2,3-Dichloro-5,6-dicyano-p-quinone

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. 

When testosterone acetate is refluxed with 2,3-dichloro-5,6-dicyano-p-benzoquinone andp-toluenesulfonic acid in benzene for 5 h, a 60% yield of 6-dehydrotestosterone acetate is obtained [966]. The treatment of an-drost-4-ene-3,17-dione in dioxane with the same quinone and gaseous hydrogen chloride gives a 72% yield of androsta-4,6-diene-3,17-dione [966] (equation 30). 

Very selective oxidizing agents suitable for the conversion of primary alcohols into aldehydes are high-potential quinones such as tetrachloro-o-benzoquinone, tetrachloro-p-benzoqninone, and 2,3-dichloro-5,6-dicyano-p-benzoquinone [973]. Such dehydrogenations are carried out in chloroform, carbon tetrachloride, or ethanol, usually under very mild conditions at room temperature or in refluxing ether, and give fair to good yields (equation 216) [973]. 

A solution of 0.65 g (0.005 mol) of tetralin and 1.14 g (0.005 mol) of 2,3-dichloro-5,6-dicyano-p-benzoquinone in 5 mL of benzene is refluxed for 45 min, during which period the initidly red solution becomes colorless. A solution of an additional 1.14 g (0.005 mol) of the quinone in 2 mL of benzene is added, and the refluxing is continued for an additional 75 min. After dilution with light petroleum, the solution is filtered, passed through a column of alumina, and evaporated to give 0.42 g (70%) of naphthalene, mp 79-80 °C. The petroleum-insoluble residue yields 0.7 g (61%) of colorless 2,3-dichloro-5,6-dicyanohydroquinone, mp 263 °C (dec), after crystallization from aqueous ethanol. 

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. 

General methods for o-quinones Benzoyl /-butyl nitroxide, 28 General methods for p-quinones Dicarbonylcyclopentadienylcobalt, 96 2,3-Dichloro-5,6-dicyano-1,4-benzoqui-none, 104 Squaric acid, 284 Benzoquinones... 

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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