Investigation of polymerization and metathesis reactions

Journal of Molecular Catalysts, 36 (1986)

39 - 45

39

INVESTIGATION OF POLYMERIZATION AND METATHESIS REACTIONS PART VIII*. INFLUENCE OF ALKYNES ON THE RING-OPENING METATHESIS OF CYCLOPENTENE WITH WC16 KARIN WEISS** and RAINER GOLLER Laboratorium (F.R G )

fur Anorganzsche

Chemre

der

Unwersrtat Bayreuth,

D-8580

Bayreuth

Summary The ring-opening metathesis of cyclopentene is catalyzed by WC16 m the presence of 1-alkynes. In an mvestigation of the reaction mechanism, several mono- and disubstituted alkynes were tested for their activating effect and their influence on the stereochemistry of the resulting poly-l-pentenylene. With l-alkynes yields of poly-l-pentenylene are as high as 90%. The tmns content of the polymer approaches 80%. Increasing chain length and stenc bulk of the alkyne substituent reduce the yield and the tram content significantly. Disubstituted alkynes inhibit the metathesis reaction. The oligomeric reaction side products which are formed in low yield were analyzed by GC/MS and NMR. The initial reaction of the system WCl,/phenylacetylene/cyclopentene is the formation of polyphenylacetylene and some polycyclopentene by WC16. As the active metathesis catalyst is produced, metathesis ensues, producing a high tram content of poly-1-pentenylene which remains constant over a period of days. Based on these results a reaction mechanism is proposed involving a carbyne complex as the starting metathesis catalyst.

Introduction Masuda et al. [l] observed the polymerization of phenylacetylene catalyzed by WC&. Some years later Makovetskii et al. [ 21 and Katz and Han [3] found the system WCI,/phenylacetylene to be a highly active catalyst for the ring-opening metathesis of cyclopentene to yield poly-1-pentenylene. n

0*For Part VII see [lo]. **Author to whom correspondence

0304-5102/86/$3.50

+

. . .

n

(1)

should be addressed. @ Elsevier Sequola/Prmted

in The Netherlands

40

It was suggested by Katz that a catalyst which polymerizes alkynes should be able to polymerize alkenes if small amounts of alkynes are added for activation. For the reaction of cyclopentene with WClh/phenylacetylene a mechanism was proposed by Katz-implicating the formation of a carbene complex m the initial step, thus providing the active catalyst:

WCI, + R-C-C-R’

-

Y Cl ,W-C=qR

/R

-

Cl

cc

In a second step the cyclopentene

Cl,W=C

- z + p’ 0 /\ Cl Cl YR

(2)

c’4 W=CYR’ ‘Cl

inserts into the W=C carbene bond.

Cl,W=C c=c-C-Cl u bl lj

lfRF;7’

(3)

This reaction pathway involves no significant influence of the alkyne substituents R and R’ on the yield and stereochemistry of the resulting polymer: after a few insertions of cyclopentene, the alkyne is far away from the metathetically active centre, so that the alkyne exerts no stereochemically directing influence. In an attempt to elucidate the reaction mechanism, we have tested several 1-alkynes and disubstituted alkynes for their activating effect on the metathesis reaction and their influence on the stereochemistry of the resulting poly-l-pentenylene. Experimental All reagents and solvents were purified, dried and stored under an argon atmosphere. The reactions were carried out under argon atmosphere. ‘H NMR and 13C NMR: Jeol FX 9OQ; IR: Beckman 4240; GC/MS: Varian MAT 312 and Varian 3700 GS. Time-dependent stereochemistry and yield of poly-1 -pentenylene by reaction of cyclopentene with WC.&and phenylacetylene To WCls (0.25 g = 0.6 mmol) dissolved in 60 ml chlorobenzene are added 7.7 g = 120 mmol cyclopentene and 0.15 g = 1.5 mmol phenylacetylene at 20 “C. The mixture is shaken under argon. 20-ml fractions of the mixture were taken after 5 min 20 s, 15 min and 1 h and the polymer isolated as described in [3]. After 1 h a mixture of 7.7 g = 120 mmol cyclopentene in 60 ml chlorobenzene is added to the rest of the reaction mixture, and again 20 ml fractions were taken after 5 mm 20 s, I.5 min, 1 h, 2 h, 4 h, and 3 days, and the polymers isolated. For yield and stereochemistry see Table 1.

