Acetoxylation of alkenes

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... 

Scheme 14 Possible outcomes for the palladium-catalyzed oxidative acetoxylation of alkenes...

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. 

Alkenes can be transformed to carbonyl compounds through the oxidation of the vinylic carbon atom. A special case of vinylic oxidation is acetoxylation of alkenes and dienes. 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

Acetoxylation of Alkenes, Dienes and Aromatic Hydrocarbons 61.3.4.3.1 Vinyl acetate from ethytene... 

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 ... 

Thallium triacetate, TI(0C0CH3)3 1.5H20 (mp 182 C), like the monoacetate, is used for the stereoselective acetoxylation of alkenes [411] and for oxidations of alkenes to epoxides [412]. 

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. 

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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