Solvent effects radical rearrangements

Over the years several different mechanisms for the singlet oxygen ene reaction have been considered as depicted in Sch. 2. Biradical (mechanism B in Sch. 2) [10,11] and zwitterionic (mechanism C in Sch. 2) mechanisms have been considered as very unlikely for a variety of reasons including the facts that Markovnikov directing effects are not observed [4], radical scavengers have no influence on the reaction, only minor solvent effects are observed [12-14], and cis-trans isomerizations of the substrates do not occur [6], A dioxetane intermediate (mechanism D in Sch. 2) was considered very early [6] but this suggestion was dismissed when isolated dioxetanes were shown to cleave to the carbonyl compounds rather than rearrange to the allylic... 

The aqueous solvent effects on the rates of Claisen rearrangement have been extensively investigated experimentally and theoretically.2i8-229 Tiie reaction is of particular interest because of its mechanistic and synthetic importance in organic chemistry. In addition, the Claisen rearrangement in chorismate mutase is the only known pericydic reaction catalyzed by a naturally occurring enzyme. 2 A fundamental issue is the dipolar versus the radical nature of the transition state struaure, which will determine the origin of the TS stabilization observed in water. 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

A possible mechanism for these reactions is shown in Scheme 21 for compound 101. The absence of a solvent polarity effect on the efficiency of photoreactions of 119 and 124 might be due to a very fast rearrangement of the radical-anion 126 within solvent cages (Scheme 21, path a). In cases in which this intermediate escapes from the cage before rearrangement occurs, a significant influence of the polarity of the solvent would have been observed. This is the situation in DMA-sensitized reactions of 101,117, and 118 (Scheme 21, path b). 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

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