Facial Diastereoselectivity in the PaternЦBchi Reaction of Chiral Silyl Enol Ethers.код для вставкиСкачать
COMMUNICATIONS E.~ppc~rirlzl~/lro/ Proc~c.hrre I n ii Iypical experiment, [ R h ( x a c ) ( C O (0.1 6 inmol). P(f,iciii-C,H,SO,NII), ted w1 ynthesized a s reported by R. Giirtner (0.8 mino1 t h i s ~ ~ i l l i i i ~ iphosphane et ill. [ I 51). ,ind chemically modified cyclodextrin (1.12 mmol) were ditsolvcd in u'iter (45 1111.) .\ \t;rinless steel autoclave (100 mL) w a s charged under an xtino\phere 0 1 N1 with thc resulting q u c o u s phase and a n organc phase composcd of dcc-I-enc (80 nrinol) ;ind undecxne (4 mmol. GC internal standard), and then hcatcd :it X O C and pressuriaxl w i t h SO atm of CO:H, ( l : l ) . A meclianical ctirrer (1000 i p m j equipped with :I inultipaddle unit was then started. The pressure was kept con\tant thr~rughoutthe whole reaction by uiing a gas reservoir along w ~ t ha prc\surc regul;itoi The reaction % a s monitored by quantitative faschromatographic sn,il>\i\ (('I'SiI5-C'B. 25 in. inner diameter 0.32 mmj. Received: June 3. 1995 [Z7X121E] German version: An,Toi.. Cheiii. 1995. 107. 2450 2457 Keywords: cyclodextrins . hydroformylation * phase transfer catalysis . rhodium compounds . Wacker oxidation [I] \I, >\. Herrmann. C. W Kohlpaintner. Angm.. C/wui. 1993. 105. 1588: 4 n y ~ i i ~<.' / I ( I U . Inr Ed. Ens/. 1993. 32. 1524. h) B. Cornila. E. Wiebus. ( ' / l t h 1 7 ' E ( / I 1995. 33: c ) Cheni. Iirg Z d i . 1994, 66. 916: d ) W. A . Herrmann. C ' W. Kohlp;iintner. R. B Manetsherper. H. Bahrmann. H. Kottinan. .1. ,Mo/. ( ' u r n / 1995. V i . 65. ;I) 121 S S Divekai. R. M . Bhanagc. R. M. Deshpande. R. V. Gholap. R . V Chaud11<111../. , M I , / ( ~ u r u l .1994. Y I . L I .  a ) M. 1 H . Russel. 8. A. Murrer (Johnson Matthey). US-A 4399312. 1983 [ < / w n i .Ah.\/, 1982. Y7. P23291nI: b) H . Bahrmann. P. Lappe (Hoechst AG). Fur.. Pur. Appl. 602463. 1994 [Chem. A h r r . 1994. 121. P107982rl. c) T. Bartik. H. tlai-Ilk. 8 k. Hanbon. .I Mu/. Ccod. 1994. 88, 43. 141 Ti V. ('h;iudhari. B. M Bhanage, R. M. Deshpande. H Delmas. .Vn/irr.c 1995, 373. 501 151 a ) H. Ding. H.k.Hailson. T. Bartik. B. B a t i k . O ~ ~ q f i ~ i o / i r ~ t1994. r r / / / i13. . ~ 3761 : b j H. Ding. T E. Glass. B. E Hanson, Inorg. Chrm. AL./<J 1995. 229. 3289: c) B Fcll. ( i Papadogianakis. J. Mol. Curd. 1991. 66. 143: d ) H Bahrmann. G. I)ci.krrs. VY Grcb. P. Heyinanns. P. Lappe, T. Mueller. J. Szameitat. E. Weibus ( l i o c c h d A(;). t'iir.. Prrr. A p p / . 602442. 1994 [C%riri. A h w . 1994. 121. P I 0797XU] [h] I . J'. Arhmcct. M. E Davis. .I. S. Merola. B. E. Hanson, J C'ural. 1990. 121. 377  1; Montcil. K . Queau. P. Kalck. ./. Or.gaiu)i?ict.Chern. 1994. 480. 177. [XI a ) 1. Monllier. E. Blouet. Y. Biirbaux. A. Mortreux. A i ~ ~ j i e i iC'/wn?. .. 1994. 1116. 2183: 41i,qcii, ('hrni. In/. 0 1 . Etigl. 1994, 33. 2100: h) E. Monflier. S. Tilloy. G. Freiny. Y. ILirbnux. A. Mortreuw. Tc~trirheilronLerr. 1995, 36, 387. [Y] t.Monllici. Y. C~isranet.A. Mortreux (Centre National d e 12 Recherchc Scientilique). t [<-A 9500466. 