Journal of Molecular Catalysis, 45 (1988) 127 - 142 127

SYNTHESES AND CHARA~E~ZA~ON OF MERCAPTO- HYDROXYL-PALLADIUM MACROMOLECULAR CHELATES AND THEIR CATALYTIC PROPERTIES

YUAN WANG and HANFAN LIU*

Institute of Chemistry, Academia Sinica, Beijing, 100080 (China)

(Received July 21, 1987; accepted October 14,1987)

An unusual macromolecular mercapto-hydroxyl bidentate ligand has been synthesized by the reaction of epoxy-phenolic oligomer with hydrogen sulfide. The palladium complexes are incorporated into the macromolecular ligand. This catalyst precursor was characterized by means of IR, XPS, UV-vis and elemental analysis.

The macromolecular palladium chelates were absorbed onto a silica support. The catalytic behaviour of the silica-supported palladium complexes was investigated by the Heck reaction. The activation energy, 67 kJ mol-i, is different from that of the same reaction catalyzed homogeneously by palladium acetate. The activity of the chelates depends on the molar ratio of sulfur to p~adium; thus, an active catalyst is obtained at a S:Pd ratio of 2 and the catalyst is inactive at ratios of 6 or greater. The catalyst has a better stability than the other polymeric palladium catalysts reported in the literature.

A study was made of the loss of palladium from the supported macro- molecule under reaction conditions. The results showed that the elution of palladium could not be explained on the basis of coordination equilibrium. A mechanism for metal elution is proposed accordingly.

Polymer-immobilized transition metal complex catalysts have been the subject of attention over the past one or two decades. A number of excellent reviews [ 1 - 51 and monographs [6, 73 are available in which the achieve- ments and limitations in this field have been discussed. The purpose of generating such a hybrid catalyst is to combine the advantages of homo- geneous and heterogeneous catalysts, while eliminating the disadvantages of each system. Thus, a suitable heterogeneous catalyst which can be easily

*Author to whom correspondence should be addressed.

0304-5102/88/$3.50 @ Elsevier S~uoia~~inted in The Netherlands

128

separated from the reaction products would have similar activity and equi- valent or better selectivity than its homogeneous counterpart. To date, immobilization attempts have met with only limited success, mainly due to the problems caused by metal elution [ 4, 51. Consequently, research on the stability of polymer-bound catalysts is a very important endeavor.

Deutch et al. pointed out that mercapto compounds have an extremely strong coordination ability to the platinum metals [8]. We used mercapto- containing polymers as ligands to improve the stability of polymer-bound platinum group metal complexes, and have achieved several positive results [9]. However, when the reactions were carried out at higher temperatures and coordinating reactants such as amines were present in solution, serious palladium loss occurred in each batch manipulation [lo]. Metal elution can be reduced by using chelating ligands. But chelating ligands often cause sharp changes in reactivity and selectivity, when they are introduced into homogeneous catalyst systems [4]. An alternative approach to solving this difficulty is to utilize metal complexes with unsymmetrical bidentate lig- ands, in which one l&and is significantly more labile than the other [ 111. In our study, the macromolecular mercapto-hydroxyl ligand was prepared and was used as a support for immobilizing the palladium complexes. The mercapto end of the chelate acts as a stronger ligand, which binds the metal to the support and will not dissociate during the catalytic reaction, while the weakly coordinated hydroxyl end will readily be substituted by an incoming substrate and act as a good leaving group. We thought that utilization of such an unsymmetrical chelating ligand should prevent metal leaching during a catalytic cycle due to the chelate effect [ 121.

Since the Heck reaction is an important reaction, in which a vinylic hydrogen is substituted by the aryl group of aryl halides [13] or aroyl halides [14] under severe conditions, such as in the presence of a great amount of amines and at high temperatures, the Heck-type reaction seems to be a good model system to test our idea on the application of these un- symmetrical ligands to the polymer-bound catalysts. Although Kaneda and co-workers [ 151 and Andersson et al. [ 161 using phosphinated polystyrene/ palladium catalyst, and Daly and Sun [17] using polyvinylpyridine/pal- ladium catalyst studied the Heck arylation, the stabilities of these catalysts were not satisfactory. We wish to report herein our experimental results.

Experimental

Materials and equipment AlI reagents and solvents were of analytical grade. They were used as

purchased, except the aroyl chloride which was distilled under reduced pressure prior to use. Palladium tetrakis(triphenylphosphine) was prepared according to a reported method [ 181.

