Oxidative acetoxylation

Other large-volume esters are vinyl acetate [108-05-4] (VAM, 1.15 x 10 t/yr), methyl methacrylate [80-62-6] (MMA, 0.54 x 10 t/yr), and dioctyl phthalate [117-81-7] (DOP, 0.14 x 10 t/yr). VAM (see Vinyl polymers) is produced for the most part by the vapor-phase oxidative acetoxylation of ethylene. MMA (see Methacrylic polymers) and DOP (see Phthalic acids) are produced by direct esterification techniques involving methacryHc acid and phthaHc anhydride, respectively. 

When the initial compound was irani-stilbene, the nnconsnmmated part was recovered with no change in configuration. When di-stilbene was employed as the initial reactant, the recovered olefin was a mixtnre of trans and cis isomers. Hence, the trans confignration is more favorable for oxidative acetoxylation than the cis confignration. In accordance with this conclnsion, the mechanism shown in Scheme 2.30 is proposed. 

Pd/Cu-zeolites are also catalysts for the oxidative acetoxylation of propylene to allylacetate [32-39]. The best results are obtained on a catalyst which is pretreated with an alkali solution to neutralize the acidic centres and containing Pd and Cu in an atomic ratio of 1.1 [37]. The alkali treatment suppresses the acid catalyzed addition of acetic acid to propylene, resulting in the formation of isopropyl acetate, which is observed over non-neutralized Na- and H-Y, as well as over unreduced and reduced Pd/Cu-NaY. Experiments with... 

Scheme 14 Possible outcomes for the palladium-catalyzed oxidative acetoxylation of alkenes...

The oxidation of 1-alkenes usually gives 2-acetoxy-l-alkenes.571,572 Oxidative acetoxylation of propylene with Pd(OAc)2 may yield allylic or vinylic acetates depending on reaction conditions573 (see Section 9.2.6). 

The positional reactivity of dibenzofuran in electrophilic substitution reactions depends on the electrophile. Reaction occurs mostly at the 2- and 3-positions but the ratio of the two products varies (91JOC4671). The reaction of cyanogen bromide catalyzed by aluminum chloride gives an 80% yield of the 2-substituted product together with 15% of the 3-cyano-derivative (92ACS312). Oxidative acetoxylation of dibenzofuran occurs predominantly at the 3-position ( 60%) together with attack at the 1-position ( 30%) (92ACS802). In this latter reaction, the attack by acetate is on the dibenzofuranium radical cation. 

Soon after the invention of the Wacker process the formation of vinyl acetate by the oxidative acetoxylation of ethylene using Pd(OAc)2 was discovered by Moiseev [16], and the industrial production of vinyl acetate based on this reation was developed. At present, vinyl acetate is produced commercially by a gas-phase reaction of ethylene, acetic acid and O using Pd catalyst supported on alumina or silica (eq. 1.11). 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

Oxidative nucleophilic substitution is, however, a more versatile technique and a much better choice for target-oriented synthesis (Sections 15.1.1 and 15.1.2.2). In 1950, Wessely and co-workers examined the use of lead tetraacetate (LTA) in acetic acid to determine the structure of phenols and, in doing so, they developed their oxidative acetoxylation reaction, referred to herein as Wessely oxidation (Figure 13) [68-76]. If both an ortho- and a para-position are available to accommodate the entry of the acetoxy nucleophile, ortho products often predominate even when the ortho position is already occupied by a resident alkyl (e.g. 40 —> 41a/b) or allcoxy group (Figure 13) [69, 74]. 

The convenient generation of bicyclo[2.2.2]octenones through the use of ortho-quinol derivatives in Diels-Alder reactions recently inspired Wood and co-workers in their studies toward the total synthesis of CP-263,114 (110) [148]. They relied on the Wessely-Yates tandem oxidative acetoxylation/intramolecular Diels-Alder sequence to build bicyclo[2.2.2]octenones such as 114 en route to advanced isotwistane intermediates such as 111b, which could eventually be fragmented to furnish the carbocyclic core of 110 (i.e. 111a —> 110, Figure 29) [149-153],... 

