Tetrakis(dimethylamino)ethylene (TDAE)/cat. PdCl2(PhCN)2-promoted reductive coupling of aryl bromides having either electron-donating or electron-withdrawing groups on their para- and/or meta-position proceeded smoothly to afford the corresponding biaryls in good to excellent yields. Notably, TDAE is such a mild reductant that easily reducible groups, such as carbonyl and nitro groups, are tolerate. A similar reductive coupling of ortho-substituted aryl bromides did not occur at all. The proper choice of palladium catalysts is essential for the reductive coupling; thus, PdCl2-(PhCN)2, PdCl2(MeCN)2, Pd(hfacac)2, Pd2(dba)3, PdCl2, and Pd(OAc)2 were used successively for this reaction, but phosphine-ligated palladium catalysts such as Pd(PPh3)4, PdCl2(PPh3)2, and Pd-(dppp) did not promote the reaction. The reductive coupling did not occur with nickel catalysts such as NiBr2, NiCl2(bpy), and Ni(acac)2. The TDAE/cat. palladium-promoted reductive coupling of aryl halides having electron-withdrawing groups took place more efficiently than that of aryl halides substituted with electron-donating groups. A plausible mechanism of TDAE/cat. palladium-promoted reaction is discussed.
ASJC Scopus subject areas
- Organic Chemistry