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).