Sensitizer methyl benzoate

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

The reaction can be also applied to azoles (e.g., imidazoles, pyrazoles, triazoles, tetrazole) as the N -nucleophile for the hydroaminations of cyclic olefins. In this case, the presence of methyl benzoate sensitizes the photoisomerization of the double bond to form a highly strained ( )-cycloalkene. Protonation of this intermediate by triflic acid (TfOH, 20mol%) and addition of the azole nucleophile completes the reaction sequence [41]. As an example, the expeditious synthesis of l-(l-methylcy-clohexyl)- H-imidazole (27) in 72% yield is shown in Scheme 3.17 [41]. 

Grant, F. W., Martin, W. C. and Quackenbush, R. W., A simple sensitive specific filed test for cocaine based on the recognition of the odour of methyl benzoate as a test product , Bull. Narcotics, 27(2), 33-35 (1975). 

Tokumaru and his co-workers have demonstrated that sensitized irradiation of the cis-alkene (35) yields the trans-isomer in 100% yield. There is no evidence for the formation of the cis- from the trans-isomer. The sensitized trans-cis isomerization of the alkene (36) has been studied. An investigation of the photoreactions of the alkene (37) and their dependence on oxygen concentration has been reported. A detailed study of the singlet-state cis-trans isomerization of cis-cycloheptene at -78 °C induced by methyl benzoate as sensitizer has been reported. 

Singlet photosensitized polar addition of methanol to (A )-(>)-limonene (102) in nonpolar solvents afforded a mixture of the diastereomeric ethers 103 and 104 and the rearrangement product 105 (Scheme 6.42).677 The diastereomeric excess (de) of the photoadduct was optimized by varying the solvent polarity, reaction temperature and nature of the sensitizer. The first step of the reaction is the Z E photoisomerization (Section 6.1.1) of 102 to a highly strained /i-isomer, followed by protonation and methanol addition. The initial formation of a carbocation via the protonation step has been excluded under those reaction conditions. The Markovnikov-oriented methanol attack on the less-hindered (Rp)-(E)-102 compared with that of (Sp)-(E)-U)2 explains why 103 can be obtained in up to 96% de upon sensitization with methyl benzoate in a methanol solution. The hypothesis that Z E isomerization of the cyclohexene moiety affords a strained (reactive) alkene, whereas isomerization of the exocyclic double bond does not, was supported by the observation of an exclusive nucleophilic addition to the cyclohexene double bond. 

The copper(I) trifluorosulfonate catalyzed photoreaction of cycloheptene (38a) leads to the dimer 95 (114c). Formation of the latter was taken as support for the generation of frans-cycloheptene (39a) as an intermediate. However, isomerization of free (ran,s-cycloheptene (39a) seems to be faster than its cyclodimerization. 7 ran.s-cycloheptene (39a), obtained by photoreaction with methyl benzoate as sensitizer, isomerizes to ds-cycloheptene (38a) at 0°C (114b) but reacts at low temperature with acidified methanol to form 96 or with diazomethane to form 97 (114a). The stereochemistry of the pyrazoline 97 is trans, in accord with the stereospecific addition to trans-cycloheptene (39a). 

SENSITIZERS AP = acetophenone BP = benzophenone A = acetone B = methyl 3,5-bis(trifluoromethyl)benzoate C = (-)-tetrabomyl 1,2,4,5-benzene tetracarboxylate D = (-)-hexakis(l-methylheptyl) benzenehexacarboxylate MB = methyl benzoate P = phenan-threne CNN = 1-cyanonaphthalene DCA = 9,10-dicyanoanthracene DCB = 1,4-dicyano-benzene DCN = 1,4-dicyanonaphthalene CuOTf = copper trifluoromethanesulfonate benzene complex TPT = tetraphenylpyrylium tetrafluoroborat. 

