Theoretical Studies on Reaction Pathways of Samarium(II) Carbenoid- Promoteded Cyclopropanation Reaction of Ethylene

CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 19, NUMBER 1 FEBRUARY 27, 2006

ARTICLE Theoretical Studies on Reaction Pathways of Samarium(II) Carbenoid- Promoteded Cyclopropanation Reaction of Ethylene

Zhi-yuan Geng∗, Xing-hui Zhang,Yong-cheng Wang, Ran Fang, Li-guo Gao, Xiao-xia Chen, Cun-yuan Zhao College of Chemistry and Chemical Engineering, Gansu Province Key Laboratory of Polymer Materials Northwest Normal University, Lanzhou 730070, China

(Dated: Received on January 26, 2005; Accepted on June 17, 2005)

In order to have eﬃcient and highly stereoselective cyclopropanating reagents, the cyclopropanation reaction of ethylene promoted with Samarium(II) carbenoid [Simmons-Smith(SS)reagent] were studied by means of B3LYP hybrid density functional method. The geometries for reactants, transition states and products are completely optimized. All transition states were veriﬁed by the vibrational analysis and the intrinsic reaction coordinate (IRC) calculations. The results showed that, identical with the lithium carbenoid, CH3SmCH2X(X=Cl, Br and I) can fairly react with ethylene via both methylene transfer pathway (pathway A) and carbometalation pathway (pathway B). And the cyclopropanation reaction via methylene transfer pathway proceeds with a lower barrier and at lower temperatures. Key words: Samarium carbenoid, Cyclopropanation, Simmons-Smith reaction, Density functional theory

I. INTRODUCTION actions for ISmCH2I carbenoid with CH2CH2 was reported by Zhao [7] and co-workers. They found the Cyclopropane moieties have been found in a wide ISmCH2I carbenoid reaction barriers computed for the range of natural and artificial compounds that exhibit methylene transfer pathway was 53.95 kJ/mol and the important biological activities and in an array of sub- reaction barriers for cyclopropanation via the carbomet- stances used as starting materials and intermediates alation pathway was 71.51 kJ/mol. The studies indi- in organic synthesis [1]. This has motivated a large cated that ISmCH2I carbenoid reactions with ethylene number of research groups to develop new and wide- can experimentally occur with relatively lower barriers, range methods to produce cyclopropanated products. and they also studied the additive effect of THF solvent. A reaction between iodomethylzinc iodide and an olefin These results is in accordance with the experiment re- that produces a cyclopropane compound was first reports [3–6]. However, the effect of the different substi- ported by Simmons and Smith in 1958 [2] and is now tute atom (Cl, Br, or I) on Samarium carbenoid species called the Simmons-Smith(SS) reaction, which is the (XSmCH2X, where X=Cl, Br, I) was not studied in method for synthesizing cyclopropanated products from that work. In this paper, we employ a hybrid density olefins using a metallic carbenoid (SS) reagent. Fol- functional method to investigate the cyclopropanation lowing this a great deal of works have been done to reaction mechanisms and pathways of the CH3SmCH2X improve and develop alternative methods to produce (where X=Cl, Br, I) with ethylene. The likely effects Simmons-Smith-type reagents by a number of research of the different atom Cl, Br or I on the cyclopropana- groups. Many researchers have been trying to find more tion reaction of Sm(II) carbenoid species was explicitly efficient and highly diastereoselective cyclopropanating considered. The computational results are briefly com- reagents. Metallic carbenoid is an important interme- pared with other carbenoid reactions [8–19] and related diate in organic synthesis for cyclopropanation reac- species in order to have better understandings for car- tion. Recently there were experimengtal reports that benoid catalyst. Samarium carbenoid is an efficient S-S reagents. Sev- eral stereoselective reactions with Sm/CH2I2 and/or the Sm/CH ICl reagents have also been reported for cyclo- 2 II. COMPUTATIONAL DETAILS propanation [3–6]. The Sm/CH2I2 carbenoid is believed to be one of the most efficient and highly diastereoselective cyclopropanating reagents. The cyclopropana- The hybrid B3LYP (DFT) density functional method tion reactions are usually performed in THF solvent was used to optimize geometries for all reactants, in- at −78oC and high yields of cyclopropanated products termediates, transition states and products. Analyti- can be achieved at low temperatures. To our knowl- cal frequency calculations were performed in order to edge, there have been only a few theoretical compu- confirm the characters of optimized structures and to tational study of the cyclopropanation reaction. Re- obtain the zero-point energy correction. IRC calcula- cently, a computational study of cyclopropanation re- tions were performed to confirm the optimized transition state correctly connecting related reactants and products. All calculations used the relativistic core po- tentials (RECPs) for the Sm(II) atom, in which 5s, 5p, ∗Author to whom correspondence should be addressed. E-mail: and 6s electrons were explicitly treated as “valence” [email protected] electrons with the remaining electrons replaced by the

