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.