Oxidation of Alcohols by Hydrogen Peroxide,Catalyzed by Methyltrioxorhenium(MTO):
A Hydride Abstraction
Timothy H.Zauche and James H.Espenson*
Ames Laboratory and Department of Chemistry,Iowa State University,Ames,Iowa50011
Recei V ed June17,1998
Primary and condary alcohols are oxidized using hydrogen peroxide as an oxygen donor and methyltrioxorhenium
(MTO)as a catalyst.The methylrhenium di-peroxide,CH3Re(O)(η2-O2)2(H2O),was the dominant and reactive
form of the catalyst.Reprentative rate constants k/L mol-1s-1are1.02×10-4for4-Me-R-methylbenzyl alcohol
and4.9×10-5for4-Cl-R-methylbenzyl alcohol.There was a kinetic isotope effect of3.2for the R C-H bond.
When c-phenethyl alcohol was labeled with18O,80%of the oxygen was retained in the ketone.Tests for the
possible intervention of a free radical intermediate were carried out;the evidence was entirely negative.A
mechanism featuring hydride abstraction is propod,the first time for the H2O2/MTO system.Also,a cocatalytic
t of reaction conditions has been developed on the synthetic scale,using bromide and MTO as cocatalysts,
which cuts the reaction time from hours to minutes.
Introduction
The lective oxidation of C-H bonds has always been a challenging task.Typical of this is the oxidation of alcohols to aldehydes or ketones.Usually only the strongest oxidizing agents,such as KMnO4,Br2,MnO2,SeO2,RuO4,and acid dichromate can perform this reaction.1Only a few of the reagents have been ud in a catalytic system;one example is SeO2.2It has recently been shown th
at methyltrioxorhenium (CH3ReO3,or MTO)can catalyze reactions of hydrogen peroxide with alcohols.3
Hydrogen peroxide,ud with a catalytic amount of MTO, has been shown to oxidize catalytically a variety of substrates besides alcohols,such as sulfides,4alkenes,5,6amines,7,8hy-droxylamines,9and halides.10,11The mechanisms of the oxidations follow a general pathway where the substrate acts as a nucleophile and attacks an electron-poor peroxorhenium oxygen.
The previous study of alcohol oxidations by MTO and H2O2 focud on the synthetic aspects of the catalytic system.3We were intrigued by the oxidations since the alcohol,unlike other substrates oxidized by this combination,has no center of electron density to act as a nucleophile,nor does it have a site to which an oxygen atom can be easily transferred.Anticipating a new mechanism of oxidation for the catalyst MTO,we undertook a study of the oxidation of alcohols by MTO and hydrogen peroxide.Experimental Section
Materials.Hydrogen peroxide and HPLC grade solvents were purchad from Fisher.Water was purified by distillation and then filtered by a Milli-Q water purification system.The various alcohols,ketones,and aldehydes were purchad either from Aldrich or Lancaster and ud as receiv
ed.The10%18O-labeled water and MTO were purchad from Aldrich.1-Phenylethanol (1,2,2,2-D4,98%)was purchad from Cambridge Isotope Laboratories.The methyl(1-phenyl)ethyl ether was made by a standard literature procedure.12
The products of alcohol oxidation were identified using various methods.1H NMR spectra were recorded using Varian VXR300MHz NMR or Bruker DRX400MHz NMR spectrometers,mass spectra were obtained using a Finnigan TSQ 700GC-MS,and the UV-vis spectra were determined using Shimadzu2501or3101spectrophotometers.
