Recent Advances in Vanadium Catalyzed Oxygen Transfer Reactions,

Other uses, including reproduction and distribution, or selling or

licensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of the

article (e.g. in Word or Tex form) to their personal website or

institutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies are

encouraged to visit:

ContentslistsavailableatScienceDirect

Coordination

Chemistry

Reviews

j ou rna l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c c r

Review

Recent

advances

in

vanadium

catalyzed

oxygen

transfer

reactions

Giulia

Licini

_{, Valeria}

_Conte

_,

_Alessia

_Coletti

_,

_Miriam

_Mba

_,

_Cristiano

_Zonta

a_{DipartimentodiScienzeChimiche,UniversitàdiPadova,ViaMarzolo1,35131Padova,Italy}

b_{DipartimentodiScienzeeTecnologieChimiche,UniversitàdiRoma“TorVergata”,ViadellaRicercaScientificasnc,00133Roma,Italy}

Contents

1. Introduction... 2345

2. Sulfoxidations. . . ... 2346

2.1. Diandtrialkanolaminederivatives... 2346

2.2. Diandtriphenolaminederivatives. . . ... 2346

2.3. Polyhedraloligomericsilsesquioxanetrisilanolate(POSS)... 2347

2.4. Schiffbasesystems. . . ... 2349

3. Epoxidations... 2350

3.1. Epoxidationofallylicandhomoallylicalcohols. . . ... 2351

3.2. Epoxidationofsimpleolefins... 2354

4. Haloperoxidations. . . ... 2354 5. Conclusions... 2356 References. . . ... 2356

a

r

t

i

c

l

e

i

n

f

o

Keywords: Vanadium Oxidations Epoxidation Sulfoxidation Haloperoxidation Alkylhydroperoxides Hydrogenperoxide

a

b

s

t

r

a

c

t

Vanadiumcomplexeshaveproventobeeffectivecatalystsfortheactivationofperoxidesandtheselective oxidationofsubstrateslikebromides,sulfidesandalkenes.Besidestheircapabilitytoformmetalloperoxo species,whicheffectivelytransferoxygenatomstothesubstrate,thesesystemsaresyntheticallyuseful forobtainingvaluableoxidizedmoleculesonapreparativescale,withahighdegreeofselectivityand TONs.Furthermore,theuseofenvironmentallyfriendlyoxidantslikehydrogenandalkylhydroperoxides increasessignificantlytheirpotentialapplicationatanindustriallevel.

Herewereportacriticalsurveyonthemosteffectivehomogeneousvanadiumcatalystsreported inthelastdecadeconcerningtheirsyntheticapplicationinoxygentransferreactions(sulfoxidation, epoxidation,haloperoxidation)usinghydrogenperoxideoralkylhydroperoxides,demonstratingthe differentclassesofligandsandcomplexes,theircatalyticperformances,theirreactivity,chemo,stereo andsubstrateselectivity.Someexamplesoftheuseofnonconventionalreactionmediaortechniques andcatalystrecyclingstudieswillbealsodiscussed.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Thecapabilityofd0_{metalcomplexestocatalyzeoxygentransfer}

reactionshasreceivedmuchattentionduringthepastthirtyyears

[1].Thesemetalsareabletoactivateperoxidessuchashydrogen peroxide,cumylort-butylhydroperoxidefortheoxidationtoa largevarietyofsubstrates,and,withtheadditionofproper chi-ralligands,highdegreesofstereoselectivityhavebeenachieved. Inourgroupswehavebeenlargelyinterestedinoxygentransfer reactionsinvolvingzirconium[2],titanium[3],molybdenum[4]

∗ Correspondingauthor.

E-mailaddress:giulia.licini@unipd.it(G.Licini).

andvanadium[5]basedcatalysts.Inparticularvanadiumcatalyzed oxidationshaveattractedourattentionbyvirtueoftheinteresting propertiesofthismetalintermsofselectivity,reactivityand stere-oselectivity.Indeed,vanadiummediatedoxygentransferreactions arefrequentlyappliedinchemicalresearch,andinrecentyears, importantadvanceshavebeenachieved.Inthiscontext,vanadium catalyzed oxygentransferreactionshavegained renewed inter-estduetothediscoveryofvanadium-dependenthaloperoxidase (VHPO)enzymesabletoactivatehydrogenperoxideforthe oxi-dationofhalideswiththeconsequentproductionofhalogenated compoundsandthestereoselectivesulfoxidations[6].

Peroxovanadium complexes,dependingonthenatureofthe ligands coordinatedtothemetaland ontheexperimental con-ditions,can act eitheras electrophilicoxygentransfer reagents

0010-8545/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

Fig.1.Synthesisofenantiopurebis-alkanolamines1–3derivedfrom(R)-styreneoxideor(S)-propeneoxide[8].

Scheme1.InterconversionofV(V)diastereomericcomplexesobtainedfromligands1–3,presentinganequatorialoranaxialvanadiumoxofunction[8].

or as radical oxidants. Typical electrophilic processes are the oxidationofsulfidesandtertiaryaminesandtheepoxidationof allylicalcoholor simplealkenes. Theoxidationof alcohols and the hydroxylation of aliphatic and aromatic hydrocarbons are examplesofhomolyticreactivity. Thiscontributionreviewsthe vanadium-catalyzed oxidations in which electrophilic oxygen transferreactionsaretakingplace.Thistranslatestotheoxidation ofsulfides,alkenes,halideswhichhavebeenstudiedinthe pres-enceof terminal oxidants suchas molecularoxygen,hydrogen peroxide,and alkyl hydroperoxides. Thus, Section 2 is devoted tosulfoxidations,whileSection3presentsepoxidations.Inboth these sections great emphasis is given to the stereoselective processes.Theoxidationofhalides,haloperoxidation,isdiscussed inSection4.Alltheprocessesinwhichvanadiumcatalystsreact in a heterolytic fashion have been considered. Several review articles have appeared on vanadium-catalyzed oxidation [6,7], consequently,thepresentreviewemphasizestheworkpublished duringthepast7–10yearscoveringstudiesupto2010.

2. Sulfoxidations

Vanadiumcatalyzedsulfoxidationisanimportant transforma-tionbothfortherelevanceoftheproductsobtained(sulfoxidesand sulfones)andfortheinformationthatthisreactioncangiveonthe natureoftheactiveperoxospeciesinvolvedintotheprocessand themechanismoftheoxygentransfer.Furthermoretheprocesscan bechemoandstereoselective,yieldingmainlysulfoxideswithhigh enantiomericexcesses[7].

2.1. Diandtrialkanolaminederivatives

In2003aseriesofenantiopurebis-alkanolamineswere synthe-sizedbyRehderandcoworkersasmodelsofvanadate-dependent peroxidases[8].Theligandswerepreparedbytheaminolysisof enantiopure(R)-styreneoxideor(S)-propyleneoxidewithdifferent primaryamines(Fig.1).

Thecoordinationchemistryofthenewclassofligands1–3has beeninvestigatedbyreactionwithVO(O-i-Pr)3.Usuallyamixture

ofmononuclearcomplexeswithadistortedtrigonalbipyramidal geometrywasobtained,equilibratingamongthepossible diastere-omeric forms having an equatorial or an axial vanadium oxo function(Scheme1).

ThesecomplexeshavebeencharacterizedbothviaX-ray diffrac-tionand,insolution,1_Hand51_{VNMR,alsoatvariabletemperature.}

Thecatalyticperformancesofthenewcomplexesinsulfoxidation havebeentestedusingalkylhydroperoxidesasoxygensource [tert-butylperoxide(TBHP)orcumylperoxide(CHP)]inchloroformon methylp-tolylsulfide4orbenzylphenylsulfide5.Thebestresults obtainedinthesestudies,withrespectmainlytothereactivityand stereoselectivityoftheprocess,arereportedinTable1.

Stereoselectivities up to 38% were obtained in satisfactory chemicalyieldsandselectivitytowardssulfoxideformation.The absolutestereochemistryofthedialkanolaminecontrolsthe stere-oselection of the oxygen transfer process [(S,S) ligands afford (S)-sulfoxides,(R,R)ligandsgivethe(R)-ones].

