Photolysis of Ethers

The photolysis of dimethyl ether (gas phase) and diethyl ether (liquid phase) was first undertaken by Berthelot and Gaudechon (21c). The main products detected were hydrocarbons (methane from dimethyl ether, ethane from diethyl ether) with some CO and H2. Ethers were later shown also to form dehydrodimers (41).  

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The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

Eloranta and Linschitz (15) have investigated the flash photolysis of ether solutions of the alkali metals. The observed bands in tetrahydro-furan and dimethoxy ether are discussed in terms of the trapped electron pair species, e2, which undergoes photodissociation ... 

V. PHOTOLYSIS OF ETHERS A. Open-Chain Ethers in the Liquid Phase... 

Irradiation as well as the action of ferrous salts results in homolytic cleavage of the three-membered ring of 34. By photolysis of ethereal solutions of 34 in the presence of triplet sensitizers, Kobayashi " obtained 47% n-caproic amide 45 thus the conversion had involved uptake of two atoms of hydrogen. Striegler and Timm converted 34 into to 6-chlorocaproic amide 46 by ferrous salt in hydrochloric acid. ... 

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

Direct proof of an oxaziridine intermediate was achieved in photolysis experiments in an organic glass at 77 K (80JA5643). Oxaziridine (75), formed by photolysis of A/-oxide (74) and evidenced by UV spectroscopy under the above conditions, decomposed at higher temperature to form the imino ether (76) by N—O bond cleavage and C -> O migration of an aryl group. 

Ring expansion of haloalkyloxiranes provides a simple two-step procedure for the preparation of azetidin-3-ols (Section 5.09.2.3.2(f)) which can be extended to include 3-substituted ethers and O-esters (79CRV331 p. 341). The availability of 3-hydroxyazetidines provides access to a variety of 3-substituted azetidines, including halogeno, amino and alkylthio derivatives, by further substitution reactions (Section 5.09.2.2.4). Photolysis of phenylacylamines has also found application in the formation of azetidin-3-ols (33). Not surprisingly, few 2-0-substituted azetidines are known. The 2-methoxyazetidine (57) has been produced by an internal displacement, where the internal amide ion is generated by nucleophilic addition to an imine. 

During 1961-2 four independent groups almost simultaneously reported the first syntheses of D-norsteroids, based on the photolysis of 16-diazo-17-ketones. In a typical procedure. Cava and Moroz ° convert the 16-oximino-17-one (93) derived from estrone methyl ether (92) to the diazoketone (94)... 

Methylfuran, irradiated in the presence of mercury vapor, gave carbon monoxide and a fraction containing 1,3-butadiene and 3-methylcyclopropene (45 55) (67JA1758). Subsequently, it was found that in both sensitized and direct photolysis of 2-methylfuran a more complex mixture of products was obtained, where 3-methylfuran was present (Scheme 5) (68JA2720 70JPC574). 3-Methylfuran was the only product when 2-methylfuran was irradiated in diethyl ether (68JA2720). 

Another route to enantiomcrically pure iron-acyl complexes depends on a resolution of diastereomeric substituted iron-alkyl complexes16,17. Reaction of enantiomerically pure chloromethyl menthyl ether (6) with the anion of 5 provides the menthyloxymethyl complex 7. Photolysis of 7 in the presence of triphenylphosphane induces migratory insertion of carbon monoxide to provide a racemic mixture of the diastereomeric phosphane-substituted menthyloxymethyl complexes (-)-(/ )-8 and ( + )-( )-8 which are resolved by fractional crystallization. Treatment of either diastereomer (—)-(/J)-8 or ( I )-(.V)-8 with gaseous hydrogen chloride (see also Houben-Weyl, Vol 13/9a, p437) affords the enantiomeric chloromethyl complexes (-)-(R)-9 or (+ )-(S)-9 without epimerization of the iron center. 

Photolysis of the sulphinyl-3H-pyrazole 587 in ether or methylene chloride leads to the formation of a relatively stable carbene 588 that can be identified by physical methods. When the irradiation is performed in ethyl vinyl ether or in furan, the expected cyclopropanes are formed smoothly and stereospecifically683 (equation 374). 

The course of the photolysis of a number of cyclic sulfoxides, however, has been shown not to involve simple photoextrusion processes. In fact, the work of Schultz and Schlessinger19,20 and Still and coworkers21 has shown the existence of a novel desulfurization pathway leading to cyclic ethers or to carbonyl compounds by formal loss of the sulfur atom only, by certain cyclic sulfoxides. 

Finally photolysis of substituted pyrazolenines 140 in ether is reported (118) to yield allene 142 and diene 143 as products via vinyl cation 141. 

Another photocyclization to a benzo[c]phenanthridine was reported (127). Oppenauer oxidation of ( )-ophiocarpine (92) with potassium fm-butoxide and benzophenone in dioxane effected C-6—N bond cleavage to afford the hydroxyisoquinoline 219 via berberinephenolbetaine (121) (Scheme 39). Although photolysis of 219 gave only the oxepine 221, that of its methyl ether 220 furnished directly norchelerythrine (222) through electrocyclization followed by spontaneous elimination of methanol. 

In ethanol solution 1,2-benzocyclobutenedione undergoes reaction to produce a lactol ether, the analog of the products produced upon photolysis of the tricyclic ketones and cyclobutanones discussed above,<30)... 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

Photolysis of diazonium salts in alcohols produces aromatic ethers and aromatic hydrocarbons<51,52) ... 

The isolation of benzvalene (61) from the irradiation of benzene at 254 nm and the observation that this compound produces the expected bicyclic ethers when treated with acidified methanol lend credence to the intermediacy of (61).(90> Photolysis of benzene in acetic acid was found to result in formation of acetates (64)—(67), with the product composition changing with time ... 

Recently, results of careful experiments were reported by Ito et a/.101 They claimed that formic acid, formaldehyde, and methanol, which had been previously reported as photoelectrochemical reduction products of carbon dioxide, were observed also by photolysis of cell materials, such as electrolytes, including 15-crown-5 ether, and epoxy resin, which has often been used as the molding material of semiconductor electrodes in aqueous solutions. Previously reported reduction products were obtained also under nitrogen with (Table 4) and without (Table 5) a p-GaP photocathode under illumination. These precise experiments under improved conditions, where no photolytic products were observed, gave the result that the main reduction product of carbon dioxide at a p-GaP photocathode in aqueous electrolytes was formic acid. Thus, many kinds of products reported in previous papers83,97,100 were suggested to be due to photolysis of cell materials. 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

Suggestive evidence for the protonation of diphenylcarbene was uncovered in 1963.10 Photolysis of diphenyldiazomethane in a methanolic solution of lithium azide produced benzhydryl methyl ether and benzhydryl azide in virtually the same ratio as that obtained by solvolysis of benzhydryl chloride. These results pointed to the diphenylcarbenium ion as an intermediate in the reaction of diphenylcarbene with methanol (Scheme 3). However, many researchers preferred to explain the O-H insertion reactions of diarylcarbenes in terms of electrophilic attack at oxygen (ylide mechanism),11 until the intervention of car-bocations was demonstrated by time-resolved spectroscopy (see Section III).12... 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

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