[7722-86-3] · H2O5S · Monoperoxysulfuric Acid · (MW 114.07)
(strong oxidizing agent for many functional groups;1 can selectively oxidize amines;2 in alcohol solvent can directly convert conjugated aldehydes to esters;3 can effect regioselective Baeyer-Villiger oxidations;4 can promote regioselective oxidative rearrangement of tertiary alcohols to o-hydroxy ketones5)
Alternate Name: Caro's acid.
Physical Data: mp 45 °C.
Analysis of Reagent Purity: a spectrophotometric method is available for analysis of the components of an H2SO5/H2S2O8/H2O2 mixture.6
Handling, Storage, and Precautions: may react explosively with acetone, and primary or secondary alcohols.
Neutralized Caro's acid can chemoselectively oxidize arylamines to nitroso compounds (eq 1);1b,2a,7 a simpler procedure for this transformation uses Acetic Acid/Hydrogen Peroxide 30%.2b An alternative pathway leading to the formation of azoxybenzenes (eq 1);7 or nitro compounds (eq 2)8 is also possible and depends upon substrate, solvent, pH, and reaction conditions. Caro's acid oxidation of polyhalodiazines is unfruitful, except for pyrazines. While chloropyrazines and chloroquinoxalines react with 30% H2O2 in glacial acetic acid to give mono-N-oxides with selective N-4 oxidation,9 Caro's acid affords mono-N-oxides with regioselective oxidation at N-1 (eq 3).
Conjugated aldehydes can be oxidized directly to a,b-unsaturated esters by Caro's acid in alcoholic media (eq 4).3 The reaction probably proceeds through a hemiacetal and not the carboxylic acid.
Caro's acid has proven useful in the Baeyer-Villiger oxidation of cyclic ketones and steroids.4 The regioselective Baeyer-Villiger oxidation of isatins to 2,3-dioxo-1,4-benzoxazines observed with Caro's acid contrasts sharply with formation of isatoic anhydrides using Peracetic Acid (eq 5).4a The related buffered reagent Bis(trimethylsilyl) Monoperoxysulfate is a strong Baeyer-Villiger oxidant. This aprotic reagent is soluble in methylene chloride and generally gives higher yields and purer product than Caro's acid.10
Caro's acid oxidizes tertiary alcohols to hydroperoxides, which rearrange in the acid medium. The procedure is useful for the conversion of cycloalkanols to o-hydroxy ketones (eq 6).5 Cyclic o-hydroxy ketones exist as oxonium ions in sulfuric acid and thus are resistant to further Baeyer-Villiger oxidation.
Grant R. Krow
Temple University, Philadelphia, PA, USA