ISSN 1003-7713/DOI:10.1360/cjcp2006.19(1).69.7 69 °c 2006 Chinese Physical Society 70 Chin. J. Chem. Phys., Vol. 19, No. 1 Zhi-yuan Geng et al.

FIG. 1 Methylene transfer and carbometalation pathways for reaction of Samarium carbenoid CH3SmCH2Cl with ethylene. Geometry optimization for all reactants, intermediates, transition states and products were carried out at B3LYP/6-311G**.

RECP [20–22]. The partially filled 4f6 electrons which Analytical frequency calculations were performed in or- do not participate actively in the bonding were also in- der to confirm the optimized structures to be either a cluded in the core for a total of 52 electrons. Thus, 10 minimum or a first-order saddle point, which are sta- valence electrons RECP were used for the Sm(II) cen- tionary structures on the potential energy surfaces of ter. The composite basis set is also composed of the the reactions. The transition state structure had one 6-311G** basis set for the CH2CH2 and the CH3, CH2 imaginary frequency. IRC calculations were performed groups in the Sm(II) carbenoid moiety, the LANL2DZ to confirm the optimized transition state correctly con- basis set for the atom Cl, Br, and I. All calculations nects the relevant reactants and products. The poten- were carried out using the Gaussian 98 program suite. tial energy profiles along the reaction coordinations of All geometry figures were completed from calculational the transition state TS4 and TS6 are showed in Fig.5. results by GaussView 3.07 program.

A. Cyclopropanation reactions of the Sm(II) carbenoid III. RESULTS AND DISCUSSION CH3SmCH2Cl with ethylene

The optimized key geometry parameters for all reac- The cyclopropanation reactions of the Samarium car- tants, intermediates, transition states and products of benoid CH3SmCH2Cl with ethylene proceed through the reaction system of CH3SmCH2X (X=Cl, Br, I) with two possible pathways: methylene transfer and car- C2H4 are depicted schematically in Figures 1, 2, and bometalation. The former pathway involves a one-step 3. The total energies with ZPE correction, the relative mechanism through a three-centered transition state energies of starting molecules and vibration frequencies where the pseudotrigonal methylene group of the car- are listed in Table I. The relative energies including ZPE benoid adds to the ethylene -bond to form two new C−C for the diﬀerent reaction pathways are shown in Fig.4. bonds asynchronously. Another pathway is a carbomet- Chin. J. Chem. Phys., Vol. 19, No. 1 Theoretical Studies on Sm(II) Carbenoid 71

FIG. 2 Methylene transfer and carbometalation pathways for reaction of Samarium carbenoid CH3SmCH2Br with ethylene. Geometry optimization for all reactants, intermediates, transition states andproducts were carried out at B3LYP/6-311G**.

