Aryl esters irradiation

If radicals diffuse from the solvent cage, fragmentation products are formed. Abstraction of hydrogen from the solvent by a phenoxy radical results in phenol, which can almost always be observed among the photoproducts of aryl esters in solution. Chemical evidence for the reaction of phenoxy radical with solvent is the formation of nearly stoichiometric amounts of 4-methyI-phenol and acetone from the irradiation of 4-methylphenyl benzoate (60) in isopropyl alcohol.34... 

Direct experimental evidence relating to the existence of free radicals 65-67 and 68 among the photoproducts of aryl esters has not yet been obtained. On the photolysis of diaryl carbonates, no features attributable to trapped radical parts were seen.49 In the esr spectra obtained at 77°K only a single weak line at g = 2.005 due to phenoxy radical was detected.49 A much more intensive spectrum of the phenoxy radical was observed by irradiation... 

The aryl esters of formic and oxalic acid (exclusively) undergo very efficient decarbonylation to form phenol.5 It is noteworthy that aryl esters of formic acid, when irradiated in the presence of olefins, are added to the latter in the same way as phenols.83... 

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. 

The comparison of the published structures of the irradiated aryl esters with the yields of photodecarboxylation products points out that increasing size and probably also the increasing number of alkyl substituents on a phenolic nucleus positively influence the yields. 

Formation of 131 in a high yield (85%) on the photolysis of ethyl 2-biphenylyl carbamate (134) is not surprising on the basis of the discussion for the conversion 110 -> 112. Other carbamates 27,7,11 rearranging via sterically strained 4- and 6-ring transient states, afford normal rearranged products 28-30. Here also, as was the case with the irradiation of aryl esters, higher yields of ortho and para products were obtained in polar solvents (ethanol, t-butanol) than in the nonpolar n-hexane.11... 

The synthesis of a-aryl esters can be achieved through the reaction of aryl and heteroaryl halides with CH2C02Bu-t, CHMeC02Bu-t and CHPhC02Lt ions under photoinitiation in 57-89% yields [20]. Moreover, phenylacetic add dianion reacts with ArX upon irradiation to give aryl substitution produds at the p- or a-carbon, depending on the counterion used (Scheme 10.9) [21]. 

SINGLET-STATE RADICAL PAIRS FROM IRRADIATION OF ARYL ESTERS AND ALKYL ARYL ETHERS... 

PHOTO-REACTIONS OF ARYL ESTERS IN POLYMER MATRICES. KINETIC INFORMATION FROM CONSTANT INTENSITY IRRADIATIONS... 

Relative Rate Information from Irradiation of Aryl Esters in Which Acyl Radicals Do Not Decarbonylate Rapidly... 

In each of the aryl esters discussed above, the acyl radical formed upon lysis of the excited singlet state of the ester loses CO very slowly at the temperatures of the irradiations. At 296K, the rates of loss of CO by acetyl and propanoyl radicals in the gas phase are 4.0 and 2.1 x 10 s respectively." As aresult, no products from decarbonylation and rearrangement are expected" (or have been found) when either of the NA or NM isomers is irradiated in liquid solvents or bulk polymers, and kinetic information from photoproducts alone is limited to relative rates of radical pair processes (Scheme 13.3). For example, if no Fries products from 1-NA or 1-NM emanate from reencounter of radicals that have escaped from their initial cages, [2-AN]/[4-AN]/[l-NOL] = /c2a/ 4a/ nol-... 

The Cj p3-O bond of 2-tert-butoxy-furan and -thiophene is cleaved catalytically by BiCl3 or Bi(OTf)3 to produce 2(5H)-furanone and -thiophenone, respectively [254]. The selective hydrolysis of aryl esters is catalyzed by bismufh(III) mandelate in DMSO [255]. In the presence of 50 mol% Bi(NO3)3-5H2O, 10 mol% Cu(OAc)2, and Montmorillonite KIO ketoximes undergo facile deoximation in acetone-H2O [256]. Under microwave irradiation the BiCls-promoted hydrolytic cleavage of the C=N bond of dimethylhydrazines, tosylhydrazines, semicarbazones, and oximes proceeds in wet THF (Scheme 14.120) [257-259]. 

The mechanism of the photo-Fries reaction, both in the vapour and condensed phases, has been thoroughly established, and the many reported examples of the process with a wide variety of systems indicate the generality of the reaction.10 Although o- and p-migration products normally result from irradiation of aryl esters, structural constraints can lead to meto-rearrangement products, and the first useful meta-Fries reaction has now been described.188 Thus irradiation of the amino-lactone (147) in THF leads to 57% of the red crystalline isomer (148) via,... 

The reaction took place either via the ArSsl or via the AtSn2 mechanism (see Scheme 14.1, paths h,c), and occurred successfully via the first mechanism when using aryl hahdes or esters with electron-donating substituents. Among halides, both aryl chlorides and fluorides underwent photosubstitution when irradiated in an aqueous MeCN solution of KCN (Scheme 14.10, left part) [59]. It should be noted that, in transition metal-catalyzed reactions, the substitution of a chloro- by a cyano-group occurs only under relatively harsh reaction conditions, whereas such a process does not take place at all with aryl fluorides [57]. In the case of aryl esters the photoinduced cyanation occurred conveniently. As esters are easily... 

More than half a century ago, Anderson and Reese reported the first example of a photo-Fries rearrangement (PFR), where 2-hydroxyphenylacetate was converted into 2,3- and 3,4-dihydroxy acetophenone upon UV irradiation [1]. Since then, hundreds of articles have appeared on this reaction, which has been extended from the initial aryl esters to amides, carbonates, carbamates, thioesters, sulfonates, etc. [2-4]. 

Trehan et al. (1997) reported a simple procedure for carrying out the Fries rearrangement of aryl esters to ortho- and para-hydroxy acetophenones in dry open media in ordinary glassware using a commercial microwave oven. The Fries rearrangement normally requires a Lewis acid and long reflux times or photochemical conditions. It has been observed that protected iV-allylanilines with different substitution on the aiyl ring can be transformed by microwave irradiation in the presence of BF -Et O within 1-2 min at 170°C. 

For instance, photolysis of aryl esters of phenylpropiolic acid (21) gives rise to o-hydroxyaryl phenyl-ethynyl ketones (22). The latter compounds can undergo cyclization in two different ways, depending upon the reaction conditions When potassium carbonate in acetone is used, flavones (23) are formed by a 6-endo-dig process by contrast, the use of sodium ethoxide in ethanol favors the 5-exo-dig process,to give aurones (24) as the major products (Scheme 6). The side-chain functionality can be masked, as in the case of the protected ketoesters (25), whose irradiation and subsequent deprotection provide an alternative entry to the flavone system (Scheme 6). ... 

In a typical reaction, a series of C - O linked pyrazinone-triazoles were irradiated with an excess (3.5 equiv) of DMAD in o-DCB. The reactions were completed in 6-20 min at 180 °C, using a maximum irradiation power of 200 W. After spontaneous elimination of the corresponding isocyanate or cyanogen chloride, the pyridines and pyridinones were formed in combined yields of 71-99%. It is noteworthy that the pyrazinones with an aryl moiety at the N-1 position yielded pyridines as major products, while those with ben-zyUc or alkyl ester moieties at the N-1 position furnished the corresponding pyridinones as major products. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

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