TRANSITION-METAL COMPLEXES OF DIAZENES .35. SYNTHESIS OF 1-(ARYLAMINO)INDOLES BY RODIUM-CATALYZED ADDITION OF ALKYNES TO 1,2-DIARYLDIAZENES

Addition of 1,2-diaryldiazenes to aliphatic or aromatic internal alkynes in the presence of catalytic amounts of RhCl(PPh(3))(3) (Wilkinson's catalyst) in refluxing propanol affords N-(arylamino)indole derivatives. Electron-withdrawing substituents in the alkyne induce a decrease of the turnover rate, while electronic properties of the diazene have only a minor influence. When unsymmetrically substituted alkynes are employed, a mixture of regioisomers is obtained except in the case of very large substituents like tert-butyl and adamantyl which bring about a regioselective reaction. Steric alterations in the diazene induce significant effects. While the nonsubstituted phenyl ring is attacked in the case of 3,5-dichloroazobenzene, no reaction occurs with 2,4,6-trimethylazobenzene. Kinetic measurements reveal a first-order dependence on RhCl(PPh(3))(3) and azobenzene, while a broken order of -0.3 is found for tolan. The activation parameters are E(a) = 75 +/- 6 kJ/mol and Delta S-double dagger = -96 +/- 14 J mol(-1) K-1. A mechanism is proposed wherein the key steps are orthometalation of the diazene followed by insertion of the alkyne into a Rh-H bond and reductive G(sp(2))-C(sp(2)) elimination to afford a 2-vinylazobenzene intermediate. This is converted to the N-(arylamino)indole derivative in an acid-catalyzed reaction step.