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The Ionic Palladium Porphyrin as a Highly Efficient

The Ionic Palladium Porphyrin as a Highly Ef?cient

and Recyclable Catalyst for Heck Reaction in Ionic Liquid Solution Under Aerobic Conditions

Qing-Xia Wan ?Ye Liu

Received:16September 2008/Accepted:11November 2008/Published online:26November 2008óSpringer Science+Business Media,LLC 2008

Abstract The ionic palladium porphyrin featured with pyridinium tags at four meso -positions,palladium tetrakis-(N -methyl-4-pyridinium)-porphyrin iodide ([Pd(II)TM-Py)P][I]4(3),embedded in the similar structured ionic liquid of N -butyl pyridinium tetra?uoroborate ([Bpy]BF 4)was found to be a highly ef?cient and recyclable catalytic system for the Heck cross-coupling of iodobenzene (derivatives)and ethyl acylate,with high turnover fre-quency of 56,000h -1under aerobic conditions.The active charge transfer transition occurred in 3-[Bpy]BF 4system during the Heck reaction could account for the ef?ciency in the catalytic coupling,which was observed in the UV–visible spectra.

Keywords Palladium porphyrin áHeck reactions áIonic liquids áUV–visible spectrophotometer

1Introduction

The Heck reaction,a palladium-catalyzed carbon–carbon bond formation between aryl halides and ole?ns,is one of the most important tools for the synthesis of elaborated styrene derivatives due to its wide variety of functional groups on both reactants [1,2].In the past decades,the considerable progresses have been made in palladium-catalyzed Heck reactions.[3–5]The development of homogeneous/heterogeneous catalysts led to a variety of different ligand systems,among which phosphine-

containing palladacycles [6–9],bulky and electron-rich monodentate phosphine [10],and carbene donors like N -heterocyclic carbenes (NHCs)which are strong r -donors with negligible p -accepting ability [11–15],are exempl-arily attractive owing to the high activities of the derived Pd catalysts.The N-containing ligands recently became preferable in considerations of non-toxicity and insensi-tivity to oxygen/moisture,having the advantage of performing the reactions under aerobic conditions;how-ever the rapid deactivation of the derived palladium catalysts in the recycling use was still a frustrated problem [16–19].Recently,the phosphine-free systems of the CNC-pincer palladium complexes have been found to be the highly ef?cient and recyclable catalysts for the Suzuki reaction [20].

Porphyrin free bases are important N -pincer ligands to which most of the metal elements in the periodic table can coordinate.Palladium porphyrins have found applications as luminescent markers [21],oxygen sensor [22],sensi-tizers for singlet oxygen formation [23],and photo-induced protein cross-linking agents [24],due to their high yield of intersystem crossing and long lifetime of the resulting triplet state in diverse media [25,26].Anyway,the examples of palladium porphyrins as phosphine-free cata-lysts in C–C coupling reactions were few,except for the ?rst case in the Suzuki reaction reported very recently [27].In contrast to the common N-ligand,the porphyrin base as a tetradentate pincer ligand can provide more stable Pd complexes suitable for Heck reactions and thereby avoid the use of exogenous ligands and high loadings of palla-dium.However,the conventional palladium porphyrins give poor solubility even in polar organic solvents (DMF,DMSO)limited their use in homogeneous catalysis,which is a highly ef?cient process but with disadvantage of dif-?cult separation from the catalyst with the reaction mixture

Q.-X.Wan áY.Liu (&)

Shanghai Key Laboratory of Green Chemistry and Chemical Processes,Chemistry Department,East China Normal University,200062Shanghai,China e-mail:yliu@http://www.wendangku.net/doc/333375d7240c844769eaee5e.html

Catal Lett (2009)128:487–492DOI 10.1007/s10562-008-9780-2

for reuse.In recent years,the use of ionic liquids is of much interest topic in homogeneous catalysis.Ionic liquids(ILs) can dissolve many organic compounds and metal com-plexes but they are in turn immiscible in some apolar organic solvents such as alkanes or ethers,able to develop a liquid system in which the reaction is carried out in the ionic liquid media,and the reaction products are removed by simple liquid–liquid extraction using alkanes or ether for the catalyst reuse.In order to increase the compatibility of the metal complexes(catalysts)with the IL and avoid the metal catalysts leaching out of the IL phase,the efforts have been made to enhance the ionopholicity of the metal catalysts through incorporating imidazolium or pyridinium tags into them[28,29].

