Advanced sol–gel process for efficient heterogeneous ring-closing metathesis 2021 Shiran Aharon.pdf

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OPENAdvancedsol–gelprocess
for efcientheterogeneous
ring‑closingmetathesis
Shiran Aharon1,2,Dan Meyerstein1,3,Eyal Tzur2*,Dror Shamir4,Yael Albo5 &Ariela Burg2*
Olefnmetathesis,apowerfulsyntheticmethodwithnumerouspracticalapplications,canbe
,asingle-stage
processfortheentrapmentofruthenium-basedcatalystswasdevelopedbythesol–gelprocess.
Systemefectivenesswasquantifedbymeasuringtheconversionofthering-closingmetathesis
reactionofthesubstratediethyldiallylmalonateandtheleakageofthecatalystsfromthematrix.
,
matricespreparedwithtetraethoxysilaneatanalkalinepHexhibitabetterreactionratethanin
,thisisthe
frststudytopresentaone-stepprocessthatissimplerandfasterthanthemethodsreportedinthe
literatureforcatalystentrapmentbythesol–gelprocessunderstandardconditions.
Olefnmetathesisisafundamentalchemicalreactioninvolvingtherearrangementofcarbon–carbondouble
bondsthatcanbeusedtocouple,cleave,ring-close,ring-open,orpolymerizeolefnicmolecules­1–,
powerful,mild,versatile,andselectivemethod,olefnmetathesisisusedinresearchinavarietyoflifesciences,
includingthosewithapplicationsinthepolymerandpharmaceutical­industries1–,thismethodso
revolutionizedthediferentfeldsofsyntheticchemistrythatthe2005NobelPrizeinChemistrywasawarded
toYvesChauvin,,“forthedevelopmentofthemetathesismethodin
organicsynthesis"14.
Olefnmetathesisreactionsrequireacatalyst­28,forexample,theruthenium-basedcatalysts(asecond-gen-
3,6,15,18,25–27
erationGrubbscatalystandasecond-generationHoveyda-Grubbscatalyst)usedinthisstudy­.Dueto
catalystsignifcance,muchresearchhasbeendonetodevelopanefcientcatalystandefcientcatalyticprocesses.
Commonly,homogenous­catalysis4,6,15,18,21–23,25,26hasbeenused;however,owingtotheirhigh­costs29,theability
,itismoreefcientandproductivetousethecatalystsaspart
ofaheterogeneoussystem,,theheterogeneoussystemwillnot
onlyfacilitateeasyseparationofthecatalyst,itwillalsoenabletheby-productstobeeasilyrecoveredfromthe
,especiallyinpharmaceuticalproduction,whereinthe
fnalproductsmustmeetstringentpuritycriteria­30–33.
-basedcatalysts
toasupportmaterial,suchasmesoporoussilica,usingthecatalystionligand(forexample,seecatalystsA-Cin
Fig. 1)34–––gelprocess,aporousmatrixisformedbymixing
precursorssuchastetramethylorthosilicate(TMOS)andtetraethylorthosilicate(TEOS)andwatertoproduce
–gelprocessisitseaseofadaptability:matrixproperties,
includingparticlesizeandsurfacearea,canbeeasilyandinexpensivelycontrolledbychangingthenatureand
theconcentrationoftheprecursorsandthepHofthewaterusedinthesol–gel­process30,31,38––gel
processenablestheentrapmentofalargevarietyofreagentsinthematricesincludinginorganicmolecules­40,
metalnanoparticles,metal-oxidenano-particles42,44,53–56,­bacteria57,and­enzymes45,58.
