The kinetics of the coupling of indole (1a), N-methylindole (1b), 5-methoxyindole (1c), and 5-cyanoindole (1d) with a set of reference benzhydryl cations have been investigated in acetonitrile and/or dichloromethane. The second-order rate constants for the reactions correlate linearly with the electrophilicity parameter E of the benzhydryl cations. This allows the determination of the reactivity parameters, N and s, characterizing the nucleophilicity of 1a-d according to the linear free enthalpy relationship log k(20 degrees C) = s(N + E) (Acc. Chem. Res. 2003, 36, 66). The nucleophilicity parameters thus defined describe nicely the reactions of 1a-d with 4,6-dinitrobenzofuroxan (2), a neutral superelectrophilic heteroaromatic whose electrophilicity (E) has been recently determined. On this ground, the kinetics of the coupling of 2 with a large variety of indole structures have been studied in acetonitrile, leading to a ranking of this family of pi-excessive carbon nucleophiles over a large domain of the nucleophilicity scale N. Importantly, two linear and parallel correlations are obtained on plotting the measured N values versus the pK(a)(H(2)O) values for protonation at C-3 of 5-X-substituted indoles and 5-X-substituted 2-methylindoles, respectively. This splitting reveals that the presence of the 2-methyl group causes steric hindrance to the approach of 2 from the adjacent C-3 position of an indole structure. The N vs pK(a)(H(2)O) correlation for 5-X-substituted indoles is used for a rapid determination of the C-3 basicity of indoles whose acidity constants cannot be measured through equilibrium studies in strongly acidic aqueous media.

Keywords: Nitrogen heterocycles / Carbocations / Kinetics / Electrophilic substitution / Linear free energy relationshipsThe second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the σ adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k 2 (20°C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reac-

[reaction: see text] Indoles are allylated and benzylated in moderate to quantitative yield when stirred with allyl and benzyl halides in 80% aqueous acetone in the presence of NH(4)HCO(3) at room temperature.

Aliphatic and aromatic epoxides react regio- and stereoselectively with indoles and pyrroles in 2,2,2-trifluoroethanol without the use of a catalyst or any other additive. While aromatic epoxides are selectively attacked at the benzylic position, aliphatic epoxides react at the less-substituted position. Chiral epoxides react with >99 % ee (ee=enantiomeric excess).

The cover picture is misleading! With the determination of the nucleophilic reactivities of pyrroles, our electrophilicity‐nucleophilicity puzzle, where synthetically useful reactions are found in the green corridor (see: R. Lucius, R. Loos, H. Mayr, Angew. Chem. Int. Ed. 2002, 41, 91–95), is not yet complete. The picture illustrates, however, that the reactions of pyrroles with electrophiles also follow the linear free‐energy relationship log k20°C = s(N + E), where N and s characterize the reactivities of nucleophiles and E describes the reactivities of electrophiles. Alkyl substitution increases the nucleophilicity of pyrrole by a factor of up to 10 million, and from the nucleophilicity parameters reported in this article by H. Mayr et al. on p. 2369 ff, one can derive suitable electrophilic reaction partners for alkyl‐substituted pyrroles.