Al2O3

JOURNAL OF

MOLECULAR CATALYSIS Journal of Molecular

Catalysis 91 ( 1994) 291-302

Evidence for intrinsically distinct active sites on rhenium-based metathesis catalysts. Norbornene metathesis with Re,O, /A120, Kenneth G. Moloy Catalyst Skill Center, Union Carbide Corporation, P.O. Box 8361, South Charleston, WV25303, (USA) (Received

January 25, 1994; accepted March 2, 1994)

Abstract Treatment of calcined Re20,/A1203 with norbornene results in extensive polymerization within the catalyst pores. 13C NMR analysis shows the polymer is exclusively ck-poly( norbomene), resulting from ROMP (ring opening metathesis polymerization). In contrast, a Re207/A1203 catalyst pretreated with Me,Sn also polymerizes norbomene but to a ROMP polymer favoring truns double bonds. These observations suggest that intrinsically different active sites are generated from Re20,/ A1203, depending on the presence or absence of organotin cocatalyst. Active sites produced on untreated catalyst are highly selective, preferentially reacting with strained, high energy double bonds. Active sites generatedupon treatment with Me,Sn are much less selective and react both with strained

and unstrained double bonds. These observations have resulted in a catalyst for the preparation of cis1,3-divinylcyclopentanes via ethenolysis of norbomenes.

1. Introduction

Heterogeneous olefin metathesis catalysts based on alumina-supported Re,O, have been the subject of a great amount of study [ I-31.This attention has been warranted by a variety of factors, perhaps most notably the low temperatures at which this catalyst performs and its ability to metathesize functionalized olefins. In addition to academic interest, Re,O,/ Al,O, catalysts are reportedly in commercial operation where they are used for the manufacture of specialty olefins via metathesis of ethylene with cyclooctene and cyclooctadiene [41. A variety of promoters and cocatalysts *Corresponding

author; fax.

( + l-304)7475571,

have been employed with the rhenium catalysts,

e-mail: [email protected].

0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10304-5102(94)00053-x

292

K.G. Moloy /Journal of Molecular Catalysis 91 (1994) 291-302

resulting in significant performance enhancements. In particular, the addition of organometallic reagents and especially tetraalkyl tin compounds has been found to lead to marked increases in reaction rate [ 31. The resulting catalysts are also tolerant of a variety of functional groups, resulting in catalysts for the metathesis of functionalized olefins [ 5,6]. The studies conducted to date have partially elucidated the role of the tetraalkyl tin cocatalysts. However, there is not a general understanding of how an untreated catalyst differs from a catalyst pretreated with an organotin cocatalyst. Specifically, it is not clear if addition of the tetraalkyl tin simply increases the number of active sites, or if intrinsically different active sites are produced. Recent experimental evidence has quantitatively confirmed the suspicion that the number of active sites present on Re207/A1203 catalysts is small, encompassing approximately 0.5% of the total rhenium [ 71. It has not yet been reported how this value is changed upon pretreatment with tetraalkyl tin reagents. Other reports suggest that tetraalkyl tin reagents serve to reduce rhenium to lower oxidation states [ 8,9]. However, these reports do not address the differences between catalysts with and without the tetraalkyl tin cocatalyst. There is also evidence which suggests that addition of a tetraalkyl tin to these catalysts results in alkylation of rhenium and is accompanied by the formation of -SnR, groups, also bonded to rhenium [ 10,111. If this ReOSnR, species is an active metathesis site, or a precursor to one, then chemically distinct active sites may be present on catalysts depending on whether they have been treated or not with R&r. Herein are presented reactivity data which suggest that chemically distinct active sites may be present on Re,O,/ Al,O,-based metathesis catalysts, depending on the pretreatment procedure.