41

TABLE 1 Trme-dependent stereochemrstry and yield of poly-1 -pentenylene by reaction of cyclopentene with WCle/phenylacetylene (trans linkage by 13C NMR) Reactron time

Polymer yreld (%)

Trans

linkage (%)

6a

a

15 mm lh

7b 37

83 86

Addition of mononer 1 h 5 mm 20 s lh15s 2h 3 days

1 9 11 99

86 85 85 a3

5 mm 20 s

aNo poly-lpentenylene. bPoly-l-pentenylene + polyphenylacetylene

By this method the poly-1-pentenylene formed is completely dissolved in chlorobenzene during the reaction time, and representative samples of the reaction mixture are thus available. Metathesis of cyclopentene with WC& and variously substituted alkynes To WCl, (0.25 g = 0.6 mmol) dissolved in 10 ml chlorobenzene at 20 “C, 7.7 g = 120 mmol cyclopentene and 1.5 mmol alkyne are added. The mixture is shaken under argon atmosphere for 5 days. Then the sticky polymer is diluted with CHzClz and precipitated by the addition of methanol, washed with aqueous NH3 followed by CHsOH, and dried in vacua. The oligomers remaining in the organic phases are combined, dried over NazS04 and freed from solvent m vacua. For yield and stereochemistry see Table 2. Results and discussion To investigate the reaction pathway of the metathesis of cyclopentene with WC& and phenylacetylene, we tested first the dependence of polymer yield and stereochemistry. Samples of the reaction mixture were taken after 5 min 20 s*, 15 min and 1 h. When after 1 h it was obvious that the active *Personal commumcation: Katz et al. tested the stereochemrstry of the poly-l-pentylene after 5 min 20 s reaction time because they found m the beginning of the reaction a lower yield in tins-poly-lpentenylene We think these results are due to WC16 catalysis and can be avolded by using more solvent The preliminary formation of polyphenylacetylene was already observed by Katz

42 TABLE 2 The influence of alkynea on the ring-opening metatheses of cyclopentene with WC16. Yield of cis- and transpoly-lpentenylene and percentage of tranapoly-lpentenylene (by 13C NMR) Alkyne

Yreld polymer + ohgomer (%)

Yield polymer 6)

Trans linkage of polymer (%)

phenylacetylene 1 -hexyne 1 octyne 1 -decyne 3,3-dlmethyl-1-butyne hthmm phenylacetylide without alkyne drphenylacetylene bis(trrmethylsrlyl)acetylene 4octyne

>90 90 80 80

>90

B

80 80 70 70 65 65 -

a

-

a

-

85 80 70 60 25 6

60 30 14 5 2 1

aNo polymer at all

catalyst was formed and an equilibrium in stereochemistry had been achieved, more cyclopentene was added (dissolved in chlorobenzene) to test whether the active catalyst continues to work with the same stereochemistry (under these circumstances the yield of the catalyst decreased to l/7 of the starting concentration). The results listed in Table 1 indicate that after a short reaction time of 5 min 20 s, no poly-l-pentenylene is formed. By 13CNMR and IR, the main products of this polymer fraction were identified as polyphenylacetylene and polycyclopentene. After 15 min reaction time the main product of the polymeric fraction is poly-1-pentenylene. The truns content (determined by % NMR) is cu. SO%, however polyphenylacetylene is still detectable. The buns content of further fractions is nearly constant even if a high excess of cyclopentene is added after 1 h. These experimental data suggest that the active metathesis catalyst is formed after an induction period in which polyphenylacetylene and polycyclopentene are produced. The metathesis of poly-1-pentenylene begins with a truns content of 80%, and the stereochemistry remains unchanged during the reaction time (3 days). Therefore we tested the influence of alkyne substituents on the ringopening metathesis of cyclopentene with WC16 catalyst to yield poly-lpentenylene over a constant reaction time of 5 days. The yield of poly-lpentenylene and trans linkage in % are given in Table 2. With

monosubstituted

alkynes,

the

yields

of poly-1-pentenylene

vary

60 to 90%. Increasing chain length and steric bulk of the alkyne substituent has a significant influence, reducing the yield. Disubstituted alkynes

from

43

inhibit the metathesis reaction and no poly-1-pentenylene is formed. If the acidic proton of phenylacetylene is substituted by Li, the yield and the tmns linkage of the poly-1-pentenylene decreases. These results indicate the importance of the acidic proton for the reaction mechanism. WC16 as a catalyst in the absence of alkyne gives polycyclopentene and poly-l-pentenylene in very low yield. The stereochemistry of the polymer was determined by i3C NMR and IR spectroscopy [4]. The steric properties of the alkyne substituent have a remarkable influence on the truns content of poly-1-pentenylene; on increasing steric bulk the truns content decreases, as do the yields. The highest yield and trans content of poly-1-pentenylene is achieved by the use of phenylacetylene. In addition to polymers, some oligomers are also formed in the reaction of cyclopentene with WCl,Jl-alkynes. Four types of oligomeric reaction products can be distinguished by GC/MS: (a) regular oligomers of cyclopentene formed in the initial reaction step by WCl,; (b) oligomeric alkynes; also formed by WC16 in the early stages of the reaction; (c) chlorinated products of cyclopentene: WC16 chlorinates cyclopentenes as previously shown by Ceausescu et al. [5]. By these reactions W(V1) is reduced to W(V) and lower oxidation states; (d) some reaction products of cyclopentene oligomers with chlorobenzene. No oligomeric reaction products of cyclopentene with alkynes were observed.

