Palladium catalyzed oxidations of cyclohexene

Aerobic Palladium-Catalyzed Oxidation of Cyclohexene to 1,4-Dioxospiro-[4,5]-decane 11.2.1.1 Optimization of the Reaction Conditions... 

Figure 11.1 Palladium-catalyzed oxidation of cyclohexene to either cyclohexanone or to 1,4-dioxaspiro[4,5]decane. Cyclohexanone is susceptible to overoxidation, while the 1,4-dioxaspiro[4,5]decane is stable.

Table 11.2 Palladium-catalyzed oxidation of cyclohexene with different Cu and Fe cocatalyst compositions showing that the Pd/Cu/Fe combination gives a stable catalyst which gives clean oxidation.

Diacetoxylation of 1,3-dienes. Palladium-catalyzed oxidation of 1,3-cyclo-hcxadicnc with bcnzoquinone (used in catalytic amounts with MnO, as the external oxidant) in acetic acid gives a 1 1 mixture of cis- and //-an.v-1,4-diacctoxy-2-cyclohexene. Addition of LiCI or LiOAc has a profound effect on the stereochemistry. Oxidation in the presence of lithium acetate results in selective tran. -diacetoxylation. whereas addition of lithium chloride results in selective r/.v-diacetoxylation (equation I). ... 

A molybdenum-mediated oxidative coupling of aniline 1 with cyclohexene 2a provides carbazole 3. Alternatively, the same overall transformation of aniline 1 to carbazole 3 is achieved by iron-mediated oxidative coupling with cyclo-hexa-1,3-diene 2b or by palladium-catalyzed oxidative coupling with arenes 2c. The use of appropriately substituted anilines and unsaturated six-membered hydrocarbons opens up the way to highly convergent organometallic syntheses of carbazole alkaloids. 

The stereoselective allylation of aldehydes was reported to proceed with allyltrifluorosilanes in the presence of (S)-proline. The reaction involves pentacoordinate silicate intermediates. Optical yields up to 30% are achieved in the copper-catalyzed ally lie ace-toxylation of cyclohexene with (S)-proline as a chiral ligand. The intramolecular asymmetric palladium-catalyzed allylation of aldehydes, including allylating functionality in the molecules, via chiral enamines prepared from (5)-proline esters has been reported (eq 15). The most promising result was reached with the (S)-proline allyl ester derivative (36). Upon treatment with Tetrakis(triphenylphosphine)palladium(0) and PPh3 in THF, the chiral enamine (36) undergoes an intramolecular allylation to afford an a-allyl hemiacetal (37). After an oxidation step the optically active lactones (38) with up to 84% ee were isolated in high chemical yields. The same authors have also reported sucessful palladium-catalyzed asymmetric allylations of chiral allylic (S)-proline ester enamines" and amides with enantiomeric excesses up to 100%. 

Zeolite-encapsulated Fe-phthalocyanine and Co-salophen catalysts were used in the palladium-catalyzed aerobic oxidation of hydroquinone to benzoquinone, in the oxidation of 1-octene to 2-octanone and in the allylic oxidation of cyclohexene to 3-acetoxycyclohexene. These catalysts proved to be active in the above reactions and they were stable towards selfoxidation and it was possible to reuse them in subsequent runs. The specific activity of the encapsulated Fe-phthalocyanine catalyst was about four times higher than those of the free complex. 

Mitsubishi Chemicals Liquid Phase Palladium-Catalyzed Oxidation Technology Oxidation of Cyclohexene, Acrolein, and Methyl Acrylate to Useful Industrial Chemicals... 

As noted above, the oxidative reaction of ethylene with acetic acid forms vinyl acetate (Equation 16.107), and this reaction has been conducted on industrial settings in both the liquid phase and in the gas phase with a supported palladium catalyst. Allyl alcohol is also produced by palladium-catalyzed oxidations. The reaction of pro-pene with acetic acid in the presence of a combination of palladium and copper forms allyl acetate (Equation 16.108), which is hydrolyzed to the alcohol. Cyclic olefins also undergo allylic oxidation, in this case to generate allylic esters. A variety of combinations of palladium acetate and co-oxidants and additives give rise to this allylic ester in good to excellent yields. Equation 16.109 summarizes results for the reactions of cyclohexene. ... 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