The sulphur, carbon, hydrogen and oxygen contents of the samples were determined in the Laboratory of Microorganic Analysis in our institute.

129

The palladium content was analyzed by the atomic absorption method. The epoxy-value of the starting oligomer was determined by the methyl ethyl ketone method. The molecular weight of the samples was measured by GPC.

The IR, UV-vis and XPS spectra were recorded on Bruker IFS 113V Hitachi 340 and Kratos AEI ES-300 instruments, respectively. Analytical GLC was performed with a Shang-Fen Model 103 gas chromatograph using a 2M OV-17 column.

Preparation of mercapto-hydroxyl-containing macromolecular ligand To a 100 ml autoclave containing a magnetic stirrer, was charged a

solution of 2.7 g epoxy-phenolic oligomer in 70 ml methanol. The autoclave was cooled to -78 “C, and a slow stream of hydrogen sulfide was bubbled through the stirred solution. After 2 h the autoclave was closed, and the hydrogen sulfide-saturated solution was then heated to 50 - 60 “C with agitation for 24 h. The solvent methanol was distilled off under reduced pressure. The macromolecular ligand was isolated by precipitation in water and was reprecipitated in ether from the dioxane solution three times, to remove low molecular weight impurities. After drying at room temperature under vacuum, a white powder was obtained (designated Pr-SH).

Preparation of macromolecular palladium chelates To a solution of 0.45 g mercapto-hydroxyl ligand (Pr-SH) in 100 ml

dioxane in a 250 ml flask containing a magnetic bar, 95.8 mg palladium acetate in 30 ml dioxane was added dropwise. The final solution was stirred for 24 h at room temperature. Within this period of time the color of the solution gradually deepened. Addition of 100 ml ether to it gave a yellow precipitate. The product (designated Pr-S-Pd-A?) was collected by filtra- tion, washed with ether several times and dried under vacuum at room tem- perature.

In order to increase the surface area of the catalyst, an appropriate amount of silica (0.82 g) was added to the solution prior to the precipitation of the chelate. A silica-supported macromolecular chelate catalyst (Si-S- Pd-AZ) resulted.

Catalytic reaction A typical procedure was as follows:

Iodobenzene as reactant A 50 ml three-neck round-bottom flask was equipped with a nitrogen

inlet, a thermometer, a reflux condenser and a magnetic stirring bar. Poly- meric chelate catalyst (Si-S-Pd--A*, 0.3 g, containing 0.09 mg-atom Pd), iodobenzene (1.2 ml, 10.6 mmol), tributylamine (2.2 ml, 9.25 mmol), ethyl acrylate (1 ml, 9.22 mmol), n-hexadecane (0.5 ml, as an internal standard for GLC determination) and dioxane (20 ml) were added to the flask. After stirring at 80 “C under nitrogen for 4 h, the mixture was cooled and the product cinnamate was determined by GLC. Then the catalyst was

130

separated by centrifugation, washed with dioxane, ether and dried at room temperature. The recovered catalyst was reused for another run.

Benzoyl chloride as reactant The reaction proceeded as above, except that benzoyl chloride and

xylene were used in place of iodobenzene and dioxane, and the reaction. conditions were 140 “C, 2 h instead of 80 “C, 4 h.

Results and discussion

Preparation and characterization of the mercapto-hydroxyl-containing macromolecular bidentate ligand and its palladium chelates

The macromolecular bidentate &and was prepared as described by Woodward [19]. The results of elemental analysis of sulfur, IR and M.W. for the starting material and resulting &and are listed in Table 1.

From the data in Table 1, it is proposed that the preparation of the bidentate ligand occurs as in the following equation:

(Pr--o) ( PI-SH)

The mercapto-hydroxyl-palladium chelates were synthesized by ligand exchange reaction with palladium tetrakis(triphenylphosphine) or palladium acetate, followed by supporting on silica. By reacting the palladium com- pounds with different amounts of the macromolecular ligand, a series of catalysts with varying S/Pd ratio were obtained.

