Metal-Catalyzed Epoxidations of Alkenes with Hydrogen Peroxide

18 Pages·2003·0.35 MB·English

Volume 103, Number 7
Metal-Catalyzed Epoxidations of Alkenes with Hydrogen Peroxide
Benjamin S. Lane and Kevin Burgess*
ContributionfromtheDepartmentofChemistry,TexasA&MUniversity,P.O.Box30012,CollegeStation,Texas77842-3012
Received October17,2002
Contents
1. Introduction 2457
2. Heterogeneous Systems 2458
2.1. Zeolites and Hydrotalcite Systems 2458
2.2. “Homogeneous Catalysts” Attached to Solid 2459
Supports
3. Soluble Metal Oxides 2459
3.1. Polyoxometalates 2459
3.2. Peroxotungstates 2459
3.3. Peroxomolybdates 2460
3.4. Methyltrioxorhenium 2461
3.5. Other Metal Oxides 2461
4. Metal Oxides Generated in Situ 2461
4.1. Selenium and Arsenic Compounds 2461 KevinBurgessisaprofessoratTexasA&MUniversity,wherehehas
4.2. Simple Metal Salts 2462 beensinceSeptember1992.Hisresearchinterestfocusesonprojectsin
5. Coordination Complexes 2463 peptidomimeticsofprotein- proteininteractions,fluorescentdyesforDNA
sequencing,andcatalysis,allwithanemphasisonhigh-throughputand
5.1. Manganese Porphyrins 2463
combinatorialchemistry.Mostofthecatalysisresearchperformedinthis
5.2. Iron Porphyrins 2464 grouprelatestoasymmetricsyntheses,butrecentprojectsonepoxidation
5.3. Manganese Salen Complexes 2466 chemistryprovidedthemotivationtowritethisreview.
5.4. 1,4,7-Triazacyclononane (TACN) Complexes 2466
5.5. Iron and Manganese Pyridyl-Amine 2468
Complexes
5.6. Other Coordination Complexes 2469
6. Conclusions 2469
7. References 2471
1. Introduction
Hydrogen peroxide is probably the best terminal
oxidantafterdioxygenwithrespecttoenvironmental
and economic considerations.1-4 Indeed, in certain
circumstances,itisbetterthanoxygeninsofarasO /
2
organic mixtures can sometimes spontaneously ig-
nite. As a result, epoxidation systems that use
hydrogen peroxide in conjunction with catalytic
BenLanecompletedhisB.S.degreefromTheRichardStocktonCollege
amounts of cheap, relatively nontoxic metals are
ofNewJerseyin1997.HisPh.D.work,onmetal-catalyzedepoxidation
potentially viable for large-scale production of inex-
reactions,wasundertakenatTexasA&MUniversityunderthedirection
pensiveproducts,andforspecializedapplicationsin ofDr.KevinBurgess.Hecompletedhisdegreein2002andiscurrently
development, process, and research. The literature atColumbiaUniversityforaspostdoctoralassociatewithDrDaliborSames.
10.1021/cr020471zCCC:$44.00 ©2003AmericanChemicalSociety
PublishedonWeb05/17/2003
2458 ChemicalReviews,2003,Vol.103,No.7 LaneandBurgess
Scheme1. HydrotalciteEpoxidationsUsing30%HydrogenPeroxide: (a)WithBenzonitrileasPeroxide
Carrier;(b)withiso-ButyramideasPeroxideCarrier;and(c)thePresumedMechanismoftheReaction
Showninpartb
inthisareaisextensiveanddifficulttosegregateinto 1-type zeolites are inherently acidic, they are some-
sharply delineated categories, but a fair way to times modified prior to use to prevent inactivation
attempt this is according to the catalyst precursors: of the catalyst or epoxide-decomposition. Even with
“heterogeneous”,“solublemetaloxides”,and“homo- such modifications, zeolites are generally limited to
geneouscoordinationcomplexes”.5Organiccatalysts production of small, fairly stable epoxides.
designedforasymmetricepoxidationmayalsofunc- Hydrotalcite systems are more widely applicable
tion with hydrogen peroxide as a terminal oxidant, than zeolites insofar as they can be used with a
buttheyarebeyondthescopeofthisreview.6-9The greatervarietyofsubstrates.Thesesyntheticanionic
focusofthisreviewismethodsfortheproductionof clays are basic; in fact, they are basic enough to
fine chemicals, but heterogeneous systems (more promote some nucleophilic epoxidations.23 For ex-
suitableforproductionofchemicalcommodities)are ample, Mg Al (OH) CO and Mg Al (OH) CO
10 2 24 3 9.5 2.6 24.8 3
outlined for completeness. mediate epoxidations of R,(cid:226)-unsaturated ketones
withH O intheabsenceofotherinorganicbases.15,24
2 2
2. Heterogeneous Systems TheMg10Al2(OH)24CO3hydrotalcitesystemdeveloped
byKanedaalsogiveshighconversionsandselectivi-
2.1. Zeolites and Hydrotalcite Systems ties for electrophilic epoxidations, even when acid-
sensitive epoxides are produced, Scheme 1. Unfor-
Heterogeneoussystemsforepoxidationofalkenes tunately, this system requires more than one equi-
with H O are typically mineral-type catalysts, in- valent of an amide13 or nitrile14 additive to act as a
2 2
cluding zeolites10-12 or hydrotalcites.13-16 Zeolites peroxide carrier. This additive is converted into a
most commonly used for alkene oxidations feature carboxylic acid byproduct, which can complicate
four-coordinatetitaniumcentersinmicroporoussili- isolation of the desired epoxide. Epoxidations medi-
ceous frameworks.17-19 The first Ti-containing sili- atedbytheKanedasystemcanbeacceleratedusing
calite zeolite, TS-1, has a relatively small pore size microwave-heating,25suchthatreactiontimescanbe
of 5.5 Å. The small pore size of TS-1 precludes reducedfrom1dayto1min.TheKaneda’sepoxida-
reactions of larger substrates4 though the isomorph tionreactionismechanisticallysimilartothePayne
ZSM-5orzeoliteswithlargerporesizes,suchasTi- epoxidation,whichisoftenusedforindustrialbatch-
(cid:226), have less severe limitations in this regard.20-22 typeprocesses,26themajordifferencebeingthatH O
2 2
Zeolites such as TS-1 are most reactive toward activation is achieved using hydrotalcites at 60 °C
terminal akenes and less reactive to Z-alkenes, and ratherthanbicarbonateatroomtemperature(Payne).
