Merging C–H Bond Activation, Alkyne Insertion, and Rearrangements by Rh(III)-Catalysis: Oxindole Synthesis from Nitroarenes and Alkynes

We report a Rh(III)-catalyzed ortho-C–H
bond functionalization of nitroarenes with 1,2-diarylalkynes and carboxylic
anhydrides. The reaction unpredictably affords 3,3-disubstituted oxindoles
with the formal reduction of the nitro group under redox-neutral conditions.
Besides good functional group tolerance, this transformation allows
the preparation of oxindoles with a quaternary carbon stereocenter
using nonsymmetrical 1,2-diarylalkynes. This protocol is facilitated
by the use of a functionalized cyclopentadienyl (CpTMP*)Rh(III)
[CpTMP* = 1-(3,4,5-trimethoxyphenyl)-2,3,4,5-tetramethylcyclopentadienyl]
catalyst we developed, which combines an electron-rich character with
an elliptical shape. Mechanistic investigations, including the isolation
of three rhodacyle intermediates and extensive density functional
theory calculations, indicate that the reaction proceeds through nitrosoarene
intermediates via a cascade of C–H bond activationO-atom
transfer[1,2]-aryl shiftdeoxygenationN-acylation.