The possible structure of these resultant chelates will be discussed later, based on the spectral data together with other physical properties. Table 2 summarizes the XPS data for the macromolecular palladium chelates and other relevant compounds, It can be seen from Table 2 that there are distinct differences in the Pd 3d 5/2 binding energies between the macro- molecular chelates and the small molecular compounds. The value of Pd 3d 5/2 in Pr-S-Pd-B which is derived from the starting material Pd(PPh& is 1.7 eV higher than that of Pd(PPh&. (It is within the range for the palla- dium(I1) compounds.) Furthermore, there is no signal for P 2p in the XPS spectrum of the macromolecular chelate Pi-S-Pd-B. This implies that all PPh3 in Pd(PPh,), are replaced by the macromolecular ligand, and the palladium exists in a +2 oxidation state. The Pd 3d 5/2 in Pr-S-Pd-A is about 1.0 eV lower than that of Pd(OAc),, which is expected because the electron density on sulfur is shifted to the palladium atom during the forma-

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132

TABLE 2

KPS data for macromolecular chelates and other relevant compounds

Compound S/Pd (molar ratio)

Binding energya (eV)

Pd 3d 512 S 2P P 2P

Pd (metal) Pd(PPh& Pd( OAc)* Pr-SH Pr-S-Pd-B Pr-S-Pd--A6 Pr-S-Pd-A2 RSH RSR RSK RSNa

8

6 2

335.5 335.6 338.7

331.5

337.1 337.5

163.8 163.4

163.4 163.0 163.7c 163.7e 161.3c 161.2c

131.0

_b

aThe binding energy values are referred to C 1s (285.0 eV). bNo P 2p signal is observed. CFrom [ 211.

tion of the mercaptide, and increases the covalent character of the palla- dium-sulfur bond [20]. As shown in Table 2, the binding energy of S 2p in the macromolecular ligand Pr-SH is comparable to those of the small molecular analogs RSH or RSR reported in the literature [21]. The S 2p binding energy in Pr-S-Pd-A and Pr-S-Pd-B are about 0.6 eV lower than that of Pr-SH, but about 2.1 eV higher than those observed for alkali mercaptides. These results can be rationalized since the transition metal mercaptides tend to be much more covalent than the corresponding alkali compounds.

The infrared spectrum of the macromolecular ligand contained a characteristic vibration at 2560 cm-’ due to the Y(S-H) absorption, which disappeared as the macromolecular chelate formed. According to the results of XPS and IR, it can be concluded that the macromolecular palladium mercaptide complexes form as they occur in the small molecular mercapto- ligand system reported in the literature [22 - 251.

Although the Pd 3d 5/2 and S 2p binding energies in the XPS spectra of the chelates are independent of the variation of S/Pd molar ratio, they behave differently with respect to their solubilities in organic solvents and their catalytic activities for the Heck reaction (see Table 3).

As the sulfur/palladium ratios decrease (S/Pd S 2), insoluble products result, while with the higher sulfur/palladium ratios (S/Pd > 3), the products can easily be dissolved in dioxane or THF. For example, chelate Pr-S-Pd- A, (with S/Pd = 6) is soluble in dioxane. Figure 1 shows the UV-vis spec- trum of Pr-S-Pd-A, (curve a) together with those of Pd(OAc)2 (curve b) and Pd(OAc),-(EtO),Si(CH,),SH system (curve c). As seen in Fig. 1, spectrum of Pd(OAc), shows no absorption band in the region of 320 - 500

133

TABLE 3

Properties of the macromolecular palladium chelates

Chelate S/Pd Dissolve in Activity* (molar ratio) dioxane or THF (% yield of cinnamate)

Pr-S-Pd-A6 6 yes Ob PrS-Pd-_A4 4 yes 5c PrS-Pd-As 3 slightly 10 Pr-S-Pd-A2 2 no 30

*Experimental conditions: palladium content, 0.022 mg-atom Pd; temperature, 80 “C; reaction time, 3 h. bReaction time, 48 h. CReaction time, 8 h.

I

0.4 -

g Ii9 8

0.2 c

3

0.0 I \ l

300 400 500 WAVELENGTH, NM

Fig. 1. UV-vis spectra of the macromolecular chelate and relevant compounds in dioxane solution; (a) Pr-S-Pd-As, ,1.57 X 10-l mg atom Pd 1-l; (b) Pd(OAc)z, 6.25 x 10-l mg atom Pd 1-l; (c) Pd(OAc)2-(EtO)sSi(CH&SH system, 0.40 x 10-l mg atom Pd 1-l; (d) PI-SH, 0.83 g 1-l.

nm except a slanting background. However, if a definite amount of mercapto ligand such as (EtO)$i(CH&SH is introduced into the Pd(OAc)z solution, two new bands appear, one at 326 m-n and the other at 390 nm, with about equal intensity. These are due to the mercapto ligand exchanged for the acetate ion.