More recently Martinez and Dutasta reported on the syn-thesisand catalytic performance inthesulfoxidation of methyl p-tolyl sulfide 4 and benzyl phenyl sulfide 5 of enantiopure hemicryptophane–vanadiumoxocomplexes6and7(Fig.2)[9].

Thesupramolecularcatalysts6and7catalyzethesulfoxidation ofboth substratesinthepresence ofCHPwithcatalystloading downto0.5%,chemicalyieldsupto95%,highsulfoxide/sulfone selectivity(98–100%)andTONupto180.Interestingly,reactions carried out with the model catalyst 8 give much lower yields (25–46%).Asfarasthestereoselectivityofthesystemsisconcerned, theauthors reportenantiomeric excesses around10% indepen-dentlyfrom theenantiopure vanadium complex employedand fromtheratioofthetwopossible/diastereomers.

2.2. Diandtriphenolaminederivatives

C3 V(V)amino triphenolateshave beenprepared andtested

ashaloperoxidasestructuralandfunctionalmodelsbyLiciniand

Table1

Oxidationofmethylp-tolylsulfide4orbenzylphenylsulfide5byTBHPorCHPinthepresenceofV(V)/1–3catalystsat0–20◦_{C.Substrate:oxidant:V(V)=10:10:1}

([sulfide]0=0.1M)[8].

Entry Substrate Ligand Time(h) Conv(%) SO/SO2 Ees(conf) Ref.

1 4 2a 2.5(0◦C) 100 85:15 31(S) [8a]

2 5 2a 4.5(0◦C) 100 79:21 23(S) [8a]

3 4 1a 2.0(−20◦_{C) 70 93:7 38(R) [8b]}

4 5 1a 0.33(−20◦_{C) 70 96:4 37(R) [8b]}

Fig.2. Dutastaenantiopurehemicryptophane–vanadiumoxocomplexes6,7andthemodelsystem8[9].

Scheme2.Aminotriphenolatevanadiumcomplexes10a–c[10].

coworkers[10].Triphenolamine9canbeeasilypreparedvia three-fold reductive amination of salicylic aldehyde derivatives [11]. Reactionof9withVO(O-i-Pr)3affordsinhighyieldsofthe

mononu-clearC3 complexes10b (R=Me)and 10c(R=t-Bu) asdeepred

crystallinesolids,whichwerecharacterizedby1_Hand51_VNMRand

X-raydiffractometric analysis(Scheme2).Complex10aderived fromthelesshinderedligand10ashowsmorecomplexNMR spec-traconsistentwiththeformationofaggregates/mixtureofspecies. The catalytic performances of complexes 10a–c (10%) were testedinthesulfoxidationofthioanisole11withhydrogen perox-ideasprimaryoxidantunderhomogeneousconditions(methanol). Allthreemononuclearcatalystsgavefastandtotallyselective con-versionintothesulfoxideandin thecaseof10c(R=t-Bu)with quantitativeyields.Thissystemwasfurtheroptimizeddecreasing thecatalyst loadingdownto0.01%.Methylphenylsulfoxide12

wasobtainedinquantitativeyieldsandveryhighselectivitywith respecttomethylphenylsulfone13,reachingTONsupto9900 andTOFupto8000h−1workingwith[sulfide]0=1.0Mandmore

concentratedH2O2(70%)(Table2,entries3and5).

Theoptimizedprocedurewasusedfordeterminingthescope ofthereactionwithaseriesofsulfides(Fig.3).Inallcasechemical yieldsover94%wereobtainedwithveryhighSO/SO2selectivities,

resultsthatareinlinewithanelectrophilicoxygentransferprocess andprovethesyntheticapplicabilityofthemethodonapreparative scale.

OtherV(V)catalystscontainingaminophenolate-basedligands werereportedbyMartinsandcoworkers[12].Ligands14a,bwere preparedviaMannichcondensationfromthecorresponding o,p-di(tert-butyl)phenols and different bis-amines(Fig. 4)[13].The authorspreparedthevanadium(III)complexes15a,bbyreaction withVCl3(THF)3.Oxidation byairgavethecorresponding

vana-dium(V)complexes16a,b,whose1_Hand13_{CNMRarecompatible}

withanaverageCsoctahedralgeometrywithtransphenolate

coor-dinationandanaxialoxogroup.Additionofi-PrOLigave17aasa

Fig.3. Sulfoxidesobtainedbysulfoxidationby30%aqueousH2O2catalyzedby9c

(0.1%)(Chemicalyields,sulfoxide,sulfoneselectivity.)[10].

diastereomericmixturehavingthevanadiumoxofunction equa-torialoraxial.Onthecontrary,thechiralcomplex(R)-17bshows

1_Hand13_{CNMRspectraconsistentwithaC}

1octahedralcomplex,

withanaxialoxogroup.Mostofthesecomplexeshavebeen char-acterizedviaX-rayanalysis(Fig.4).

Thecatalyticactivityofthesecomplexeshasbeeninvestigated usingH2O2(20%)inslightexcessinthepresenceof1%catalystusing

differentsolventsandreactiontemperatures.Thebestresultsare reportedinTable3.

Themostreactiveand selectivecatalystis(R)-16bin homo-geneous phase (acetone) at 0◦C (Table 3, entry 4). Complete conversionwithhighSO/SO2selectivitywasobtainedalreadyafter

4h. The same systemprovided much lower conversions under biphasicsystems(DCE/H2O,Tables3and4,entries2–3).The

bipha-sicsystemprovidesbetterresultswhenthecatalystsareprepared insitufromVO(acac)2reaching91%conversionsafter24h(Table3,

entries6and7).Inallthecasesnoenantiomericexcesseswere obtainedusingthechiralcatalysts.

2.3. Polyhedraloligomericsilsesquioxanetrisilanolate(POSS)

Polyhedral oligomeric silsesquioxanetrisilanolate V(V) com-plex18catalyzesthesulfoxidation,aswellasamineoxidationto nitronesorN-oxides(Scheme3)[14].

Quantitativeyieldswereobtainedintheoxidationofmethyl p-tolylsulfide4withCHPworkingatroomtemperaturewithhigh sulfoxide/sulfoneselectivity(Table4,entry1).Furthermoreinthe presenceofLewisbasecoligandsasignificantincreaseofthe reac-tivity(upto24fold)couldbeobtainedassociatedwithdecreased sulfoxide/sulfoneselectivity(Table4).

Table2

Oxidationofthioanisole11byaqueoushydrogenperoxidecatalyzedby10c.a_{Effectofconcentrationandcatalystloading[10].}

Entry [11]0(M) 9c(%) Yields(%) 12:13 Time(min) TON TOF(h−1)

1 0.1 1 97 96:4 25 97 240

2 1.0 0.1 98 98:2 80 980 1330

3 1.0b _{0.1 98 97:3 20 980 8000}

4 1.0 0.01 97 97:3 850 9700 1790

5 1.0b _{0.01 99 98:2 255 9900 2667}

a_{Reactionswerecarriedoutat28}◦_{CinCD3ODusinga1:1molarratioofsubstrate/H2O2(35%inwater).} b_{ReactionsperformedwithH}

2O2,70%inwater.

Fig.4. Aminobis-phenols14andthecorrespondingV(V)complexes15–17[13].

Table3

Oxidationofthioanisole(11)byH2O2withcatalystsVO(acac)2/14,15–17a[13].

Entry Catalyst Solvent Temp.(◦C) Time(h) Conv(%) Sulfone13(%)

1 (R)-16b DCE 10 18 96 11

2 (R)-16b DCE 0 24 27 1

3 (R)-16b DCE 25 4 78 3

4 (R)-16b Acetone 0 4 >99 2

5 (R)-17b DCE 10 20 93 5

6 VO(acac)2/14a DCE 0 24 91 14

7 VO(acac)2/(R)-14b DCE 0 24 91 6

a_{Reactionscarriedoutat0–25}◦_{Cwitha1:1.2molarratioofsubstrate/aqH2O2;1%catalyston0.55mmolscale.}

Table4

Oxidationofmethylp-tolylsulfide4byCHPcatalyzedbyVO–POSScomplex18(1%)a_[14].