1995. [lo] Thc lack ol activity of the a-cyclodestrin has also observed by W. R. Jackson during the biplrasic hydrolormylation but with hex-I-ene; J. R. Anderson. E. M Cainpi. W. R. Jackson. Co/o/. Lett. 1991. 9, 55. 1111 a ) 1). Ihchcne. D Wouessidjewe. J. Coorrl. Chem. 1992, 27. 223: b) H. Ikeda. I<K o p . ( ' .I. Yoon. T.Ikeda. J In</u.\ionPhmoiri. Mu/. Kcr.og17it.Chfwi. 1989. 7. 1 I , c ) C'. M. Spencer. J. F. Stoddart. R. Zarzycki, J. C/i?iii.Soc. Perkiri 7kun.v 7 1987, 1323. d ) Y. Kubota. T. Tanimoto. S. Horiyama. K . Koiruini. Cur.ho/i~il. K<..,. 19x9. /Y-'. 159. [I21 M C ~ u g l e rE. Ecke. J. 1. Stezowski. J. C h i . Soc. Chcni. C'oii?muii. 1981, 1294. 1131 I f . M ('olquhoun. J. F. Stoddart. D. J. Williams. An,&yiv Chcrri. 1986. YH. 483: Airgvii. ( ' / i w i . In!. E d Engl. 1986. 75. 487. 1141 G Wcw. 4i/,qc,11.C/WII.1994. 106. X51: An,Tcti.. C h ~ n ?Iiit. . €d. B i g / . 1994. 33, Facial Diastereoselectivity in the Paterno - Buchi Reaction of Chiral Silyl Enol Ethers** Thorsten Bach," Kai Jodicke, Kristian Kather. and Jiirgen Hecht The [2 + 21 photocycloaddition of alkenes t o carbonyl compounds commonly known as the Patern6 - Buchi reaction provides an efficient and straightforward route to functionalized oxetanes.['] By employing chiral auxiliaries it has become possible to photochemically produce enantiomerically pure oxetanes.lZ1 Studies by Scharf et al. impressively demonstrated that the two diastereotopic carbonyl faces can be well differentiated in phenyl glyoxylates of 8-phenylmenthol and related concavc alcohol^.'^' Besides the auxiliary-directed methodology there are several procedures that employ cyclic substrates to establish a stereoselective carbonyl photocy~loaddition.[~~ In these cases a htcreogenic center within the ring system is responsible for it successful side differentiation. Contrary to the auxiliary-directed method, the chirdl information is not removed after the reaction but is utilized for further synthetic operations. Analogously acyclic chiral substrates have not yet been employed for the preparation of enantioinerically pure oxetanes. Chiral aldehydes, which showed a remarkable Cram selectivity in nucleophilic addition reactions, proved less selective as carbonyl substrates in Paterno- Buchi reactions and yielded an unsatisfactory mixture of diastereoisomers.['] In connection with our studies on the regio- and stereoselective photocycloaddition of silyl enol ethers to aromatic aldehydes,"] we were intrigued by a possible synthesis of enantiomerically pure oxetanes according to the principles of acyclic stereoselection. We started our investigations with a single carbony1 compound (benzaldehyde) and left the z-substituent on the silyl enol ether unchanged.