o alation process, in which addition of ethylene to the C2−Sm−Cl and Sm−C1−Cl bond angles are 122.25 o Sm−C bond for the carbenoid proceeds to produce an and 78.81 , respectively. The C2−Sm−C1−Cl dihedral o intermediate through a four-centered transition struc- angle is −111.69 in RC1. The ethylene CH2CH2 coor- ture. They are denoted as the pathway A and pathway dination to the Sm(II) carbenoid to form the reactant B in Fig.1, respectively. We studied cyclopropanation complex RC1 does not significantly change the geom- reactions of the Sm(II) carbenoid CH3SmCH2Cl with etry of the carbenoid species except for Sm−C1 and ethylene through two different pathways. Figure 1 dis- Sm−Cl bond lengths that become slightly elongated. plays the optimized geometry parameters found for the Examination of Fig.1 for the methylene transfer path- Sm (II) carbenoid CH3SmCH2Cl, the reactant complex way (pathway A) shows that the transition state TS1 RC1, and the transition states TS1 and TS2 for reac- is a three-centered struture. The C3−C4 and Sm−C1 tions with ethylene through two different pathways to bond lengths are elongated by 1.1 and 5.8 pm respec- produce cyclopropane (c-C3H6) and CH3SmCl. The tively upon going from the reactant complex (RC1) Sm−C2, Sm−C1, Sm−Cl and C1−Cl bond lengths are to the transition state (TS1). The interaction of the 0.2572, 0.2594, 0.2926 and 0.1592 nm in the Samarium CH3SmCH2Cl moiety with the olefin orbitals is mainly carbenoid CH3SmCH2Cl. The C2−Sm−Cl bond an- responsible for the slight lengthening of the C3=C4 o gle and the C2−Sm−C1−Cl dihedral angle are 119.49 and C1−Sm bonds. Relatively large changes are as- o and −109.37 for CH3SmCH2Cl. A π-complex RC1 sociated with the C2−Sm−C1−Cl dihedral angle, the is formed when the two molecules(CH3SmCH2Cl and C2−Sm−Cl angle, the Sm−Cl, C1−Cl, C1−C3 and o o C2H4) move toward one another and can be regarded as C1−C4 distances that vary from −111.69 , 122.25 , reactant complex for these two reaction pathways. The 0.2943, 0.1953, 0.3333 and 0.3350 nm, respectively in o o bond lengths for both Sm−C3 and Sm−C4 are 0.3333 RC1, to −114.95 , 122.19 , 0.2807, 0.2414, 0.2335 and and 0.3350 nm for the reactant complex RC1. The 0.2576 nm, respectively in TS1. Meanwhile, The C1−Cl 72 Chin. J. Chem. Phys., Vol. 19, No. 1 Zhi-yuan Geng et al.

FIG. 3 Methylene transfer and carbometalation pathways for reaction of Samarium carbenoid CH3SmCH2I with ethylene. Geometry optimization for all reactants, intermediates, transition states andproducts were carried out at B3LYP/6-311G**.

bond becomes nearly broken and the Sm−Cl bond be- and is on its way to make the propyl group as found comes almost formed in TS1. These changes in the in IM1. The C2−Sm−Cl bond angle undergoes a large bond lengths and bond angles are attributed to partial change from 122.25o in RC1 to 114.49o in TS2. This formation of the CH3SmCl byproduct in the transition suggests the Sm interaction with the C3 and C4 atoms state (TS1). Vibrational analysis showed that the TS1 become strong enough to make it appear with more structure has one imaginary frequency (316.15i cm−1). four coordination character in TS2 than in TS1, which o It is evident that TS1 is the transition state of the con- still has a large C2−Sm−Cl angle of 122.19 . These certed reaction from the RC1 reactant complex to PD1 changes in structure as RC1 goes to TS2 appear some- (c-C3H6 + CH3SmCl) products by IRC calculations. what larger than those experienced as RC1 goes to TS1, which was discussed in the preceding paragraph. This In the carbometalation pathway (pathway B), an in- suggests that there should have a larger barrier for RC1 sertion reaction of ethylene to the Sm−C1 bond occurs to TS2 than to TS1. Vibrational analysis found that the to produce intermediate IM1 through a four-centered optimized TS2 structure had one imaginary frequency −1 TS2 transition state. The Sm−C3 interaction increases (320.87icm ) and that TS2 was confirmed to be the significantly as the bond length changes from 0.3333 first-order saddle-point connecting the corresponding nm in RC1 to 0.2663 nm in TS2. The C3−C4 bond reactants and products by IRC calculations. weakens somewhat from 0.1333 nm in RC1 to 0.1412 nm in TS2 and the C1−C4 bond forms to a significant Inspection of Table I and Fig.4 shows that the reac- extent and goes from a distance >0.3000 nm in RC1 tion for the methylene transfer pathway has a barrier to 0.1979 nm for TS2. This is accompanied by weak- of 12.52 kJ/mol (with ZPE correction) and is exother- ening of the Sm−C1 bonds from 0.2612 nm in RC1 mic by about 235.93 kJ/mol at the B3LYP/6-311G** to 0.2768 nm in TS2. The preceding changes suggest level. The reaction for carbometalation pathway has a that as RC1 goes to TS2 the C−C−C moiety forms barrier of 29.69 kJ/mol (with ZPE correction). The re- Chin. J. Chem. Phys., Vol. 19, No. 1 Theoretical Studies on Sm(II) Carbenoid 73