modified forms can allow for epoxidation of E- Typicalhydrotalcitesusedascatalystsinepoxida-
isomers.11 For instance, Ti-MWW (a zeolite having tion reactions are polynuclear-alumina clays (i.e.,
large, 10-membered ring, channels in the siliceous Mg Al (OH) CO ); however, simple alumina will
10 2 24 3
framework,alsoknownasMCM-22)mediatesepoxi- also catalyze this reaction without amides or
dation of C -C linear aliphatic alkenes with 30% nitriles.27-30 A variety of different alumina sources
6 8
H O to produce a 4:1 rate preference in favor of can absorb hydrogen peroxide onto that surface,
2 2
E-alkenes over their Z-isomers. This preference is forming an active oxidant (alumina-OOH) that can
related to the larger pore size, which also allows epoxidizeunfunctionalizedalkenesinlowtomodest
formationofC -C epoxidesin80-85%yield.How- yieldsatambienttemperatures.27Higheryieldscan
6 8
ever, such systems tend to be used at slightly be obtained by using anhydrous H O in refluxing
2 2
elevated temperatures (e.g., 60-70 °C), which may ethylacetate,butthepresenceofasmallamountof
leadtodecompositionofsensitiveproducts.SinceTS- wateriscritical.28-30Aqueous60%H O canalsobe
2 2
Metal-CatalyzedEpoxidationsofAlkenes ChemicalReviews,2003,Vol.103,No.7 2459
phasetechniquescanbeusedtopreparesupported,
peptide-derivedtransitionmetalcomplexes(particu-
larly Mn and Fe), which may then be screened for
potential catalytic activities.42-45
3. Soluble Metal Oxides
3.1 Polyoxometalates
Polyoxometalates are salts composed of complex
anionsincorporatedwithtwoormoremetalcations.
Theyactashomogeneousepoxidationcatalysts,but
theconversionsobtained,theselectivitiesforepoxide
products,andtherangeofsolventsthatcanbeused
forthereactionaremostlyinferiorcomparedtoother
methods.46 For example, (R N) SiW Fe(OH ) O
4 6 10 2 2 38
(where R ) alkyl) salts epoxidize aliphatic alkenes
slowlyat32°CinacetonitrilebutalsomediateC-H
activation of alkanes, causing lack of selectivity.47
Betterselectivitieswereobtainedforsomesubstrates
usingsystemswith(R N) Ni(OH )H F NaW O but
4 9 2 2 6 17 55
at60°Cinbiphasicsystemsthatincludehalogenated
solvents.48
3.2. Peroxotungstates
Systemsderivedfromtungsticacid(H WO ),phos-
2 4
phate, and ammonium or phosphonium counterions
that act as phase transfer agents are prominent
amongsimple,solublemetaloxidesaltsforcatalytic
epoxidation with H O .49-60 They are nearly always
2 2
formedinsitu,butcatalyticallyactivecomplexessuch
as (R N) {PO (W(O)(O ) ) } (Figure 2) have been
4 3 4 2 2 4
Figure1. Strategiesforattachingcoordinationcomplexes
tomodifiedsilica.
used with the addition of a Dean-Stark apparatus;
1-tert-butylcyclohexene was epoxidized in 91% yield
in 4 h using these conditions.28 These types of
systems have generally been tested on simple, un-
functionalized alkenes.
2.2. “Homogeneous Catalysts” Attached to Solid
Supports
Heterogeneous catalysts may be constructed by
attaching or impregnating homogeneous catalysts Figure2. Venturelloepoxidationcatalyst.
onto solid supports. This may be achieved by using
ion-exchange resin to bind anionic catalysts,31,32 isolated and characterized crystallographically by
encapsulating inorganic complexes onto chemically Venturello and co-workers.61 Tungsten catalysts of
modifiedsilicaorzeolites,12,16,33-35orcovalentlybind- thistypeuseH O moreefficientlythanmanyother
2 2
ing coordination complexes onto modified silica. epoxidationcatalysts,insofarastheiruniquechem-
Figure1illustratesthelatterapproachinthecontext istryfavorsoxygentransferoverperoxidedispropor-
of porphyrin (1 and 2)36,37 or triazacyclononane (3) tionation. Biphasic systems involving chlorinated
complexes.38-41 solvents or, less commonly, aromatic solvents such
Supported homogeneous catalysts tend to have asbenzenewereusedforsuchreactionsuntilNoyori
reducedactivitiesrelativetotheirtrulyhomogeneous showed that terminal aliphatic alkenes could be
analogues. However, this drawback can be offset by epoxidized at 90 °C without organic solvents by
the advantages of easy catalyst recovery, reduction means of rapid-stirring, Figure 3.62,63 Under these
oftracemetalcontamination,andfacilemethodsfor conditions, 1-dodecene was converted to its corre-
parallelscreening.Recentworkillustrateshowsolid sponding epoxide in 99% yield with only 1.5 equiv
2460 ChemicalReviews,2003,Vol.103,No.7 LaneandBurgess
Figure3. Noyori’ssolventfreeepoxidationsystem.
