Triplet energy sensitization

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

Salt crystals of the benzonorbomadiene carboxylic acid 68 with optically pure triplet sensitizer amines 70, 71 also underwent enantioselective di-TT-meth-ane photorearrangement to afford the optically active tetracyclo[5.4.0.02,4.03 6]undecane derivative 69 in high enantiomeric excess [81]. The 4-acetylbenzyl ester of L-phenylalanine 70 and the 4-benzoylphenyl ester of l-valine 71 perform the dual roles of asymmetric induction and triplet energy sensitization. 

CA is both chiral 25 auxiliary and triplet energy sensitizer... 

Sensitized photolysis of both salts resulted in the formation of the same major products as was observed in direct irradiation, iodobenzene, or phenylsulfide and acid. Quantum yields of organic and acidic products were measured using nine sensitizers. Product quantum yields were relatively high (< 1) under conditions where electron transfer sensitization was expected to occur. Quantum yields >2 were obtained in solvents likely to be substrates for hydrogen abstraction, indicating that a chain reaction was involved. Triplet energy sensitization resulted in low product quantum yields. 

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

Preliminary observations indicate that stilbenes can act as quenchers for piperylene triplets.<69) This, along with the effect of low-energy sensitizers upon piperylene or 2,4-hexadienes, is another area that needs more experimental work before a complete picture can be obtained. 

As the triplet energy of the sensitizer becomes less than that necessary to excite the trans-diene triplet, energy transfer to the cis triplet becomes increasingly important and the product derived from this state (vinylcyclohexene) increases proportionally. This explanation is summarized in Figure 10.1.(11>14)... 

Another explanation has been offered to explain the large proportion of cyclobutane derivatives produced by low-energy sensitizers, especially for the anthracene derivatives.<17) This is that energy transfer to diene occurs from the second excited triplet state of the sensitizer rather than the first. Experiments using a large number of anthracene derivatives as sensitizers... 

By studying the photolysis of sulfone (33) in the presence of sensitizers of varying triplet energy, it was found that the triplet state of (33) lies approximately in the range of 53.0-59.5 kcal/mole. 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

For compounds that are very weakly phosphorescent or that phosphoresce at wavelengths out of the normal range of sensitivity of the spectrometer this method of triplet energy determination cannot be applied. For these compounds triplet energies can sometimes be determined by measuring their E-type or P-type delayed fluorescence. 

For sensitizers with triplet energies greater than 60 kcal/mole the following mechanism is consistent with the experimental observations ... 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

Significantly, there is no variation in the relative product distribution in this case as a function of the sensitizer triplet energy. 9-Anthraldehyde (Et = 42 kcal/mole) was ineffective as sensitizer for this reaction, presumably because its lowest triplet is lower in energy than that of cyclo-pentadiene.<23>... 

As with cyclopentadiene, the relative yields of products (19)—(21) were relatively insensitive to the sensitizer triplet energy(37) although an effect of temperature on dimer distribution has been noted.0,28 The direct photolysis of cyclohexadiene with wavelengths greater than 330 nm yielded products (19)—(21), although the product distribution in this case was more nearly statistical [(19) (20) (21) = 44% 2470 33%] 28 . 

The product distribution in this reaction was found to be dependent upon the triplet energy of the sensitizer. Variation of the relative amounts of cyclobutanes (32) and (33) to cyclohexene (34) with sensitizer triplet energy... 

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