Cyclohexadiene, hydrogenation

The product 16 is identical with the one expected from direct [1,5] methyl migration [path (A)]40 42 (see also refs 41 and 42). For 5,5-diethyl-l,3-cyclohexadiene it is concluded that no more than 3% of the reaction can result from a direct [1,5] alkyl migration42. However, in 5,5-disubstituted 1,3-cyclohexadienes, hydrogen and other substituents, such as the formyl35 and acetyl groups35, should migrate by the [1,5] sigmatropic shift pathway. 

Isaeva VI, Tarasov AL, Tkachenko OP, Kapustin GI, Mishin IV, Solov eva SE, et al. 1,3-cyclohexadiene hydrogenation in the presence of a palladium-containing catalytic system based on an MOF-5/caUxarene composite. Kinet Catal (Russ) 2011 52 95-8. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

Tetracyanobenzoquinone [4032-03-5] 3,6-dioxo-l,4-cyclohexadiene-l,2,4,5-tetracarbonitrile, is a remarkably strong oxidizing agent for a quinone it abstracts hydrogen from tetralin or ethanol even at room temperature (50). It is a stronger TT-acid than TCNE because it forms more deeply colored TT-complexes with aromatic hydrocarbons. 

Vanadium pentafluoride replaces benzylic hydrogen by fluonne but also adds fluonne to the aromatic system, giving fluonnated cyclohexadienes and cyclohexenes [5] (equation 5)... 

FIGURE 11.2 Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexa-triene, and benzene. All heats of hydrogenation are in kilojoules per mole. 

Pd-C using transfer hydrogenation. A number of methods have been developed in which hydrogen is generated in situ. These include the use of cyclohexene (1-8 h, 80-90% yield)," cyclohexadiene (25°, 2 h, good yields)," HC02H, ammonium formate (MeOH, reflux, 91% yield), ... 

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. 

We can get a quantitative idea of benzene s stability by measuring heats of hydrogenation (Section 6.6). Cyclohexene, an isolated alkene, has ff ydrog = -118 kj/mol (-28.2 kcal/mol), and 1,3-cyclohexadiene, a conjugated diene, has A/Chydrog = 230 kj/mol (-55.0 kcal/mol). As noted in Section 14.1, this value for 1,3-cyclohexadiene is a bit less than twice that for cyclohexene because conjugated dienes are more stable than isolated dienes. 

As described ( 6.5.3.1) l-phenylbora-2,5-cyclohexadiene (cyclic divinylborane) reacts with FeCCOj under irradiation to the Fe(CO)3 complex. The dinuclear borabenzene complex [(C5H5BPh)Fe(CO>2]2 is found as a by-product. It is to be compared with the well-known [Tj -CpFe(CO)2]2. CjHjBPh reacts spontaneously with Co2(CO)u to form three products. Above 60°C (C5H5BPh)Co(CO)2 is the only product . At 30°C yellow isomers III and IV having partially hydrogenated borabenzene ligands form above 40°C III isomerizes to IV. 

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