Photoreduction and .alpha. cleavage of aryl alkyl ketones

Abstract

The photochemical a cleavage and reduction reactions of several aryl alkyl ketones have been investigated. Electron-releasing aromatic substituents decrease the excited state reactivity of ieri-alkyl aryl ketones toward both primary photoprocesses to a similar extent. The aromatic carbonyl 3( , *) state is concluded to be responsible for a cleavage as well as hydrogen abstraction reactions. The rate constant for intermolecular hydrogen abstraction by the carbonyl triplet state decreases with -methyl substitution. Pinacol and carbinol products are formed via the combination and disproportionation of ketyl radicals. A primary deuterium isotope effect on isobutyrophenone ketyl radical disproportionation of 2.7 is observed. -Methyl substituents increase the percentage of carbinol products due to steric hindrance of free-radical combination.Synthetic8'4 and mechanistic5-7 aspects of aryl alkyl ketone photoreduction have been extensively investigated. A number of «-substituted acetophenones form pinacols upon irradiation in 2-propanol or other suitable hydrogen-donor solvents;3•4 however, quantitative data on such ketones was unavailable prior to our investigation.7 The elegant studies of Yang and coworkers5 on aryl-substituted acetophenones estab-

A Photodissociation Reaction: Experimental and Computational Study of 2-Hydroxy-2,2-dimethylacetophenone†

“…Particularly, the increase of the rate constant with increasing solvent polarity is well reproduced by the calculations. This is clearly ascribed to the decrease of the energy barrier, possibly through the stabilization of polar transition state as suggested on the example of aromatic ketone derivatives (1–3).…”

“…Creation of radicals through a cleavage process occurring from the excited states of molecules is a fascinating problem. Many molecules exhibiting such a property have been known for a long time and this process, Norrish I, is still of interest as shown by earlier works (1–2). Several papers devoted to the α‐cleavage reaction in alkylphenylketones (1–3) gave some evidence for the involvement of polar effects.…”

Studies on the Occurence of Hydrogen Transfer, 58. ‐ Possibilities and Limits of Photochemically Induced Asymmetric Synthesis

“…Irradiation of acetophenone in the presence of DL-ethyl lactate (16) affords a 90% yield of acetophenone pinacol, 70% yield of diethyl2,3-dimethyltartarate (18) and the mixed pinacol 19 from acetophenone and ethyl pyruvate. -Irradiation of methyl o-acetylbenzoate (20) and the corresponding menthyl ester 3 on irradiation in isopropyl alcohol result in the formation of the R,R-dilactone 22 and not in the expected pinacol 21. The p-menth-3-yl ester 3 gives a rnesohL-mixture of the lactone in a ratio of ca.…”

“…Verhaltnis meso/DL = 48: 52; [a]578 = + 10.3" (c = 4, CHCI,). Auf den Anteil an D,L-Dilacton umgerechnet: = + 19.8"; optische Reinheit 20.4%.Versuch I ) der Tabelle 3: 10 g (56 mmol) o-Acetylbenzoesaue-methylester(20), gelost in 500 ml Benzol/Isopropanol (1 : 3), werden 132 h belichtet.…”

Kinetic parameters for step and flash imprint lithography photopolymerization

“…Although Φ is highly dependent on reaction environment, a literature survey of Φ values in similar free‐radical systems provides a valuable point of reference. The quantum yield of Darocur 1173 in benzene was reported to be 0.163 for 313 nm light 37. Fouassier reports the quantum yield of Darocur 1173 to be 0.2 under practical conditions of bulk polymerization, based on time‐resolved laser‐spectroscopy 38.…”