Abstract
A method for synthesis without protecting groups of bis(indolyl)methanes by the (η3-benzyl)palladium system generated from a palladium catalyst and benzyl alcohol in water is developed. This domino protocol involves C3–H bond activation/benzylation of indole–carboxylic acids and benzylic C–H functionalization. Mechanistic studies indicate that the (η3-benzyl)palladium(II) complex, which is formed via oxidative addition of benzyl alcohol 2 to a Pd(0) species, activates the C–H bond at the C3-position of indole 1. Notably, water plays an important role in our catalytic system for sp3 C–O bond activation and stabilization of OH− by hydration for the smooth generation of the activated Pd(II) cation species, as well as for nucleophilic attack of indoles to hydrated benzyl alcohols.
Highlights
The (η3-benzyl)palladium catalysts are gaining increasing interest
We have developed a unique strategy for benzylation and C–H activation [18,19,20,21,22,23,24] by the (η -benzyl)palladium system from a palladium catalyst and benzyl alcohol in water [25,26,27,28]
In our previous paper [25], we reported a method for the synthesis of bis(indolyl)methanes via palladium-catalyzed domino reactions of indoles with benzyl alcohols in water and suggested a plausible mechanism for the formation of bis(indolyl)methanes
Summary
The (η3-benzyl)palladium catalysts are gaining increasing interest. The powerfulness of these complexes is still far from other well-established Tsuji–Trost reactions, merely due to a more recent development [1,2,3,4,5,6,7,8,9]. We became interested in further expanding the substrate scope of the (η3-benzyl)palladium system to water-soluble unprotected indole carboxylic acids 1, since we have been studying the development of synthesis without protecting groups and selective reactions towards various reactive functional groups [29,30,31,32]. In our previous paper [25], we reported a method for the synthesis of bis(indolyl)methanes via palladium-catalyzed domino reactions of indoles with benzyl alcohols in water and suggested a plausible mechanism for the formation of bis(indolyl)methanes. We explore the synthesis without protecting groups of bis(indolyl)methanes 3 from indole-carboxylic acids 1 and propose a more detailed mechanism based on various control experiments. We report the development of synthesis without protecting groups of bis(indolyl)methanes 3 via palladium-catalyzed domino reactions of indole–carboxylic acids 1 with benzyl alcohols 2 in water. This paper describes mechanistic investigations aimed at providing a rational explanation for the formation of bis(indolyl)methanes 3
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