2,3,5,6-Tetrachloro 4-benzoquinone

The most useful class of inhibitors are molecules that react with chain radicals to yield radicals of low reactivity. Quinones such as benzoquinone and chloranil (2,3,5,6-tetrachloro-benzoquinone) are an important class of inhibitor. The behavior of quinones is quite complex [Eastmond, 1976a,b,c George, 1967 Small, 1975 Yamamoto and Sugimoto, 1979 Yassin and Risk, 1978a, 1978b]. Two major types of products are obtained—quinone and ether—formed by reaction at the C and O atoms of a quinone, respectively. Attack of a propagating radical at oxygen yields the aryloxy radical XXVIII, which can terminate by... 

Some inhibitors such as 2,3,5,6-tetrachloro-l,4-benzenedione (tetrachloro-benzoquinone) act as inhibitors by adding to the growing chain radicals to give radicals too stable to continue the chain ... 

The oxidation of 123 with chloranil (tetrachloro-/)-benzoquinone) in toluene at reflux temperature yielded the isoxazole 124, whereas treatment of 123 with Ra-Ni and H3BO3 in Me0H/H20 led to the oxime 125 in moderate yield. Attempts at reductive ring opening of 123 failed (Scheme 27) <1996T14323>. 

MCPBA m-chloroperbenzoic acid" — TCQP Z3,5,6-tetrachloro- -benzoquinone... 

Insertion of aldehydes into the P-halogen bond of phosphoranes referred to in Section 3 (ref.8) is also exemplified by the reaction of the pyrocatecholtribromophosphorane (22) with fluoral (23) to give (24) with 5 P,-55ppm. Further studies in this area by Pudovik et al. have shown that the reaction of the phosphite (25) with one mole of pyruvic ester (26) results in the formation of (29) via (27) and (28) but with two moles of (26) the phosphorane (30) is formed. The reaction of (25) with tetrachloro-benzoquinone to form a cyclic phosphorane (5 P,-53) is also reponed in this paper. 

The effect of the Galvani potential difference on the photocurrent responses is illustrated in Figure 11.24 for Fc and tetrachloro benzoquinone (TCBQ). In the case of the electron donor, the photocurrent increases as increases (Figure 11.24a). The opposite dependence is observed for the acceptor TCBQ. The behaviour of Jpjj in Figures 11.23 and 11.24 is self-consistent within the correlation... 

In the aryl nitro compounds, the powerful electron-withdrawing (—1, -M) effect of the nitro group results in the sulfonation of nitrobenzene being more difficult than that of benzene. Early work claimed that heating nitrobenzene with excess chlorosulfonic acid (two equivalents) afforded m-nitrobenzenesulfonyl chloride, but under drastic conditions (150 °C, 20 hours) the only product was reported to be tetrachloro-/>-benzoquinone (chloranil). 

DDQ has proved to be the most versatile of the high-potential quinones although certain others, notably chloranil (previous section) and tetrachloro-o-benzoquinone have been used on occasion. 

Recently Swan has employed tetrachloro-o-benzoquinone in the oxidation of the 3,4-dihydro-j8-carbolinium cation 124 to the j8-carbo-linium cation 252. Dehydrogenation with palladium black at 175° or at a higher temperature and with selenium has also been successfully used for the purpose. 

Tricarbonyl[t/M-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with the 2-oxyallyl cation derived from 2,4-dibromo-2,4-dimethylpentan-3-one and nonacarbonyldiiron(O) yields a mixture of adducts which, after oxidative decomplexation with tetrachloro-l,2-benzoquinone (o-chloranil), affords the tetrahydrofuro[2,3-A)azcpine derivative 33 and the 3-substituted 1H-azepine-l-carboxylate 34.227... 

Ethyl 1//-azepine-1-carboxylate (1), on treatment with 3,4,5,6-tetrachloro-l,2-benzoquinone in benzene at room temperature, deposits a mixture of the [6 4- 4] 7t-adduct 29, the expected [2 + 4] 7T-adduct 30, and the unexpected regioisomer 31, the product of a rare C2-C3 cycloaddition.265 The [6 + 4] adduct 29 is thermally unstable and rearranges in high yield (63%) to adduct 31 in refluxing benzene. Cycloaddition fails with the more electron-rich 1,2-naph-thoquinone. 

The isomeric pyridazino[4,5-6]azepine 19 is obtained directly during the decomplexation of the [4 + 2] adduct 17 formed from tricarbonyl(ethyl +17/-azepine-l-carboxylate)iron and 1,2,4,5-tetrazine-3,6-dicarboxylate, with trimethylamine A-oxide.113 Surprisingly, decomplexation of adduct 17 with tetrachloro-l,2-benzoquinone yields only the dihydro derivative 18 (71 %), aromatization of which is achieved in high yield with trimethylamine A-oxide in refluxing benzene. 

Ethyl H-, 2-diazepine-l-carboxylate functions as a 2 -component in the Diels-Alder reaction with tetrachloro-l,2-benzoquinone to give a mixture of the regioisomers 20 and 21.100... 

Cycloaddition-cyclorevcrsion reaction of 9-oxabicyclo[6.1.0]nona-2,4,6-triene (6) with 3,6-diphenyl-1,2,4,5-tetrazine followed by dehydrogenation with tetrachloro-1,2-benzoquinone (o-chloranil) yields the annulated oxonin 7.8... 

Transformations which alter the bacteriochlorin chromophore are quite rare. An important reaction in the structural elucidation of the bacteriochlorophylls is the dehydrogenation to chlorophyll derivatives. Thus, bacteriopyromethylpheophorbide a (1) can be smoothly dehydrogenated with 3,4,5,6-tetrachloro-l,2-benzoquinone to the corresponding chlorin 3-acetyl-pyromethylpheophorbide a (2) in high yield.1 la,b... 

Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion °... 

The allylidenephosphorane (64) with phenanthraquinone gave the pyran (65) together with small amounts of (66) and (67). Similar pyrans were also obtained from (64) and a-naphthoquinone and tetrachloro-o-benzoquinone. 

Quinoxalinone charge-transfer complexes with tetrachloro-l,4-benzoqui-none and tetrabromo-l,4-benzoquinone.168... 

---