Highlighted by the good coordinating ability of the porphyrin as a tetradentate ligand with characters of non-toxicity and insensitivity to moisture/oxygen,and the advantages of ILs as the ideal reaction media,an ionic palladium porphyrin featured with four pyridinium tags, palladium tetrakis-(N-methyl-4-pyridinium)-porphyrin iodide ([Pd(II)TMPy)P]][I]4(3),was synthesized and applied as the catalyst in the Heck reaction of ethyl acrylate with aryl iodides under aerobic conditions,along with the IL of N-butyl pyridinium tetra?uoroborate([Bpy]BF4)as the sol-vent in consideration of the matched ionophilicity and compatibility.The behavior of the palladium catalyst(3)in [Bpy]BF4was investigated by a UV–visible technique to elucidate the catalytic performance of3.To our knowl-edge,this is the?rst case to use the ionic palladium porphyrin in the IL solution to catalyze the Heck reaction. 2Experimental

2.1Materials and Methods

Pyrrole was puri?ed by re-distillation before use.Pyridine-4-carbaldehyde,methyl iodide(CH3I),and the other reagents/ solvents were of commercial grade and used as received.The 1H NMR(500MHz)spectra were recorded on a Bruker Avance500spectrometer.The IR spectra were recorded on a Nicolet NEXUS670spectrometer.The elemental analyses were performed by the Analytical Center in ECNU(Ele-mentar Vario EL III).GC analyses were performed on a SHIMADZU-2014chromatography equipped with Rtx-wax capillary column(30m90.25mm).GC-MS analyses were recorded on an Agilent6890instrument equipped with Agilent5973mass selective detector.

The UV–visible spectra were recorded on a SHIMA-DZU-UV2550spectrophotometer at ambient temperature (ca.25°C).The spectral resolution was about1nm.Upon completion,the left IL phase(0.05mL)containing3and the formed salt of Et3NáHI after extraction by diethyl ether was taken out and then diluted with2mL[Bpy]BF4(or other solvent)into a UV cuvette(3mL)by single mixing. The reference sample is the same as the solvent used for the test sample.

2.2Synthesis

2.2.1Palladium Tetrakis-(N-Methyl-4-Pyridinium)-

Porphyrin iodide([Pd(II)TMPy)P][I]4(3)

The neutral porphyrin free base of tetrakis(4-pyridyl)-por-phyrin([H2TPyP],1)was prepared according to the reported work[30].The ionic porphyrin free base of tetrakis(N-methyl-4-pyridinium)-porphyrin iodide([H2TMPyP][I]4,2) was prepared according to our reported work[31].The obtained2(0.5mmol)and PdCl2(0.6mmol)was added into 20mL deionized water.The resultant mixture was re?uxed until the absorbance peak at425nm(Soret band of2)blue-shifted to416nm(Soret band of3)without any change afterwards.The obtained black solids were washed with hot water thoroughly and ethanol to give3(Scheme1)after dryness in vacuo,with yield of89mol%.1H NMR (500MHz,DMSO-d6,ppm):d H9.5(d,J=5Hz,8H, pyridinium,C H N?C H),9.1(s,8H,pyrrolyl,b-H),9.0 (d,J=5,8H,Hz,pyridinium,C H CC H).UV–vis(3in [Bpy]BF4):k max=416(s,Soret band),525(w,Q band), 560(Q band)nm.

