Esterification of camphene over heterogeneous heteropoly acid catalysts : synthesis of isobornyl carboxylates.

ContentslistsavailableatSciVerseScienceDirect

Applied

Catalysis

A:

General

j ourna 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

Esterification

of

camphene

over

heterogeneous

heteropoly

acid

catalysts:

Synthesis

of

isobornyl

carboxylates

Augusto

L.P.

de

Meireles

_,

_Kelly

_A.

_da

_Silva

_Rocha

_,

_Ivan

_V.

_Kozhevnikov

_,

_Elena

_V.

_Gusevskaya

a_{DepartamentodeQuímica,UniversidadeFederaldeMinasGerais,31270-901BeloHorizonte,MG,Brazil}

b_{DepartamentodeQuímica,UniversidadeFederaldeOuroPreto,35400-000OuroPreto,MG,Brazil}

c_{DepartmentofChemistry,UniversityofLiverpool,LiverpoolL697ZD,UK}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16August2011

Receivedinrevisedform

21September2011

Accepted25September2011

Available online 1 October 2011 Keywords:

Biomass-basedfeedstock

Esterification Camphene

Heterogeneousacidcatalysis

Heteropolyacid

a

b

s

t

r

a

c

t

SilicasupportedH3PW12O40(PW),thestrongestheteropolyacidintheKegginseries,isanactiveand

environmentallyfriendlysolidacidcatalystforliquid-phaseesterificationofcamphene,arenewable

biomass-basedsubstrate,withC2,C4andC6 short-chainfattyacids.Thereactionprovidesisobornyl

carboxylates, useful asfragrances, in virtually100% selectivity and 80–90% yield. The reaction is

equilibrium-controlledandoccursundermildconditionswithacatalystturnovernumberofupto3000.

TheuseofhydrocarbonsolventpreventsPWfromleachingtoalloweasycatalystrecovery.Thecatalyst

canbereusedseveraltimeswithoutlossofactivityandselectivity.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Terpenes arethe main component ofessential oils of many plantsandflowersandrepresentanimportantrenewable hydro-carbon feedstock for fine chemical industry, in particular, for the synthesis of flavors and fragrances [1,2]. Various terpenic ingredients of fragrance compositions are commercially pro-ducedbyacid-catalyzedtransformationsofabundantmono-and sesquiterpenes.Intheseprocesses,mineralacidsarestillusedas homogeneouscatalysts,whichresultsinserioustechnologicaland environmentalproblems.Thedevelopmentofefficientsolidacid catalystsfortheconversionofterpenescancontributesignificantly tovalorizationofessentialoils.

Heteropolyacids(HPAs)oftheKegginseriesarewellknownas promisingacidcatalystsforthecleansynthesisoffineandspecialty chemicals[3,4].Duetotheirstrongeracidity,HPAsusuallyshow highercatalyticactivitiesthantheconventionalacidcatalysts,such asmineralacids,ion-exchangeresins,mixedoxidesandzeolites. Incontrasttomineralacids,HPAsdonotpromoteundesirableside reactions,suchassulfonationandchlorination,aswellascause lesscorrosionproblems.Inourpreviousstudies,HPAshavebeen

∗ Correspondingauthor.Tel.:+553134095741;fax:+553134095700.

E-mailaddress:elena@ufmg.br(E.V.Gusevskaya).

usedascatalystsforvarioustransformationsofterpenoids,such asisomerization[5–8]andacetoxylation[9–11],includingthe ace-toxylationandhydrationofcamphene[10].Throughacid-catalyzed additionofwaterandcarboxylicacids,camphenecanbeconverted toisoborneolanditsesters,respectively,whichpossessastrong characteristic fragranceandareused insoap formulations, cos-meticperfumesandpharmaceuticalproductsmakingpopularpine andfruit-berryaromasalmostidenticaltothenaturalones[1,12]. Isoborneolisalsousedfortheproductionofsyntheticcamphor[1]. Sincetheequilibriumofcamphenehydrationisshiftedtowards thereagents, onlylow yieldsof isoborneolhave beenobtained inthis reaction[13,14].Ontheotherhand,theacetoxylationof camphene,whichislessstrictlyequilibrium-controlled,canresult inexcellentyieldsofthecommerciallyimportantpine-fragranced isobornylacetate[10,15–17].Anumberofsolidacidcatalysts,such aszeolites[15,18],surface-functionalizedsilica[19]andsulfonated polystyreneresins[16,17],havebeenusedfortheacetoxylationof camphene.Inordertoshifttheequilibriumandobtainisobornyl acetateinhighyields,aceticacidisusuallyusedasasolvent.Such conditions, however,arehardly suitablefor solid HPA catalysts due to highsolubility of HPAs in polar solvents. HPAs, includ-ingH3PW12O40(PW),havebeenusedashomogeneouscatalysts

