Biphasic benzene hydroxylation

The Fenton-type system, Fe(II)/Cu(n)/H202, produced phenol with a moderate selectivity [28b]. In the absence of Cu(II) or Fe(III), biphenyl was the main product. Quinones, in particular l,2-naphthoquinone-4-sulfonate, played a vital role in the Fe(III)-promoted hydroxylation of benzene with HjOj, giving moderate yields of phenol [70]. The addition of polyethyleneglycol to Fe(III)/ HjOj in aqueous acetonitrile enhanced the phenol yield and selectivity [71]. A heterophase reaction in the presence of surfactants (quaternary ammonium salts, crown ethers, and polymers) has led to a further improvement. Biphasic benzene hydroxylation in the system composed of aqueous H O, Fe SO modified with 2-pyrazinecarboxylic acid (Hpca) derivatives, and CF COOH produced phenol with a selectivity of 97% (based on benzene) at substrate conversion of 8.6% (Eq. 14.22)... 

The hydrophobicity of TS-1 could also explain why the oxidation of hydrocarbons in aqueous H2C>2 is faster without added organic solvent (triphase catalysis) than in organic solution (biphase catalysis) e.g. benzene hydroxylation under triphase conditions was up to 20 times faster than in acetonitrile or acetone (biphase conditions).1741 Indeed, benzene competes more favourably with water than with organic solvents for adsorption within the micropores of hydrophobic TS-1, as furthermore confirmed through adsorption experiments.1471... 

Compared with the starting arenes, phenols are more reactive toward the hydroxyl radical, resulting in overoxidation to quinones and decreasing selectivity and yield of the reaction. Physical separation of products from the starting materials by means of a biphasic reaction mixture can be successfully used in this case [15], Yields as high as 80% (based on benzene) have been reported with cetyl-... 

The hydroxylation of benzene to give phenol was carried out using a biphasic mixture of hydrophobic ionic liquids such as [OMIM][BF4] or [OMIM][Pp6] and water/hydrogen peroxide [283]. A range of transition metal dodecylsulfate catalysts were employed with the iron giving the best results. Conversions of benzene to phenol of 54% were obtained and the ionic liquid/catalyst combination could be... 

A green aqueous-ionic liquid biphasic hydroxylation of benzene was performed using metal dodecanesulfonate salts as catalysts [73]. With 1 equiv. of H O, the phenol selectivity reached 90% at 54 and 60% conversion of benzene and oxidant, respectively. 

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