Ketones photochemical dimerization

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Photochemical sensitizer. DeBoer, Turro, and Hammond1 selected Michler s ketone as sensitizer for the photochemical dimerization of butadiene to cis-and trans-1,2-divinylcyclobutane the m-isomer undergoes thermal rearrangement... 

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. 

The photochemical reduction of benzophenone to benzopinacol in the presence of ferrocene31 apparently relates to a high concentration of ketone so that sufficient unquenched benzophenone triplets are present and proceed to the dimer in the usual fashion. 

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... 

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

Table XXXVI is a list of some catalytic photochemical redox transformation of organic reactants by (Q or H)3PW 204o. In the presence of UV light, Q3PW12O40 reacts with paraffins, arenes, alcohols, alkyl halides, ketones, nitriles, thioethers, and water. Under either anaerobic or aerobic conditions, decarboxylation, dehydrogenation, dimerization, polymerization, oxidation, and acylation takes place.

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

The 2,6-dimethyl derivatives 353a gives the tricyclic ketone 354 resulting from an intramolecular electrocyclic reaction. However, the parent thieno-tropone (51) and its 2- (353b) or 7-methyl derivatives give [4 + 2] dimers (e.g., 355) of the head-to-head type. These dimers are formed by reaction of the 5,6-ene bond of one molecule with the 5,7-diene system of another. The dimerization seems to be the normal photochemical reaction of this group of thienotropones and to be suppressed by a 6-methyl substituent (e.g., in 353a). 

Silylene complexes show high reactivities toward nucleophiles such as water, alcohols, ketones, and isocyanates [3, 4]. Some typical reactions of (OC)4Fe=SiR2(HMPA) are summarized in Scheme 1 [2]. The dimerization forming the diferradisilacyclobutane is the only known [2+2]-cycloaddition starting from 1. The photochemical reaction of 1 with 2,3-dimetylbutadiene... 

Chemical Quenching. Many photochemical reactions of organic compounds are known to occur via intermolecular interactions of the excited species with reactant molecules in solution. Common examples which may involve this mechanism include the photo-reduction of ketones by hydrogen donor solvents and many photo-dimerizations (227). The scarcity of quenching reactions of metal complexes that have been attributed to chemical processes has already been discussed by Balzani et al. (204). With the increasing interest in the mechanistic aspects of OTM complex photochemistry, however, many more examples should be discovered. 

In the pinacol coupling, two ketones are reductively coupled to give a 1,2-diol. (Compare the photochemical pinacol coupling discussed in Section 5.3.1.) The two ketones are usually identical, but intramolecular dimerizations can give unsymmetrical 1,2-diols. The reaction proceeds by electron transfer to the ketone to give a ketyl radical anion. This compound dimerizes to give the 1,2-diol. 

Carbon attack of the carbonyl group of cyclopropyl ketones has also been achieved in dimerization reactions under McMurry and photochemical conditions. Conceivably, the reactions take place via radical anion intermediates. Photodimerization does not occur if structural features of the substrate facilitate intramolecular reactions such as ring opening, ring expansion, and ring closure. Radical... 

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