2.3General Procedures for the Heck Reaction

and Recycling of the Catalyst

In a typical example,a solution of iodobenzene(5mmol) with ethyl acrylate(6.0mmol)in[Bpy]BF4(2mL)was mixed with3and triethylamine(Et3N,7.5mmol).The mixture sealed in the glass vials reacted at100°C for1h on an Advantage Series TM2410Personal Screening

The Ionic Palladium Porphyrin as a Highly Efficient

488Q.-X.Wan,Y.Liu

Synthesizer(Argonaut Technologies Inc.).Upon comple-tion,the reaction mixture was cooled to room temperature and then extracted with diethyl ether(2.5mL93).The ether fractions were combined,and then analyzed by GC to determine the conversions(1-dodecane as internal stan-dard)and the selectivities(normalization method).The structures of obtained products were further con?rmed by GC-Mass.The remaining IL phase containing3and the formed salt of Et3NáHI was dried in vacuo at room tem-perature and then used directly without further treatment for the next run.Due to the stoichiometric consumption of the base,Et3N(7.5mmol)was added additionally per pass.

All manipulations were conducted in air.

3Results and Discussion

In UV-visible spectra(Fig.1),the characteristic Soret bands of the free base2is observed at425nm along with4 Q-bands at517,550,590,644nm.As for the palladium porphyrin3,its Soret band blue-shifts to416nm.It was also noted that after Pd insertion,the spectrum pattern changes from a four Q-band spectrum,indicating D2h symmetry for2,to a two Q-band spectrum(525,560nm), indicating D4h symmetry,which is characteristic of por-phyrin coordinated to a metal ion through the four N-heteronuclei[32].The1H NMR spectra of2and3are nearly the same except for the high-?eld resonance signal at-3.08ppm for N–H units in2.

3.1Catalytic Performance of3in Heck Reaction

To evaluate the catalytic performance of3in Heck reac-tion,the coupling reaction of iodobenzene with ethyl acrylate has been chosen as a model reaction.The investigations on the solvents([Bmim]BF4,[Bmim]PF6, [Bpy]BF4,DMF)indicated that the activity of the fresh3 was mostly favored by using[Bpy]BF4as a solvent.Under the mild reaction conditions(100°C,1h),the coupling product of trans-ethyl cinnamate was obtained in excellent yield(100%,Table1,No.1).As shown in Table2,even at the concentration of0.00125mol%,70%yield of the coupling product was obtained with high TOF of 56,000h-1in3-[Bpy]BF4system(No.5).Anyway,com-pared to PdCl2,the mixture of the equivalent PdCl2and porphyrin free base2(or1)did not show obviously improved effect on the catalytic activity,implying that the palladium porphyrin complexes derived from the ligation of Pd(II)ion to the porphyrin free bases could not be formed in situ under the conditions of the coupling reac-tions in Table2.Especially,for the mixture of PdCl2-1, it was impossible to insert Pd(II)ion into1even after re?uxing in PhCN for48h.

The recovery and reuse of catalysts are important in the practice of Heck reactions from an economic point of view

The Ionic Palladium Porphyrin as a Highly Efficient

Table1Effects of the solvents on the catalytic performance of3for

the Heck reaction

No.Sol.Conv.

(%)

Sel.

(%)a

1[BPy]BF4100100

2[BMIM]PF696100

3[BMIM]BF474100

4DMF94100

30.02mol%,Et3N7.5mmol;PhI5mmol;ethyl acrylate6mmol;

solvent2mL;reaction time1h,reaction temperature100°C

a To trans-ethyl cinnamate

Table2Comparison of the catalytic performance of3,PdCl2-2,and

PdCl2-1for the Heck reaction

No.Cat.Conc.of Pd

(mol%)

Conv.

(%)

Sel.

(%)a

TOF

(h-1)