Teimmobilizationofruthenium-basedcatalyststhroughacovalentbond,whichhasbeendoneinseveral
­studies2,47,59,entailsbindingthecatalysttothesol–
syntheticstages;however,inthisstudy,wesoughttodevelopasimplermethodofcatalystconfnementthatdoes
notinvolvecovalentbindingtothematrix­2,47,,ourmethodreliesonintermolecularbonds,whichare

1ChemicalSciencesDept,ArielUniversity,Ariel,,SamiShamoonCollegeof
Engineering,BeerSheva,Ashdod,,Ben-GurionUniversityoftheNegev,Beer‑Sheva,
,Beer‑Sheva,,ArielUniversity,Ariel,
Israel.*email:******@;******@
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Figure 1.  —GrII:Grubbssecondgenerationcatalyst;HGII:Hoveyda-
—(A)[1,3-Bis(2,4,6-trimethylphenyl)-4-[(4-ethyl-4-methylpiperazin-
1-ium-1-yl)methyl]imidazolidin-2-ylidene]-(2-i-propoxybenzylidene)dichlororuthenium(II)chlorideAquaMet;
(B)(1,3-Bis(2,6-diisopropylphenyl)-4-((4-ethyl-4-methylpiperzain-1-ium-1-yl)methyl)imidazolidin-2-ylidene)
(2-isopropoxybenzylidene)ruthenium(II)chloridedihydrateFixCa;and(C)1,3-Bis(2,4,6-trimethylphenyl)-4-
[(trimethylammonio)methyl]imidazolidin-2-ylidene]-(2-i-propoxy-5-nitrobenzylidene)dichlororuthenium(II)
chloridenitro-StickyCatCl.
thusfarbythetendencyofmatricespreparedviathismethodtoexhibitcatalyst­leakage30,31,39–44,48,49,adrawback
thatweworkedtoavoid.
Inthecurrentstudy,weentrappedtwodiferenttypesofruthenium-basedcatalysts,Fig. ,neutral
catalysts,comprisedGrubbssecond-generationcatalystsandHoveyda-Grubbssecond-generationcatalysts(.,
GrII&HGII).Type2wereruthenium-basedcationiccatalysts,.,A-C,inFig. 1,whicharewater-solubledue
totheiraqueousquaternaryammoniumgroup­60–63.
CatalystactivityandleakageweremeasuredbytheconversionofDDM(diethyldiallylmalonate)inaRCM
(RingClosingMetathesis)reaction(reaction1).TeRCMofDDMisofenusedasabenchmarkmetathesis
catalyst­comparisons64.
Experimentalsection
Type1catalystscomprisedtheGrubbssecond-generationcatalyst(GrII);Hoveyda-Grubbssecond-generation
catalyst(​​HGII);Tetramethylorthosilicate(TMOS);Tetraethylorthosilicate(TEOS);Diethyldialylmalonate
(DDM);MethyleneChloride(Dichloromethane,DCM);Toluen;NaOH;HNO­3werepurchasedfromAldrich
andwereofanalyticalpurity.
Type2catalystscomprisedthefollowing—A:[1,3-Bis(2,4,6-trimethylphenyl)-4-(4-ethyl-4-methylpiperazin-
1-ium-1-yl)methyl[imidazolidin-2-ylidene]i-propoxybenzylidene)dichlororuthenium(II)chlorideAquaMet;
B:(1,3-Bis(2,6-diisopropylphenyl)4-ethyl-4-methylpiperzain-1-ium-1-yl(methyl-imidazolidin-2-ylidene)
(2-isopropoxybenzylidene)Ruthenium(II)chloridedihydrateFixCa;andC:1,3-Bis(2,4,6-trimethylphenyl)
4-[trimethylammoniomethyl]imidazolidin-2-ylidene](2-i-propoxy-5-nitrobenzylidene)dichlororuthenium
(II)chloridenitro-StickyCatCl,allofanalyticalpurity,werepurchasedfromStremChemicals,Inc.
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Allwaterusedinthisresearchwasultrapurewater,purifedbyaTrekatypeTKA-GenPuresystemwithafnal
∙
inthe­literature30,31,39–44,48,(additionalinformation
inSI,Sect. ).
GC–MSmeasurements. Conversionandleakage(indirecttest)percentagesweremeasuredbyusinggas
chromatographycombinedwithmassspectrometry(GC–MS)fromAgilentTechnologiesGC--
%
columnusedwasaJ&WHP-5 msUltraInertGCColumn(30 m, mm, μm,7-inchcage)connectedto
a5977Bmassspectrometer(MS)detector.
–6  mLofthesolvent(dichloromethane/tolu-
ene)whichcontainedthesubstrate(DDM).If1%ofthecatalysthadleakedfromthematrix,accordingtothe
literature­65(whereconversionhasbeenreportedwhileusing < 1 ppmofcatalyst),themetathesisreactionwould
haveoccurredoutsidethematrix,andaproductpeakintheGC–,
theleakagemeasurementwasdoneintwosteps:First,leakagewastestedusingGC–MS,-
ond,leakageofthosesamplesthatshowedgoodresultsintermsofconversionandleakage,wastestedviaICP.