Unhindered simple olefins are usually rapidly hydrogenated under very mild conditions over platinum metal catalysts such as platinum, palladium, and rhodium as well as over active nickel catalysts such as Raney Ni, nickel boride, and Urushibara Ni. For example, 0.1 mol of cyclohexene is hydrogenated in 7 min over 0.05 g of Adams platinum oxide in ethanol at 25°C and 0.2-0.3 MPa H2 (eq. 3.1).5 1-Octene and cyclopentene (eq. 3.2) are hydrogenated in rates of 11.5 and 8.6 mmol (258 and 193 ml H2 at STP) g Ni 1-min 1, respectively, over P-1 Ni in ethanol at 25°C and 1 atm H2.18 Hydrogenation of cyclohexene over active Raney Ni proceeds at rates of 96-100 ml H2 at STP (4.3-4.5 mmol) g Ni min-1 in methanol at 25°C and 1 atm H2 49,50 and can be completed within a short time, although usually larger catalyst substrate ratios than required for platinum catalyzed hydrogenations are employed (eq. 3.3).50... 

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... 

Recently, palladium-catalyzed asymmetric allylic substitution of an activated cyclohexenol derivative has allowed two enantioselective syntheses of (—)-galantha-mine (75) (234,235). Both approaches rely on the enantioselective preparation of the same tricyclic intermediate, which is subsequently converted to the alkaloid via stereocontrolled transformations the most efficient of which comprised stereoselective allylic oxidation of the cyclohexene moiety (Scheme 5). The same methodology allowed the synthesis of (—)-codeine and (—)-morphine (236). The same group had earlier reported the synthesis of (-l-)-pancratistatin following a related strategy (237). Use of a tosylamide as the nucleophile in the displacement of an activated aryl-cyclohexenol derivative enabled the preparation of a chiral intermediate which... 

One example of such a reaction was reported in 1971 by Brown and Davidson [55], who studied oxidation reactions of 1,3- and 1,4-cyclohexadiene. These authors observed that reaction of 1,3-cyclohexadiene with p-BQ in acetic acid in the presence of catalytic amounts of Pd(OAc)2 produced l,4-diacetoxy-2-cyclohexene of unknown configuration. At the time. Brown and Davidson were uncertain about the mechanism, and suggested possible involvement of radicals. A related palladium-catalyzed 1,4-diacetoxylation of butadiene employing as an oxidant and a heterogeneous Pd-Te catalyst has been developed and commercialized by Mitsubishi Chemicals [56]. 

By incorporating the quinone molecule into the macrocycle, a more efficient palladium-catalyzed aerobic 1,4-oxidation was developed [69]. Thus, with catalytic amounts of 48 and Pd(OAc)2, 1,3-cyclohexadiene was oxidized to 1,4-diacetoxy-2-cyclohexene at more than twice the rate achieved with a system having the... 

The chloroacetoxylation proceeds via the same type of intermediate as that involved in the palladium-catalyzed 1,4-diacetoxylation that is, via a 4-acetoxy-l,2,3- [t-allylpalladium intermediate (cf Scheme 11.9). The high selectivity for unsymmetrical products (usually >98%) is remarkable. Since chloride anion is the strongest nucleophile of the two present (Cl and AcO ), 4-chloro-7t-aUyl-palladium intermediate 68 is initially formed (Scheme 11.22). However, the chloride in the 4-position is rapidly exchanged for acetate to give a more stable rt-allyl intermediate 69, which leads to product. The presence of the intermediate 68 was confirmed by its trapping with a faster oxidant (isoamyl nitrite) than p-BQ, which furnished l,4-dichloro-2-alkene [89],(J.E. BackvaU, unpublished results). In the case of 1,3-cyclohexadiene, this product was ds-l,4-dichloro-2-cyclohexene (J.E. BackvaU, unpublished results). 

Oxidative dehydrogenation of cyclohexanones with [Pd(CF3C02)2] is believed to occur by the mechanism shown (Scheme 5) in which the palladium remains in the +II oxidation state throughout. A related mechanism has been proposed for the [Pds(PPh)2]-catalyzed oxidative dehydrogenation of cyclohexene to benzene. ... 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-CYCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

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