Cyclohexene, conversion reactions

Cyclohexene, conversion to cyclopen-tanecarboxaldehyde, 44, 26 purification of, 41, 74 reaction with zinc-copper couple and methylene iodide, 41, 73 Cyclohexene oxide, 44, 29 2-Cyclohexenone, 40,14 Cyclohexylamine, reaction with ethyl formate, 41,14... 

Moreover, this catalytic reaction could be employed in a continuously operated membrane reactor [105,106]. A stirred membrane reactor module equipped with a solvent-stable Koch MPF-50 membrane [107] was operated at 40 atm. After exchange of a few reactor volumes a steady conversion is achieved, e.g., 30% cyclohexene conversion for the example shown in Fig. 9 [32], corresponding to a catalytic activity of 1200 TO h 1. Over 30 exchanged reactor volumes, corresponding to a time of operation of 30 h, a productivity of a total of 29 000 turnovers was observed. 

These results have profound effects for the selective catalytic dehydrogenation of cyclohexane to benzene, a prototypical hydrocarbon conversion reaction. On Pt(lll), the intermediates, cyclohexene and a species, have been identified and the rate constants for some of the sequential reaction steps measured [56]. Adsorption and reaction studies of cyclohexane [39], cyclohexene [44], 1,3-cyclo-hexadiene [48], and benzene [39] on the two Sn/Pt(lll) alloys provide a rational basis for understanding the role of Sn in promoting higher selectivity for this reaction. One example of structure sensitivity is shown in Fig. 2.7, in which a monolayer of cyclohexyl (C H ) was prepared by electron-induced dissociation (EID) of physi-orbed cyclohexane to overcome the completely reversible adsorption of cyclohexane... 

Titanium framework-substituted aluminophosphate, TAPO-5, gave a 30.3% selectivity to AA at total cyclohexene conversion, in 72 h reaction time at 80 °C [35c]. However, when the filtered TAPO-5 catalyst was reused its activity was significantly diminished. The major by-product was 1,2-cyclohexanediol, which is formed as reaction intermediate. The trans isomer was more slowly transformed into 2-hydroxycyclohexanone than the cis isomer. A detailed investigation of the mechanism indeed showed that the trans-diol formed by ring opening of the cyclohexene epoxide, whereas the cis isomer formed via a free-radical mechanism. Scheme 7.14 shows the mechanism proposed. 

Initial experiments were done in water and resulted in low cyclohexene conversions, low product selectivities, and extensive palladium deactivation by Pd black formation. The low cyclohexanone yield originated from overoxidation of cyclohexanone to 2-cyclohexenone, which undergoes further oxidation to a plethora of by-products. The low cyclohexene conversion can be attributed to the aforementioned low reactivity of the internal double bond as well as the low solubility of cyclohexene in water. Several reaction media have been described in which higher alkenes are oxidized to ketones in organic solvent-based systems. Some typical examples are DMF [4], water mixtures with chlorobenzene, dodecane, sulfolane [5], 3-methylsulfolane andM-methylpyrrolidone [6], or alcohols [7]. These solvent systems indeed lead to increased cyclohexene conversions but still suffer from overoxidation and catalyst deactivation by Pd black formation. Hence, the goal of our research was to find a variation to the Wacker oxidation without over-oxidation of the product and deactivation of the palladium catalyst. 

Fig. 2. Effect of reaction temperature on cyclohexene conversion when using catalysts fresh (A) spent (B) decoked before (D) and after (E) leached by 2%.

Humphries et al. [92,93] gave an overview of different test reactions used to characterize the acidity of zeolites. The reactions included cracking, isomerization, disproportionation, and alcohol dehydration as well as hydride transfer reactions involving cyclohexene conversion. In the following sections, we will discuss each class of these test reactions for their ability to give information about the nature, concentration, and strength of the active sites involved in acid-catalyzed reactions. 

A dry 5(X)-mI flask equipped with a thermometer, pressure-equalizing dropping funnel, and magnetic stirrer is flushed with nitrogen and then maintained under a static pressure of the gas. The flask is charged with 50 ml of tetrahydrofuran and 13.3 ml (0.15 mole) of cyclopentene, and then is cooled in an ice bath. Conversion to tricyclo-pentylborane is achieved by dropwise addition of 25 ml of a 1 M solution of diborane (0.15 mole of hydride see Chapter 4, Section 1 for preparation) in tetrahydrofuran. The solution is stirred for 1 hour at 25° and again cooled in an ice bath, and 25 ml of dry t-butyl alcohol is added, followed by 5.5 ml (0.05 mole) of ethyl bromoacetate. Potassium t-butoxide in /-butyl alcohol (50 ml of a 1 M solution) is added over a period of 10 minutes. There is an immediate precipitation of potassium bromide. The reaction mixture is filtered from the potassium bromide and distilled. Ethyl cyclopentylacetate, bp 101730 mm, 1.4398, is obtained in about 75% yield. Similarly, the reaction can be applied to a variety of olefins including 2-butene, cyclohexene, and norbornene. 

Chauvin s group described the selective hydrogenation of cyclohexadiene to cyclohexene through making use of the biphasic reaction system [46]. Since the solubility of cyclohexadiene in [BMIM][SbFg] is about five times higher than the solubility of cyclohexene in the same ionic liquid, the latter was obtained in 98 % selectivity at 96 % conversion. 

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

In the Monsanto/Lummus Crest process (Figure 10-3), fresh ethylbenzene with recycled unconverted ethylbenzene are mixed with superheated steam. The steam acts as a heating medium and as a diluent. The endothermic reaction is carried out in multiple radial bed reactors filled with proprietary catalysts. Radial beds minimize pressure drops across the reactor. A simulation and optimization of styrene plant based on the Lummus Monsanto process has been done by Sundaram et al. Yields could be predicted, and with the help of an optimizer, the best operating conditions can be found. Figure 10-4 shows the effect of steam-to-EB ratio, temperature, and pressure on the equilibrium conversion of ethylbenzene. Alternative routes for producing styrene have been sought. One approach is to dimerize butadiene to 4-vinyl-1-cyclohexene, followed by catalytic dehydrogenation to styrene ... 

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

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). 

In this work, highly active epoxidation catalysts, which have hydrophobic surface of TS-1, were synthesized by the dry gel conversion (DGC) method. Ti-MCM-41 was synthesized first by a modifed method and the TS-l/MCM-41 catalysts were subsequently synthesized by the DGC method. The catalysts were characterized by the XRD, BET, FT-IR, and UV-VIS spectroscopy. TS-l/MCM-41 catalysts were applied to the epoxidation of 1-hexene and cyclohexene with aqueous H202to evaluate their activities for the epoxidation reaction. ... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

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