Experimental and DFT Studies on Cp*Co(III)-Catalyzed Selective C8-Olefination and Oxyarylation of Quinoline N-Oxides with Terminal Alkynes

Abstract

Herein we report Cp*Co(III)-catalysed site-selective (C8)-H olefination and oxyarylation of quinoline N-oxides with terminal alkynes. The selectivity for C8-olefination and oxyarylation is sterically and electronically controlled. In case of quinoline N-oxides (unsubstituted at C2-position), only olefination product is obtained irrespective of the nature of alkynes. In contrast, majorly oxyarylation is observed when 2-substituted quinoline N-oxides are reacted with bulkier alkynes such as 9-ethynyl phenanthrene. However, alkynes with electron-withdrawing groups provided only olefination products with 2-substituted quinoline N-oxides also. The developed strategy allowed a facile functionalization of naturally derived quinoline N-oxides and terminal alkynes to deliver corresponding olefinated and oxyarylated products. In the developed protocol, the 'N-O’ bond played a dual role i.e., as a traceless directing group and an oxygen atom source (in case of oxyarylation), which is confirmed by 18O-labeling and crossover experiments. In addition, control experiments, deuterium labeling experiments, KIE studies and DFT studies are performed to understand the mechanism and origin of selectivity for different substrates. DFT studies revealed that the alkyne addition into Co-C bond is the rate limiting step. The observed product selectivity is reproduced by DFT methods. Furthermore, the energy decomposition analysis is performed to understand the origin of selec-tivity.