Benzoquinones acetoxylation

Allylic acetoxylation.2 Pd(OAc)2 in HOAc can effect allylic acetoxylation of alkenes, probably via a TT-allylpalladium complex, and only a catalytic amount is required in the presence of a cooxidant such as benzoquinone-Mn02. The reaction is not useful in the case of simple alkenes because of lack of discrimination between the two allylic positions, but this acetoxylation can be regioselective in the case of alicyclic alkenes. 

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... 

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

Allylic acetoxylation of cyclohexene can be selectively effected by palladium acetate in the presence of Mn02 and p-benzoquinone (bq) (equation 298).640... 

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). 

On controlled current electrolysis (200 mA), 2,6-di(terf-butyl)-p-cresol (69) underwent nucleophilic hydroxylation, methoxylation or acetoxylation depending on the solvent system used (IM H2O, MeOH or 0.2 M NaOAc-AcOH in MeCN) to afford the corresponding cyclohexa-2,5-dienones 70, 71 and 72 in 86, 88 and 91% yields, respectively. In the case of 2,4,6-tri(tert-butyl)phenol (73), 2,6-di(tert-butyl)-p-benzoquinone (74) was produced in 96% yield through cyclohexa-2,5-dienone 75 , as shown in Scheme 13. 

Palladium-catalyzed allylic acetoxylation has also been applied to terpene substrates [17], e. g. (/ )-limonene (Eq. 2), albeit using stoichiometric amounts of Cu(II) or benzoquinone as the oxidant. 

Palladium-catalyzed aUyUc C-H acetoxylation (acyloxylation) of alkenes is one of the synthetically most established C-H functionabzation methods [57-62]. These reactions are conducted under oxidative reaction conditions. In the most commonly used approach, the reaction proceeds via a Pd(II)/Pd(0) catalytic cycle and benzoquinone (BQ) is used for reoxidation of Pd(0) and activation of the nudeophihc (acetate) attack [61, 62],... 

By starting from the diene, a 1,4-bis acetoxylation can be carried out to give cis or trans product, according to the exact conditions. The intermediates 14.12a and 14.12b are invoked to explain these products. The benzoquinone serves to reoxidize Pd(0) and make the reaction catalytic (Eq. 14.83) ... 

The cyclohexenyl acetate arises either via acetoxypalladation or 7r-allylpalladium intermediate, or via both, depending on the conditions used. Using 1,2-dideuteriocyclo-hexene, the allyhc acetoxylation of cyclohexene by a catalyst system of Pd(0Ac)2-Mn02-benzoquinone has been demonstrated to proceed via Tr-aUylpaUadium intermediate (Scheme... 

Allylic C-H Bond Activation and Allylic Oxidations. A new system has been developed for the allylic acetoxylation of alkenes. This uses Pd(OAc)2 as catalyst, 1,4-benzoquinone (BQ) as a co-catalyst/electron-transfer mediator, hydrogen peroxide as the stoichiometric oxidant and acetic acid as the solvent (eq 73). ... 

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