Synthesis of unsymmetrically disubstituted ethynes by the palladium/copper(I)-cocatalyzed sila-Sonogashira-Hagihara coupling reactions of alkynylsilanes with aryl iodides, bromides, and chlorides through a direct activation of a carbon-silicon bond

In this paper, we explore the copper/palladium-cocatalyzed cross-coupling reactions of 1-aryl-2-trimethylsilylethynes with aryl iodides, bromides, and chlorides as coupling partners, to furnish unsymmetrically disubstituted ethynes in moderate to excellent yields. Various aryl iodides were subjected to reaction under the optimized conditions with 5 mol % of Pd(PPh ₃) ₂ and 50 mol % of CuCl. The steric properties of the aryl iodide proved more influential to the outcome of the cross-coupling reaction than electronic factors. In addition, we succeeded in synthesizing unsymmetrical diarylethynes using two different aryl iodides in one-pot. Furthermore, under the same reaction conditions with 10 mol % of PdCl ₂, 40 mol % of P(4-FC ₆H ₄) ₃, and 50 mol % of CuCl as catalyst, we succeeded in synthesizing unsymmetrical diarylethynes from various aryl bromides. Finally, we explored reactions with aryl chlorides and duly discovered that unsymmetrical diarylethynes were obtainable in moderate to good yields when 10 mol % of Pd(OAc) ₂, 10 mol % of (-)-DIOP, and 10 mol % of CuCl were used. These reactions proceed through a direct activation of a carbon-silicon bond in alkynylsilanes by CuCl to generate the corresponding alkynylcopper species via transmetalation from silicon to copper. Mechanistic investigations on the reaction of alkynylsilanes with aryl bromides confirmed that the trimethylsilyl bromide generated in situ retarded both transmetalation steps between CuCl and alkynylsilane, and between palladium(II) species formed by oxidative addition and alkynylcopper species.