['] The only parameter varied was the 7-substituent on the silyl enol ether. The starting materials rac-1 were readily accessible in racemic form bq addition of the appropriate cuprates to 2,2-dimethyl-4-hexen-3-one and subsequent silylation. Upon irradiation of these substrates in the presence of benzaldehyde the desired oxetanes were produced as one of four possible pairs of diastereoisomers r~ic-2/ruc-3(the formula of only one enantiomer is shown in each case). Based on our previous studies[hc.dlit appears likely that the relative configuration of the three substituents within the oxetane ring is fixed. The ratio of the two diastereoisomers found therefore reflects the facial diastereoselectivity which is induced by the stereogenic center. This ratio increases with increasing size of the substituent R and is considerably high for some examples (Table 1 ) . $R + O f RH - XO.3. [Is] R . Gdrtnci. B Cornils. H. Springer. P. Lappe (Ruhrchemre AG). DE-B 3215030. 19x2 [ C ~ P I Ahvrr. I I . 1984, 101, PSS331tl t Bu racl Ph " t Bu OTMS 'OTMS Ph' rac2 tBU fac3 ~ [*] Dr. T. Bach, Dipl.-Chem K. JBdicke. DipLChem K . tixther. Dip1 -Chem. 3. Hccht'" Orfanisch-chemisches lnstitut der Universitdt Orlcansring 23. D-48149 Munster (Germany) Telcfax: Int. code + (251)X3-977? e-mail: bachtcn uni-rnuenster.de [ ' 1 X-ray crystallography [**I This project was generously supported hq the Deutschc F;orschungsgemeinschaft. the Fonds der Chemirchen Industrie, the Geselljchaft zur Fiirdcrung der Westliditchen Wilhelms-Universitit. and the Dr. O t l o Riihm-Gediichtnisstil'tung. We thank Mrs. B. Wihbeling for her help w i t h the X-ray crystallographic measurement. The continuing support of Prof. I)r. D. Hoppe is gratefully acknowleged COMMUNICATIONS Table I . Yields and diaatereomeric ratios (d.r.) observed in the photocycloaddition of benzaldehyde to the silyl enol ethers rac,-l with benzene as solvent. Alkene R T [ CI /[h][~] Yield[%][b] (/I la Et rPr rBu Ph Ph SiMe,Ph 30 30 30 30 -30[d] 30 4 4 4 4 61 51 64 72 51 44 61:39 70.30 92:8 69:31 58:42 Ib Ic Id Id I f 6 9 (2 3j[c] 95:s Scheme 2. Diastercoselective synthesis of the oxetane rrrc-2f from the ketone ruc-6. a) Mc,OBF, ( 1 . S equiv). 1.8-bis(dimethylamino)napiithaleiie(2 equiv) in CH,CI,. RT. 10d. 5 8 % ; b) TMSCI, L D A in T H E -7X C to RT, 7 2 % ; c) PhCHO in hcnzenc. hi., 4 h. 54%. [a] Irradiation time. [h] Total yield of both diastercoisomers ( r w - 2 and ruc-3) [ c ] Diastereomeric ratios (r/.r.) were determined by GLC aiialyses of the crude product mixture and were checked by 'H NMR spectrorcopy. [d] n-Hexme as solvent. The 3-(sily1oxy)oxetanes rac-2 and rac-3 are easy to identify by 'H NMR spectroscopy because of their characteristic singlet (2-H of the oxetane). The detection and quantification of the regioisomeric 2-(sily1oxy)oxetanes is difficult. In general. they are not produced stereoselectively in the examples under scrutiny and they are sensitive to hydrolysis. Based on the GLC data an upper limit for the amount of regioisomers formed may be estimated and one finds a decrease of regioselectivity with increasing size of the 8-alkyl substituent. Silyl enol ether rcic-I e gave the worst ratio (70:30), a result which explains at least partially the low yield obtained in this example. In order to elucidate the configuration of the major diastereoisomers, we have studied