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

In the BASF process the 1,2-diacetate is the substrate for the hydroformylation step. It can be prepared either directly via oxidative acetoxylation of butadiene using a selenium catalyst or via PtCl4-catalyzed isomerization of the 1,4-diacetate (see above). The latter reaction affords the 1,2-diacetate in 95% yield. The hydroformylation step is carried out with a rhodium catalyst without phosphine ligands since the branched aldehyde is the desired product (phosphine ligands promote the formation of linear aldehydes). Relatively high pressures and temperatures are used and the desired branched aldehyde predominates. The product mixture is then treated with sodium acetate in acetic acid to effect selective elimination of acetic acid from the branched aldehyde, giving the desired C5 aldehyde. 

Table 1 Isomer Ratios for Oxidative Acetoxylation or Aromatic Hydrocarbons...

Oxidative acetoxylation provides a direct access from alkenes to alkenyl esters the alkene molecule undergoes replacement of an H atom by an acetate (or generally OCOR) group in its vinylic (v), allylic (a), or homoallylic (h) position according to Scheme 1, where Ox is an oxidant such as O2, Cu p-benzoquinone, and Red a reduced form of Ox such as H2O, Cu hydroquinone. A typical example is the Pd-catalyzed co-oxidation of ethylene and acetic acid to vinyl acetate (eq. (D). 

The oxidative acetoxylation of ethylene was discovered while studying the reactivity of palladium(II) 7r-complexes [1]. It was found that 7r-ethyl-ene-palladium chloride, the so-called Kharash complex jt-C- 4 PdCl2)2 [3], is readily decomposed by hydroxyl-containing reagents such as water or alcohols, yielding Pd metal and acetaldehyde or acetyl, respectively (eq. (2)) [1, 4]. 

When Pd compounds (PdfOAc) ", Pd2(OAc)i , or Pd3(OAc)e) are used as starting material, even small additions of water (1-3%) to the NaOAc/AcOH solvent give rise to a great deal of acetaldehyde instead of vinyl acetate [11-13]. In contrast to this, the Pd metal catalysts (e. g., supported Pd or Pd black, prepared by H2 reduction of Pd" complexes in combination with NaOAc) provide vinyl ester from alkene and AcOH with high selectivity, regardless of the water content up to 10% [11, 14, 15]. Further differences in the selectivity of reaction (1) with Pd" and Pd° catalysts were found for the oxidative acetoxylation of higher alkenes, viz., propylene, 1-hexene, and cyclohexene [7]. All these facts apparently implied that the alkene activation came from two different origins one from Pd" and another from Pd metal or, more exactly, low-valent Pd clusters formed upon Pd" reduction with H2. 

The outer-sphere OAc anions can be replaced by other anions. For instance, the and PF anions readily substitute for OAc anions in an aqueous solution containing KPFft, affording the giant cluster with the idealized formula [Pdsei LeoOeoKPFeleo [Ik 16, 17]. The Pd-561 clusters exhibit a high catalytic activity in alkene acetoxylation in an AcOH solution under mild conditions (20-60 °C at 0.1 MPa). Besides reaction (1), the clusters provide the oxidative acetoxylation of propylene to allyl acetate (eq. (6)) or of toluene to benzyl acetate (eq. (7)). 

The selectivity of these reactions with respect to the products of oxidative acetoxylation is at least 95-98 %. No decrease in the selectivity was found even in... 

The Michaelis-Menten character of the kinetics suggests that the formation of the reaction product is preceded by reversible coordination of the alkene, O2, and AcOH molecules by the cluster. The kinetic isotope effects give evidence that the mechanisms of oxidative acetoxylation (eq. (1)) catalyzed with Pd and low-valence Pd clusters are different. On the basis of kinetic data, including the H/D kinetic isotope effects [9], the reaction mechanism represented by Scheme 6 has been proposed for Pd-561-catalyzed reaction. 

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