Entry Co-ligand t1/2s(equiv.coligand)b SO:SO2

1 None 6000 98:2 2 MeSOpTol 5580(5) 98:2 4 HMPA 3042(5) 93:7 5 HMPA 456(150) 68:32 6 (nOct)3PO 290(5) 95:5 7 HexMe2N+-O− 233(5) 87:13

a_{ReactionswerecarriedoutinCHCl}

3at28◦Cusing[4]0=[CHP]0=0.1Mand1mol%of18. b_{Timerequiredfora50%decreaseoftheinitialconcentrationof4.}

Scheme3.SynthesisofVO–POSScomplex18[15].

ThemodifiedreactivityofthecatalystsinthepresenceofLewis basescanbeascribedtotheformationof newcatalyticspecies insolution viacoordination oftheLewis basetotheV(V) cen-ter.Experimentalevidenceofthecoordinationofthecoligandto thecatalystwasobtainedby51_{VNMRstudies.Infact,in}

chloro-formin thepresenceofonlytwo equivalentsofHexMe2NOthe

signalof18(−690.9ppm)evolvescompletelyintoanewspecies (−605.9ppm). Furthermorethereactionperformedin the pres-enceofanenantiopurephosphoricamidegaveastereoselective oxidation(ee=14%).

2.4. Schiffbasesystems

Oneofthemostefficientcatalyticsystemsforstereoselective sulfoxidationsofarreportedistheBolmprotocol,basedon vana-diumcomplexesofchiralSchiffbases19a,bandhydrogenperoxide asprimary oxidant(Fig.5)[16].Thereactions, under the stan-dardconditions,arecarriedoutinatwo-phasesystem(aqueous hydrogenperoxide/dichoromethane)atroomtemperatureusing VO(acac)2asvanadiumsourceandachiralligand.Theoperative

conditionsaresimple,thecatalyst(1%)isformedinsituandthe stereoselectivitiesobtainedaregenerallyveryhigh.Thesystemis highlymodularandtherefore,afteritsdiscoveryin1995, inten-siveinvestigationswerecarriedoutforfurtheroptimizations,also withthe significantcontribution of other researchgroups. The influenceofparameterslikethenatureoftheSchiffbaseligands

19–23,thereaction protocoland scopehave beenexaminedin detail toallowfurtherincreasingof thestereoselectivityof the process(ees>99.5%)[7a].

Thisparticularaimwasobtainedviaoveroxidationtosulfone, takingadvantageofthecooperativestereoselectivekinetic resolu-tionprocesswhichbuildsupthesameenantiomericsulfoxide.

WhiletheBolmprotocoldidnotindicatesignificantkinetic res-olution onthesecondoxidation step(oxidation of sulfoxideto sulfone),areportbyZengetal.[17]showedthatatlower tempera-tures(0◦Cvs20◦C)thepreformedvanadiumcomplexes(S)-24a

and (S)-24b (Fig. 6), in the presence of increasing amounts of hydrogenperoxideaffordincreasedstereoselectionsinthemethyl phenylsulfoxideformation(ees fromca. 65 to99%)in reason-able chemical yields (40%). The selectivity of both in situ and preformedcatalystswasexaminedandcompared,findingslightly betterresultsinthesecondcase.Thepreformedsystems(S)-24a

and(S)-24bhave beentestedwithdifferentsubstratesreaching eesintherangeof75–99%

Jacksonandcoworkersreportedafurthermodificationofthe Bolm original oxidation protocol which increased the selectiv-ityfactor(S)[18]ofthekineticresolutionofthesulfoxidefrom almostone (S=1.1)to muchhigher values (S=28) [19]. In par-ticulartheeffectofthereactiontemperatureandthesolventare critical.InitiallythesystemwasoptimizedusingtheVO(acac)2

/3,5-diiodosalicylicaldehyde19c(R=R=I)/H2O2system.Workingin

dichoromethanethekineticresolutionoccursonlyat20◦C,while at0◦Cthestereoselectivityofsulfideoxidationishigher.Aprotocol

withtwosubsequentadditionsofoxidantatdifferenttemperatures (1.2equiv.at0◦C+0.3equiv.at20◦C.)wassetup.Thisnewprotocol allowstherecoveryofthe(R)-sulfoxidewithees=97%(44%). How-ever,thebestresultswereobtainedworkinginchloroform,at0◦C withonly 1.2equiv.ofoxidant(catalyst:chiralligand=1%:1.5%): (R)-p-tolyland2-naphthylmethylsulfoxideswereobtainedwith ees>99.5%inverygoodchemicalyields(70and73%).Themethod couldbeeffectivelyusedwithaseriesofalkylarylsulfideswithees intherangeof87–99.5%.Thereactionscouldbecarriedoutalsoon multigramscale.

Maguireandcoworkersoptimizedthereactionprotocolforthe arylbenzylsulfides[20].Arylbenzylsulfidescanhaveapeculiar behaviorinstereoselectivesulfoxidation[21]andthisdependson theoccurrenceofaromaticinteractionsbetweenthesubstratesand thecatalysts.AlsointhiscasethediiodoSchiffbase19c(R=R=I) affordsthebeststereoselectivitiesworkingindichloromethaneat roomtemperature(ees>99%,40–50%yield)withaseriesofaryl benzylsulfideswithselectivityfactorsupto17.6.Interestinglyalso inthiscasehigherselectivitieswereobtainedatroom tempera-tureindichloromethane(Srt=4.6,S0◦C=3.3)orat0◦Cinchloroform

(Srt=5.8,S0◦C=7.2).

MorerecentlyLiandcoworkerstookadvantageofthecombined stereoselectivesulfoxidation/kineticresolutionforobtaininghigh eesintheoxidationofarylalkylsulfides[22].Theyexamineda seriesofdifferentSchiffbaseligandsbearingtwostereocenterat the␤-aminoalcoholarm.Amongtheligandstested23(1%)gave thebestresults(ees≥99%)withalkylarylsulfidesinchloroform at0◦Cinthepresenceof1.35equiv.ofH2O2.Alsointhiscasethe

beststereoselectivitieswereobtainedinchloroformatlow tem-perature.

Gau and coworkers[23] determined the X-raystructures of vanadium(V) complexes 24b–f and the reactivity of both iso-lated and in situ formed complexes reactivity in sulfoxidation wasexamined.Theauthorsobtainedcomparable stereoselectivi-tiesandthisisastrongindicationthattheactivespeciesarethe sameinbothprocesses.Alltheisolatedcomplexeshaveasquared pyramidalgeometry. ComplexeswithR3_{=Bn (24c,e,f) adoptan}

endo-structure, in which theBn substituentand the oxogroup areonthesamesideofthepyramidalbase,while24dwithR3₌

t-Buadoptsanexostructurewiththet-Butranstotheoxomoiety (Fig.7).

Pentadentate Schiffbase25 vanadium complexeshave been synthesizedandtestedinsulfoxidationbyMaeda(Fig.8)[24].The ligandsareobtainedfrombis(aminoalkyl)aminessalicylaldehyde derivatives. The vanadium(IV) complexes displayed a distorted octahedralcoordinationinsolidstate,andtheyaffordedlowerrates ofoxidationofthesulfidethanthetetradentatesalenligands.The samegroupalsoreportedonfourteenaminoacidsandaminoacid esterSchiffbase26(Fig.8)andthecorrespondingV(V)complexes

[25].Peroxocomplexes preparedfromSchiff-basecomplexes of oxovanadium(V)convertedmethylphenylsulfideinhighyieldbut lowees(5–20%).OtherSchiffbaseshavebeenobtainedusingsugars

27,28[26,27].Intheoxidationofthioanisolewithhydrogen per-oxideV(V)/27complexesaffordedeesupto60%[26]whileV(V)/28 eesup26%(Fig.8)[27].