Conclusions The results indicate that the metathesis of cylcopentene with WClJ phenylacetylene begins with an induction period in which polymeric phenylacetylene and cyclopentene are formed. After some minutes, the metathesis of cyclopentene yields poly-1-pentenylene, the trans content of which is constant during a reaction time of some days. (To obtain representative polymer samples for this test, it is necessary to avoid polymer precipitation in the reaction mixture by using sufficient dilution.) The active metathesis catalyst is obviously formed after an induction period. It continues to show catalytic activity for some days with high yield and constant stereochemistry. The results of some 1-alkynes tested as cocatalysts indicate that there is a significant influence of the alkyne substituents on yield and stereochemistry of the cyclopentene metathesis. With increasing bulkiness of the substituents, the yield and the trans content diminish. These results suggest that the alkyne substituents are associated with the active metathesis centre, throughout the reaction at the W atom and not far away on the other end of the polymer chain. Disubstituted alkynes inhibit the metathesis reaction; they

44

form stable tungsten cyclopropene complexes [ 61. The reaction with LiCrCC6H5 gives significantly lower yields and truns content of poly-l-pentenylene. The acidic protons of the 1-alkynes seem to be an essential factor for the synthesis of the active catalyst. These results are not consistent with the reaction mechamsm suggested by Katz. We therefore propose another mechanism which is compatible with the new results The Schrock carbyne complex C13(dme)WGCCMe3 reacts with isbcyanates and carbodiimides [ 7,8] in a metathesis-like reaction of the WXcarbyne triple bond with the double bond systems of the heteroallenes. In addition, C13(dme)WXCMe3 catalyzes the metathesis of cyclopentene to yield poly1-pentenylene in high yield and with 80% tmns content [ 91. The results are very similar to those of the metathesis reaction of cyclopentene with WC16 and 1-alkynes, suggesting that the starting metathesis catalysts in both reactions are carbyne complexes of high-valent tungsten.* WC16 and 1-alkynes may form a carbyne complex via alkyne and vinylidene complex intermediates (Fig. 1). This type of carbyne complex synthesis has been developed by Birdwistell et al. [ll, 121 starting with W(0) alkyne complexes. The W(V1) alkyne complex which results from the first HCl elimination yields a vinylidene ligand by reaction with a chloride ligand (the /3-C atom of the alkyne ligand should be electrophilic in contrast to those of the Birdwistell W(0) alkyne complexes). The vmylidene complex may then react in a way similar to carbene complexes and initiate the 1-alkyne polymerization. If no more l-alkyne is available a HCI elimination, forming a tungsten-carbyne complex, may occur. This reaction step is only possible with 1-alkynes and not with dlsubstituted alkynes. The tungsten-carbyne complex is able to coordinate cyclopentene and to start the metathesis reaction of the Wscarbyne triple bond with the cyclopentene double bond to yield a metallacycle with a W-C single bond and a W=C double bond. Further metathesis reactions may then occur at the W=Ccarbenedouble bond. The W-C single bond with the former alkyne substituent R at C, remains unchanged during the metathesis reaction, and the bulkiness of R may contribute to the stereochemistry of the metathesis reaction. The chain growth of the poly-1-pentenylene can be terminated by a back-biting process which regenerates the starting carbyne complex (eqn. 4). This last reaction step explains the observation that no oligomeric reaction products between cyclopentene and l-alkynes could be detected. C13WC-Rn

+

*New results indicate that metathesis reactlons of alkenes with C13(dme)W=CCMe3 are inhibited by disubstituted, but not by monosubstituted, alkynes [lo]

45 WCI, + H-EC-R - HCI I

/R Cl ,w=c=c,

Cl

I

+ H-EC-R

I

+ n H-EC-R

=R

I CI,WX-R

n - HCI n

and so on

Fig. 1

References 7 (1974) 728. 1 T. Masuda, K. Hasegawa and T. Higashimura, Macromolecules, 2 K. L. Makovetskii, L. I. Red’Kma and I. A. Oreshkin, Zzu. Akad. Nauk, SSR, Ser. Khlm, (1981) 1928 3 T. J. Katz and C.-C. Han, Organometallics, 1 (1982) 1093. 4 K. J Ivm, Olefin Metathesis, Academic Press, London, 1983. 5 E Ceausescu, A. CorniIescu, E. Nicolescu, M. Popescu, S Coca, C. Belloin, M. Dimonie, M. Gherorghui, V. Dragutan and M. Chipara, J. Mol. Catal., 28 (1985) 337. 6 K. H. Theopold, S. J. Holmes and R. R. Schrock, Angew. Chem., 95 (1983) 1012 7 K. Weiss, U. Schubert and R. R. Schrock, Organometallics, 5 (1986) 397. 8 K. Weiss, unpublished results. 9 K. Weiss, Angew Chem, 98 (1986) 350. 10 K. Weiss and R. Goller, unpublished results. 11 K. R. Birdwlstell, T L. Tonker and J. L Templeton, J. Am. Chem SOC., 107 (1985) 4474.

12 K. R. Birdwistell and J. L. Templeton, Organometallics,

4 (1985) 2062.