Carbon nucleophilicities of indoles in S_NAr substitutions of superelectrophilic 7-chloro-4,6-dinitrobenzofuroxan and -benzofurazan

Superelectrophilic 7-chloro-4,6-dinitrobenzofuroxan (DNBF-C1) and 7-chloro-4,6-dinitrobenzofurazan (DNBZ-C1) are shown to undergo facile carbon-carbon couplings with a series of weak carbon nucleophiles consisting of a number of differently substituted indoles, 1,2,5-trimethylpyrrole and azulene, in acetonitrile. Despite the fact that steric effects preclude a coplanarity of the donor and acceptor moieties, the resulting substitution products are subject to an intense intramolecular charge transfer. A kinetic study of the various substitutions has been carried out. The absence of a significant dependence of the rates of coupling on the hydrogen or deuterium labeling at the reactive center of the nucleophiles indicates that the reactions take place through an S_EAr-S_NAr mechanism with the initial nucleophilic addition step being rate-limiting. A vicarious-type substitution is shown to be unreasonable. Referring to Mayr nucleophilicity parameters (N), which have become recently available for a large set of indoles, the electrophilicity of DNBF-C1 and DNBZ-C1, could be ranked on the general electrophilicity scale E developed by this author (Acc. Chem. Res. 2003, 36, 66). With essentially similar E values of -6.1, these two compounds have an electrophilicity which approaches that of cationic stuctures such as 4-nitrobenzenediazonium cation or tropylium cations. Most important in the context of S_NAt substitutions, DNBF-C1 and DNBZ-C1 are 7 orders of magnitude more electrophilic than picryl chloride, the conventional reference electrophile in this field. It is this so far unique behavior which allows the facile coupling of DNBF-C1 and DNBZ-C1 with such weak carbon nucleophiles as indoles. Based on a nice Brönsted-type correlation for 5-X-substituted indoles, the unknown pK^CH _a values measuring the Brönsted C-basicity of several N-benzylindoles could be readily estimated. The influence of some steric effects in 2-methylindole systems is pointed out.