compound roc-2d by X-ray crystallography.18] The preferred attack of the photoexcited carbonyl compound on the silyl enol ether can be obviously explained by invoking the influence of 1.3-allylic strain.1', l o ] The conformational rigidity Facilitates the differentiation of the diastereotopic alkene faces, and the approach of the electrophile takes place preferentially from the less shielded side. Stereoelectronic considerations lead to a similar result.'"' In the course of the reaction, after addition of the carbonyl triplet to a putative 1.4triplet biradical (3D) the relative configuration of the oxetane ring is directed by previously discussed phenomena (free rotation in 3D, intersystem crossing.['*' cleavage vs. ring closure in a 1.4-singlet biradical).[6b,d1 Since there is a plethora of methods for the stereoselective 1,4-addition to Michael acceptors,'13] enantiomerically pure silyl enol ethers can be synthesized in a few steps. As an example we converted the previously described acid 4" 41 into the corresponding ketone 5 from which the silyl en01 ether was readily obtained. Photocycloaddition gave oxetane 2d as the major product; its enantiomeric purity was determined by shift experiments as > 95 'YO w I 1 (Scheme 1). The reaction proceeds without raceniization at the stereogenic center responsible for the stereoselectivi ty. As a substrate with a polar substituent R we chose the silyl enol ether roc-1 f which displayed a good diastereoselectivity ( d r . = 85/15) upon irradiation in the presence of benzaldehyde. The major diastereoisomer roc-2 f was isolated in 54 "/o yield and T HO 4 (>95%ee) t Bu p ' 5 Ph ''( h t Bu has the depicted configuration (Scheme 2). To determine the relative configuration the noncrystalline product roc-2f was deprotected (K,CO, , MeOH) and the 3-oxetanol rat-7 obtained in this way was crystallized from ether and characterized by a crystal structure analysis (Fig. Pig. 1 . Crystal structure of the 3-oxekinol rrfc-7 obtained by dcsilylation from the .i-(sily1oxy)oxetaiie rrrc-2 f. As a possible explanation for the mode of attack the "inside alkoxy" model first suggested by Houk may be invoked." 'I The steric difference between a methyl group and a hydrogen atom attached to the stereogenic center is responsible for the facial diastereoselectivity. Two examples were used to demonstrate that the oxetanes which are available according to the methodology described herein may also be employed for the stereoselective construction of acyclic molecules. The hydrogenolytic ring of oxetane rcic-2f (cleavage of the bond between 0 and C-2) leads to a partially protected trio1 rac-8 and the new fluoride-promoted fragmentation'''] of rczc-2e (cleavage of the bond between 0 and C-4) to the (E)-alkenediol rac-9. rac-6 (76%) rac2f OTMS 2d (>95% ee) Scheme I . Preparation of the enantiomerically pure oxetane Zd. a j SOCI, (1.Sequiv). i-eflux. 6 h . Y Z % ; b) tBuMgCI (1.5equiv). CuCl (02cquiv) in Et,O. - 7 8 ' C . 