Hydrogen transfer from 1,4-cyclohexadiene

In another example, the hexaacetylene 109 - after deprotection with potassium carbonate in methanol - is subjected to typical Bergman trapping conditions, resulting in the formation of the anthracene derivative 110 [61]. As a third, more complex illustration, the aroma-tization of the triacetylene 111 may be considered. Here, the 1,4-diradical intermediate faces another triple bond as an internal trap, and, after hydrogen transfer from 1,4-cyclohexadiene, the tricyclic allylic alcohol 112 is produced [61]. 

A priori, the mechanism of hydrogenation of benzene may be represented as a series of hydrogen transfers from the catalyst to the adsorbed benzene and the adsorbed intermediates (Scheme 5)." The often observed first order reaction in H2 suggests that the addition of the second hydrogen atom is the more difficult step, indicating that the largest energy barrier lies between adsorbed arene and adsorbed diene. However, no cyclohexadienes have been detected as intermediates. 

Related catalyst. A related homogeneous rhodium catalyst, RhHCI[Tj -CsCCHslellPICeHsIa], has recently been reported to catalyze hydrogenation of benzene to cyclohexane, also without intermediate formation of cyclohexadienes or cyclohexene. However, this catalyst functions in the absence of an added base. It also has high catalytic activity for hydrogen transfer from secondary alcohols to alkenes and alkadienes and is recovered unchanged after the transfer. ... 

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). 

A study of a transfer hydrogenation reaction provides a useful example of the mechanistic information available from a computational study and some of the caveats to be considered." A transfer hydrogenation reaction involves the net transfer of two hydrogen atoms from one reactant to another, as illustrated for the reaction of 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DIDQ, 51) and 1,4-cyclohexadiene (52) to produce 53 and benzene (equation 6.14). 

The order of quenching rate constants of triplet-excited azoalkanes and ketones cannot solely be explained on basis of energy transfer processes. Hydrogen abstraction from olefins and dienes plays a role as well, especially for double allylic systems such as 1,4-cyclohexadiene [206]. The resulting bis-aUylic radicals are highly stabilized, thus, hydrogen transfer to ketones and azoalkanes is thermodynamically favored. The superior reactivity of the stronger electron acceptor benzophenone can be partly... 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

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