Vinyl acetate palladium catalysts

Although there have been reports of thiophene vinylation using palladium catalysts, the requirement of acetic acid in the reaction mixture precludes the inclusion of many functional groups. In order to mitigate this, Satoh and Miura investigated... 

Vinyl acetate reacts with the alkenyl triflate 65 at the /3-carbon to give the 1-acetoxy-1,3-diene 66[68]. However, the reaction of vinyl acetate with 5-iodo-pyrimidine affords 5-vinylpyrimidine with elimination of the acetoxy group[69]. Also stilbene (67) was obtained by the reaction of an excess of vinyl acetate with iodobenzene when interlamellar montmorillonite ethylsilyl-diphenylphosphine (L) palladium chloride was used as an active catalyst[70]. Commonly used PdCl2(Ph3P)2 does not give stilbene. 

Olefins add anhydrous acetic acid to give esters, usually of secondary or tertiary alcohols propjiene [115-07-1] yields isopropyl acetate [108-21-4], isobutjiene [115-11-7] gives tert-huty acetate [540-88-5]. Minute amounts of water inhibit the reaction. Unsaturated esters can be prepared by a combined oxidative esterification over a platinum group metal catalyst. Eor example, ethylene-air-acetic acid passed over a palladium—Hthium acetate catalyst yields vinyl acetate. 

Vinyl acetate (ethenyl acetate) is produced in the vapor-phase reaction at 180—200°C of acetylene and acetic acid over a cadmium, 2inc, or mercury acetate catalyst. However, the palladium-cataly2ed reaction of ethylene and acetic acid has displaced most of the commercial acetylene-based units (see Acetylene-DERIVED chemicals Vinyl polymers). Current production is dependent on the use of low cost by-product acetylene from ethylene plants or from low cost hydrocarbon feeds. 

Most of the vinyl acetate produced in the United States is made by the vapor-phase ethylene process. In this process, a vapor-phase mixture of ethylene, acetic acid, and oxygen is passed at elevated temperature and pressures over a fixed-bed catalyst consisting of supported palladium (85). Less than 70% oxygen, acetic acid, and ethylene conversion is realized per pass. Therefore, these components have to be recovered and returned to the reaction zone. The vinyl acetate yield using this process is typically in the 91—95% range (86). Vinyl acetate can be manufactured also from acetylene, acetaldehyde, and the hquid-phase ethylene process (see Vinyl polymers). 

A one-stage process for producing vinyl acetate directly from ethylene has also been disclosed. In this process ethylene is passed through a substantially anhydrous suspension or solution of acetic acid containing cupric chloride and copper or sodium acetate together with a palladium catalyst to yield vinyl acetate. 

Vinyl acetate was originally produced hy the reaction of acetylene and acetic acid in the presence of mercury(II) acetate. Currently, it is produced hy the catalytic oxidation of ethylene with oxygen, with acetic acid as a reactant and palladium as the catalyst ... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

The situation has now changed and currently an area of considerable research interest is in heterogenizing homogeneous catalysts. One such instance is to be found in the ethylene based manufacture of vinyl acetate (11). A homogeneous catalytic process based on palladium and copper salts was first devised, but corrosion problems were made much less serious in a heterogeneous system based on the same chemical principles. 

The commercial process for the production of vinyl acetate monomer (VAM) has evolved over the years. In the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate carbon-supported catalyst. This chemistry and process eventually gave way in the late 1960s to a more economically favorable gas-phase conversion of ethylene and acetic acid over a palladium-based silica-supported catalyst. Today, most of the world s vinyl acetate is derived from the ethylene-based process. The end uses of vinyl acetate are diverse and range from die protective laminate film used in automotive safety glass to polymer-based paints and adhesives. 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

In order to probe the influence of Au and KOAc on the vinyl acetate synthesis chemistry, four different catalysts were synthesized. All of these catalysts were prepared in a manner exemplified in prior patent technology [Bissot, 1977], and each contained the same palladium loading in an egg-shell layer on the surface of a spherical silica support. The palladium content in the catalyst was easily controlled by adjusting the solution strength of palladium chloride (PdClj) added to the porous silica beads prior to its precipitation onto the support by reaction with sodium metasilicate (Na SiOj). The other two catalyst components (Au and KOAc) were either present or absent in order to complete the independent evaluation of their effect on the process chemistry, e.g., (1) Pd-i-Au-hKOAc, (2) Pd-i-KOAc, (3) Pd-hAu, and (4) Pd only. 