FIG. 4 Energy proﬁle along the isomerization pathways of reaction for Samarium carbenoid with ethylene

TABLE I Total energies Et relative energies Er, and vibration frequencies v in diﬀerent geometries −1 Geommetry ZPE/a.u. Et/a.u. Er/(kJ/mol) v/cm

CH3SmCH2Cl 0.05745 −568.23948 − 31.97

CH3SmCH2Br 0.05678 −121.18245 − 32.25

CH3SmCH2I 0.05649 −119.40153 − 25.10

CH2=CH2 0.05085 −78.55611 − 840.31 RC1 0.10913 −646.81096 −40.35 12.14 RC2 0.10831 −199.75438 −41.54 8.57 RC3 0.10816 −197.97329 −40.98 15.52 TS1 0.10926 −646.79082 12.52 316.15i TS2 0.11070 −646.78428 29.69 320.87i TS3 0.10889 −199.73730 3.31 282.82i TS4 0.10961 −199.72900 25.10 367.97i TS5 0.10882 −197.95394 9.71 306.21i TS6 0.10933 −197.94836 24.36 258.39i IM1 0.11349 −646.82097 −66.64 34.96 IM2 0.11251 −199.76166 −60.65 38.0 IM3 0.11205 −197.97888 −55.77 31.62

CH3SmCl 0.03289 −529.03560 − 19.77

CH3SmBr 0.03266 −81.97918 − 25.13

CH3SmI 0.03255 −80.19283 − 26.56

c-C3H6 0.08100 −117.84985 − 744.21 FIG. 5 Potential energy proﬁles along the reaction coordination benoid was found to possess a “samarium carbene complex” character with a structure, properties, and chemical reactivity similar to previously studied lithium car- actant complex RC1 is lower in energy by 40.35 kJ/mol benoids [2]. of theory than the starting materials. The relatively low barriers found for the two diﬀerent pathways for the Sm(II) promoted (CH3SmCH2Cl) cyclopropanation re- B. Cyclopropanation reactions of the Sm(II) carbenoid actions indicate that the cyclopropanation reactions of CH3SmCH2Br with ethylene the CH3SmCH2Cl carbenoid with ethylene could proceed via a methylene transfer pathway or carbometa- Figure 2 shows the geometry of Sm(II) carbenoid lation pathway, but obviously the methylene transfer species CH3SmCH2Br. The Sm−Br, C1−Br, and pathway is more favorable. The CH3SmCH2Cl car- Sm−C1 bond lengths are 0.2943, 0.2187 and 0.2611 nm 74 Chin. J. Chem. Phys., Vol. 19, No. 1 Zhi-yuan Geng et al.

