Acetophenone Sensitizers

The [2 + 2] cycloaddition of benzene derivatives with alkenes was also carried out using photosensitization. Maleimide 36 was added to benzene in high yields (Scheme 5.8, reaction 16) [42]. In this case, the sensitizer acetophenone 37 transfers its triplet energy to 36, after which the cycloadduct VII reacts immediately with an... 

This example demonstrates the most challenging problem of flavor chemistry, ie, each flavor problem may require its own analytical approach however, a sensory analysis is always required. The remaining unknown odorants demand the most sensitive and selective techniques, and methods of concentration and isolation that preserve the sensory properties of complex and often dehcate flavors. Furthermore, some of the subtle odors in one system will be first identified in very different systems, like o-amino acetophenone in weasels and fox grapes. 

The reaction was conveniently carried out in DME in the presence of benzophenone, and the yields were in the range 20-30%. In ethanol solution, 1,3,5-trimethylpyrazole (177 R = R = R = Me, R = H) yielded (183) and (184) (69T3287). In contrast with the previously observed sensitized photorearrangements (67HCA2244, 67CC488), this last reaction was not sensitized by acetophenone or benzophenone. 

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 products are the same for both direct irradiation and acetophenone sensitization. When the reactant B is used in homochiral form, the product D is nearly racemic (6% e.e.). Relate the formation of the cyclobutanones to the more normal products of type E and E Why does the 5-aryl substituent favor formation of the cyclobutanones Give a complete mechanism for formation of D which is consistent with the stereochemical result. 

Sensitized by Acetophenone. A -butanol solution of (114) (2.10 M) and acetophenone (0.8 M) is irradiated for 6 hr at 30° under nitrogen with a Hanau Q 81 high-pressure mercury lamp through a Pyrex-acetone filter (path length 1 cm, cut-off of wavelengths below 3270 A). Better than 98 % of the incident light is absorbed by the acetophenone. A 70% conversion of (114) to the same products as listed above is observed. The ratio (118) (120) is again -2 1. 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

Similarly quantitative yields of (141) are obtained from (140) in benzene solution by triplet sensitization using acetophenone. 

This behavior has been verified with bithiazole derivative 94 (Scheme 36). Neither acetophenone nor benzophenone sensitized the reaction, in agreement with an ICI mechanism via the excited singlet state (86TL6385 87JA938). 

The overall sensitivity of the sampling and concentrating system was examined using acetophenone as the solute. Purified water (water that had been passed through a reversed-phase column) was used to make... 

It is seen that with 1 ml samples the peak for a concentration of 1 ppm is well off scale and a clearly defined peak was observed for 10 ppb. When a 10 ml sample was used, acetophenone at a concentration of 1 ppb could just be detected. Under the conditions used, the ultimate mass sensitivity of the system was about 10 ng. This level of sensitivity was achieved, on the one hand, by the sample concentrating process, and on the other, as a result of the high mass sensitivity of small bore columns. 

This reaction is sensitized with acetophenone and quenched by naphthalene, di-f-butylnitroxide, or piperylene. 

As with 2-cyclopentenone, the ratio (70) (71) varies with the molar concentration of the enone, the head-to-head dimer (71) becoming increasingly important at higher concentrations/133 This reaction is efficiently sensitized by acetophenone, benzophenone, thioxanthone, and naphthalene. The same enone concentration effect was observed in the sensitized photo-dimerization as in the direct photolysis. Similarly, quenching of the dimerization by piperylene was not accompanied by a change in dimer ratio. Systematic... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

The quantum yield for isomerization in the direct photolysis was found to be d> = 0.94. The reaction could also be sensitized with acetophenone ( = 1.02) and quenched with piperylene, indicating a reactive triplet species with a rate constant kr of 3 x 10l°sec-1. With a 3-(p-methoxyphenyl) derivative two products were obtained<81) ... 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

Several mechanisms have been postulated in order to account for ketone-sensitized photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as singlet sensitizers via a collisional deactivation process (13). An alternative mechanism proposed for benzophenone involves abstraction of a methylene hydrogen from PVC by the triplet ketone (Equation 2), followed by 3 scission of a... 

A different MS-based ee-assay makes use of a proline-derived mass-tagged acylating agent.95 In the course of derivatization it is necessary that some degree of kinetic resolution comes about. The sensitivity of the method was reported to be 10% ee. It can also be applied to the reaction of a prochiral compound lacking enantiotopic groups, as in the transformation of acetophenone to phenylethanol. 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

Bicyclo[2.2.l]hepta-2,5-diene, irradiation sensitized by acetophenone to give qua-dricyclane, 51, 133 Bicyclo[2.2.0]hexa-2,5-diene,... 

Acetic anhydride, with 2-hep-tanone to give 3-n-butyl-2, 4-pentanedione, 51, 90 ACETIC FORMIC ANHYDRIDE, 50, 1 Acetone azine, 50, 2 ACETONE HYDRAZONE, 50, 2, 28 Acetophenone, 54, 93 as sensitizer for irradiation of bicyclo[2.2.1]hepta-2,5-diene to give quadricyclane,... 

The reaction in its basic form is illustrated by the conversion of 274 into 275. This transformation shows the control exercised within the two biradicals A and B on the final outcome of the reaction151. Another typical di-jr-methane process, this time with electron-withdrawing substituents on one of the vinyl moieties, arises on acetophenone-sensitized irradiation of the diene 276. This yields the cyclopropane 277 in 35% yield. The... 

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