Inoue et al. have described their study of the singlet sensitized isomerization of cyclo-octene. They report that the use of methyl benzoate or other aryl esters results in the anomalously high trans-cis ratio of 0.25. This result is only anomalous if a triplet mechanism is operative but the authors have shown that in this instance a singlet state is the active species. The same sensitizer has been used in a study of the photochemical behaviour of the diene (44), which is converted into the isomer (45) and subsequently into (46). ... 

Wasserman and Saito have used diphenyl sulfide to intercept dioxetanes formed in the sensitized photooxidations of certain unsaturated systems. Thus reaction of cw-dimethoxystilbene (1) with singlet oxygen yields only the diox-etane (2) and the product of cleavage, methyl benzoate. However, addition of diphenyl sulfide to the reaction results in formation of benzil dimethyl ketal... 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas phase basicity for a series of substituted acetophenones and methyl benzoates. It was found that sensitivity of the free energy... 

Evidence has also been obtained for the intermediacy of the (E)-cycloheptene (E)-7 in the formation of methyl ethers 8 and 9 on direct, p-xylene-sensitized and methyl benzoate-sensitized irradiation of the 3,7-disilacycloheptene (Z)-7 in acidified methanol. Sensitized irradiation of (Z)-7 in methylcyclohexane solution at -75°C followed by siphoning of the irradiation mixture into acidified methanol similarly afforded ethers 8 and 9, showing that a long-lived intermediate is involved. Addition of cyclopentadiene to the cold irradiation mixture in the dark afforded a Diels-Alder adduct shown to have a trans ring juncture. 

Shim, S.C., Kim, D.S., Yoo, D.J., Wada, T., and Inoue, Y., Diastereoselectivity control on photosensitized addition of methanol to ( R)-(-t)-limonene,/. Org. Chem., 67, 5718, 2002. The trans isomer of ether 49 was formed with high diastereoselectivity (>96%) using 0.5 M methanol in ether solution at -75°C and either methyl benzoate or dimethyl phthalate as sensitizer. However, chemical yields were low. 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

For more remote substitution p is expected to be smaller. Thus Hay and Porter-28 found p = 0.6 for the alkaline hydrolysis of the methyl esters of a-amino acids, RCH(NH2)COOCH.3 in water at 25°C. However, higher sensitivities are possible if the geometry of the system is such that the direct transmission of polar effects becomes more important. For example, Roberts and Moreland245 found that the effects of substituents in the 4 position of ethyl bicyclo-[2.2.2]octane-l-carboxylates (28) on the hydrolysis in 88% ethanol at 30°C are comparable to those observed for meta- anti para-substituted benzoates (p = 2.24), and a similar, though smaller, effect, is observed for methyl rrarts-4-substituted cyclohexanecarboxylates (29). 

A recently reported regioselective photoreduction of benzoates by photosensitized electron transfer reaction was applied to nucleosides [85]. In presence of jV-methyl-carbazole as the electron donor sensitizer and in an isopropanol, water solution, w-trifluoromethylbenzoates of adenosine 83 or benzoates of uridine 84 give deoxygenated products in good yields (73 %). 

The first phenolic derivative of ferrocene to be reported, di(l-hy-droxy-3-methyl-cyclopentadienyl) iron, was prepared by the reaction between 3-methyl-2-cyclopentanone, sodamide, and ferrous chloride in liquid ammonia. This compound is remarkably unstable, being extremely sensitive to air. It is a yellow substance which sublimes at 130-140° at 0.1 mm pressure, and is readily soluble in methylene dichloride but insoluble in carbon tetrachloride and in n-heptane. In water saturated with nitrogen a yellow solution is produced which rapidly bec( nes blue-green on exposure to air. The compound forms a benzoate (mp 121-123.5°) which is stable in air. An analogous derivative of the dicyclo-pentadienyl cobalt (III) cation has also been obtained (6). 

However, in contrast to strychnidine, vomicidine is phenolic (0-acetate (21) and 0-benzoate (14) ), it being soluble in alkali. While this sensitive aminophenol has been methylated with dimethyl sulfate, only an intractable black resin results from the action of diazomethane. 

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