Efficient copper-free aerobic alkyne homocoupling in polyethylene glycol

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• 作者：Garazi Urgoitia ; Raul SanMartin ; María Teresa Herrero…
• 关键词：Homocoupling ; Alkynes ; Diynes ; Palladium ; Molecular oxygen ; Sustainable media
• 刊名：Environmental Chemistry Letters
• 出版年：2017
• 出版时间：March 2017
• 年：2017
• 卷：15
• 期：1
• 页码：157-164
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Environmental Chemistry; Ecotoxicology; Pollution, general; Analytical Chemistry; Geochemistry;
• 出版者：Springer International Publishing
• ISSN：1610-3661
• 卷排序：15

文摘

1,3-Diynes are valuable building blocks of biologically active molecules and precursors of polymers, macrocycles and supramolecular structures. The most direct synthesis of 1,3-diynes is the metal-catalyzed homocoupling of alkynes, a process where molecular oxygen is a convenient oxidant due to its availability, low cost and innocuous by-products. Concerning the catalyst, palladium species are efficient and selective but they often need copper salts as co-catalysts and stoichiometric amounts of reoxidants. Here we found that heating terminal aromatic alkynes with small amounts of pincer-type palladacycles (10−4 mol%, 10−6 M) and a weak base (NaOAc, 10 mol%) under oxygen atmosphere (1–1.2 atm.) in polyethylene glycol 400 provided the corresponding 1,4-diarylbuta-1,3-diyne. Yields ranged from 50 to 92% by using CNC-type complex, whereas a NCN palladacycles provided the coupled products with slightly higher yields, of 61–92%. Overall, our findings show an environmentally friendly method for the aerobic homocoupling of arylacetylenes based on the use of palladium pincer complexes as only catalyst in the absence or any other copper co-catalyst or additional oxidant. The use of the above palladacyclic complexes allows for very low catalyst loading, and the method tolerates the presence of halogens, alkyl, alkoxy and hydroxy groups in the starting alkyne. In addition, the extremely low content of metal traces present in final products is in accordance with regulations for drug products.