Ketones photochemical reactions

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decaibonylation and sidsoetprait reactions of the alkyl free radicals that result ... 

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

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. 

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

Photochemical reaction of aldehydes or ketones bearing y-hydrogens... 

The dimerization of ketones to 1,2-diols can also be accomplished photochemi-cally indeed, this is one of the most common photochemical reactions. The substrate, which is usually a diaryl or aryl alkyl ketone (though a few aromatic aldehydes and dialkyl ketones have been dimerized), is irradiated with UV light in the presence of a hydrogen donor such as isopropyl alcohol, toluene, or an amine. In the case of benzophenone, irradiated in the presence of 2-propanol, the ketone molecule initially undergoes n — k excitation, and the singlet species thus formed crosses to the T, state with a very high efficiency. 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

It is important to point out at this point that the rate constant k and the quantum yield for a photochemical reaction are not fundamentally related. Since the quantum yield depends upon relative rates, the reactivity may be very high (large kr), but if other processes are competing with larger rates, the quantum yield efficiency of the reaction will be very small. That there is no direct correlation between the quantum yield and the rate is clearly seen from the data in Table 1.2 for the photoreduction of some substituted aromatic ketones in isopropanol ... 

In general ketones undergo three main types of photochemical reactions. These are ... 

Finally, Podoplelov and coworkers41 have prepared trimethyltin hydride and benzyltrimethyltin labelled with tin-117 from a tin sample enriched to 92% in 117Sn. The photochemical reaction between dibenzyl ketone and trimethyltin-117 hydride was used to investigate the chemically induced dynamic nuclear polarization of tin-containing radicals. 

In spite of its low vapor pressure, endrin has been found to volatilize significantly (20-30%) from soils within days after application (Nash 1983). In air, endrin will be primarily absorbed to particulates which may be re-entrained to soil or surface water via wet or dry deposition. Laboratory studies have indicated that a predominant mechanism for the transformation and degradation of endrin in air under field conditions is via photochemical reactions and rearrangements to yield primarily endrin ketone, with minor... 

From the foregoing discussion it might seem fruitless to utilize MNP to investigate photochemical reactions. However, the monomer is transparent between ca. 270 and 550 nm, and by irradiating reaction mixtures in this window excellent results have been obtained without complications from DTBN formation (Leaver et al., 1969 Leaver and Ramsay, 1969a,b Torssell, 1970). This expedient is unfortunately not infallible, there being good evidence that aromatic ketones can photosensitize MNP dissociation (Ikeda et al., 1978). 

In some photochemical reactions both Si and T take part in the reaction. For example, both the Si and Tt states of aliphatic ketones, such as hexan-2-one, take part in Norrish type 2 reactions ... 

A thermal oxidation of 2,3-dimethyl-2-butene, 16, occurs in NaY when the temperature of the oxygen-loaded zeolite in raised above — 20°C [35], Similar thermally initiated oxidations were not observed for the less electron rich tram-or cix-2-butene. Remarkably, pinacolone was conclusively identified as one of the products of the reaction of 16, This ketone is not a product of the photochemical Frei oxidation (vide supra) and underscores the very different character of these two reactions and the complexity of the oxygen/16 potential energy surface, A rationale for the different behavior could lie in the different electronic states of the reactive oxygen-CT complex in the thermal and photochemical reactions. Irradiation could produce an excited triplet-state CT complex ( [16 O2] ) and/ or ion pair ( [16 02 ] ) with different accessible reaction channels than those available to a vibrationally excited ground-state triplet complex ( [16 "02]) and/... 

There are several other photochemical reactions which ketones can undergo in the absence of olefins. Here again, the products obtained depend upon the relative rates of the competing reactions. Some of these reactions are shown in Table XI. Since it is beyond the scope of this review to discuss these reactions in detail, only a few interesting examples germane to the competition with photocycloaddition will be mentioned. 

Some Photochemical Reactions of Ketones in the Absence of Olefins Photochemical hydrolysis97,98... 

The photochemical reactions of l,l,4,4-tetramethyl-2,3-dioxotetralin (14) have been studied in methanol, isopropyl alcohol, cyclohexane, toluene, -butyraldehyde, methyl formate, acetic acid, dioxane, and benzene.75 In isopropyl alcohol the reduced ketone (15) is formed almost exclusively, while... 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

The dimerization of ketones to 1,2-diols can also be accomplished photochemically indeed, this is one of the most common photochemical reactions.688 The substrate, which... 

Figure 15.7 Examples of direct photochemical reaction pathways (a) substituted chlorobenzenes, (b) trifluralin, and (c) a ketone (from Mill and Mabey, 1985).

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