134

By comparison of the spectrum of Pr-S-Pd-A, with the spectrum of the Pd(OAc),-(EtO),Si(CH,),SH system, a rather close resemblance is observed, i.e. both have absorption bands at the same positions. This implies that the mercapto group in the macromolecular ligand complexed with palladium ion is similar to the small molecular mercapto ligand. However, the absorption of the two peaks at 326 nm and 390 nm were much weaker, and the spectrum background varied considerably in the case of the macro- molecular system. This change in the UV-vis spectra might be due to the presence of hydroxyl groups in the macromolecular ligand, which might act as weak coordinating groups, affording the weaker interaction with the palladium ion, and therefore changing the d-orbital splitting of the central atom. It is easy to imagine that the hydroxyl group located at the /3 position to the mercapto group will respond most to this action. This fact can also account for the chelate effect causing the enhanced stability of a complex system.

From the results of elemental analysis, M.W. measurement, XPS, IR and UV-vis spectrometries, together with the other properties of the chel- ates, it can be deduced that two different species, viz. (Fig. 2), are formed. For higher ratios of sulfur to palladium (S/Pd > 3), the linear complex (I) was dominant, in which rz is a small integer, and which could be dissolved in common organic solvents. As excess mercapto groups are present around the palladium atom, they will block the coordination site and consequently lead to poor catalytic activity for the Heck reaction. At lower ratios of sulfur to palladium (S/Pd < 2), however, the crosslinked complex (II) was the dominant form. According to the results of elemental analysis and M.W. determination of PrSH, two kinds of sulfur atoms are present in the ligand molecule. One is mercapto sulfur, which constitutes 2/3 of the total

HO SH

Y S

A 7 FH Pd

HC&

Y S I n

A s-pd+ .0&-CH3

S H

HS OH

(1) (11) For the sake of clarity we neglect the hydrocarbon skeleton and designate as the macromolecular ligand molecule.

Fig. 2. Proposed structure of the macromolecular chelates.

135

sulfur content, and the other is thioether, comprising l/3 of the total sulfur. When the mercapto groups surrounding the palladium ion decrease, this offers the possibility for the .weaker thioether ligand to complex with the palladium ion. The palladium ion acts as a crosslink and results in the forma- tion of species (II). Apparently its insolubility is due to the crosslinked structure, and its high activity results from the weakly bonded thioether which is readily replaced by an incoming substrate molecule during the catalytic reaction.

It should be mentioned that although important information is ob- tained from the IR determination of v(S-H) and XPS measurement of S 2p, it is impossible using the same methods to determine if the hydroxyl group near the mercapto group has coordinated with palladium, since many phenol hydroxyl groups are present in the macromolecular ligand molecule. We have attempted to use laser Raman spectroscopy to characterize its structure, but failed because of interference of the strong scattering fluores- cence.

Catalytic reaction

Iodo benzene as reactant The Heck reaction in this case was carried out as follows:

Pd catalyst C,H,-I + CH2=CH-COOEt -

Base C,H,-CH=CH-COOEt (2)

Since the silica-supported chelate Si-S-Pd-A2 with a S/Pd ratio of 2 shows higher activity, it is used throughout the study with tri-n-butylamine as the base and dioxane as solvent. The conversion curves at different tem- peratures are plotted in Fig. 3.

From the results obtained, a general pattern can be seen: after an induc- tion period, the rate of appearance of the product is nearly constant until

0 20 40 60 130 100

AMCTION TIME (MIN)

Fig. 3. Conversion as a function of time for the arylation of ethyl acrylate with iodo- benzene at different temperatures: l 80 “C; 0 90 “C; x 100 “C.

136

-70% conversion, after which it declines. The induction period becomes shorter as the temperature increases. It is generally believed that the Heck reaction involves the oxidative addition of an aryl halide to a palladium(O) complex [ 131. Thus, when a palladium(I1) species is used as a precatalyst, initial reduction of palladium(I1) is a prerequisite for the arylation to take place. The generation of such active palladium(O) species is reflected in the presence of the induction period. The reducibility of the chelate increases with increasing temperature, this being consistent with the fact that the in- duction period becomes shorter at higher temperature.