InterestingresultshavebeenobtainedbyAhnandcoworkers usingSchiffbasesof␤-aminoalcohols29and30[28].Thevanadium complexesobtainedfromtheseligandsareveryeffectiveinterm ofstereoselectivityofarylbenzylsulfides(eesupto96%).

Zhu and coworkers reportedhighenantioselectivities in the asymmetricoxidationofsulfidesusingchiralvanadiumcomplexes withsalanligands31inchloroformat0◦C[29].Thecomplexwith

31a(R=R=H)showssuperiorresultsintermofenantioselectivity thanthecorrespondingsalenanalogueandprovidesthe sulfox-ideswithoppositeconfiguration.Thepresenceofsubstituentsin ortho–parapositions ofthearomaticring,N-methylationorthe

Fig.5.Schiffbaseligands19–23forV-catalyzedstereoselectivesulfoxidation[16–23].

Fig.6.IsolatedV(V)catalysts24a–f[17].

Fig.7.Isolatedcomplexesendo-24c,exo-24d,endo-24eandendo-24f[23].

useofdifferentbis-aminebackbones,solventsorhigherreaction temperaturessignificantlydecreasetheselectivityofthesystem. MorerecentlyotherresearchgroupslikeBryliakovandTalsi[30]

orCorreiaandcoworkers[31]exploredthereactivityofanalogous systemfindingmuchlowerstereoselections.InparticularCorreia

[31],working withisolated salan-complexes,reported opposite stereoselectivitieswithrespecttotheZhusystem.

Polymer boundvanadium complexes have beensynthesized andusedinsulfoxidation,inparticularforthedesulfurizationof sulfurcontainingaromaticcompoundslikethiophenederivatives

[32].

Schiff base ligands containing an histamine unit or 3-formylsalicylicacidhavebeencovalentlybondedto chloromethy-lated polystyrene [32b] in some cases cross linked with divinylbenzene(5%)[32a].TreatmentwithVO(acac)2eventuallyin

thepresenceofoxygengavethepolymericcatalysts.Thecatalytic desulfurizationhasbeencarried outin apolar solvents at60◦C with3–15%catalystloadinginthepresenceofthreefoldexcess ofhydrogenperoxide.Dependingonthesubstrate,desulfurization

of70–99%wasachievedin2h.Thesystemswererecycledupto threetimeswithoutlostofactivity.

3. Epoxidations

Chiral epoxides are key buildingblocks for thesynthesis of naturalproductsandbiologicallyactivecompounds.Amongthe different methodologies available for theirsynthesis, the Ti(IV) catalyzedoxidationofallylicalcoholsdeveloped byKatsukiand Sharplessis,todate,themostefficientone[33].Ontheotherhand, this methodology presentssome limitations, mainly therather highcatalystsloadingsandanhydrousworkingconditions. Alter-natively, vanadium catalyzed epoxidations have beenexplored. Vanadiumislessmoisturesensitivethantitaniumandpermitted procedureswithlowercatalystloading.Afterthepioneeringwork ofSharpless,whoreportedenantioselectivitiesupto80%usinga proline-basedhydroxamicacid[34],anotherimportant achieve-mentwasreportedbyYamamotoandcoworkersin2000[35].They designedanewchiralhydroxamicacidderivedfromtert-leucine

Fig.8. Schiffbaseandsalantypechiralligands24–31[24–30].

abletoaffordgoodenantioselectivitiesintheepoxidationof disub-stitutedallylicalcoholsinreasonablereactiontimeswithonly1% catalystloadingandTBHPasoxidant.

Inmorerecentyears,researchhasbeenmainlyfocusedonthe improvementofthestereoselectivity,reducingcatalystloadings andusingmildreactionconditionsmaintainingreasonablereaction rates.Anotherimportantissueofvanadiumcatalyzedepoxidations thathasbeenaddressedistheliganddecelerationeffect.Infact,in thesereactionsthecoordinationoftheligandreducestheactivity ofthecatalystsothereactivityofthesystemisusuallylowerthan thatofthevanadiumprecursors(VO(acac)2orVO(O-i-Pr)3).A

com-monstrategytosolvethisproblemistoworkwithexcessofligand. However,theexcessofligandmustbecontrolledinorderto dis-favortheformationofthecomplexL2VO(ligand/vanadium=2:1)

thatisinactiveascatalystintheepoxidation.

Inadditiontoallylicalcoholepoxidation,studieshavebeen per-formedinordertoextendtheapplicabilityofvanadiumcatalysts totheepoxidationofhomoallylicalcoholsandlessreactiveolefins.

3.1. Epoxidationofallylicandhomoallylicalcohols

Afterthediscoveryofhydroxamicacidsaseffectiveligandsfor allylicalcoholoxidation,Yamamotodevelopedaminoacidbased chiralhydroxamicacidvanadiumcomplexesforthe stereoselec-tiveepoxidationofhomoallylicalcohols[36].Inthesesystemsthe natureoftheaminoacidresiduehasanimportanteffecton

enan-Fig.9.tert-Leucinebasedhydroxamicacid32developedbyYamamoto[36].

tioselectivity,whereas theimido andthehydroxylaminemotifs havelittleornoinfluence.Hydroxamicacid32derivedfrom tert-leucine(Fig.9)affordshighlystereoselectiveepoxidationofvarious homoallylicalcoholsusing6%oftheligand,2%ofVO(O-i-Pr)3and

cumene hydroperoxideasoxidant (Fig. 10).The systemis par-ticularlyeffective inthecaseof 3-monosubstituted homoallylic alcoholsandenantioselectivitiesupto91%wereobtainedat0◦Cin reasonablereactiontimes(10h).Theauthorssuccessfullyapplied thisprotocolinthekeyepoxidationstepofthesynthesisof(−)-␣ and(−)-8-epi-␣-bisabolol.

Thequestfornewligandsabletolowercatalystsloading,to workwithaqueousoxidantsandavoidliganddecelerationledto thesynthesis of a seriesof newchiral bis-hydroxamicacids34

and35[37–39]readilyavailablefromtheenantiomericallypure diaminetartratesalt(Fig.11).

Ligands34a,34band35awereinitiallytestedintheepoxidation ofallylicalcohols[37,39].Thebis-hydroxamicacidcatalytic

sys-Fig.10.Homoallylicepoxides33a–ibyoxidationwithVO/hydroxamicacid32catalyst.Conditions:32(6%),VO(O-i-Pr)3(2%),cumenehydroperoxide(1equiv.)intoluene

at0◦_C[36].

Fig.11. Yamamoto’sbis-hydroxamicacids34and35[37–39].

temsweresuperiortotheonesbasedonmonohydroxamicacids.In fact(1)higherenantioselectivitiesthanTi(IV)-basedsystemswere obtained,(2)aqueousTBHPcouldbeusedasoxidant,(3)the reac-tionscouldberunwithaslittleas0.2%ofcatalystwithoutdecrease

inenantioselectivityorreactivity,and(4)noliganddeceleration effectwasobservedwhenaligand/vanadiumratio3:1wasused. Undertheoptimizedconditions(2%ligand,1%vanadium, DCM, −20◦_{C),trans-monosubstitutedanddisubstitutedallylicalcohols}

Fig.13. Epoxidationofhomoallylicalcoholsusingthebis-hydroxamicacidligand30c.Reactionconditions:1mol%catalyst,CHP,toluene,rt[38].

Scheme4.Desymmetrizationofallylicalcoholswithligand34a[40].

Fig.14.Sulfonamide-based hydroxamicacidligands37and38developedby Malkov[41–43].

wereepoxidizedwithexcellentenantioselectivitiesusingligand

34a,whereasligand35awasusedfortheenantioselective epoxida-tionofcis-substitutedallylicalcohols[37](Fig.12).Lowmolecular weightallylicalcoholswerealsoepoxidizedwithhigh enantiose-lectivitiesusingthecatalystsderivedfrom34a,34bor35a[37,39]

andtheproductscouldbeobtainedafterextractionfromthe aque-ousphase.

