novel usage of palladium complexes with p–n–p ligands as catalysts for diphenyl carbonate...

Novel usage of palladium complexes withP±N±P ligands as catalysts for diphenylcarbonate synthesis²

Meenakshi Goyal,1,2,3 Josef Novosad,4 Marek Necas,4 Hirotoshi Ishii,2 RitsukoNagahata,1,2 Jun-ichi Sugiyama,1,2 Michhiko Asai,1,2 Mitsuru Ueda1,2,5 andKazuhiko Takeuchi1,2*1National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan2Joint Research Center for Precision Polymerization, Tsukuba, Ibaraki, Japan3New Energy and Industrial Technology Development Organization, Tokyo, Japan4Masaryk University, Brno, Czech Republic5Tokyo Institute of Technology, Tokyo, Japan

Two palladium complexes with P–N–P ligands[Pd{(SPPh2)2N}2]{Pd(S,S)} and [Pd{(SePPh)2N}2]{Pd(Se,Se)} were prepared and investigated asnovel polladium catalysts for oxidative carbony-lation of phenol using carbon monoxide andoxygen along with a redox catalyst (for in situregeneration of palladium) and ammonium ha-lide. The efficiency of these new catalysts wascompared with that of a PdCl2-based catalystsystem. In order to obtain the maximum effi-ciency, the effects of various parameters such asconcentration of redox catalyst and ammoniumhalide, the effect of solvent, the influence of aquinone-type redox catalyst in addition to inor-ganic redox catalyst, and the effect of tempera-ture were studied. Under the reaction conditionsemployed, the Pd(S,S) catalyst was found toperform better than PdCl2-based catalyst system,whereas the Pd(Se,Se) catalyst had extremely lowefficiency. Copyright# 2000 John Wiley & Sons,Ltd.

Keywords: palladium; complexes catalyses; oxi-dative carbonylation; diphenyl carbonate

INTRODUCTION

Polycarbonates are highly useful as engineeringthermoplastics, owing to their good thermal andmechanical properties. For their synthesis, atransesterification process using diphenols anddiphenyl carbonate (DPC) has been given morerecognition lately than interfacial polycondensationbecause of its advantages, such as the absence oftoxic phosgene, solvent and salt formation. How-ever, the most common method employed for DPCsynthesis is based on the reaction of phenol withphosgene in the presence of bases. Therefore, it isof the utmost importance to find a phosgene-freesynthesis route for DPC synthesis. Direct synthesisof DPC by oxidative carbonylation of phenol in thepresence of a palladium-based catalyst systemusing CO and O2 (Eqn [1]) seems to be a goodalternative, but in the reactions reported units nowthe efficiency of catalyst is too low for suchprocesses to be commercially viable.1–13

Recently we demonstrated the successful usageof a PdCl2–Ce(OAc)3–bis(triphenylphosphoranyli-dene) ammonium bromide (PPNBr) - based catalystsystem for DPC synthesis in yields as high as 76%,and for direct synthesis of polyarylcarbonates in90–95% yield and average molecular weight (Mw)up to 5000.14–16An efficient catalyst system is thekey to the success of this process and there is stillscope for finding new palladium complexes whichcan act as highly efficient catalysts for oxidativecarbonylation of phenols.

Dichalcogenoimidodiphosphinato anions,

APPLIED ORGANOMETALLIC CHEMISTRYAppl. Organometal. Chem.14, 629–633 (2000)

Copyright# 2000 John Wiley & Sons, Ltd.

* Correspondence to: Kazuhiko Takeuchi, National Institute ofMaterials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki305-8565, Japan.† Presented at the XIIIth FECHEM Conference on OrganometallicChemistry, held 29 August–3 September 1999, Lisbon, Portugal.Contract/grant sponsor: Grant Agency of the Czech Republic;Contract/grant number: 203/97/0955; Contract/grant number: 203/98/0676; Contract/grant number: 203/99/0067.

[R2P(E)—N—(E)PR2] (E = O, S,Se;R = Ph,OPh)are versatile P–N–P-typeligands with a strongtendencyto form inorganic (carbon-free)chelaterings.17 For manyyearsafter their introductionbySchmidpeteret al.18 dichalcogenoimidodiphosphi-natesreceivedonly scantattention,but recentlytheinterestin their complexeshavebeenrenewed.19–21

Although known for severalyears,diselenoimido-diphosphinatewas only recently been used as aligand.22 In this work we prepared two[Pd{Ph2P(E)—N—(E)PPh2} 2] complexes withP–N–PligandswhereE waseither S [Pd(S,S)]orSe [Pd(Se,Se)], and tested their activity foroxidativecarbonylationof phenol.