2. Results and discussion

Addition of norbornene to a hexane slurry of Re207/A1203 catalyst at room temperature results in a rapid darkening of the solid catalyst. Repeating this procedure in the presence of C2H4, even at exceedingly high pressures (7000 psig) and with slow addition of norbornene, yields at most only a trace of the cross-metathesis product cis-1,3-divinylcyclopentane, as determined by gas chromatography analysis. However, analyses of catalysts exposed to norbornene in this manner indicate that significant quantities of a non-volatile hydrocarbon residue are formed in the catalyst pores (see Experimental section for catalyst isolation procedure). Elemental analysis (Table 1) shows more than 21 wt.% hydrocarbon Table 1 Analyses of Me,Sn treated and untreated ReZ0,/A1203

untreated catalyst Me,Sn treated catalyst unused catalyst

catalysts used for norbomene

metathesis

Elemental analysis

Pore volume

Pore area

(ml/g)

(m’/g)

19.15% c 2.49% H 18.16% C 2.51% H not determined

0.29

141.3

0.26

100.6

0.57

242.9

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Catalysis 91 (1994)

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residue present, even after extensively washing the spent catalyst with hexane, followed by high vacuum treatment. Consistent with the elemental analysis, the pore volume is reduced to less than one-half its original value. The persistence of this residue, even after extensive solvent extraction, suggested that it might be polymeric in nature. Two polymer structures were considered to be possible, shown in Scheme 1. One is the ROMP (ring opening metathesis polymerization) polymer, A. This polymer has been the subject of numerous investigations and is readily formed in the presence of metathesis catalysts. Alternatively, polymer B could result from acid catalyzed polymerization, a reaction possible in the presence of an acidic support such as the y-alumina used for rhenium-based catalysts. 13CNMR spectroscopy was used to investigate the identity of the residue, and specifically to distinguish between polymer structures A and B. 13C CP-MAS NMR spectroscopy was conducted on an untreated Rez0,/A1203 catalyst (e.g., one which had not received prior treatment with Me$n) after reaction with norbornene in the manner described above. The resulting spectrum is shown in Fig. 1 and shows the presence of an unsaturated hydrocarbon species. Thus, the resonance at 6 134.2 is attributable to an olefinic resonance, and three aliphatic resonances are observed at 6 43.3, 39.9, and 33.4. The spectrum is reminiscent of that reported in the literature for

.& ” -k3---+ B Scheme 1.

--;iJ,h pm

140.0

120.0

100.0

80.0

60.0

Fig. I. 13C CP-MAS NMR spectrum of untreated Re207/A1203

40.0

20.0

0.0

catalyst after reaction with norbomene.

K.G. Moloy/Joumal

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of Molecular Catalysis 91(1994) 291-302

poly (norbornene) prepared by ROMP [ 2,121. The spectrum is clearly not consistent with a polymer of structure B. In order to provide further confirmation for the ROMP structure, the spent catalyst was subjected to ultrasonication in toluene to extract the polymer residue. This technique resulted in the isolation of sufficient quantities of polymer for high resolution solution phase i3C NMR, the spectrum of which is shown in Fig. 2. These data are entirely consistent with the ROMP poly( norbornene) depicted in A. Moreover, a comparison of the chemical shifts with those reported in the literature [ 21 indicates that the polymer consists of essentially 100% cis double bonds. Chemical shifts and assignments are given in Table 2. It is clear that Re,0,/A1203 (no Me& treatment) not only catalyzes the metathesis polymerization of norbomene, but does so with remarkable selectivity to the cis-polymer. Similar experiments were then conducted with a catalyst pretreated with Me,Sn. Thus, a slurry of Re207/A1203 in hexane was treated with Me,Sn (Sn/Re = 1) for approximately 15 min at 25°C followed by addition of norbornene. After 1 h the catalyst was isolated in the manner described. Analysis (Table 1) again shows the presence of a large amount of hydrocarbon residue; the isolated catalyst contains > 20 wt.% of a hydrocarbon residue and the pore volume is reduced to less than half of its original value. 13C CP-MAS NMR of this catalyst shows (Fig. 3) that the polymer is unsaturated and the similarity to the spectrum in Fig. 1 strongly suggests that it is also a poly( norbornene) of structure type A. However, there are slight differences in the aliphatic region in terms of both chemical shifts and relative peak heights, suggesting a slightly different polymer structure between the two samples. The most likely structural difference is a greater truns double bond content in polymer derived from the Me$n treated catalyst. The olefinic carbons of cis- and trans-poly(norbornene) differ very slightly in their NMR chemical shifts, but significant differences are observed for the aliphatic carbons [ 21. To further address this possibility, the polymer was extracted from the catalyst by the ultrasonication technique previously described. The 13C NMR spectrum of polymer isolated in this manner