Furthermore, when the recovered catalyst was used again in the aryla- tion reaction, the induction period disappeared. XPS measurement of the catalysts also revealed that palladium(O) was formed during the reaction. This corresponded to a change in the color of the chelate from yellow to greyish-yellow. The binding energy of Pd 3d 5/2 became lower in the case of reused catalysts, even though some palIadium(I1) still remained* (see Table 4).

The most likely mechanism for the activation of palladium species is via olefin reduction [13]. The immobilization of macromolecular ligand prevents the palladium(O) species from agglomerating to form larger metallic particles. Hence the catalyst maintains good activity and stability after recycle (see Table 5). The turnover number was determined to be more than 13 000 mmol (mg-atom Pd))‘. The rate constants of the chelate-catalyzed

TABLE 4

XPS data for used catalysts

Catalyst S/Pd (molar ratio)

Binding energy* (eV)

Pd 3d 512

Pd( OAc)? 338.7 Pr-S-Pd-AZ 2 337.5 Si-S-Pd-A2 2 337.6 Si-S-Pd-AZ-lb 2 337.1 Si-S-Pd-A2-3c 2 336.9

*The binding energy values are referred to C 1s (285.0 eV). bRecovered from catalyst Si-S-Pd-AZ. CCatalyst Si-S-Pd-A2 after reusing 3 times.

*In fact, comparing the spectrum of the catalyst (Si-S-Pd-A2) with that of the recovered catalyst (Si-S-Pd-AZ-l), it is seen that the hand maximum has moved 0.5 eV toward lower binding energy; in addition, the Pd 3d 512 hand of Si-S-Pd-AZ-l now is skewed, indicating a peak around 336.2 eV. Upon reusing the catalyst three times (Si-S-Pd-As-s), the peak component at 336.2 eV increases in intensity and the hand maximum moves 0.2 eV further toward lower binding energy. The low energy com- ponent can be assigned to palladium oxide, which is caused by air oxidation of the palladium metal phase in the catalyst, as verified by Andersson and Larsson [ 261.

137

TABLE 5

Catalytic properties of the macromolecular chelate for the Heck reaction

Catalyst

Pd( OAc)* Si-S-Pd-A2

Pd content (mg-atom)

0.09 0.09

Yield of cinnamate (%) Time Temperature (h) (“C)

Run 1 Run 2 Run 3

80 4 86 80 79 78 4 86

2.65 2.70 2.75 2.80 2.85

l/TX ( 1O-3 ) K-’

Fig. 4. Arrhenius plot for the arylation reaction, where Ink = In A - (EJRT), E, = 6.7 x

lo4 J mol-l.

arylation were obtained from the slope of the linear part of the conversion curve at different temperatures. Then the apparent activation energy was deduced from an Arrhenius plot (Fig. 4).

Benzoyl chloride as the reactant When the arylation is carried out using aroyl halide instead of aryl

halide, decarbonylation of the aroyl halide occurs:

C,H,COCl + CH,=CH-COOEt e C6H,--CH=CH-COOEt + CO (3)

This can be regarded as an extension of the Heck reaction. The decar- bonyl-arylation proceeds as described in earlier, except that it must be carried out under much more severe conditions. As reported in our previous paper [lo], a variety of polymer-bound species, such as polymeric amine-, phosphine-, pyrrolidone-, nitrile and quinoxaline-palladium complexes, have been tested as catalysts. These all easily lost metal and decreased in activity during the decarbonyl-arylation. Although monodentate mercapto- palladium polymeric complexes show better stability than the others, the

138

TABLE 6

Activity and reuse of the catalysts

Catalyst Pd Yield of cinnamate (%) Time Temp. (mg atom) (h) (“C)

Run 1 Run 2 Run 3 Run 4 Run 6

Pd( OAc)* 0.04 80 2 130 Pd(PPh& 0.04 71 2 130 P)-CN-Pd( OAc),” 0.04 62 0 2 130 P)-NCO-Pd( OAC)~* 0.04 66 0 2 130 SiOz)-NH,-Pd( OAc)za 0.04 73 7.5 2 130 SiO+PPh,-Pd(PPh3)aa 0.04 52 11 2 130 SiO+PPQ-Pd(OAc)za 0.04 75 12 2 130 SiO+PPQ-PdClza 0.04 83 6 2 130 SiO+S-Pd( OAc)za 0.04 75 54 2 130 SiO+S-PdClza 0.04 69 39 2 130 SiOz )-S-Pd(OAc)~8 0.04 75 33 16 11 1 130 Si-S-Pd-Azb 0.036 71 68 67 2 140 Si-S-Pd-A2b 0.062 96 95 79 77 61 2 140