EXPERIMENTAL

Materials

All chemicalswerecommercialproductsandwereusedwithout any further purification.The startingmaterials Ph2PNHPPh2, Ph2P(S)NHP(S)Ph2 andPh2P(Se)NHP(Se)Ph2 were preparedaccordingtothe literature methods.22–24 The solvents weredistilled and stored in activated 3 A molecularsieves before use. 3 A molecular sieves wereactivatedby heatingat 350°C undernitrogenfor10h.

Instrumentation

ElementalanalysiswasperformedonaFisoin’sEA1108 instrument. The 31P NMR spectra wererecordedin CH2Cl2 on a Bruker Avance DRX500 instrument using H3PO4 (85%) as externalreference.The oxidative carbonylation reactionproducts were identified and quantified by gaschromatographyusinga Hewlett-PackardHP6890seriesGC system(TC-1 column).

Synthesis of Pd(S,S) and Pd(Se,Se)

Though the synthesis of [Pd{(SPPh)2N} 2] and[Pd{(SePPh2)2N} 2] is already reported in theliterature,22 in thepresentwork weusedamodifiedmethodreportedbelow.

[Pd{(SPPh)2N}2]A solution of Pd(OAc)2 (0.112g; 0.5mmol) inmethanol(40mmol) wasaddedto a clearsolutionof Ph2P(S)NHP(S)Ph2 (0.449g; 1 mmol) andKOtBu (0.112g; 1 mmol) in methanol(50mmol)

andthesewerestirredtogetherfor 2 h. Theproduct(orangecrystals)was recrystallizedfrom CH2Cl2.Yield 0.371g, 74% (after recrystallization). Ele-mentalanalysis:Found:C, 57.24;H, 3.84;N, 2.42.Calcd for C48H40N2P4PdS4: C, 57.45;H, 4.00; N,2.80%. 31P{1H} NMR: � = 37.8. IR data (cmÿ1):n(P2N) = 1207m,1174m,804s;n(P=S)= 565vs.

[Pd{(SePPh2)2N}2]A solution of Pd(OAc)2 (0.112g; 0.5mmol) inmethanol(40mmol) wasaddedto a clearsolutionof Ph2P(Se)NHP(Se)Ph2 (0.543g; 1 mmol) andKOtBu (0.112g; 1 mmol) in methanol(50mmol)andthesewerestirredtogetherfor 2 h. Theproduct(brown–red crystals) was recrystallized fromCH2Cl2. Yield 0.423g, 78% (after recrystalliza-tion). Elementalanalysis:Found:C,48.12;H, 3.04;N, 2.22.Calcdfor C48H40N2P4PdSe4: C, 48.40;H,3.40; N, 2.35%. 31P{1H} NMR: � = 26.7[1J(31P–77Se)= 543Hz]. IR data(cmÿ1): n(P2N) =1172m,1174m,800s;n(P=Se)= 537vs.

Oxidative carbonylation of phenol

Palladium complex (0.0125mmol), Ce(OAc)3(25mg; 0.075mmol), PPNBr (231mg; 0.375mmol), and 1 g activated 3 A molecular sieveswere loadedinto a 50-ml stainlesssteelautoclaveanddriedat 70°C for 2 h undervacuumbeforethereaction.

After the drying, additionof 3.011g (32mmol)phenoland5 ml dichloromethanewasfollowed bychargingwith 6.0MPaCOand0.3MPaO2, andtheautoclavewas placedin an oil bath preheatedto100°C.After thedesiredreactiontime,thereactionwas quenchedimmediately by cooling the auto-clavein a waterbath.

RESULTS AND DISCUSSION

Theutility of Pd(S,S)andPd(Se,Se)complexeswasinvestigatedas novel catalystsfor the oxidativecarbonylation of phenol to diphenyl carbonate.Othercomponentsof the catalystsystemincludedPPNBr and Ce(OAc)3 as an inorganic redoxcatalystfor in-situ regenerationof reducedpalla-dium.Shortreactiontimeswereusedwith unexcessof phenol,in order to understandthe efficiencyofthesecomplexesclearly.3 A molecularsieveswereusedas dehydratingagentsto remove the waterproduced during the reaction and prevent thehydrolysis of the DPC producedto phenol. The

Copyright# 2000JohnWiley & Sons,Ltd. Appl. Organometal.Chem.14, 629–633(2000)

