Oxidovanadium complexes with tridentate aroylhydrazone as catalyst precursors for solvent-free microwave-assisted oxidation of alcohols

ContentslistsavailableatScienceDirect

Applied

Catalysis

A:

General

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

Oxidovanadium

complexes

with

tridentate

aroylhydrazone

as

catalyst

precursors

for

solvent-free

microwave-assisted

oxidation

of

alcohols

Manas

Sutradhar

_,

_Luísa

_M.D.R.S.

_Martins

_,

_M.

_Fátima

_C.

_Guedes

_da

_Silva

_,

Armando

J.L.

Pombeiro

a_{CentrodeQuímicaEstrutural,InstitutoSuperiorTécnico,UniversidadedeLisboa,Av.RoviscoPais,1049-001Lisboa,Portugal} b_{ChemicalEngineeringDepartment,ISEL,R.ConselheiroEmídioNavarro,1959-007Lisboa,Portugal}

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r

t

i

c

l

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f

o

Articlehistory:

Received26August2014 Receivedinrevisedform 16December2014 Accepted5January2015 Availableonline13January2015 Keywords: Oxidovanadiumcomplexes Aroylhydrazone X-raystructure Microwave-assistedoxidation

a

b

s

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Aroylhydrazoneoxidovanadiumcompounds,viz.theoxidoethoxidovanadium(V)[VO(OEt)L](1)(H2L

=salicylaldehyde-2-hydroxybenzoylhydrazone), the salt like dioxidovanadium(V) (NH3CH2CH2OH)+

[VO2L]−(2),themixed-ligandoxidovanadium(V)[VO(hq)L](Hhq=8-hydroxyquinoline)(3)andthe

vana-dium(IV)[VO(phen)L](phen=1,10-phenanthroline)(4)complexes(3and4obtainedbythefirsttime), havebeentestedascatalystsforsolvent-freemicrowave-assistedoxidationofaromaticandalicyclic secondaryalcoholswithtert-butylhydroperoxide.Afacile,efficientandselectivesolvent-free synthe-sisofketoneswasachievedwithyieldsupto99%(TON=497,TOF=993h−1_{for3)and58%(TON=291,}

TOF=581h−1for2)foracetophenoneandcyclohexanone,respectively,after30minunderlowpower (25W)microwaveirradiation.

©2015ElsevierB.V.Allrightsreserved.

1. Introduction

Functionalizedoxygenatedproducts, namelyketones,areon the basis of important industrial synthetic strategies, as sol-vents,polymerprecursorsandsubstratesforthesyntheses,e.g.,of pharmaceuticals,agrochemicalsandfragrances[1–4].Withinthe vastketonesyntheticmethods,recenteffortsfocusonenergy-and atom-efficiency,eliminationoftheuseofhazardoussubstances, andreductionoftimeandofthegenerationoftoxic(e.g.organic sol-vents)andheavy-metalwastes[5–9].Hence,aerobic[10–13]and peroxidative[14–16]selectiveoxidationsofsecondaryalcoholsare regardedassimpleandveryusefulsyntheticmethodsforthe prepa-rationofketones,thisconversionbeingofapivotalsignificancein organicsynthesis.Moreover,itisknownthatmicrowave(MW) irra-diationcanprovideamuchmoreefficientsyntheticmethodthan conventionalheating,allowingtoachievesimilar(orhigher)yields inashortertimeand/ortoimprovetheselectivity[17–27].

Thereisalsoastronginterestinthedesignofoxidationcatalysts basedonearth-abundantmetals,suchasvanadium. Oxidovana-diumcomplexes are good candidatesas catalystsfor oxidation reactions,e.g.,ofalkanes[28–34].Theyalsocatalyze[35–39](or co-catalyze)theaerobicoxidationofsecondaryalcoholswithgood

∗ Correspondingauthor.Tel.:+351218419237;fax:+351218464455. E-mailaddress:pombeiro@tecnico.ulisboa.pt(A.J.L.Pombeiro).

yields and highselectivities, either in liquid or supported sys-tems.Theuseofpolyoxidovanadateshasalsobeenreportedforthe efficientoxidationofbenzylicalcoholstothecorresponding car-bonylcompoundswithp-toluenesulfonicacid[40].However,the vanadiumcatalyzedperoxidativeoxidationofsecondaryalcoholsis comparativelyscarce[35–39,41–43].Someefficientsystems con-cerntheuseoftert-butylhydroperoxide(TBHP)inthepresenceof silica[41]orgraphene[42]supportedoxidovanadiumSchiffbases, orofhydrogenperoxideinahomogeneousmixturecomposedof vanadate,acidandTEMPOfunctionalizedionicliquids[43].Toour knowledgethereisnoreportforsolvent-freeMW-assisted oxida-tionofsecondaryalcoholscatalyzedbyvanadiumcomplexes.