2 h, Y3%; c ) TMSCI, lithium diisopropylamide (LDA)in THF, - 7 8 C to RT. 6 5 % ; d) PhCHO in benzene, /IF. 5 h. 51 '!!" ( + 2Z'% 3dj. THF rac2e H d iBu rac-9 (85%) COMMUNICATIONS ErpcritizcviId Procx~chrrc Sly1 en01 etlici (4.5 nirnol) was dissolved in henzene (8 mL) in a quartz tuhe. The irted (300 nm: light source: Rayonet R P R 3000) and a solution of [I iiig. 1.5 mmol. 152 ILL)in henzene ( 2 mL) was slo\rly added to the rc:iction mixtiirc h y ii syringe (within cii. 2 h ) . As soon as the aldehyde was fully con'iiiined (TLCJ llie irsxli~itionw a s slopped and the solvent ecaporated i n v~icuo. The dinstcrconicric tiitlo was detemiincd hy GLC' m d ' H N M R iinnlyses. Escccs s l y 1 e n d ethcr and the desired oxetanc were obtained by coluinn chromatography ( i l a s l i c1iroinatogr;ipliy: eluent. cyc1ohexane;ethyl acetate as ii gradient 200:l + 95, 5 ) . boi- oset;ine\ m -2d.riic~3da n d r u ( . - 2 f ; r u ~ 3 fii complcte separation of diii\ t c r c o i ~ u m c i - uiis \ posiblc. In the other eximples only ;in enrichment of the major diwtercoiwmera could he ~ h i e v e d All . compounds not yet reporled uere charac- tcrircd by \pccti-o\copic methods ( ' H , "C. IR, MS) and gave correct elemental a n a l y x s ( C . H i- 0 . 3 ' h ) . Received: May 9. I995 [Z79681E] <;erman version ' A i i , y m . C ' h i i . 1995. 107. 2455-2457 - Keywords: asymmetric syntheses cycloadditions Paternh- Ruchi reactions * photochemistry oxetanes . [I]t i ) .) A . Porco. S L. Schreiher i n ~ ' i i i i i / i ~ ~ , / i i , i0i;puirc t . ~ ~ r [ ~ .SI.II//~P.Y;\ l.b/ i (Eds. H l - i o h i . I. I leniing. L. A. Paquette). Pergamon. Oxford. 1991. pp. I 5 1 -192. b) H A. J (',irle\s in .Swir/ie/k Orgmitr Phoroi/i~,iii!srr~.(Ed.: W. M . Horspool). Plenum. Ncn York. 19x4. pp. 42.5 4x7: c) G. Jones I 1 i n Orxirnir. Pliorodwiri . t / i i Lo/ .r ( 1 x 1 : A. Piidha). Dekker. New York. 1981. pp. 1 - 1 3 ; d) D. A . Arnr)ld. 4111. P/iii/iii/wiii. 1968, 6 , 301 423. v.: Y.Inoiie. C ' / i i ~ i i i .Rev. 1992, Y2, 741 770. 131 H . Bu\chm,inn. H -D. Scharl'. N . Hol'linann. P. h e r . A i i , q i v . C'/rci?i.1991, 103. 5x0 5 l X . iq(,ii. C'/iiwi / I ] / . Ed. Oix/. 1991, 30. 477-515 iind references thesell1  t x ; i m p l e ~ .i i ) 1) K . blurton. K.A . Morge. J. Org. C/iiwi. 1978. 41. 2093 -2101 , h ) L Ar;ihi. K Senn;r. K . M a t w u r a . Y. Ishido. C'rir/io/ij.i/r. K F . ) . 1978. 60. 3X'l .3Y5. C I S. Vahudccan. C. P. Brock. 0 . S. Watt. H . Morita. .I Orx. C ' / i i w i 1994. YO. 4677 5679. c 227. 857- X63. 151 S I. Schrcrhcr. S i w ~ i i ~1985. [h] ii) 'I: Hach. / ( , r i ~ d i i v / r o i iLcrr. 1991. 33, 7037- 7038: b) T. Bach. K Jikiicke. ( ' / i < , i i i . 8i.r 1993. 126- 2457 2466: c ) T. Bach. 7 i , / r o / i d r o i r L r / / . 1994. 3s. 5x45 5X4X. <I) T. BiKtl. Li1,h;q .Aii?l. 199s. 855 -X66. 