I

140.0

I 120.0

I

100.0

I 80.0

I 00.0

I 40.0

I 20.0

I 0.0

PPm

Fig. 2. ‘3C( ‘H) NMR spectrum of polymer extracted from the sample shown in Fig. 1. The CD2Cl, solvent resonates at 53.8 ppm and the resonance at 1.1 ppm is attributed to an impurity. Chemical shift positions and assignments are given in Table 2.

K.G. Moloy/Journal Table 2 13C NMR chemical shifts and assignments= Rez0,/A120, catalysts

for poly (Thomene)

Me,&

Untreated catalys? Assignment

Chemical shift Solid State

Liquid

134.0 43.3 39.9 33.4

134.2 43.1 39.1 33.6

produced

from untreated and Me,.% treated

treated catalystC Assignment

Chemical shift Solid State

Cz. C, C, C,, C, C5. C,

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ofMolecular Catalysis 91(1994) 291-302

Liquid 134.0 133.8 133.7

cis-C,, C,

134.0

43.6

39.9

33.2

133.1 133.0 132.8 43.4 43.1 42.7 42.1 41.4 38.6 38.4 33.2 32.9 32.4 32.2

trans-C,,

C3

trans. cis-C,, C, trans. tram-C,, C, cis, cis-C, cis, trans-C, Pans, tram-C, cis, cis-C, , C, cis, tram-C,, C, cis, cis-c,, c, cis, trans-C5, C, trans. cis-C5, C, tram, tram-C,, C,

“Assignments are for liquid phase chemical shifts which are based on work described in ref. 121. YZD2C12 solvent. ‘CDC13 solvent.

is shown in Fig. 4 and clearly shows the presence of two types of double bonds, presumably cis and trans. The two olefinic resonances at S 133.8 and 6 133.0 are assigned to cis and tram carbons, respectively. In addition, the aliphatic region now consists of eleven resonances, corresponding to the various isomers possible due to the presence of the cis and tram double bonds. The number and chemical shifts of these aliphatic resonances are in direct agreement with the literature [ 21, and those assignments are given in Table 2. Close inspection of the aliphatic region suggests that the distribution of cis and fruns double bonds is nearly random [ 121. Unfortunately, there is insufficient data thus far to identify the polymer end group. These results demonstrate that both Me,Sn-treated and untreated Re20,/A1,03 catalysts polymerize norbomene in a ROMP fashion. The untreated catalyst is highly selective, exclusively producing cis-poly( norbomene) . However, the untreated catalyst is ineffective at catalyzing the metathesis of norbornene with acyclic olefins, such as ethylene. In contrast, the Me,Sn-treated catalyst yields poly (norbomene) with both cis and truns double bonds apparently randomly distributed throughout the polymer. One explanation for this obser-

K.G. Moloy /Journal ojMolecular Catalysis 91 (1994) 291-302

296

I 140.0

I 120.0

1 100.0

I 80.0

I 60.0

I 40.0

I 20.0

I 0.0

pm

Fig. 3. 13C CP-MAS NMR spectrum of Me,Sn treated Re,0,/A1203 feature at 17 ppm is attributed to a spinning side band.

I

I 140.0

I 120.0

I 100.0

I 80.0

I 60.0

catalyst after reaction with norbomene.

