How rhodium (III) complexes catalyze alkenylation of C(sp3)–H bond of 8-methyquinolines

• The C(sp3)–H activation preferentially undergoes 4-membered ring process.
• The relaxed Rh complexes structure of 16e system is favorable for the alkyne insertion.
• Rh complexes prefer to keep 18e system rather than to 20e system in the hydrogen transfer process.

DFT studies on rhodium(Ш)-catalyzed alkenylation of 8-methyquinolines show that the activation of C(sp3)–H bond of 8-methylquinoline prefers to undergo four-membered ring process with free energy barrier of 29.8 kcal/mol, rather than six-membered ring process because of the electron-delocalized effect of oxygen atom to Rh center. The diphenylacetylene coordinates into the Rh center of the catalyst, in which Rh center tends to maintain 16e structure rather than 18e structure, and then inserts into the activated Rh–C bond with a free energy barrier of 21.2 kcal/mol. The binding of HOAc process is a premise for the catalyst regeneration, in which the Rh center directly accommodate HOAc instead of experiencing seven-membered rhodacycle proposed by the experiment. The catalyst was eliminated via a direct hydrogen transfer with an energy barrier of 12.8 kcal/mol, rather than Rh-mediated hydrogen transfer, which is attributed to the formation of unstable 20e species in the Rh-mediated process. The substituted effect follows the H < F < NMe2 trends, being consistent with its experimental results.

The alkenylation of 8-methylquinolines subsequently involves the binding of rhodium (III) complexes to 8-methylquinolines, the C(sp3)–H activation, alkyne insertion and catalyst regeneration. The H migration process in C(sp3)–H activation is the rate-determining step in the whole catalytic cycles.Figure optionsDownload as PowerPoint slide