Figure4. Phosphate/tungsticacidcatalystfortheforma-
H O . However, only simple aliphatic alkenes are
2 2 tionofacidsensitiveepoxides.
usually cited as substrates; slightly acid-sensitive
epoxides such as phenyl oxiranes are not stable to
teristics. Tributyl phosphate is not an ideal solvent
such reactions conditions, resulting in low yields.62
withrespecttoenvironmentalconsiderations,sothe
This is common for systems with Lewis-acidic cata-
system could be improved in that respect. Another
lysts that use aqueous H O at high temperatures.
2 2 method to recover Venturello tungsten catalysts is
Duetotheabovelimitations,suchtungstencatalysts
to support them on ion-exchange resins.31,32 The
may only be used to produce relatively stable ep-
anion {PO [W(O)(O ) ] } supported on Amberlite
oxides, particularly terminal aliphatic ones, despite 4 2 2 4
IRA-900 resin by ion exchange61 gives a heteroge-
their high conversions and selectivities.
neous system for the epoxidation of a variety of
The type of ammonium salt used with phosphate/
commercially interesting epoxides; high yields are
tungstic acid catalyst is important. For example,
obtained using only 2 equiv of 30% H O in acetoni-
n-octylammonium hydrogen sulfate is critical in 2 2
trile at 38 °C.
Noyori’swork(Figure3);chloridecausesdeactivation
and other ammonium hydrogen sulfates produce Noneofthesystemsmentionedabovearesuitable
catalyststhatarenotaseffectiveorevencompletely for preparing acid-sensitive epoxides such as that
inactive. It seems that the catalyst must have an from1-phenylcyclohexene,butamodificationdevel-
appropriate partition coefficient, one that allows it opedbyJacobsaddressesthis.31TheVenturelloanion
tointeractwithboththeaqueousoxidantandorganic wascombinedwiththeNoyori’soptimalammonium
substrate. There is no reliable way to predict which cation to form an epoxidation catalyst for use in a
ammonium salt is ideal, so selections are usually biphasicreactionwith(aminomethyl)phosphonicand
madethroughtrialanderror.Theaminomethylphos- tungstic acids at pH 5, Figure 4. Under such condi-
phonic acid required in this system is relatively tions, 1-phenylcyclohexene was epoxidized in 81%
expensive. yield and successfully isolated from the reaction
Recoverable catalysts reduce product-impurities mixture.Experimentallythereactionhassomedraw-
and costs, and both these factors are concerns with backs. Solutions of catalyst and reagents must be
tungsten-basedsystems.Toaddresstheseissues,the adjusted to appropriate pH’s prior to mixing, the
tungstic acid epoxidation catalyst shown in Scheme optimalreactionconditionsaresubstratedependent,
2wasdesigned.Thekeytothissystemistheunique and relatively large quantities of tungsten are re-
quired.
Scheme2. ARecoverablePhosphate/Tungstic
In general, the tungstate/phosphate systems dis-
AcidSystemBasedonCatalystSolubility
cussedinthissectionareeffectivecatalysts.Applica-
tionofthesesystemstendstoberestrictedbytoxicity
factorsrelatedtotungstencatalystsandsomeofthe
solvents (often chlorinated media), though the cata-
lysts are active and selective.
3.3. Peroxomolybdates
Molybdenumcatalysts(e.g.,[NMe ] [(PhPO ){MoO-
42 3
(O ) } (cid:226){MoO(O ) (H O)}],[(NH ) Mo O (cid:226)4H O],etc.,
2 2 2 2 2 2 4 6 7 24 2
similartoperoxotungstates),havebeenpreparedand
investigated for the epoxidation of alkenes with
H O .59,65-70Theygivelowturnoversandselectivities
2 2
for the desired products, and their reactions are
performedunderharshconditions.Solublemolybde-
numoxidecomplexesareoftenusedinstoichiometric
quantities in which an active oxidant is generated
from 30% H O and commercially available am-
2 2
monium molybdate. Some peroxomolybdate com-
solubility properties of the catalyst 6. This soluble plexesbearingachiralbidentateR-hydroxyamideor
material oxidizes propene with 30% H O in xylene a chiral monodentate amine N-oxide ligand have
2 2
andtributylphosphateat35°C,butthereducedform been prepared and tested in asymmetric epoxida-
5 precipitates from the medium once H O is con- tions,71,72buttheresultingcomplexeshavetobeused
2 2
sumed.64Manyammoniumcationswerescreenedto in stoichiometric quantities to afford low enantiose-
find a system with such desirable solubility charac- lectivity and yields.