forthehydration[13,14,20]andacetoxylation[10]ofcamphene, albeitwithacomplicatedcatalystrecovery.Inourpreviouswork, silica-supportedPWhasbeenappliedasaheterogeneouscatalyst forthesolvent-freeacetoxylationofcamphene,witheasycatalyst

0926-860X/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

recovery,butyieldinglessthan30%ofisobornylacetate[10].Thisis becausetheconcentrationofaceticacidshouldbekeptlowenough topreventPWleachingfromthecatalyst.

Hereweattempttodevelopanefficientheterogeneousprocess forcampheneesterificationwithC2,C4 andC6 short-chain

car-boxylicacidsinliquidphase usingasacatalystsilica-supported H3PW12O40(PW),thestrongestHPAintheKegginseries.Thisis

achievedbyperformingthereactioninanon-polarhydrocarbon solventcontainingdissolvedcampheneand carboxylicacidin a molarratioof1:5–1:10.Undersuchconditions,thecatalystisfound tobestableenoughtowardsPWleaching,andreactionequilibrium appearstobeshiftedtowardstheproducttoallowhighyieldsof isobornylesterstobeobtained.Toourknowledge,noattemptto useHPAcatalystsfortheesterificationofcamphenehasbeenmade sofar,exceptforourpreviousworkonthecampheneacetoxylation [10].

2. Experimental

2.1. Chemicals

Allchemicalswere purchasedfrom commercialsources and usedasreceived,unlessotherwisestated.H3PW12O40hydratewas

fromAldrichandAerosil300silicafromDegussa. 2.2. Characterizationtechniques

31_{PMASNMRspectrawererecordedatroomtemperatureand}

4kHzspinningrateonaBrukerAvanceDSX400NMR spectrom-eterusing 85% H3PO4 as a reference. PowderX-ray diffraction

(XRD)ofthecatalystswasperformedonaRigakuGeigerflex-3034 diffractometer with Cu K␣ radiation. The textural characteris-ticsweredeterminedfromnitrogenphysisorptionmeasuredona MicromeriticsASAP2000instrumentat77K.Tungstenand phos-phoruscontent in thecatalysts wasdetermined by inductively coupledplasma(ICPatomicemissionspectroscopy)onaSpectro CirosCCDspectrometer.

2.3. Catalystpreparationandcharacterization

The20wt%H3PW12O40/SiO2catalysts(PW/SiO2)wasprepared

by impregnating Aerosil 300 (SBET, 300m2g−1) with an

aque-ous PW solution followed by calcination at 130◦C/0.2–0.3Torr for 1.5h, as described elsewhere [21]. The PW content deter-minedbyICP was20wt%.Thespecificsurfaceareaofthefresh catalystdeterminedbytheBETmethodwas200m2_g−1_.The

aver-agepore diameterandthesinglepointtotal porevolumewere 144 ˚Aand 0.53cm3_g−1_{,respectively. Afterreaction the surface}

areaslightlyreducedto170–180m2_g−1 _{probablydueto}

deposi-tionofoligomers,whereastheporositydidnotchangesignificantly afterreaction.The31_{PMASNMRspectraoffreshaswellasspent}

catalystsdisplayedonlyasinglepeakatca.−15ppm character-isticofPWconfirmingtheintegrityoftheKegginstructure[22]. FromXRD,thecatalystsincludedfinelydispersedPWonthe sil-icasurface,withasmallamountPWcrystallinephasealsopresent. Theacidstrengthofsilica-supportedPWwascharacterized calori-metricallyusingammoniaandpyridineadsorptionanddiscussed previously[23].