Kinetics.The experiments were carried out in20%water/ acetonitrile containing0.1M HClO4.This solvent mixture was chon for solubility reasons and becau the kinetics and thermodynamics of the H2O2/MTO system are known under the conditions.The reactions were monitored by the absor-bance ri in the region of240-255nm,due to the formation of the carbonyl product.The reactions were carried out in a quartz cuvette under air,since there was no effect upon saturating the solutions with air or argon.Becau of the large molar absorptivities of the products( ∼1×104L cm-1mol-1) and the background from hydrogen peroxide and MTO,a0.01 cm path length cell from Spectrocell was ud throughout the study.Reactant stock solutions were made fresh daily.A typical reaction procedure is as follows:10-50mM MTO,200mM H2O2,and0.10M HClO4were mixed togethe
r in the cell and allowed to stand for1-2min to allow the complete formation of the catalytically active peroxorhenium compounds;20-50 mM of neat alcohol was added at this point and the solution mixed thoroughly(∼25s).Data acquisition was then started, and the reaction was kept at a constant temperature of25°C.
generaltool>香港小黄鸭
(1)March,J.Ad V anced Organic Chemistry:Reactions,Mechanisms,and
Structure,4ed.;John Wiley&Sons:New York,1992.
(2)Larock,R.C.Comprehensi V e Organic Transformations;VCH Pub-
lishers:New York,1989.
(3)Murray,R.W.;Iyanar,K.;Chen,J.;Wearing,J.T.Tetrahedron Lett.
spca
1995,36,6415-6418.
(4)Vasll,K.A.;Espenson,J.H.Inorg.Chem.1994,33,5491-5498.
(5)Herrmann,W.A.;Fischer,R.W.;Scherer,W.;Rauch,M.U.Angew.
trespass
派特森英语
Chem.,Int.Ed.Engl.1993,32,1157.
(6)Al-Ajlouni,A.;Espenson,J.H.J.Org.Chem.1996,61,3969-3976.
(7)Yamazaki,S.Bull.Chem.Soc.Jpn.1997,70,877.
(8)Zhu,Z.;Espenson,J.H.J.Org.Chem.1995,60,1326-1332.
(9)Zauche,T.H.;Espenson,J.H.Inorg.Chem.1997,36,5257-5261.
(10)Espenson,J.H.;Pestovsky,O.;Huston,P.;Staudt,S.J.Am.Chem.
Soc.1994,116,2869.
(11)Hann,P.J.;Espenson,J.H.Inorg.Chem.1995,34,5839-5844.(12)Meinwald,J.;Schneider,R.A.;Thomas,A.F.J.Am.Chem.Soc.
1967,89,70.
6827
Inorg.Chem.1998,37,6827-6831
10.1021/ic9806784CCC:$15.00©1998American Chemical Society
Published on Web12/01/1998
The reactions were typically monitored until1-2%of the alcohol had reacted,since over longer periods of time the catalyst is susceptible to decomposition.The absorbance versus time data were analyzed by the initial rate method.To do so, the absorbance traces were first converted to concentrations bad on the molar absorptivities of the alcohols and products determined from authentic samples.The concentrations varied linearly with time over the time frame;the data were fit by a linear least-squares analysis using the computer program Ka-leidaGraph.
Oxygen Labeling.An18O isotope labeling experiment was
carried out with c-phenethyl alcohol.The10%labeled alcohol was synthesized by the standard literature preparation13from acetophenone mixed with10%18O water and a catalytic amount of acid.After16h,the labeled acetophenone was isolated and reduced using sodium borohydride.The resulting alcohol was then purified and stored in a desiccator.