I 40.0

I 20.0

The

I 0.0

pm

Fig. 4. ‘%[ ‘H) NMR spectrum of polymer extracted from the sample shown in Fig. 3. The CD& resonates at 77.0 ppm. Chemical shifts and assignments

solvent

may be found in Table 2.

vation is that the Me,Sn treated catalyst is able to metathesize both strained and unstrained double bonds with approximately equal efficacy. The catalyst metathesizes not only new monomer molecules but is also able to metathesize double bonds in the polymer backbone. Homogeneous catalysts which metathesize both strained cyclic double bonds and unstrained acyclic double bonds typically yield poly (norbornenes) with high truns contents [ 1,131. If the Me,Sn-treated catalyst does indeed metathesize norbornene (strained double bond) and the polymer backbone (unstrained double bonds), then it should be possible to use this catalyst for the metathesis of norbornene with acyclic olefins such as ethylene. In fact, this reaction works exceptionally well. Norbornene “ethenolysis” with Me4Sn treated Re,?O,/

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A1203 proceeds with 100% selectivity to cis-1,3-divinylcyclopentane and a distribution of higher oligomers, as shown in Scheme 2. This distribution of oligomers is common for the metathesis of cyclic olefins with ethylene [ 41. This reaction occurs smoothly at 25”C, relatively low pressure, and with norbornene conversions nearing 100%. Of course, high C&H4pressure shifts the distribution of oligomers towards monomer but otherwise has no effect on the overall metathesis selectivity. The formation of cis- 1,3-divinylcyclopentane by metathesis of norbornene with ethylene at high temperature in the gas phase has been reported [ 141. However, many substituted norbornenes possess low volatility and are thermally sensitive (with respect to retro Diels-Alder cracking reactions), and metathesis under the mild conditions described in here may be advantageous. The 13CCP-MAS NMR spectrum of the spent, Me,Sn treated catalyst used for norbornene ethenolysis is similar to those described above, but in this case it is apparent that low molecular weight oligomers, rather than polymer, are present within the catalyst pores. Thus, Fig. 5 shows the familiar resonance at S 134.0 attributable to a 1,2-disubstituted olefin. In addition, the weak absorptions at 6 113 and 143 are assignable to the (Y-and P-carbons, respectively, of a vinyl group. This is most likely the oligomer end group. The aliphatic region of the spectrum suggests a much different ratio of cis and tram double bonds than seen previously. These chemical shift assignments and conclusions are confirmed by the liquid phase i3C{ ‘H) NMR of the oligomer isolated by extraction from the spent catalyst (Fig. 6). The oligomer contains mainly tram double bonds, with a trunslcis ratio of ca. 4, and much closer to that expected based on thermodynamic considerations. It has already been mentioned that catalysts that metathesize cyclic and acyclic olefins tend to favor trunspoly( norbornene) . It has also been reported that the cisltruns content of poly (norbornenes) produced with these catalysts tends to vary with time, and the amount of truns increases with increasing contact with the catalyst [ 1,131. It is possible that the difference in cisl truns content observed for the polymers/oligomers shown in Figs. 3-6 is attributable to a difference in contact time with the catalyst. Me&n

cis-1,3-divinylcyclopentane

‘-o-+-oetc

oligomers

11 “e*T I Scheme 2.

K.G. Moloy/Journal

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ho.0

hO.0

of Molecular Caialysis 91 (1994) 291-302

‘so.0

hO.0

pm

60.0

Fig. 5. 13C CP-MAS NMR spectrum of spent Re20,/AI,03 catalyst from the preparation pentane. The feature at 20 ppm is attributed to a spinning side band.

-

J__

wm

I 120.0

Fig. 6. ‘%[ ‘H] NMR spectrum solvent resonates at 77.0 ppm.