Ontheotherhand,aroylhydrazonesareknowntoformstable complexeswithvanadiumin+4or+5oxidation states[44–48]. Therefore,themainobjectiveofthepresentstudywastoestablish theviabilityofoxidovanadiumcomplexesbasedonan aroylhydra-zoneligandaspossiblecatalystprecursorsforalcoholsoxidation undertheaboveconditions(intheabsenceofaddedsolventand under MWirradiation). We thus reportherein the preparation and characterization of the novel complexes [VO(hq)L] (H2L=

salicylaldehyde-2-hydroxybenzoylhydrazone, Hhq= 8-hydroxy-quinoline) (3) and [VO(phen)L] (phen=1,10-phenanthroline) (4), and theiruse, aswell asof theknownrelated compounds [VO(OEt)L](1)and(NH3CH2CH2OH)+[VO2L]−(2),ascatalystsfor

theperoxidativeoxidation of1-phenylethanolandcyclohexanol undergreenandmildreactionconditions.Thosemodelreactions http://dx.doi.org/10.1016/j.apcata.2015.01.005

Table1

Spectroscopiccharacterizationdatafor3and4.

Catalystprecursor IR(KBr,cm−1) 1_HNMR 51_{VNMR ESI-MS(+)}

m/z max (CH3CN,nm(ε, LM−1_cm−1₎₎ (DMSO-d6,ı) 3 1599␯(C N),1257 ␯(C O)enolic,1103 ␯(N N),955␯(V O) 11.12(s,1H,OH),8.59 (s,1H, CH N), 8.26–6.78(m,14H, C6H4andC9H6NO) −478 466[3+H]+ (100%) 552(4480),331 (13,440),273(21,280), 240(34,720). 4 1603␯(C N),1252 ␯(C O)enolic,1056 ␯(N N),961␯(V O) – – 502[4+H]+ (100%) 701(41.2),412 (15,600),351(18,540), 269(45,320).

arejustifiedbytheirimportanceinorganicsynthesisandchemical

industry[3,49].

2. Experimental

2.1. Preparationofcatalystprecursors

The Schiff base pro-ligand salicylaldehyde-2-hydroxybenzoylhydrazone (H2L) was prepared by a reported

method[50,51]fromcondensationof2-hydroxybenzoylhydrazide withsalicylaldehyde.Thecatalystprecursors[VO(OEt)L](1)and (NH3CH2CH2OH)+[VO2L]− (2)weresynthesizedaccordingto

lit-eraturemethods[50–52].Thesynthesesoftheothertwocatalyst precursors,theoxidovanadium(V)complex[VO(hq)L](3)andthe oxidovanadium(IV)complex[VO(phen)L](4),aredescribedbelow. Oxidovanadium(V)complex[VO(hq)L](3)–Toa30mL acetoni-trilesuspensionofH2L(0.256g,1.00mmol),0.265g(1.00mmol)of

[VO(acac)2]wasaddedandthereactionmixturewasrefluxedfor

1hinanoilbath,inopenair.Tothismixture0.145g(1.00mmol)of 8-hydroxyquinoline(Hhq)wasaddedandtherefluxwascontinued foranotherhour.Theresultantdarkvioletsolutionwasfilteredand thefiltratewaskeptinair.Afterca.2d,X-rayqualitydarkviolet singlecrystalswereisolated,washed3timeswithcoldacetonitrile anddriedinopenair.Yield76%(0.353g,basedonvanadium).Anal. CalcdforC23H16N3O5V:C,59.37;H,3.47;N,9.03.Found:59.28;H,

3.39;N,8.91.SpectroscopicandESI-MSdataaregiveninTable1. Oxidovanadium(IV)complex[VO(phen)L](4)–Thiscomplexwas isolatedasorangeredcrystalsusingtheprocedureadoptedinthe preparationof3butwith1,10-phenanthroline(phen)insteadof 8-hydroxyquinoline(Hhq).Yield72%(0.360g,basedonvanadium). Anal.CalcdforC26H18N4O4V:C,62.28;H,3.62;N,11.17.Found:

61.96;H,3.56;N,11.09.SpectroscopicandESI-MSdataaregiven inTable1.

2.2. Characterizationofcatalystprecursors 2.2.1. Generalmaterialsandprocedures

Allsyntheticworkwasperformedinair.Thereagentsand sol-ventswereobtainedfromcommercialsourcesandusedasreceived, i.e., without further purification or drying. Complexes 1 and 2

weresynthesized accordingtothereportedprocedure[50–52]. [VO(acac)2]wasusedasthemetalsourceforthesynthesisof

com-plexes3and4.C,H,andNelementalanalyseswerecarriedout bytheMicroanalyticalService oftheInstituto SuperiorTécnico. Infraredspectra(4000–400cm−1)wererecordedonaBRUKER VER-TEX70orJascoFT/IR-430instrumentinKBrpellets,wavenumbers areincm−1.MassspectrawereruninaVarian500-MSLCIonTrap MassSpectrometerequippedwithanelectrospray(ESI)ionsource. Forelectrosprayionization,thedryinggasandflowratewere opti-mizedaccordingtotheparticularsamplewith35p.s.i.nebulizer pressure.Scanningwasperformedfromm/z100to1200in ace-tonitrilesolution.Thecompoundswereobservedinthepositive

mode(capillaryvoltage=80–105V).The1_{Hspectrawererecorded}

at roomtemperatureona BrukerAvanceII+300 (UltraShieldTM

Magnet)spectrometeroperatingat300.130MHzforproton.The chemicalshiftsarereportedinppmusingtetramethylsilaneasthe internalreference.51_{VNMRspectrawererecordedonaBruker400}

