Para rearrangement

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

Irradiation of 5-phenyl thiolacetate (203) in benzene gives diphenyl disulfide (206) as major product (52%). However, minor amounts of ortho-rearranged (204) and para-rearranged (205) products are also formed, along with thiophenol (17%) and methyl phenyl thioether (208) (19%) (Scheme 54) [155],... 

The selectivity has been found to depend on the size of the cation of the faujasite. Thus, with large cations like K+ no para rearrangement is observed, whereas with small cations like Na or Li, a bit of it is observed [277],... 

Saczewski and Debowski reported the l,4-diaza-3-oxa-Cope rearrangement of N-cyanate anilides (equation 52). Prototropic rearomatization of 176 and internal nucleophilic addition afford the corresponding benzimidazolinone 177, usually in moderate yields (32-78%). A concerted [3,3]-sigmatropic rearrangement is proposed based on the absence of para rearrangement product that usually results from homolysis or heterolysis of the N—O bond followed by recombination of the two radicals or ions. 

In both viscosity33,34 and 14C-isotopic51 experiments, it has not been possible to monitor the formation of para-rearranged product because of structural reasons (substituted para-position). Such studies would be desired to support the assumption that in para-rearrangement both Path A and Path B participate equally (vide supra). 

Ortho- and para-rearrangement and phenol formation on uv-irradiation of aryl esters are accompanied in several cases by decarboxylation,37,60,62,64,80,81 represented for 3,5-di-t-butylphenyl benzoate by the equation 118 -> 119-122. It was shown that this reaction cannot be sensitized,64 but the dramatic differences in product distribution could be observed by changing of the solvent.60,84 The results in Table VI indicate that in polar solvents the decarboxylation process is minimized while the formation of the photo-Fries rearrangement 119 is enhanced. The reverse appears to be true when nonpolar ethereal solvents are used. A considerable amount of biaryls are formed, and hence this reaction may prove useful for the preparation of biaryls and alkylary Is. 

Table III. para Rearrangements of Allyl Aryl Ethers. 44...

Allyl ethers of orf/io-disubstituted phenols rearrange to the corresponding p-allylphenols. It is noteworthy that the para rearrangement is not usually accompanied by inversion of the allyl group.8-4-6-6-7 For example, cinnamyl 2-carbomethoxy-6-methylphenyl ether (III) rearranges without inversion8 to yield the p-cinnamyl derivative (IV). 

The crotyl ether of the same phenol also rearranges without inversion.8 The only known example of para rearrangement accompanied by inversion is the reaction of a-ethylallyl 2-carbomethoxy-6rmethylphenyl ether (V), which yields the p-(y-ethylallyl) derivative (VI).8... 

The para Rearrangement (Table III). If both ortho positions of an allyl aromatic ether are blocked, the allyl group migrates to the para position. If both ortho positions and the para position are occupied, complex decomposition ensues, but the allyl group never goes to the... 

The generalizations above apply only to the migration to the ortho position. When a substituted allyl ether of the type ArOCH2CH=CHR rearranges to the para position, inversion does not occur. As mentioned earlier (p. 3), he only known para rearrangement of an ether of the type ArOCH (R) CH=CH2 proceeds with inversion. No evidence for the formation of abnormal products in the para rearrangement has been reported. 

The observed photobehavior of the benzaldehyde-CDx complexes in the solid state is unique and completely different from that of these complexes in aqueous solution and also from that of benzaldehyde 36 in organic solvents. The substantial formation of 4-benzoylbenzaldehyde 38 upon irradiation in (3- and y-CDx cavities indicates that these medium-sized CDx s provide the radical pair within a fairly spacious supercage environment, thus allowing the para-rearrangement (Scheme 13). The formation of practically racemic 37 upon irradiation of the y-CDx complex may also be attributed to the looser orientation of benzaldehyde 36 in the y-CDx cavity than in the (3-CDx cavity. It was thus demonstrated that the chiral hydrophobic cavity of native cyclodextrins not only modifies the photoreactivity of the included guest but also functions as a chiral supramolecular environment for photochirogenesis, albeit resulting in only modest ee%. 

Cyclodextrins form inclusion compounds in aqueous solution with various molecules which may then exhibit modified reactivity (see reference 185 above). It was reported in 1975 that the presence of p-cyclodextrin modified the ortho para ratio of hydroxyacetophenones formed photochemically from phenyl acetate,and the photoinduced rearrangements of acetanilide, ben-zanilide, and ethyl phenyl carbonate under similar conditions are now reported. In each case the para rearrangement isomer is favoured, and this is rationalized in terms of the aromatic nucleus being bonded into the cyclodextrin cavity such that the ortho and weta positions are shielded whDe the para position is accessible. This proposal is supported by observations that para but not ortho disubstituted arenes are well bonded into p-cyclodextrin cavities. The by-product formation of aniline from the amides and of phenol from the carbonate is also markedly reduced for reactions in the presence of the cyclodextrin. 

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