I loo.0

I 80.0

(CDCll solution)

1,3-divinylcyclo-

L

L I 140.0

of cis-

I 60.0

I 40.0

I 20.0

I 0.0

of polymer extracted from the sample shown in Fig. 5. The

3. Conclusions This study further underscores the large chemical differences between untreated and R,Sn treated Re,O,/Al,O, metathesis catalysts. Both catalyze the ring opening metathesis polymerization of norbornene. However, these results suggest that chemically distinct active sites are present in the two catalysts. Sites present in the untreated catalyst are highly selective for the metathesis of strained, cyclic double bonds and selectively produce a &-polymer from norbornene, obviously a kinetic product. It has been reported that homogeneous ROMP catalysts which yield predominantly c&polymers are poor catalysts for the metathesis of acyclic double bonds [ 11. Furthermore, it has been postulated that the polymer is chelated to the metal during ROMP with homogeneous catalysts that give high cis contents [ 21. The

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active site present in the Me&i treated catalyst is non-selective in that it metathesizes all double bonds, strained (cyclic) and unstrained (acyclic), apparently with equal ease. A polymer containing the thermodynamically favored tram double bonds results. By analogy with homogeneous metathesis catalysts it is possible that the Me,Sn treated catalyst does not coordinate with the growing polymer chain in a chelating fashion. Clearly, the two catalysts contain active sites with quite different chemical behavior. It is also possible that the Me& treated catalyst actually contains two types of active sites. One is the same “selective” site present in the untreated catalyst. The second is a new site generated upon addition of Me,Sn. This latter site is non-selective and metathesizes incoming monomers as well as the backbone double bonds of the cis polymer produced at the “selective” site. Because Re207/A1203 based catalysts are known in many cases to metathesize acyclic olefins without prior addition of tetraalkyl tin reagents, it is not necessarily true that the difference in active site is attributable to the direct involvement of tin. Instead, it is more likely that the nature of these active sites, which are presumably rhenium carbenes, depends on their mode of formation from the Re20,/Al,0, catalyst precursor and the activating agent employed, be it an olefin or tetraalkyl tin. Further experimentation is required to further elucidate the chemical nature of these active sites.

4. Experimental 4.1. General Hexane (Fisher OptimaTM) was sparged with nitrogen before use. Me,Sn and norbornene were used as received from Aldrich Chemical Company. Re,O, was obtained from Aesar. A high purity y-Al,O, (SA 6273) obtained from Norton Company was used for all catalysts reported herein. This 3/-A1203 has a surface area of 214 m*/g, a pore volume of 0.57 ml/g, and is supplied in the form of l/ 16 in. spheres. Combustion (C, H) analyses were performed by Galbraith Laboratories, Knoxville, TN. Pore volumes were measured by mercury porosimetry with a Micrometrics Auto-Pore 9200. Metathesis products were analyzed by gas chromatography on a Hewlett Packard 5890 Chromatograph with flame ionization detector and a 30 m Durabond 1701 capillary column. 4.2. “C NMR spectroscopy Solid-state 13C spectra were obtained on a Nicolet NT-200 spectrometer operating at 50.313 MHz (4.7 Tesla magnetic field, 200.069 MHz ‘H frequency), modified for solids operation by the addition of high-power RF amplifiers (Henry Radio, 200 W ‘H decoupling; Dentron, 1 kW observe) and CP/MAS probes from Doty Scientific, Inc. Samples were ground to pass an 80-mesh sieve and packed in 7 mm OD sapphire rotors with Vespel end caps (6 kHz maximum spinning speed and ca. 350 ~1 sample volume). The magic angle was set using the 79Br resonance of KBr spinning at 5.5 kHz by maximizing the duration of the rotational echo train. The magnetic field was shimmed on the ‘H signal of ca. 5% acetone in acetone-d, spinning at 1.25 kHz to obtain a linewidth of ca. 2 Hz. The same sample was used to set the decoupling frequency to ca. 5.0 ppm relative to tetramethylsilane.