Metal-CatalyzedEpoxidationsofAlkenes ChemicalReviews,2003,Vol.103,No.7 2461
Scheme3. CatalyticCyclein
MethyltrioxorheniumMediatedEpoxidationswith
HydrogenPeroxide
Figure5. Additivesinmethyltrioxorheniumepoxidations
with30%hydrogenperoxide.
pyridine,84,88-90notablypyrazole91-93and3-cyanopy-
ridine,94,95 have now been investigated as additives
inmethyltrioxorhenium-mediatedepoxidations,Fig-
ure 5. Despite these studies, the effects of the basic
3.4. Methyltrioxorhenium additivesarestillamatterforconjecture.Theyseem
to function as phase transfer catalysts. The most
Thediscoveryofmethyltrioxorhenium,MeReO3or effective ones are resistant to oxidation themselves,
MTO,epoxidationsbyHerrmannandco-workers’is and not so basic that catalyst decomposition is
a milestone in this area. A variety of alkenes was favored over the desired epoxidation event. Basic
epoxidized with anhydrous H2O2 in tBuOH or THF characteristicsoftheadditivesarecrucialbecauseif
at room temperature or below using 0.1-1.0 mol % theyaretoobasicthenMTOdecomposestocatalyti-
ofthiscatalystintert-butylalcoholorTHF.73,74This cally inert perrhenic acid and methanol. For these
innovation inspired a variety of research efforts, reasons, pyrazole and 3-cyanopyridine tend to be
including a switch to 30% hydrogen peroxide as preferred.
oxidant,75 mechanistic studies,75-81 and theoretical There are relatively few disadvantages of MTO/
simulations of involved intermediate and transition basesystemsinepoxidation.Insomecasesitcanbe
states, Scheme 3.82,83 With a few exceptions, MTO difficult to separate the additive from the product;
does not undergo redox chemistry during catalytic for instance, separation of acid-sensitive epoxides
reactions; it remains in the +VII oxidation state. from bases having similar boiling points can be
MTOactivatesH2O2throughanequilibriumforma- experimentally difficult since the product may not
tion of the Ł2-peroxo species 8 and 9. The structure survivetreatmentofthecrudereactionmixturewith
of 9 was confirmed crystallographically, and the acid or exposure to silica. With few exceptions,96-98
methyl resonances of 8 and 9 were detected by both additives only work when used in an aprotic media
1H and 13C NMR. like nitromethane or chlorinated solvents, but such
ThemaindisadvantageinMTO/H O epoxidations solvents are not suited for large-scale reactions due
2 2
is that they are inherently acidic and this tends to to the risk of explosions and toxicity issues. Never-
causeringopeningofsensitiveepoxidestodiols.None theless, MTO-catalyzed epoxidations are a very at-
of the procedural-modifications to MTO-mediated tractiveoption,particularlyforsmall-scaleepoxida-
epoxidations satisfactorily addressed the issue of tion reactions. MTO has been used to epoxidize a
acidity until Sharpless and co-workers tested pyri- variety of different alkenes with aliphatic and aro-
dine in the system.84 This additive confers two matic substituents.
importanteffects.First,itmakesthemediumslightly
basic, thus protecting acid-sensitive epoxides from 3.5. Other Metal Oxides
ring opening, and second it accelerates the rate of
Other than MeReO , soluble metal oxides for alk-
the desired epoxidation reaction. Herrmann had 3
ene epoxidation using H O have relatively major
2 2
previously investigated the use of nitrogen bases in
limitations.5,99 For instance, tetraperoxoniobate,
MTO-catalyzed epoxidations but did not observe {Nb(O ) }3-, is a catalyst, but it produces relatively
significantligand-acceleratedcatalysis85becausethe 2 4
poor conversions and selectivities for the desired
aminesusedwereoxidizedtoamineN-oxidesunder
epoxide product;100 other peroxyniobium complexes
thereactionconditions.86,87Besidesdecomposingdur-
havesimilaractivities.101Reactivityprofilessuchas
ing the reaction, amine N-oxides also served as
this are characteristic of systems that operate via
weakerdonorligandsthanthecorrespondingamines,
production of free hydroxyl radicals, i.e., “Fenton
resultinginlessselectiveandactivecomplexes.The
chemistry”.102,103
pK of the amine used is important in achieving
a
ligand-accelerated catalysis since the extent of in-
4. Metal Oxides Generated in Situ
teraction between the rhenium and amine can be
correlatedtothelatter’spK ;morebasicaminestend
a 4.1. Selenium and Arsenic Compounds
to deactivate the MTO. Sharpless’ discovery hinges
ontheuseoftherightbaseinthecorrectquantities Someselenium104-107andarsenic108-110compounds
to ensure high catalytic activity and diminished inperfluorinatedsolventsaresurprisinglyactiveand
epoxide ring-opening. Other nitrogen donors beside selectiveepoxidationcatalysts.Epoxidationofelectron-
2462 ChemicalReviews,2003,Vol.103,No.7 LaneandBurgess
Scheme4. CatalyticCycleforManganeseSulfate
MediatedEpoxidations
All the systems mentioned in this section utilize
perfluorinated solvents to obtain high yields of ep-
oxide. The perfluorinated solvent activates H O by
2 2
serving as a hydrogen bond acceptor prior to its
reaction with the catalyst.112 Despite their remark-
ably efficient catalytic abilities, these systems are
thereforelimitedbyenvironmentalconcernsregard-
ing fluorinated media and safety issues related to
high concentrations of H O .
