Olefins oxidative acetoxylation

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. 

Acetoxylations (oxyacylations) have to be seen in context with olefin oxidation to carbonyl compounds (Wacker process, Section 2.4.1). With the lowest olefin, ethylene, acetaldehyde is formed. In water-free acetic acid no reaction takes place. Only in the presence of alkali acetates - the acetate ion shows higher nu-cleophilicity than acetic acid - ethylene reacts with palladium salts (eq. (1)) to give vinyl acetate, the expected product, as first reported by Moiseev et al. [1]. Stem and Spector [2] independently used [HP04] as base in a mixture of isooctane and acetic acid. This reaction could be exploited for a commercial process to produce vinyl acetate and closed the last gap replacing acetylene by the cheaper ethylene, a petrochemical feed material, for the production of large-tonnage chemical intermediates. 

Quite analogously to the olefin oxidation in aqueous medium, acetoxylation of olefins can also be carried out catalytically by addition of oxidants such as ben-zoquinone [1], cupric chloride, and cupric acetate (a survey of the patent literature has been given by Krekeler and Schmitz [19] and Miller [20]) which oxidize the metallic palladium to the active oxidation state Pd (eq. (2)). Cuprous chloride is reoxidized by oxygen (eq. (3)) and the overall reaction according to eq. (4) becomes catalytic. 

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... 

As a major step in the evaluation of the above mentioned high-throughput tools and techniques, a scale-down of different types of catalysts for several applications was performed. For that purpose, two well established commercial catalysts, one of the mixed metal oxide type for selective olefin oxidation and one impregnated catalyst for ethylene acetoxylation to vinyl acetate monomer (VAM), respectively, were prepared in the small-scale and their catalytic performance was compared. As shown in Fig. 1 with the selective oxidation catalyst, the scale-down of this catalyst was successful, since both, the commercial and the high-throughput prepared catalyst are showing identical performances. Regarding the calcination procedure one can point out, that only if this step is carried out in the 5-fold rotary kiln, equal catalysts were obtained. 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use is made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-x-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast... 

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. 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

The Chemistry of Ring B.—The isolation of taxodione (49), a diterpenoid tumour inhibitor, has stimulated interest in the introduction of oxygen functions at C-6. Taxodione itself has been synthesized from podocarpic acid. The latter was converted to ferruginol benzoate (46). The C-11 hydroxy-group was introduced via the diazo-compound and the product acetoxylated at C-7 to afford (47) this was converted into the A -olefin (48), which was epoxidized and isomerized to the C-6 ketone. The product was then oxidized to give the quinone-methide of taxodione (49). 

The anodic dimethoxylation of simple cyclic olefins, such as cyclohexenes, results in the predominant formation of ra/w-dimethoxycyclohexanes [303,305]. However, Barba and CO workers [306] reported that the stereochemistry of l,2-dimethoxy-l,2-dihydroace-naphthene derived from acenaphthene is drastically influenced by the anode material. Palasz and Utley [307] have reported that jV-acetylpiperidines are methoxylated via the enamide intermediate species formed by the oxidation. 1,3-Dienes are also 1,4-dimethoxylated in low stereoselectivity [293]. The anodic methoxylation of or-pinene proceeds similarly to the acetoxylation, resulting in the formation of two sets of stereoisomeric products [308]. 

Acetoxylation of olefins according to eq. (1) is an oxidative reaction which can be widely applied. However, it does not occur in such a distinct manner as olefin... 

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]. 

Acetoxylation of hydrocarbons. I n a paper of 1923Dimroth reported preliminary observations on the oxidation of aromatic hydrocarbons and olefins with lead tetraacetate. Toluene, he found, affords benzyl acetate in very low yield oxidation of diphenylmethane and triphenylmethane proceeded more readily but offered nothing of preparative promise. Dimroth observed also that anethole reacts to give in small yield a product of addition of two acetoxyl groups to the olefin linkage. 

Acetoxylation of olefins. The oxidation of olefins with selenium dioxide in acetic acid results in allylic oxidation (1, 994-996 2, 360-361) however, if the reaction is catalyzed by sulfuric acid, the main product results from acetoxylation. Thus oxidation of cyclohexene under these conditions (110°, autoclave) gives 1,2-cyclohexanediol diacetate as a mixture of cis- (55%) and trans- (45%) isomers in 32% yield. Similarly, oxidation of 1-hexene gives 1,2-hexanediol... 

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. 

As with the allylic oxidation of olefins (see above) the giant Pd-561 cluster was also found to catalyze benzylic acetoxylation under mild conditions in acetic acid [10]. 

When the use of nucleophiles other than water in the presence of terminal alkenes under Pd(II) catalysis Wacker-type products frequently predominate. Use of White s electrophilic ftts-sulfinyl Pd(ll) acetate catalyst with terminal alkenes in the presence of acetic acid enabled the preparation of terminal acetoxylated olefins via allylic C—H oxidation and subsequent regioselective nucleophilic trapping of a Pd 7i-allyl complex (Table 3.3). The substrates screened afforded the desired end products with high levels of fi-selectivity and in moderate yields with excellent linear branched ratios. Recent updates to this method include the use of A-tosylcarbamates as nucleophiles. ... 

In fact, Wacker type oxidations (largely applied for aldehyde synthesis, acetoxylation reactions) can be considered as an intra or, more probably according to the recent literature, as an out-of-sphere nucleophilic attack on a palladium-olefin 7r-complex. 

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