Alkene To methyl ketone

Higher terminal alkenes are oxidized to methyl ketones and this unique oxidation of alkenes has extensive synthetic applications[23]. The terminal alkenes can be regarded as masked methyl ketones, which are stable to acids, bases, and nucleopliiles[24]. The oxidation of terminal alkenes to methyl ketones has been extensively applied to syntheses of many natural products[77]. 

Wacker oxidation of l-alkenes. The Wacker oxygenation of 1-alkenes to methyl ketones involves air oxidation catalyzed by PdCl2 and CuCU, which is necessary for reoxidation of Pd(0) to Pd(II).1 This oxygenation is fairly sluggish and can result in chlorinated by-products. A new system is comprised of catalytic amounts of Pd(OAc)2, hydroquinone, and 1, used as the oxygen activator.2 The solvent is aqueous DMF, and a trace of HClOj is added to prevent precipitation of Pd(0). Oxygenation using this system of three catalysts effects Wacker oxidation of 1-alkenes in 2-8 hours and in 67-85% yield. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

Similar 0—4 generations silica-supported Pd-PAMAM dendrimers with various spacer lengths were used by Alper et al. as recyclable catalysts for the hydroesterification reaction of alkenes (55) and the oxidation of terminal alkenes to methyl ketones (56). The hydroesterification experiments (Scheme 16) showed that (PPh3)2Pd-PPh2-PAMAM-Si02 complexes were highly active catalysts for styrene derivatives and linear long-chain alkenes (numbers of turnovers up to 1200). 

SCHEME 137. Proposed mechanism for the palladium-catalyzed oxidation of terminal alkenes to methyl ketones using TBHP oxidant... 

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

The Cope rearrangement of 24 gives 2,6,10-undecatrienyldimethylamine[28], Sativene (25j[29] and diquinane (26) have been synthesized by applying three different palladium-catalyzed reactions [oxidative cyclization of the 1,5-diene with Pd(OAc)2, intramolecular allylation of a /i-keto ester with allylic carbonate, and oxidation of terminal alkene to methyl ketone] using allyloctadienyl-dimethylamine (24) as a building block[30]. 

Oxidation of 1-alkenes to methyl ketones. This Pd catalyst allows air oxidation of 1-alkenes to alkyl methyl ketones in yields of about 350% (based on Pd). The oxidation is also possible under nitrogen (about 90% isolated yield), but then I is not functioning as a catalyst (equation 1). 2-Alkenes can be oxidized slowly in this way but a number of products are formed. 

Methyl ketones. Hydrogen peroxide in the presence of Pd(OAc)-. as catalyst converts terminal alkenes to methyl ketones in high yield and with high selectivity (equation I).1 Internal and cycloalkenes are inactive. The actual oxidant is... 

Oxidation of —CH=CH2 to —COCH3. Mimoun et al. have prepared a number of reagents in which the CF3 group of I is replaced by other groups. However 1 is the most effective for conversion of terminal alkenes to methyl ketones. Yields are high and the reaction is usually complete within an hour. The reaction can be catalytic with respect to 1 if r-BuOOH is present (equation I). 

Wacker oxidation. Tsuji et al.s have developed two procedures for oxidation of 1-alkenes to methyl ketones with oxygen that are catalyzed by PdCl2 (7, 278 9, 327). The solvent in both cases is aqueous DMF. One method uses PdCl2-CuCl (molar ratio 1 10) the other uses PdCl2 and p-benzoquinone (molar ratio 1 100). Both procedures are about equivalent for oxidation of simple l-alkenes to methyl ketones, but the former method is usually more effective for oxidation of more complex 1-alkenes. 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

The overall catalytic oxidation of terminal alkenes to methyl ketones by 02 has been tentatively interpreted as resulting from the consecutive consumption of each oxygen atom by two moles of alkene in two complementary reactions as shown in Scheme 3. 

