Site-Selective C–H Functionalization on Coumarins Directed by Manganese: Mechanistic Insights from Time-Resolved Spectroscopy and Catalytic Development

Mn-catalyzed C–H bond functionalization offers great potential in the synthesis of highly functionalized aromatic and heteroaromatic compounds, involving the intermediacy of manganacycles and their interception through reactions with appropriate acceptor compounds. The coumarin motif is widely embedded in an eclectic array of compounds possessing interesting biological and photophysical properties. In this study, we examined the effect of changing the position of a manganese 2-pyridyl-directing group from C3 to C4 on a coumarin moiety, directing C4–H functionalization or C3–H functionalization, respectively. The reactions of cyclomanganated intermediates with phenylacetylene have been studied using time-resolved infrared spectroscopic methods on a pico- to microsecond time scale, unraveling the bond-forming processes that underpin the differing site C–H bond functionalizations. The findings facilitated the development of a catalytic methodology for the C3-alkenylation of a 4-(2-pyridyl)coumarin using phenylacetylene.