Photo-Claisen rearrangement

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-... 

Irradiation of the a-cyclodextrin complexes of m-alkoxyphenyl allyl ethers (35) gave only a single ortho isomer in each case, although in solution each ether gave two ortho and one para isomers via photo-Claisen rearrangements (Schemes 17 and 18) [156,157], This can be attributed to the difference in free volume available within the cavity near the two ortho centers and to the difference in accessibility of the radical fragments to them. In support of this conclusion, irradiation of the /1-cyclodextrin complexes of 35 results in no... 

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). 

A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

The enone (130), the pesticide Coumaphos, undergoes regioselective dimerization on irradiation at wavelengths > 313 nm. The product was identified as the dimer (131). Irradiation of the furocoumarin, Imperatorin (132), in methanol affords a low yield of the (2t2)-dimer (133). This product is accompanied by the dealkylated compound (134) and the rearrangement product (135). This last compound is formed by a photo-Claisen rearrangement. The influence of solvent was studied and the dimer was obtained only in methanol. The other two products were formed in a variety of polar and non polar solvents. In any solvent the dealkylated compound (135. was the main product. ... 

Pitchumani, K., Warrier, M., and Ramamurthy. V. 1996. Remarkable product selectivity during photo-Fries and photo-Claisen rearrangements within zeolites /. Am. Chem. Soc. 118 9428-9429. 

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 photoproduct distribution is strongly affected by CD in the photo-Claisen rearrangement of m-alkoxyphenylallyl ethers 95 (Scheme 17), where significantly higher orthpselectivity was obtained as compared to water or... 

Acceleration of the aromatic Claisen rearrangement under microwave irradiation conditions [46] and the photo-Claisen rearrangement has also been reported [47]. 

A similar mechanism is accepted for the photo-Claisen rearrangement of aromatic ethers, where allyl, benzyl, or other activated alkyl groups migrate from the oxygen to the o- andp-carbons of the aromatic ring [17]. 

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