Inthecaseofhomoallylicalcoholsligand35cwasthemost effec-tiveintermsofyieldsandenantioselectivitiesintheoxidationof substrateswithhighlyhinderedphenylgroups[38,39].Excellent stereoselectivitiesweresystematicallyobtainedintheepoxidation ofbothcis-andtrans-substitutedhomoallylicalcoholswithCHP; forexampleinthesynthesisofepoxide33dtheuseofligand35c

increasestheeesfrom46%(obtainedwithligand32,seeFig.9[36]) to96%(Fig.13).Inaddition,thesystemworksatroom tempera-tureandallowstheuseoflowercatalystloading(2mol%ligand, ligand/metalratio2:1).

bis-Hydroxamicacid ligands were then used for thekinetic resolution of racemic allylic and homoallylic alcohols [37,38]. Workingundertheconditionsoptimized forthestereoselective epoxidationofachiralsubstratesit waspossibletoobtainboth thestarting alcoholand theepoxide withgood stereoselectivi-ties.Thedesymmetrizationofmesosecondaryallylicalcoholsand homoallylicalcoholswasalsosuccessfullyachieved[40].Using1% catalyst,theoxidizingsystemVO(O-i-Pr)3/34a/CHPtransformed

trans-substitutedand1,1-disubstitutedallylicalcoholswithhigh yieldsandenantioselectivities(Scheme4),while35awasmore effi-cientforcis-substitutedsubstrates.Inthecaseofmesohomoallylic secondaryalcoholsitwasobservedthatthesystemderivedfrom ligand35c(1mol%VO(O-i-Pr)3,2mol%ligand)waseffectiveforcis

alkenesbutnotforthetransones.

Contemporarily tothe developmentof bis-hydroxamic acid-basedligands,Malkovdescribedanewseriesofsulfonamide-based hydroxamicacids(Fig.14)[41].

Reactions carried out in the presence of ligand 37 afforded frommoderatetogoodenantioselectivitiesintheepoxidationof transmonosubstitutedallylicalcoholsintolueneat−20◦_Cusing

TBHPasoxidant,butmuchlowerreactivityand

enantioselectiv-Scheme5. Synthesisofepoxy-alcohol36bviaepoxidationwithV(V)/sulfonamide basedhydroxamicacids37and38[41–43].

ityintheepoxidationofnerol,acisallylicalcohol,wereobtained

[41].Althoughintermsofenantioselectivitiesislessefficientthan thebis-hydroxamicligand-basedsystemsreportedbyYamamoto

[35,37],thissystemexhibitsahighreactivityevencomparableto thatofVO(acac)2alone.Theobservedacceleratingeffectofthe

lig-andwasattributedtothepresenceofhydrogenbondsbetweenthe sulfonamidegroupandtheincomingallylicalcoholthattetherthe tworeactantstogether.

Animportantimprovementofthismethodologycamefromthe useofaqueousreactionmedium[42].Inthiscase,VOSO4·H2Owas

usedasmetalsourceand70%aqueousTBHPasoxidant.Though ageneraldecreasedreactivitywasobserved,the enantioselectivi-tieswerenotaffectedandthehydrolysisoftheresultingepoxides countedlessthan5%atlowtemperatures(Scheme5).The reac-tioncarriedoutwithonlyVOSO4·H2Oisveryslowandtheaddition

of0.5equiv.ofligandledtocompletereagentconversion.A fur-therimprovementinenantioselectivityandreactivitywasachieved usingligand38,bearingaN-chiralsubstituent[43].Optimization ofthereactionconditions(5%catalystloading,carefullyadditionof toluene)allowedobtainingtheepoxide36binshortreactiontimes with98%yieldand90%ee(Scheme5)[43].

TerpenoidtypeligandshavebeenalsostudiedbyBryliakovand coworkersintheoxidationof(−)-(R)-linalool[44],with stereose-lectivitiesnotexceeding50%ee.

An interesting strategy in stereoselective epoxidation is the useofchiralalkylhydroperoxides.Zhangandcoworkersachieved the stereoselective epoxidation of allylic alcohols catalyzed by sandwichtypepolyoxometalates(POMs)usingachiral hydroper-oxide[45].First,ascreeningofvarioustransitionmetalsubstituted sandwich type POMs (V(V), Mn(II), Fe(III), Zn(II), Pd(II), Pt(II)) demonstratedthesuperiorityofoxovanadium-substituted polyox-ometalate[ZnW(VO)2(ZnW9O34)2]12−intermsofreactivityand

selectivityintheoxidationofamodelsubstrate.Asurveyof chi-ralhydroperoxidesindicatedaTADDOLderivedhydroperoxide39

(TADOOH)asthebestchoice,enantioselectivitiesupto91%could beobtainedworkingatrt(Scheme6).Thankstothehighrobustness ofPOMcatalyst,loadingsdownto0.002%couldbeemployedand TONsupto42,000wereachievedwithoutanydecreasein enan-tioselectivity.Theprotocolhasanadditionalvalue:TADDOLcan beeasilyrecoveredwithoutlossofenantiopurityandrecycledinto thechiralhydroperoxide[46].

TADOOH has been also effective in stereoselective epoxida-tions(upto72%ee)whenVO(O-i-Pr)3wasusedascatalystinthe

presenceofhydroxamicacid40(Scheme8)[47].Morerecently, Lattanziandcoworkersdescribedtheuseofthechiral hydroper-oxidederived from (S)-norcamphor 41 (Scheme6)inreactions catalyzedbybothVO(acac)2andVO(O-i-Pr)3using

N-hydroxy-N-Scheme6. Vanadiumcatalyzedepoxidationofallylicalcoholsusingchiralhydroperoxides39and41[45,47,48].

phenylbenzamide42asligand[48].Stereoselectivitiesupto61% wereachievedwithepoxide36busing10%ofcatalystand15%of ligand(Scheme6).Goodstereoselectivitieswereobtainedinfor someselectedsubstrates,howeverbothsystemsarelessactiveand stereoselectivethanthePOMsystemdescribedbyZhang.

Stereoselectivevanadiumcatalyzedepoxidationofenantiopure allylicalcoholsbearingan_␣sulfinylmoiety(VO(acac)2/TBHP)has

beenreportedbyFernándezdelaPradillaandcoworkers[49].

3.2. Epoxidationofsimpleolefins

Polyoxometalates(POMs)havebeenextensivelystudiedas oxi-dationcatalyst duetotheirrobustness,easeofpreparation and compatibilitywithdifferentoxygensources.Mizunoand cowork-ersreportedthatvanadium-based␥-Keggintypepolyoxometalate [␥-1,2-H2SiV2W10O40]4−whichhasa{VO-(␮-OH)2-VO}coreisan

effectivecatalystfortheepoxidationofalkenes[50].Aseriesof lin-earandcyclicalkeneswereefficientlyoxidizedusingthetetrabutyl ammoniumPOMsaltascatalyst(5%)andH2O2asoxidant(1equiv.)

(Fig.15).Variousnonconjugateddieneswerealsooxidizedwith highregioselectivityforthelesssubstituteddoublebond.

The epoxidationofcyclohexenewithsandwichtype polyox-ometalatesusingchiralTADOOHasoxidantwasreportedbyZhang

[45b].Muchlowerreactivitiesandbasicallynostereoselectivities comparedwiththeallylicalcoholswereobtained(27%yieldwith cyclohexene).

The oxidationof alkeneswithoxygenusingvanadium/Schiff base type ligands gave reasonable results only in the case of cyclooctene, while for other olefins very low conversions and selectivitieswereobserved[51].Thecatalyticactivityof 4-acyl-5-pyrazoloneligandsintheoxidationofstyrene,␣-methylstyrene and cis-␤-methylstyrene withTBHP wasalso investigated [52], but as in the case of Schiff base ligands, very poor conver-sionandselectivitywereobtained.Vanadiumcomplexesderived from 2-(2-butoxyethoxy)ethanolate epoxidizecyclooctene with 68%conversions [53].Aminetriphenolate vanadium complexes haveascarcecatalyticactivityinstyreneandtrans-stilbene epoxi-dation[54].

