Selective C-H Bond Functionalization of 2-(2-Thienyl)-pyridine by a Rhodium N-Heterocyclic Carbene Catalyst

Rh(mu-Cl)(H)(2)(IPr)](2) (IPr= 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) catalyzes the selective functionalization of 2-(2-thienyl)pyridine efficiently with a range of alkenes and internal alkynes. A catalytic cycle is proposed on the basis of the identification of key reaction intermediates and the study of their reactivity by NMR spectroscopy. Theoretical calculations at the DFT level support that the reaction proceeds by initial kappa N-1 coordination of 2-(2-thienyl) pyridine followed by the loss of H-2 to afford the active catalyst. Subsequently, cyclometalation of 2-(2-thienyl) pyridine, coordination of the unsaturated substrate (alkyne or alkene) at the vacant position trans to the hydride, and reductive elimination of the thiophene moiety occur. Finally, cyclometalation of the thiophene moiety renders the hydride cis to the unsaturated substrate, which leads to migratory insertion into the Rh-H bond and subsequent reductive elimination of the functionalized product.