Ortho rearrangement

The relatively small dependence of the rate on solvent is demonstrated in Table 5. The rate coefficients are a little greater in the more polar solvents but the overall effects are small. This is taken to indicate that there is only little polar character to the transition state. Another probe to determine the polarity of the transition state is the effect of substituents on the rate. The results obtained for the ortho rearrangement by varying the para substituents are shown in Table 6131 for the conversion... 

The mechanism is a concerted pericyclic [3,3] sigmatropic rearrangement and accounts for all these facts. For the ortho rearrangement ... 

Nitrogen, carbon-13 and carbon-14 kinetic isotope effects have been determined38 for the analogous acid-catalyzed ortho,ortho -rearrangement of the Af-2-naphthyl-Ar/-phenylhydrazine (equation 24). The labelled compounds required for this study were prepared by the sequence of reactions shown in Schemes 20-22. 

However, solvent viscosity, rather than polarity, has been a useful tool for mechanistic purposes. Although the quantum yield of the ortho-rearranged product of 4-methylphenyl acetate (20) does not change with viscosity of the medium, the formation of 4-methylphenol (22) is highly sensitive to this factor. Thus, its quantum yield is 0.45 in ethanol (1.00 cP) but only 0.02 in Carbowax 600 (109 cP) (Scheme 8 Table 3) [13], This clearly supports the mechanism involving caged radical pairs. A related aspect is the intramolecular nature of the process confirmed by the lack of cross-coupling products in crossover experiments with mixtures of different esters [10]. 

As can be seen, the pathway from the cyclohexadienone to the ortho-rearranged phenol does not imply a jump over an energy barrier (34 kcal/mol), but a tunneling from the vibrational energy levels located at E(vo) = 0 kcal/mol and E(vi) = 3.9 kcal/mol (4.4 kcal/mol for the deuterated compound). Analogous results have been reported for the photo-Fries rearrangement of 2,4-dimethoxy-6-( ara-tolyloxy)-v-triazine (28) to give 2,4-dimethoxy-6-(2-hydroxy-5-meth-ylphenyl)-v-triazine (29) (Scheme 11) [43]. 

The polarized signals correspond to the starting ester (30), the ortho-rearranged product (31) and para-cresol (32). The variation in intensity of the signals... 

Phenol esters of a,(3-unsaturated carboxylic acids have an interesting reactivity due to the synthetic utility of the resulting hydroxychalcones (Scheme 19). This aspect will be illustrated in Section IV. However, from the basic point of view, it is worth mentioning that the cis or trans configuration of the olefinic part of the acyl moiety can have a marked influence on the photochemical reactivity of the ester. When para-methoxyphenyl fumarates are irradiated, the normal ortho-rearranged products are obtained. By contrast, irradiation of para-me-thoxyphenyl maleates does not lead to rearrangement. Instead, cyclization products are obtained (Scheme 20). 

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],... 

Diacetylhexahydromarmesin 276 has been converted into the ortho-rearranged product 277 in 30.6% yield among other photoproducts. Transformation of 277 into methyl hexahydrorutaretin methyl ether 278 served to determine the absolute configuration of rutaretin methyl ether (279) (Scheme 70) [199]. 

Another problem for the synthetic use of PFR is that the ortho-rearranged products may act as internal light filters, stopping the reaction. In synthetic routes leading to chromanones, chromones, and related compounds, this is of vital importance because the overall yield is limited by the photochemical step. Improved yields can be obtained if the a, 3-unsamrated orr/io-hydroxyphenones resulting from PFR are removed and cyclized to chromanones. This can be acomplished in one pot by irradiation in a two-phase system benzene/10% aqueous NaOH, whereby chromanones are directly obtained from phenyl crotonates in 80-90% yields [210]. 

Figure 6 shows the kinetics of appearance of the ortho-rearranged product emission. It is noteworthy that after only few minutes, it is possible to detect photoproducts by means of this technique, whereas by UV-vis or IR, it usually takes longer to appreciate spectral changes. 

The values presented in the second column show the quantum yields of the aryl ester conversion and can be correlated with other values in Table V, only because in ethanol at 20°, 70-80% of the ester is converted to ortho-rearranged product.59 It was shown that the given data for para-substituted esters could be also correlated with Hammett s a constants (Eq. 2). 

The reversed dependence of this concerted decarboxylation reaction, as well as the concerted ortho-rearrangement on the polarity of solvents (cf. Tables II and VI), and the competition of decarboxylation to phenol formation in nonpolar solvents, are striking. Lack of information on the influence of substituents, viscosity, sensitization, or possible quenching in direct comparison with the formation of rearranged products and phenol do not yet... 

Ortho-Rearranged Products. They seem to be formed for the major part or exclusively via concerted Path B with subsequent aromatization (70 -> 71). 

Fries rearrangement of aromatic formate esters suggests that phenols are the major products (.24) obtained in the reaction. As poly(p-hydroxystyrene) is remarkably clear in the deep UV, it is likely that poly(p-formyloxystyrene) will not suffer from the same problem of photostabilization upon exposure as was the case with poly (p-acetoxystyrene). This expectation was confirmed by our study of the photo-Fries reaction of p-cresyl formate no ortho rearranged product was isolated after reaction while p-cresol and a small amount of starting material were obtained. 

The expected photorearrangement is observed and can again be monitored conveniently by IR spectroscopy (Figure 5). The intensity of the ester carbonyl band at 1747 cm-1 decreases while a small hydroxyl and a new carbonyl band appear at 3384 and 1626 cm-1 respectively. This new carbonyl band corresponds to the hydrogen-bonded ortho rearranged ketone. Although... 

The 7V,N-dimethylcarbamate 351, when reacted with LDA and aliphatic or aromatic aldehydes, afforded surprisingly ortho-rearranged products 353 (Scheme 109) (87JHC1487). 

Af-Phenylhydroxylamine may be converted into an Af-benzoyl derivative by treating it in aqueous solution with benzoyl chloride while simultaneously removing, with sodium hydrogen carbonate, the liberated hydrogen chloride.32 On reaction with thionyl chloride this benzoyl derivative is converted into o-chlorobenzanilide (Expt 6.61) by way of an intermediate chlorosulphite ester, which then undergoes a specific ortho rearrangement.25... 

Photolyses of 31-34 in homogeneous solution results in the formation of diphylethanes 39 (5-15%), phenols 38 (5-15%), ortho-hydroxyphenone 36 (40-60%), and para-hydroxyphenones 37 (20-25%). Small amounts of phenyl benzyl ether 35 (3-8%) were also detected. However, photolyses of all of the four esters on NaY zeolite and Nafion only produce ortho rearrangement products 36. Molecular models suggest that esters 31-34 can enter into NaY zeolite internal surface and the inverse micelle of Nafion. We believe that the preference for formation of ort/zo-hydroxyphenones 36 in the products is a consequence of the restriction on diffusional and rotational motion of the geminate radical pair. 

Table II. ortho Rearrangements of Allyl Aryl Ethers. 30...

C. ortho Rearrangements with Displacement of Carbon Monoxide or Car-... 

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