Carboxylic acids as cocatalysts

EFFECT OF CARBOXYLIC ACIDS AS COCATALYSTS IN TUNGSTATE-CATALYZED EPOXIDATIONS... 

Ackermann L, Vicente R, Althammer A (2008) Assisted ruthenium-catalyzed C-H bond activation carboxylic acids as cocatalysts for generally applicable direct arylations in apolar solvents. Org Lett 10 2299-2302... 

Ackermann probed carboxylic acids as cocatalysts in ruthenium-catalyzed direct arylations. Mesityl carboxylic acid was found to be a very active preligand, which displayed a broad scope for effective arylations of pyridine, 1,2,3-triazole, oxazoline or pyrazole derivatives. Regarding the electrophile, diversely functionalized aryl bromides, but, interestingly, also aryl chlorides were found to be practicable substrates (Scheme 23). These transformations were proposed to occur through a transition state like 90, where carboxylate assisted during CMD process. 

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

One of the most efficient methods for oxidation of primary alcohols to either aldehydes or carboxylic acids is the one, commonly known as the Anelli oxidation. This reaction is carried out in a two-phase (CH2Cl2/aq.buffer) system utilizing TEMPO/NaBr as a catalyst and NaOCl as the terminal oxidant The new system described here is an extension of the Anelli oxidation, but surprisingly, does not require the use of any organic solvents and replaces the KBr co-catalyst with the more benign, Na2B40y (Borax). The use of the new cocatalyst reduces the volume of the buffer solution and eliminates completely the need of a reaction solvent. The new system was successfully applied in the industrial synthesis of the 3,3-Dimethylbutanal, which is a key intermediate in the preparation of the new artificial sweetener Neotame. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Although catalytic hydration of ethylene oxide to maximize ethylene glycol production has been studied by a number of companies with numerous materials patented as catalysts, there has been no reported industrial manufacture of ethylene glycol via catalytic ethylene oxide hydrolysis. Studied catalysts include sulfonic acids, carboxylic acids and salts, cation-exchange resins, acidic zeolites, halides, anion-exchange resins, metals, metal oxides, and metal salts (21—26). Carbon dioxide as a cocatalyst with many of the same materials has also received extensive study. 

Catalysts for ethylene/carbon monoxide copolymerisation were initially obtained from Ni(II) derivatives, such as K2Ni(CN)4 and (w-Bu4N)2 Ni(CN)4, and Pd(II) derivatives, such as [(w-Bu3P)PdCl2]2, Pd(CN)2 and HPd(CN)3, often combined with alcohol or protonic acid as a cocatalyst [241]. It must be emphasised that, in contrast to titanium-, zirconium- or vanadium-based catalysts, nickel- and palladium-based catalysts tolerate polar functional groups (including hydroxyl, carboxylic and sulfonic groups)... 

And thirdly, cocatalysts are scavengers for impurities such as moisture, excess carboxylic acids etc. 

For the reaction of TDI with a polyether triol, bismuth or lead compounds can also be used. However, tin catalysts are preferred mainly because of their slight odor and the low amounts required to achieve high reaction rates. Carboxylic acid salts of calcium, cobalt, lead, manganese, zinc, and zirconium are employed as cocatalysts with tertiary amines, tin compounds, and tin—amine combinations. Carboxylic acid salts reduce cure time of rigid foam products. Organic mercury compounds are used in cast elastomers and in RIM systems to extend cream time, ie, the time between mixing of all ingredients and the onset of creamy appearance. 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

The presence of free Lewis acid in these types of systems tends on the contrary to favour the polymerisation of the olefin, or its isomerisation. This situation has been exploited in a large number of investigations in which carboxylic acids have been used as cocatalysts for the cationic polymerisation of alkenyl monomers. The majority of these reports are qualitative in that they do not examine the kinetics and mechanism of these cocatalytic processes. But a few excellent studies have been carried out dealing with these fundamental aspects, and these are reviewed below. 

Much faster reaction can be achieved with strong bases (/07). The chain is started by an N-acylimide which may be N-caproyl-caprolactam produced in a slow reaction from the monomer during an induction period or an N-acylamid produced by action of a cocatalyst like for example carboxylic acid chlorides or anhydrides on caprolactam. The cocatalyst action speeds up the reaction such, that fast polymerization below the melting point of the polymer becomes possible. The strong base, such as alkali metal, metal hydride, metal amid, or oiganometallic compound, activates the monomer by lactam anion formation ... 

DCC can be used to prepare 5-alkyl and 5-aryl thiocarboxylates (1) from carboxylic acids and thiols according to equation (5). This method has been successfully applied to the synAesis of thiol esters with sensitive substituents, e.g. 5-methyl thioacrylate, a natural product. In particular, N-protected amino acid and peptide 5-phenyl esters, which are useful building blocks in peptide synthesis, are obtained in excellent yields without racemization. N-Hydroxyphthalimide and DMAP have been used as cocatalysts to facilitate the reaction. The preparation of the Wittig reagent (5) by this route is shown in equation (6). 

