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 b een prepared via Pd(II)/Cu(I) catalyzed cross-coupling of 1,4-diiodobe nzene (5) and p-substituted phenylethynes (4a-h). Similarly, the unsym metric analogues (9a-c) were obtained from 1-iodo-4-(p-nitrophenylethy nyl)benzene (8) and p-substituted phenylethynes (4c, 4d, 4g). Quantita tive analysis of 1,4-(trimethylsilyl)butadiyne (10), produced in the c atalytic coupling of ethynyltrimethylsilane with aryl halides using Pd Cl2(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 exactl y one equivalent of 10 based on Pd(II). If air is present, the PdCl2(P Ph3)2/CuI/amine mixture provides a very effective system for catalytic oxidative homo-coupling of terminal alkynes to diynes and thus air mu st be rigorously excluded from the cross-coupling reactions. Hydrodeha logenation can compete effectively with the cross-coupling reaction fo r highly fluorinated aryl halides. Under certain conditions, the fluor inated aryl bromide or iodide can serve as the oxidant for the alkyne to diyne oxidative homo-coupling reaction. This can be avoided by appr opriate choice of reaction conditions and reagents. These competing pa thways have significant implications for the cross-coupling of aryl ha lides with terminal alkynes and are discussed herein.