Diacetoxylation reaction

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

The acetoxytellurium tribromides are converted into the diacetates by the same treatment employed for the diacetoxylation reaction (HOAc, 120°C). The formation of an overall syn- or anft-addnct depends on the competition between a rearward attack by the acetate ion at the tellnrinm atom (as in the case of cyclohexene and 2-butenes), and a front attack by the neighbonring acetate moiety (as in cyclopentenes, where the almost the planarity of the five-membered ring makes the conformation of the acetoxytellnrinm tribromide susceptible to frontal attack). 

This diacetoxylation reaction is general for cyclic and acyclic dienes. In the latter reaction, the new double bond has the (E)-configuration. ... 

The reaction conditions are similar to those employed in the diacetoxylation reaction, with the difference that the halide concentration (usually CI ) has been increased. Thus, palladium-catalyzed oxidation of 1,3-dienes with / -benzoquinone in the presence of lithium chloride and lithium acetate gives l-acetoxy-4-chloroalk-2-enes [78]. For example cyclohexa-1,3-diene and cyclohepta-1,3-diene afforded the corresponding chloroacetates 58a and 58b in good yield and >98% cis selectivity [Eq.(41)]. Cycloocta-1,3-diene gave a 61% yield of acetoxychlorination product (>98% cis), but in this case a 3 1 mixture of 1,4-and 1,2-addition products was formed. A number of substituted cyclic conjugated dienes work well, and, in all cases tried, the reaction proceeds with >97-98% cis selectively [52,78-81]. 

In the catalytic cycle of the palladium-benzoquinone-based 1,4-oxidation of 1,3-dienes, benzoquinone is reduced to hydroquinone. The diacetoxylation reaction is conveniently performed with p-benzoquinone in catalytic amounts, employing Mn02 as the stoichiometric oxidant. In this process, the hydroquinone formed in each cycle (cf. Scheme 8-6) is reoxidized to p-benzoquinone by MnO,. For example, the catalytic reaction of cyclohexa-1,3-diene using catalytic amounts of both Pd(OAc)2 and p-benzoquinone with stoichiometric amounts of Mn02 in acetic acid in the presence of lithium acetate afforded a 93% yield of rranj-l,4-diacetoxycyclohex-2-ene (>91% tram) [51b]. The corresponding reaction in the presence of lithium chloride gave ci5-l,4-diacetoxycyclohex-2-ene in 79% yield (>96% cis). 

A short synthesis of Conduritol C was achieved utilizing the diacetoxylation reaction (Scheme 11.21). In this way, racemic Conduritol C was obtained, which was transformed via enzymatic kinetic resolution into enantiomerically pure (—)-Conduritol C (49%, >99.5% ee) and (+)-Conduritol C (48%, >99.5% ee) [86]. 

The di-diacetoxylation reaction together with subsequent enzymatic resolution of the corresponding mero-diacetate constitutes a rapid and efficient route to enantiomerically... 

Difunctionalization with similar or different nucleophiles has wide synthetic applications. The oxidative diacetoxylation of butadiene with Pd(OAc)i affords 1,4-diacetoxy-2-butene (344) and l,2-diacetoxy-3-butene (345). The latter can be isomerized to the former. An industrial process has been developed based on this reaction. The commercial process for l,4-diacetoxy-2-butene (344) has been developed using the supported Pd catalyst containing Te in AcOH. 1,4-Butanedioi and THF are produced commercially from 1,4-diacetoxy-2-butene (344)[302]. 

The diacetoxylation of E,E)- and ( ,Z)-2.4-hexadiene (351 and 353) is stereospecific, and 2,5-dimethylfurans (352 and 354) of different stereochemistry have been prepared from the isomers. Two different carboxylates are introduced with high cis selectivity by the reaction of 1,3-cyclohexadiene and... 

In 1971, Brown and Davidson reported that 1,3-cyclohexadiene undergoes a palladium-catalyzed 1,4-diacetoxylation of unspecified stereochemistry28. The oxidant employed was p-benzoquinone. They were uncertain about the mechanism at the time but later work has shown that the reaction proceeds via a (jr-allyl)palladium intermediate and subsequent nucleophilic attack by acetate6,7. 

In 1981, a stereoselective palladium-catalyzed 1,4-diacetoxylation of 1,3-dienes with p-benzoquinone (BQ) as the oxidant was reported33. It was found that chloride ions can be used as a stereochemical switch. Thus, in the absence of chloride ions trans diacetoxylation takes place, whereas in the presence of a catalytic amount of chloride ion (as added LiCl) a cis diacetoxylation takes place (Scheme 4). In both cases the reaction is highly 1,4-regioselective. The explanation for... 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

In this reaction, the redox couple hydroquinone/benzoquinone promotes the second redox couple Pd(0), Pd(II) and Pd(II) causes the oxidative transformation of the diene to the 1,4-diacetoxylated compound. The most remarkable characteristic of this reaction... 

Stereo- and regioselective palladium-catalyzed oxidation of 1,3-dienes in acetic acid to give l,4-diacetoxy-2-alkenes has been accomplished using Mn02 and catalytic amounts of p-benzoquinone (BQ)11. The reaction can be made to take place with cis- or trans-1,4-diacetoxylation across the diene in cyclic systems as shown in equation 6. 

As shown for the preceding method employing phenyltellurinic anhydride, the diacetoxylation prefers a yyn-stereochemistry, especially for cyclic aUcenes and dy-linear aUcenes, whereas for frany-alkenes the preference for the yyn-stereochemistry is decreased. In accordance with the mechanism proposed in the case of the tellurinic anhydride, the reaction can be rationalized as involving the intermediacy of a frany-adduct followed by an SN2-type detellurative acetolysis. 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

Palladium-catalyzed 1,4-diacetoxylation of butadiene is a useful reaction of commercial interest which provides an interesting alternative for the synthesis of butanediol and tetrahydrofuran, previously based on acetylene feedstocks (equation 165). 

In 1981, a stereoselective palladium-catalyzed 1,4-diacetoxylation of conjugated dienes was reported [51-53]. By ligand control it was possible to direct the reaction to either 1,4-trans- or 1,4-cw-diacetoxylation (Scheme 8-7). 

One example of such a reaction was reported in 1971 by Brown and Davidson [49], who studied oxidation reactions of cyclohexa-1,3-diene and cyclhexa-1,4-diene. They observed that reaction of cyclohexa-1,3-diene with p-benzoquinone in acetic acid in the presence of catalytic amounts of Pd(OAc)2 produced l,4-diacetoxycyclohex-2-ene of unknown stereochemistry. At the time, they were uncertain about the mechanism and suggested a possible involvement of radicals. A related palladium-catalyzed 1,4-diacetoxylation of... 

In this reaction a useful stereocontrol was obtained by the use of LiCl as a catalytic additive. Without added LiCl a l,4-fr(ins acetoxylactonization took place, while in the presence of a catalytic amount of LiCl a 1,4-ds acetoxylactonization occurred. This is in analogy with the diacetoxylation of conjugated dienes discussed above where chloride ions block the coordination of acetate to palladium. At an increased chloride ion concentration (as added LiCl) a highly regio- and stereoselective 1,4-ds chlorolactonization took place. The presence of the 7r-allylpalladium intermediate 40 was demonstrated by its isolation and stereochemical assignment. The tram stereochemistry between palladium and oxygen in the 7r-allylpalladium complex 40 was established by the use of reporter ligands and NOE measurements . 

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