130.020********,000

230.010*********,000

330.00509810019,600

430.00259410037,200

530.001257010056,000

6PdCl2-2b0.00505810013,600

7PdCl2-1b0.00505710012,000

8PdCl20.00505410012,200

PhI5mmol,ethyl acrylate6mmol,Et3N7.5mmol,[Bpy]BF42mL,

temperature100°C,time1h

a To trans-ethyl cinnamate

b Free base2(or1)and PdCl

2

were mixed and used directly for the

reaction

The Ionic Palladium Porphyrin as a Highly Ef?cient and Recyclable Catalyst489

due to high cost of palladium compounds and ligands,as well as for preventing contamination of?nal products by toxic metals.The recycling uses of3in[Bpy]BF4were examined in Table3.At concentration of0.0025mol%,3 exhibited very good activity and stability even after7runs without any precipitation of Pd black.Anyway,the cumulative slurry ammonium salts(Et3NáHI)showed degraded impacted on the activity of3,mainly due to the mass transfer limitation.In each run,the pure product of ethyl cinnamate was obtained in good yield by extracting with diethyl ether and concentrating under vacuum.The ICP(inductive coupled plasma emission spectrometer) analysis indicated that the leaching of Pd into organic phase was below the detection limit(\0.1l g/g).Due to the porphyrin ligand2is non-sensitive to oxygen and moisture,all the manipulations for performing the reaction, separating the product from the IL phase,and recycling the IL mixture,could be handled without N2protection,which facilitated the workup greatly.

3.2UV–Visible Analysis of3in the Different Solvents

after Used in the Heck Reaction

In order to understand the behaviors of3in different sol-vents in Table1,the resultant mixtures after performing the reaction were analyzed by the UV–visible spectro-photometer.Figure2showed that,in the solvents of [Bmim]BF4,[Bmim]PF4,or DMF,the characteristic absorbance of3was observed unexceptionally.Surpris-ingly in[Bpy]BF4,the typical Soret band at ca.416nm of 3was completely overwhelmed by an intensive absorbance at ca.350–450nm.

Highlighted by the appearance of the intensive absor-bance at ca.350–450nm,the detailed investigations on 3-[Bpy]BF4system for the Heck reaction was carried out as shown in http://www.wendangku.net/doc/333375d7240c844769eaee5e.htmlparatively,the fresh3in[Bpy]BF4, without performing the Heck reaction,exhibited the typical Soret band and Q bands(line1).Even when3in[Bpy]BF4

was treated with ethyl acrylate and Et3N at100°C for1h, its characteristic Soret and Q bands were observed nor-mally(line2),but with the slight red-shift Soret band compared to that in line1.Only when3in[Bpy]BF4was treated with PhI,Et3N,and ethyl acrylate at100°C for1h to proceed the Heck reaction,did the intensive absorbance at350–450nm appear,which completely overlaid the characteristic Soret band of3similar to line4in Fig.2.At the?rst sight of such intensive absorbance,it was supposed that a new palladium complex probably was formed derived from3-[Bpy]BF4system during the Heck reaction. However,the further analysis on3-[Bpy]BF4system

Table3The recycling use of3for the Heck reaction in[Bpy]BF4

Run1234567

Conv./Sel.a94/10098/10094/9990/9987/10084/9981/99

30.0025mol%(0.125l mol),PhI5mmol;ethyl acrylate6mmol,

Et3N7.5mmol,[Bpy]BF42mL,reaction time1h,temperature

100°C

a To trans-ethyl cinnamate

The Ionic Palladium Porphyrin as a Highly Efficient

The Ionic Palladium Porphyrin as a Highly Efficient

490Q.-X.Wan,Y.Liu

indicated that the intensive absorbance at ca.350–450nm could not be ascribed to a new palladium complex,but still responsible to the palladium porphyrin3or its derivative. Firstly,the characteristic spectrum pattern of the two Q-bands at525and560nm revealed that the palladium porphyrin3remained intact after the?rst use in the Heck reaction in[Bpy]BF4(line3).Secondly,when water was used to dilute the mixture of3-[Bpy]BF4corresponding to line3in Fig.3,the intensive absorbance at350–450nm was subsided,and then the Soret band of3at418nm was observed clearly(line4),con?rmatively indicating the intactness of3after the Heck reaction.Similarly,even after the7th use of3in[Bpy]BF4,the typical Q bands of3were clearly observed(line5),but with indication of more intensive band at350–450nm,from which the Soret peak (420nm)could be released again after treated with water (line6).