In a typical labeling experiment a reaction solution contained 30mM of10%18O-labeled alcohol,25-30mM MTO,and ∼200mM urea hydrogen peroxide(UHP)in acetonitrile.Not
all of the UHP was dissolved,but enough was dissolved to ensure that the dominant form of the catalyst was the di-peroxorhenium compound,determined by obrvation of its characteristic360nm absorption.UHP was ud in the experiments to limit the amount of16O-bearing water in the reaction solution.good luck什么意思
All of the gas chromatography-mass spectrometry experi-ments were performed using a Finnigan TSQ700.The system was configured in the electron impact ionization mode.The first quadrupole was ud as the analyzer to scan from m/z35to m/z400at0.5s/scan.The cond and third quadrupoles were kept in the RF-only mode.Unit mass resolution was achieved using FC43as calibration and tuning reference.A DB1701gas column was ud for all experiments.This allowed the ketone to flow through the column first,and any tailing of the larger alcohol peak was not a worry.This was a concern lest tailing from the alcohol peak,were it first off the column(as was the ca when a DB5gas column was ud),may overlap with the ketone peak.The ratio of labeled to unlabeled compounds was determined by the ratio of mass to charge peaks for both the alcohol and ketone with each injection by taking an average of three mass spectral scans at the maximum of the GC peak and subtracting a background scan.This allowed direct comparison of the starting alcohol labeling and the product ketone,eliminat-ing any variations the instrument might have from day to da
y. Each data point was the average of three injections,and each reaction was carried out two or three times.
Synthetic Scale Reactions.The reactions were carried out on the2.5mmol alcohol scale.A typical procedure is as follows:4mol%MTO and2equiv of30%H2O2were combined.The alcohol was then added and the biphasic mixture stirred at40(2°C for1day.No acid was added,to avoid acid-catalyzed byproducts.Periodically,the NMR spectrum of an aliquot taken from the organic layer was obtained in chloroform-d to monitor the progress of the reaction.
Other synthetic scale reactions were carried out using Br-as a cocatalyst with MTO.The reactions were performed with an equal volume of acetonitrile as compared to H2O2and alcohol added,which rendered the reaction mixture homogeneous.The 1.1equiv of H2O2was added by syringe pump infusion to limit the amount of peroxide decomposition by bromide species in the solution.For the reactions4.0mol%of HBr and0.50mol%of MTO(relative to the alcohol)were ud as the
catalysts with addition of hydrogen peroxide over30-45min. Results
The Catalyst.MTO can reversibly coordinate one or two
d unithydrogen peroxides to yield the corresponding mono-and di-
peroxorhenium complexes A and B,as shown in Scheme1.
Without a substrate prent MTO,A,and B will reach
equilibrium concentrations governed by[H2O2]and the values
of K1()k1/k-1)and K2()k2/k-2),the equilibrium constants.
The same does not hold true during a catalytic reaction cycle
where steady-state conditions apply.With the oxidation of a
substrate at rates defined by the cond-order rate constants k3
and k4and the concentration of substrate,the steady-state
concentrations of the various rhenium complexes will be
obtained.
With such a number of reactions possible it proved difficult
to determine the steady-state concentration of each rhenium
complex during the reaction.For example,when the rates of
partyanimal
oxidation are faster than the rates of reformation of A or B,
then after a few catalytic cycles the major form of the catalyst
will be MTO,with the rate-determining step being the formation
of A.On the other hand,when the rates of oxidation are slow
compared to the formation of A or B,then the dominant form
of the catalyst during the reaction will be B at high[H2O2].
With alcohols the rate of oxidation is much slower than the
rate of reformation of the oxidizing species B from A,leading
to a well-behaved catalytic system.
Reaction Kinetics.It has been shown that the initial rate
method can be applied to catalytic MTO oxidations.9Owing to
the long reaction times for alcohol oxidations(typically t1/2∼30h)and the slow decomposition of the catalyst,the initial
rate method was employed.For the reactions MTO and H2O2
were equilibrated before addition of the alcohol.With the high
concentration of H2O2ud(0.2-0.3M),the predominant form
of the catalyst will be B in accord with the assumption that[B]
=[Re]
T
)[MTO]+[A]+[B],which will hold throughout the reaction.This treatment allows the determination of the bimolecular rate constant k4for the reaction of B with an alcohol as written in eq1,expresd by the r
ate law,eq2,written in terms of initial rates.14
Bad on eqs1and2a plot was made of the initial rates versus the product[Re]T×[R2CHOH]0.According to eq2, the values of V i should define a straight line that pass through the origin with the slope equal to k4.The data for4-Me-R-
(13)Stasuik,F.;Sheppard,W.A.Can.J.Chem.1956,34,123-127.(14)Espenson,J.H.Chemical Kinetics and Reaction Mechanisms,2ed.;
McGraw-Hill:New York,1995.
