Multifold C–C Coupling and Unorthodox Cyclization Catalysis for Selective Synthesis of Indolotriarylmethanes, Indolocarbazoles, and Their Analogues: A Control Experiment Study

The selective construction of medicinally
and synthetically important
indole-based unsymmetrical triarylmethanes using indoles and aldehydes
is challenging because the significant nucleophilicity of indole leads
to C–C coupling with an azafulvene intermediate to build up
the alternative bis­(indolyl)­methane products, which may be useful
synthons. A new, straightforward, ligand-free CuII catalytic
strategy for easy syntheses of unsymmetrical indolotriarylmethanes
and new bisindolylbenzoyl analogues is established through the dual
C–C coupling of an assembly of three reaction partners comprising
aldehydes, indoles, and arylboronic acids. More importantly, this
approach is exploited for multifold C–C coupling cyclization
reactions with C–C cleavage using symmetrical bisindolylbenzoylmethanes
in the presence of an organic base and aerial molecular oxygen as
a stoichiometric oxidant. In contrast to the formation of a simple
cyclocondensation product indolocarbazole, it undergoes unprecedented
selective pseudo-four-component tandem oxidative cyclization with
fragmentation from a 1,3-dicarbonyl compound to produce valuable fused
5,7-dihydroindolo­[2,3-b]­carbazoles through the functionalization
of two indole C­(sp2)–H and one C­(sp3)–H
bond of the active methylene residue. For a better understanding of
the new reactions, we have studied various competition experiments
and ESI-MS and 3D Mid-IR-ATR spectral analyses of the ongoing reactions.
The predicted DFT transition state model is also in agreement with
the experimental results.