2.4. Catalyticreactions

The reactions were carried out in a glass reactor equipped with a magnetic stirrer and a condensor. In a typical run, a mixture(totalvolumeof5.0mL)ofcamphene(0.35–2.25mmol, 0.07–0.45M), carboxylicacid (3.5–22.5mmol, 0.7–4.5M), dode-cane(0.5–1.0mmol,0.10–0.2M,GCinternalstandard)andPW/SiO2

(10–50mg,0.7–3.5␮molofPW)inaspecifiedsolventwasintensely stirredunderairataspecifiedtemperature(40–80◦C).The reac-tionswerefollowedbygaschromatography(GC)usingaShimadzu 17instrumentfittedwithaCarbowax20Mcapillarycolumnanda flameionizationdetector.Atindicatedreactiontimes,stirringwas stoppedandafterquickcatalystsettlingdownaliquotsweretaken andanalyzedbyCG.Themassbalance,productselectivityandyield werecalculatedusingdodecaneasinternalstandard.Any differ-enceinmassbalancewasattributedtotheformationofoligomers, whichwereGCunobservable.

Catalystrecyclingexperimentswereperformedasfollows:after thereaction,thecatalyst wascentrifuged,washed withhexane andreused.Tocontrolcatalystleachingand thepossibility ofa homogeneousreaction,thecatalystwasremovedby centrifuga-tionofthereactionmixtureatthereactiontemperaturetoavoid re-adsorptionofactive componentsontosilica, thenthe super-natantwasaddedwithafreshportionofsubstrate,ifnecessary, andallowedtoreacton.Nofurtherreactionwasobservedinsuch experiments,indicatingabsenceofPWleaching.

Isoborneol acetate (2) was identified by GC/MS (Shimadzu QP2010-PLUSinstrument,70eV)bycomparisonwithanauthentic sample.Theesters3and4wereseparatedbyacolumn chromatog-raphy(silicagel60)usingmixturesofhexaneandCH2Cl2aseluents

andidentifiedbyGC–MS,1_H,and13_CNMR.The1_Hand13_CNMR

signalswereassignedusingbidimensionaltechniques.NMR spec-trawererecordedinCDCl3usingaBruker400MHzspectrometer,

withTMSasaninternalstandard.Massspectrawereobtainedon aShimadzuQP2010-PLUSinstrumentoperatingat70eV.

Dataforisobornylacetate2:MS(m/z/rel.int.):154/10;136/47; 121/50;108/25;95/100;93/48. 1_{HNMR, ı} H: 0.83 (s,3H, C8H); 0.84(s,3H,C10_H 3);0.98(s,3H,C9H3);1.05–1.15(m,1H,C5HH); 1.10–1.20(m,1H,C6_HH ax);1.50–1.60(m,1H,C6HeqH);1.65–1.75 (m,1H,C5_{HH);1.65–1.75(m,1H,C}4_{H);1.75–1.85(m,2H,C}3_H 2); 2.08(s,1H,C12_H 3);4.67(dd,1H,3J=8.0and3.0Hz,C2H).13CNMR, ıC:11.38(C10),19.87(C9),20.17(C8),21.31(C12),27.02(C5),33.72 (C6_),38.75(C3_),44.99(C4_),46.94(C7_),48.63(C1_),80.94(C2_),170.66 (C11_).

Dataforisobornylbutyrate3:MS(m/z/rel.int.):154/22,137/20, 136/89, 121/67, 110/44, 109/40, 108/50, 95/100, 93/55, 71/89, 69/25.1_HNMR,ı H:0.84(s,6H,C8H3,C10H3);0.96(t,3H,3J=7.4Hz, C14_H 3);0.98(s,3H,C9H3);1.05–1.10(m,1H,C5HH);1.10–1.18(m, 1H,C6_HH ax);1.50–1.56(m,1H,C6HeqH);1.58–1.64(m,2H,C13H2); 1.64–1.70(m,1H,C5_{HH);1.70–1.75(m,2H,C}4_H,C3_{HH);1.75–1.85} (m,1H,C3_{HH);2.26(t,2H,}3_J=7.4Hz,C12_H 2);4.67(dd,1H,3J=8.0 and3.2Hz,C2_H).13_CNMR,ı C:11.70(C10),13.98(C14),18.82(C13), 20.15(C9_),20.39(C8_),27.32(C5_),34.05(C6_),37.04(C12_),39.14(C3_), 45.34(C4_),47.18(C7_),48.89(C1_),80.99(C2_),173.51(C11_).

