Dichloro- 5,6-dicyano-p-quinone DDQ

Interestingly, the simple p-quinone (84a) is also able to oxidize certain unsaturated alcohols under harsh conditions.98 Because of its lower oxidation potential, p-quinone only oxidizes unsaturated alcohols devoid of steric hindrance and able to generate very stabilized carbocations. Thus, it is able to react with primary cinnamyl alcohols but not with secondary cinnamyl alcohols, simple allylic alcohols and benzylic alcohols. 

As early as in 1956, Braude et al.92 suggested that the selective oxidation of unsaturated alcohols with the quinone o-chloranil (82), can be explained by the intermediacy of a resonance-stabilized cation resulting from a hydride abstraction. Later, detailed mechanistic studies confirmed this hypothesis94c,95e in oxidations performed with the more common quinone DDQ. 

The speed of alcohol oxidation with DDQ correlates with the following factors  

Because of the very facile oxidation of benzylic alcohols possessing phenol at the ortho or para position, DDQ has been described as the preferred oxidant in those cases over other oxidants, such as Mn02.95f 

Electron-withdrawing groups close to the alcohol functionality may likewise destabilize intermediate carbocations and result in very slow oxidations. For instance, sterol 88 is oxidized with DDQ at the allylic alcohol two hundred times slower than the corresponding compound lacking the fluorine atom,94 and the treatment of hydrobenzoin (89) with DDQ results in the oxidation of a single alcohol because a second oxidation would involve a carbocation highly destabilized by the presence of a carbonyl group.95f 

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A variety of organic compounds such as 2,3-dichloro-5,6-dicyano-p-quinone (DDQ) and related benzoquinones, m-chloroperbenzoic acid, or dioxirane have been utilized as oxidants in organic synthesis. This section will focus on the synthesis of natural products and related compounds using DDQ. 

Most aliphatic alcohols react slowly if at all with 2,3-dichloro-5,6-dicyano-p-benzo-quinone (DDQ), allowing selective allylic or benzylic alcohol oxidation. 

The oxidation of hydroarenes to arenes by quinones such as 2,3-dichloro-5,6-dicyano-l,4-quinone (DDQ) is frequently used for the synthesis of aromatic compounds. Brower et al. have already shown that the dehydrogentaion 1,4-cyclo-hexadiene to benzene [128] or tetraline to naphthalene [129] by thymoquinone is accelerated by pressure giving a negative volume of activation ((AV = —33 (75 °C) and —28 (175 °C) cm mol respectively). A similar effect of pressure has been observed for the oxidation of leuco crystal violet with p-chloranil ((AV = -25 cm mol (21 °C) [130]. The pressure-dependent kinetic isotope effect of this reaction (29 °C kn/ D = H-5 (1 bar) and 8.2 (1.5 kbar)) indicates that hydrogen transfer occurs in the rate-determining step. The large kn/ko value at 1 bar and it pressure dependence was attributed to a quantum mechanical tunneling. 

A propargylic phosphonium salt was transformed to a cyclic derivative in a cobalt-catalyzed Diels-Alder reaction that was utilized in a Wittig olefination. All the three reaction steps including the formation of the phosphonium salt and its reaction with aldehydes were accomplished in a one-pot sequence. Moreover, the dihydro derivative was aromatized by oxidation with 2,3-dichloro-5,6-dicyano-l,4-quinone (DDQ). The ratio of the E and Z isomers was in the range ca. 1-2 (Scheme 12)P... 

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