Acetals photolysis

Evaluation of quantum yield of acetic acid formation at light intensity (L=254 nm) 1=1 TO quant/cm sec gives the value =().()2 close to the value of quantum yield of ester groups destruction (=0,015), measured in similar conditions [162]. Close value of quantum yield of acetic acid formation 0=0,01 has been obtained in the case of polyvinyl acetate photolysis at lower intensity - I=5,7T0 " quant/cm sec [163]. Data on effect of temperature on the rate of acetic formation are given in Table 1. Activation energy of the reaction of acetic acid formation at CA irradiation, calculated according to these results, is E,=9,66 kJ/mole, which is characteristic for photoprocesses. 

The course of benzoin acetate photolysis in the presence of / - and y-CD in both solution and solid complexes is selective. Efficient caging for radical pair recombination induces a remarkable preference for the formation of a photocyclization product over type I products [283]. Similar cage effects improved selectivity for type II products in alkyldeoxybenzoin- -CD complexes in the solid state [284]. 

Thermolysis of trithiane (69) or carbonate (70) at reduced pressure yields methylene-thiirane which is stable in cold, dilute solution (Scheme 152) (78JA7436, 78RTC214). A novel acenaphthylene episulfide is obtained by treatment of the six-membered sulfoxide (71) with acetic anhydride (Scheme 153) (68JA1676), and photolysis of (72) gives a low yield of episulfide (73 Scheme 154) (72JA521). Low yields may be due to the desulfurization of the thiiranes under the reaction conditions. 

The 2-nitrobenzylidene acetal has been used to protect carbohydrates. It can be cleaved by photolysis (45 min, MeOH CF3CO3H, CH2CI2, 0°, 95% yield) to form primarily axial 2-nitrobenzoates from diols containing at least one axial alcohol. ... 

Benzene-sensitized photolysis of methyl 3-cyclohexene-1-carboxylate in acetic acid leads to addition of acetic acid to the double bond. Only the trans adducts are formed. What factor(s) is (are) responsible for the reaction stereochemistry Which of the two possible addition products, A or B, do you expect to be the major product ... 

The irradiation of 2-methoxytropone (A) leads to methyl 4-oxo-2-cyclopentenyl-acetate (D). The reaction can be followed by analytical gas chromatography and two intermediates are observed. These have the structures B and C. Indicate a mechanism by which each of the three successive reactions might occur. The first two steps are photochemical, while the third is probably an acid-catalyzed reaction which occurs under the photolysis conditions. 

Apparently the role of methanol is to intercept unstable species which otherwise tend to polymerize or rearrange. The methoxy peroxide (72) can be isolated in crystalline form if desired, but it is preferable to treat the methylene dichloride solution at 0° with zinc dust and acetic acid until the mixture shows a negative potassium iodide test. The resulting crude seco-aldehyde (73) is then cyclized to (74) by stirring with neutral alumina in benzene at room temperature for 3 hr. ° Wechter has recently reported a number of transformations of a 5yS-hydroxy-6yS-formyl-B-norpregnane prepared in 8% yield by photolysis and hydrolysis of a 5a-hydroxy-6 -azidopregnane. 

Dipolar cycloadditkm reactions of nitrones to olefins, 46, 1,3-Dipolar cycloadditions with 3-phenylsydnone, 45, 98 Dispiro[5 1 5 l]tetradecane-7,14-dione, photolysis to cyclohexylidene-cyclohexane, 47, 34 preparation from cyclohexanecarbonyl chlonde and triethylamine, 47,34 Displacement of bromine from 1-bromo-2-fluoroheptane to give 2-fluoro-heptyl acetate, 46, 37... 

Nitro 2-nitrosO N-rnethylaniline, green prisms from acet/w, mp 165—67° with decompn prepd by the photolysis of N-(2,4-dlnitrophenyl)-N-methyl-leucine (Ref 9)... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

Arylthallium bis(trifluoroacetates) (see 12-21) can be converted to aryl nitriles by treatment with copper(I) cyanide in acetonitrile. Another procedure uses excess aqueous KCN followed by photolysis of the resulting complex ion ArTl(CN)3 in the presence of excess KCN. Alternatively, arylthallium acetates react with Cu(CN)2 or CuCN to give aryl nitriles. Yields from this procedure are variable, ranging from almost nothing to 90 or 100%. 

A more concentrated (1000 ppm) solution of dibenzo-p-dioxin in methanol was irradiated for 1.5 hours under a 450-watt lamp fitted with a borosilicate glass filter while nitrogen was bubbled continuously through, the solution. Unchanged starting material was recovered to the extent of 85%. The principal photolysis product again was a dark brown insoluble gum similar to that described above. Its mobility on thin layer chromatography (TLC) was very low in the benzene/ethyl acetate (4 1) solvent used to separate the other products. 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

As stated previously, the photolysis of cyclobutanone in ethanol solution results in an 8% yield of an acetal, presumably formed from ethanol addition to a carbene intermediate. Alkylation of the a positions of cyclobutanone increases the yield of this rearrangement product,(22)... 

While only the 2,3-diphenylphenol is formed upon photolysis of (44) in aqueous dioxane, in 507, acetic acid the 2,3- and 3,4-diphenylphenols are formed in a 1 1 ratio. This has been explained on the basis of the intermediates (47H50) ... 

Zimmerman and co-workers<135> have reported that photolysis of p-methoxy-benzyl acetate in aqueous dioxane results in p,p -dimethoxybibenzyl and bidioxane, probably through the intermediacy ofp-methoxybenzyl radicals ... 

When 3-methoxybenzyl acetate is photolyzed, however, the products expected from free radical coupling are decreased and a substantial amount of 3-methoxybenzylalcohol results. A carbonium ion intermediate has been proposed in the photolysis of this meta ester ... 

It has been stated that bridged bicyclic systems fail to yield any cyclic acetal upon photolysis,[Pg.382]

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 ... 

Extrapolation of the data to zero time suggests that the endo acetates (65) and (67) are produced in amounts as great as or greater than the exo isomer. Solvolysis studies of the bicyclo[3.1.0]hex-2-en-6-yl cation reveal that nucleophilic capture occurs preferentially from the exo side to give (66) rather than (67). Similarly, solvolysis of cation (63) leads to exo product (64) in at least a 90% yield. Photolysis of benzene in deuteriophosphoric acid results in (68), in which all the deuterium is incorporated into the 6-endo position ... 

Photolysis of the triazepine products produces 2,2-dimethylpropanenitrile and the corresponding pyrazole in quantitative yield <1997BSF927>. Reaction of sydnone 89 with fulvene 91 proceeds by [ji4s + jt6s]-cycloaddition followed by spontaneous loss of carbon dioxide and a molecule of dimethylamine or acetic acid from the pseudo-azulene , cyclopentaMpyridazine 92 (Equation 9) <1996CC1011, 1997T9921>. 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

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