Carbon–Carbon Bond Formation and Hydrogen Production in the Ketonization of Aldehydes

Abstract

Aldehydes possess relatively high chemical energy, which is the driving force for disproportionation reactions such as Cannizzaro and Tishchenko reactions. Generally, this energy is wasted if aldehydes are transformed into carboxylic acids with a sacrificial oxidant. Here, we describe a cascade reaction in which the surplus energy of the transformation is liberated as molecular hydrogen for the oxidation of heptanal to heptanoic acid by water, and the carboxylic acid is transformed into potentially industrially relevant symmetrical ketones by ketonic decarboxylation. The cascade reaction is catalyzed by monoclinic zirconium oxide (m-ZrO2 ). The reaction mechanism has been studied through cross-coupling experiments between different aldehydes and acids, and the final symmetrical ketones are formed by a reaction pathway that involves the previously formed carboxylic acids. Isotopic studies indicate that the carboxylic acid can be formed by a hydride shift from the adsorbed aldehyde on the metal oxide surface in the absence of noble metals.

Production of acetic acid from ethanol over CuCr catalysts via dehydrogenation-(aldehyde–water shift) reaction

“…8 Orozco et al carefully investigated the role of water in the oxidation of aldehyde to carboxylic acid use ZrO 2 and CeO 2 as model catalysts. [31][32][33] Surface hydroxyl group (from water adsorption and decomposition) could combine with adsorbed aldehyde to form aldehyde hydrate, which then is able to transfer a hydride species and further form molecular hydrogen. Meanwhile, with a surface proton derived from the initial water adsorption, carboxylic acid product can be formed.…”

Ketone Formation from Carboxylic Acids by Ketonic Decarboxylation: The Exceptional Case of the Tertiary Carboxylic Acids

“…[32]. Twom olecules adsorb in adifferent fashion,n amely by dehydroxylation and double deprotonation.Anucleophilic attackf orms ac arbon-carbon bondand subsequent decarboxylation and protonation gives the ketone product.…”

“…

For the reaction mechanism of the ketonic decarboxylation of two carboxylic acids, a b-keto acid is favored as keyi ntermediate in many experimental and theoretical studies. [32].

However,i solated observations with tertiaryc arboxylic acids are not consistentw ith it and these are revisited and discussed herein.…”

Electrochemical Coupling of Biomass‐Derived Acids: New C8 Platforms for Renewable Polymers and Fuels

“…Claisen chemistry, while feasible with carboxylic acids,2 is routinely carried out with the corresponding esters, in which case the condensate is an alcohol instead of water.…”

“…Scheme1.Comparison of products derived by ac ondensation reaction versus electrochemicalcoupling. Claisen chemistry,while feasible with carboxylic acids, [2] is routinely carriedo ut with the corresponding esters, in which case the condensate is an alcoholi nsteado fw ater.…”