Elbe reaction

The AlCIs-catalyzed benzoylation of 2-methyl-5-phenyIthiophene to 2-methyl-3-benzoyl-5-phenylthiophene (118) is of certain interest for comparison of the activating effect of an alkyl and phenyl group. The structure of (118) was proved in an original way by conversion to (119) by an Elbs reaction. ... 

The hydroxylation of a phenol 1 upon treatment with a peroxodisulfate in alkaline solution, to yield a 1,2- or 1,4-dihydroxybenzene 3, is called the Elbs reaction The phenol is deprotonated by base to give a phenolate anion 4, that is stabilized by resonance, and which is activated at the ortho or the para position towards reaction with an electrophilic agent ... 

The main product of the Elbs reaction is the 1,4-dihydroxybenzene (hydro-quinone). If the para position is already occupied by a substituent, the reaction occurs at an ortho position, leading to a catechol derivative although the yields are not as good as for a hydroquinone. Better yields of catechols 7 can be obtained by a copper-catalyzed oxidation of phenols with molecular oxygen ... 

This reaction, that can be viewed as a modified Elbs reaction, often gives good yields, while the ordinary procedure often gives catechols in less then 50% yield. 

Nevertheless the Elbs reaction is a valuable method for the preparation of dihydroxy benzenes. The experimental procedure is simple, and the reaction conditions are mild a variety of functional groups is tolerated. 

Another hydroxylation reaction is the Elbs reaction In this method, phenols can be oxidized to p-diphenols with K2S20g in alkaline solution. Primary, secondary, or tertiary aromatic amines give predominant or exclusive ortho substitution unless both ortho positions are blocked, in which case para substitution is found. The reaction with amines is called the Boyland-Sims oxidation. Yields are low with either phenols or amines, generally under 50%. The mechanisms are not clear, but for the Boyland-Sims oxidation there is evidence that the S20 ion attacks at the ipso position, and then a migration follows. ... 

Hurd, C. D., and J.Azorlosa Deuterium Tracers in the Elbs Reaction. 

When aromatic amines react with potassium persulfate in alkaline aqueous solution at room temperature in the absence of a catalyst, they are converted to the corresponding phenol . Behrman has studied the ortho-para ratio in this reaction and found that the ortho product predominates over the para product (in contrast with the Elbs reaction)... 

For comments on this and the Elbs Reaction, see M. B. Smith, J. March in March s Advanced Organic Chemistry, 5th ed., John Wiley and Sons, Inc., New York, 2001, p 724 E. J. Behrman, Organic Reactions 35, 2... 

The well-known Elbs reaction has been applied to thiophene syntheses, but the reaction may involve some isomerization. Thus pyrolysis of 3-o-toluoylbenzo[6]thiophene (347) for 3 hours at 340-360 °C gave naphtho[2,1 -b]benzothiophene (348) in 35% crude yield, instead of the expected naphtho[2,3-Z>]benzothiophene. A radical mechanism (Scheme 26) was proposed to explain this result (56JCS3435). 

A number of other aromatic oxidations involving peroxygen species have been documented. Probably the best known of these is the hydroxylation of phenols using alkaline peroxydisulfates, known as the Elbs reaction.490 This... 

Cyclodehydration of diaryl ketones through an ortho methyl or methylene group by pyrolysis at 400-450° gives low yields of certain substituted anthracenes and their benzologs (Elbs reaction). ... 

This oxidation is similar to the Elbs reaction, except that ortho substitution of sulphate normally occurs, e.g. 

The yield of o-amino aryl sulphate is generally 10-40 %. If both ortho positions are blocked then para substitution results. For preparative purposes the sulphate is hydrolysed by acid, as with the Elbs reaction. Behrman studied the oxidation of 2-amino-pyridine, and showed that the kinetics are second -order for the major part of reaction, viz. 

The salt effect is small and positive, consistent with the attack of an ion on a neutral molecule. Allyl acetate has no effect on the rate or yield of product. Electron-releasing substituents accelerate the reaction. Thus the features of the reaction are similar to those of the Elbs reaction, and point to a mechanism involving nucleophilic attack of the amine on peroxide oxygen. In order to determine whether attack occurred by nitrogen or carbon, Behrman compared the rates of oxidation of amines such as 2-amino-5-methyl-pyridine and 2-amino-6-methyl-pyridine, and found that in all cases the order of reactivity is consistent with attack by nitrogen. Furthermore, the rate of oxidation of 3,5-dideutero-2-amino-pyridine is the same as for the normal material, and the yield of product is the same. Behrman proposed the reaction scheme... 

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