Mild and Efficient Synthesis of Functionalized Carbazoles via a DBU-Assisted Sequence Involving Cu- and Pd-Catalyzed Coupling Reactions

Practical access to diversely functionalized carbazoles has been developed by consecutive Cu-catalyzed Chan-Lam N-arylation of various o-iodoanilines and boronic acids, and Pd-catalyzed intramolecular aryl C-H activation of 2-iodo-N-arylanilines. Use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base was found beneficial for both steps. In the Pd-catalyzed C-H activation step, DBU acts as ligand as well as base, resulting in improved functional tolerance and higher yields than those observed with inorganic or other nitrogen bases. This DBU-assisted sequence offers access to a variety of carbazoles with various electron-donating and electron-withdrawing substituents, including halogens or other reactive functional groups. Twenty-seven cabazoles with various substitution paterns, including two naturally-occurring carbazoles - clausine L and clausine H - have been successfully synthesized using these DBU-promoted metal-catalyzed coupling reactions.