Photolysis of dibenzyl ketone

Photodecarbonylation of p-tolyl benzyl ketone Photolysis of dibenzyl ketones with CuCl2 scavenger... 

TABLE 2 Representative Examples of the Product Distribution from Photolysis of Dibenzyl Ketones in Various Environments"... 

Figure 12 Cage effect during the photolysis of dibenzyl ketone. Top GC traces of products in micelles and solution. Bottom Cage effect with respect to the detergent concentration. Note the sudden change in the cage effect at the cmc.

Figure 13 Variation of cage effect during the photolysis of dibenzyl ketone with respect to detergent chain length.

Table 1. Quantum Yields for Photolysis of Dibenzyl Ketones in HDTC119-2<,)...

Table 3. Single Stage 13C Isotope Enrichment Parameters for Photolysis of Dibenzyl Ketone Under Various Conditions...

Thus, upon photolysis of dibenzyl ketone (11) in micelles, 4-methylphenyl benzyl ketone (12) is formed as a side product according to Scheme 3. 

Table 1. Product distributions from the photolysis of dibenzyl ketone adsorbed on ion exchanged X and Y zeolites under vacuum.

Photolysis of dibenzyl ketone gives 1,2-diphenyl ethane and CO (scheme 12). 

The photochemistry of dibenzyl sulfoxide 10 was briefly reported in the mid 1960s [21,22]. It was said to decompose mainly to benzyl mercaptan (isolated as the disulfide 17) and benzaidehyde 16. Though no mechanism was suggested at the time, it is now clear that these products arise from a standard a-cleavage mechanism, followed by secondary photolysis of the sulfenic ester 13. The careful reader will note that the yield of bibenzyl (19) is very low in comparison to photolysis of dibenzyl ketone. Sulfinyl radicals rarely lose SO, though some net extrusions are discussed later. 

Isotopes do not fractionate identically among the products of most chemical reactions. If a reaction proceeds in 100% yield, the net balance of isotopes in the products ought to be identical to that of the reactants, though the distribution may differ between two products. When a reaction does not run to completion, the natural abundance in the products may vary appreciably from that of the reactants. Measurement of the distribution of natural-abundance isotopes represents an important tool in studying reaction pathways, but the partitioning does not always depend solely upon isotopic mass. In 1978 B. Kraeutler and N.J. Turro used mass spectrometry to determine the fractionation of in carbon monoxide from decomposition of the radical pair produced by photolysis of dibenzyl ketone (Eqn [1]). 

It is relevant to note here that the chemistry of benzyhc radicals from the steady-state photolysis of dibenzyl ketones inside cavities of pentash zeohtes have been reported. " From a study on the lifetime of simple benzyl radicals generated inside the zeohte cages in the photolysis of phenylacetic acid using nanosecond diffuse reflectance spectroscopy, it has been concluded that the benzyl radicals decay rapidly by a coupling reaction with a second benzyl radical, and the mobihty decreases within the series from LiY to CsY. ... 

Photoextrusion of small stable molecules is a well-known phenomenon, with SO2 being one of the standard leaving groups for that type of reaction. Photochemical extrusion of SO is also known, though less conunon, and is reviewed here. Loss of SO appears to be an uncommon process of sulfinyl radicals, at least near room temperature. This was foreshadowed by the earliest photolysis of dibenzyl sulfoxide vide supra), in which sulfenate-derived products clearly dominate any loss of SO from the benzylsulfinyl radical, in marked contrast to the ketone case. The loss of SO from CH3SO- is estimated to be endothermic by 50 kcal/mol [62,63], so it is clear that near simultaneous formation of a stable structure in the carbon portion of the molecule is a critical component in the design of extrusion reactions. 

Photochemistry of dibenzyl ketones has been examined in a number of organized media. Photolysis of 3-(4-methylphenyl)-l-phenylacetone (MeDBK) results in a 1 2 1 mixture of three products shown in Scheme 31. Photolysis of the same molecule solubilized in a micelle (hexadecyltrimethylammonium chloride) gives a single product (AB). This remarkable change in product distribution is due to the cage effect brought forth by the micellar structure. The change in product distribution occurs at or above the critical micelle concentration. 

Lebedeva et al report theoretical and experimental studies of CIDNP generated in consecutive micellised radical pairs. They measured the radical pair escape rate in the photolysis reactions of three substrates e.g., dibenzyl ketone), and compared physico-chemical aspects of the process to that occurring in biradicals, where the escape of radical pairs is inhibited by the covalent linkage. 

The facility with which this reaction occurs in solution depends on the stabiUty of the radical fragments that can be ejected. Dibenzyl ketone, for example, is readily cleaved photolytically. Similarly, f-butyl ketones that can lead to the f-butyl radical undergo a-cleavage quite readily on photolysis in solution. " ... 

Dibenzyl ketones are suitable guests for this dimeric capsule and they undergo exclusive formation of decarboxylation products in the absence of the hosting dimer, while in the presence of the capsule unusual massive formation of rearrangement products derived by recombination of radical residues and minor amounts of decarbonylation products is observed (Figure 24a). Moreover, as far as the decarbonylation products are concerned, photolysis within the capsule provided only mixed products and no homodimers, confirming that the recombination of radicals occurs in the capsule faster than the in-out exchange. 

SCHEME 10.2 Product distribution in the photolysis of o>methyl dibenzyl ketone inside the pores of a MOF. (Adapted from Ref. [34] with permission of Wiley-VCH. Copyright 2003.)... 

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