Scheme 1 Main pathways of alkenylation of (hetero)arenes with
alkynes1.
Ligand-accelerated catalysis, adjusting and controlling the efficiency
and selectivity of the catalytic reaction by appropriate ligands, has
also been used in C-H bond activation13.
In contrast, only a few suitable ligands are available for
palladium-catalyzed C-H functionalization, especially for substrates
without chelating groups13b. Generally,
Pd(OAc)2 or other simple palladium salts are the widely
used catalysts for C-H activation, and the addition of nitrogen ligands
inhibit the catalytic activity of palladium catalysts. Recently, Yu et
al.14 and Sanford et al.15 reported
that the use of monodentate pyridine ligands increase the activity of
palladium catalysts in the C-H functionalization of simple arenes. And
in this reaction, the coordination of palladium with monodentate
nitrogen ligand effectively promotes C-H activation
reaction16,17. However, bidentate nitrogen-nitrogen
ligand, 1,10-phenanthroline, is so far rarely used for
palladium-catalyzed C-H activation, although it is a ligand widely used
in catalysis and preparation for various metal complexes18. Recently, Duan’s group reported for the first time
the use of the self-developed bidentate ligand (L1 ),
2-OH-1,10-phenanthroline, to promote the palladium-catalyzed C-H
activation of simple arenes with internal alkynes 10.
As shown in Scheme 2 entry 1, the representative reaction protocol
between 1,4-dichlorobenzene (1a ) and the n-propyl-substituted
alkyne (R1 =n-propyl), 2a1 , employing L1ligand, produces a 82: 9 ratio for a mixture of alkenyl chloride
(PR11 ) and hydroarylation product
(PR12 ). In contrast, the ligand was changed toL2 (1,10-phenanthroline), the reaction did not proceed at all
and no products were observed (entry 2). In addition, alkenyl chloride
(PR11 ) in 81% yield was obtained when the
substituent of alkyne 2a was the R1 group (entry 3).
However, replacing the n-propyl group in alkyne 2a1 by the
3,5-dimethylphenyl (R2 ) group (the alkyne substrate is denoted
as 2a2 ) results in a different chemoselectivity: the final
hydroarylation product (PR22 ) with a yield of
91% (entry 4).