Synthesis of Pentasubstituted 2-Aryl Pyrroles from Boryl and Stannyl Alkynes via One-Pot Sequential Ti-Catalyzed [2+2+1] Pyrrole Synthesis/Cross Coupling Reactions

Multisubstituted pyrroles are commonly found in many bioactive small molecule scaffolds, yet the synthesis of highly- substituted pyrrole cores remains challenging. Herein, we report an efficient catalytic synthesis of 2-heteroatom-substituted (9-BBN or SnR3) pyrroles via Ti-catalyzed [2+2+1] heterocoupling of heteroatom-substituted alkynes. In particular, the 9- BBN-alkyne coupling reactions were found to be very sensitive to Lewis basic ligands in the reaction: exchange of pyridine ligands from Ti to B inhibited catalysis, as evidenced by in situ 11B NMR studies. The resulting 2-boryl substituted pyrroles can then be used in Suzuki reactions in a 1-pot sequential fashion, resulting in pentasubstituted 2-aryl pyrroles that are inaccessible via previous [2+2+1] heterocoupling strategies. This reaction provides a complementary approach to previous [2+2+1] heterocouplings of TMS-substituted alkynes, which could be further functionalized via electrophilic aromatic substitution.