UltraShieldspectrometeratambienttemperature(297K)in DMSO-d6.Thevanadiumchemicalshiftsarequotedrelativetoexternal

[VOCl3].TheUV–Visabsorptionspectraofacetonitrilesolutionsof 3(1.12×10−5M)and4(1.03×10−5M)in1.00cmquartzcellswere recordedatroomtemperatureonaLambda35UV–Vis spectropho-tometer(Perkin–Elmer)byscanningthe200–1000nmregionata rateof240nmmin−1.

2.2.2. X-raymeasurements

X-raysinglecrystalsofcomplexes3and4wereimmersedin cryo-oil, mounted in Nylon loopsand measured at a tempera-ture of 150(3) or 296K (4). Crystals of 4 wereof bad quality andveryweaklydiffracting(seeSupplementaryInformationfile). IntensitydatawerecollectedusingaBrukerAXS-KAPPAAPEXII or Bruker APEX-II PHOTON 100 withgraphite monochromated Mo-K␣(0.71073)radiation.Data werecollectedusingomega scans of 0.5◦ per frameand full sphereof datawere obtained. CellparameterswereretrievedusingBrukerSMART[53]software and refinedusingBrukerSAINT[53] onalltheobserved reflec-tions. Absorption corrections were applied using SADABS [53]. Structuresweresolvedbydirectmethodsbyusingthe SHELXS-97 package [54] and refined withSHELXL-97 [54]. Calculations wereperformed usingtheWinGX System-Version 1.80.03[55]. Thehydrogenatomswereinsertedincalculatedpositions.Least squarerefinementswithanisotropicthermalmotionparameters forallthenon-hydrogenatoms andisotropicfor theremaining atoms wereemployed. Therewere disorderedsolvents present inthestructuresofbothcompounds.Sincenoobviousmajorsite occupationswerefoundforthosemolecules,itwasnotpossibleto modelthem.PLATON/SQUEEZE[56]wasusedtocorrectthedata andpotentialvolumeof119(3)and221(4)Å3_{werefoundwith,}

respectively,30and66electronsperunitcellsworthof scatter-ing.Thesewereremovedfromthemodel,butnotincludedinthe empiricalformula.CrystallographicdataaresummarizedinTable2 andselectedbonddistancesandanglesarepresentedinTable3. CCDC 1005764(3) and 1005765 (4)contain thesupplementary crystallographicdataforthis paper.Thesedatacanbeobtained freeofchargefromTheCambridgeCrystallographic DataCentre viawww.ccdc.cam.ac.uk/datarequest/cif.

2.3. Catalytictests

2.3.1. Solvent-freemicrowave-assistedoxidationofsecondary alcohols

ThecatalytictestsunderMWirradiationwereperformedina focusedmicrowaveAntonPaarMonowave300discover reactor (25W),usinga10mLcapacityreactiontubewitha13mminternal

Table2

Crystaldataandstructurerefinementdetailsforcomplexes3and4.

3 4

Empiricalformula C23H16N3O5V C26H18N4O4V

FormulaWeight 465.33 501.38

Crystalsystem Triclinic Triclinic

Spacegroup P-1 P-1 a/Å 8.1264(10) 12.3188(12) b/Å 11.8080(15) 12.5684(12) c/Å 12.5868(15) 16.4642(15) ␣/◦ _{112.746(5) 85.024(5)} ˇ/◦ _{101.275(8) 71.596(4)} ␥/◦ _96.266(5)◦ _88.915(4) V(Å3_{) 1069.3(2) 2409.5(4)} Z 2 4 Dcalc(gcm−3) 1.445 1.382 (MoK˛)(mm−1_{) 0.504 0.451} Rfls.collected/unique/observed 19,619/3892/2841 28,101/8122/2228 Rint 0.0673 0.2054 FinalR1a_,wR2b_{(I≥2) 0.0679,0.1834 0.1299,0.3363} Goodness-of-fitonF2 _{1.037 0.927} a_R=||F o|-|Fc||/|Fo|. b _wR(F2_)=[w(|F o|2-|Fc|2)2/w|Fo|4]1/2. Table3

Selectedbonddistances(Å)andangles(◦_)incomplex3.

N1 V1 2.090(3) N3 V1 2.385(3) O1 V1 1.852(3) O2 V1 1.948(3) O4 V1 1.840(3) O10 V1 1.580(3) O10 V1 O4 98.68(14) O10 V1 O1 100.17(15) O4 V1 O1 96.55(12) O10 V1 O2 95.99(14) O4 V1 O2 98.74(12) O1 V1 O2 155.76(13) O10 V1 N1 102.94(14) O4 V1 N1 158.05(13) O1 V1 N1 83.44(12) O2 V1 N1 75.38(12) O10 V1 N3 171.66(14) O4 V1 N3 75.44(11) O1 V1 N3 86.51(13) O2 V1 N3 79.30(12) N1 V1 N3 82.65(12)

diameter,fittedwitharotational systemandanIRtemperature

detector.Gaschromatographic(GC) measurementswerecarried

outusingaFISONSInstrumentsGC8000seriesgaschromatograph

withaDB-624(J&W)capillarycolumn(FIDdetector)andthe

Jasco-Borwinv.1.50software.Thetemperatureofinjectionwas240◦C.

