Silylation as a protective method in Cadiot-Chodkiewicz couplings : Synthesis of aryl-butadiynes and -hexatriynes

Abstract

Cadiot-Chodkiewicz couplings of arylacetylenes (I), XC 6H 4CCH (X = H; m-Br, -Me; p-F, -NO 2, -OMe) with bromoethynyl(triethyl)silane(II), BrCCSiEt 3, yield the silylated diynes, XC 6H 4(CC) 2SiEt 3, from which the terminal diynes(III), XC 6H 4CC) 2H, are quantitatively liberated by treatment with aqueous methanolic alkali. Reactions of II with III in turn yield the silylated triynes, XC 4H 4CC) 3SiEt 3, which, with alkali, give the free arylhexatriynes, XC 6H 4(CC) 3H(IV). The germanium analogue of II, BrCCGeEt 3, likewise couples with I (X = H, p-NO 2) to give XC 6H 4(CC) 2GeEt 3 or with III to give XC 6H 4(CC) 3GeEt 3. The products (X = H) are identical with those obtained from the Grignard reagents, C 6H 5(CC) nMgBr ( n = 2, 3), and bromotriethylgermane. Other XC 6H 4(CC) nGeEt 3 compounds ( n = 2,3; excluding X = NO 2) were also prepared by the Grignard method. Couplings between phenylacetylene and the reagents, BrCCSiMe3(V), and ICCSiR 3 (R = Me, Et) give the appropriate silylated phenylbutadiynes in poor yield because of symmetrical oxidative coupling (iodo compounds) or of base-sensitivity of the trimethylsilyl-acetylene bonds.