Dataforisobornylcaprylate4:MS(m/z/rel.int.):154/20,137/32, 136/100, 121/92, 110/3, 109/50, 108/60, 107/26, 99/76, 96/95, 95/92, 93/96, 92/25, 81/36, 79/26, 71/60, 67/25. 1_{H NMR, ı} H: 0.77(s,6H,C8_H 3,C10H3);0.82(t,3H,3J=6.8Hz,C16H3);0.91(s, 3H,C9_H 3);0.96–1.04(m,1H,C13HH);1.06–1.12(m,1H,C6HHax); 1.10–1.26(m,2H,C5_H 2);1.22–1.26(m,2H,C15H2);1.42–1.50(m, 1H,C6_H eqH);1.50–1.56(m,2H,C14H2);1.58–1.68(m,3H,C13HH, C4_H,C3_{HH);1.68–1.76(m,1H,C}3_{HH);2.20(t,2H,}3_J=7.4Hz,C12_H 2); 4.59(dd,1H,3_{J=7.6and3.2Hz,C}2_H).13_CNMR,ı C:11.43(C10),13.88 (C16_),19.83(C9_),20.12(C8_),22.31(C15_),24.75(C14_),27.05(C13_), 31.32(C5_),33.78(C6_),34.80(C12_),38.87(C3_),45.07(C4_),46.92(C7_), 48.63(C1_),80.75(C2_),173.46(C11_).

3. Resultsanddiscussion

The results on camphene (1) esterification with acetic acid (5–10foldexcess)inthepresenceofPW/SiO2incyclohexane

Table1

Esterificationofcamphenewithaceticacidcatalyzedby20wt%PW/SiO2.a

Run Camphene(mmol) HOAc(mmol) Catalyst(mg/␮molofPW) T(◦C) Time(h) Conversion(%) Selectivityto2(%) TONb

1 0.35 3.5 50/3.5 60 1 3 67 70 100 100 70 2 0.75 3.5 50/3.5 60 1 3 68 70 100 100 150 3c _{0.75 7.5 50/3.5 60 1} 3 78 80 100 100 170 4 1.00 10.0 50/3.5 40 1 3 79 83 100 100 240 5 1.00 10.0 None 60 6 0 6 0.75 7.5 10/0.7 60 2 4 78 81 100 100 870 7 2.25 22.5 10/0.7 60 2 4 80 90 100 100 2892 8d _{2.25 22.5 10/0.7 60 0.15} 3 24 25 100 100 800 9e _{2.25 22.5 10/0.7 60 2} 4 83 88 100 100 2828 10e,f _{2.25 22.5 10/0.7 60 4 <2%} 11e _{2.25 22.5 10/0.7 80 0.5} 3 75 75 100 100 2410

a_{Reactionswerecarriedoutincyclohexaneasasolvent,withthetotalvolumeofthereactionmixtureof5.0mL.ConversionandselectivityweredeterminedbyGC.}

b_{TONinmolesofsubstrateconvertedpermoleofatotalamountofPWincatalyst.}

c _{Thecatalystwasreusedtwicewithoutsignificantlossofactivityandselectivity.}

d _{After10minthecatalystwasseparatedbycentrifugation,thesupernatantwasallowedtoreactfurther,withnofurtherconversionobservedthereupon.}

e_{Inisooctaneassolvent.}

f _{Afterrun9,thecatalystwasremoved,thesolutionwasrechargedwithfreshsubstrate(2.25mmol)andthereactionwasallowedtoproceedfurther,withnofurther}

conversionobservedthereupon.

with100%selectivitytoisobornylacetate(2),nootherproducts beingobserved(Scheme1).Thereactionwasnotcomplicatedby oligomerizationandisomerization,asoftenhappenswithterpenic compoundsunderacidicconditions.Itisremarkablethatnotrace ofborneolacetate,theendo-isomerof2,wasobserved,which indi-cateshighreactionstereoselectivity.

At60◦C,nearly70%ofcampheneconvertedtoisobornylacetate in1h.Afterthatthereactionbecamestagnant,indicating equilib-riumcontrol(Table1,run1).Withthesameamountofcatalyst,the reactionwasequallyefficientwithadoublesubstrate concentra-tion,reachingagain70%campheneconversionat100%selectivity to2(Table1,run2).Whenalarger(10fold)excessofaceticacidwas used,reactionequilibriumshiftedtowardstheproduct,allowinga 78%conversionin1h(Table1,run3).Carryingonthereactionfor another2hdidnotchangetheconversion.Afterthatthecatalyst wasseparatedbycentrifugationandreusedtwicewithout signifi-cantlossofactivityandselectivity.Atalowerreactiontemperature of40◦C,ahigheryieldofisobornylacetate(83%)wasobtainedin 3h(Table1,run4).Blankreactions,withoutanycatalystaddedor

withpuresilica,showedvirtuallynoconversionofcamphenein6h (Table1,run5).

