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

Abstract

For the reaction mechanism of the ketonic decarboxylation of two carboxylic acids, a β-keto acid is favored as key intermediate in many experimental and theoretical studies. Hydrogen atoms in the α-position are an indispensable requirement for the substrates to react by following this mechanism. However, isolated observations with tertiary carboxylic acids are not consistent with it and these are revisited and discussed herein. The experimental results obtained with pivalic acid indicate that the ketonic decarboxylation does not occur with this substrate. Instead, it is consumed in alternative reactions such as disintegration into isobutene, carbon monoxide, and water (retro-Koch reaction). In addition, the carboxylic acid is isomerized or loses carbon atoms, which converts the tertiary carboxylic acid into carboxylic acids bearing α-proton atoms. Hence, the latter are suitable to react through the β-keto acid pathway. A second substrate, 2,2,5,5-tetramethyladipic acid, reacted by following the same retro-Koch pathway. The primary product was the monocarboxylic acid 2,2,5-trimethyl-4-hexenoic acid (and its double bond isomer), which might be further transformed into a cyclic enone or a lactone. The ketonic decarboxylation product, 2,2,5,5-tetramethylcyclopentanone was observed in traces (<0.2 % yield). Therefore, it can be concluded that the observed experimental results further support the proposed mechanism for the ketonic decarboxylation via the β-keto acid intermediate.

Catalytic Pyrolysis of Aliphatic Carboxylic Acids into Symmetric Ketones over Ceria-Based Catalysts: Kinetics, Isotope Effect and Mechanism

“…Intensive decarboxylation accompanying the formation of the corresponding symmetrical dialkyl ketone is observed during propionic acid pyrolysis ( Figure 1A,B) and all studied acids (C2-C10), except pivalic acid. As was previously established [19,35], and also observed it in this work, pivalic acid forms two other major products, isobutene and 2-dimethyl-4-methyl-3-pentanone (Table 1). As was previously established [43,[47][48][49], the ketenization reaction (pathway b, Scheme 3) proceeds on the silica at 300-450 • C with a high level of selectivity.…”

“…On this basis, it is concluded that alpha hydrogen atoms take part in the reaction. The second proof is that a symmetric ketone pivalone is not formed from pivalic acid (CH 3 ) 3 C-COOH) [28,30,35]. It was postulated that the reason for the lack of ketonization of pivalic acid is the absence of α-hydrogen, which excludes enolization, so ketonization is impossible [19,30,33].…”

“…Despite the close attention to the ketonization reaction, last year's publications [15][16][17][18], including numerous studies and reviews [16,17,[19][20][21][22][23] suggested that the mechanism of this reaction is not completely clear. Over the past few decades, several basic mechanisms of this reaction have been proposed : (i) free-radical mechanism [24], (ii) direct concerted mechanism [20,25], (iii) mechanism involving an acyl intermediate [26], (iv) ketene based mechanisms [27][28][29], (v) and a betaketo-acid-intermediate based mechanisms [30][31][32][33][34][35] (Scheme 1). The free-radical mechanism, the acylbased mechanism, and ketene-based mechanism involve the participation of free radicals, acylium cation and ketene species as key intermediates of ketonization, respectively.…”

Reversibility of the catalytic ketonization of carboxylic acids and of beta-keto acids decarboxylation

“…It is less likely that 13 C/ 12 C exchange can proceed through some kind of a direct alkyl group transfer from ketone to CO2 as in the concerted mechanism of ketonic decarboxylation, or via formation of alkyl radicals, or alkyl anions, given that all such options have been ruled out over the century-long debate in favor of the stepwise mechanism of alkyl group transfer via beta-keto acid formation [10][11][12]. The most recent experimental evidence against direct alkyl group transfer is that reported in the work of Oliver-Tomas et al [14]. 13 C-labeled carboxylic acids too exchange their carboxyl group presumably through the condensation between enolized surface carboxylate with CO2 into intermediate malonic acid and CO2 by the water-gas shift reaction, which is also unlikely at temperatures below 300 °C.…”

“…Exchange via retro-Koch -Koch reaction equilibrium under chosen conditions is very unlikely. Retro-Koch decomposition of carboxylic acids requires much higher temperatures, above 500 °C, and cannot compete with the ketonic decarboxylation favored at 300 °C[14]. Only acids lacking alpha-hydrogens and not able to undergo ketonic decarboxylation may be forced into the retro-Koch reaction at extreme temperatures.…”