4. Haloperoxidations

Selectivebrominationoforganiccompoundsisaveryimportant tool inorganicsynthesis.Organic bromo-derivatives are impor-tantprecursorsforvariousselectiveandefficienttransformations inorganicsynthesis,asindustrialintermediatesand pharmaceuti-cals.Hazardsandenvironmentalproblemscorrelatedwithclassical brominationmethodshaveencouragedtheattentiontowardsthe developmentofsaferandenvironmentallyfriendliermethods[55]. In this contestvanadium haloperoxidases (VHPOs),which have beenisolatedmainlyfrommarinealgaeandfungi,havereceived muchattentionbecausetheyareabletocatalyzetheoxidationof halidestothecorrespondinghypohalousacidusinghydrogen per-oxideasprimaryoxidant[56].Studiesfordeterminingthestructure andactivityofactivesitesinVHPOsandthesynthesisofstructural VHPOsmodelshavestimulatedtheresearchonbiomimetic vana-diumsystemsascatalystsforhaloperoxidation.Attentionhasbeen mainlyfocusedoncomplexeswhichmimictheactivesitesofthe enzyme[57,58]and,morerecently,interesthasmovedtowards theuseofnonconventionalsolvents,newoxidantsorthe heteroge-nizationofthecatalyticsystems.Thispartofthereviewisdedicated totherecentadvancementsinthisfield.

Plass[59]reportedthata N-salicylidenealkoxovanadium(V) oxoperoxocomplex whichhas, inthesolidstate,a pentagonal-bipyramidalcoordinationgeometry,withaside-onbondedperoxo ligandintheequatorialplane,isabletobrominate,under stoichio-metricconditions,1,3,5-trimethoxybenzene(TMB)inthepresence oftetrabutylammoniumbromide(90%yield).MorerecentlyXing etal.reportedthatoxovanadium(IV)-carboxylatecomplexesare effectivesystemsinthebrominationreactionofphenolredto bro-mophenolblueinphosphatebuffer[60].

LiciniandcoworkersreportedtheC3vanadium(V)amino

triph-enolatecomplexes 10a–cas structuraland functional model of vanadiumhaloperoxidases[10].Thesecomplexoxidizebromide andchlorideionsbyhydrogenperoxide,leadingtothe correspond-ingmono-halogenTMB.Thecatalystloadinghasbeenloweredto 0.05%withrespecttothelimitationagentleadingtotherecovery oftheBrTMBproductin63%yieldwithTONsupto1260.

Reac-Fig.15.Epoxidationofalkenesusing␥-Keggintypevanadiumpolyoxometalates[50].

Scheme7.Halocyclizationofbis-homoallylicalcoholsinthepresenceofbromide, ROOH,andvanadiumcatalysts[63].

tionswerealsocarriedoutinthepresenceofchlorideions.Slow chlorinationofTMBcouldbeachievedobtainingthecorresponding chlorinatedproductin40%yieldsafter2days.

Conteetal.exploredthepossibilitytoperform haloperoxida-tion of styrenein ionic liquids using NH4VO3 as metal source,

andhydrogenperoxideasoxidant[61].Goodtoexcellentyields ofbrominatedproductswereobtainedwithstyreneusing imida-zoliumsaltsassolvent,muchhigherthanin thecorresponding two-phaseH2O/CHCl3system[5].Ionicliquidshavealsobeen

suc-cessfullyusedintheoxidationofsulfideswiththesamesystems

[62].

Thepossibilitytooxidizebromideionhasbeenusednotonly forbromoperoxidationsbutalsoforthecombinedhalocyclization

[63].

Hartungused(Schiff-base)vanadium(V)complexestocatalyze the oxidation of bromide for the stereoselective synthesis of functionalized cyclic ethers(Scheme 7).This methodology was effectiveforthesynthesis ofthe2,2,3,5,6,6-substituted tetrahy-dropyrannucleusofthemarinenaturalproductaplysiapyranoidA

46(Scheme8).bis-Homoallylicalcohol44wastreatedwithTBHP, pyridiniumhydrobromideand5mol%ofVOL(OEt)(EtOH)[L= N-(2-hydroxyphenyl)salicylidene imine dianion]to furnish43% of 6-endo-bromocyclizedproduct45[63].

ThemechanismofthisbrominationreactionconsistsofTBHP activationbymeanofthevanadiumcomplex,viainsituformation ofthecorrespondingtert-butylperoxocomplex.Themetalloperoxo complexoxidizesbromidetoBr2astheactivebrominatingreagent,

whichisreleasedintothesolutionandservesasreagentforthe halocyclization.

Heterogenizationof vanadiumcomplexes hasbeenobjectof studies as well. New peroxovanadate complexes anchored to

soluble polymers of the type, Na3[V2O2(O2)4(carboxylate)]–Pol

[Pol=poly(acrylate)orpoly(methacrylate)]havebeensynthesized from the reaction of V2O5 withH2O2 and the sodium salts of

therespectivemacromolecularligands [64].Carboxylate groups of polymerchainsareabletocoordinatetoV(V)centers.These supportedperoxospecieshavebeentestedinbrominationof aro-matics compoundsinacetonitrile–water mediausingtetraethyl ammoniumbromideasbromidesource.Activatedaromatics,such asaniline,werebrominatedtoproducepredominantlyp-bromo products(yields75–95%).

Heterogenization of vanadium complexes has been also reportedbythegroupofMauryaandcoworkers.Inparticular vana-diumsaltshavebeenincorporatedinpolymericSchiffbases[65], insertedinNa-Yzeolites[66],anchoredtopolystyreneresins[67]

andreactedwithpolymerboundimidazole[68].Thelattersystem furnishedthebestresultsintermofproductyieldandcapability toberecycledinthehaloperoxidationofsalicylicaldehyde.Under theoptimizedconditions,usingaqueous30%H2O2,thesupported

catalyst,KBrandperchloricacid,81.4%conversionofsalicyl alde-hydewasachieved.Thesupportedcatalystcanbereusedwithout lostofactivity(77.1%ofconversionafterfourcycles).

Vanadiumcomplexesbearingorganicligandsarenottheonly catalystsfor haloperoxidation. Vanadiumpentoxide(V2O5)also

very efficiently promotesthebrominationoforganicsubstrates inthepresenceofhydrogenperoxide[69].Howeverthemethod developed by Khanrequires high catalyst loadings and a large excess of oxidant: typical conditions require 50% of V2O5 and

16equiv.of H2O2.This methodologyis usefulfor a wide range

of substrates such asaromatic rings, olefins, chalcones, ␤-keto estersand1,3-diketonesInterestingly,thevanadiumcontentcan bereducedifamineralacid,suchassulfuricacid,isused[70].

Whileallofthesystemsdescribeduseofperoxidesas oxida-tionsource,morerecentcatalyticsystemscapableofusingoxygen asoxidanthavebeenreported.Hiraoandcoworkersdevelopeda catalyticsystemwhichperformsoxidativebrominationofarenes usingammoniummetavanadate(NH4VO3)inthepresenceofa

bro-midesalt(Bu4NBr)andaBrønstedorLewisacidinthepresenceof

molecularoxygen[71].Thiscatalyticsystemhasbeenappliedto thebrominationofalkenes andalkynestogivethe correspond-ingvic-bromides,andtoketonestogive␣-bromination.Allthese reactionsfurnishtheproductswithexcellentyields.

Scheme8.Vanadium(V)-catalyzedoxidationofbromideinthe6-endo-selectivebromocyclizationofstyrene-derivedalcoholforthesynthesisofaplysiapyranoidA46[63].

5. Conclusions

In this contributionwehave shownthat, inthelast decade, importantresultshavebeenobtainedinoxygentransferreactions catalyzedbyvanadiumcomplexes.Inthefieldofstereoselective oxidationof sulfides,andepoxidationofallylicand homoallylic alcohols important resultshave been achieved. Even though it isstillnotpossiblealwaystousehydrogenperoxideinaqueous homogenousconditions,phasetransfersystemsoralkylperoxides havebeenusedsuccessfullyintermsofyieldandstereoselectivity. ThecapabilityofPOMvanadiumcomplexestotransferoxygento unfunctionalizedolefinshasbeenamajorachievement. Haloper-oxidationshaveseenalsoanincreasinginterestinthescientific community.Importantresultshavebeenachievedinthe bromina-tionoforganicsubstratesandinthefieldofhalocyclization.

