Synthesis of symmetric 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 |
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Authors: | Paul Nguyen Zheng Yuan Lisa Agocs Gerald Lesley Todd B Marder |
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Institution: | Department of Chemistry, University of Waterloo, Waterloo, Ont., N2L 3G1 Canada |
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Abstract: | A series of symmetric 1, 4-bis(p-R-phenylethynyl)benzenes (6a-h) have been prepared via Pd11/Cu1 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 ethynyltri- methylsilane with aryl halides using PdCl2(PPh3)2/CuI in an amine solvent, confirmed that catalyst initiation proceeds via reduction of Pd11 to Pd0 with concomitant oxidative homo-coupling of two ethynyltrimethylsilane molecules producing exactly one equivalent of 10 based on Pd11. 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. |
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Keywords: | Cross-coupling reaction Aryl halide Alkyne Palladium complexes Copper complexes Non-linear optics Liquid crystal Hydrodehalogenation |
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