Isomerization-Induced Multiple Reaction Pathways in Platinum-Catalyzed C–H Acylation Reaction of 2‑Aryloxypyridines

In Pt-catalyzed C–H acylation
of 2-(2-methylphenoxy)­pyridine
(L1) with ethyl chlorooxoacetate
to produce ethyl 2-(3-methyl-2-(pyridin-2-yloxy))­phenyl-2-oxoacetate
(L2), possible reaction steps
involved in the catalytic cycle have been investigated, which include
the ligand exchange of precatalysts cis- and trans-Pt­(PhCN)2Cl2 with L1, the intramolecular C–H activation
(cyclometalation) of cis- and trans-Pt­(L1)2Cl2 (1), and the acylation of the trans-cyclometalated platinum complex (trans-2) formed from the C–H activation reaction. The cis coordination compounds including the intermediate platinum complexes
have shown much higher reactivity toward the ligand exchange and the
C–H activation than their trans isomers. Consequently,
the cis isomers exhibit higher catalytic activity
in the C–H acylation reaction. Although the C–H activation
of cis- and trans-Pt­(L1)2Cl2 produces trans-2, the cis isomer (cis-2) is very likely formed in the C–H
activation of cis-Pt­(L1)2Cl2 but undergoes isomerization to
the more stable trans-2. The results
suggest that the fast reaction pathway via all cis-platinum complexes may be responsible for the efficient catalytic
reaction, but isomerization of cis-2 to trans-2 could be highly competitive.
The acylation of the trans-cycloplatinated complex
produces both cis- and trans-platinum
complexes Pt­(L1)­(L2)­Cl2. The regaining of the cis-platinum complex revives the catalytic cycle.