As can been seen in Figs.2and3,besides the typical Pd-porphyrin Soret and Q absorbances,3dissolved in [Bpy]BF4after performing the Heck reaction shows an unique strong and broad absorbance band in the region of 350–450nm,which was tentatively contributed to a charge transfer(CT)transition originated from the interaction between the palladium porphyrin and[Bpy]BF4according to the reported work[33–39].Although the CT transition had been universally observed in metalloporphyrin cata-

lyzed oxidations and metal-ligand coordination compounds [38,39],it was?rstly observed in palladium porphyrin catalyzed Heck reaction herein.The formation of this low-lying and long-lived CT complex could be dramatically affected by‘‘environmental tuning’’[37]like the polar solvent of[Bpy]BF4,due to the strong cation-p/p-p inter-action[40,41],which was favored by the structural similarity and preferential symmetry between[Bpy]?cat-ions and3.The active electron transfer between the palladium porphyrin and[Bpy]BF4through orbital mixing resulted in the corresponding CT transition[37].Whereas, with replacement of[Bpy]BF4by[Bmim]PF6,[Bmim]BF4 or MDF as the solvent of3,the CT transition with intensive absorbance at ca.350–450nm could not be observed (Fig.2,line2).Anyway,the built-up cation-p/p-p inter-action between[Bpy]BF4and3could be collapsed by the intrusion of the third water molecule[42],and then the corresponding intensive CT transition was dramatically suppressed due to the destruction of the CT complex,as shown in Fig.3(lines4,6)after treatment by water.

As we know,the essence of the redox process is the intermolecular or intramolecular electron transfer.The essence of CT transition is characterized by the electron transfer[37].From this point of view,the active CT tran-sition occurred in the system of3-[Bpy]BF4during the Heck reaction is correlated to the rapid redox process happened on the palladium porphyrin[Pd(II)$Pd(0)]with the super-conjugated and electron-reservoir characters, which could account for the catalytic ef?ciency for the Heck reaction[43].

3.3Generality of3-[Bpy]BF4for Heck Reaction

The scope of the Heck reaction was investigated on dif-ferent iodobenzene derivatives with different electronic and steric effects.It was indicated that,due to the high reactivity of the aryl iodides,the trans-cross-coupling products were all obtained in high yields(86–100%)under mild conditions(100°C,2h)without obvious discrimi-nation of the electronic nature of the substituents.The steric effect could only be observed when the o-substituting group involved(Nos.3,6,12).However,with the activated o-nitroiodobenzene no reaction took place(No.9).As to the activation of aryl bromides,no acceptable conversions were obtained under low concentration of3and the mild conditions(100°C,2h)(Table4).

4Conclusions

In conclusion,the Heck reaction of iodobenezen(deriva-tives)and ethyl acrylate could be successfully catalyzed by ionic palladium porphyrin3feature with four pyridinium tags,along with the similar structured IL of[Bpy]BF4as Table4The generality of3for different iodobenzene derivatives in the Heck reaction

O

O

OEt

I

R

OEt

R

.

++HI

Et3N

No.R Conv.(%)Sel.(%)a 1p-CH310099

2m-CH38799

3o-CH340100

4p-OCH3100100

5m-OCH3100100

6o-OCH371100

7p-NO2100100

8m-NO2100100

9o-NO2––

10p-CF310099

11m-CF3100100

12o-CF343100 30.0025mol%;substrate5mmol;Et3N7.5mmol;ethyl acrylate 6mmol;[Bpy]BF42mL;temperature100°C,reaction time2h

a To trans-coupling product which was con?rmed by GC-Mass analysis

The Ionic Palladium Porphyrin as a Highly Ef?cient and Recyclable Catalyst491

the solvent under aerobic conditions.The combination of ionic porphyrin3with the IL of[Bpy]BF4not only gave rise to the high activity,but also endow the homogeneous catalyst3with recyclability.The formation of the CT complex in3-[Bpy]BF4system during the Heck reaction was favorably allowed due to the compatible ionopholicity, symmetry,and the strong cation-p/p-p interaction,which could account for the high catalytic ef?ciency,and was ?rstly observed in this study.

Acknowledgments The research was?nancially supported by the National Natural Science Foundation of China(No.20533010, 20590366,20673039),the Science&Technology Commission of Shanghai Municipality(06JC14023,06SR07101),and Shanghai Leading Academic Discipline Project(B409).

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