Aryl formates

The Dakin Reaction allows the preparation of phenols from aryl aldehydes or aryl ketones via oxidation with hydrogen peroxide in the presence of base. The aryl formate or alkanoate formed as an intermediate is subsequently saponified to yield the substituted phenol product. 

ArCHO — ArOH. Aromatic aldehydes undergo Baeyer-Villiger reaction when treated with H202 (30%) in the presence of several selenium compounds as catalysts, of which areneseleninic acids are most effective. The resulting aryl formates are readily hydrolyzed to phenols. 

For example, oxidation of acenaphthene by red lead in acetic acid gives 7-acenaphthenol acetate, from which 7-acenaphthenol is obtained by saponification with methanolic sodium hydroxide. Phenols may be prepared indirectly from aromatic aldehydes by oxidation with peracetic acid followed by hydrolysis of the resulting aryl formate. "... 

M.E. Jung and co-workers have developed a synthesis of selectively protected L-Dopa derivatives from L-tyrosine via a Reimer-Tiemann reaction followed by the modified Dakin oxidation. The formyl group introduced by the Reimer-Tlemann reaction had to be converted to the corresponding phenol. After trying many sets of conditions, the Syper process was chosen, which uses arylselenium compounds as activators for the oxidation. Treatment of the aromatic aldehyde with 2.5 equivalents of 30% hydrogen peroxide in the presence of 4% diphenyl diselenide in dichloromethane for 18h gave the aryl formate in excellent yield. This ester was cleaved by treatment with methanolic ammonia for 1h to afford the desired phenol in good yield. 

The oxidation of aromatic aldehydes to phenols via the aryl formates is known as the Dakin reaction and is evidently related to the Baeyer-Villiger oxidation of ketones. The use of MCPBA is well known [45]. The successful use of MMPP and UHP-acetic anhydride in these reactions (Eqs. 34 to 36) can be achieved with aromatic aldehydes that have an electron releasing substituent in an ortho- or para-position. In the absence of a suitable electron releasing substituent, aromatic aldehydes are oxidised to the corresponding carboxylic acid. 

However, given that transition metal catalysis had arguably the strongest impact through the development of efficient methods for bi(hetero)aryl formations, we focus here largely on C—H bond functionalizations for catalytic direct arylations of (hetero) arenes [24]. 

Baeyer-Villiger oxidation. The Oj/t-PrCHO/metal catalyst system transforms methoxyaryl aldehydes mainly to aryl formates. ... 

In a variation of the Bayer Villiger reaction, known as the Dakin reaction, the oxidation of aromatic aldehydes to phenols is accomplished via aryl formate... 

This reaction was initially reported by Dakin in 1909. It is the preparation of phenols from aryl aldehydes or aryl ketones involving the oxidation of corresponding aromatic compounds by hydrogen peroxide in the presence of a base and subsequent hydrolysis of the aryl formate or alkylcarboxylate intermediates. Therefore, this reaction is generally known as the Dakin reaction. It has been reported that the para-or ortho- substituents (such as OH, NH2) on aryl aldehydes or aryl ketones will facilitate this reaction, especially for the ortho OH group, which accelerates the reaction via the formation of an intfamolecular hydrogen bonding. ... 

The oxidation of aromatic aldehydes could be complicated. In addition to direct oxidation to the dicarboxylic acid (phthalic acid), an aromatic aldehyde carbonyl could also be oxidized and then rearranged and hydrated to form a phenol via aryl formates (Dakin reaction)." Thus, when treating p-hydroxy-benzaldehyde, 25, with basic hydrogen peroxide, the following mechanism is actually involved (Fig. 10). 

SCHEME 12.35 Oxidative aryl-aryl formation of nonactivated aromatic substrates. 

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