polyolefin photo-oxidation continue to be shrouded in complexities. Recently Geuskens has attempted to answer questions concerning the sensitized photolysis of hydroperoxides in these polymers in the presence of carbonyl groups. In contrast with the theory proposed above,it is suggested that a complex is formed and photolysed to produce a carboxylic acid and an ether directly (Scheme 19) rather than free hydroxyl radicals. 

Thus the formation of free radicals in the photolysis of polypropylene at —196° C can be reasonably explained by the mechanism which involves photolysis of oxidative groups such as hydroperoxides and carbonyl groups, although Kujirai et al. (50) proposed that ash residues may be responsible for light absorption. 

The macroradical formed to reaction (16) interacts with oxygen, leading to a normal chain reaction and the formation of hydroperoxides. Accordii to the data of [85], the quantum yield of carbonyl groups in the photolysis of polyethylene is no greater than 0.1. This means that the kinetic chains in photooxidation are not very long. In view of this, termination of the chains by means of antioxidants is hindered in practice, since in the case of short chains and a high rate of initiation, effective inhibition is impossible. The formation of unsaturated compoimds during photolysis [reactions (15) and (16)] facilitates further oxidation of the polyolefin. 

Figure C2.3.9. Product distribution of dissymmetrical ketone photolysis as influenced by cefyltrimethylammonium chloride (CTAC) micelles. The initial ketone, A(CO)B is photolysed to lose the carbonyl group and to produce tliree products, AA, AB and BB. These data are for benzyl (A) 4-methylbenzyl (B) ketone. Product AA is 1,2-diphenylethane, product BB is 1,2-ditolylethane and product AB is l-phenyl-2-tolyl-ethane. At low CTAC concentration, in the absence of micelles, a random distribution of products is obtained. In the presence of micelles, however, the AB product is heavily favoured. Adapted with pennission from 1571.

Photolysis of 2,3-diphenyl-A -azirine (442) generates benzonitrile ylide (443). Irradiation in the presence of ethyl cyanoformate resulted in a mixture of the oxazoline (444) and the imidazole (445) by 1,3-dlpolar cycloaddition to the carbonyl and nitrile group, respectively (72HCA919). 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

UV absorbers (UVA) act by absorbing UV light hence retarding the photolysis of hydroperoxides. Their activity is also associated with hydrogen bonding between the 2-hydroxy group and the carbonyl chromo-phore [22]. Typical examples are based on 2-hydroxy-benzophenones and 2-hydroxybenztriazoIes (e.g., Table lb, AO 23 and AOs 24-26). 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

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