630 M. GOYAL ET AL.

resultsobtainedwerecomparedwith thoseobtainedfrom PdCl2–Ce(OAc)3–PPNBr-basedcatalystsys-temsreportedearlier by us.14–16 From the resultsgivenin Table1 (Run2), it is clearthattheturnoverfrequency (TOF) of the Pd(S,S) catalyst wasmarginally lower than that of the PdCl2-basedcatalyst system under the reaction conditionsemployedand further optimization is requiredtoobtain the the best efficiency. Besidesdiphenylcarbonate, phenyl salicylate (PS) and a fewoxidationproductsof phenolwereobtainedin lowyields. The selectivity of diphenyl carbonatewasfoundto bemorethan90%.Assumingthatsolventmight be decreasingthe catalytic efficiency, areactionwasperformedin the absenceof solvent.However, as evident from the result reportedinTable 1 (Run 3), no difference in the TOF ofcatalyst could be observed,suggestingthat thesolventdoesnotaffecttheactivity of thecatalystinthese reaction conditions. Therefore, all furtherreactions were carried out in the presenceofsolvent, as usageof a solvent is essentialif wewish to usethis reactionfor direct polycarbonatesynthesis.

Choosinga suitableinorganic redox catalystisvery importantfor in-situ regenerationof palladium(0). Thus, various redox catalystswith differentligandswere investigated;the resultsare listed inTable 1, Runs 4–8. Among the different metalcomplexes,Ce(TMHD)4 was found to give thehighestTOF of 46.1mol DPC(mol Pd)ÿ1hÿ1.

Fromour previouswork we know that incorpor-atinghydroquinoneor benzoquinoneasanadditivesubstantially improved the efficiency of thePdCl2–Ce(OAc)3–PPNBr catalyst system.15,16 In

the palladium catalysedreactions,benzoquinone(BQ) is reportedto work as a redox catalyst forregenerationof reducedpalladium.In additionit isalsoknown that benzoquinonecanact asa ligandandcanpreventtheaggregationof palladium(0) bythe formation of Pd–BQ complexeswhich caneasily be oxidized to palladium(II) complexesbymetallic redox agents.25 However, addition of0.375mmol hydroquinonewith the Pd(S,S)com-plex led to the reductionof the TOF from 46.1 to28.4mol DPC (mol Pd)ÿ1hÿ1 (Table 1, Run 9).This behaviourindicatesthat a Pd(S,S)complexwhichcontainsaP–N–Pliganddoesnotrequireanyadditionalligandfor stabilizingits palladiummetal,and any additional ligand such as benzoquinonewill makepalladiumunavailablefor further reac-tion.

Althoughall the reportson oxidativecarbonyla-tion of phenolconfirm the needfor an ammoniumhalide for the reactionto progressefficiently, therole of this componentof catalyst systemis notclear.Furthermore,it hasbeenconfirmedfrom allthesereports that among the various ammoniumhalides,only ammoniumbromidesare useful forthis reaction.Ammoniumbromideis not a catalystfor the reaction,nor can it help in regenerationofpalladium.Recently,Vavasoriand Toniolo9 havesuggestedthat ammonium bromide works as astabilizerandpreventsundesirablepalladiummetalformation.If ammoniumbromideis actuallyactingasasurfactant,thenits concentrationin thereactionshouldbe aboveits critical micelle concentration.Wethereforeperformedsomeexperimentswith thePPNBr/Pdratio (mol molÿ1) varyingfrom 30to 80,to findasuitableconcentrationof PPNBr.Asshown

Table 1 Efficiency of Pd(S,S)catalystin oxidativecarbonylationof phenola

Runno. Pd.catalyst Redoxcatalyst Solvent HQ (mmol) DPCb (%)TOF (mol DPC(mol Pd)ÿ1 hÿ1) PSb (%)

1 PdCl2 Ce(OAc)3 CH2Cl2 — 4.96 21.3 0.092 Pd(S,S) Ce(OAc)3 CH2Cl2 — 3.57 15.2 0.073 Pd(S,S) Ce(OAc)3 — — 3.67 15.6 0.074 Pd(S,S) Ce(TMHD)4

a CH2Cl2 — 10.8 46.1 0.15 Pd(S,S) Ce(trop)4 CH2Cl2 — 8.3 35.4 0.086 Pd(S,S) Mn(TMHD)3 CH2Cl2 — 8.8 37.5 0.0987 Pd(S,S) Mn(trop)3 CH2Cl2 — 2.97 12.7 0.038 Pd(S,S) Co(TMHD)3 CH2Cl2 — 0.17 0.74 0.029 Pd(S,S) Ce(TMHD)4 CH2Cl2 0.375 6.6 28.4 0.07

a Reactionconditions:Pdcatalyst(0.012mmol) redoxcatalyst(0.075mmol), PPNBr(0.375mmol), CH2Cl2 (5 ml), 3 A molecularsieves(1 g), phenol(32mmol) CO (6.0MPa),O2 (0.3MPa),100°C, 3 h.b Yield basedon phenol.c TMHD, tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato).


