Palladium allylic acetoxylation

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... 

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

ALLYLIC ACETOXYLATION Palladium(II) chloride-Silver(I) acetate. 

The asymmetric allylic acetoxylation of cycloalkenes has also been reported. In this case, the catalyst is a bimetallic palladium(II) complex bearing a chiral bisox-azoline or a chiral diphosphine (DIOP). The reaction is performed in acetic acid/ sodium acetate under oxygen atmosphere at room temperature. Under these conditions, acetoxylation products of cyclohexene and cydopentene are obtained with 55 % and 78 % ee, respectively, albeit in low yields [39a]. 

ALLYLIC ACETOXYLATION Bromine-Silver acetate. Palladium(II) trifluoroacetate. 

Since the first example of catalytic reaction of palladium-catalyzed allylic acetoxylation was reported by Haszeldine and coworkers in 1966 [10], cyclohexene has been a benchmark substrate for this kind of reactions under different oxidative conditions, which are well documented in reviews and books [11, 12]. The proposed mechanism for allylic acetoxylation of cyclohexene is illustrated in... 

Figure 3. Catalytic cycle for allylic acetoxylation over palladium metal.

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. 

This one-step transformation of an alkene to an allylic acetate compares well with other methods of preparation such as hydride reduction of a, 8-unsaturated carbonyl compounds followed by esterification. The scope and limitations of the reaction have been investigated. The allylic acetoxylation proceeds via a TT-allylpalladium intermediate, and as a result, substituted and linear alkenes generally give several isomeric allylic acetates. With oxygen nucleophiles the reaction is quite general, and reactants and products are stable towards the reaction conditions. This is normally not yet the case with nitrogen nucleophiles, although one intramolecular palladium-catalyzed allylic amination mechanistically related to allylic acetoxylation has been reported. ... 

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]. 

Acetoxylation of propene to allyl acetate can be performed in the liquid phase with high selectivity (98%) in acetic acid in the presence of catalytic amounts of palladium trifluoroacetate. The stability and activity of this catalyst can be considerably increased by adding copper (II) trifluoroacetate and sodium acetate as cocatalysts (100 °C, 15 bar, reaction time = 4 h, conversion = 70%, selectivity = 97%). Gas-phase procedures for the manufacture of allyl acetate are described in several patents and use conventional palladium catalysts deposited on alumina or silica, together with cocatalysts (Au, Fe, Bi, etc.) and sodium acetate. The activity and selectivity reported for these catalysts are very high (100-1000 g l-1 h-1, selectivity = 90-95% ).427 A similar procedure has been used for the synthesis of methallyl acetate from 2-methylpropene.428... 

Acetoxylation of toluene using a Pd(OAc)2-Sn(OAc)2-charcoal catalyst selectively produces benzyl acetate with high turnover numbers ( 100).373,434 The active catalyst presumably contains Pd—Sn bonds. Tin ligands are known to increase the 7r-acceptor ability of palladium, and may favor the coordination of the toluene in the form of a benzylic 7r-allyl complex (141) which is nucleophilically attacked by the acetate anion.435... 

In the first step of the process (Fig. 1), the acetoxylation of propylene is carried out in the gas phase, using solid catalyst containing palladium as the main catalyst at 160 to 180°C and 70 to 140 psi (0.49 to 0.98 MPa). The reactor effluents from the reactor are separated into liquid components and gas components. The liquid components containing allyl acetate are sent to the hydrolysis process. The gas components contain unreacted gases and... 

FUNCTIONALIZATION OF 1,3-OIENES VIA PALLADIUM-CATALYZED CHLORO-ACETOXYLATION AND ALLYLIC AMINATION l-ACETOXY-4-DIETHYLAMIN0-2-BUTENE AND 1-ACET0XY-4-BENZYLAMIN0-2-BUTENE... 

This reaction describes the entrance of a nucleophile into the allylic position of an olefin. In aqueous medium this reaction is of minor importance but in nonaqueous medium, particularly under the conditions of acetoxylation, it attracts broad interest. As already mentioned above and outlined later (see Section 3.3.14.6), higher and cyclic olefins give exclusively allylic esters. Two mechanisms have been proposed. One possibility is according to eq. (17) hydride abstraction through the palladium of an oxypalladation moiety by -elimination from the adjacent C-atom whieh had not been added to the nucleophile [9]. 

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]. 

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. 

In his pioneering contributions Moiseev has shown that giant cationic palladium clusters , e.g. Pd56iL6o(OAc)i8o (L = phenanthroline, bipyridine), characterized by use of high-resolution TEM, SAXS, EXAFS, IR and magnetic susceptibility data, catalyze, under mild conditions (293 363 K, 1 bar), the oxidative acetoxylation of ethylene into vinyl acetate, propylene into allyl acetate, and toluene into benzyl acetate. The oxidation of primary aliphatic alcohols to esters, and the conversion of aldehydes into acetals were also studied. ... 

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