Copper-catalyzed decarboxylative arylation

A novel copper-catalyzed decarboxylative arylation of a variety of benzoxazoles with 2-nitrocarboxyhc acids (218) was reported to be most effective with electron-rich benzoxazoles by Hoover and coworkers (2015CC15059). Moreover, the presence of the ortho-nitro group was found to be essential for coupling, suggesting that this group plays a role in faciH-tating the decarboxylation step. 

Since then, experiments have been reported which indicate that (1) organocopper compounds will couple with aryl halides (2) arylcopper compounds can be oxidatively and thermally dimerized (3) arylcopper compounds are intermediates in the Ullmann reaction (4) organocopper compounds are intermediates in copper-catalyzed decarboxylations and (5) copper-promoted coupling reactions are not restricted to aromatic halides. The copper(I) oxide-promoted coupling reactions, however, have still to yield firm evidence of a copper intermediate. 

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... 

Shang, R., Fu, Y., Wang, Y., Xu, Q., Yu, H.-Z., Liu, L. (2009). Copper-catalyzed decarboxylative crosscoupling of potassium polyfluorobenzoates with aryl iodides and bromides. Angewandte Chemie International Edition, 48, 9350-9354. 

In 2010, Xue and coworkers [41] reported the viability of a copper-catalyzed decarboxylative coupling of alkynyl carboxylic acids with aryl h llides under relatively mild reaction conditions (Scheme 3.24a). On the basis of computation2il investigations, the authors proposed that in the initial step of the catalytic cycle, the copper(I) precursor was oxidized to a copper(lll) complex. 

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

The copper-catalyzed arylation of propiolic acids was disclosed by You s (Scheme 4.65) [66] and Jiao s groups [67] independently. The latter group also reported the copper-catalyzed decarboxylative amidation of propiolic acids to produce the corresponding amides (Scheme 4.66) [68]. 

As a preparative method the direct decarboxylation of olefinic acids is almost limited to the formation of styrenes and stilbenes from substituted cinnamic acids. Thermal decomposition of cinnamic acid gives styrene (41%). The yield is nearly quantitative if the reaction is carried out in quinoline at 220° in the presence of a copper catalyst. The yields of substituted styrenes where the aryl radical contains halo, methoxyl, aldehyde, cyano, and nitro groups are in the range of 30-76%. cis-Stilbene and cis-p-nitrostilbene are prepared in this way from the corresponding a-phenylcinnamic acids (65%). One aliphatic compound worthy of mention is 2-ethoxypropene, prepared by heating -ethoxycro-tonic acid at 165° (91% yield). The mechanism of acid-catalyzed decarboxylations of this type has been studied. Isomerization of the double bond from the a,/5- to the /5, y-position before decarboxylation very likely occurs in many instances. ... 

In 2013, Satoh, Miura, and coworkers [39] reported the palladium-catalyzed decarboxylative ary-lation of readily available 3-benzoylacrylic acids with arylboronic acids in the presence of a copper salt oxidant, affording chalcone derivatives (Scheme 3.22). The decarboxylative arylation could also be achieved using aryl halides as an alternative aryl source (Scheme 3.22). 

Yes, many functionalized alkynes such as ynamides and ynol ethers are well tolerated in several metal-catalyzed vinyl ester syntheses [139, 140] Decarboxylation is a common side reaction with this chemistry however, a copper-catalyzed approach has minimized this secondary process and generated good to excellent yields of the vinyl esters with electron-rich aryl carboxylic acids [130]... 

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