Figure 3. Transient response of 1,2 C-vinyl acetate (MW=88) obtained from a typical pump-probe experiment with the palladium-based catalysts and 1,2 C-ethylene.

Bayer-Hoechst A gas-phase process for making vinyl acetate from ethylene and acetic acid, using a supported palladium catalyst. Developed jointly by Bayer and Hoechst. In 1991, nearly 2 million tonnes per year of vinyl acetate were made by this process. 

Hoechst-Uhde (2) A variation of the Wacker process, which makes vinyl acetate from ethylene and acetic acid. The catalyst is an aqueous solution of palladium and copper chlorides. 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Ethanal is only used as an intermediate to acetic acid and its derivatives and in the near future production of ethanal will be replaced by other routes based on methanol and syn-gas to give acetic acid and acetic anhydride. Vinyl acetate can also be made via syn-gas routes, but the major producer in Europe employs a direct Wacker route with a heterogeneous palladium catalyst (former Hoechst ethene, acetic acid -from syn-gas-, and oxygen). 

The catalyst is generally a palladium compound promoted with a trivalent amine or phosphine in the presence of methyl iodide as described earlier. Systems proven to bias acetaldehyde are utilized, of course (e.g. see Table I, run 12). A yield of 85% acetaldehyde from methyl acetate is typical by this method. It can be utilized in stoichiometric addition to easily prepared acetic anhydride resulting in EDA formation. When considering that the "boiling pot" reaction by-products are recyclable acetic acid, acetic anhydride and small amounts of EDA, the yield to vinyl acetate related products is 95%. 

The integration of a catalyzed kinetic enantiomer resolution and concurrent racemization is known as a dynamic kinetic resolution (DKR). This asymmetric transformation can provide a theoretical 100% yield without any requirement for enantiomer separation. Enzymes have been used most commonly as the resolving catalysts and precious metals as the racemizing catalysts. Most examples involve racemic secondary alcohols, but an increasing number of chiral amine enzyme DKRs are being reported. Reetz, in 1996, first reported the DKR of rac-2-methylbenzylamine using Candida antarctica lipase B and vinyl acetate with palladium on carbon as the racemization catalyst [20]. The reaction was carried out at 50°C over 8 days to give the (S)-amide in 99% ee and 64% yield. Rather surpris-... 

Vinyl acetate is produced by the oxidation of ethylene and acetic acid (4,5). Catalysts for the gas phase oxidation are made from palladium compounds with additional metal compounds on a porous support (6). Catalysts, preferably coated catalysts, can be used for many heterogeneously catalyzed reactions such as hydrogenations and oxidations. 

In the process jointly developed by Bayer and Hoechst,933-935 a palladium-gold-on-silica or alumina catalyst impregnated with KOAc is used. A mixture of ethylene, acetic acid, and oxygen is converted at 150-170°C and about 5-10 atm to produce vinyl acetate with about 91-92% selectivity at about 10% conversion in a highly exothermic reaction. The only major byproduct is C02. KOAc requires continuous replenishment. A similar process was independently developed by U.S.I. Chemicals.932 936... 

The gas-phase process, successfully commercialized independently by Bayer and USI,417 involves passing a mixture of ethylene, acetic acid and oxygen over a supported palladium catalyst contained in a multitubular reactor at 150 °C and about 5-10 atm pressure. The overall yield in vinyl acetate is about 92%, and the major by-product is C02. The catalyst consists of a palladium salt (e.g. Na2PdCl4) deposited on silica (or alumina) in the presence of a cocatalyst (e.g. HAuC14), reduced and impregnated with potassium acetate before use.384,418 The lifetime of the catalyst is about 2... 

Essentially the same substituents as listed above may be present in the alkene being substituted, with the possible exception of chloro, alkoxy and acetoxy groups on vinyl or allyl carbons. These groups, especially chloro, may be lost or partially lost with palladium when the final elimination step occurs. For example, vinyl acetate, iodobenzene and triethylamine with a palladium acetate-triphenylphosphine catalyst at 100 C form mainly (E)-stilbene, presumably via phenylation of styrene formed in the first arylation step (equation 21 ).6 ... 

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