A copper(I)/phosphine system that homogeneously catalyses the alkynylation of formaldehyde with acetylene or terminal alkynes is reported. Using acetylene at atmospheric pressure, the catalyst consisting of < 1 mol% copper(I) phenylacetylide and a trialkyl bisphosphine ligand shows a high selectivity towards the formation of propargyl alcohol rather than 1,4-butynediol, which is not reported for common heterogeneous copper acetylide catalysts. The biphasic water/toluene reaction mixture allows the separation of targeted products with the aqueous layer and catalyst recycling via the organic layer.

Wir beschreiben die Liganden‐vermittelte C‐H‐Aktivierung/Olefinierung freier Carbonsäuren in der γ‐Position. Durch eine intramolekulare Michael‐Addition werden δ‐Lactone als Produkte erhalten. Es wurden zwei verschiedene Ligandenklassen identifiziert, welche die anspruchsvolle palladiumkatalysierte Aktivierung freier Carbonsäuren in der γ‐Position ermöglichen. Das entwickelte Protokoll zeichnet sich durch ein breites Spektrum sowohl an Carbonsäuren, als auch an olefinischen Reaktionspartnern aus und ist in einem präparativ nützlichem Maßstab durchführbar. Kinetische Untersuchungen liefern Einblicke in den zugrundeliegenden Reaktionsmechanismus.

We report a sterically controlled C–H olefination using heteroarenes as the limiting reagent. The method enables the highly C5-selective olefination of a wide range of heteroarenes and is shown to be useful in the context of late-stage functionalization.<br>

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The regioselective functionalization of heteroarenes is a highly attractive synthetic target due to the prevalence of multiply substituted heteroarenes in nature and bioactive compounds. Some substitution patterns remain challenging: While highly efficient methods for the C2‐selective olefination of 3‐substituted five‐membered heteroarenes have been reported, analogous methods to access the 5‐olefinated products have remained limited by poor regioselectivities and/or the requirement to use an excess of the valuable heteroarene starting material. Herein we report a sterically controlled C−H olefination using heteroarenes as the limiting reagent. The method enables the highly C5‐selective olefination of a wide range of heteroarenes and is shown to be useful in the context of late‐stage functionalization.