Cyclohexene selective oxidation

The epoxidation method developed by Noyori was subsequently applied to the direct formation of dicarboxylic acids from olefins [55], Cyclohexene was oxidized to adipic acid in 93% yield with the tungstate/ammonium bisulfate system and 4 equivalents of hydrogen peroxide. The selectivity problem associated with the Noyori method was circumvented to a certain degree by the improvements introduced by Jacobs and coworkers [56]. Additional amounts of (aminomethyl)phos-phonic acid and Na2W04 were introduced into the standard catalytic mixture, and the pH of the reaction media was adjusted to 4.2-5 with aqueous NaOH. These changes allowed for the formation of epoxides from ot-pinene, 1 -phenyl- 1-cyclohex-ene, and indene, with high levels of conversion and good selectivity (Scheme 6.3). 

The Pacman catalyst selectively oxidized a broad range of organic substrates including sulfides to the corresponding sulfoxides and olefins to epoxides and ketones. However, cyclohexene gave a typical autoxidation product distribution yielding the allylic oxidation products 2-cyclohexene-l-ol (12%) and 2-cyclohexene-1-one (73%) and the epoxide with 15% yield [115]. 

Other TUD-l catalysts proven for selective oxidation (16) include Au/Ti-Si-TUD-1 for converting propylene to propylene oxide (96% selectivity at 3.5% conversion see also (17), Ag/Ti-Si-TUD-1 for oxidizing ethylene to ethylene oxide (29% selectivity at 19.8% conversion), and Cr-Si-TUD-1 for cyclohexene to cyclohexene epoxide (94% selectivity at 46% conversion). 

Andrus also reported the synthesis and use of biphenyl-derived bis(oxazoline) (154) as a ligand for Cu(I) (110). In the presence of this catalyst, cyclohexene is oxidized in comparable yield and selectivity as 55c CuOTf complexes. The ni-trobenzoate perester was found to be a more reactive oxidant than perbenzoate, although the reaction still requires 5 days to proceed to completion, Eq. 93. 

The results of the olefin oxidation catalyzed by 19, 57, and 59-62 are summarized in Tables VI-VIII. Table VI shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbomene) are selectively oxidized to epoxides. Cyclopentene shows exceptional behavior, it is oxidized exclusively to cyclopentanone without any production of epoxypentane. This exception would be brought about by the more restrained and planar pen-tene ring, compared with other larger cyclic nonplanar olefins in Table VI, but the exact reason is not yet known. Linear inner olefin, 2-octene, is oxidized to both 2- and 3-octanones. 2-Methyl-2-butene is oxidized to 3-methyl-2-butanone, while ethyl vinyl ether is oxidized to acetaldehyde and ethyl alcohol. These products were identified by NMR, but could not be quantitatively determined because of the existence of overlapping small peaks in the GC chart. The last reaction corresponds to oxidative hydrolysis of ethyl vinyl ether. Those olefins having bulky (a-methylstyrene, j8-methylstyrene, and allylbenzene) or electon-withdrawing substituents (1-bromo-l-propene, 1-chloro-l-pro-pene, fumalonitrile, acrylonitrile, and methylacrylate) are not oxidized. 

Sheldon and coworkers have developed chromium-substituted molecular sieves (CrAPO-5) as recyclable solid catalysts for several selective oxidations, among them also the allylic" and benzylic ° " ° " ° oxidations using TBHP or O2 as the terminal oxidants (equation 63), which yielded the corresponding benzylic ketones in moderate yield (conv. 13-70%) and moderate to good selectivity (41%, 65-97%). The benzylic alcohols were formed as side products. Allylic oxidation also proceeded with good conversions, while selectivities were lower and both possible products, the allylic ketone (31-77% selectivity) and the allylic alcohol (0-47% selectivity), were formed. Chromium sUicalite showed activity for selective benzylic oxidation in the presence of TBHP as well as giving mainly the allylic ketone (2-cyclohexen-l-one with 74% selectivity) and the allylic alcohol as minor product (2-cyclohexen-l-ol with 26% selectivity) -. ... 

The oxidation of simple internal alkenes is very slow. The clean selective oxidation of a terminal double bond in 40, even in the presence of an internal double bond, is possible under normal conditions[89,90]. The oxidation of cyclic alkenes is difficult, but can be carried out under selected conditions. Addition of strong mineral acids such as HC104, H2S04 and HBF4 accelerates the oxidation of cyclohexene and cyclopentene[48,91). A catalyst system of PdS04-H3PMo6W604o [92] or PdCT-CuCh in EtOH is used for the oxidation of cyclopentene and cyclohexene[93]. 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

Gold-catalyzed oxidation of styrene was firstly reported by Choudhary and coworkers for Au NPs supported on metal oxides in the presence of an excess amount of radical initiator, t-butyl hydroperoxide (TBHP), to afford styrene oxide, while benzaldehyde and benzoic acid were formed in the presence of supports without Au NPs [199]. Subsequently, Hutchings and coworkers demonstrated the selective oxidation of cyclohexene over Au/C with a catalytic amount of TBHP to yield cyclohexene oxide with a selectivity of 50% and cyclohexenone (26%) as a by-product [2]. Product selectivity was significantly changed by solvents. Cyclohexene oxide was obtained as a major product with a selectivity of 50% in 1,2,3,5-tetramethylbenzene while cyclohexenone and cyclohexenol were formed with selectivities of 35 and 25%, respectively, in toluene. A promoting effect of Bi addition to Au was also reported for the epoxidation of cyclooctene under solvent-free conditions. 