Theinitialtemperaturewasmaintainedat120◦Cfor1min,then

raised10◦C/minto200◦Candheldatthistemperaturefor1min.

Heliumwasusedasthecarriergas.GC–MSanalyseswere

per-formedusingaPerkin–ElmerClarus600Cinstrument(Heasthe

carriergas).Theionizationvoltagewas70eV.Gas

chromatogra-phywasconductedinthetemperature-programmingmode,using

aSGEBPX5column(30m×0.25mm×0.25␮m).Reactionproducts

wereidentifiedbycomparisonoftheirretentiontimeswithknown

referencecompounds,andbycomparingtheirmassspectrato

frag-mentationpatternsobtainedfromtheNISTspectrallibrarystored

inthecomputersoftwareofthemassspectrometer.

2.3.2. Typicalproceduresforthecatalyticoxidationofalcohols

andproductanalysis

Oxidationreactionsofthealcoholswerecarriedoutinsealed

cylindrical Pyrex tubes under focused MW irradiation as

fol-lows: the alcohol substrate (5mmol), catalyst precursor 1–4

(1–100␮mol,0.02–2mol%vs.substrate)anda70%aqueous

solu-tionofBut_OOHorH

2O2 (10mmol)wereintroducedinthetube.

Thiswasthenplaced inthemicrowavereactorand thesystem

wasstirredandirradiated(25W)for0.25–1hat80◦C.Afterthe

reaction,the mixturewas allowedtocooldown toroom

tem-perature.300␮Lofbenzaldehyde(internalstandard)and5mLof

NCMe(toextractthesubstrateandtheorganicproductsfromthe

reactionmixture)wereadded.Theobtainedmixturewasstirred

during10minandthenasample(1_␮L)wastakenfromtheorganic

phase andanalyzed byGC usingtheinternal standard method.

Blanktestsindicatethatonlytraces(<0.7%)ofacetophenone(or

cyclohexanone)aregeneratedinaV-freesystem.

3. Resultsanddiscussion

3.1. Synthesis

Toexamine thepossible catalytic activityof oxidovanadium

complexes withan aroylhydrazoneligandtowards solvent-free

MW-assistedoxidationofsecondaryalcoholsundermildreaction

conditions,wehaveusedthevanadiumcomplexes(1–4).Twoof

thecomplexes(1and2)werealreadyknownandwere

synthe-sizedbyareportedmethod[50–62],whereastheothertwo(3and

4)werenewlysynthesizedandarenowreported(Scheme1). Complexes [VO(hq)L] (3) and [VO(phen)L] (4) are pre-pared by reaction of [VO(acac)2] (acac−=acetyleacetonate)

in refluxing CH3CN with the Schiff base

salicylaldehyde-2-hydroxybenzoylhydrazone (H2L), in the presence of

8-hydroxyquinoline(Hhq)or1,10-phenanthroline(phen), respec-tively. They were isolated as dark violet (3) or orange red (4) crystallinesolids,andcharacterizedbyIR,NMR(inthecaseof3)

andUV/Visspectroscopies,elementalanalysis,ESI-MSandsingle crystalX-raydiffraction(seebelow).[VO(hq)L](3)and[VO(phen)L] (4)arehexacoordinatemixed-ligandoxidovanadium(V)and oxi-dovanadium(IV) complexes, respectively, whereas the known [VO(OEt)L](1)and(NH3CH2CH2OH)+[VO2L]–(2)[59–61]are

pen-tacoordinateoxidoethoxidovanadium(V)compounds, wherethe sixthcoordinationpositionisavailableforfurthercoordination.

Itiswellknownthat[VO(acac)2]undergoesaerialoxidationin

solution(preferablyinalcoholmedium)inthepresenceof hydra-zoneSchiffbases[48,57,58].Sometimes,thepresenceofaneutral bidentateN,NdonorauxiliaryligandinCH3CNhelpstostabilize

the oxidovanadium(IV)centre, which can be isolated as a sta-blesolidproduct[59,60].Herein,weusedtwodifferentbidentate auxiliaryligandstostabilizeoxidovanadiumcentresin3and 4:

themono negative8-hydroxyquinolinate(hq−)and theneutral 1,10-phenanthroline(phen)whichstabilizethevanadium(V)and thevanadium(IV)centre,respectively,withthetridentate hydra-zoneSchiffbase[48,59,60].Thefourcomplexesofthisstudy(1–4) presentdifferentcoordinationenvironmentsandhavebeenused foracomparativestudyoftheircatalyticactivitiestowards solvent-freeMW-assistedoxidationofsecondaryalcohols.

