Carboxylic acids, aryl => arenes

Ar. carboxylic acid aryl esters from arenes ArH ArGOOAr... 

ProMem 10.35 Which isomer of the arene C,oH,4 resists vigorous oxidation to an aryl carboxylic acid ... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

There has been a summary of the use of insertion reactions of arynes into a-bonds to prepare nrt/io-disubs tituted arenes. A key to the success of these processes is the ability to generate benzyne under mild conditions by the reaction of readily available o-(trimethylsilyl)phenyl triflate with fluoride ions.61 Reaction of amines and their derivatives with benzynes generated in this way has been shown to be an efficient method for the production of N-arylated derivatives, as illustrated in Scheme 8. The method also works well in the O-arylation reactions of phenols and carboxylic acids 62... 

Friedel-Crafts acylation. Carboxylic acids are converted into RCOCl with Me2Si(H)Cl and InCls. In the presence of an activated arene (e.g., an aryl ether) an aryl ketone is formed. A dual role is played by InCl3. 

The experimental data reported in the previous sections leave no doubt of the intermediate formation of aryl and alkyl platinum derivatives in the reactions of Pt(ll) and Pt(IV) chloride complexes with arenes and alkanes in aqueous solutions. In the 1980s, it was discovered that the reaction of H PtCCwith arenes in aqueous carboxylic acids produces stable a-aryl complexes of platinum(IV) which can be isolated in crystalline form (see, for example, [43]). 

In addition to the industrial apphcations, in Scheme 8.1, other reactions have been the focus of extensive research and development. For example. Chapter 12 surveys the research efforts directed toward Pd-catalyzed oxidative carbonylation of phenol affords the important monomer, diphenyl carbonate (Scheme 8.2a). Other reactions of potential industrial significance highlighted in this chapter include the oxidation of alcohols to aldehydes and ketones (Scheme 8.2b), oxidative coupling of arenes and carboxylic acids to afford aryl esters (Scheme 8.2c), benzylic acetoxylation (Scheme 8.2d), and oxidative Heck reactions (Scheme 8.2e). The chapter concludes by highlighting a number of newer research developments, including ligand-modulated catalytic oxidations, Pd/NO cocatalysis, and alkane oxidation. 

Oxidative esterification of arenes with carboxylic acids produces aryl esters, which can be used as precursors to valuable phenol derivatives (Scheme 8.6). Commercial production of phenol involves the aerobic oxidation of cumene to cumene hydroperoxide, followed by conversion to acetone and phenol under acidic conditions (Hock process) [49]. Aerobic acetoxylation of benzene to phenyl acetate provides a potential alternative route to phenol, and Pd-catalyzed methods for this transformation have been the focus of considerable effort. None ofthese methods are yet commercially viable, however. 

Cyanation of aryl hahdes, boronates, boronic acids, arene carboxylic acids, and (hetero)arene C—H bonds using nonmetaUic cyano-group sources 12AG(E)11948. 

Arylcarbonyl compounds, such as carboxylic acids, aldehydes, acid chlorides, amides, and esters, have been increasingly used as an aryl source in arene-assembling cross-coupHng reactions. They are usually inexpensive and readily available compared to general cross-coupling partners [155]. In addition, these coupling reactions produce CO or CO2 as a by product instead of toxic metal halides. 

Recently, Yu [171] discovered that N-protected amino acids such as Ac-Leu-OH or Ac-Ile-OH (127) dramatically enhance the reactivity of sp C-H olefination. Subsequently, they demonstrated C-H arylations of arenes bearing sulfonamides [172] and carboxylic acids [173] as directing groups under a Pd"/(N Protected amino acids)/AgjCOj catalyst system. 

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]. 

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