300

K.G. Moloy/Journal

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13C spectra were obtained using quadrature detection with CYCLOPS phase cycling, highpower ‘H decoupling, and cross-polarization excitation with spin-temperature alternation. RF field strengths of ca. 19 gauss for ‘H and ca. 75 gauss for r3C were used. The ‘H 90” pulse length was determined with the 180”/360” method using 13C detection and crosspolarization experiments on a sample of glycine spinning at 6 kHz, after which a HartmannHahn match was obtained by varying the 13C power. Chemical shifts were externally referenced to tetramethylsilane, with higher frequencies (lower fields) positive, using the 176.4 ppm resonance of glycine as a secondary standard. No correction is made to the measured shift for the bulk susceptibility of the sample. The optimum recycle delay (2-5 s) and cross-polarization contact time (500-750 ms) were determined for each sample, after which a 2048-scan spectrum was recorded. Solution 13C{‘H} NMR spectra were obtained using General Electric GN-300NB and QE-300 NMR spectrometers. The spectra were acquired using Waltz-16 decoupling and 64K data sets with spectral widths >20 KHz. Solvent resonances were used as internal secondary chemical shift references, CDCl, at 77.0 ppm and CD&l2 at 53.8 ppm, which are relative to tetramethylsilane at 0.0 ppm. The qualitative spectra in Figs. 2 and 4 were acquired using a “one pulse sequence,” no delay between pulses, 45” flip angles, and 1.O s acquisition times. The spectrum in Fig. 6 used a “ IPDNA” sequence (decoupler on during the 1.64 s acquisitions and off during the 10 s pulse delay) with 45” pulses. Off-line data processing used the program MacFID by Tecmag, Inc., Houston, TX. 4.3. Catalyst preparation The Re207/A1203 catalyst was prepared by the incipient wetness technique. In a typical procedure a 6 wt.% Re207/A1203 catalyst is prepared by dissolving Re*O, (29.4 g) in HZ0 (238 ml). The resulting solution is then added slowly to 441 g of y-alumina support, with gentle mixing. After standing at room temperature for 30 min the impregnated support is gently mixed again and then allowed to dry overnight in a 120°C oven. The catalyst is then activated in a quartz tube at 550°C under a steady flow of oxygen for 3 h, followed by a 3 h nitrogen flow. The catalyst is then cooled to room temperature under nitrogen and stored in a nitrogen filled dry box to prevent water absorption and deactivation. In some experiments the catalyst spheres were ground to a fine powder before use. 4.4. Preparation

of catalyst samples for 13C CP-MAS NMR

In a typical experiment, 4.0 g Re,0,/A1,03 catalyst was slurried in 20 ml hexane. Then a solution of 2.7 g norbornene in 5 ml hexane was added and the mixture was stirred at room temperature for 1 h. The catalyst was isolated by filtration, washed several times with hexane, and dried under high vacuum. Samples prepared in this manner were also used for combustion (carbon, hydrogen) and pore volume analyses. 4.5. Isolation of poly(norbornene)

for solution phase NMR analysis

In a typical experiment, 4.0 g of finely powdered catalyst was slurried into 10 ml hexane. 200 ~1 Me,Sn was added and the suspension was stirred for several minutes. 20 ml of 50%

K.G. Moloy / Journal of Molecular Catalysis 91 (1994) 291-302

301

norbornene in hexane was then added and the mixture stirred at room temperature for 4 h. The mixture was filtered and the catalyst washed with hexane, then dried under vacuum. The catalyst was placed in toluene and treated in an ultrasonic bath for 7.5 h. The catalyst was removed by filtration and the solvent removed under vacuum to yield a colorless, gummy residue. The polymer thus isolated was analyzed by i3C NMR in CD&. 4.6. Metathesis