3.2. Spectroscopiccharacterizationofthecomplexes

TheIRspectraofcomplexes3and4(Table1)containallthe char-acteristicbandsofthecoordinatedtridentateanionicligandL2−_in

theenolform,viz.,1599,1257and1103cm−1for3and1603,1252 and1056cm−1for4.Inaddition,the␯(V O)bandsareobservedat 955and961cm−1for3and4,respectively.

The1_{HNMR spectrumof3inDMSO-d}

6 (Table1)showsthe

characteristicresonanceoftheligand[50,51]:theprotonofthe azomethinenitrogenat8.59ppm,thearomaticprotonsas multi-pletsintherangeof8.26–6.78ppm,andthephenolicOHprotonat 11.2ppm.

Scheme1. Synthesisofvanadiumcomplexes.

Complex3 wasalsoidentifiedbythe51_{V NMRspectroscopy}

(Table1).Thespectrum showscharacteristicpeak at−478ppm correspondstooxidiovanadium(V)centre[48].

Theelectronicspectraldataoftheoxidovanadiumcomplexes

3and4aregiveninTable1.Theelectronicspectrumofthe oxi-dovanadium(V)complex3inacetonitrilesolutiondisplaysastrong absorptionat552nmassignedtothePhO−→V(d_␲)LMCT transi-tionandthreeotherintenseabsorptionbandinthe331–240nm regionallocatedtointraligand(␲→␲*)transitions[59].The oxi-dovanadium(IV)complex4exhibitsalowintensitybandat701nm duetoligandfielddxz,dyz→dxytransitionsoftheVO2+centre[60].

Thestrongbandobservedat412nmisduetotheOenolate→V(d␲)

LMCTtransition.Bandscentredat350and269nmareassignedto intraligandtransitions[60].

TheESI-MSspectraof3and4inacetonitrilesolution(Table1) displaytheparentpeaksatm/z=466[3+H]+_{(100%)andatm/z=502}

[4+H]+_{(100%)respectively,whichalsosupporttheformulations.}

3.3. X-raystructuralcharacterization

Themolecularstructuresofcomplexes3and4aredepictedin Figs.1and2,respectively,whereascrystaldataarepresentedin Table2andselectedbonddistancesandanglesfor3aregivenin Table3.Adiscussionofthelowqualitystructureof4isgiveninthe SupplementaryInformationfilewhereasTableS1showsdistances and anglesfor this compound. Thedianionic Schiffbase ligand (L2−)in 3coordinatestovanadium(V)in theenolform(Fig.1). ThevanadiumionexhibitsadistortedoctahedralO4N2

coordina-tion(quadraticelongationof1.048andanglevarianceof104.79◦

2_{)inwhichthebasalplaneismadeofthephenolic(O1),the}

eno-lic(O2)oxygenandtheimine(N1)nitrogenatomsfromtheL2−

ligandtogetherwithaphenolic(O4)oxygenoftheauxiliary quino-lateligand.Theapicalpositionsareoccupiedbytheoxido(O10)and thepyridinenitrogen(N3)atomofthequinolate.TheL2−_ligand

bindsthevanadiumiongeneratingsix-[V1 O1 C1 C6 C7 N1] andfive-membered[V1 O2 C8 N2 N1]chelaterings.

TheL2−_{ligandin3deviatesslightlyfromplanarity.Indeed,}

rel-ativetotheleast-squareplaneformedbythecoordinatingatoms N1,O1and O2(planeA),theleast-square planesofthe pheno-lateringsC1>C6andC9>C10makeanglesof10.09and15.30◦, respectively.Themetalcationissituated0.230 ˚Aawayfromplane Aandshiftedtowardstheoxidoatom;theleast-squareplaneof

theauxiliaryligand(hq−)makesanangleof87.76◦withplaneA. TheV1 N3distanceof2.385(3)Å,withthatnitrogenlyingtrans totheOoxidoatom,ismuchlongerthantheV NSchiffbondlength

[V1 N12.090(3)Å]what isduenot onlytothetransinfluence oftheOoxidoatombutalsoto5-memberedringconstraints.The

shortV Ooxidodistance[V1 O101.580(3)Å]isconsistentwitha

vanadium–oxygendoublebond(V O)asitiscommonlyfoundin five-andsix-coordinatevanadium(IV)and(V)complexes[48];the

twoV Ophenolate lengths,V1 O1fromL2−and theV1 O4from

hq− [1.852(3)and 1.840(3) Å, inthis order],slightly differ and areshorterthantheV Oenolate[V1 O21.948(3)Å].Aspreviously

found[48,59],thefourvanadium oxygenbondlengthsin3follow theorder:oxido<phenolate<enolate.

Fig.1.Ellipsoidplotforcomplex3(drawnatthe30%probabilitylevel)withatom labellingscheme.

Fig.2.Ellipsoidrepresentationofoneofthemoleculesofcomplex4(drawnatthe 30%probabilitylevel)withatomlabellingscheme.