A somewhat similar oxidation of terminal alkenes to methyl ketone and alcohol by 02 in the presence of Co(salMDPT) [salMDPT = bis(salicylideneiminopropyl)methylamine] and in ethanol solvent has recently been reported by Drago and coworkers (equation 244).560 Only terminal alkenes were found to be reactive with this catalytic system. The reaction is alcohol dependent and occurs in ethanol and methanol but not in t-butyl or isopropyl alcohols. The alcohol is concomitantly oxidized during the reaction, and may act as a coreducing agent and/or favor the formation of cobalt hydride. This oxidation might occur according to the mechanism of equation (243). 

Table 1 Palladium(II)-catalyzed Oxidation of Functionalized Terminal Alkenes to Methyl Ketones Alkene Product...

Oxidation of allylic andhomallylic acetates (cf. 10,175-176).1 This system is an efficient catalyst for oxygenation of terminal alkenes to methyl ketones (Wacker process). Similar oxidation of internal olefins is not useful because it is not regioselective. However, this catalyst effects oxygenation of allylic ethers and acetates regioselectively to give the corresponding /i-alkoxy ketones in 40-75% yield. Under the same conditions, homoallylic acetates are oxidized to y-acetoxy ketones as the major products. 

Methyl ketones from l-alkenes. This complex selectively oxidizes 1-alkenes to methyl ketones in high yield at 70°. 

Scheme 3.4 PdCl2/CuCl-catalyzed Wacker oxidation of alkenes to methyl ketones [36]...

The Pd-catalyzed conversion of terminal alkenes to methyl ketones is a reaction that has found widespread use in organic chemistry [87,88]. These reactions, as well as the industrial Wacker process, typically employ CuCh as a co-catalyst or a stoichiometric oxidant. Recently Cu-free reaction conditions were identified for the Wacker-type oxidation of styrenes using fBuOOH as the oxidant. An NHC-coordinated Pd complex, in-situ-generated (I Pr)Pd(OTf)2, served as the catalyst (Table 5) [101]. These conditions min-... 

Acetalization of l-alkenes. The Wacker uinversion of l-alkenes to methyl ketones by oxidation catalyzed by PdCE-CuCl takes a different course when applied to vinyl ketones (1). Thus oxygenation of mixtures of I and 1,3- or 1,2-diols catalyzed by PdCE-CuCl results in cyclic acetals formed by exclusive attaek at the terminal carbon atom. A similar reaction occurs with l-alkenes substituted with COOCH,. 

Wacker oxidation.3 The oxidation of 1-alkenes to methyl ketones by oxygen catalyzed by PdCl2 and CuCl2 can be carried out under phase-transfer conditions with cetyltrimethylammonium bromide or a closely related salt as the phase-transfer catalyst. Yields are in the range 50-75%. Several rhodium and ruthenium complexes can be used as the metal catalyst, but the yields are lower. 

The rate of alkene oxidation depends on the substitution pattern of the alkene. For a series of alkenes oxidized in aqueous solution, with benzoquinone as oxidant for the PdCl2, the relative rates are ethene (850) > propene(450) > 1-butene (380) > tra i-2-pentene (90) > cfr-2-pentene (80) > cyclohexene (8) > cycloheptene (1). Thus, selective oxidation of terminal alkenes to methyl ketones can occur in the presence of internal alkenes (equation 84). 

Strukul and co workers recently prepared a series of cis and trans isomers of the platinum- (-butyl peroxide complexes Pt(R)(OOBu )L2 (R = CF3, Ph, etc. L = tertiary phosphine). The X-ray crystal structure of the (ranx-[Pt(Ph)(OOBu )(PPh3)2] (88) revealed a square-planar arrangement with end-bonded OOBu group. Interestingly, only the trans isomers were found to be capable of oxidizing terminal alkenes to methyl ketones, whereas the cis isomers were inactive. The suggested mechanism involves the coordination of the alkene leading to a five-coordinate intermediate, followed by a pseudocyclic peroxymetalation, as in Scheme 6 (equation 92). ... 

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