Acetophenone, sensitizer photochemistry

Attempts to sensitize the rearrangement of [79a] to [80a] with xanthone (ET = 74 kcal/mole) under conditions where the sensitizer absorbed essentially all of the incident radiation resulted in no observable chemical change (35). To determine if the cyclobutanone was receiving triplet energy from the sensitizer, the direct and sensitized photochemistry of syn- and anti-2-sec-butylidenecyclobutanones [81] and [82] were investigated (35). Under direct irradiation (313 nm), the isomeric acetals [83] and [84] were produced quantitatively ( = 0.1-0.2) yet under sensitized conditions (e.g., with xanthone, acetophenone, benzophenone, or triphenylene),... 

Scheme 53. Molecular models do not make this mechanism seem very likely and further work on the structures of caulolcatone A and B may be required. A further study of the photochemistry of germacrone (340) has shown that direct irradiation causes isomerization only of the A °-double bond whereas acetophenone-sensitized irradiation induces isomerization of both the and...

Acetophenone sensitization and cyclohexadiene quenching were held to demonstrate (not very conclusively) that the unsensitized process involves triplet intermediates, as in Scheme 7. A similarity to cyclohexenone photochemistry... 

Acetophenone-sensitized irradiation of the quinuclidinones (63) brings about an oxa-di-TT-methane rearrangement to afford (64) in 70% yield. Singh and Lahiri have reported the odd photochemistry of the enone (65, R = H, = Et). On irradiation of this compound at 300 nm in acetone, only a low yield of... 

The photochemistry of benzoin ethers (a-alkoxy-a-phenyl-acetophenones) has been examined in considerable detail, recently, by quenching, sensitization (, CIDNP (, and radical scavenging studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This abscission is not retarded by conventional triplet... 

Later,triplet alkylititrenes 12a-c were generated by intramolecular sensitization in solution. The photochemistry of three a-benzoyl-ft)-azidoalkanes (PhCO(CH2)oN3 (13a-c, n = 3-5) was studied and two competitive processes were found to proceed from the triplet ketone (Scheme 11.8) energy transfer to azide to yield triplet alkyl nitrene and 7-hydrogen abstraction to yield Norrish type II products. Photolysis of azides with a longer methylene chain (13b,c) yields mainly acetophenone, a product of Norrish type II reaction. The major products of the azide 13a photolysis were substituted pyrrole and pyr-roline derived from triplet nitrene 12a. ... 

Bicyclo[2.2.2]octenones with a-methoxy groups undergo a remarkable photoreaction. Rogers and Parker investigated the photochemistry of 114 and its isomer 117, and other related ketones, upon sensitized as well as direct irradiation. Sensitized (acetone or acetophenone in benzene) irradiation of the enone 114, with a methoxy group at the a-position, was found to give the dione 116 in major quantity the expected ODPM product was obtained only in minor amounts (Scheme 22). It was... 

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