On the polarizability of azulene ring, very stable 1-azulenyl-substituted methyl cations have been reported [1]. We also found that the 3-indolyl group stabilizes the carbocations [2]. Herein, we present the synthesis and properties of a series of indole analogues of triphenylmethyl cation. (1-Alkyl-3-indolyl)diphenylmethyl cations 1a,b, bis(1-alkyl-3-indolyl)phenylmethyl cations, 2a,b, and tris(1-alkyl-3-indolyl)methyl cations 3a,b were prepared by DDQ oxidation of the corresponding methane derivatives. The pK_R+ values were determined spectrophotometrically and shown in Fig. 1. From these values, it was found that the 3-indolyl group highly stabilize cations by an extended conjugation effect among the central cation and the indolyl rings in comparison with triphenylmethyl cation [3].

The Hammett analysis of pK_R+ values of (p-substituted aryl)bis(1-methyl-3-indolyl)methyl cations is also discussed.

[1] a) S. Ito, N. Morita, T. Asao, Bull. Chem. Soc. Jpn. 1995, 68, 1409. b) S. Ito, S. Kikuchi, N. Morita, T. Asao, J. Org. Chem.1999, 64, 5818.

[2] S. Kikuchi, M. Iki, C. Ikeda, K. Imafuku, Heterocycles 2005, 66, 275.

[3] E. M. Arnet, R. D. Bushick, J. Am. Chem. Soc. 1964, 86, 1564.

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