Arene carboxylates decarboxylation

The reductive transformation of arene carboxylates to the corresponding aldehydes under aerobic conditions has already been noted. In addition, aromatic aldehydes may undergo both reductive and oxidative reactions, with the possibility of decarboxylation of the carboxylic acid formed ... 

In the presence of palladium(II) and silver(I) salts, arene carboxylates could be converted to aryl palladium species, which were engaged in Heck coupling reactions.90 Since the more electrophilic palladium trifluoroacetate proved to be the bestcatalyst, decarboxylation probably occurred by aromatic electrophilic substitution... 

Myers AG, Tanaka D, Mannion MR (2002) Development of a decarboxylative palladation reaction and its use in a Heck-type olefination of arene carboxylates. J Am Chem Soc 124 11250-11251... 

Tanaka D, Romeril ASP, Myers AG (2005) On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes. J Am Chem Soc 127 10323-10333... 

The phthalimide functional group has absorption and ET properties which make it very attractive in terms of PET processes in which it is the oxidizing species. The application of phthalimide derivatives in carbon-carbon bond forming processes with electron-donating groups such as ethers, thioethers, amines, arenes, carboxylates, etc., has been reviewed. A PET decarboxylation-cycliz-ation sequence has been used for the synthesis of medium-sized (8-16) heterocyclic ring systems (15) from AT-phthaloylanthranilic amides coupled to ca-aminoacids (14). The same PET protocol has been used to convert di-, tri-, and tetrapeptides into cyclopeptides with a minimum of protection and activation... 

SCHEME 22.9 Pd(ll)-catalyzed decarboxylative aUylation of arene carboxylates with allylic haUdes. 

An unexpected Pd-mediated conpling reaction of arene carboxylic acids with nitroethane via a combination of decarboxylation and dehydrogenation has been reported by Su and coworkers [42]. This method provides exclnsively ( )- 3-nitrostyrenes. Su et al. varied the reaction conditions... 

Wang, J., Cui, Z., Zhang, Y, Li, H., Wu, L.-M., Liu, Z. (2011). Pd(II)-catalyzed decarboxylative allylation and Heck-coupling of arene carboxylates with allylic halides and esters. Organic and Biomolecular Chemistry, 9, 663-666. 

Various enzymes are known to catalyze arene carboxylation or decarboxylation in nature [41]. The first report [42] of a preparatively useful biocatalytic arene carboxylation was the para-carboxylation of phenol using the enzyme phenyl phosphate carboxylase. As the name imphes, this requires the phenol to be phosphorylated prior to reaction. Subsequently, phenol para-carboxylation without prior phosphorylation was demonstrated using enzymes such as 4-hydroxybenzoate decarboxylase [43-45] and 3,4-dihydroxybenzoate decarboxylase [46, 47] (Scheme 32.10). Although the natural function of these latter enzymes is to catalyze catalyze reactions in either direction, dependent on the reaction conditions. 

It has become clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkylated arenes such as toluene and xylenes, and that carboxylation, hydroxylation, and reductive dehydroxylation are important reactions for phenols that are discussed in Part 4 of this chapter. The simplest examples include alkylated benzenes, products from the carboxylation of napthalene and phenanthrene (Zhang and Young 1997), the decarboxylation of o-, m-, and p-phthalate under denitrifying conditions (Nozawa and Maruyama 1988), and the metabolism of phenols and anilines by carboxylation. Further illustrative examples include the following ... 

The hemidecarboxylation of sodium phthaiate on reaction with mercuric acetate in boiling water [Eq. (82), X = H] (90) was the first reported thermal decarboxylation. The reaction has been observed for a number of arenes with two adjacent carboxylate groups (1-4,91) and has been named the Pesci reaction (91). Studies of 3-substituted sodium phthalates or of preformed mercuric 3-substituted phthalates have shown that the sterically hindered carboxyl group (the 2-carboxyl) is preferentially eliminated whether X is electron-donating or electron-withdrawing [Eq. (82), X = Me (91), Cl, N02 (91,93), Br (93), or C02H (94)]. A similar conclusion was drawn from the decomposition of mercuric 1,2-naph-thalenedicarboxylate and 3,4-phenanthrenedicarboxylate (91). 

Sarca and Laali386 have developed a convenient process for transacylation of sterically crowded arenes such as acetylmesitylene [Eq. (5.150)] and tetramethyl- and pentamethylacetophenones to activated aromatics using triflic acid in the presence of imidazolium-type ionic liquids under mild conditions. When the reactions are run without an activated arene acceptor, efficient deacylation takes place. Simple 4-methoxyaryl methyl ketones can be transacetylated with toluene and para-xylene as acceptors with triflic acid.387 Nafion-H has been found to be an efficient catalyst for the decarboxylation of aromatic carboxylic acids as well as deacetylation of aromatic ketones.388... 

Bis(acyloxy)iodo]arenes 1 can serve as precursors to alkyl radicals 2 via decarboxylative radical decomposition initiated by irradiation with a mercury lamp (Hg-hv) or heating (Scheme 1) [3]. Generated under these conditions alkyl radicals 2 can be effectively trapped with the appropriate organic substrates affording products with a new C-C bond. The starting [bis(acyloxy)iodo]arenes 1 can be prepared in situ from the readily available [bis(trifluoroacetoxy)iodo]ben-zene or (diacetoxy)iodobenzene and a carboxylic acid. 