of norbornene

with ethylene: preparation

of cis-1,3-divinylcyclopentane

A 100 ml Parr autoclave was heated to 150°C under a flush of nitrogen for 2 h to remove moisture. The reactor was then charged, under a nitrogen purge, with 1.O g 6 wt.% Re,O,/ Al,O, catalyst, 5 ml hexane, and 40 ~1 Me,Sn (Re/Sn = 1). The catalyst was added via a long stemmed ampoule which had been previously loaded in a nitrogen filled dry box and sealed with a septum to prevent contact with atmospheric moisture. The reactor was then sealed and pressurized to 600 psi C2H4. A solution of 5 ml norbornene in 20 ml hexane was then fed into the reactor over a period of 2.5 h using a Gilson liquid pump. After ca. 12 h GC analysis showed 100% norbornene conversion to cis- 1,3-divinylcyclopentane and oligomers. The product is readily purified by distillation at 14&142”C (760Torr). The spectroscopic parameters are consistent with those reported in the literature [ 15-171. NMR (CDCI,): ‘H: 65.82 (m, 2H), 4.9 (m, 4H), 2.56 (m, 2H), 1.98 (m, lH), 1.86 (m, 2H), 1.48 (m, 2H), 1.21 (m, 1H). i3C: 6 143.0 (=CH), 112.4 (=CH,), 44.3 (CH), 40.1 (CH,), 31.6 (CH,). IR (neat film): 3088m, 2990m, 2961s, 2875m, 1646m, 147Ow, 145Ow, 1423w, 992m, 909s. Mass spectrum: 122 (M+).

Acknowledgements T.L. Fortin is sincerely thanked for expert technical assistance. This work would not have been possible without the expertise of K.G. Canterbury, A.M. Harrison, A.D. Ronemus, and C. St. Clair of the Union Carbide NMR Skill Center. Union Carbide Corporation is thanked for supporting this work and for granting permission to publish the results.

References [I] R.H. Grubbs. in G. Wilkinson (Ed.), Comprehensive Organometallic Chemistry, Vol. 8, Pergamon Press, Oxford, 1982, p. 499. [2] K.J. Ivin, Oletin Metathesis, Academic Press, London, 1983. [3] J.A. Moulijn and J.C. Mol, J. Mol. Catal., 46 (1988) 1. [4] P. Chaumont and C.S. John, J. Mol. Catal., 46 (1988) 317. [5] M. Sibeijn and J.C. Mol, J. Mol. Catal., 76, (1992) 345. [6] J.C. Mol. J. Mol. Catal., 65 (1991) 145. I71 Y. Chauvin and D. Commereuc, J. Chem. Sot., Chem. Commun., ( 1992) 462. 181 X. Yide, H. Jiasheng, L. Zhiying and G. Xiexian, J. Mol. Catal., 65 ( 1991) 275. [91 X. Xiaoding, A. Andreini and J.C. Mol. J. Mol. Catal., 28 (1985) 133. f 101 K.P.J. Williams and K. Harrison, J. Chem. Sot., Faraday Trans., 86 (1990) 1603. [ 111 R. Spronk, A. Andreini and J.C. Mol. J. Mol. Catal., 65, ( 1991) 345.

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K.G. Moloy/ Journal of Molecular Catalysis 91 (1994) 291-302 K.J. Ivin, D.T. Laverty, J.H. O’Donnell, J.J. Rooney and C.D. Stewart, Makromol. Chem., 180 (1979) 1989. R.R. Schrock, J. Feldman, L.F. Cannizzo and R.H. Grubbs, Macromolecules, 20 ( 1987) 1169. US Patent 3 424 811 ( 1969) to F.D. Mango (Shell Oil Company) P.W. Jennings, R. E. Ekeland, M.D. Waddington and T.W. Hanks, J. Organometal. Chem., 285 (1985) 429. W. Trautmann and H. Musso, Chem. Ber., 114 (1981) 982. G.C. Corfield, A. Crawshaw, G.B. Butler and M.L. Miles, J. Chem. Sot., Chem. Commun., ( 1966) 238.