CATALYTIC SYNTHESISOF DIPHENYL CARBONATE 631

in Table2 (Runs1–4),theDPCyield aswell astheTOF of the catalyst increasedsubstantially onraisingthePPNBr/Pdratio from 30 to 60, but withfurther increasesin the PPNBr/Pdratio to 80 nosignificantimprovementin catalyticactivity couldbe observed.Thus it can be assumedthat, for thecatalystsystemunderinvestigation,the amountofPPNBrrequiredto achievethebestresultsis 60–70timesof thatof thepalladium.

In further reactions,the concentrationof theoxidationcatalyst,Ce(TMHD)4 (TMHD is definedin Table 1) was optimized. The Ce(TMHD)4/Pdratio (mol molÿ1) wasvariedfrom 2 to 6, keepingthe PPNBr/Pd ratio constant at 60; the resultsobtainedaregivenin Table2 (Runs2, 5 and6). OnreducingtheCe/Pdratio from 6 to 2 theefficiencyof thecatalystsystemin termsof TOFwasreducedfrom 71.6 to 48.2mol DPC(mol Pd)ÿ1 hÿ1. Thistrendshowsthatmoreredoxcatalystis requiredforanefficient regenerationof palladium.

Furthermore,the temperaturedependenceof theefficiency of the catalyst systemwas studiedbyvaryingthereactiontemperaturefrom 80to 140°C.

It is evidentfrom the resultsgiven in Table3 thatmaximum TOF of 71.6mol DPC(mol Pd)ÿ1 hÿ1

andDPC yield of 16.8%wereobtainedat 100°C.Any further increasein temperatureled to sub-stantialdecreasesin theTOFof thecatalystaswellasin theDPCyield,presumablydueto degradation.

After various reaction parametershad beenoptimizedwith the Pd(S,S)catalyst,the efficiencyof thePd(Se,Se)catalystsystemwasinvestigatedatoptimum temperature,using optimum concentra-tions of Ce(TMHD)4 and PPNBr. The resultsshoweda TOF of 0.17mol DPC(mol Pd)ÿ1 hÿ1.At presentwehavenoexplanationfor suchadrasticdifferencein the efficiencyof thesetwo palladiumcomplexeswith sameligand but different donoratoms.

In summary,the Pd(S,S)complexwith P–N–PligandsandanS donorwasfoundto beanefficientcatalyst for oxidative carbonylationof phenol todiphenylcarbonate.ThePd(Se,Se)complex,on theother hand, did not show any activity for thisreaction. Efforts to understandthis substantialdifference in the efficiency of thesetwo similar

Table 2 Optimizationof PPNBr/PdandCe/Pdratiosfor oxidativecarbonylationof phenola

Runno.PPNBr/Pdratio

(mol molÿ1)Ce/Pdratiob

(mol molÿ1) DPCc (%)TOF (mol

DPC(mol Pdÿ1) hÿ1) PSc (%)

1 30 6 10.8 46.1 0.12 60 6 16.8 71.6 0.153 70 6 17.3 73.7 0.124 80 6 16.0 68.4 0.135 60 4 12.8 54.5 0.126 60 2 11.3 48.2 0.09

a Reactionconditions:Pd catalyst(0.012mmol), CH2Cl2 (5 ml), 3 A molecularsieves(1 g) phenol(32mmol) CO (6.0MPa), O2(0.3MPa),100°C, 3 h.b CeasCe(TMHD)4.c Yield basedon phenol.

Table 3 Temperaturedependenceof theefficiencyof thePd(S,S)complexfor oxidativecarbonylationof phenola

Runno. Temp.( °C) DPCa (%)TOF (mol DPC(mol Pdÿ1) hÿ1) PSa (%)

1 80 12.1 52.0 0.092 100 16.8 71.6 0.153 120 6.18 26.4 0.104 140 2.06 8.82 0.09

a Reactionconditions:Pd(S,S)(0.012mmol), Ce(TMHD)4 (0.15mmol) PPNBr(0.75mmol), CH2Cl2 (5 ml), 3 A molecularsieves(1 g), phenol(32mmol), CO (6.0MPa),O2 (0.3MPa),3 h.b Yield basedon phenol.


632 M. GOYAL ET AL.

complexesandto furtherimprovetheactivity of thePd(S,S)complexarecurrentlybeingmade.

Acknowledgements One of the authors (JN) thanks theAgency of Industrial Science and Technologyfor providingan AIST fellowship and the Grant Agency of the CzechRepublicfor grantsnos203/97/0955,203/98/0676and203/99/0067.

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novel usage of palladium complexes with p–n–p ligands as catalysts for diphenyl carbonate...

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