1.4- Cyclohexadiene, reactions with hydrogen atoms

The substrate scope and mechanism of Rh2(cap)4-catalysed TBHP oxidation of phenol and aniline was discussed. The rate of oxidation of para-substituted phenols to 4-(f-butyldioxy)cyclohexadien ones increased significantly in aromatic hydrocarbon solvents. Comparative results with RuCl2(PPh3) and Cul were provided. The results were consistent with hydrogen atom abstraction by the f-butyl peroxy radical followed by combination of the phenoxy and the f-butylperoxy radicals. Under similar reaction conditions,para-substituted anilines were oxidized to the corresponding nitroarenes, and primary amines were oxidized to carbonyl compounds in moderate to good yields. ... 

Von Sonntag and coworkers14 repeated Michael and Hart s study of the reaction of OH radical with 1,3- and 1,4-cyclohexadienes and extended it. They found that in the case of 1,4-cyclohexadiene, 50% of the OH radicals abstract an hydrogen atom, while only about 25% of the OH radicals abstract an hydrogen atom from 1,3-cyclohexadiene. The remaining OH radicals probably add to the double bond. The addition to the double bond was confirmed by final products analysis in the case of the 1,4-isomer. When N20-saturated aqueous solution of 1,4-cyclohexadiene (10-2 M) together with lower (10-4 M) concentration of the thiol (1,4-dithiothreitol) was y-radiolysed, it was found that 4-hydroxycyclohexene was produced with a yield of 0.29 prnol J 1, i.e. a yield of 50% of the OH radicals (equation 9). 

Although O2 reacts with proton sources to form HOO- (which dispropor-tionates via a second 02), with limiting fluxes of protons to control the rate of HOO- formation from 02, the rate of decay of HOO- is enhanced by reaction with the allylic hydrogens of excess 1,4-cyclohexadiene (1,4-CHD).25 Because HOO- disproportionation is a second-order process, low concentrations favor hydrogen-atom abstraction from 1,4-CHD. This is especially so for MezSO, in which the rate of disproportionation for HOO- is the slowest (PhCl > MeCN > H20 > DMF > Me O).16... 

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

Vitamin E and vitamin C are also good hydrogen atom donors in living bodies. The rate constants for the reaction of an alkyl radical and an alkoxyl radical with vitamin E are 1.7 X 106 and 3.8 X 109 M-1 s-1, respectively [58, 59]. The rate constants of hydrogen atom abstraction from R-H such as cyclopentane, 1,4-cyclohexadiene, tetrahydrofuran, Bu3SnH by tert-BuO are shown in Table 1.12. 

To study the effects of water and other solvents on titanocene(III)-mediated processes we used the transannular cychzation of epoxygerma-crolides as a model reaction [47]. Thus, we found that in anhydrous, non-halogenated solvents such as THF the reaction led selectively to decalins with an exocyclic double bond (Scheme 5). In an aqueous medium (THF/H2O), however, the characteristic lime green color of Cp2TiCl turned deep blue and the main product was a reduced decalin (Scheme 5). Under these conditions, water (either H2O or D2O) proved to be more effective than the toxic and expensive hydrogen-atom donor 1,4-cyclohexadiene for the reduction of tertiary radicals [47]. This is an unusual phenomenon in free-radical chemistry [48-50], subsequently exploited by us for the selective reduction of aromatic ketones as we shall see later [51,52]. 

The reaction of tetrachlorothiophene 1,1-dioxide 53 with a dienophile 64 affords the primary product 65 in a stereospecific manner because of steric demand [162, 163]. In 65, the cyclohexadiene ring protons are placed in close proximity to the other ring double bond. This enables the hydrogen atom transfer to take place stereospecifically to give 66 (Scheme 37). In the case of the dienophile 67, such a reaction is impossible, and the normal product 68 was obtained (Scheme 38). An additional two examples are also shown below (Scheme 39) [162-165]. 

Figure 6.13 illustrates two pathways that had been proposed for the transfer hydrogenation reaction of acridan (54) and a-methylstyrene (55) to give acridine (56) and cumene (57). The top pathway proceeds by transfer of a hydride ion from 54 to 55 to give an ion pair intermediate, which subsequently transfers a proton to the anion to complete the reaction. The bottom pathway is a radical mechanism that involves a sequence of two hydrogen atom transfers. Previous reports suggested that the reaction of acridan with a-methylstyrene and the reaction of DDQ with 1,4-cyclohexadiene most likely involved the radical pathway but that some other transfer hydrogenation reactions followed the ionic pathway. 

James and Troughton ( ) obtained ethylene and 1,3,5-hexatriene as the primary products in their study on the reaction of diallyl with the ethyl radical at 134- 175 C. Furthermore, they obtained 1,3-cyclohexadiene as a successive product. Recently Orchard and Thrush (19) reported the thermal isomerization of 1,3,5-hexatriene to 1,3-cyclohexadiene at ca. 400 C and the consecutive formation of benzene at ca. 550 C. In the present work, 1,3-cyclohexadiene (reaction 17) and benzene (reaction 18) were obtained as the secondary products. The hydrogen atom produced in reactions 12,... 

From a mechanistic point of view, in the initial step, the (Z)-l,2,4-heptatrien-6-yne, or compounds containing an equivalently unsaturated core, undergoes a mild, thermal electrocyclization reaction to form an a,3-alkylbenzenediyl, a diradical intermediate with substantial polar character. Dehydroaromatic intermediate 7, when trapped by the solvent or compounds (e.g., 1,4-cyclohexadiene) present in the reaction medium, forms than aromatic products of type 8, 9, and 10. In methanol, a mixture of hydrogen atom abstraction and polar addition product are obtained. ... 

Many side reactions compete efficiently with the HAS process. Under non-oxidizing conditions, the cyclohexadienyl radical 1 is rather long lived and can dimerize to 5 (radical-radical coupling) or disproportionate to cyclohexadiene 6 (H-atom abstraction) and substitution product 3 (Scheme 9.2). Long-lived radical 1 may also couple with radicals derived from the radical initiator [8]. These reactions can be considered as termination steps [9]. In addition, radical R can be reduced by hydrogen atom abstraction from the solvent to yield R-H, before the attack on the benzene core preventing the HAS reaction. 

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