Aromatic compounds oxidative carbonylation

The oxidative cleavage of carbon-carbon bonds in vicinal diols [756, 759] is a reaction widely used in saccharide chemistry. Besides its application in this reaction, periodic acid achieves the oxidative coupling [757] or oxidation to quinones [758] of polynuclear aromatic hydrocarbons, the oxidation of methyl groups in aromatic compounds to carbonyl groups [760], the conversion of epoxides into dicarbonyl compounds [761], and the oxidative cleavage of trimethylsilyl ethers of acyloins to carboxylic acids [755]. 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

TRAHANOVSKY Ether oxidation Oxidation of aromatic ethers to carbonyl compounds with cemim ammonium nitrate... 

Oxidative carbonylation generates a number of important compounds and materials such as ureas, carbamates, 2-oxazolidinones, and aromatic polycarbonates. The [CuX(IPr)] complexes 38-X (X = Cl, Br, I) were tested as catalysts for the oxidative carbonylation of amino alcohols by Xia and co-workers [43]. Complex 38-1 is the first catalyst to selectively prepare ureas, carbamates, and 2-oxazolidinones without any additives. The important findings were the identity of the counterion and that the presence of the NHC ligand influenced the conversions. 2-Oxazohdinones were formed from primary amino alcohols in 86-96% yield. Complex 38-1 also catalysed the oxidative carbonylation of primary amines to ureas and carbamates. n-Propylamine, n-butylamine, and t-butylamine were transformed into the... 

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

Synthetically especially valuable is the oxidation of carbonyl compounds and nitroalkanes by manganese(III) salts to form carboxymethyl and nitromethyl radicals, respectively. These radicals can be trapped by olefins like 1,3-butadiene or aromatic compounds to yield synthetically interesting products. In this case it is very advantageous to generate and regenerate the oxidizing species in situ by indirect electrolysis. This was the basis for the development of a process for the synthesis of sorbic acid viay-vinyl-y-butyrolactone Equations (31)—(35) summarize the im-... 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

The oxidative carbonylation of arenes to aromatic acids is a useful reaction which can be performed in the presence of Wacker-type palladium catalysts (equation 176). The stoichiometric reaction of Pd(OAc)2 with various aromatic compounds such as benzene, toluene or anisole at 100 °C in the presence of CO gives aromatic acids in low to fair yields.446 This reaction is thought to proceed via CO insertion between a palladium-carbon (arene) allyl chloride, but substantial amounts of phenol and coupling by-products are formed.447... 

Oxidation/hydroxylation of aromatic compounds by OH and HOONO is expected to enhance their degradation rate and hence decrease their lifetime on particulate matter, which in the case of pollutants is beneficial from the point of view of human health. Oxidation of PAHs could also lead to the production of photosensitizers such as quinones and aromatic carbonyls [10, 40, 41]. These compounds, if present in the gas phase, are also able to form aggregates and are therefore involved in the formation of secondary organic aerosol [42]. In contrast, nitration induced by OH + N02 or HOONO could lead to highly mutagenic nitro-PAHs [43] or phytotoxic nitrophenols [44, 45], in which case the health and environmental impact of the reaction intermediates is not negligible and is sometimes higher than that of the parent molecules. 

The use of the above methods does not generally result in the coupling reaction of aromatic compounds, ArX, because of the strong bond of C(sp2)-X in ArX. However, the coupling reaction of a cation radical formed from the single-electron oxidation of aromatics readily occurs. For example, 4-methylquinoline coupled to give bis[2-(4-methylquinolyl)] in 90% yield, by electrolysis [14-19]. Direct irradiation (300 nm) of carbonyl compound (13) in dimethylaniline without a solvent gives rise to ethanolamine (14) as the major product as shown in eq. 2.7 [20]. 

Ce Compounds oxidize side chains of aromatic compounds effectively and selectively, methylene carbons at the benzylic positions being oxidized to carbonyl groups. Polymethy-lated aromatics generally are oxidized to a single aldehyde group. 

Certain aromatic compounds containing alkyl groups have been converted to carbonyl derivatives by liquid-phase oxidation of these groups with air in the presence of chromium oxide catalysts. 

N03)j, a newcomer to the arena of oxidants, is useful for the acetoxylation of aromatic side chains in benzylic positions [415, 416] and for the oxidation of methylene or methyl groups that are adjacent to aromatic rings to carbonyl groups [238, 415, 417]. The reagent also oxidizes alcohols to aldehydes [418, 419, 420, 421] and phenols to quinones [422, 423], cleaves vicinal diols to ketones and a-hydroxy ketones to acids [424, 425], and converts diaryl sulfides into sulfoxides [426]. A specialty of ammonium cerium nitrate is the oxidative recovery of carbonyl compounds from their oximes and semicarbazones [422, 427] and of carboxylic acids from their hydrazides [428] under mild conditions. 

Sodium dichromate hydroxylates tertiary carbons [620] and oxidizes methylene groups to carbonyls [622, 623, 625, 626, 631] methyl and methylene groups, especially as side chains in aromatic compounds, to carboxylic groups [624, 632, 633, 634, 635] and benzene rings to quinones [630, 636, 637] or carboxylic acids [638]. The reagent is often used for the conversion of primary alcohols into aldehydes [629, 630, 639] or, less frequently, into carboxylic acids or their esters [640] of secondary alcohols into ketones [621, 629, 630, 641, 642, 643, 644] of phenylhydroxylamine into nitroso-benzene [645] and of alkylboranes into carbonyl compounds [646]. 

Potassium dichromate, K2Cr207, is applied under similar conditions as its sodium analogue to oxidize benzene rings to quinones [647, 648, 649, 650], methylene groups adjacent to aromatic rings to carbonyls [514], primary alcohols to aldehydes [651, 652, 653], secondary alcohols to ketones [644, 652, 654, 655], and aldehydes to acids [656]. Phenylhydroxylamine is transformed into nitrosobenzene [657], and an aromatic nitroso compound, into a nitro compound [655]. 

The spectrum of applications of potassium permanganate is very broad. This reagent is used for dehydrogenative coupling [570], hydrox-ylates tertiary carbons to form hydroxy compounds [550,831], hydroxylates double bonds to form vicinal diols [707, 296, 555, 577], oxidizes alkenes to a-diketones [560, 567], cleaves double bonds to form carbonyl compounds [840, 842, 552] or carboxylic acids [765, 841, 843, 845, 852, 869, 872, 873, 874], and converts acetylenes into dicarbonyl compounds [848, 856, 864] or carboxylic acids [843, 864], Aromatic rings are degraded to carboxylic acids [575, 576], and side chains in aromatic compounds are oxidized to ketones [566, 577] or carboxylic acids [503, 878, 879, 880, 881, 882, 555]. Primary alcohols [884] and aldehydes [749, 868, 555] are converted into carboxylic acids, secondary alcohols into ketones [749, 839, 844, 863, 865, 886, 887], ketones into keto acids [555, 559, 590] or acids [559, 597], ethers into esters [555], and amines into amides [854, 555] or imines [557], Aromatic amines are oxidized to nitro compounds [755, 559, 592], aliphatic nitro compounds to ketones [562, 567], sulfides to sulfones [846], selenides to selenones [525], and iodo compounds to iodoso compounds [595]. 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

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