4 Conclusion
We investigated the detailed mechanism and chemoselectivity in the
ligand-promoted PdII-catalyzed C–H alkeylation of
internal alkynes with simple arenes without directing group using DFT
calculation. The calculated results revealed that the substituents (R)
in alkyne 2a effect on the mechanism and the chemoselectivity:
when R is n-propyl (alkyl) group, the major product is alkenyl chloride
and the rate-determining step of the catalytic reaction is the concerted
metalation-deprotonation (CMD) process. The origins of chemoselectivity
are related to oxidative addition with the C-Cl bond cleavage and
protonation process. While R is 3,5-dimethylphenyl (aryl) group, the
reaction almost exclusively afford the corresponding trisubstituted
olefins and the key step of the reaction is protonation process.
Reductive elimination relative to protonation process is responsible for
the large barrier difference. The electrostatic attractions between Pd
&C3 and Pd &Cl, repulsive force between C2 and Cl atom and a electron
density decrease of alkynyl carbon caused by the aryl substituent result
in reverse chemoselectivity. Bidentate ligand L1(2-OH-1,10-phenanthroline) reacts with Pd(OAc)2 to form
a most stable square-planer species bearing only one bidentate acetate
anion on the palladium atom, which is different from the structure
formed by the coordiantion of ligand L2 (1,10-phenanthroline)
with palladium acetate. The OH group of ligand is crucial to the
C(sp2)-H bond cleavage of aromatic ring through CMD
process. Theoretical results provide valuable information for a better
understanding of the ligand effects and substituent-controlled
chemoselectivity in Pd-catalyzed C-H arylation of alkynes with simple
arenes to furnish trisubstituted alkenes or vinyl chlorides through a
1,4-chlorine migration.