Vinyl acetate oxidative coupling

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]. 

Rhodium complexes catalyze the oxidative coupling of benzene with ethene to produce styrene directly.45,45a,45b Using Rh(ppy)2(OAc) (ppyH = 2-phenylpyridine), the reaction of benzene with ethene in the presence of 02 and Cu(OAc)2 in benzene and acetic acid at 180 °C gives styrene and vinyl acetate in 77% and 23% selectivities, respectively. 

The formation of vinyl acetate via the oxidative coupling of ethylene and acetic acid was among the earliest Pd-catalyzed reactions developed (Sect. 2) [19,20]. Subsequent study of this reaction with higher olefins revealed that, in addition to C-2 acetoxylation, allylic acetoxylation occurs to generate products with the acetoxy group at the C-1 and C-3 positions (Scheme 14). The synthetic utihty of these products imderhes the substantial historical interest in these reactions, and both BQ and dioxygen have been used as oxidants. 

The industrial synthesis of vinyl acetate [14] via palladium-catalyzed oxidative coupling of acetic acid and ethene using direct 02 reoxidation has already been mentioned (Scheme 3, d). Some NaOAc is required in the reaction medium, and catalysis by Pd clusters, as alternative to Pd(II) salts, was proposed to proceed with altered reaction characteristics [14]. Similarly, the alkenyl ester 37 (Table 5) containing an isolated vinyl group yields the expected enol acetate 38 [55] whereas allylphenol 39 cyclizes to benzofuran 40 with double bond isomerization [56]. 

When PdCl2 —LiOAc was used instead of Pd(OAc)2, vinyl acetate was converted to a mixture of acetaldehyde and acetic anhydride [see Eq. (370)]. No oxidative coupling took place under these conditions.566 ... 

Isoprene also underwent the intermolecular coupling with vinyl acetate (Scheme 4.47) [95[. In the presence of 0.7 mol% 17, isoprene and vinyl acetate were heated at 100 °C in MeOH for 14 h to give dienes 132 and 133 with a ratio of 96 4. The present selectivity was attributed to the regioselective oxidative cyclization of the more substituted alkene moiety of isoprene and vinyl acetate giving rise to the ruthenacyclopentane intermediate 134. 

Oxidative coupling of specific alkenes such as styrene derivatives and vinyl acetate to 1,3-diene derivatives can also be achieved in the presence of palladium catalysts. " This coupling essentially occurs head to head , i.e. the C—C bond formation involves the least substituted carbon atoms of the double bonds (equation 188). ... 

Biphenyls are also by-products of acetoxylation of aromatics [92]. Their formation is favored with a palladium metal catalyst in the absence of oxidants [93-95]. Vinyl acetate undergoes oxidative coupling under similar conditions to form 1,4-diacetoxy-1,3-butadiene [99], and aromatics and heterocycles can substitute an olefinic H-atom [100] according to eq. (28) (with X = H, CN, AcO, EtO) [100-102]. 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

Pd(II) can be expressed by the following general schemes. As a whole, two hydrogens are abstracted from two substrates A-H and B-H, generating Pd(0) and the product A-B. This reaction is stoichiometric with Pd(II), but the reaction becomes catalytic when Pd(0) is oxidized in situ to Pd(II) with appropriate oxidants (OX), and the whole reaction can be summarized by a third equation. For example, formation of vinyl acetate from ethylene and oxidative coupling of benzene can be understood formally as dehydrogenation reactions. These oxidative reactions using Pd(ll) are considered in Chapter 2. 

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