Hydrogen peroxide tautomerization

Thiourea dioxide, or formamidine sulfinic acid, is an oxygenated thiourea derivative synthesized by the oxidation of thiourea with hydrogen peroxide. It has the chemical formula (NH2)NHCS02H and is tautomeric. 

Only sulfoxidation, and no elimination, occurs when the hydrochloride of 2 (R = Me) is treated with sodium periodate at room temperature.3 Ketone 4, which represents the tautomeric form of dibenzo[6,/]thiepin-10-ol, can be sulfoxidized with hydrogen peroxide at room temperature to provide sulfoxide 5 in 56% yield however, at reflux temperature oxidation occurs additionally at the carbon atom next to the oxo group and sulfone 6 is isolated in 79% yield.3... 

This gives tautomeric mixtures119 when the tert-butyl group is removed. The methyl ether has been used to obtain 3-hydroxy-2-carbonyl derivatives in the selenophene series.120 The unsubstituted 2-hydroxyselenophene system has been prepared by hydrogen peroxide oxidation of 2-selenophene-boronic acid.121 However, in the 5-methyl-substituted system deboronation became such an important side reaction that 5-methyl-2-hydroxyselenophene had to be prepared by acid-catalyzed dealkylation of 5-methyl-2-fert-butoxy-selenophene. Both 2-hydroxy- and 5-methyl-2-hydroxyselenophene exist mainly as 3-selenolene-2-ones (93) and for the 5-methyl derivative it was possible to isolate the / ,y-unsaturated form (92) and follow the tautomeric isomerization. The activation parameters thus obtained were compared with those for the corresponding furan and thiophene systems. 

Biochemical oxidation of the ellipticine derivative 241 to the o-quinone 242 has been achieved using hydrogen peroxide and horseradish peroxidase (HRP) as a catalyst (83JMC574). Compound 242 was easily protonated to form a tautomeric equilibrium between 243 and 244 it gave an addition product with methanol and was reduced by cysteine (Scheme 41). 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

The hydroboration reaction is also useful with alkynes. As is shown in the following example, the product after treatment with basic hydrogen peroxide is an enol. As we have seen before, the enol cannot be isolated because it spontaneously tautomerizes to a ketone. This provides another way to hydrate alkynes to produce ketones. 

Hexyne has the triple bond in the middle of a carbon chain and is termed an internal alkyne. If, instead, an alkyne with the triple bond at the end of the carbon chain, a 1-alkyne or a terminal alkyne, were used in this reaction, then the reaction might be useful for the synthesis of aldehydes. The boron is expected to add to the terminal carbon of a 1-alkyne. Reaction with basic hydrogen peroxide would produce the enol resulting from anti-Markovnikov addition of water to the alkyne. Tautomerization of this enol would produce an aldehyde. Unfortunately, the vinylborane produced from a 1-alkyne reacts with a second equivalent of boron as shown in the following reaction. The product, with two borons bonded to the end carbon, does not produce an aldehyde when treated with basic hydrogen peroxide. 

Oxidation of the vinylborane (using basic hydrogen peroxide) gives a vinyl alcohol (end), resulting from anti-Markovnikov addition of water across the triple bond. This end quickly tautomerizes to its more stable carbonyl (keto) form. In the case of a terminal alkyne, the keto product is an aldehyde. This sequence is an excellent method for converting terminal alkynes to aldehydes. 

With strong oxidizing agents such as chromium trioxide in aqueous sulfuric acid, alkylpyrroles are converted into maleimides, e.g., 154. This oxidative technique played an important part in the classical determination of porphyrin structures. Milder oxidizing agents, such as hydrogen peroxide, convert pyrroles into pyrrolinones, e.g., oxidation of the parent heterocycle gives a tautomeric mixture of pyrrolin-2-ones 155 and 156. 

Peroxidic reagents may dehydrogenate C-8-Na. The example of this involves the conversion of isostrychnic acid (XXXIII) by hydrogen peroxide in formic or acetic acids in the presence of catalytic quantity of cobalt salt or by potassium nitrosodisulfonate, into the lactone bases CLXXIV (R = H and OH) (144, 145). In both cases, the initially formed simple 3-H indole is oxidized further, probably by way of the tautomeric enamine, to the 13-oxy derivative, which then lactonizes. 

The same authors found that thieno[2,3-c - and - 3,2-c]pyridones (e.g., 58) yielded substitution products (59) when treated with hydrogen halides and hydrogen peroxide, or with nitric acid. The orientation of substitution and increased reactivity compared with the parent thieno-pyridine were ascribed to the +M effect of the hydroxy group in that tautomeric form however, it is clear that the same results would be expected from the pyridone tautomer. 

Hydrogen peroxide is a more selective reagent and can convert pyrrole itself into a tautomeric mixture of pyrrolin-2-ones in good yield (section 13.17.1). 

Reactions.—All the mono- and poly-bromo-derivatives, as well as iodo-derivatives, of thieno[2,3-h]thiophen have been prepared by direct bromination or iodination, or from the lithium compounds, and characterized by their n.m.r. spectra. Boronic acids were obtained from several alkyl- and aryl-thienothiophens via lithiation and were converted by hydrogen peroxide oxidation into the tautomeric hydroxy-derivatives. It was shown by n.m.r. that all thieno[2,3-h]-thiophen systems exist as thieno[2,3-h]thiophen-2(3 )-ones, while in the case of... 

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