[23115-33-5] · C6H18O5SSi2 · Bis(trimethylsilyl) Monoperoxysulfate · (MW 258.44)
Physical Data: colorless viscous liquid at -30 °C. 1H NMR (CD2Cl2, 200 MHz, -30 °C) d 0.38 (s);2 (-50 °C) 0.28 (s) and 0.42 (s) due to nonequivalence of the Me3Si groups.4 13C NMR (CD2Cl2, 50 MHz, -30 °C) d 0.53 (s).2
Solubility: sol CH2Cl2, CCl4, and diethyl ether; sparingly sol petroleum ether.
Analysis of Reagent Purity: the oxidation of I- to I2 serves for the determination of the peroxide content (iodometric titration). Moreover, bis(trimethylsilyl) monoperoxysulfate is a sufficiently strong oxidizing agent to oxidize Cl- or HCl gas to Cl2 in CH2Cl2 solution at -20 °C.3
Preparative Method: the reagent is conveniently prepared1,3 by adding dropwise a solution of Sulfur Trioxide (1 equiv or slightly less) in dry CH2Cl2 to a solution of Bis(trimethylsilyl) Peroxide6 in dry CH2Cl2 at -30 °C, followed by stirring further for 30 min at this temperature (eq 1). The yield is quantitative and the crude reagent can be used directly for oxidation reactions. Use of more than 1 equiv of SO3 affords bis(trimethylsilyl) peroxydisulfate.3
Handling, Storage, and Precautions: the reagent should be handled below -20 °C under anhydrous conditions and used immediately after preparation. It decomposes (explosion) on warming to rt, with the evolution of sulfur trioxide.5 The reagent rearranges at rt in CH2Cl2 solution to the isomeric trimethylsilyl methoxydimethylsilyl sulfate within 1-2 d;5 the rearrangement is slower at -30 °C and requires about 36 h for complete conversion.2 The reagent undergoes hydrolysis in the presence of water to form hexamethyldisiloxane and peroxymonosulfuric acid.3,5 A good fume hood, a safety shield, and all safety precautions required for peroxides are essential.
Bis(trimethylsilyl) monoperoxysulfate is a convenient reagent for Baeyer-Villiger oxidations under mild conditions, in which high yields (88-98 %) of esters or lactones are obtained from the corresponding ketones (eq 2).1,2 Usually 3-5 equiv of the reagent are required and the reaction is complete within a few hours (3-8 h). Its solubility in CH2Cl2 makes it a useful reagent for Baeyer-Villiger oxidations in nonprotic solvents, unlike Monoperoxysulfuric Acid (Caro's acid) or Potassium Monoperoxysulfate (Oxone). An important advantage of this reagent over monoperoxysulfuric acid is the fact that after oxygen transfer the byproduct, bis(trimethylsilyl) sulfate, is not acidic and therefore undesirable side products are usually minimized.
Bis(trimethylsilyl) monoperoxysulfate in CH2Cl2 oxidizes 4-heptanone, benzophenone, fluorenone, adamantanone, and tetracyclone to their corresponding Baeyer-Villiger rearrangement products in high yield.1 Acetophenone gives mainly phenyl acetate, together with an appreciable amount (20 %) of methyl benzoate, despite the poor migratory aptitude of the methyl vs. phenyl group in Baeyer-Villiger oxidations.2 Cyclohexanone affords ε-caprolactone in high yield, along with small amounts of g-caprolactone.2 Through the use of 18O-labeled Me3SiOSO218O18OSiMe3 it was shown that this reaction proceeds via an intermediate dioxirane.2
The limitations of this reagent in Baeyer-Villiger oxidations are that a,b-unsaturated ketones react sluggishly and that double bonds are epoxidized.1
Bis(trimethylsilyl) monoperoxysulfate in CH2Cl2 at low temperature oxidizes alkenes to ketones in high yield via the intermediacy of the corresponding epoxide (eqs 3 and 4). Addition of ketones to the reaction mixture remarkably increases the rate of this reaction.2
Bis(trimethylsilyl) monoperoxysulfate reacts instantaneously with sulfides such as methyl p-tolyl sulfide to give the corresponding sulfoxide and bis(trimethylsilyl) sulfate (eq 5).2 The reagent oxidizes pyridine to pyridine N-oxide at -30 °C in CH2Cl2 solution (eq 6).3
Waldemar Adam & Pralhad A. Ganeshpure
University of Würzburg, Germany