Tisisadirectmeasurementoftherutheniuminthesolventintheeventthatitleaksfromthematrix(limitof
detectionequals600 ppbofruthenium).
Foradditionalinformationaboutconversionandleakagemeasurements,seesupplementarySect. .
ICP‑OES(inductivelycoupledplasma‑opticalemissionspectrometry)instrument. Direct
measurementofrutheniumwasdonebyICP-OES,ARCOSmodel,fromSpectroCorp.,usingArgonplasmaat
6000˚CandCCDdetectoratthewavelengthrangefrom167to766 nm.
BET(Brunauer–Emmett–Teller)measurements. Surfaceanalysisofthetestedmatriceswasmeasured
3
byaQuantachromeNOVAtouch­LXsurfaceanalyzer­(N2 at77 K).Temeasurementwascarriedoutusing
nitrogengas(%purityfromMaxima),withthespecifcsurfaceareacalculatedaccordingtotheBET
curve.
Forsomeofthematricestested,thesurfaceareaappearstohavebeenbelowthemeasurementlimit,and
therefore,largeerrorsarepossible.
Resultsand discussion
Sol–gelmatricescontainingtypes1and2ruthenium-basedcatalystsweremadeaccordingtotheprocedure
describedinourrecent­studies30,31,39–44,48,49andintheSupportingInformation(SI)Sect. 
informationaboutconversionandleakagemeasurements,seesupplementarySect. -
densationreactionsinthesol–,allofthematrices
inthisstudywerepreparedinacidicmedia()orinalkalinemedia(wateratpH12)38,50,
leakageofthetype1catalysts(GrIIandHGII)wasmeasured,andlowconversionrateswereobserved(Table.
S1).Toexplaintheseresults,theporeradiirangemeasurementsbyBETofsol–gelmatricesthatdidnotcontain
– nm,Fig. -dimensionalsizesofthe
 nm ×  nm ×  ­nm66.
Hence,thecatalystmaybesmallerthantheporeradiiofthesol–gelmatrix,whichwouldcausethemtoleak
fromthematrixduringthewashingofthelatterbeforetheactivitytest,therebyresultinginlowconversionrates.
Inviewofthepoorconversionandleakagefndingsofthetype1catalyst,wedecidedtofocustheremainder
ofthestudyontheactivityoftype2catalystsentrappedinsol–gelmatricesFig. 1,A-
thequaternaryammoniumgroupinthetype2catalyststostrongly(butnotcovalently)bindtothesilicasurface
byadsorption,probablyviaelectrostaticbondswiththesilanolgroupsand/ortheoxidesonthesurfaceofthe
sol–gelmatrix­34,35,37,,evenduringametathesisreactionconducted

homogenoussystem,Fig. ,matrixactivi-
tieswerestudiedusingtwocommon­solvents68—tolueneanddichloromethane(thelatterofwhichisamore
polarsolvent)
catalyst,itmayalsoafectsubstratefowbetweentheporesandproductexitfromthematrixduetotheintermo-
lecularbondsthatformedbetweenthematrixandthe­substrate70,(catalystsA-C)were
(> 85%,60 min,Fig. S2)wereobtainedindichloromethane
(DCM)andtoluene,forcatalystsAandBduetotheirgoodsolubilityinthesesolvents­,theconver-
sionsobtainedforcatalystCintoluenewererelativelylowerthanthoseobtainedinarelativelypolarsolvent
(DCM)duetothelowsolubilityofcatalystCtoluene­62.
Sincecatalystactivitycanbeafectedbyseveralparameters,theheterogeneouscatalysiswasstudiedasafunc-
tionofthefollowing:catalyst,solvent,precursor,pH,andthemolarratiobetweenthecatalystandthesubstrate.
Table 1showstheconversionandsurfaceareaasafunctionofcatalysttypeandsolventidentity.
Whentype2catalystswereentrappedinsol–gelmatrices,theleakageofthecatalystfromthematrixwas
signifcantlyimproved,andhigherconversionrateswereobtained,comparedtotheseoftype1catalystsin
table ,inthetype2
catalysts,tothesilicasurfacebyadsorption,probablyviaelectrostaticbondswiththesilanolgroupsand/orthe
oxidesonthesurfaceofthesol–gel­matrix34,35,37,