Alkenes allylic 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 of alkenes is achieved using acetic acid with sodium acetate as solvent and nucleophile. Cyclohexene gives 1 -acetoxycyclohexene as the... 

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). 

The mechanistic role of BQ in the allylic acetoxylation of alkenes suggests that it may not be possible to achieve direct dioxygen-coupled turnover. Recently, however, Kaneda and coworkers reported BQ-free conditions for aerobic allylic acetoxylation that feature a solvent mixture of acetic acid and M,M-dimethylacetamide (DMA) and O2 as the sole oxidant for the Pd catalyst (Eq. 55) [209]. The reactions are highly selective for C-1 acetoxylation (C-1 C-3 = 7-45 1). High pressures of O2 (6 atm) are required to achieve these results. 

Pd(H) complexes with strongly electron-withdrawing ligands can insert into the allylic C—H bond (path c) to form directly the Jt-allyl complex via oxidative addi-tion.502,694,697 Pd(OOCCF3)2 in acetic acid, for example, ensures high yields of allylic acetoxylated products.698 The delicate balance between allylic and vinylic acetoxylation was observed to depend on substrate structure, too. For simple terminal alkenes the latter process seems to be the predominant pathway.571... 

Allylic acetoxylation.1 The combination of r-butyl hydroperoxide and Se02 has been used for allylic hydroxylation of alkenes (8, 64-65), but this system is not useful for oxidation of cycloalkenes. Allylic acetoxylation of cycloalkcnes is possible, but in modest yield, with PdCl2 and AgOAc, which probably form a reactive species such as [PdCl(OAc)] . This system can be used in catalytic amounts in the presence of t-butyl hydroperoxide for a reoxidation step. The yield is improved by addition of TcO, which seems to accelerate the oxidation. The most satisfactory ratios of... 

Some other reactions involving oxidation of the C—Hg bond have been known for some time, but these are either of limited synthetic appeal or have experienced no significant development in recent years. Thus ozonolysis of the C—Hg bond to form carboxylic acids or ketones falls into the first category, whereas allylic acetoxylation of alkenes by Hg(OAc>2 falls into the second category. Nevertheless, this allylic oxidation (Treibe s reaction) has considerable synthetic utility, and has been reviewed quite recently.5 ... 

Scheme 4 DMSO promoted intermolecular allylic acetoxylation of terminal 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 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

This Pd-catalyzed allylic acetoxylation has recently been extended to numerous unsubstituted cycloalkenes and linear alkenes [249], using MnO as terminal oxidant. 

Very efficient systems for Pd-catalyzed allylic acetoxylation have thus been developed. They work well with cycloaUcenes and internal aUcenes but are less efficient with terminal alkenes. The result probably reflects the operation of different mechanisms for terminal alkenes and the other classes of alkenes. 

At present, the allylic acetoxylation reaction, especially in combination with catalytic allylic substitution, is useful for functionalizing cyclic and internal alkenes. There is ample room for improvement of the selectivity of the reaction of terminal alkenes but with an increased knowledge of the structure of the intermediate complexes and with an improved understanding of the influence of reaction conditions, this reaction should also have considerable synthetic potential. 

The acetoxylation of higher alkenes with Pd(OAc>2 is synthetically less useful, since it is always accompanied by allylic acetoxylation and/or diacetoxylation.t ( For instance, the acetoxylation of methyl oleate gives various products as shown in Scheme 3,3-Dimethyl-4-pentenoates, in which the allylic position is blocked by two Me groups, react with AcOH in the presence of Pd(OAc>2 catalyst to give the corresponding vinyl acetate, selectively (Scheme The catalysis is exerted by the use of O2 alone as the co-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. ... 

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). ... 

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]. 

The oxidation of 1-alkenes usually gives 2-acetoxy-l-alkenes.571,572 Oxidative acetoxylation of propylene with Pd(OAc)2 may yield allylic or vinylic acetates depending on reaction conditions573 (see Section 9.2.6). 

In contrast to ethylene, which gives only vinylic or oxidative addition products, the acetoxylation of higher alkenes results in the formation of a mixture of allylic and vinylic acetates.367 The... 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

Other transition metal salts mediate in similar oxidations. For example, mercury(II) acetate, a milder reagent than LTA, effects a-acetoxylation through a comparable mechanism. However the corresponding yields for these processes are poor. 3,3-Dimethylcyclohexanone, for example, is oxidized to the a-acetoxy derivative in only 14% yield.The, 7-unsaturated ketone, isopugelone, exhibits no oxidation at the a- or a -positions, but affords a product derived from isomerization of the alkene and allylic oxidation. Not surprisingly therefore the reagent has found little synthetic application for this transformation. 

Acetoxylation of alkenes takes place in allylic position [40,115], concurrent with addition of acetoxy groups across the double bond or double-bond system. The mechanism is almost certainly analogous to that discussed previously for methoxylation [Eq. (20)]. Because AcO is difficult to oxidize, the anodic discharge of simple, unactivated alkenes may be achieved in its presence. Subsequent reactions of the cationic intermediates are not very selective, but several experimental parameters may be controlled and the method can compare well with the few competing chemical methods. 

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