Sn-b molecular sieve catalysed Baeyer–Villiger oxidation in ionic
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Sn-bmolecular sieve catalysed Baeyer–Villiger oxidation in ionicliquid at room temperature
Sharad P. Panchgalle,aUttam R. Kalkote,*aPrashant S. Niphadkar,bPraphulla N. Joshi,b
Subhash P. Chavanaand Gopal M. Chaphekarb
aDivision of Organic Chemistry: Technology, National Chemical Laboratory, Pune-411008,India. E-mail: kalkote@dalton.ncl.res.inbCatalysis Division, National Chemical Laboratory, Pune-411008, India
Received 22nd April 2004, Accepted 25th May 2004First published as an Advance Article on the web 21st June 2004
Efficient oxidation of aryl ketones to esters at room temperaturewith 30% aqueous H2O2and catalytic Sn-b-molecular sieve inionic liquid was established.
IntroductionMore than 100 years ago Baeyer and Villiger opened a new area toorganic chemists by reporting the oxidation of ketone to ester,commonly known as Baeyer–Villiger oxidation.1Baeyer–Villiger
oxidation is attractive for practical applications into building blocksfor complex bioactive molecules. Traditionally this reaction isperformed with peroxy acids as oxidant. This oxidation was madesimpler by replacing traditionally used peroxyacids with hydrogenperoxide,2a cheaper and less polluting reagent. More benefits were
expected from the catalytic version of this oxidation by minimizingreactant use and waste production. Efficient activation of ketoneoxidation by hydrogen peroxide was achieved by employingdissolved platinum complexes,3zeolites,4,5and sulfonated res-
ins.6
Recently Sn-bmolecular sieve was reported as a potentialheterogeneous catalyst for this oxidation in 1,4-dioxane at elevatedtemperature.7Excellent selectivity and yields of ester or lactones
were described from saturated as well as unsaturated ketones.Corma and co-workers employed a concept that involves selectiveactivation of a carbonyl group with a catalyst7which has been
activated earlier by Lewis acids,8,9followed by reaction withhydrogen peroxide.As a part of our ongoing programme to develop mild andefficient protocols for functional group transformation, we haveundertaken a study on developing an efficient methodology forBaeyer–Villiger oxidation. Earlier we have communicated oxida-tion of 4-substituted 1,4-dihydropyridines to the correspondingpyridine at ambient temperature with hydrogen peroxide in ionicliquid.10In recent years the use of room temperature ionic liquids
(ILs) as ‘green’ solvents in organic synthetic processes has gainedconsiderable importance due to their solvating ability, negligiblevapour pressure, easy recyclability and reusability.11Many reac-
tions have been reported recently using ionic liquids as reactionmedia12and as rate enhancers.13
Recently Bernini14reported methyltrioxorenium catalysedBaeyer–Villiger oxidation in ionic liquid. In order to avoid the useof costlier methyltrioxorenium, we decided to develop an efficientand practical method for Baeyer–Villiger oxidation using Sn-bmolecular sieve and hydrogen peroxide in ionic liquid.
Results and discussionHerein, we wish to report an efficient oxidation method of ketones(1) to esters (2) with 30% aqueous hydrogen peroxide in 1-n-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4])15usingcatalytic Sn-bmolecular sieve in excellent yields Scheme 1.4-Methyl acetophenone (1a) on reaction with H2O2(30%) in
presence of a Sn-bmolecular sieve7catalyst (20%) in ionic liquid
[bmim][BF4] at room temperature gave Baeyer–Villiger oxidationproduct (2a) in 88% yield in 10 hours. The workup procedureinvolved simple filtration of the catalyst and extraction of theproduct with ethyl acetate.4-Methyl acetophenone (1a) on reaction with H2O2(30%) in
presence of Sn-bmolecular sieve catalyst (20%) in dioxane at roomtemperature remains unchanged. This “confirms” that the ionicliquid is essential for the transformation at room temperature.However, Corma reported the Baeyer–Villiger oxidation using Sn-bmolecular sieve in dioxane at 80 °C.Similarly it was also observed that 4-methyl acetophenone (1a)remains unchanged when treated with H2O2(30%) at room
temperature and 80 °C in presence of Ti-silicalite-1,16Sn-silicalite-117and all-silica-b18in ionic liquid. This study “confirms” that thepresence of the ionic liquid and Sn are critical for the Baeyer–Villiger oxidation (Table 1, entry 2–4).
In order to examine whether the reaction occurs over thetetrahedral Sn-bframework or on the extra framework Sn in theform of SnO2, Sn was impregnated on all-Si-b19and was used as a
