SYNTHESIS OF SYMMETRICAL AND UNSYMMETRIC 1,4-BIS(P-R-PHENYLETHYNYL)BENZENES VIA PALLADIUM COPPER-CATALYZED CROSS-COUPLING AND COMMENTS ON THE COUPLING OF ARYL HALIDES WITH TERMINAL ALKYNES

A series of symmetric 1,4-bis(p-R-phenylethynyl)benzenes (6a-h) have been prepared via Pd(II)/Cu(I) catalyzed cross-coupling of 1,4-diiodobenzene (5) and p-substituted phenylethynes (4a-h). Similarly, the unsymmetric analogues (9a-c) were obtained from 1-iodo-4-(p-nitrophenylethynyl)benzene (8) and p-substituted phenylethynes (4c, 4d, 4g). Quantitative analysis of 1,4-(trimethylsilyl)butadiyne (10), produced in the catalytic coupling of ethynyltrimethylsilane with aryl halides using PdCl2(PPh3)2/CuI in an amine solvent, confirmed that catalyst initiation proceeds via reduction of Pd(II) to Pd(I) with concomitant oxidative homo-coupling of two ethynyltrimethylsilane molecules producing exactly one equivalent of 10 based on Pd(II). If air is present, the PdCl2(PPh3)2/CuI/amine mixture provides a very effective system for catalytic oxidative homo-coupling of terminal alkynes to diynes and thus air must be rigorously excluded from the cross-coupling reactions. Hydrodehalogenation can compete effectively with the cross-coupling reaction for highly fluorinated aryl halides. Under certain conditions, the fluorinated aryl bromide or iodide can serve as the oxidant for the alkyne to diyne oxidative homo-coupling reaction. This can be avoided by appropriate choice of reaction conditions and reagents. These competing pathways have significant implications for the cross-coupling of aryl halides with terminal alkynes and are discussed herein.