Quantum yield oxetane formation

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

Further work by Turro and coworkers70 led to the discovery of a singlet complex in this reaction. A plot of the reciprocal of the quantum yield for oxetane formation versus the reciprocal of the concentration of 12 according to Eq. (34) p. 275 gave a slope of 2.6 but an intercept of 13.2. This suggests that oxetane formation proceeds via an intermediate (complex) which is frequently deactivated. Since the value of Ks obtained from this plot agreed... 

Yang144 has reported a study of 3-methyl-2-pentene (30), which undergoes isomerization and oxetane formation when irradiated with benzophenone or benzaldehyde. The quantum yields for these processes are shown in Table V. 

The cycloaddition of enones to olefins is a reaction of considerable synthetic interest 14°). Oxetane formation and cyclobutane formation are sometimes competitive 141>, but the latter reaction is the more common. The photodimerization of enones 142> is a special case of such cycloaddition. It has been shown that triplets are involved in these cycloadditions, since intersystem crossing quantum yields are unity 143> and cycloaddition is totally quenchable by triplet quenchers. Careful kinetic analysis indicates an intermediate which can partially revert to ground state reactants, since quantum yields are lower than unity even when extrapolated to infinite substrate olefin concentration. That a diradical is... 

Figure 5.34 shows the Stern-Volmer plot for triplet quenching of the photochemical addition of benzaldehyde to 2,3-dimethyl-2-butene (cf. Section 7.4.3) by piperylene. The ratio of the quantum yield 4> of oxetane formation without quencher to that with quencher is plotted against the quencher concentration [Q]. From the resulting straight line it may be concluded that the reaction proceeds via a single reactive state which is assigned as (n, r ). 

Morrison has made a detailed mechanistic study (70) of [77c] and [77d]. On irradiation, compound [77d] leads only to [77c] with a quantum yield of 0.029. Compound [77c], however, forms oxetane [78] ( =0.015) in competition with isomerization to [77d] ( =0.02) no crossed adduct [79] was observed. Triplet quenching did have very little effect on Isomerization or oxetane formation, while on sensitization cis-trans Isomerization but no oxetane formation occurred. The authors concluded that the reaction occurred via that intramolecular complex formation in... 

Simple carbonyl compounds, whose photochemistry in solution are fully investigated(4), are also photoreactive in solids. Typical reactions in the solution phase, e.g., hydrogen abstraction, a-cleavage, oxetane formation, and elimination of CO, are all observed in the solid phase. Recently we have studied some of these solid-state reactions and estimated their quantum yields, which will be reviewed here. 

The quantum yield of oxetane formation is then equal to the rate of photocycloaddition divided by the sum of rates of all processes which deactivate the ketone triplet (Eq. 54) ... 

Clearly, all indications are that (6—4) photolyase binds DNA and repairs its substrate by a mechanism quite similar to that of classical photolyase. However, there appears to be a fundamental difference in the photochemical reaction catalyzed by the two enzymes. The quantum yield of repair by excited singlet-state flavin by classical photolyase is near unity, whereas the quantum yield of repair by excited flavin in (6-4) photolyase is 0.05-0.10. Whether this low quantum yield of repair by (6—4) photolyase is a result of the low efficiency of formation of the oxetane intermediate thermally, low efficiency of electron transfer from the flavin to the photoproduct, or low efficiency splitting of the oxetane anion coupled with high rate of back electron transfer is not known at present. Furthermore, it was found that (6-4) photolyase can photorepair the Dewar valence isomer of the (6-4) photoproduct (Taylor, 1994) that cannot form an oxetane intermediate, casting some doubt about the basic premise of the retro [2+2] reaction. However, the Dewar isomer is repaired with 300-400 lower quantum yield than the (6-4) photoproduct, and it has been proposed (Zhao et ai, 1997) that the Dewar isomer may be repaired by the enzyme through a two-photon reaction in which the first photon converts the Dewar isomer to the Kekule form and a second electron transfer reaction initiated by the second photon promotes the retro [2+2] reaction. 

Two recent reports have appeared of oxetan formation via an exciplex derived from singlet-state olefin and ground-state carbonyl, rather than the much more usual excited-state carbonyl and ground-state olefin. In one of these the oxetan (31) and the cyclobutane (32) were formed from phenan-threne and dimethyl fumarate, whereas in the other the oxetan (33) and olefin dimers were obtained on irradiation of methyl coumarilate and benzo-phenone. In the latter case, the increase in the quantum yield of oxetan at the expense of the dimers on increasing the concentration of ground-state benzo-phenone relative to olefin is compelling evidence for the proposed mechanism. 

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