RCO2H, R OH, DCC/DMAP, EtjO, 25°C, l-24h, 70-95% yield. This method is suitable for a large variety of hindered and unhindered acids and alcohols. The use of Sc(OTf)3 as a cocatalyst improves the esterification of 3° alcohols. Carboxylic acids that can form ketenes with DCC react preferentially with aliphatic alcohols in the presence of phenols whereas those that do not show the opposite selectivity. In some sterically congested situations the 0-acyl urea will migrate to an unreactive A-acyl urea in competition with esterification. Carbodi-imide I was developed to make the urea by-product water soluble and thus easily washed out. Isoureas are prepared from a carbodiimide and an alcohol which upon reaction with a carboxylic acid give esters in excellent yield. A polymer supported version of this process has been developed. This process has been reviewed. Note that DCC is a potent skin irritant in some individuals. 

Scheme 33 Ruthenium-catalyzed direct arylation using carboxylic acid 88 as cocatalyst...

Most or perhaps all of the Lewis acids are seldom effective alone as initiators or catalysts they are used in conjunction with a second compound, called a cocatalyst , which very often is water or some other proton donor protogen) such as hydrogen halide, alcohol, and carboxylic acid, or a carbocation donor cationo-gen) such as f-butyl chloride and triphenylmethyl chloride. On reaction with the Lewis acid, they form a catalyst-cocatalyst complex that initiates polymerization. For example, isobutylene is not polymerized by boron trifluoride if both are dry, but immediate polymerization takes place on adding a small amount of water. The initiation process is therefore represented by... 

Esterification. Heating carboxylic acids with alcohols in toluene at 80° in the presence of Ph2NH20Tf (1 mol%) furnishes esters (12 examples, 78-96%). The same catalyst can be used in transesterification. Improved yields are obtained by adding McjSiCl as cocatalyst. 

Under different reaction conditions, vicinal diol production [70] or C=C double bond oxidative cleavage to carboxylic acids occurs [59, 71], Dialdehydes are produced from cycloolefins, by tungstic acid as catalyst in t-butanol [72], Secondary alcohols yield ketones, while primary alcohols produce aldehydes or carboxylic acids [59, 68-69, 73-74], Different products are obtained from glycols, under different reaction conditions, 1,2-Diols are cleaved to ketocarboxylic acids and dicarboxylic acids [58, 75], or oxidised to a-hydroxy ketones [76], The latter can be obtained directly from the olefins, with lower selectivity [77], Lactones are formed by 1,4-diols and other a,o)-diols [78], Internal alkynes predominantly yield a,p-epoxyketones [79], or 1,2-diketones and carboxylic acids if HgfAcO) is added as the cocatalyst [80], Terminal alkynes yield a-ketoaldehydes and carboxylic acids. 

As shown in Table 3.2, 5% BTCA in the presence of 10% SHP and 0.1% TiO (as a cocatalyst) was nsed, and the addition of TiO as a cocatalyst further increased WRA by 58.5%. This was becanse both TiO and SHP accelerated the catalytic reaction throngh the formation of ester bonds between the cyclic anhydride ring and the hydroxyl gronp of cellulose. The improvement of WRA by the addition of TiO in the BTCA treatment was probably dne to the nniqne photocatalytic properties of TiO, which is a kind of N-type semicondnctor. The hydroxyl radical (-OH) and snperoxide anion (-0 -) formed may have acted as catalysts to accelerate the formation of anhydrides from poly (carboxylic) acids. Fnrthermore, the effect of hydroxyl radical (-OH) and superoxide anion (-O -) on the increase of charge localization of the sohd cellulose medium in which esterfication and cross linking occur may also have been significant. Therefore, WRAs of cotton fiber treated with 5% BTCA, 10% SHP, and 0.2% TiO further increased to 61.3% compared with those of the untreated cotton fabric. The increment was proportional to the increase of TiOj from 0.1 to 0.2% in the BTCA treatment bath (Lam et al., 2011). 

An example of an asymmetric induction from optically inactive monomers in an anionic polymerization of esters of butadiene carboxylic acids with (/ )-2-methylbutyllithium or with butyllithium complexed with (-)-menthyl ethyl ether as the catalyst. The products, tritactic polymers, exhibit small, but measurable, optical rotations. Also, when benzofiiran, that exhibits no optical activity, is polymerized by cationic catalysts like aluminum chloride complexed with an optically active cocatalyst, like phenylalanine, an optically active polymer is obtained. ... 

Catalytic and highly enantioselective fluorination of acyl chlorides was reported by Lectka et al. where O-benzoyl quinidine (O-Bz-QD), is combined with a transition metal-based cocatalyst, (l,3-dppp)NiCl2 or trans-(PPh3)2PdCl2, to generate chiral ketene enolates from acyl chlorides, which are fluorinated with NFSI to produce ot-fluorinated carboxylic acid derivatives. These derivatives are then in situ reacted with different nucleophiles such as methanol, water or variety of amines affording a-fluoro esters, acids... 

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