3.4. Oxidationofsecondaryalcohols

Complexes 1–4 were tested as catalyst precursors for the homogeneous oxidation of aromatic and alicyclic secondary alcohols (1-phenylethanol and cyclohexanol were chosen as modelsubstrates)totherespectiveketonesusing aqueous tert-butylhydroperoxide (But_{OOH, TBHP) as oxidizing agent, under}

typicalconditionsof80◦C,lowpower(25W)microwave irradia-tion(MW),30minreactiontimeandinasolvent-andadditive-free medium(Scheme2).

OtherC6linearsecondaryalcohols(Table5)werealsotestedas

substrates,andtheketonesaretheonlydetectedoxidation prod-ucts.Theeffectsofvariousreactionparameters,suchas,thetype andamountofoxidant,time,temperature,amountofcatalyst pre-cursorandpresenceofadditives,wereinvestigatedandtheresults aresummarizedinTables4and5andFigs.3–6.

Undertypicalsolvent-andadditive-freeconditions(Table4)all catalystprecursors1–4exhibitahighactivity[TON(TOF)valuesup to1.8×103_(3.6×103_h−1_{)(3)],leadingtoyields(basedonthe}

alco-hol)ofacetophenoneandcyclohexanoneintherangesof77–94% and30–38%,respectively.Ahighselectivitytowardstheformation ofketoneswasdisplayedbytheseMW-assistedtransformations, sincenotracesofby-productsweredetectedbyGC–MSanalysisof thefinalreactionmixtures(onlytheunreactedalcoholwasfound, apartfromtheketone).

Table4

MW-assistedsolvent-freeoxidationofselectedaromaticandalicyclicsecondaryalcoholsusing1–4ascatalystprecursors.a

Entry Catalyst precursor

Substrate Yieldb_{(%) TOF(h}−1₎c _{Selectivity(%)}d

1 1 1-Phenylethanol 94.3 943 98 2 Cyclohexanol 36.1 361 90 3 2 1-Phenylethanol 77.1 771 94 4 Cyclohexanol 37.6 376 92 5 1-Phenylethanol 92.6 926 97 6e _{36.3 3.64×10}3 ₉₃ 7f _{3 90.9 91 86} 8 Cyclohexanol 30.3 303 91 9e _{12.3 1.2×10}3 ₈₃ 10f _{30.1 30 72} 11 4 1-Phenylethanol 86.4 863 92 12 Cyclohexanol 35.4 354 87

a_{Reactionconditionsunlessstatedotherwise:5mmolofsubstrate,10␮mol(0.2mol%vs.substrate)of1–4,10mmolofTBHP(2eq.,70%inH}

2O),80◦C,30minofMW

irradiation(25Wpower).

b _{Molesofketoneproductper100molesofalcohol.}

c _{TOF=numberofmolesofproductpermolofcatalystprecursor(TON)perhour.} d _{Molesofketonepermoleofconvertedsubstrate.}

e_{1␮mol(0.02mol%vs.substrate)of3.} f _{100␮mol(2mol%vs.substrate)of3.}

Table5

MW-assistedsolvent-freeoxidationofselectedlinearsecondaryalcoholsusing1–4ascatalystprecursorsa_.

Entry Substrate Catalystprecursor Yieldb_{(%) TOF(h}−1₎c _{Selectivity(%)}d

1 1 29.4 59 79 7 2-Hexanol 2 40.2 80 87 8 3 35.6 71 82 9 4 31.9 64 67 11 1 32.3 65 84 13 3-Hexanol 2 32.8 66 79 19 3 28.9 58 73 20 4 28.1 56 52

a_{Reactionconditionsunlessstatedotherwise:5mmolofsubstrate,10␮mol(0.2mol%vs.substrate)of1–4,10mmolofTBHP(2eq.,70%inH}

2O),80◦C,2h30minofMW

irradiation(25Wpower).

b _{Molesofketoneproductper100molesofalcohol.}

c _{TOF=numberofmolesofproductpermolofcatalystprecursor(TON)perhour.} d _{Molesofketonepermoleofconvertedsubstrate.}

Scheme2.Solvent-freehomogeneousoxidationof1-phenylethanoland cyclo-hexanoltoacetophenoneandcyclohexanone,respectively,bythe1–4/TBHP/MW system.

Fig.3. Influenceofdifferentadditives(TEMPO,K2CO3,HNO3,radicaltraps)onthe

yieldofacetophenone( )orofcyclohexanone( )obtainedfromMW-assisted peroxidativeoxidationof1-phenylethanolinthepresenceof1orofcyclohexanol inthepresenceof2.

Asexpected, thealiphaticcyclohexanol is less reactivethan 1-phenylethanolleading,underthesamereactionconditions,to moderateyields(Table4),asreportedinothercases[22,23,61].In contrastwiththecomparableyieldsobtainedforcyclohexanone byMW-assistedperoxidativeoxidationofcyclohexanolwiththe variouscatalystprecursors1–4(entries2,4,8and11,respectively, Table4),theoxidationof1-phenylethanol,underthesame condi-tions,appearstobemoresensitivetothemetalcatalystprecursor. Thebestyieldswereobtainedinthepresenceofthe oxidovana-dium(V)complexes1(94%,entry1,Table4)and3(93%,entry5, Table4).Theywerefollowedbytheoxidovanadium(IV)4 (86%,

Fig.5.Influenceofthetemperatureontheyieldofacetophenoneorcyclohexanone obtainedbyMW-assistedoxidationof1-phenylethanolinthepresenceof3( ) orcyclohexanolinthepresenceof4( )(0.5hreactiontime).