Thesuggestionthattheincompleteconversionisdueto equi-librium control rather than catalyst deactivation was further confirmedbyrunningthereactionwithmuchsmalleramountsof catalyst(cf.runs3and6,Table1).Thusinrun6withonefifth of thecatalyst amountusedin run3, thereaction reachedthe sameconversion(78%),albeitmoreslowly(2hratherthan1h),as expectedfortheequilibrium-controlledreaction.Thiscorresponds toaturnovernumber(TON)of870permolofthetotalamount ofPWinthecatalyst.Consideringthatapartofacidsitesmaybe locatedinthebulkofPWphaseandhencenotaccessibletothe sub-strate,therealefficiencyofthesurfaceactivesitescouldbeeven higher.

Attemptingtoimprovetheyieldof2,weincreasedtheinitial concentrationofcampheneata1:10camphene/aceticacidmolar ratio.Thisgaveacampheneconversionof90%at100%selectivity to2(90%isobornylacetateyieldandTONabout3000)(Table1,run 7).

2

1

R

=

CH

3 R

=

CH

CH

CH

4 R

=

CH

CH

CH

CH

CH

20

wt%

H

PW

O

/SiO

40-80

C

hydrocarbon so

lven

t

5-10

eq.

RCOOH

OC(O)R

Table2

Esterificationofcamphenewithcarboxylicacidscatalyzedby20wt%PW/SiO2.a

Run Acid(mmol) Catalyst

(mg/␮molof PW)

Time(h) Conversion(%) Selectivityto2,

3,or4(%) TONb 1 Acetic(22.5) 10/0.7 2 4 83 88 100 100 2828 2 n-Butyric(11.2) 10/0.7 2 7 23 52 100 100 1670 3c _{n-Butyric(11.2) 25/1.8 2} 4 6 69 80 80 100 100 100 1000 4 n-Butyric(11.2) 50/3.5 2 78 100 500 5c _{n-Hexanoic(11.2) 25/1.8 2} 4 33 46 100 100 575 6 n-Hexanoic(11.2) 50/3.5 2 3 6 65 80 80 100 100 100 515

a_{Reactionconditions:2.25mmolcamphene,isooctanesolvent,5.0mLtotalvolumeofthereactionmixture,60}◦_{C.ConversionandselectivityweredeterminedbyGC.}

b _{TONinmolesofsubstrateconvertedpermoleoftotalamountofPWincatalyst.}

c _{Thecatalystwasreusedtwicewithoutsignificantlossofactivityandselectivity.}

AlthoughPWisinsolubleinnon-polarsolventsandpoorly sol-ublein dry carboxylic acids, thepresence of acetic acidin the reactionmixturecouldpromotesomePWleachingfromthe cat-alystandhenceinitiatehomogeneouscatalysisbythedissolved PW.Toexamineanycontributionofhomogeneouscatalysis,the catalystwasremovedafter10minreactiontimebycentrifugation andthesupernatantwasallowedreactingfurther(Table1,run8). Nofurthersubstrateconversionwasobservedinthisexperiment, whichshowsthatthereactionoccursbyheterogeneouscatalysis andhomogeneouscatalysisdoesnotplayanysignificantroleinour system,implyingtheabsenceofsubstantialPWleachingfromsilica underreactionconditionsused.Inourpreviousstudiesofterpene reactionsoverPW/SiO2incyclohexane,ICPanalysisofthe

super-natantdidnotfindanytungstenleachingfromthecatalyst[5].The characterizationoffreshversusspentcatalysts(Section2.3)didnot revealanysignificantchangeinthecatalysttexture,nordiditshow anystructuralalterationofthePW,asmaybeexpectedforthis reactionoccurringundermildconditionsinnon-polarhydrocarbon solvent.

TheacetoxylationofcampheneoverPW/SiO2catalystwasalso

performedinisooctane.Thisyielded88%of2,almostthesameas incyclohexane(Table1,run9vs.run7).Higherboilingpointof isooctane(b.p.99◦C)canbeanadvantageforpracticalapplications ascomparedtocyclohexane(b.p.81◦C).Asexpected,thereaction at80◦Cwasfasterthanat60◦C;however,theequilibrium con-versiondecreasedfrom88to75%(cf.runs9and11,Table1).To controltheleachingofPW,thecatalystafterrun9wasremoved fromthereactionmixturewhichwasthenrechargedwithfresh substrateandallowedreactingfurther(Table1,run10).Practically nocampheneconversionwasobservedafterthecatalystremoval. Thisresultalsosupportstheaboveconclusionregardingthe reac-tionoccurringbyheterogeneouscatalysisandtheabsenceofany significantPWleachinginoursystem.