171 Since the \ i l k 1 enol ethcrs r u - I a l l h e (%)-conliguration the m e of the r-subs t i ~ u c i iI ~ i i i t prcrtim~ihlq no influence o n tlie iacial diastereo5clectivit).. The choice (11' tlic i w - b u t y l group pro\ed advantageous i n view of the desired d c t c r i i i i n a t i ~ ~onf the enantiomcric purity tot- o s e t m e 2d. [XI 7 ILich. K Jndicke. M . Grehl, R Friihlich. unpublished [Y] Re\le\\: I<W H ~ f f ~ f i n ~ Ci n' k~i i .i R1.v. 1989. XY. 1x41 -1860. [ l o ] T h e c o i i r ~ eoi i-~idicdl.idditions to :I prostereogenic C C double bond IS oltcn .ing htereogcnic center i n a related \ray. However. almost include ii nucleophilic .ittack on ;in electron deficient Srnadja. Sw/c,// 1994. 1 -26: b)T. Morlkawa, Y. Washio. S 1iiit;id;i. R !Linai. 7 Kayashila. H. Nrmoto. M. Shiro. T. Tiiguchi. .I Climi. S,IC l J < i / \ / i i /?<i!i\ I IYYS. 271 2x1 and references therein. [ I 11 ( i . J. M d L ~ r b c ) .J. bl Williams. J Aiu C / w i i i . S i c . . 1985, 107. 1435 1437 and reicic~iccs~ I ~ ~ I - c I I ~ . [I?] For :I coinpreIicnsi\'c r e ~ i on e ~ thc \ariou\ intersystem crossing geometries rclated to o w t x i i e Sorin~ition,see: A. G . Griesheck. H. Mouder, S. Stadtniuller. . A < < . C'/i<,lii K P ~ IY94. . 27. 70 75. 1131 B L: Kv\sitcr. N. M . S\r#ingle. C'heiii. R r r . 1992, 92. 771 -X06. 114) 1. M u k a i ~ m a N. . I\rasa~\a,C ' h i w . L r r r r . 1981. 913 916. [ I 51 The shil't cpei-iinent\ were conducted with tris-[3-(lieptafluoropropS.lhydrux?inelliyleiie)-~~-c.irnphor;ito~europiiiiii [Eu(hfc),]. .ind the ec \dues were determined h! mtegi iition of the separated signals obtained for the !rrr-hutyl group :II ('-.3 o i ' t l i e oycI:inc. Base line separation was achieved by employing I 0 m g oitlic rciigeiit ioi- 15 inig o f w . - 2 d i n I m L CDCI,. Under these conditions 2d d i \ p l ; i ~ s d,I wigle line only. [ 161 ( ' r y t d l <I;it*i l o i ?-o\ctanol w - 7 I C , , > H 2 d 0 ,A(, 3 , = 264.35). c r y s t d m e 0.25 x 0.2 x 0 115 n i i i i . ( I = lI).27X(?). /I = 18.493(4), 1 . = X.458(1) i\. /i= lOX.40(2) , 1 = lii0.7(5)..\'. pc,,cc, =1.147gcnY'. lrni = 6 . l c r n - ' . Z = 4. monoclinic. (', lNc>.O), Enrnf-Nonius-CAD4diffractoineter. .; = 1 54178 A. I f A X sellections ( - / I , + h . &/I, [sin/) 0.62 A I. 1647 independ e n t iiiid 1275 <)bservedrcllcction\ [F>4n(F)].refinement of I78 parameters, li = K 0 5 l . I I I<' = 0.131. direct niethods. hydrogen atoms calculated. Further det;iiI\ ol'tlic c i y s t n l structure invesliyition may be obtained from the FachinIbrini~iti~~nrtciitrutii K;irlsruhe, D-76344 Eggenstein-Leopo1dshali.n (Gerin;in?.i.tm qut"ing the depository number CSD-501760. 1171 I.: N H m h . M . N. P d d o n - R o w . N G Rotidan, Y.-D. Wu. F. K . Brown. U. ( ' . Spelliiie~ei-.J. 1. Met/. Y LI. R. J. Loncharich, . S " i m i , 1986. 231. 1 1 O X I 1 17. 11x1 i l l 'I 13iich. / ? r r d i ~ / i ~1.ivr. ii 1994. 35. 1855 lX5X: b) 1. Bach. L;[,hi,q.\ .41ii1 1995. 104.5 I O i 3 . [ IY] 1.0s ii prulccccd ring upcniiig o f t h i \ typc. see: T. Bach. K . Kiither. T ( ~ r r a h i 4 r o i i 1994.10.1 2 \ 1 0 12328. \p;icc pioiip r+2I) xiin. A Tailor-Made Metallocene for the Copolymerization of Ethene with Bulky Cycloalkenes** Walter Kaminsky," Rudiger Engehausen. and Jiirgen Kopf Dcdic ~ i t 10 ~ d PI ofcJ\ T O Y Jocic liii1i Klern tlw o ( c L i ( r o i 1 of h r ~60th hrrlho'nl 011 Metallocenes in combination with methylaluminoxane (MAO) have gained great prominence as catalysts for the polymerization of olefins. Remarkable is their ability to polymerize cycloolefins such as cyclopentene. norbornene, and dimethanooctahydronaphthalene (DMON) to give homo- and copolymers in which only the double bond is opened and not the ring.