Fig.6. Influenceofthetypeoftheoxidantontheyieldofacetophenone( ) orcyclohexanone( )obtainedbyMW-assistedoxidationofthecorresponding alcoholinthepresenceof1–4.

entry11,Table4)andthenbytheanionicdioxido-V(V)2(77%, entry3,Table4).Toachieve considerablybetterconversionsof 2- or 3-hexanol, although still moderate, to the corresponding ketones(selectivitiesof52–87%)anextendedreactiontime(2.5h) isrequired(seeTable5andFig.4b).

AdditionofTEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl) rad-ical, anefficient mediator for theaerobic oxidation of alcohols [23,24,61–65],althoughscarcelyusedfortheperoxidative oxida-tion[35,43,66,67]ofthosesubstrates,providedalmostquantitative formationof acetophenone(Fig.3)anda significantincreasein cyclohexanoneyield(from38to58%(2),Fig.3).Itshouldbenoted

Fig.4. Yieldofketones(obtainedfromMW-assistedperoxidativeoxidationofthecorrespondingalcohols)alongthetime:(a)acetophenoneinthepresenceof1( )and cyclohexanoneinthepresenceof2( );(b)2-hexanoneinthepresenceof3( )and3-hexanoneinthepresenceof4( ).

thatreactionscarriedoutwithTEMPO,butintheabsenceof1–4,

ledtoverylowketoneyields(<4%,see,e.g.,entries4and8,Table S2).

Toourknowledge,noMW-assistedTBHP/oxidovanadium cat-alytic system has been previously reported and this study demonstratesitsviabilityfortheperoxidativeoxidationof alco-hols.Afavourableeffect ofMWirradiationwasobserved, even applyingthelowpowerof25W,asalreadyreportedforother sys-tems[27].Forexample,only12%ofproductwasobtainedunder thesameconditionsofthoseadoptedfor1(Table4,entry1)but usingconventional heating(anoil-bath).Moreover,an increase oftheacetophenoneyieldto89%required15hofreactionwith conventionalheating,inthepresenceof1.

Thepresentcatalyticsystempresentsseveraladvantagesover thoseusingconventionalheating.Forexample,itallowsthefast (30min.)quantitativeformationofacetophenone(Fig.4)whereas, e.g.,the3hoxidationof1-phenylethanolwithTBHP(5Mindecane) inthepresenceofa silicasupportedoxidovanadiumSchiffbase [41]leadstoamaximum90%yieldofacetophenone[41]. How-ever,thelattersystemleadstohigheryields(93%)ofcyclohexanone fromcyclohexanol[41].Aerobic(byO2)conversions(upto90%)

ofcyclohexanoltocyclohexanonewerepreviouslyobtainedinthe presenceofN-hydroxyphthalimideasa radicalproducingagent andtheco-catalyst[VO(acac)2][38],althoughrequiringatleast

18hreactiontime.

Anoteworthyfeatureof1–4concernstherelativelylowloading (0.2mol% vs. substrate) necessary to reach high yields of ace-tophenone (i.e.up to 99% (1 or 3) in thepresence of TEMPO) withsubstantialTOFvaluesof987and993h−1,respectively.The effectoftheamountofcatalystprecursor3wasstudiedforthe 1-phenylethanol(entries5–7,Table4)andcyclohexanol(entries 8–10,Table4)oxidations.Itsincreasefrom1␮mol(0.02mol%vs. substrate)to10␮mol (0.2mol%vs. substrate)resultsin a yield enhancementfrom36to93%(acetophenone)orfrom12to30% (cyclohexanone).However,beyond10_␮molofcatalyst,theyield remainedalmostunchanged,leadingtotheexpectedTONlowering (comparee.g.entries5and7or8and10,Table4).

Blank tests (in the absence of any catalyst precursor) were performedundercommonreactionconditionsandnosignificant conversionwasobserved(<0.7%,see,e.g.,entries1and5,Table S2).Moreover,theuseof[VO(acac)2](thestartingcomplexforthe

catalyticprecursors)orofV2O5,insteadofourvanadiumcatalysts,

resultedinmuchloweryields(e.g.,47%or13%for1-phenylethanol, and24%or9%forcyclohexanol,respectively,TableS2,entries9–10, versus94%for1-phenylethanoland36%forcyclohexanolinthecase of1,Table4,entry1).

Theoptimalreactiontemperatureis80◦CasdepictedinFig.5for theMW-assistedoxidationsof1-phenylethanolandcyclohexanol, with3or4,respectively.Attemptstoperformtheoxidationatroom temperaturefailedandtheminimumrequiredtemperatureisca. 45◦C.Moreover,temperaturesabove80◦Cdonotleadtohigher ketoneyields.Theoveralltemperaturecoefficientoftheoxidations isca.2.2(temperaturerangeof50–80◦C).

