Acetoxylation of olefins

The anodic acetoxylation of olefinic terpenes was used for the synthesis of new fragrances (Kuraray 38-42 )... 

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... 

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. 

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... 

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... 

Vicinal iodo carboxylates may also be prepared from the reaction of olefins either with iodine and potassium iodate in acetic acid/ or with N-iodosuccinimide and a carboxylic acid in chloroform. " A number of new procedures for effecting the hydroxylation or acyloxylation of olefins in a manner similar to the Prevost or Woodward-Prevost reactions include the following iodo acetoxylation with iodine and potassium chlorate in acetic acid followed by acetolysis with potassium acetate reaction with iV-bromoacetamide and silver acetate in acetic acid reaction with thallium(III) acetate in acetic acid and reaction with iodine tris(trifluoroacetate) in pentane. ... 

Pd-hydroquinone-mediated electrochemical 1,4-diacetoxylation of cyclohexa-1,3-diene (118), leading to 1,4-diacetoxycyclo-hex-2-ene (119), has been investigated (Scheme 46) [156]. Palladium-catalyzed indirect electrochemical monoacetoxylation of olefins has been attained in an MeCN/Ac0H-NaC104/Ac0Na/Pd(0Ac)2-Cu(OAc)2-(C) system. The acetoxylation of cyclohexene produces unsaturated esters with less current efficiency, giving a 1 1 mixture of allylic and vinylic products [118]. 

The cation-radicals of stilbene have been detected by ESR spectroscopy. These cation-radicals are accumulated and then consumed in the course of consecutive reactions. The stereoisomeric composition of the final products occurs to be constant and does not depend on the configuration of the initial substrate. Acetoxylation of the olefinic bond in cix-stilbene is almost one order of magni-tnde slower than in trany-stilbene. This kinetic feature deserves a special explanation, because cis-stilbene is less stable thermodynamically than irani-stilbene and should react faster. The products obtained are depicted in Scheme 2.29. 

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) ) ... 

Fig. 4.38 PdCI2// /,/ /-dimethylacetamide system for regioselective acetoxylation of terminal olefins.

Acetoxylation is a valuable method for the introduction of an OH group into organic compounds, which can be used for further syntheses. In Section 3.3.14.2 it has been mentioned that acetoxylation of higher and cyclic olefins with palladium salts, or catalyzed by palladium salts or metal, mostly leads to allylic derivatives. This also takes place in the catalytic acetoxylation of terpenic olefins [79, 80]. 

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. 

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]. 

The product from fluonnation of sodium acetate is acetyl hypofluorite [64], which IS isolated and characterized [65] The value of this reagent lies in its relative mildness, because it reacts cleanly with most olefins adding the elements of acetoxyl and fluorine [66] Tnfluoroacetyl hypofluorite adds cleanly only to benzylic or electron-rich double bonds... 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

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. 

The reactions are performed treating diarylteUurium dichlorides or aryltellurium trichlorides with excess olefin (5-10 mol equiv) in the presence of Pd(II) chloride-sodium acetate in refluxing acetic acid or acetonitrile. The yields are moderate to good with the dichlorides and poor with the trichlorides. Minor amounts of biaryls are the usual by-products beyond acetoxylated arylalkenes (formed by the addition of acetic acid to the olefins). 

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. 

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