Photolysis of ketones

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. 

The experiments described in this and in the earlier paperi show that the HALS derivatives studied exercise a marked controlling influence on the product mixture resulting from the photolysis of ketones in the presence of oxygen. A detailed study of the results leads to the further following conclusions ... 

Much also depends on how the carbene is generated. Duncan et al showed that carbene generated by the photolysis of ketone adds to the above substrate in a non-stereospecific manner. 

In solution, the Norrish type 1 reaction of ketones results in the non-selective free-radical combination reactions to give products AA, AB and BB in the ratio of 1 2 1, whereas photolysis of ketones in zeolites produces ... 

Figure 60. Temperature dependence on the ratio of elimination to cyclization products for photolysis of ketones 97 (n = 10) (O) and 97 (n = 19) (A) in n-butyl stearate.

The photolysis of ketones into radicals is called Norrish type I cleavage. The efficiency of this cleavage depends strongly on the ketone s structure and on the medium. 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

The main difficulty with the mirror techniques as a quantitative tool lies in the extreme sensitivity of the mirrors to contamination by oxygen, nitrogen, parent compounds, or other radicals. Thus in the photolysis of ketones and fatty acids the original reactants as well as acyl radicals are capable of desensitizing the mirror unless it is heated above 100°C. Another difficulty is that the mirror may not only react with the free radicals but also catalyze secondary reactions and recombination of radicals. 

Pitts, IN., Ir., and Norman, I. 1954. Structure and reactivity in the vapor-phase photolysis of ketones, i. methyl cyclopropyl ketone. JAm Chem Soc 76, 4815—4819. 

The occurrence of primary step III was observed in the photolysis of ketones having several carbon atoms. In spite of the limited information available, it seems likely that only ketones with y-hydrogen atoms show this type of reaction. 

The mechanism of steps II and III seems to be related somehow to the magnitude of the sum of the primary quantum yields, (ptoM- Namely, is considerably less than 1 in the photolysis of ketones being able to decompose according to... 

The values of the primary quantum yields, found in the photolysis of ketones with y-H atoms, were explained by Brunet and Noyes on the basis of steric effects that diminish the probability of the formation of the cyclic complex. Following the original suggestion of Whiteway and Masson, Martin and Pitts proposed the internal conversion of the cyclic structure to be responsible for the low primary quantum yields observed in the photolysis of ketones capable of forming such a structure. In contrast to other interpretations. Wagner and Hammond explained the low quantum yields by an elementary chemical process, suggesting that the y-H atom transfer is reversible, i.e. that the biradical, after vibrational relaxation, may convert back into the ground state ketone molecule, viz. 

Photolysis of ketones in micelles with simultaneous application of an external magnetic field permits a C isotope enrichment. (Cf. Section 6.1.5.5.) This is the case because C nuclei have a magnetic moment and thus accelerate the spin inversion by the hyperfine interaction mechanism. (Cf. Example 4.9.) Due to the more efficient recombination of radicals containing C, the initial product formed after photolysis in a back reaction is C enriched (TUrro et al., 1980b). 

Polycyclic hydrocarbons (atmospheric pollution) Carbonyl (by photolysis of hydroperoxides) Unsaturation (by photolysis of ketones)... 

Formaldehyde and other aldehydes and ketones formed during the atmospheric oxidation of more complex organic compounds are photolysed with a loss of a hydrogen atom or an alkyl radical. Photolysis of ketones (6.59) provides one way in which C-C bonds break another is the fragmentation of alkoxyl radicals mentioned earlier (reaction 6.40). Eventually, all organic compounds end up as carbon dioxide and water. 

Environmental exposure Polycyclic hydrocarbons (atmospheric pollution) Carbonyl (by photolysis of hydroperoxides) Unsaturation (by photolysis of ketones) Singlet oxygen and derived hydroperoxides (by quenching of carbonyl triplet etc with triplet oxygen) Transition metal ions (particularly iron and copper)... 

The first two processes occur with quantum yields very close to unity, probably through a dissociative excited state. The photolysis of ketones is more complex and it is suggested that the Norrish type I split partly occurs through excitation above the dissociation limit of an upper state. 

The type I photolysis of ketones, generates free radicals and also causes chain scission. Acyl radicals, reactions (63a) and (66a), are produced by UV irradiation from carbonyl groups situated at chain ends or at some points along the chain. The formation of acyl radicals was observed by ESR measurements [262]. Light of wavelength below 6500 A can convert... 

A55. Janzen, E.G., LG. Lopp, and T.V. Morgan Detection of fluoroalkyl and acyl radicals in the gas-phase photolysis of ketones and aldehydes by electron spin resonance gas-phase spin trapping techniques J. Phys. Chem. 77 (1973) 139. 27A71. 

In the presence of a hydrogen atom donor (usually an alcohol or an amine), carbonyl derivatives and halides can be photochemically reduced. Photolysis of ketone 585 gave the henzocyclobutane 586 1 hut photolysis in the presence of ec-butylamine gave alcohol 587 as the product. Photoreduction is often less... 

Method c) is exemplified in the chiral modification of surfaces of zeolites with chiral compounds. Sundarababu et al. modified NaX and NaY zeolites with (-)-ephedrine and used them in the asymmetric photolysis of ketones with ee s above 10%. A strange behavior of the nature of carriers for the configuration of products in the latter reaction was observed the NaY-(-)-ephedrine zeolite system gave (+)-rotating products, while the NaX-(-)-ephedrine zeolite system gave (-)-rotating products. Zeolite H-Y modified with R)- or (5)-dithiane-l-oxide showed catalytic activity in the asymmetric decomposition of racemic 2-butanol (Hutchings)... 

CaHjO]- CH3CO Photolysis of ketones/ MTHF EPR/ 170 - 2.0005 3H 0.40 73Paul... 

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