Sulfides are generally oxidized much faster than olefins. For example, with r-BuOjH-VO(acac)2 in ethanol at 25°C, the relative rates decreased in the order Bu"S(100) > PhSBu"(58) >Bu"SO (1.7) > cyclohexene (0.2).480 Unsaturated sulfides are selectively oxidized at the sulfur atom as shown in the following example477 ... 

The results of the olefin oxidation catalyzed by 1 to 6 are summarized in Tables 1-3. Table 1 shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbornene) are selectively oxidized to epoxides. Cyclopentene shows an exceptional behavior it is oxidized exclusively to cyclopentanone without any produc-... 

The active oxidant was proposed to be a Ru(V)=0 species and access of benzene towards the Ru=0 bond is facilitated by the flat structure of the salicyldiimine ligand (see Fig. 8). This catalytic system was also applied to the epoxidation of stilbene, C-H bond activation of cyclohexane or cyclohexene and the oxidation of tetrahydrofuran to y-butyrolactone [37]. We conclude however, that a suitable and catalytic system for the selective oxidation of benzene to phenol has not yet been forthcoming. 

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). 

The entrapped complexes are known to catalyze selective oxidation or hydrogenation reactions, depending mainly on the complexed transition metal cation [4, 82, 84]. Recently, two exciting examples have been published describing the synthesis of adipic acid from cyclohexene [93] or even from cyclohexane [94], respectively (cf Figure 6). 

PIFA easily converts succinic acid derivatives (32) to -alanine derivatives (33). Limited use of PIFA (1 equiv.) allows the rearrangement of 3-cyclohexene-1-carboxamide (34) without oxidation of the double bond, as shown in Scheme 12. Cyclohexanone is obtained by the PIFA oxidation of 1-cyclohex-enecarboxamide (35). Selective oxidation of the primary amide (36) occurs without effect on secondary or tertiary amides in the same molecule. The rearrangement of the cyclopropane derivative (37) accompanies the ring cleavage to give the -alanine derivative (38) after treatment with benzyloxycarbonyl chloride. ... 

Autoxidation of cyclohexene is mediated by 812(804)3 to give 1-cyclopentene-l-carboxylic acid in 90-92% yield [232], The selective oxidation of sulfides to sulfoxides is accomplished by Bi(NO3)3, both stoichiometrically [233] and catalytically with air [234]. 

Cr-exchanged zeolite NaY is found to convert cyclohexene selectively to cyclohexen-1-one by oxidation with molecular oxygen, in contrast to an equivalent CoNaY system. The initiator, NMP, is found to play an important role in the transformation, both components being necessary to achieve the high selectivities observed. A reaction mechanism consistent with the experimental data is proposed. 

Fig. 4. Cyclohexene selectivity as a function of conversion in the cyclohexane oxidation at 450°C over nickel containing catalysts (a) influence of the metallic substrate A1 (Cl), Ti (C2) and Mg substrate (C3) as well as of the MO/AI2O3 catalyst (C7), respectively (b) influence of the pore length on the selectivity pattern.

The liquid phase hydrogenation of benzene on carrier-fixed ruthenium colloid catalysts suspended in an aqueous solution of sodium hydroxide proceeds with 59% cyclohexene selectivity at 50% benzene conversion. The catalysts are prepared by adsorbing a hydrophilic stabilized ruthenium metal colloid on lanthanum oxide. Protection of metal colloids with chiral molecules can lead to a new type of enan-tioselective catalyst combining good selectivity control with extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidinel °°l (Fig. 7). 

Ethylene is more rapidly oxidized than propylene. Furthermore, the substituted ethylenes do not display the dependency on reactivity of allyl C—H bonds shown over bismuth molybdate (Tables XI and XII). It is clear that the C02-producing reaction is favored by unsaturation, but not by allyl hydrogens. In fact, over Pt ter<-butylethylene, without any allyl hydrogen, was oxidized about as fast as the methylethylenes. Dienes and acids were found to inhibit the oxidation of olefins over the metals. Acetone, like acetic acid from ethylene over Pd, is considered a side reaction product rather than an intermediate. The only selective oxidation observed was an oxidative dehydrogenation of cyclohexene to benzene over Pd at —20 to +30° here no CO2 was produced. 

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