['] This distinguishes catalysis with metallocenes from that with common Ziegler-Natta catalysts o r metiithesis catalysts which are known to produce polymers containing ring-opened structures and elastomeric polymers by ring-opening polymerization.['] Polycycloolefins prepared with rnetallocene catalysts exhibit extraordinarily high melting points of 400 'C and above due to their rigid, tactic structures. The high melting points prevent melt processing of these polymers. However, the copolymerization of cycloolefins with ethene results in cycloolefin copolymers (COC) with interesting properties. These copolymers are amorphous if more than 15 moly" of cycloolefin is incorporated. feature glass transition temperatures exceeding 200 C, have high mechanical strength, and arc stable towards common solvents. Since they contain no aromatic groups or double bonds their interaction with light waves is modest. Therefore they are well suited for optoelectronic applications in data transfer and storage (polymer optical libels, compact discs); other possible applications have been discussed recentIy.I3l C2-and C',-symmetric metallocenes perform much better in the homopolymerization of cycloolefins and their copolymerization with ethene than compounds with higher symmetry like bis(cyclopentadieny1)zirconium dichloride which shows only low polymerization activities. Zirconocenes like lC4]and 215] containing unequally sized x ligands incorporate sterically demanding cycloolefins at high speeds into the polymer chain. The best copolyinerization results are obtained with the C,-symmetric 3 which up to now has not been characterized in detail and has only been applied in the polymerization of styrene.t61 [IMelC(lluo)(Cp))ZrC'l~]1 [IPh,C(fluo)(C'p)JZrClz] 2 [ I PhzC(tnd)(Cp))ZrCI,] 3 The structure of 3, which crystallizes as yellow needles, was elucidated by single-crystal X-ray diffraction studies (Fig. I ) .I8' The zirconium atom resides in a tetrahedral environment surrounded by two chlorine atoms and the centroids (Cen) of the five-membered rings of the indenyl and the cyclopentadienyl groups. The centroid-Zr-centroid angle (1 17.14 ) in 3 which is [*I Prof. Dr. W. K;iminsky. Dr. R . Engehauseii Institut l i r Technische und M;ikromolekul;ir ('heinie dti 1Jiii~ersit;iI Bunde~strasx!45. D-20146 Hamburg (Gel-inmy) Trlet'au. Int. code ~ ( 4 04123hI)OX ) Dr. J. Kopf I n s t t t n t fur Anorg;inische und Angeuandte Chemie dei l!nivcrstit M a i - t i n - L u t h e r - K i i i ~ - P I ~D-20146 it~. Hamburg ( G c r i i i ~ i n ~ ~ [**IThis work w a s fin;inciallq u p p o r t e d by the Bundesiiiini~tctiuiiifur Fimchung iiiid Technologie. t h e Hoechst AG. and the Fonds der clicniischen Iiidustrie.