The utilization of perfluorodiacyl peroxides for this purpose has been more widely developed. The rate of decomposition of perfluorodiacyl peroxides in the presence of electron-rich benzene derivatives is enhanced by a significant factor via a process of electron-transfer [66, 280], As can be seen by the contrasting examples below [281], highly reactive arenes are capable of trapping the per-fluoroalkyl carboxyl radical before it decarboxylates to RF, a result which can diminish the synthetic utility of this process. 

Hydroxyphenazine-l-carboxylic acid (li) and 2-hy-droxyphenazine (lb) are believed to be derived sequentially from phenazine-1-carboxylic acid (Ih) via known biotransformation reactions, presumably via an arene oxide intermediate (Scheme 3). In Pseudomonas aureofaciens, an NADPH-dependent reductase is responsible for the hydroxylation to form li, whereas the subsequent decarboxylation to give lb occurs spontaneously and nonenzymatically. ... 

Decarboxylation of A -Boc-L-valine has been performed in polar solvents, using electron acceptors such as dicyanobenzenes, methyl 4-cyanobenzoate, and 1,4-dicyanonaphthalene as photosensitizers, in combination with several arenes (phenanthrene, naphthalene, 1-methylnaphthalene, biphenyl, triphe-nylene, and chrysene) as co-sensitizers. The best result is achieved using biphenyl and 1,4-dicyanonaphthalene in aqueous acetonitrile. This type of reaction has been applied to the photodecarboxylation of A -Boc protected amino acids and other free carboxylic acids, in the presence of thiol and a small amount of D2O, to obtain products with high deuterium content. 

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]. 

In a handful of cases, two CCXIH groups have been activated for the synthesis of biaryls. Larrosa and coworkers reported for the first time the decarboxylative homocoupling of aromatic acids mediated by Pd and Ag [62a]. The reaction makes use of Pd(TFA)j as a catalyst and Ag CO as an additive to afford the desired biaryls in 76-95% yields. The only by-products observed were due to the proto-decarboxylation of the aryl carboxylic acid. Both metals are essential for the reaction, and the role of the Ag salt is not only as the terminal oxidant but also as a mediator of the decarboxylation process. The method is subject to some limitations on the substituents on the benzoic acids. Thus, m- and p-nitrobenzoic acids as well as benzoic acids ortho substituted with F, Br, or MeO failed to give decarboxylative homocoupling products. In all cases, protodecarboxylations to the corresponding arenes were the main products observed. The same problem was reported in the protocol developed by Deng and coworkers, where the best results were obtained with PdCl and PPhj in the presence of Ag COj [62b]. 

Myers oxidative decarboxylative Heck reaction became the prototype for a whole series of regiospecific oxidative couplings in which carboxylic acids adopt the reactivity of aryl electrophiles in the corresponding redox-neutral processes [67-72]. Crabtree et al. developed a process in which arenes react with aromatic carboxylates under C-H activation in the presence of a palladium catalyst and excess silver carbonate to yield biaryls. This reaction is useful especially for intramolecular couplings (Scheme 20) [73, 74]. Recently, a palladium-free, silver-catalyzed radical variant has been disclosed [78]. 

It has long been known that aromatic acids undergo decarboxylation to produce arenes when rigorously heated in the presence of copper and quinoline [73]. This reaction would involve decarboxylation of a copper carboxylate intermediate. A more explicit example has been reported with copper pentafluoroben-zoate (Scheme 1.57) [74]. When copper(I) pentafluorobenzoate was heated at 60 C in quinoline, pentafluorophenylcopper was produced with liberation... 

The intramolecular decarboxylative coupling of an aromatic carboxylic group with an arene C-H moiety is useful for construction of tricyclic compounds. For example, the reactions of 2-phenylbenzoic acids and 2-benzoylbenzoic acids give rise to dibenzofuran and fluorenone derivatives, respectively (Schemes 4.17 and 4.18) [21, 22]. In the latter case, radical conditions using AgOAc catalyst in the presence of K2S20g are effective. 

Decarboxylative couplings were accomplished using polyfluo-rinated arenes and heteroaryl carboxylic acids using silver(I) carbonate and tricyclohexylphosphine as a Ugand (eq 39). ... 

Yoshimi, Y, Hayashi, S., Nishikawa, K., Haga, Y, Maeda, K., Morita, T., Itou, T., Okada, Y, Ichinose, N. and Hatanaka, M. 2010. Influence of solvent, electron acceptors and arenes on photochemical decarboxylation of free carboxylic acids via single electron transfer (SET). Molecules. 15(4) 2623-2630. 

A practical, mild and highly selective protocol for the monodeuteration of a variety of arenes and heteroarenes has been reported. Catalytic amounts of Ag(I) salts in DMSO/DjO are shown to facilitate the deutero-decarboxylation of ortho-substituted benzoic and heteroaromatic a-carboxylic acids in high yields with excellent levels of deuterium incorporation. 

---