Mechanism of Photoinduced Reactions between 1‐Acetylisatin and Aldehydes

Abstract

AbstractHydrogen atom abstraction from aliphatic (2a, 2b) and aromatic (2c, 2d, 2e) aldehydes by the triplet nπ* state of 1‐acetylisatin (1) yielded triplet (isatin ketyl‐aldehyde acyl) radical pairs, which follow an out‐of‐cage pathway in subsequent reactions to give a series of radical coupling products 4−7. This out‐of‐cage mechanism is supported by examination of the primary products at various degrees of conversions during the course of the reaction, by the results of addition reactions of thermally generated isatin acyl radical with neutral 1, and by calculation of the charge density distribution in the acyl radicals and in ground state 1. Therefore, the oxygen‐philic attack of the acyl radical, which had escaped out of cage from the triplet radical pair, at the C(3) carbonyl oxygen atom of a ground state 1 selectively afforded the thermodynamically more stable addition radical B. The recombination of two radicals B gave the 3‐indolinone dimers 4 and 5 as diastereomers, while coupling of B with the isatin ketyl furnished the dihydroisoindigo derivative 6. Isatide 7 was also formed by ketyl radical recombination. The product ratio is dependent on the structure of the aldehyde. For 2d and 2e, each with an electron‐donating substituent in the benzene ring, the corresponding acyl radicals are less electrophilic because of a diminished positive charge density at the carbonyl carbon atom, and the formation of the addition radical B is a slow process relative to the encounter and recombination of the isatin ketyl radicals. As a result, products 6d and 6e from cross‐coupling of the isatin ketyl and radical B are formed in much lower yields than 6a, 6b, and 6c. The 3‐acetyloxy‐2‐indolinone 3 turned out to be a secondary product derived from the decomposition of 6 during the workup procedures. Since all the primary products are formed by out‐of‐cage radical pair combination processes, no chain mechanism is involved. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)