Scheme1
B+R
2
CHOH98
k4
A+R
2
CO+H
2
O(1)
V
i
)k
4
[B][R
2
CHOH]
=k
4
[Re]
T
[R
2
CHOH]
(2)
6828Inorganic Chemistry,Vol.37,No.26,1998Zauche and Espenson
methylbenzyl alcohol are displayed in Figure S-1,Supporting Information.The least-squares slope of t
he line gives the value k4)1.02×10-4L mol-1s-1(25°C,in20%water/acetonitrile, 0.1M HClO4).The rate constant k4determined for each alcohol is listed in Table1.The spread of rate constants is not large, all being within a factor of5.Included in this study was the oxidation of methyl(1-phenyl)ethyl ether.This compound has the lowest rate constant of all compounds;it reacts3.3times more slowly than the analogous alcohol,c-phenethyl alcohol, entries3and13.The k4rate constants could not be determined accurately for some alcohols,but the substrates were also listed to show the widespread applicability of the MTO/H2O2oxida-tions.
A linear free energy relationship plot was made of the para-substituted R-methylbenzyl alcohols as shown in Figure1.From this plot the slope F)-0.51was determined.The negative F value implies that electron-donating groups increa the reaction rate,while electron-withdrawing groups decrea the rate.This agrees well with an oxidation mechanism in which the rhenium peroxo-oxygen performs an electrophilic attack on the C-H bond.One of the ven substrates ud to make the LFER plot did not react as expected.4-Bromo-R-methylbenzyl alcohol has a rate constant that is approximately half of the expected value.The alcohol sample was pure by1H NMR.We have no explanation for this deviation at this time,but we note that this compound also gave a low product yield(e later). Isotopic Labeling.A number of reactions were performed using10%18O-labeled c-phenethyl alcoh
上海胡姬港湾幼儿园ol.When it was treated with MTO/UHP,it was found by GC-MS monitoring that80(5%of the labeled oxygen remains in the ketone.UHP was ud for the experiments to limit the amount of added water,thus minimizing oxygen exchange between the ketone and solvent.When30%aqueous H2O2was ud,there was significant exchange of the labeled ketone with the solvent oxygens.Even without added acid,when MTO decompos,it forms perrhenic acid,which will catalyze exchange.With UHP as the oxygen donor the control experiments gave no detectable exchange of the labeled ketone over24h.Multiple trials were performed and the reactions were monitored over time to verify that the ketone did not exchange its oxygen during the oxidation experiment.A deuterium kinetic isotope study was performed using1-phenylethanol(1,1,1,2-D4).As shown in Table1,entries 3and11,the k H/k D ratio is3.2.
Radical Mechanism Possibilities.Another strong oxidizing compound,dimethyldioxirane(DMDO),can also oxidize alco-hols.15The mechanism of C-H bond activation by DMDO has been investigated by a number of groups.16-19The intermediate in DMDO oxidations can be described as a biradical trapped within a solvent cage.Under different conditions the radical nature of this intermediate can be exploited.MTO has been compared to DMDO in the past for alkene oxidations;6therefore a radical intermediate for MTO oxidations was examined.We first tested whether O2affects the reactio
n rates.For reactions carried out under an air or argon atmosphere,no difference in the rate of the reactions was noted.Another test for radicals was to add freshly distilled BrCCl3.17The reaction rate was not changed when25-50mM BrCCl3was added.(No halogenated product was checked for since such a product would rapidly lo HBr to form the ketone.)Other radical trapping agents such
(15)Kovac,F.;Baumstark,A.L.Tetrahedron Lett.1994,35,8751-8754.