aFrom [lo]. Palladium complexes of polymers bearing nitrile, P)-CN-Pd(OAc)z; pyr- rolidone, P)-NCO-Pd( OAc),; amine, SiOa)-NH*-Pd( OAc)z; phosphine, SiO+PPhz- Pd(PPhs),; quinoxaline, SiO+PPQ-Pd(OAc)z, SiO+PPQ-PdClz; monodentate mer- capto group, SiOz)-S-Pd( OAc)z, SiO+S-PdCl2. bMacromolecular mercaptohydroxyl-palladium chelate.

recycling ability of this catalyst needs to be improved. For comparison, some relevant data are listed in Table 6, from which it is obvious that a marked improvement in catalyst stability can be made by using the mer- capto-hydroxyl bidentate ligand.

Leaching of palladium from the catalyst In order to prevent metal leaching from the catalyst, a series of sub-

strate combination experiments were performed. The palladium concentra- tions in solution under the experimental conditions were determined by atomic absorption spectrometry. The results are summarized in Table 7. The data show that the loss of palladium is not serious except in the simul- taneous presence of all three substrates, and that the catalyst is in solution (entries 1 - 4). This means that the metal leaching is caused by the coopera- tive action of several substrates (Phi + CH2=CH-COOEt + NBus) with the catalyst, other than the simple coordination equilibrium between polymeric ligand and soluble small molecular ligand amine. From the results obtained, metal leaching is proposed to occur as shown in Scheme 1.

In step 1, the catalyst precursor is reduced to a palladium(O) species by acrylate. This represents the activation of the catalyst during the induc- tion period. Because the stability constant of palladium(O) species with mercapto ligands is much smaller than that of palladium(I1) species [8], the combination of ethyl acrylate with tributylamine causes a larger loss of metal than in the case of tributylamine alone (entries 1, 3). For the loss of

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palladium, the presence of PhX (X = I, Br) plays an important role (entries 5 - 7), probably due to the addition of phenyl halide to the low-valent palladium complex to give a o-phenyl palladium compound, in which the halide group exerts a high trans effect, thus weakening the mercapto- palladium linkage. In the presence of excess amine, palladium therefore dissolves. With the dissociation of the mercapto-palladium complex, the free mercapto group will be destroyed easily by the oxidative impurities or byproducts, and then equilibrium 3 in Scheme 1 will shift to the left. It should be mentioned that other factors, such as the breakdown of the macromolecular chelate skeleton, may also cause the loss of palla- dium.

141

A separate experiment was done as follows: After terminating an arylation reaction, the catalyst was separated by centrifugation, and an appropriate amount of silica-supported mercapto-containing polymeric ligand was added. The mixture was stirred at room temperature for about 24 h, and the concentration of palladium in the solution decreased from 15 ppm to less than 1 ppm. Thus, most of the soluble palladium species was recovered from the solution. The resulting solid which contained a definite amount of palladium was collected by filtration, washed with ether, then tested as a catalyst for the Heck reaction. It showed fairly good activity. A blank experiment was carried out by using the silica support without a mercapto functionalization as the absorbent, but no palladium was recovered.

These results imply that a decisive factor in the palladium leaching probably is the destruction of free mercapto groups during the reaction, otherwise the recovered palladium-containing solid would not have shown activity for the Heck reaction caused by the coordination saturation (uide supra).

Acknowledgement

Financial support from the Fund of Academia Sinica is gratefully acknowledged. The authors wish to express sincere thanks to Professor Liangzhong Zhao for valuable help with XPS studies, to Mr. Xiaobing Zeng for the atomic absorption spectrometric determinations of palladium, to Ms. Shunzi Jin, Shujie Li, Qifang Ying for IR determination, to Ms. Guifang Li for GPC measurements, and to Ms. Guangqin Feng and Guanzhi Qu for elemental analyses.

References

1 J. C. Bailar, Jr., Catal. Rev., 10 (1974) 17. 2 Y. Chauvin, D. Commereuc and F. Dawans, Prog. Polym. Sci., 5 (1977) 95. 3 D. C. Bailey and S. H. Langer, Chem. Rev., 81 (1981) 109. 4 C. U. Pittman, Jr., in G. Wilkinson, F. G. A. Stone and E. W. Abel (eds.), Compre-

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