Synthesis of Tris(indol-3-yl)methanes From Indoles, CO2, and Dimethylphenylsilane With BPh3: Two-Step Construction of π-Conjugated Systems.

Tris(indol-3-yl)methanes were synthesized from indoles, CO2, and PhMe2SiH in the presence of BPh3 in acetonitrile at 50°C. This is the first CO2 fixation reaction forming the aliphatic methine (>CH─) group. The isotope-labeling experiments clearly demonstrated that the carbon and hydrogen atoms of the methine group originated from carbon dioxide and dimethylphenylsilane, respectively. The mechanism for the multicomponent cascade reaction was elucidated by DFT calculations. One of the CO2-derived products, tris(4-bromo-1-methylindol-3-yl)methane, underwent palladium-catalyzed intramolecular cascade reactions for the selective formation of two novel polycyclic heteroaromatic compounds, where the methine C(sp3) atom originating from CO2 was converted into aromatic C(sp2) atoms. One is formally produced via double direct C─H arylation, Wacker-type oxidation, dehydrogenation, and debromination, while the other seems to be produced via the intramolecular homocoupling of aryl bromide, direct C─H arylation, Wacker-type oxidation, and dehydrogenation. Both of them are new intramolecular cascade reactions that are useful for the short-step construction of nitrogen-doped nanographenes. On the other hand, one of the CO2-derived products, tris(1-pentylindol-3-yl)methane, was converted into tris(1-pentylindol-3-yl)methylium methanesulfonate, which is a potential antitumor drug. This is the first derivatization of CO2 to a cationic C(sp2) center.