(16)Baumstark,A.L.;Beeson,M.;Vasquez,P.C.Tetrahedron Lett.1989,
30,5567-5570.
(17)Minisci,F.;Zhao,L.;Fontana,F.;Bravo,A.Tetrahedron Lett.1995,
plan b是什么意思36,1697-1700.
(18)Vanni,R.;Garden,S.J.;Banks,J.T.;Ingold,K.U.Tetrahedron Lett.
1995,36,7999-8002.
(19)Murray,R.W.;Gu,H.J.Org.Chem.1995,60,5673-5677.
Table1.Rate Constants and Yields for the Catalytic Oxidation of Alcohols
entry no.alcohol105×k4/L mol-1s-1a%yield on synthetic scale b%conversion 14-Me-R-methylbenzyl alcohol10.2849293 2c4-MeO-R-methylbenzyl alcohol8.85
3c-phenethyl alcohol7.7899798 41-phenyl-1-propanol 6.8699293 54-F-R-methylbenzyl alcohol 6.2919999 6(R)-(+)-2-methyl-1-phenyl-1-propanol 5.5648687 7benzyl alcohol d 5.341(7)e40(12)54
28(21)16(38)55 8benzhydrol 5.0464849 94-Cl-R-methylbenzyl alcohol 4.9849596 104-(CF3)-R-methylbenzyl alcohol 4.4608585 111-phenylethanol-1,2,2,2,-D4 2.4
124-Br-R-methylbenzyl alcohol 2.3586464 13methyl(1-phenyl)ethyl ether 2.3225353 141-cyclohexylethanol749293 154-phenyl-2-butanol586164 161-phenyl-2-propanol262730
a Determined in20%water/acetonitrile,0.10M HClO4,at25°C.
b Neat alcohol,2molar equiv H2O2,4mol%MTO,40°C.The first column is the yield after8h,the cond after24h,bad on NMR integrations.c4-MeO gave a number of products,which were not identified;entry11 was not carried out on a syntheti
c scale owing to a shortage of the starting material.
d Th
e top entry is with1equiv o
f H2O2;the bottom entry is with2equiv.e The first value refers to benzaldehyde,the parenthetic one to benzoic acid.
Figure1.Linear free energy relationship between the rate constant
for the oxidation of4-X-R-methylbenzyl alcohols by MTO and H2O2
and the Hammettσconstant for X.27K0is defined as X)H.The
deviant X)Br point was left out of the F calculation.The F value of
-0.51describes the effect that electron-donating and-withdrawing
groups have on the transition state.
Oxidation of Alcohols by Hydrogen Peroxide Inorganic Chemistry,Vol.37,No.26,19986829
as Tempo could not be ud for this reaction,becau the oxidations were performed under acidic conditions.From the negative results we conclude that radical reactions are not significant in the mechanism of the reactions.The basis for this is that BrCCl3is so reactive toward carbon-centered radicals that they would certainly have trapped any in competition with any radical lf-reaction.
Pha-Transfer Reactions.The slow rates of oxidation for the various alcohols necessarily mean that long reaction times are required to reach completion.Recently,it was reported that the oxidation of alcohols occurs with WO42-and H2O2under “solvent-free”conditions.20That study ud neat alcohol and dissolved the catalyst in30%H2O2while using a pha-transfer catalyst(PTC)to assist in WO42-transportation between the phas.Since MTO is soluble in many organic solvents,no PTC would be needed were MTO ud under similar reaction conditions.
Synthetic preparations of ketones were carried out on the neat
alcohol.Typically2.5mmol of the alcohol was ud with5 mmol of hydrogen peroxide as a30%solution and4mol% MTO.The reactions were stirred at40°C for1day,and then the yields were determined by NMR as found in Table1.As en in Table1many of the yields are often better than80%. This is a great improvement over the previous report of alcohol oxidations by MTO/H2O2(<30%yield,19mol%
catalyst3). The temperature of40°C was chon instead of the previously ud60°C,to avoid MTO decomposition at higher tempera-tures.21At40°C the decomposition of MTO is less,allowing good yields of the product while shortening the reaction times. The reactions were monitored for up to24h,but as en in the table most of the reactions were almost complete after only8 h.The percent conversion is also listed in the table,demonstrat-ing the low amounts of byproducts in the reactions.