Experiments withthe cheaperand environmentally friendly hydrogenperoxide(30%aqueoussolution)asoxidantareless effec-tive,asattestedbythemarkedyieldlowering,e.g.from94%to26% (1)(Fig.6), inaccordwiththeexpecteddecompositionofH2O2

undertheusedreactionconditions(80◦C).Moreover,theuseof higheramountsofoxidantdoesnotleadtoabetterconversion.

Thepreviouslyrecognizedpromotingeffectofbasicadditives, whichfacilitatethedeprotonationofthealcohol[61,62,69–75],was notdetectedforthepresentsystems.Incontrast,astronginhibitor effectofthecatalyticactivity(Fig.3)wasobservedforthe reac-tionscarriedoutinthepresenceof1MK2CO3solution.Moreover,

thepresenceof HNO3 alsoexhibitedaninhibitoryeffectonthe

acetophenoneyield(Fig.3).

Theperoxidativeoxidationofthetestedsecondaryalcoholsis believedtoproceedmainlyviaaradicalmechanismwhichinvolves bothcarbon-andoxygen-centredradicals[76–78],inviewofthe stronginhibitioneffectobservedwhenitiscarriedoutinthe pres-enceofeitheracarbon-radicaltrapsuchasCBrCl3(Fig.3)oran

oxygen-radical trapsuchas Ph2NH (Fig.3).It may involvee.g., t_BuO• _andt_BuOO• _{radicalsproducedintheVpromoted}

decom-position of TBHP [79,80] according to the Eqs. (1)–(6), where VOn+ _{represent arylhydrazonederived oxido-vanadiumspecies.}

Theoxidoperoxidospecies[VO(OO)L]−,conceivablyinvolvedinthe catalyticprocess,wasdetectedbyESI-MS(seeSI).

VO3++t_{BuOOH→ VO}2+₊t_BuOO• _+H+ ₍₁₎

VO2++t_{BuOOH→ VO}3+ _OH+t_BuO• ₍₂₎

VO3+ OH₊tBuOOH _→VO3+ OO tBu₊H2O (3)

t_BuO• _{+R2CHOH →}t_BuOH+R2C• _{OH (4)}

t_BuOO• _+R

2CHOH →tBuOOH+R2C• OH (5)

VO3+ OO tBu+R2C• OH →R2C O+tBuOOH+VO2+ (6)

Itmayalsoproceedviathecoordinationofthealcoholsubstrate toanactivesiteofthecatalyst,anditsdeprotonationtoformthe alkoxideligand,followedbyametal-centred(andTEMPOassisted) dehydrogenation[68,71,72].

Otherimportantfeaturesofthe1–4/TBHP/MWsystemconcern theuseofweakMWirradiation(25W)andofsolvent-and additive-freeoxidation conditions,whichcontrastwiththecommonuse oforganicsolventsorcostlyionicliquidsinmanystate-of-the-art methodsfortheoxidationofalcohols[1–4,17–19,25,26].

4. Conclusions

In this study, we have successfully explored the catalytic activity of four differenttypes of vanadium complexes, i.e., an oxidoethoxidovanadium(V)(1), a salt likedioxidovanadium(2), a mixed-ligandoxidovanadium(V) (3) and a mixed-ligand oxi-dovanadium(IV)complex(4)towardsMW-assistedhomogeneous oxidationofsecondaryalcohols.1–4actasefficientandselective catalyst precursors for themild MW-assisted oxidation of sec-ondaryalcohols(1-phenylethanol andcyclohexanol)in solvent-andadditive-freesystems.Acomparativestudyoftheircatalytic efficiencyhasbeendrawn.Cyclohexanoneisobtainedin compa-rableyieldsinthepresenceof1–4,whiletheoxidovanadium(V) complexes1and3arethemostefficientonesfortheperoxidative oxidationof1-phenylethanol.

The application, for the first time, of a MW-assisted TBHP/oxidovanadiumcomplexcatalyticsystemfortheoxidationof secondaryalcoholswidensthescopeofperoxidativecatalytic sys-temssuitableforMW-assistedoxidativetransformationsofsuch substrates.Moreover,acooperativeactionoftheradicalTEMPO with1–4towardstheperoxidativeoxidationofthetestedalcohols wasfound.Thisstudyshouldopenanewwindowforthecatalytic applicationofvanadiumcomplexesinsolvent-andadditive-free systems.

Supportinginformation

Description of thecrystal structure of compound 4 is given aselectronicsupplementaryinformation.CCDC1005764(3)and 1005765(4)containthesupplementarycrystallographicdatafor this paper.Thesedatacanbeobtainedfreeof chargefromThe CambridgeCrystallographicDataCentreviawww.ccdc.cam.ac.uk/ datarequest/cif.

Acknowledgments

We are grateful for the financial supportreceived fromthe Fundac¸ãopara a Ciência ea Tecnologia(FCT), Portugal,and its projects PTDC/EQU-EQU/122025/2010 and PEst-OE/QUI/UI0100/ 2013.M.S.acknowledgestheFCT,Portugal,forapostdoctoral fel-lowship(SFRH/BPD/86067/2012).

AppendixA. Supplementarydata

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.apcata. 2015.01.005.

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