Importantchallengesremain.Forexample,thedevelopmentof avanadiumcatalystabletotransferoxygenstereoselectivelyto unfunctionalizedolefinsorsimplesulfideslikedialkylsulfides,and thedevelopmentofcatalystabletoworkefficientlyinthe haloper-oxidations.For the existing catalytic systems,the possibilityto immobilizethecatalystinsolidsupportsortoworkin unconven-tionalsolventsinordertoprovidea“greener”processremainsan importantchallenge.

References

[1](a)G.Strukul(Ed.),CatalyticOxidationswithHydrogenPeroxideasOxidant, KluwerAcademicPublishers,Dordrecht,1992;

(b)W.Adam(Ed.),PeroxideChemistry.MechanisticandPreparativeAspects ofOxygenTransfer,Wiley-VCHVerlagGmbH,Weinheim,2000;

(c)V.Conte,O.Bortolini,in:Z.Rappoport(Ed.),TheChemistryofPeroxides– TransitionMetalPeroxidesSynthesisandRoleinOxidationReactions,Wiley Interscience,2006.

[2](a)M.Bonchio,G.Licini,F.DiFuria,S.Mantovani,G.Modena,W.A.Nugent,J. Org.Chem.64(1999)1326;

(b)G.Licini,M.Bonchio,G.Modena,W.A.Nugent,PureAppl.Chem.71(1999) 463.

[3](a)C.Zonta,E.Cazzola,M.Mba,G.Licini,Adv.Synth.Catal.350(2008)2503; (b)M.Forcato,M.Mba,W.A.Nugent,G.Licini,Eur.J.Org.Chem.(2010)740; (c)M.Mba,L.J.Prins,C.Zonta,M.Cametti,A.Valkonen,K.Rissanen,G.Licini, DaltonTrans.39(2010)7384;

(d)G.Santoni,M.Mba,M.Bonchio,W.A.Nugent,C.Zonta,G.Licini,Chem.Eur. J.16(2010)645.

[4]F.Romano,A.Linden,M.Mba,C.Zonta,G.Licini,Adv.Synth.Catal.352(2010) 2937.

[5](a)V.Conte,B.Floris,Inorg.Chim.Acta363(2010)1935; (b)V.Conte,B.Floris,DaltonTrans.40(2011)1419. [6](a)H.Vilter,Phytochemistry23(1984)1387;

(b)M.A.Andersson,A.Willetts,S.G.Allenmark,J.Org.Chem.(1997)8455; (c)A.G.J.Ligtenbarg,R.Hage,B.L.Feringa,Coord.Chem.Rev.237(2003)98. [7](a)C.Bolm,Coord.Chem.Rev.237(2003)245;

(b)E.Wojaczyska,J.Wojaczyski,Chem.Rev.110(2010)4303. [8](a)G.Santoni,G.Licini,D.Rehder,Chem.Eur.J.9(2003)4700;

(b)C.Wikete,W.Pingsong,G.Zampella,L.DeGioia,G.Licini,D.Rehder,Inorg. Chem.46(2007)196;

(c)W.Pingsong,C.C¸elik,G.Santoni,J.Dallery,D.Rehder,Eur.J.Inorg.Chem. (2008)5203.

[9]A.Martinez,J.-P.Dutasta,J.Catal.267(2009)188.

[10]M.Mba,M.Pontini,S.Lovat,C.Zonta,G.Bernardinelli,E.P.Kündig,G.Licini, Inorg.Chem.47(2008)8616.

[11]L.J.Prins,M.Mba,A.Kolarovi ´c,G.Licini,TetrahedronLett.47(2006)2735. [12]S.Barroso,P.Adão,MadeiraFP.,M.T.Duarte,J.CostaPessoa,A.M.Martins,Inorg.

Chem.49(2010)7452.

[13] E.Y.Tshuva,M.Versano,I.Goldberg,M.Kol,H.Weitman,Z.Goldschimidt,Inorg. Chem.Commun.2(1999)371.

[14]S.Lovat,M.Mba,H.C.L.Abbenhuis,D.Vogt,C.Zonta,G.Licini,Inorg.Chem.48 (2009)4724.

[15]F.Carniato,E.Boccaleri,L.Marchese,A.Fina,D.Tabuani,G.Camino,Eur.J.Inorg. Chem.4(2007)585.

[16]C.Bolm,F.Bienewald,Angew.Chem.Int.Ed.Engl.34(1995)2640.

[17] Q.Zeng,H.Wang,T.Wang,Y.Cai,W.Weng,Y.Zhao,Adv.Synth.Catal.347 (2005)1933.

[18](a)H.B.Kagan,J.C.Fiaud,in:E.L.Eliel,S.H.Wilen(Eds.),Topicsin Stereochem-istry,vol.18,JohnWiley&Sons,1988,p.249;

(b)C.J.Sih,S.H.Wu,in:E.L.Eliel,S.H.Wilen(Eds.),TopicsinStereochemistry, vol.18,JohnWiley&Sons,1988,p.63,vol.19.

[19](a)C.Drago,L.Caggiano,R.F.W.Jackson,Angew.Chem.Int.Ed.44(2005)7221; (b)M.Hinch,O.Jacques,C.Drago,L.Caggiano,R.F.W.Jackson,C.Dexter,M.S. Anson,S.J.F.Macdonald,J.Mol.Chem.A:Chem.251(2006)123.

[20] P.Kelly,S.E.Lawrence,A.R.Maguire,Eur.J.Org.Chem.(2006)4500. [21](a)F.DiFuria,G.Licini,G.Modena,R.Motterle,W.A.Nugent,J.Org.Chem.61

(1996)5175;

(b)M.I.Donnoli,S.Superchi,C.Rosini,J.Org.Chem.63(1998)9392; (c)P.Axe,S.D.Bull,M.G.Davidson,M.D.Jones,D.E.J.E.Robinson,W.L.Mitchell, J.E.Warren,DaltonTrans.46(2009)10169.

[22]Y.Wu,J.Liu,X.Li,A.S.C.Chan,Eur.J.Org.Chem.(2009)2607. [23]S.-H.Hsieh,Y.-P.Kuo,H.-M.Gau,DaltonTrans.(2007)97.

[24] R.Ando,S.Mori,M.Hayashi,T.Yagyu,M.Maeda,Inorg.Chim.Acta(2004)1177. [25]R.Ando,H.Inden,M.Sugino,H.Ono,D.Sakaeda,T.Yagyu,M.Maeda,Inorg.

Chim.Acta(2004)1337.

[26]M.E.Cucciolito,R.DelLitto,G.Roviello,F.Ruffo,J.Mol.Catal.A:Chem.236 (2005)176.

[27] I.Lippold,J.Becher,D.Klemm,W.Plass,J.Mol.Catal.A:Chem.299(2009)12. [28](a)Y.-C.Jeong,S.Choi,Y.D.Huang,K.-H.Ahn,TetrahedronLett.45(2004)9249; (b)Y.-C.Jeong,Y.D.Huang,S.Choi,K.-H.Ahn,Tetrahedron:Asymmetry16 (2005)3497.

[29] J.Sun,C.Zhu,Z.Dai,M.Yang,Y.Pan,H.Hu,J.Org.Chem.69(2004)8500. [30]K.P.Bryliakov,E.P.Talsi,Eur.J.Org.Chem.(2008)3369.

[31]P.Adão,J.CostaPessoa,R.T.Henriques,M.L.Kuznetsov,F.Avecilla,M.R.Maurya, U.Kumar,I.Correia,Inorg.Chem.48(2009)3542.