An interesting point is the oxidation of a primary alcohol beyond the aldehyde to the carboxylic acid.When2equiv of hydrogen peroxide was ud to oxidize benzyl alcohol,not only was benzaldehyde made but a large portion was further oxidized to benzoic acid(21%).When1equiv of hydrogen peroxide was ud,the product ratio became40%benzaldehyde,7% benzoic acid.Most of the R-phenyl alcohols were oxidized in good yields in reference to large-scale reactions.The only two exceptions were the4-bromo-R-phenylethanol and the4-meth-oxy-R-phenylethanol.The reason for the decread yield for the4-bromo could be due to the slower rate of reaction than expected as en in the LFER plot,while the4-methoxy compound gave a number of products that were not character-ized.The non-R-phenyl alcohols varied in amounts of yields from27%for1-phenyl-2-propanol to92%for1-cyclohexyl-ethanol.
Cocatalysts:MTO and Br.Synthetic scale reactions can be made more environmentally friendly by limiting the number and types of side products.Another goal is for the reactions to be halide-free.We have accomplished this by using nonhalo-genated solvents and nonhalogenated oxidizing agents.Now that the oxidation of alcohols by MTO/H2O2has been defined,we have explored shortening the reaction time by adding bromide as a cocatalyst.
Previously,MTO/H2O2has been shown to oxidize catalyti-cally Br-to BrO-,10which can form Br2when in the prence of another Br-and H+.22It is also known that Br2can oxidize alcohols.23When HBr was added as a cocatalyst for the oxidation of alcohols,the reaction is significantly faster,as en in Figure2.This figure demonstrates that the uncatalyzed reaction of the alcohol with hydrogen peroxide is almost nonexistent.When MTO(0.5molar equiv)is added,the reaction becomes faster.However,if HBr is added as well(2mol%), the reaction is much faster.Also if the reaction is done on a synthetic scale,99%of the ketone can be achieved in just minutes,instead of hours without HBr.
There is a competing decomposition of hydrogen peroxide by BrO-or Br2.24Even when hydrogen peroxide is initially added to5times excess over the alcohol,it was necessary to add more hydrogen peroxide to oxidize all of the alcohol in this cocatalytic system.If the hydrogen peroxide is added drop
wi though,only10%excess was needed for the reaction to reach completion.
Discussion
Oxidation Mechanism.As stated earlier the oxidation of alcohols by MTO and hydrogen peroxide must proceed by a different mechanism than has been determined for other substrates such as sulfides,alkenes,and hydroxylamines.All of the previous compounds had a center of electron density to which an oxygen atom can be transferred.Scheme2best describes the oxidation of alcohols by MTO/H2O2bad on our obrvations.This mechanism shows the formation of an intermediate in which there are interactions between the peroxorhenium oxygen with both the carbon and the hydrogen of the R C-H bond.This is typically how a hydride abstraction is depicted.The assignment of this mechanism is supported but not defined by the kinetic isotope effect k H/k D of3.2.The electron-poor oxygen performs an electrophilic attack on the electron density of the C-H bond.On the basis of our obrvations there is no radical intermediate.
This intermediate can then proceed along two pathways in parallel.The major path is followed when the C-H bond is vered,after which the carbocation los a proton to produce the ketone.This proton could be lost either to the solvent or to the other peroxorhenium oxygen to give a di-hydroxo r
henium product,which rapidly eliminates water to give A.The di-hydroxo rhenium species has not been isolated,but is not unreasonable in that MTO oxygens are known to exchange with
(20)Sato,K.;Aoki,M.;Takagi,J.;Noyori,R.J.Am.Chem.Soc.1997,
119,12386-12387.
