Decarboxylative cross-coupling

GooBen LJ, Zimmermann B, Knauber T (2008) Palladium/copper-catalyzed decarboxylative cross-coupling of aryl chlorides with potassium carboxylates. Angew Chem Int Ed 47 7103-7106... 

A complex consisting of Cu 1.10-phenanthroline [which mediates decarboxylation of arylcarboxylic acids with formation of aryl Cu species] and Pd(acac)2 [for coupling] was made and used to catalyse the decarboxylative cross-coupling of the Cu species with aryl halides. This bimetallic system allows direct coupling of a variety of aryl, heteroaryl or vinyl carboxylic acids with aryl or heteroaryl bromides, chlorides or iodides at 160° in N methylpyrrolidine in the presence of K2CO3. [Goossen et al. JAm Chem Soc 129 4824 2007]. 

An interesting example of decarboxylative cross-coupling is the Pd(ll)-mediated aUylation reaction between aromatic carboxylates and aUylic halides affording the corresponding allyl arenes in good to excellent yields (Scheme 22.9) [17]. 

Pd-Catalyzed Reactions in the Presence of Ag Salts A bimetallic system containing a Pd catalyst has been used in some cases to achieve Ar-Ar bond formation. Palladium-mediated decarboxylative cross-coupling of aromatic acids and aryl iodides takes place efficiently under microwave heating (MW), affording the corresponding biaryls, as shown in Scheme 22.40 (path a) [55]. 

The use of aryl sulfonates (mainly triflates) allows slightly lower temperature (130°C) for the decarboxylative cross-coupling with potassium carboxylates, with a Pd/Ag catalytic system [59]. The corresponding biaryls are obtained with catalytic loads of PdCl and Ag CO in the presence of inexpensive PPh and catalytic amounts of 2,6-lutidine. This ARCIS reaction proceeds efficiently under microwave irradiation in only 5 min. The reaction also meets with some success with aromatic carboxylates functionalized in the ortho position. 

SCHEME 22.42 Ar-Ar bond formation via decarboxylative cross-coupling of (a) substituted benzoic acids and (b) perfluorobenzoic acid. 

SCHEME 22.44 (a) Synthesis of biaryl compounds from Pd-catalyzed decarboxylative cross-coupling reaction between aryl carboxylic acids and (b) proposed reaction mechanism. 

SCHEME 22.47 Decarboxylative cross-coupling of aryl sulfonates with aromatic carboxylates. 

Goofien, L. J., Lange, P. R, Rodriguez, N., Linder, C. (2010). Low-temperature Ag/Pd-catalyzed decarboxylative cross-coupling of aryl triflates with aromatic carboxylate salts. Chemistry-An European Journal, 16, 3906-3909. 

GooBen, L. J., Knauber, T. (2008). Concise synthesis of telmisartan via decarboxylative cross-coupling. The Journal of Organic Chemistry, 73, 8631-8634. 

Scheme 3.6 An effective decarboxylative cross-coupling involving a 3,4-dioxypyrrole substrate, as described by Arroyave and Reynolds [17].

Scheme 3.7 Decarboxylative cross-coupling reaction between two aryl-carboxylic adds, as described by Su...

In 2011, Goossen, Underwood, and coworkers [36] developed a practical protocol that allowed performing the decarboxylative cross-coupling reactions in continuous flow reactors. The advantage of this method was that the reaction time was reduced and the formation of side products was minimized. 

In 2009, Liu and coworkers [40] reported the decarboxylative cross-coupling of potassium polyflourobenzoates with aryl iodides mediated by a copper-only system (both the decarboxylation and the cross-coupling steps are cattilyzed solely by copper). The reaction is very practical for the synthesis of polyfluorobiaryls from readily available and nonvolatile polyflourobenzoate salts (Scheme 3.23). On the basis of DFT ctilculations, the authors proposed a mechanism in which the initially formed copper(I) carboxylates extrude COj with formation of polyfluorophenylcopper(I) species (Scheme 3.23), which then react with aryl h llides in an oxidative addition/reductive elimination sequence, yielding the unsymmetrical biaryl products [40]. 

Scheme 3.19 Decarboxylative cross-coupling of aryl tosylates with aromatic carboxylate salts, as described by Goossen and coworkers [35].

Scheme 3.23 Copper-catalyzed decarboxylative cross-coupling of potassium polyfluorobenzoates with aryl halides, as described by Liu and coworkers [40].

Scheme 5 First design of a catalytic decarboxylative cross-coupling...

This first plan for a decarboxylative cross-coupling carried with it certain weaknesses for potential industrial applications. It was to be expected that the salt metathesis between alkali metal carboxylates and late transition metal halides would be thermodynamically disfavored. We expected the formation of a palladium benzoate complex i from palladium bromide complexes c and potassium benzoate (g) to proceed well only in the presence of a stoichiometric quantity of silver to capture bromide ions [27]. However, we did not like the idea of using stoichiometric quantities of silver salts or of expensive aiyl triflates in the place of aryl halides. Finally, the published substrate scope of the oxidative Heck reactimi led to concerns that palladium catalysts mediate the decarboxylation rally of a narrow range of carboxylates, precluding use of this reaction as a general synthetic strategy. 

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