Hydroxylation, Boyland-Sims Elbs reaction

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. ... 

Hydroxylation of arylamines with persulfate ion, or Boyland-Sims oxidation, gives ortho-substituted aminophenols in good yields [29]. As with the Elbs oxidation, the procedure is also carried out in two steps - first, treatment with the oxidant to obtain an aminophenyl sulfate ester and, second, hydrolysis to obtain the final product. Primary, secondary and tertiary amines can all be used in this reaction. The ortho product is formed, except when no ortho-positions are available, which leads to para-substitution. Electrophilic attack on the ipso-carbon is believed to be the most likely mechanism, although minor radical pathways also seem to be present. 

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