Photo-Claisen reaction

Figure 34 Photo-Fries rearrangement of phenyl acetate and phenyl benzoate, and photo-Claisen reaction of allyl phenyl ether in solution and within zeolites.

The reactions analyzed here (and used to generate the initial radical pairs in the polymer media) can be separated into two distinct categories those that involve lysis of one molecule into two radicals (such as the Norrish Type 1, photo-Fries, and photo-Claisen reactions) and those that require bimolecular processes in which a part of one molecule is abstracted by another (e.g., H-atom abstractions from a phenol or an amine by the lowest energy triplet state of benzophenone). Each reaction produces either singlet or triplet radical pairs and, thus, allows the influence of spin multiplicity on radical pair reaction rates to be separated somewhat from other influences, such as the natures of the polymer matrices and the radical structures. Different methods for extracting rates of processes for the radicals from both static and dynamic data will be discussed. ... 

For further corroboration, the photodissociations of arylethers [104d, 104e] and arylthioethers [104e] (photo-Claisen reactions) were investigated. 

Two other photochemical reactions that appear to proceed through a bond homolysis are the photo-Claisen reaction of phenyl allyl ether (106, equation 12.75), which produces phenol (107) plus o- (108) and p-allylphenol (109) ° and the photo-Fries reaction of phenyl acetate (110,equation 12.76) to produce o- (111) and p-hydroxyacetophenone (112) plus phenol. In the case of the photo-Fries reaction, both concerted and dissociative mechanisms were proposed for the formation of the acylphenols. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

These ideas will be discussed in the following subsections, where most of the attention will be devoted to the mechanistic smdies with aromatic esters, which have been the subject of an overwhelming majority of the research efforts. Nevertheless, the same reaction mechanism has been shown to be valid for the PFR of anilides, thioesters, sulfonates, and so forth. Furthermore, it is also applicable to the photo-Claisen rearrangement [i.e. the migration of alkyl (or allyl, benzyl, aryl,)] groups of aromatic ethers to the ortho and para positions of the aromatic ring [21,22]. 

A unified theoretical explanation using molecular orbital theory has been proposed. Grimme [65] investigated the PFR of phenyl acetate as well as the photo-Claisen rearrangement of allyl phenyl ether and the 3-cleavage of para-substituted phenoxyacetones. A unified description of the three reactions has been invoked according to MNDOC-CI and AMl/AMl-HE calculations. No matter what ex-... 

Ramamurthy and coworkers studied the photo-Fries rearrangement of phenyl acetate and phenyl benzoate and photo-Claisen rearrangement of allyl phenyl ether (Fig. 34) included in two types of zeolite (faujasites X and Y and pen-tasils ZSM-5 andZSM-11) [192], The photolysis was performed with the zeolite slurry in either hexane or iso-octane. One of the most remarkable observations is that the product distribution is altered within zeolites from that in isotropic solvent. Furthermore, while in solution, nearly a 1 1 mixture of ortho and para isomers 40 and 41 (Fig. 34) was obtained, within zeolites one is able to direct the photoreaction selectively toward either the ortho or the para products by conducting the reaction either within faujasites or pentasils, respectively (Fig. 34). 

All experimental observations with respect to these reactions, including the dependence on neither orbital character nor spin multiplicity of the excited state (which is in striking contrast to a-cleavage of carbonyl compounds) and the influence of substituents of the aryl moiety on the reaction rate, could be rationalized by correlation diagrams that were obtained on the basis of semiempirical MO calculations [105]. /1-Cleavage of phenoxyketones as well as photo-Fries and photo-Claisen rearrangements were characterized to be no photodissociations. 

The mechanism of the thermal Claisen rearrangement is discussed in Chapter 11. Photo-dissociative reactions are discussed in Chapter 12. 

The photo-Claisen and photo-Fries reactions are shown below. Propose an experiment for each to test whether these reactions are concerted or stepwise, involving initial bond homolysis. 

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