Acetic acid as cocatalyst

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Lewis AcidICo-catalyst Systems. These continue to be investigated in some detail. Thus SnCU with water, a range of phenols, and acetic acid as cocatalysts have been used to polymerize isobutene s and rates of initiation obtained by using 2,6-di-t-butylphenol as a retarder. Generally good correlation... 

An important contribution elucidating the potential of primary amines derived from Cinchona alkaloids has been the aldol cyclodehydration of achiral 4-substituted-2,6-heptanediones to enantiomerically enriched 5-substituted-3-methyl-2-cyclohexene-l-ones, presented by List and coworkers in 2008 (Scheme 14.26). Both 9-deo>y-9-amino-epr-quinine (QNA) and its pseudoenantiomeric, quinidine-derived amine QDA, in combination with acetic acid as cocatalyst, proved to be efficient and highly enantio-selective catalysts for this transformation, giving both enantiomers of 5-substituted-3-methyl-2-cyclohexene-l-ones with very good results. The authors observed that proline and the catalytic antibody 38C2 delivered poor enantioselectivity in this reaction. Furthermore, the synthetic utility of the reaction was exemplified by the first asymmetric synthesis of both... 

Gu and coworkers reported an improved protocol for the Miehael reaction between acetone and aromatic nitroalkenes by employing Ma s saccharide-derived catalyst 37 combined with acetic acid as cocatalyst (Scheme 19.40, left). The aeidie counterpart enhanced the performance of the catalytic system delivering the products in high yields (76-94%) and with high to excellent enantioselectivity (88-96% enantiomeric excess), whilst the catalyst... 

Enamides were explored as substrates for Brpnsted acid organocatalytic reductions. While this reaction formally involves hydrogenation of a C=C bond, substrate protonation yields an iminium ion which is subsequently reduced by Hantzsch ester 4a [66] (Scheme 2.13). Optimization of the reaction conditions revealed that the loading of phosphoric acid catalyst could be dramatically decreased while retaining high degrees of stereocontrol by using acetic acid as cocatalyst to increase the concentration of iminium ion in the reaction mixture. 

Co-catalysts other than water. Trichloro- and monochloro-acetic acids, when used as cocatalysts, induced instantaneous polymerisation at -140°. With the following co-catalysts the rate of polymerisation at -78° decreased in the order acetic acid > nitroethane > nitromethane > phenol > water [75a]. Since this is also the sequence of the acid dissociation constants of these substances in water, it appears that the catalytic activity , as shown by the rate of polymerisation, is correlated with the acidity of the cocatalyst in aqueous solution. Flowever, there are two reasons for questioning the validity of this correlation. 

The second reason is that Satchell [78] has shown that in the protonation of m-xylene by catalysts composed of stannic chloride and acetic acid or the three chloroacetic acids as co-catalysts, the rate of reaction is inversely related to the aqueous acidity of these acids. Satchell rightly points out that, since the polymerisations are complicated reactions the rates of which are also affected by the terminating efficiency of the anion derived from the co-catalyst, no valid conclusions can be drawn from such studies about catalytic efficiency in any fundamental sense. He interprets the order of effectiveness of the cocatalysts in terms of the stability of the complexes which they form with the metal halide. 

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). 

The consumption of nitric and acetic acids is plotted as a function of percent water in the system in Figure 4. The minimum in the nitric acid curve at the 5% level coincides with the maximum in yield (Figure 3). This is not surprising it merely indicates that the yield depends on the activity of the cocatalyst. 

Acetoxylation of propene to allyl acetate can be performed in the liquid phase with high selectivity (98%) in acetic acid in the presence of catalytic amounts of palladium trifluoroacetate. The stability and activity of this catalyst can be considerably increased by adding copper (II) trifluoroacetate and sodium acetate as cocatalysts (100 °C, 15 bar, reaction time = 4 h, conversion = 70%, selectivity = 97%). Gas-phase procedures for the manufacture of allyl acetate are described in several patents and use conventional palladium catalysts deposited on alumina or silica, together with cocatalysts (Au, Fe, Bi, etc.) and sodium acetate. The activity and selectivity reported for these catalysts are very high (100-1000 g l-1 h-1, selectivity = 90-95% ).427 A similar procedure has been used for the synthesis of methallyl acetate from 2-methylpropene.428... 

