Synthesis of 2,2'-biindolyls by coupling reactions

Coupling reactions of indoles forming 2,2'-biindolyls have been investigated. 2-Iodo-N-methyl-indole and unactivated copper gave N,N'-dimethyl-2,2'-biindolyl whereas activated copper yielded N-methyl-indole and the symmetrical trisindolobenzene 5. A mechanism involving the elimination of copper hydride to form a hetaryne intermediate is suggested.Coupling of N-benzenesulfonyl-2-lithioindole with CuCl₂ yielded 2,2'-biindolyl after hydrolysis. N-methyl-2-lithioindole likewise gave N,N'-dimethyl-2,2'-biindolyl.