114 CHINESE JOURNAL OF CHEMISTRY 2002, 20, 114-116

Immobilization of Pd( II) Catalysts for Cyclopropanation in Ion- ic Liquid

Homogeneous catalysis offers many advantages over heterogenous catalysis with respect to activity, selectivi- ty, flexibility of operation. However, the continuous cat- alyst consumption, catalyst removal from the pduc t s and catalyst disposal have still to be improved. An at- tractive approach to ovemme the problem is biphasic catalysis, a method to heterogenize a catalyst and p d - uct into two separate and immiscible phases without la- ing the efficiency and selectivity inherent in homogenous catalysis. 'Ihis method also provides an elegant solution to the catalyst/p&ct separation and catalyst recyclmg associated with homogenous catalysis. Among Herent strategies to biphasic catalysis, non-aqueous ionic liquids, i . c . a salt mixture with a melting point below or at ambient temperature, constitute a unique clans of

versatile solvents with a great potential. This concept has been receiving increased attentions in the last few years. Recent applications include Friedel-Crafts reactions, Diels-Alder reactions, alkylations, olefin dimerisation and oligomerisation , Heck reaction, hydmfonnylation, palladium catalyzed allylation reaction,' epoxidation* and asymmetric ring opening reaction of epoxides.

It is well known that palhhum acetatddiazo esters is an efficient reagent for the cyclopmpanation of tenni- nal ole fin^.^*' As a homogeneous catalysis, it is usually difficult to reuse it after the reaction. Herein, a practi- cal recyclmg procedure of palhhum catalyst involving the use of air- and moisture-stable ionic liquid,

methylimidazolium cation) is reported. 'Ihe ionic liquid 1 is conveniently prepared by literature method and has a

cy- clopropadon of styrene with ethyl diazoacetate, a m- tion which has been widely used for assessing the &- ciency and selectivity in cyclopropane was

used to examine the biphasic catalysis. Palladium ac- etate, pallad~um chloride and cycloplldated complexes

[&I- [BFd] (11, ([&I+ = l-0ctyl-3-

limited miscibility with low polar organic

2,3 were employed 89 palladium catalysts in OUT study.

E-mail: tangecnu@263.net Received August 29, 2001 ; revised November 11, 2001 ; accepted November 20, 2001. Project supported by the Foundation of shanghru Science and Technology Development (No. OlJC14002).

Vol. 20 No. 1 2002 Chinese Joumal of Chemistry 115

Cyclopmpanation reactions were carried out at am- bient tempera-, and the catalystlpduct separation is easily achieved by simple phase decantation. The organ- ic phase was analyzed, and the ionic liquid which con-

tained palMum catalyst was reused for further reac- tions. 'Ihe results using pallachum acetate and palladium chloride were summanzed ' in Table 1.

Table 1 Cyclopropanahon of styrene using Pd( OAc), and PdQ in [ omim] [ BF4] '

Yield (%)* apru/Cic Ratio' Run Tune (h)

Pd( OAC)~ PdQ Pd(Ok)2 p a 1 10 9a 9s 1.7 1.6 2 10 9a 94 1.7 1.6 3 10 96 93 1.7 1.7 4 10 73 93 1.6 1.6 5 20 34 9s 1.6 1.6 6 20 a 95 1.6 1.6

'Reaction conditiom: styrene, 7 mL (6 x lo-' mol) ; catalyst, 4 x lo-' mol; ethyl dice, 4 x lo'' md; [ omim] [ BF,], 1.5 mL. * Determined by GC. ' Ratio of area in the CC peaks, tmns/cic pro$ucts mre identified by canparison with authentic samples.

Usually palhhum chloride is a less efficient cata- lyst than palladium acetate due to its poor solubility in organic media. However, its catalytic activity was p a t - ly enhanced by the use of ionic liquid 1. This was at- tributed to its higher solubility in ionic liquid. As shown in Table 1 , in the case of palladium chloride, the recov- ery and reuse of the catalyst were more successful com- pared with the case of palladium acetate. For example, the above pmedure was repeated for 6 times with a sat-

isfactory yield when palladium chloride was used (Run 1 4 , Table i 1, while this procedure was repeated sat-

isfactorily for only 3 times with a satisfactory yield using palladrum acetate (Run 1-3, Table 1). During the re- action, the color of ionic liquid containing palladium ac- etaie tumed black after one run indicating the formation of palladrum(0) species, which pmbably resulted from the reducing ability of ethyl diazoacetate. "he catalytic activity was decreased greatly as the palMum(0) species was formed. In contrast, for palliuhum Chloride, the color of ionic liquid remained pale yellow in the most of the time and only a trace of palladrum deposit was ob- served. The catalytic activity still remained unchanged after PdCl2 was recycled for 6 times. "he reason of the decrease of the catalytic activity for Pd( 0Ac)z along with the increase of recycling times maybe is that the f o m - tion of palladium(0) is easier for Pd(OAc)z than that for PdC12.

