Acetophenone, sensitizer for irradiation

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 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. 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

Acetophenone is also a convenient sensitizer for the ODPM rearrangement. Bicyclo[2.2.2]oct-7-ene-2,5-diones 53a-c undergo a highly chemoselective ODPM rearrangement under acetophenone-sensitized irradiation (350 nm) to yield 54a,c and 55b, in excellent yields (Table 4.2) [33]. However, phenols 56a,b, which presum-... 

Irradiation of simple 1,3-dienes in the presence of a sensitizer leads to dimers, with the product ratio dependent on the triplet energy of the sensitizer. For example, acetophenone 14 converts 1,3-butadiene 3 almost exclusively to the [2+2] adduct 6 (80 20 trans. cis), (Sch. 4) [31], whereas the use of pyrene 15 gives mostly the [4+2] adduct 7 [25]. 

Cyclobutanone formation becomes more bothersome in the acetone-sensitized rearrangements of the homologs (20b-d) to (21b-acetone solutions are irradiated. For these substrates, and generally for all 1-substituted bicyclooctenones of type (20), it is therefore advantageous to employ acetophenone sensitization at >340 nm, where the absorbance of the enone chromophore is negligible. I paratively attractive high substrate concentrations of up to 10% of (20) can be employed under the latter conditions. 

Acetone- or acetophenone-sensitized irradiation of the enones yields the reeurranged ketones (213, 214) which are described as useful substrates for the synthesis of polyquinane natural products. The enones (215) can be converted by photosensitized irradiation into the diquinanediones (216). Irradiation of (215) in acetone gives a low yield of the tricyclic ketone (217). This ketone, while stable in the dark, is photochemically converted into the diquinanedione (215). The authors present evidence that this conversion results from fission of the cyclopropyl bond to yield the biradical (218). Demuth and Hinsken have reported the use of the oxardi-ir-methane rearrangement in the synthesis of annelated triquinanes. Thus the photo conversion of the enone (219) affords the tetracyclic ketone (220,72%). In an analogous reaction the enone (221) is converted into the isomeric ketone (222,70%). ... 

A synthesis of substituted phenanthrenes has been reported using the bicyclooctadienes (61) as starting material. The process makes use of the nucleofugal group on C-8 and follows the path outlined in Scheme 3. This involves a di-rr-methane bridging process followed by the collapse of the intermediate biradical (62). Normal di-Ti-methane behaviour is reported in the acetophenone-sensitized irradiation of the isoquinolinone derivative (63a). This yields the two products (64) and (65) as a 3 1 mixture in a total yield of 75%. An N-oxide derivative gave a brown polymer with little evidence for the formation of di-n-methane products. The influence of ring substituents was also studied for the derivatives (63b, d) and the results of this are shown in Scheme 4. The authors conclude that the cyclization process is under LUMO control. Irradiation of the dihydropyridine (66) affords the oxidised pyridine (67) as the major product. A minor product (68) is also formed by a di-ir- methane process. ... 

Experimental details660 A benzene solution of 88 (0.653 mmol) and acetophenone (sensitizer 65 mmol) was irradiated with a medium-pressure mercury lamp (450 W) through a Pyrex filter (1 > 280nm) (Figure 3.9) for 30 min. The reaction mixture was concentrated under reduced pressure and acetophenone was then removed by bulb-to-bulb distillation at 40 °C under reduced pressure to give 89 as a yellow oil in nearly quantitative chemical yield. 

A solution of 200 mg (0.41 mmol) of l,l-dicarbomethoxy-3,3,5,5-tetraphenyl-l,4-penta-diene and 8.03 g (66.9 mmol) of acetophenone in 200 mL of benzene was irradiated for 30 min through Pyrex using a 450 W Hanovia medium-pressure lamp and the standard immersion well. The solvent was removed under vacuum leaving a yellow oil. The acetophenone sensitizer was removed by bulb to bulb distillation at 0.015 Torr and 30 °C with dry ice cooling of the receiving bulb. The residue consisted of 202 mg (98%) of colorless crystals whose NMR spectrum show complete conversion. Recrystallization fix)m ether/ hexane afforded 161 mg of colorless crystals, mp 216 - 217 °C. 

There are a number of processes that can conpete with the di-n-rearrangement and related reactions. As indicated earlier, in the case of the parent DPM rearrangement, unproductive isomerization of the olefins within the substrate can occur when triplet-mediated conditions are applied to acyclic substrates. Electrocyclic processes can also conpete. For exanple, while acetophenone-sensitized irradiation of benzobarrelene (44) affords benzosemibullvalene (8), direct irradiation of the same substrate yields benzocyclooctatetraene (45) via a [2+2] photocycloaddition/cycloreversion sequence. Such electrocyclic processes tend to proceed preferentially from the singlet excited state and take place exceptionally rapidly. In broad terms, cyclic substrates are more likely to suffer from conpeting electrocyclic reactions by conparison with their acyclic counterparts, one reason being the entropic advantage conferred on such processes by more conformationally rigid frameworks. 

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. 

Sensitization by acetophenone and benzophenone could be demonstrated for the cyclization of 74, R=4—C(CH3)3 to 20, R = 6—C(CH3)s. With the latter sensitizer, enhanced yields of N-hydroxy-2-indoUnone formation have been observed relative to direct irradiation runs. Triplet benzophenone does not act as dehydrogenating agent in the step 19 20, since the pinacolization of benzo-... 

The first compound selected for this study was the 2-aza-1, 4-diene 68 (Scheme 11). Triplet-sensitized irradiation of azadiene 68, using acetophenone, for 25 min affords two new products, identified as the cyclopropylimine 69 (11%)... 

For related reasons, and because their excited-state energies are lower than for dialkyl ketones, diaryl ketones and simple alkyl aryl ketones do not fragment on irradiation in solution, even at higher temperatures. This leads to a photostability that is one factor contributing to the successful employment of ketones such as benzophe-none tPh-CO) or acetophenone (PhCOMe) as triplet sensitizers. a-Cleavage for ketones in solution at room temperature is promoted if structural factors cause the bond adjacent to the carbonyl group to be somewhat weaker than normal. Hence t-alkyl ketones give decar-borylation products readily (4.5), as do benzyl ketones (4.6 and benzoin derivatives (4.7). 

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