2 2
Figure6. Selenium/arsenic-basedcatalystsfortheepoxi-
4.2. Simple Metal Salts
dationofalkenes: (a)phenylselenidecatalyst,(b)diselenide
catalyst,and(c)benzenearsonicacid.
Parallel screening led to a method wherein epoxi-
dationispromotedbycatalyticamounts(1.0-0.1mol
rich alkenes by H2O2 in perfluorinated solvents at %) of manganese (2+) salts with 30% H O as a
2 2
80 °C occurs in the absence of catalysts,111,112 but terminaloxidant.116Morethan30d-blockandf-block
addition of diselenide and arsenic catalysts allows
transitionmetalsaltswerescreenedforepoxidation
epoxidationtobeachievedatlowertemperaturesand
activityundersimilarconditions(allinthepresence
shorter times. The phenylselenide, diselenide, and ofbicarbonate);onlychromium(2+)chlorideandiron
arsenic systems can epoxidize alkenes with only 2 (3+)sulfateexhibitedsomeactivity,andMnSO gave
4
equivorlessofH O (Figure6).Whilemostsystems
2 2 the highest activity (Scheme 4). Bicarbonate is an
involvingthesecatalystsrequiretemperaturesof60-
essentialcomponentinthistransformation.Itforms
70 °C, the diselenide system catalyzes epoxidations peroxymonocarbonateion,HCO -,inthissystem;the
4
atroomtemperature.Themechanismofthisprocess
presence of this intermediate was observed using
involves in situ oxidative formation of selenic acids NMR on mixing H O and HCO -. Such equilibria
2 2 3
or oxides that further react with H O to form
2 2 had previously been observed by Richardson et al.
peroxyselenicacids.105,106,113Peroxyselenicacidisthe for formation of HCO - from H O and bicarbonate
4 2 2
active epoxidizing agent, so the diselenide and phe- in other solvents,117 and HCO - without metal was
4
nylselenide compounds are catalyst precursors. The
foundtobeamoderatelyactiveepoxidizingagentin
mechanism of benzenearsonic-acid-catalyzed epoxi- aqueous acetonitrile.118 Uncatalyzed epoxidation by
dations is similar, occurring via formation of phe-
bicarbonate/H O mixtures is slow in the solvents
2 2
nylperarsonic acid.
used in Mn-catalyzed processes, i.e., DMF and
Tertiary arsines can serve as cocatalysts in per- tBuOH.Theinferencefromtheseobservationsisthat
rhenic acid (HReO )-catalyzed epoxidations.114 Rhe- the formed HCO - reacts with Mn ions to produce
4 4
nium compounds, other than MTO, tend not to be active epoxidation reagent. EPR studies show that
efficient epoxidation catalysts with H O because although Mn (2+) is initially consumed in the cata-
2 2
their Lewis acidities, and/or the acidic media they lyticreaction,itisregeneratedtowardtheendofthe
require for catalytic activity, lead to formation of process when H O is spent.119 The implication of
2 2
diols.114,115Diphenylmethylarsineattenuatestheacid- suchfindingsisthatMn(2+)isfirstoxidizedtoMn
ityoftherheniumsaltsandallowsagreatervariety (4+), and then reduced once again in the catalytic
of solvents to be used such that epoxides form cycle;thesesaltsdonotsimplyactasLewisacids.A
without any ring opening. Perrhenic acid (1 mol %) variety of aryl-substituted, cyclic, and trialkyl-
anddiphenylmethylarsine(1.5mol%)with60%H O substituted alkenes were epoxidized under these
2 2
intrifluoroethanolcanepoxidizeterminalandcyclic conditions,butmonoalkylalkeneswereshowntobe
alkylalkenes in high yields at 75 °C. Interestingly, unreactive.
tertiary arsines are the most useful cocatalysts ToimprovethesubstratescopeoftheMn-mediated
tested. The next best is triphenylantimony, which process and to increase the efficiency of H O con-
2 2
produces less than half the amount of epoxide. sumption,catalyticamountsof68diversecompounds
Metal-CatalyzedEpoxidationsofAlkenes ChemicalReviews,2003,Vol.103,No.7 2463
Table1.ManganeseMediatedEpoxidationsUsing30
%HydrogenPeroxidewithoutandwithCatalytic
SalicylicAcid
Figure 7. Additives in manganese porphyrin mediated
were tested as additives to potentially enhance the epoxidations: (a) imidazole and (b) an N-alkyl imidazole
andbenzoicacid.
rate of the epoxidation reaction relative to dispro-
portionation of H O .119 It was shown that 6 mol %
2 2 and additives such as imidazole122-125 or combina-
sodium acetate in the tBuOH system and 4 mol %
tions of imidazoles and carboxylic acids126-129 were
salicylic acid in the DMF system were effective; in
used to ensure high reactivities, Figure 7. Cy-
thepresenceoftheseadditives,thereactionsrequired
cloocteneoxidewasproducedin91%yieldin45min
less H O , decreased reaction times, and the yields
2 2 using only imidazole under the original conditions,
ofthelessreactivealkeneswereenhanced,Table1.
andacomparableyieldwasobtainedinonly15min
Themanganesesystemdescribedaboveissimilar
when N-n-hexylimidazole/benzoic acid was added.
