Cyclohexene formation

Azabicyclo[2.2.0]hexa-1,3,5-triene reactions, 7, 360 2-Azabicyclo[2.2.0]hex-5-ene reactivity, 7, 342 synthesis, 2, 371 2-Azabi cyclop. 1.0]hexene synthesis, 2, 371 2-Azabi cyclop. 1.0]hex-2-ene synthesis, 7, 67 2-Azabicyclo[k 1.0]hex-3-ene synthesis, 4, 258 6-Azabicyclo[k 1.0]hex-2-ene synthesis, 2, 305 Azabi cyclohexenes formation from azirines, 7, 58... 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

Figure 15.17 Orbital correlation diagram for cyclohexene formation...

Fig. 25. Inhibition of benzene (O) from cyclohexane and increasing cyclohexene formation ( ) with time on Pt(S)-[6(111) x (100)] surface. All catalysts with (111) orientation terraces behave similarly. T, 150°C 4 x 10 8 Torr reactant H2 HC, 20 1.

Indirect evidence that intermediate radicals are involved stems from the formation of two regioisomers when an unsymmetrical olefin is employed, as indicated by the NMR spectrum of 13e. More direct evidence is obtained upon conducting the reaction in the presence of the radical scavenger diphenylpicrylhydrazyl [194]. In the case of cyclohexene, formation of the corresponding addition product is indicated by MS analysis [142]. The cyclohexenyl radical was postulated also in the electrochemical oxidation of cyclohexene in methanolic solution [195]. 

Most partial benzene hydrogenations use a ruthenium catalyst since it is a good species for promoting the hydrogenation of aromatic systems but not very effective for double bond saturation. The reaction is commonly run at 150°-200°C under 10-70 atmospheres of hydrogen. The maximum cyclohexene formation of about 40-70% selectivity is usually observed at low to moderate... 

Although the hydrogenation of benzene by standard supported platinum or rhodium catalysts does not lead to cyclohexene formation, platinum/polyamide... 

Diene generation followed by a Diels-Alder reaction can be carried out as a one-pot reaction. Methyl 9-(3-cyclopropylcyclobut-2-enyl)nonanoate was heated with dimethyl acetylenedicarboxylate, ring opening of the cyclobutene resulted followed by cyclohexene formation due to dienophile trapping of the buta-1,3-diene intermediate giving methyl 9-[5-cyclo-propyl-2,3-bis(methoxycarbonyl)phenyl]nonanoate after oxidation. ... 

A more general rule emerges from these considerations Concerted cycloaddition reactions are "forbidden" if involving 4n 7r-electrons, as in ethene dimerization (n is any integer), and are "allowed" if involving 4n + 2, as in cyclohexene formation [32,35,36],... 

The successive transfer of hydrogen atoms from the metal surface to the adsorbed aromatic ring and subsequently adsorbed intermediates, i. e. cyclohexa-diene and cyclohexene, finally results in the fully saturated ring system [7]. Although cyclohexene formation has been observed (cf. Section 8.6.3), no cyclohex-adienes have yet been detected as intermediates [8]. Several explanations have been proposed to explain why formation of a diene on the metal is by-passed,... 

Although the reaction mechanism at high temperatures is not clear, the formation of the secondary products, such as cyclohexene, cyclohexadiene, and benzene, may be attributed to the devinylation and dehydrogenation of 4-vinylcyclohexene. The possibility of cyclohexene formation by the reaction of butadiene and product ethylene can be neglected because of the prohibitively low concentration of ethylene (discussed next). 

In addition to the examples shown above, there are alternative pathways (Scheme 4) to construct cyclohexene rings by a cationic (mechanism A), a nucleophilic (mechanism B), and a radical mechanism (mechanism C). Therefore, it is difficult to distinguish the Diels-Alder reaction and alternative reactions in cyclohexene formation, and to determine enzymatic and nonenzymatic [4 + 2] adducts. Through our studies on Diels-Alderases,we recognized that Diels-Alderase catalyzes not only the formation of reactive species but also cycloaddition at the same active site. This indicates that Diels-Alderase is, at least part of them, a producer of a reactive substrate for cycloadditions. If this is general, a Diels-Alderase could be any type of enzyme such as an oxidase, a dehydrogenase, a decarboxylase, or... 

The ratio of divinylcyclobutanes to cyclohexene product depends on the 7. of the sensitizer that is used. With sensitizer Ej- > 60kcal/mol, either the s-trans (dominant) or s-cis conformation can be excited. With Ej. 55kcal/mol, the s-cis is preferentially excited (see Figure 12.15). The excited s-trans conformer can give only cyclobutanes, whereas the excitedr-d5 structure can also form the cyclohexene product. Isoprene shows a similar effect with cyclohexene formation being at a maximum with Ej at 46-54 kcal/mol. 

Scheme 4. Statistical probabilities of subsequent random addition of H atoms to the benzene ring. The probabilities of elementary steps has been indicated on the arrows. The thicker arrow indicates the 1,3,5-addition avoiding cyclohexene formation. Adapted after Ref. 15.

Whereas cyclohexene and cyclohexane were formed by hydrogenation of benzene with a Pt/nylon catalyst, only cyclohexane was detected with a Pt/Si02 catalyst [119]. Increased selectivity was observed for cyclohexane at elevated temperatures and after air pretreatment. Apparently oxidized platinum particles were responsible for cyclohexene formation. 

Cyclobutanes from the addition of 1-chloro-l-fluoroethylene to chlorotrifluoro- or tetrafluoro-ethylene, of 2-fluoropropene to tetrafluoroethylene, and of 1-chloro-2,2-difluoroethylene to vinyl fluoride or propene have been claimed to be anaesthetics. Diels-Alder addition of cyclopentadiene to acids of the type, trans-RpCHiCH COaH (Rf = CHaF, CHFa, CFs, C2F5, or n-CsFv) and a number of related esters and other derivatives, at 25 °C, results in predominant formation of adduct witii an endo fluoroalkyl group. The addition of cyclopentadiene to the 1,1-difluoroethylenes, CFalCFa, CFa CF CFs, CFa CCla, CFa C(CF3)2, CFaiCCl-CFaCl, and CFa.CFH has been recorded and the fiee-radical bromina-tion of the resulting norbomenes studied. Buta-1,3-diene and trifluorovinyl-sulphur pentafluoride yield essentially a mbcture of cis- and rmns-cyclobutanes (116), with little or no cyclohexene formation the olefin resembles perfluoropropene which yields ca. 5% of cyclohexene, in this respect. Perfluoroindene, which adds... 

Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of 2+2 and 2+4 addition products on sensitizer energy. The s-cis geometry is suitable for cyclohexene formation the... 

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