[32] (a)M.R.Maurya,A.Arya,A.Kumar,J.CostaPessoa,DaltonTrans.(2009)2185; (b)M.R.Maurya,A.Arya,A.Kumar,M.L.Kuznetsov,F.Avecilla,J.CostaPessoa, DaltonTrans.49(2010)6586.

[33] T.Katsuki,K.B.Sharpless,J.Am.Chem.Soc.102(1980)5974.

[34] (a)R.C.Michaelson,R.E.Palermo,K.B.Sharpless,J.Am.Chem.Soc.99(1977) 1990;

(b)K.B.Sharpless,T.R.Verhoeven,Aldrichim.Acta12(1979)63. [35]Y.Hoshino,H.Yamamoto,J.Am.Chem.Soc.122(2000)10452.

[36]N.Makita,Y.Hoshino,H.Yamamoto,Angew.Chem.Int.Ed.42(2003)941. [37]W.Zhang,A.Basak,Y.Kosugi,Y.Hoshino,H.Yamamoto,Angew.Chem.Int.Ed.

44(2005)4389.

[38]W.Zhang,H.Yamamoto,J.Am.Chem.Soc.128(2007)286. [39]A.U.Barlan,W.Zhang,H.Yamamoto,Tetrahedron63(2007)6075. [40]Z.Li,W.Zhang,H.Yamamoto,Angew.Chem.Int.Ed.47(2008)7520. [41]A.V.Malkov,Z.Bourhani,P.Koˇcovsk ´y,Org.Biomol.Chem.3(2005)3194. [42]Z.Bourhani,A.V.Malkov,Chem.Commun.(2005)4592.

[43]A.V.Malkov,L.Czemerys,D.A.Malyshev,J.Org.Chem.74(2009)3350. [44]K.P.Bryliakov,E.P.Talsi,S.N.Stas’ko,O.A.Kholdeeva,S.A.Popov,A.V.Tkachev,

J.Mol.Catal.A:Chem.194(2003)79.

[45](a)W.Adam,P.L.Alster,R.Neumann,C.R.Saha-Möller,D.Seebach,R.Zhang, Org.Lett.5(2003)725;

(b)W.Adam,P.L.Alster,R.Neumann,C.R.Saha-Möller,D.Seebach,A.K.Beck, R.Zhang,J.Org.Chem.68(2003)8222.

[46]M.Aoki,D.Seebach,Helv.Chim.Acta84(2001)187.

[47] W.Adam,A.K.Beck,A.Pichota,C.R.Saha-Möller,D.Seebach,N.Vogl,R.Zhang, Tetrahedron:Asymmetry14(2003)1355.

[48]A.Lattanzi,S.Piccirillo,A.Scettri,Eur.J.Org.Chem.(2005)1669.

[49]R.FernándezdelaPradilla,A.Castellanos,J.Fernández,M.Lorenzo,P.Manzano, P.Méndez,J.Priego,A.Viso,J.Org.Chem.71(2006)1569.

[50](a)Y.Nakagawa,K.Kamata,M.Kotami,K.Yamaguchi,N.Mizuno,Angew.Chem. Int.Ed.44(2005)5136;

(b)N.Mizuno,Y.Nakagawa,K.Yamaguchi,J.Mol.Catal.A:Chem.251(2006) 286.

[51](a)D.M.Boghaei,A.Bezaatpour,M.Behzad,J.Mol.Catal.A:Chem.245(2006) 12;

(b)S.Mohebbi,A.H.Sarvestani,Transit.Met.Chem.31(2006)749;

(c)S.Rayati,N.Torabi,A.Ghaemi,S.Mohebbi,A.Wojtczak,A.Kozakiewicz, Inorg.Chim.Acta361(2008)1239;

(d)S.Rayati,M.Koliaei,F.Ashouri,S.Mohebbi,A.Wojtczak,A.Kozakiewicz, Appl.Catal.A:Gen.346(2008)65;

(e)H.H.Monfared,R.Bikas,P.Mayer,Inorg.Chim.Acta363(2010)2574. [52]F.Marchetti,C.Pettinari,C.DiNicola,R.Pettinari,A.Crispini,M.Crucianelli,A.

DiGiuseppe,Appl.Catal.A:Gen.378(2010)211.

[53]E.C.E.Rosenthal,H.Cui,M.Hummert,Inorg.Chem.Commun.11(2008)918. [54]S.Groysman,I.Goldberg,Z.Goldschmidt,M.Kol,Inorg.Chem.44(2005)5073. [55] A.Podgorˇsek,M.Zupan,J.Iskra,Angew.Chem.Int.Ed.48(2009)8424. [56](a)A.Butler,Coord.Chem.Rev.187(1999)17;

(b)V.M.Dembitsky,Tetrahedron59(2003)4701.

[57](a)R.deLaRosa,M.J.Clague,A.Butler,J.Am.Chem.Soc.114(1992)760; (b)M.J.Clague,N.L.Keder,A.Butler,Inorg.Chem.32(1993)4754.

[58](a)D.C.Crans,J.J.Smee,E.Gaidamauskas,L.Q.Yang,Chem.Rev.104(2004)849; (b)G.J.Colpas,B.J.Hamstra,J.W.Kampf,V.L.Pecoraro,J.Am.Chem.Soc.118 (1996)3469;

(c)A.Butler,M.J.Clague,G.E.Meister,Chem.Rev.94(1994)625. [59]S.Nica,A.Pohlmann,W.Plass,Eur.J.Inorg.Chem.(2005)2032.

[60] (a)Z.-P.Li,Y.-H.Xing,Y.-Z.Cao,X.-Q.Zeng,M.-F.Ge,S.-Y.Niu,Polyhedron28 (2009)865;

(b)Z.-P.Li,Y.-H.Xing,C.-G.Wang,J.Li,Y.-Z.Cao,X.-Q.Zeng,M.-F.Ge,S.-Y.Niu, Z.Anorg.Allg.Chem.63(2009)2601.

[61]V.Conte,B.Floris,P.Galloni,A.Silvagni,PureAppl.Chem.77(2005)1575.

[62]V.Conte,F.Fabbianesi,B.Floris,P.Galloni,D.Sordi,I.C.W.E.Arends,M.Bonchio, D.Rehder,D.Bogdal,PureAppl.Chem.81(2009)1265.

[63](a)M.Greb,J.Hartung,F.K ˝ohler,K. ˇSpehar,R.Kluge,R.Csuk,Eur.J.Org.Chem. (2004)3799;

(b)J.Hartung,PureAppl.Chem.77(2005)1559.

[64]D.Kalita,S.Sarmah,S.PrasadDas,D.Baishya,A.Patowary,S.Baruah,N.S.Islam, React.Funct.Polym.(2008)876.

[65] M.R.Maurya,A.Kumar,P.Manikandan,S.Chand,Appl.Catal.A:Gen.277(2004) 45.

[66] M.R.Maurya,H.Saklani,S.Agarwal,Catal.Commun.5(2000)563.

[67](a)M.R.Maurya,S.Sikarwar,T.Joseph,P.Manikandan,S.B.Halligudi,React. Funct.Polym.63(2005)71;

(b)M.R.Maurya,U.Kumar,P.Manikandan,DaltonTrans.(2006)3561. [68] M.R.Maurya,M.Kumar,A.Arya,Catal.Commun.10(2008)187.

[69](a)U.Bora,G.Bose,M.K.Chaudhuri,S.S.Dhar,R.Gopinath,A.T.Khan,B.K.Patel, Org.Lett.2(2000)247;

(b)A.T.Khan,P.Goswami,TetrahedronLett.46(2005)4937;

(c)A.T.Khan,P.Goswami,L.H.Choudhury,TetrahedronLett.47(2006)2751. [70]G.Rothenberg,J.H.Clark,Org.ProcessRes.Dev.4(2000)270.

[71] (a)K.Kikushoma,T.Moriuchi,T.Hirao,AsianJ.Chem.4(2009)1213; (b)K.Kikushima,T.Moriuchi,T.Hirao,TetrahedronLett.51(2010)340; (c)K.Kikushima,T.Moriuchi,T.Hirao,Tetrahedron66(2010)6906.