1.4- diacetoxylation

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]. 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

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... 

The diacetoxylation works well with a number of cyclic and acyclic conjugated dienes and has been applied to the synthesis of natural products33,34. For example, the meso diacetate from 2,4-hexadiene was used for the enantiodivergent synthesis of the carpenter bee pheromone343. 

The 1,4-diacetoxylation was also extended to the use of other acyl groups than acetyl. Thus, an unsymmetrical 1,4-acetoxy-trifluoroacetoxylation of 1,3-dienes was developed by the use of added trifluoroacetic acid to the acetic acid used as the solvent330. With the use of acetone as the solvent with an added carboxylic acid a general diacyloxylation was obtained and, for example, the 1,4-dibenzoates of 2-cycloalkene-l,4-diols were prepared directly from the corresponding l,3-cycloalkadienes33d. 

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. 

Further development of this aerobic oxidation was done by utilizing a quinone containing cobalt tetraphenyl porphyrin47. This gives a more efficient electron transfer between quinone and porphyrin and results in a faster aerobic 1,4-diacetoxylation of the diene. The... 

When a palladium(II)-hydroquinone system is used as the mediator4 in the anodic oxidation of 1,3-cyclohexadiene in acetic acid, either trans- or cis- 1,4-diacetoxy-2-cyclohexene is formed with rather high selectivity, though the possible formation of 1,2-diacetoxylated compound is not discussed. 

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. 

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,... 

Scheme 103 Anodic side chain diacetoxylation of furan.

The anodic diacetoxylation of A7-acety-lindoles (or A7-acetylindoline) in acetic acid and in the presence of a base affords the corresponding 2,3-diacetoxyindoline (Scheme 104) [191]. A 2,3-diacetoxylation of A7-carbomethoxypiperidine is also observed [192]. 

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]. 

Olefins are diacetoxylated by benzenetellurinic anhydride in boiling acetic acid. ... 

Diacetoxylation of olefins with benzenetellurinic anhydride (typical procedure) To a solution of (PhTe0)20 (0.95 g, 2.1 mmol) in dry HOAc (15 mL) is added styrene (0.208 g, 2.0 mmol) in HOAc (4 mL) and 98% H2SO4 (0.020 g, 0.2 mmol) in HOAc (1 mL). The mixture is gently refluxed for 24 h, which turns red with deposition of a small amount of elemental tellurium. After removal of tellurium (0.071 g, 0.56 mmol) by fdtration, the solvent is evaporated, the residue extracted with ether and the ether extract dried (MgS04). Chromatography on Si02 gives the v/c-diacetate (0.363 g (82%)) and 1-phenylethyl acetate (0.029 g (9%)). 

Other methods achieving the iyn-diacetoxylation of olefins use the TeOj/LiBr/HOAc (i) or the TeCl4/LiOAc/HOAc ° (ii) systems. 

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. 

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). 

By submitting conjugated dienes to a treatment with the Te02/LiBr/H0Ac system, a mixture of 1,2- and cis- and fran -l,4-diacetoxylated addncts is formed. ... 

Diacetoxylation of 1,3-butadiene (typical procedure) To a mixture of TeOj (0.80 g, 5 mmol), LiBr (2.17 g, 25 mmol), AcOH (18 mL) and AC2O (2 mL) in a glass pressure bottle, chilled at -20°C, is added chilled 1,3-butadiene (1.35 g, 25 mmol). The resulting suspension is heated (oil bath) at 120-130°C for 20 h under magnetic stirring, producing a... 

The diacetoxylation of isoprene and 2,3-dimethyl-l,3-butadiene is performed similarly. 

The observation that the previously described diacetoxylation of olefins by means of tel-lurinic anhydrides produces quantitative yields of the corresponding ditellurides suggested the oxidative functionalization of olefins employing diphenyl diteUuride combined with an oxidizing agent instead of the tellurinic anhydride. ... 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

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). 

Diacetoxylation of 1,3-butadiene is a process that drew much attention since the product l,4-diacetoxy-2-butene may be converted to 1,4-butanediol and tetrahydro-furan by further transformations (see Section 9.5.2). The liquid-phase acetoxylation of 1,3-butadiene in a Wacker-type system yields isomeric 1,2- and cis- and trans-1,4-diacetoxybutenes ... 

Other conjugated cyclic dienes undergo a similar palladium-catalyzed stereoselective 1,4-diacetoxylation.578... 

Palladium-hydroquinone electrochemical 1,4-oxidation,579 aerobic oxidation via the Pd(II)-hydroquinone-metal macrocycle triple catalytic system,529 and Mn02 as a reoxidant578 were also applied in the 1,4-diacetoxylation of 1,3-dienes. 

A somewhat similar hydrogenation problem arose in a different approach to 1,4-butanediol and tetrahydrofuran.345 In the process developed by Mitsubishi, 1,3-butadiene first undergoes Pd-catalyzed diacetoxylation to yield 1,4-diacetoxy-2-butene. To avoid the further transformation of the diol as in the abovementioned process, l,4-diacetoxy-2-butene is directly hydrogenated in the liquid phase (60°C, 50 atm) on traditional hydrogenation catalysts to produce 1,4-diacetoxybutane in 98% yield, which is then hydrolyzed to 1,4-butanediol. 

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