Amine, enamine/imine, or enamide moieties, as are accessible by hydroamination or -amidation reactions, are widely encountered in the scaffolds of natural products or synthetic drugs (Figure 1). Monomorine is one of the first natural products that became accessible by a synthetic sequence involving a hydroamination step. 2

In organic molecules carboxylic acid groups are among the most common functionalities. Activated derivatives of carboxylic acids have long served as versatile connection points in derivatizations and in the construction of carbon frameworks. In more recent years numerous catalytic transformations have been discovered which have made it possible for carboxylic acids to be used as building blocks without the need for additional activation steps. A large number of different product classes have become accessible from this single functionality along multifaceted reaction pathways. The frontispiece illustrates an important reason for this: In the catalytic cycles carbon monoxide gas can be released from acyl metal complexes, and gaseous carbon dioxide from carboxylate complexes, with different organometallic species being formed in each case. Thus, carboxylic acids can be used as synthetic equivalents of acyl, aryl, or alkyl halides, as well as organometallic reagents. This review provides an overview of interesting catalytic transformations of carboxylic acids and a number of derivatives accessible from them in situ. It serves to provide an invitation to complement, refine, and use these new methods in organic synthesis.

Within the past years, several catalyst systems have been discovered that promote the carboxylation of terminal alkynes with formation of propiolic acids. This reaction concept is particularly topical in that it involves the functionalization of a C–H bond and the incorporation of CO2 into an organic molecule without need for strong bases or aggressive organometallic reagents. The present article reviews the development of this new reaction type and critically compares the reaction and catalyst concepts disclosed in the literature.

In organischen Molekülen zählen Carbonsäuregruppen zu den häufigsten Funktionalitäten. Seit langem dienen aktivierte Derivate der Carbonsäuren als vielseitige Anknüpfungspunkte bei Derivatisierungen und beim Aufbau von Kohlenstoffgerüsten. In den letzten Jahren sind zahlreiche katalytische Transformationen entdeckt worden, mit deren Hilfe Carbonsäuren ohne zusätzlichen Aktivierungsschritt direkt als Bausteine nutzbar werden. Über vielfältige Reaktionspfade wurde so aus dieser einzelnen Funktionalität eine große Zahl unterschiedlicher Produktklassen zugänglich. Der Vortitel illustriert einen wichtigen Grund dafür: In den Katalysezyklen kann aus Acylmetallkomplexen Kohlenmonoxidgas, aus Carboxylatkomplexen gasförmiges Kohlendioxid freigesetzt werden, wobei jeweils unterschiedliche metallorganische Spezies gebildet werden. So können Carbonsäuren zu Syntheseäquivalenten von Acyl‐, Aryl‐ oder Alkylhalogeniden sowie von metallorganischen Reagentien werden. Dieser Aufsatz gibt einen Überblick über interessante katalytische Transformationen von Carbonsäuren und einigen aus ihnen in situ zugänglichen Derivaten. Er soll dazu einladen, diese neuen Methoden zu ergänzen, weiter zu entwickeln und in der organischen Synthese einzusetzen.

A series of metal-catalyzed processes are presented, in which carboxylic acids act as sources of either carbon nucleophiles or electrophiles, depending on the catalyst employed, the mode of activation, and the reaction conditions. A first reaction mode is the addition of carboxylic acids or amides over C-C multiple bonds, giving rise to enol esters or enamides, respectively. The challenge here is to control both the regio- and stereoselectivity of these reactions by the choice of the catalyst system. Alternatively, carboxylic acids can efficiently be decarboxylated using new Cu catalysts to give aryl-metal intermediates. Under protic conditions, these carbon nucleophiles give the corresponding arenes. If carboxylate salts are employed instead of the free acids, the aryl-metal species resulting from the catalytic decarboxylation can be utilized for the synthesis of biaryls in a novel cross-coupling reaction with aryl halides, thus replacing stoichiometric organometallic reagents. An activation with coupling reagents or simple conversion to esters allows the oxidative addition of carboxylic acids to transition-metal catalysts under formation of acyl-metal species, which can either be reduced to aldehydes, or coupled with nucleophiles. At elevated temperatures, such acyl-metal species decarbonylate, so that carboxylic acids become synthetic equivalents for aryl or alkyl halides, e.g., in Heck reactions.