Electron transfer in the type II photoelimination of .alpha.-aminoacetophenones

Abstract

The type II photoelimination reaction of a series of -aminoacetophenones has been studied. Quantum yields and rate constants for product formation were determined in several solvents. These ketones were found to undergo photoelimination with extraordinarily high rates but with modest quantum efficiency. Photoelimination of «-amino ketones with low-lying -* triplet states also occurs with enhanced reaction rates. Attachment of an electron-withdrawing benzoyl group to the nitrogen atom decreases the rate of photoelimination. The results can best be rationalized by an electron-transfer route for which the rate constant may exceed diffusion control.tudies of the photochemistry of ketones that possess a hydrogen-bearing y carbon have shown that two major reaction pathways are available.2-21 The (1) Alfred P. Sloan Foundation Research Fellow, 1968-1970 (2) This work was presented in part at the 161st National Meeting of the American Chemical Society, Los Angeles, Calif., March 1971. the formation of cyclobutanols.23 Both reactions appear to be intramolecular with little or no detectable side reactions and have been visualized as arising from a common biradical intermediate. In the case of aryl alkyl ketones, both reactions proceed by way of a triplet which leads to a 1,4-biradical intermediate which may either cyclize, undergo cleavage, or revert to starting ketone by reabstraction of hydrogen. The lines of evidence implicating the reversibility of the hydrogen transfer step are based on kinetic data4 24 and are reinforced by stereochemical25•26 and deuterium isotope ef-

4-Hydroxyphenacyl Ammonium Salts: A Photoremovable Protecting Group for Amines in Aqueous Solutions

“…Alternatively, amino acid derivatives 1d and 1e could undergo Norrish type II photochemical reactions. − This reaction could be initiated by an intramolecular electron transfer from the nitrogen atom (free base) to the excited carbonyl group followed by a proton shift from the γ-position to give a 1,4-biradical intermediate that fragments into a ketone and imine or cyclizes to a four-membered azetidine derivative. The contribution of such a process would also be limited by pH where the form D would be available.…”

Photochemical transformations of small ring carbonyl compounds. XXIII. Photoinduced ring expansion of arylaroylazetidines. Mechanistic studies and characterization of the excited state

“…All runs at 250; irradiation conditions described in the Experimental Section. 5 Azetidine concentration was 1.5 X 10-2 M in 95 % ethanol. c Absorbing over 98 % of incident light.…”

Aza-Yang Cyclization—Buchner Aromatic Ring Expansion: Collective Synthesis of Cycloheptatriene-containing Azetidine Lactones

“…More facile than γ Hradical abstraction is quenching of the triplet excited state through a rapid electron transfer from nitrogen to form a ketyl radical, which opens up a more favorable degradation pathway through the scission of the C -N bond, yielding methylketones and imine oligomers, or amines through complex reductive processes (Figure 2B). 17,18 One way to avoid this has been to protect the amine so that its lone pair engages in conjugation with other groups (e.g. by converting it to amides, carbamates, sulfonamides; Figure 2C), [19][20][21] but this necessitates additional steps (introduction and removal of the electron-withdrawing group) and precludes the use of tertiary amines.…”