Ortho-quinonoid

Figure 6. Suggested mechanism for the reaction of ortho-quinonoid structures in lignin derivatives with hydrogen peroxide under alkaline conditions (according to Gierer, ref. 3).

When the ortho-para-directing group is one with an unshared pair (this of course applies to most of them), there is another effect that increases the amount of para product at the expense of the ortho. A comparison of the intermediates involved (p. 509) shows that C is a canonical form with an ortho-quinonoid structure, while D has a para-quinonoid structure. Since we know that para-quinones are more stable than the ortho isomers, it seems reasonable to assume that D is more stable than C and therefore contributes more to the hybrid and increases its stability compared to the ortho intermediate. 

All these special cases involve benzologs with an ortho quinonoid structure. But even this pattern is not universal, a deviation being 2-methylindazole (40), which quaternizes 2.2 times more slowly than its parent compound 1-methylpyrazole (37).122 Indeed, except for the molecules just considered and 1,2-benzisothiazole,122 benzo-fusion is rate retarding. Thus, 1,2-benzisoxazole (indoxazene, 39), reacts 3.2 times, and 1-methylindazole (39) 7.1 times, more slowly than their parent compounds.122 Fusing a benzene ring onto an azole where the heteroatoms are situated 1,3 leads to decreases in rate constants by factors of 5.0, 6.3, and 6.8, respectively, when X of 38 is NMe, S, and O.122 These factors are not much smaller than that obtained from a comparison of pyridine and quinoline reactivities.61,78,79... 

The stability of betaines of type 246 and 248 is thus mainly determined by the environment of the deprotonated hydroxyl group, and their further transformations may be sterically retarded (cf. Section III,E,2). At the same time, on treatment of salt 249 with triethylamine in anhydrous solvents, betain 250 was detected by electronic absorption spectra as a shortlived compound. Its instability is obviously explained by the disturbance of aromaticity in the annelated benzenoid ring, resulting in an ortho-quinonoid fragment (87RRC417). 

A corroboration of the key role of the intermediate cation 291 is the observation of a sharp yield increase of disproportionation products 297 and 298, compared to their direct formation from salt 280 on treatment of dimer 281 with catalytic amounts of triethylammonium perchlorate, which acts as the protonating agent generating intermediate 293. The formation of unsaturated dimers 296 is possible not only by the direct hydride transfer, followed by deprotonation of dimeric salt 294 (pathway a, Scheme 16), but also by equivalent 1,5-hydrogen transfer in the ortho-quinonoid intermediate 295 formed by deprotonation of the acidic methine group in intermediate 291 (pathway b). 

The formation of o-quinones by the above oxidative methods is less reliable since the ortho quinonoid system is more susceptible to attack by electrophilic and nucleophilic species mild conditions are therefore essential. The use of the silver carbonate/Celite reagent noted above for phenol coupling reactions is particularly suitable the conditions are those described in Expt 6.129, and they have been applied to the oxidation of catechol, 4-methylcatechol, 4-t-butylcatechol, and 3,5-di-t-butylcatechol to yield the corresponding o-quinones in almost quantitative yield. 

Lebo, S.E., JR., Lonsky, W.F.W., McDonough, T.J., Medveca, PJ. and Dimmel, D.R., "The Occurrence and Light-Induced Formation of ortho-Quinonoid Lignin Structures in White Spruce Refiner Mechanical Pulp", J. Pulp Paper Sci., 1990,16, J139. 

The LCGO calculations of dipole and second moments for a number of five-membered heterocycles agreed with the experimental absolute dipole moment (74JCS(P2)42o). The dipole moments of 4-, 5- and 6-substituted 2,1,3-benzazoles (1), (2) and (5), determined in benzene at 25 °C (73JHC773), show that the 2,1,3-benzoxadiazole (5) exists primarily in the ortho-quinonoid form, while (1) and (2) are more in the benzenoid form. The greatest amount of mesomeric contribution occurs in the selenium compound. 

The phosphonic acid analog of NSAID (Non-Steroidal Anti-Inflammatory Drug) diclofenac was successfully synthesized in the laboratory of B. Mugrage using a novel acid catalyzed Arbuzov reaction as the key step followed by a TMSBr promoted dealkylation. It needs to be pointed out that the nucleophilic attack takes place on the ortho-quinonoid intermediate in a non-SN2 process. 

In a Wheland intermediate, a disproportionate amount of the positive charge would reside at the para position. Hence, if there were a substituent that could stabilise that charge, it would have greater effect than a substituent at the ortho position thus, the amount of para substitution would be increased over that predicted on purely statistical grounds. In line with this deduction, it is observed that para-quinonoid structures are more stable than those with ortho-quinonoid structures. 

In both chemical and enzymic (phenylalanine hydroxylase) oxidation of tetrahydropteridines.ortho-quinonoid dihydropterins... 

The usefulness of benzocyclobutenyl derivatives for the synthesis of isoquinoline alkaloids (see Vols. 2—4 of these Reports) has been extended to the preparation of the protoberberine bases (Schemes 9 and 10). In one case (Scheme 9), compound (155) was thermolysed to afford the protoberberinium salt (157), the reaction plausibly proceeding via the ortho-quinonoid intermediate (156), which then undergoes electrocyclization and dehydrogenation to give the product. Reduction of (157) then provided the protoberberine (158 R = H, R = R = Me). This route was used for the synthesis of (+ )-discretine (158 R = H,... 

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