toMeReO -mediatedepoxidations,butthetwometh-
3 The improved conditions with benzoic acid also
ods are complementary in some respects. Less hy-
resulted in quantitative formation of terminal ali-
drogenperoxideisusedintherheniumsystem;itis
phaticepoxides,arelativelydifficultcase,inonly15
suitableforepoxidationofterminalalkenes,whereas
min. Epoxidation of cis-alkenes was shown to be
the manganese-based catalyst is not, and MeReO
3 stereospecific,buttrans-alkenesarepoorsubstrates
does not cause isomerization in the epoxidation of
for these catalysts, e.g., trans-stilbene did not react
mostcis-alkenes,whereastheMnsystemsdo.Onthe
even after an extended period.
other hand, the manganese catalyst is cheaper, far
Quici et al. constructed modified porphyrins with
lesstoxic,andusesmoreenvironmentallycompatible
pendantligandstodeterminetheeffectofcovalently
solvents(tBuOHandDMF,asopposedtohalogenated
linkingunitsoftenusedasadditives.128,129Aporphy-
hydrocarbons or nitromethane).
rin with tethered carboxylic acid, and imidazole
groups gave enhanced epoxidation rates relative to
5. Coordination Complexes
unmodified porphyrin systems, i.e., cyclooctene was
completelyepoxidizedin3minwithperfectselectiv-
5.1. Manganese Porphyrins
ity and a high initial rate of 500 turnovers min-1
Porphyrinshavebeenwidelyinvestigatedasligands (Scheme 5), and similarly high reactivities were
to stabilize metals with respect to undesirable de- observed for other substrates. Scheme 5 shows the
composition pathways and tune their reactivities. possiblerolesoftheseadditivesinthecatalyticcycle.
Manganeseandironporphyrinsarethemostimpor- ImidazoleremainscoordinatedtotheMnthroughout
tant catalyst types for epoxidation reactions. Por- thereaction,whereascarboxylicacidhelpscleavethe
phyrins of other metals, such as molybdenum, give peroxide O-O bond leading to a reactive Mn-oxo
inferior conversions and selectivities.120,121 It was intermediate 14. Oxomanganese species are well-
Mansuyandco-workerswhodiscoveredtheeffective- established intermediates in Mn-porphyrin-medi-
ness of Mn-porphyrin complexes for alkene epoxi- ated epoxidations with H O .130-133 The oxo-Mn(V)
2 2
dationinthepresenceofH O .Chlorinatedporphy- intermediate has been isolated and its formation
2 2
rinswererequiredtoresistoxidationofthecatalyst, further studied using spectroscopic techniques.134
2464 ChemicalReviews,2003,Vol.103,No.7 LaneandBurgess
Scheme5. PossibleCatalyticCycleShowingthe
ImportanceofImidazoleandCarboxylatesinthe
QuiciModifiedManganese-Porphyrin
EpoxidationSystem
Figure 8. Iron porphyrin mediated epoxidation with
electron-poorligandsandproticsolvent.
madetwoimportantdiscoveriesinthisarea.141First,
contrary to prior assumptions, peroxides in the
presence of Fe-heme systems do not undergo ho-
molytic cleavage to produce radicals and oxoiron
intermediates (>FedO; high valent ironV oxo) but
insteadgiveheterolyticcleavagetoproduceanoxene
species(the>FedO¥+ironIVradicalcation,fromone
electron oxidation of the porphyrin ligand). This
oxenespeciesisthedesiredepoxidationcatalyst,but
unconstructive side reactions (i.e., catalyst decom-
position, unselective oxidation) arise when the oxi-
dantandoxenereactpriortotheepoxidationevent.
In that case, alkoxy radicals are produced and
undesirable side reactions result. Traylor therefore
proposed that radical production is minimized by
using protic solvents and by keeping the concentra-
tionoftheoxidantlow(e.g.,byslowaddition),Figure
8.Second,Traylorfoundthatmoreelectron-deficient
porphyrins favored epoxidation under the experi-
mental conditions. For instance, highly fluorinated
porphyrinsandthosewithotherelectron-withdraw-
ing groups were shown to catalyze epoxidation of
alkenes with H O (Figure 8).142-145 More recently,
2 2
electron deficient porphyrins were also shown to be
catalytically active in ionic liquids.146
Although other additives including ammonium ac-
Recentresearchsuggeststhatcomplicationsinthe
etate,amineN-oxides,andsodiumbicarbonatewere
epoxidation reaction can have several origins,147,148
also investigated, their effects were comparable to
and competing mechanisms, especially for electron-
those of imidazole and/or benzoic acid.135-137
rich porphyrins, can complicate mechanistic inter-
Asymmetric Mn-porphyrins have also been syn-
pretations. For instance, direct oxidation of the
thesized by attaching threitol to the aryl substitu-
porphyrin ligands by Fe-oxene intermediates can
ents.138 These catalysts gave up to 88% ee and 85%
compete with catalytic epoxidation, even if haloge-
yields in the epoxidation of 1,2-dihydronaphthalene
nated porphyrins are used. Consequently, reactions
withiodosylbenzene,butinferiorresultsusingH O
2 2 ofFe-oxenespecieswiththeoxidanttogeneratefree
as the oxidant, i.e., 29% ee, 68% yield.
alkoxy radicals may partially explain why Fe-
porphyrins in catalytic epoxidations can have poor
5.2. Iron Porphyrins
activities.
