Ruthenium-Mediated C–H Functionalization of Pyridine: The Role of Vinylidene and Pyridylidene Ligands

A combined experimental and theoretical study has demonstrated
that [Ru­(η5-C5H5)­(py)2(PPh3)]+ is a key intermediate, and active
catalyst for, the formation of 2-substituted E-styrylpyridines
from pyridine and terminal alkynes HCCR (R = Ph, C6H4-4-CF3) in a 100% atom efficient manner under
mild conditions. A catalyst deactivation pathway involving formation
of the pyridylidene-containing complex [Ru­(η5-C5H5)­(κ3-C3-C5H4NCHCHR)­(PPh3)]+ and subsequently a 1-ruthanaindolizine complex has
been identified. Mechanistic studies using 13C- and D-labeling
and DFT calculations suggest that a vinylidene-containing intermediate
[Ru­(η5-C5H5)­(py)­(CCHR)­(PPh3)]+ is formed, which can then proceed to the pyridylidene-containing
deactivation product or the desired product depending on the reaction
conditions. Nucleophilic attack by free pyridine at the α-carbon
in this complex subsequently leads to formation of a C–H agostic
complex that is the branching point for the productive and unproductive
pathways. The formation of the desired products relies on C–H
bond cleavage from this agostic complex in the presence of free pyridine
to give the pyridyl complex [Ru­(η5-C5H5)­(C5H4N)­(CCHR)­(PPh3)]. Migration of the pyridyl ligand (or its pyridylidene tautomer)
to the α-carbon of the vinylidene, followed by protonation,
results in the formation of the 2-styrylpyridine. These studies demonstrate
that pyridylidene ligands play an important role in both the productive
and nonproductive pathways in this catalyst system.