Thereactioncanbecarriedoutwithothercarboxylicacidsto obtainthecorrespondingisobornylcarboxylatesingoodto excel-lentyields.These estersarealsousefulasfragrancessimilarto

2.Representativeresultsobtainedwiththeuseofn-butyricand n-hexanoic(n-caproic)acidsareshowninTable2.For compari-son,theresultsforaceticacidundersimilarconditionsarealso includedinTable2(run1).Intheexperimentswithn-butyricand n-hexanoicacids,duetotheirhighermolecularweights,smaller acid/camphenemolarratios(upto5:1)wereusedtoavoidPW leachingfromthecatalysttothereactionmixturealthoughitcould resultinlowerproductyields.

Thereactionofcamphenewithn-butyricacidoccurredmuch slowerthanwithaceticacid,reachinganon-equilibriumcamphene

conversionof52%in7hwithaTONof1670(Table2,run2).To acceleratethereaction,weincreasedtheamountofcatalystand attainedanearlyequilibriumconversionof80%in1–2hreaction time(Table2,runs3and4).Afterthatthereactionbecame stag-nant.Thisreactiongaveisobornylbutyrate(3)(Scheme1)inalmost 100%selectivity.Theproductwasisolatedbyacolumn chromatog-raphyandidentifiedbyGC–MSandNMR.TheNOESYspectrumof3 revealsthatinthismoleculethehydrogenH-2givesNOEwiththe hydrogenH-6axshowingtheirspatialproximity.Thisclearly

indi-catestheexo-configurationfor3,in whichthecarboxylicgroup atC-2and thehydrogenH-6ax areattheopposite sidesofthe

six-memberedring,asshowninScheme1.

Theesterificationofcamphenewithn-hexanoicacidisalso fea-siblewiththePW/SiO2catalyst(Table2,runs5and6),although

itwentslowerthanwithn-butyricacid(cf.runs3and5,Table2). Withn-butyricacid,thereactionalmostreachedequilibrium in 4h, whereas withn-hexanoic acidonly46% camphene conver-sionwasobtainedwithinthattimeinterval.Thus,thelongerthe hydrocarbonchainof carboxylicacid,theslowerthecamphene esterification, which can be plausibly explained by steric con-straints.Underoptimizedconditions,isobornylcaprylate(4)was obtainedwith80%yieldin3h(Table2,run6).Thisyieldwasalso limitedbyequilibriumastheselectivityto4wasvirtually100%and continuingthereactionbeyondthe3hintervaldidnotincreasethe conversionanymore.Unconvertedcamphenecouldberecovered duringproductseparation.This,however,wasnotattemptedinthe presentstudy.Thecatalystseparatedafterruns3and5wasreused twiceinthesamereactionsandshowednearlythesameactivity andselectivity.

Stereochemistry of ester 4 wasalso confirmed by a NOESY experiment.Theresultssuggestthatin4thehydrogenatC-2and theaxialhydrogenatC-6areatthesamesideofthecyclohexane ring,asshowninScheme1,becausethereisastrongcorrelation sig-nalbetweenH-2andH-6ax.Thus,theobtainedesterisexo-isomer,

i.e.derivedfromisoborneol.

4. Conclusions

Heteropoly acid H3PW12O40 (PW) supported onsilica is an

efficientandenvironmentallyfriendlysolidacidcatalystforthe liquid-phase esterification of camphene with carboxylic acids under mild reaction conditions. This reaction is an effective route for the synthesis isobornyl carboxylates, potential fra-grancespossessing a strongcharacteristic aroma, starting from a renewable biomass-based substrate. The reaction yield is equilibrium-controlled, with ester selectivity of virtually 100%.

Under optimized conditions, the esters have been obtained in 80–90%yieldandupto3000PWturnovernumber.Thecatalystcan berecoveredandreusedwithoutlossofactivityandselectivity.

Acknowledgments

FinancialsupportandscholarshipsfromCNPq,CAPES,FAPEMIG, andINCT-Catálise(Brazil)areacknowledged.

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