Iron porphyrins can be highly active epoxidation Traylor’sobservationsregardingtheadvantagesof
catalysts, but the conversions and selectivities ob- electron-deficient porphyrins, protic solvents, and
tained from them tend to be inferior to their Mn slow addition of the oxidant spawned further devel-
counterpartsunlessthereactionsareexecutedunder opments in this field. Formation and reactivity of
strictly controlled conditions. An appreciation of ironintermediateswerestudiedusing18Oincorpora-
suitableconditionsarosefromresearchtounderstand tion experiments.149,150 When 18O-labeled water was
why these epoxidations sometimes work only with presentinthereaction,differentlevelsofincorpora-
otheroxidantssuchasiodosylbenzene.139,140Traylor tion into the epoxide were observed at different
Metal-CatalyzedEpoxidationsofAlkenes ChemicalReviews,2003,Vol.103,No.7 2465
Scheme6. TwoDistinctPathwaysforIronPorphyrin-MediatedEpoxidationsDependingonthe
ConditionsUseda
aPathwayaisfavoredinpolarsolventsandlowhydrogenperoxideconcentrations,whereaspathwaybpredominatesinapolarsolvents
andhighperoxideconcentrations.
temperatures; a reaction carried out at -78 °C Table2.EffectsofCatalystsStructureandAdditives
inEpoxidationReactionsMediatedbyIron
showed no incorporation, but almost 60% was ob-
Porphyrins
served at 40 °C. This, combined with EPR experi-
ments showing that the oxidation state of Fe does
not change at low temperatures, suggests that a
stable intermediate can epoxidize alkenes at low
temperature,andthisisdistinctfromthehigh-valent
Fe-oxene species formed at around 40 °C. Nam
concluded that this intermediate is an iron hydrop-
eroxide complex (>FeOOH).149,150
Experiments involving incorporation of labeled
water were used to deduce the role of imidazole in
theironporphyrinmediatedepoxidation.Ifimidazole
wasaddedtothereaction(at25°C),noincorporation
of 18O from water was observed. This suggests that
imidazole coordinates the open axial position of the
complexthatwateroriginallyoccupied,thusprevent-
ingaredox-liketautomerizationofcoordinatedwater
withthejuxtaposedoxo-ligand.Thistautomerization
would otherwise deliver labeled oxygen into the
reactiveoxeneintermediate,leadingtopartially18O- yield yieldwith
labeledepoxides.Thus,imidazoleadditivescanpre- catalyst (%)a additiveb(%)a
ventaxialcoordinationofwaterandsimultaneously 15 0 51
attenuate the reactivity of the imidazole containing 16 0 63
porphyrininusefulways.Infact,ifthereactionsare 17 <2 79
performed in aprotic solvents with a near-optimal 18 <2 65
imidazoleadditive(e.g.,5-chloro-1-methylimidazole), aYieldbasedonH O used.bAdditive)5mol%5-chloro-
2 2
even electron-rich porphyrins could be competent 1-methylimidazole.
catalysts, Table 2.151
Competition experiments using different alkenes oxidant. This suggests that the oxidizing species
have been carried out to explore the effects of using varies in aprotic solvent with the oxidant used. In
protic or aprotic media (Scheme 6).152 In this study, controlexperiments,withstoichiometric,preformed
reactions using either cis- and trans-stilbene or high-valent Fe(IV)-porphyrin complexes in aprotic
cycloocteneandtrans-stilbenewereconductedusing solventstheproductratiosweredifferenttoanyother
different oxidants such as H O , m-CPBA, or t- oxidant used in those aprotic solvents, and similar
2 2
BuOOHinbothproticandaproticmedia.Theratios toonesobtainedinproticsolvent.Itthereforeseems
ofproductsformedineachepoxidationreactionwere that at least two active intermediates are involved,
compared. If a common intermediate was formed in depending on the solvent used. For protic solvents,
two or more reactions, it would be expected that the reactive intermediate appears to be the high-
similarratiosofproductswouldbeobserved.Inprotic valent Fe(IV)-porphyrin radical cation complex
solvents, all the oxidants gave the same ratio of formedfromanironhydroperoxideprecursor(Scheme
products for a given catalyst. However in aprotic 6pathA),basedonthesimilarproductdistributions
solvents, the product ratios were dependent on the for all the oxidants. In aprotic solvents, Scheme 6
2466 ChemicalReviews,2003,Vol.103,No.7 LaneandBurgess
path B, the reactive intermediate is more likely to Epoxide ee’s and yields from salen complexes and
be the iron hydroperoxide complex itself. Scheme 6 anhydrous hydrogen peroxide approach those ob-
onlyaddressesthecaseofhydrogenperoxide.Other tainedwithotheroxidants,155butremovalofureaor
oxidants in aprotic solvents must give different triphenylphosphine oxide side-products is an issue.
intermediates, e.g., a tert-butyl hydroperoxide iron Moreover,quitelargemolpercentagesofsalencata-
complex(>Fe-O-O-tBu,ironIII)ift-BuOOHisusedas lystsareused,typicallyinchlorinatedsolventswith
theoxidant.Thisaccountsforthedifferentratiosof additives.Nonetheless,salencomplexesarethebest
products formed under aprotic conditions with dif- catalysts, to date, for asymmetric epoxidation using
ferent oxidants. (anhydrous or aqueous) H O as an oxidant.
