Ruthenium-Catalyzed Ortho-Selective C–H Alkenylation of Aromatic Compounds with Alkenyl Esters and Ethers

A direct, regioselective alkenylation of aromatic C–H bonds of aryl- and heteroarylpyridines and related compounds with alkenyl esters was developed using Ru­(cod)­(cot) as a catalyst. Aromatic compounds bearing electronically diverse substituents and various heterocyclic directing groups are reacted with alkenyl acetates bearing mono- and disubstituted alkenyl groups with aliphatic and aromatic substituents to give ortho-alkenylation products in high yields. The results of deuterium-labeling experiments and competition reactions using different ratios of the E and Z isomers of β-styryl acetate suggested that the C–H alkenylation proceeded via a ruthenium–alkene intermediate and the C–O bond of the alkenyl acetate was cleaved by β-acetoxy elimination. Two types of catalytically relevant species were identified, and the reactivities of these species, combined with the results of the kinetic studies, suggest that the rate-limiting step is the exchange of the COD ligand with the alkenyl ester. On the basis of the elucidated mechanism, the first catalytic coupling reaction using aromatic C–H bonds with C–O bonds of ethers was also developed.