(21)Jacob,J.;Espenson,J.H.Unpublished results.
(22)Eigen,M.;Kustin,K.J.Am.Chem.Soc.1962,84,1355.(23)Stewart,R.Oxidation Mechanisms;W.A.Benjamin,Inc.:New York,
1964.
(24)Bray,W.C.Chem.Re V.1932,10,
161.
Figure2.Increa in absorbance at240nm accompanying the buildup of the ketone from the reaction of c-phenethyl alcohol and hydrogen peroxide in20%water/acetonitrile containing0.10M HClO4.Without a catalyst the reaction is nonexistent,while adding MTO or HBr and MTO increas the rate of the reaction to different extents.
6830Inorganic Chemistry,Vol.37,No.26,1998Zauche and Espenson
tho of water,through a similar di-hydroxo rhenium form.25 This major pathway is further supported by the retention of labeled oxygen in the ketone.The events are depicted in Scheme2.
The minor pathway is propod to account for the20%of labeled oxygen that does not remain in the product and to account for the fact that the methyl ether can be oxidized as well.With the rate constants for the ether and the respective alcohol agreeing within a factor of4,the mechanism for oxidation of both must be somewhat similar.The difference between the major and minor pathway is where the di-hydroxy group resides when the intermediate breaks apart.Since the active oxidizing form of MTO has capabilities similar to tho of DMDO,there appear to be similarities between the rhenium center of MTO and the carbon center of DMDO.The pathway with the di-hydroxo rhenium pr
oduct is preferred in our mechanism over the carbon gem-diol product,showing that the two centers are not truly identical.
Listed in Table1are the substrates oxidized by MTO/H2O2 in this study.A LFER plot of the para-substituted R-methyl-benzyl alcohols demonstrates the effect of electronic variations. If an electron-donating group is at the para position,the reaction becomes faster,and vice versa.This also supports a hydride abstraction mechanism.It can also be noticed that steric factors play a role in the rate of the reaction bad on the difference in yields for the various non-R-phenyl alcohols.The yields start at92%for cyclohexylethanol and decrea with additional steric bulk to27%for1-phenyl-2-butanol.A last point of interest is that a primary ,benzyl alcohol)was oxidized not only to the aldehyde with excess peroxide to the carboxylic acid as well.This cond oxidation can be limited by the amount of peroxide added,since the two steps have quite different rate constants,k alc>k ald.
Synthetic Considerations.This study has produced a new synthetic scale preparation of ketones from alcohols in good yields.The yields are somewhat dependent on the starting alcohol,with the majority being above80%.Further,the u of bromide cocatalyst decread the reaction times by a factor of at least1000with decread reaction temperatures.We feel that this cocatalytic system26has great potential. Acknowledgment.This rearch was supported by a grant from the National Science Foun
dation(CHE-9007283).Some experiments were conducted with the u of the facilities of the Ames Laboratory.The authors also thank Dr.Kamel Harrata for assistance with the GC-MS experiments and also Dr.Walter Trahanovsky for informative discussions.
Supporting Information Available:Figure of kinetic data for oxidation of4-Me-phenethyl alcohol by hydrogen peroxide,catalyzed by MTO(1page).Ordering information is given on any current masthead page.
IC9806784
(25)Pestovsky,O.;van Eldik,R.;Huston,P.;Espenson,J.H.J.Chem.
Soc.,Dalton Trans.1995,133.(26)Espenson,J.H.;Zhu,Z.;Zauche,T.H.J.Org.Chem.,submitted for
publication.
(27)Lowry,T.H.;Richardson,K.S.Mechanism and Theory in Organic
Chemistry,3rd ed.;Harper Collins Publishers:New York,1987.
Scheme2
Oxidation of Alcohols by Hydrogen Peroxide Inorganic Chemistry,Vol.37,No.26,19986831