Considerable work in this field has been done at Hoechst AG by Leupold and coworkers [147]. As can be seen from Table XI, the selectivity to ethanol can be significantly increased by the addition of cocatalysts such as Zr or B(. It is also possible to increase the selectivity to acetic acid or acetaldehyde, if Rh/Mg catalysts on SiOj are used. Under these conditions, C3-C4 olefins are the major side products [ 148]. 

Indeed, a combination of tributyltln hydride, Pd° catalyst and a weak acid, such as ammonium chloride, forms an effective, yet mild tool for conjugate reduction of a, -unsaturated aldehydes and ketones. Similar results are obtained with other acidic cocatalysts, such as zinc chloride, acetic acid and tributyltln triflate. With this system, reductions occur with high regloselectivity, providing a useful approach for deuterium incorporation into either the P- or a-position by using either tributyltln deu-teride or D2O, respectively (Scheme 57). ... 

Catalytic oxidation of p-xylene with air is the chief commercial method used to produce terephthalic acid. A solution of p-xylene in acetic acid, together with manganese or cobalt derivative and heavy metal bromides, which serve as cocatalysts, is fed to a continuous reactor, vigorously stirred, and heated to 200°C while under about 25 atm pressure. Air is continuously fed into the reactor at the same time as a small stream of partially reacted solution is removed (Eq. 19.66). 

Although most depictions of the catalytic cycle include Equation (3) as one of the reaction steps, this is, in fact, an oversimplification. For the continuous process, under typical industrial conditions, the principal component of the reaction medium is acetic acid, and so in a working catalytic system, esterification leads to conversion of most of the alcohol substrate into methyl acetate (Equation (4)). It is then methyl acetate (rather than methanol) that is activated by reaction with the iodide cocatalyst (Equation (5)), the net result of Equations (4) and (5) being Equation (3). 

Recently, Yamaji and coworkers reported that the Pd(OAc)2-catalyzed oxidative coupling of benzene in the presence of 02 and acetic acid gave high selectivity of biphenyl (88%) when Mo02(acac)2 was used as a cocatalyst, but the turnover number remained low (up to 10) (Eq. 1, Scheme 6) [46]. 

Very recently, White and coworkers introduced the chiral Lewis acid Crm(salen) as cocatalyst into Ll/Pd11 catalytic system. The oxidative allylic acetoxyaltion of terminal olefins 1 afforded the corresponding branched allylic acetates 3 in high regioselectivity and moderate enantio-selectivities (up to 63% ee) (Scheme 6) [22], The asymmetric induction possibly results from the coordination between Cr salen) and BQ, and the adduct of Cr,n(salen) BQ promotes the acetoxylation of rc-allyl-palladium complex to form enantioenriched branched allylic acetates. 

In the well-known Wacker process ethylene is converted to acetaldehyde by aerobic oxidation in an aqueous medium in the presence of PdCl2 as catalyst and CuCl2 as cocatalyst [7], Terminal olefins afford the corresponding methyl ketones. Oxidative acetoxylation of olefins with Pd(II) salts as catalysts in acetic acid was first reported by Moiseev and coworkers [8], The addition of an alkali metal acetate, e. g. NaOAc, was necessary for the reaction to proceed. Palladium black was also found to be an active catalyst under mild conditions (40-70 °C, 1 bar) in the liquid phase, if NaOAc was added to the solution before reducing Pd(II) to Pd black, but not afterwards [9,10]. These results suggested that catalytic activity... 

It has long been known that Pd(OAc)2 catalyzes the benzylic oxidation of toluene in acetic acid [18,19]. The catalyst was more active when reactions were performed in the presence of active charcoal [19] and low oxidation-state palladium was implicated in benzylic oxidations [20]. Optimum results were obtained in the presence of active charcoal and Sn(OAc)2 as cocatalyst [19]. Similarly, Lyons [4] reported that Pd/C is an effective catalyst for benzylic oxidations under mild conditions, with little or no competing ring oxidation (Eqs. 3 and 4). 

A particularly broad potential for application in syngas reactions is shown by ruthenium carbonyl clusters. Iodide promoters seem to favor ethylene glycol (155,156) the formation of [HRu3(CO),i]" and [Ru(CO)3l3]" was observed under the catalytic conditions. These species possibly have a synergistic effect on the catalytic process. Imidazole promoters have been found to increase the catalytic activity for both methanol and ethylene glycol formation (158-160). Quaternary phosphonium salt melts have been used as solvents in these cases the anion [HRu3(CO)i,] was detected in the mixture (169). Cobalt iodide as cocatalyst in molten [PBu4]Br directs the catalytic synthesis toward acetic acid (163). With... 

---