In the last decades, cyclopaUadated cnrnplexes have attracted people' s interests increasingly.'" The com-

plexes, with paUadium center stabilized by a five-mem- ber ring, are thermally stable and not sensitive to oxygen and moisture. They have shown super catalytic activity for some paUachum-rnediated reactions. The previous study by our p u p demonstmted that complexes 2 and 3 are good homogeneous catalysts for Heck reaction and cyclopmpanation . Is Their applications in biphasic c d y - sis for cyclopmpanation were also conducted here. "he

results were summanzed * in Table 2. The results showed that complex 2 is more efticient

and recyclable than complex 3. This was also indicated by respective color of ionic liquid layer. The color of ionic liquid layer containing complex 2 remained pale yellow for all runs. However, for complex 3, the color tumed black after one run indicating formation of paUa- dium(0) species. We suppose that the Merences of catalytic activities result fmn the triphenylphoephme lig- and which is the substantial Werence between structures 3 and 2. It is assumed then that the triphenylphoephine ligand promotes the formation of palladium( 0 ) . So the

compared with complex 2, which leada to the de- crease of the catalytic activity of complex 3. Moreover, the results in Table 2 also showed that biphasic catalysis using cyclopalladated complex 2 was more efficient than that using pallarllum chloride.

In summary, we have developed a practical nxy- cling procedure of palMum catalysts for cyclopmpana- tion by use of ionic liquid 1. Our results showed that palladium chloride , which was not an efficient catalyst

Palladium(0) was more d y formed fbm complex 3

116 Palladium catalyst8 YANG et al.

Table 2 Cyrlopmpmtion of styrene using complexes 2 and 3 in [ami~nl[BF,]~

Yield ( % ) b rmnr/& Ratio'

2 3 2 3 Run T - /( h)

1 10 99 98 1.8 1.7 2 10 98 97 1.7 1.7 3 10 98 93 1.7 1.6 4 10 98 62 1.6 1.6 5 10 98 21 1.6 1.6 6 10 97 8 1.6 1.6 7 20 96 7 1.6 1.6

O ~ ~ t i ~ : styrene, 7 m L ( 6 ~ 1 O - ~ m l ) ; catalyst, 2x10-Jmolforcomplex2and4~10-5molforcomplex3; ethyldia- ~ ~ ~ c e t a t e , 4 x m l ; [omim][BF~], 1.5 mL. 'Determined by GC. 'Ratio of area in the GC peaks.

for cyclopmpanation in ordinary cases, became more ac-

Meanwhile, biphasic catalysis using cyclopalladated complex 2 tumed out to be successful in our study. At the end of the reaction, the catalyst contained in ionic liquid could be easily recycled for several times- 'Ihe use of ionic liquid provides not only a simple mycling of pahd~um catalyst, but also the additional advantage Of enhancing the catalytic activity of a catalyst. A further studv concerning the detailed interaction between the

N.; Teysaib, P. 1. Org. Ckm. 1980, 40, 695. e, A. J . ; - 9 A * ; N ~ e l s . A. F.; W h , R. ; Hubert, A. J . ; Tepsi6, P. T ! h n 1983, 39, 2169.

6 H o l b r e y , J . D . ; S e d d o n , K . R . J . C h a n . S o c . l 9 9 9 , 2133.

7 Fik,i, H. ; Leutenegger, u. ; Hdtz, A. Hel,, . h. Ada l9B. 71, 1553.

8 Ryabov, A. D. +U ME, 233. 9 N m l r m , G. R.; Puck&, W. E.; Gupta, V. K . ;

Kiefer, G. E. Chem. Reu. 1986, 86, 451.

tive in biphasic catalysis mediated by ionic liquid. 5

Y

palladium salts and the ionic liquid and the application of this methodology to other homogeneous catalysis is currently in pmgress.

lo RY*, A. D. 11 Wang, H. X.; Ding, L.; Wu, Y. J. Chin. J . Org.

12 B o s e , A . ; S a h a , C . R . J . M d . C a r n l . 1 9 [ 1 9 , 4 9 , 2 7 1 .

1990, 90,403-

Chan. m, 20,44(in h).

13 H e r m ~ n , W. A . ; B&u, V. P. W.; Rekinger, C. P. J . OrgMMCr. Chan. 1999, 576, 23. Chi, X.; Grigg, R . ; Raman, M. I.; Sridharan, V . ;

1 W e l t o n , T . h . R a r . 1 9 9 9 , 9 9 , 2 0 7 1 . C$lard,S.;Muir, J. E. Chem. Gmmun. 2000,2053. 2 3

References 14

sons, C. E.; Roh, E. J. Chem. Canmun. m, 837. 15 zhsng, Y. M.; Y-9 F.; Re; Tang, J. chin. Song, C. E . ; O h , C . R . ; R o h , E . J . ; C h o o , D . J.

4 Ancisln, A. J.; Hubat, A. J . ; Nab, A. F.; Petiniot,

1. org. h. 2001, 21, 533(in Chinese).

2177. chsn. Gnmurn. 2000, 1743. 16 O n m a , F . ; K u b o , A . ; H a ~ , T . C h a n . L t t . 1 9 9 t ,

(Ell108293 SONG, J. P. ; HUANG, W. Q.)