2 2
Overall, the story of epoxidations with iron por- There are a few reports of Mn-salen mediated
phyrincatalystsreflectsthedelicatebalanceofsteps epoxidationsofalkeneswithhydrogenperoxidethat
inthistypeofcatalyticcycleandhoweasilytheyare do not feature asymmetric syntheses. For example,
perturbed. The data obtained depend on the elec- polymeric,polynuclearSchiff-baseMncomplexes23,
tronicstructureoftheporphyrin,additives,andthe withrepeatingsalen-likecores,weresynthesizedand
solvent used and illustrate the fact that several characterizedinsitu.161Thegoalofthisworkwasto
reactiveintermediatesareaccessible,andtheyeach testifthepolynuclearcharacteristicsofthesemateri-
can deliver oxygen to alkene substrates. alsenhancetheirreactivities,and/ortoeliminatethe
need for additives. However, they proved to be
5.3. Manganese Salen Complexes moderate catalysts for the epoxidation of simple
alkenes, and additives such as imidazole were re-
Manganesesalencomplexeshavebeeninvestigated
quiredtoachievecatalyticactivitywithH O .Other
in alkene epoxidations with H O , largely from the 2 2
2 2 Schiffbaseligandshavebeenpreparedbytemplate-
perspective of asymmetric catalysis. Berkessel, the
inducedmacrocyclization/condensationofdiethylen-
first to report them in epoxidations with H O ,
2 2 etriamine(H NCH CH NHCH CH NH )witheither
noticed that most Mn coordination complexes re- 2 2 2 2 2 2
pentane-2,4-dioneor1,3-diphenyl-propane-1,3-dione.
quired additives such as imidazole to generate the
Thebinuclearmanganese-complexesproducedinthis
requisiteMn-oxointermediatesviaheterolyticcleav-
way were moderately active in epoxidations, giving
age of H O .153,154 Consequently, his group con-
2 2 yields from 9% to 82% for isoprene and limonene
structedcatalyst19containingatetheredimidazole;
when additives (e.g., ammonium acetate) were in-
itwasshowntoepoxidize1,2-dihydronaphthalenein
cluded.162
77%yieldand64%ee,Table3.Unfortunately,better
results were obtained using other oxidants, notably
5.4. 1,4,7-Triazacyclononane (TACN) Complexes
sodiumhypochlorite,withthesecatalysts.155Concur-
rently, Katsuki developed conditions relying on an Interest in TACN-derived catalysts for alkene
unfunctionalizedMn-salencomplex,20,andimida- epoxidation with H O originates largely from work
2 2
zole in solution,156 and obtained high enantiomeric atUnileveronpotentialdetergentadditivestooxidize
excess (96%) but low yields (17%). away organic staining materials.163-166 Many Mn-
Pietika¨inenbeganinvestigatingimidazolesasad- TACN ligands have been used to form epoxidation
ditivesinsalenmediatedepoxidationswithhydrogen catalysts with H O . Interestingly, the same ligand
2 2
peroxide157 but realized that higher yields and ee’s canformvariousdinuclearcomplexesdependingon
couldbeobtainedusingcarboxylatesasadditives.158 the synthesis conditions. For example, dinuclear
Highenantioselectivities(92%)andyields(86%)were complexes bearing the Me TACN ligand 24 have
3
obtainedbyaddingammoniumacetatetoJacobsen’s either bridging oxo, peroxo, and/or carboxylate cen-
catalysts 21. Such conditions are reminiscent of the ters as shown in Figure 9.163 The reactions of com-
ammoniumacetate/Mn-porphyrinsystemdeveloped plexes 25-27 are inefficient with respect to conver-
by Mansuy, and there are likely to be mechanistic sion of H O to epoxide; complex 27 completely
2 2
similarities.136 epoxidizes styrene, but 100 equiv of 30% H O were
2 2
Catalyst deactivation is a problem in salen-medi- used. The efficiency was improved by using oxalate
ated epoxidations with aqueous hydrogen peroxide, buffer,167 or adding ascorbic and squaric acid.168 In
caused by radical formation via homolytic cleavage retrospect it is evident that additives change the
oftheweakO-Operoxidebond.Theadditivesused reactivities and selectivities of these reactions in
in the examples above do elevate this to a certain unpredictable ways. For example, 0.1 mol % of the
degree; however, an alternative approach uses an- complex formed from ligand 24 and manganese(II)
hydrousH O adducts,suchasPh P(O)(cid:226)H O (Ph P- sulfate quantitatively epoxidizes 1-hexene in the
2 2 3 2 2 3
(O)(cid:226)H O )triphenylphosphineoxide/H O complex) presence of 0.3 mol % oxalate buffer and 1.5 eq of
2 2 2 2
can be used to produce peracids in solution with 30%H2O2,butcatalyst27evenoxidizeshydrocarbons
maleic anhydride, thus avoiding this problem.159 when acetic acid is present.169 Activated carbonyl
Urea(cid:226)H O complex (UHP) is also viable for epoxi- additives,suchasglyoxylicacidmethylestermethyl
2 2
dations with salen complexes.159 It can be used hemiacetal(CH3OCHOHCO2CH3)and27,canresult
directly as an oxidant with the dimeric catalyst 22 in production of significant amounts of vicinal cis-
and ammonium acetate as an additive;160 cyano- diols in addition to peroxides.170
chromene was epoxidized in nearly 100% yield and The modified TACN ligands 28-31 have been
enantioselectivityusingthisapproach.However,this made,butmanganesecomplexesofthesehaveactivi-
system may not be particularly general for other ties comparable to the ones previously mentioned.
types of alkenes. Ligand 31 was designed to support the TACN core