o respectively. The C2−Sm−Br bond angle is 120.96 , or Sm−Br bond lengths. There are some systematic the C2−Sm−C1−Br dihedral angle is calculated to changes of CH3SmCH2X (where X=Cl, Br , I) with be 111.58o.A π-complex RC2 is formed when the ethylene in the structures. The geometry optimization two molecules of the CH3SmCH2Br with C2H4 move and reaction mechanism are similar to those of the reac- towards each other. The methylene transfer path- tion of the Sm(II) carbenoid CH3SmCH2Cl with ethy- way shows both C1−C3 and C1−C4 bond lengths(the lene. Vibrational analysis showed that the optimized C1−C3 and C1−C4 bond lengths are calculated to be TS5 and TS6 structures each have one imaginary fre- 0.2341 and 0.2592 nm ) in TS3 are noticeably elongated quency of 306.21i cm−1 and 258.39 i cm−1. compared to the C1−C3 and C1−C4 bond lengths in Examination of Table I and Fig.4 shows that the TS1. Meanwhile, the C1−Br bond becomes nearly bro- methylene transfer pathway has a barrier of 9.71 ken and the Sm−Br bond becomes almost formed in kJ/mol (with ZPE correction) and is exothermic by TS3. Vibrational analysis showed that the TS3 struc- −1 about 223.27 kJ/mol relative to the starting materi- ture has one imaginary frequency (282.82i cm ). It als. The barriers increase by only 4.40 kJ/mol to is evident that TS3 is the transition state of the con- the CH3SmCH2Cl carbenoid, the reaction for the car- certed reaction from the RC2 reactant complex to c- bometalation pathway has a barrier of 24.36 kJ/mol C3H6+CH3SmBr products by IRC calculations. In including the ZPE correction. Our studies indicated the carbometalation pathway, all structures are simi- that the reactions promoted by the Sm(II) carbenoid lar to the structures in Fig.1. In transition state TS4, CH3SmCH2Br and CH3SmCH2I occur with relatively the Sm−C3 and C1−C4 bond lengths are 0.2738 and low barriers to the carbenoid CH3SmCH2Cl. There 0.1907 nm, respectively. The C2−Sm−Br bond angle is o are lower barriers for the two different pathways, but 112.02 and the C2−Sm−C1−Br dihedral angle is cal- o the reaction barriers computed for the methylene trans- culated to be 98.33 . Vibrational analysis found that fer pathway is lower than that of the carbometalation the optimized TS4 structure had one imaginary fre- pathway. The reaction mechanism of Sm(II) carbenoid quency (367.97i cm−1) and TS4 was confirmed to be CH3SmCH2X (where X=Cl, Br, I)+C2H4 is similar to the first-order saddle-point connecting the correspond- the lithium carbenoid [2]. ing reactants and products by IRC calculations. As seen in Table I and Fig.4, The reactant complex As shown in Fig.4, (i)The cyclopropanation reac- RC2 is lower in energy by 41.54 kJ/mol of theory than tions of the Sm(II) carbenoid CH3SmCH2X (where the starting materials at the B3LYP/6-311G** level. X=Cl, Br, I) with ethylene proceeds through two com- The reaction for the methylene transfer pathway has peting pathways, and the relatively low barriers are a barrier of 3.31 kJ/mol (with ZPE correction) and is found for the two different pathway; (ii)The barriers exothermic by about 237.53 kJ/mol. The reaction for for the Sm(II) carbenoid methylene transfer pathway carbometalation pathway has a barrier of 25.10 kJ/mol reactions become lower than the carbometalation path- (with ZPE correction). The barriers for the two dif- way, the methylene transfer pathway is strong exother- ferent pathway reduce 9.21 kJ/mol (with ZPE correc- mic by about 223.27−237.53 kJ/mol; (iii)The reaction tion) and 4.59 kJ/mol(with ZPE correction) respec- of Sm(II) carbenoid CH3SmCH2X (where X=Br, I) oc- tively from Fig.2 to Fig.1. The relatively low barriers cur more easily than CH3SmCH2Cl carbenoid; (iv)Our found for both of the two different pathways. However, studies indicated that Sm(II) carbenoid CH3SmCH2X the barrier for the carbometalation pathway stayed rela- (where X=Cl, Br, I) promoted cyclopropanation with tively higher. Consequently, the reaction for the methy- ethylene take place at low temperature as to relatively lene transfer pathway occurs more easily. low barriers. The barriers for the reactions of the EtZnCHI2 and IZnCHI2 carbenoid [8] with ethylene are reported to C. Cyclopropanation reactions of the Sm(II) carbenoid be 83.60 and 102.41 kJ/mol respectively. The reaction CH3SmCH2I with ethylene for LiCH2Cl carbenoid [2] reported methylene transfer pathway and the carbometalation pathway had barri- To further improve our understanding of the ers of 8.78 and 15.88 kJ/mol respectively, Both two- samarium-promoted reactions, we carried out calcula- pathway barriers for ISmCH2I carbenoid [7] are 70.64 tions on the reaction of the CH3SmCH2I carbenoid pro- and 36.78 kJ/mol. Our studies indicate that the re- moted cyclopropanation with ethylene. Our calculation action for the Sm(II) carbenoid CH3SmCH2X (where found that the structures are Similar to the structure X=Cl, Br, I) with ethylene have barriers of 3.30−29.69 of CH3SmCH2Cl and CH3SmCH2Br carbenoid. It was kJ/mol, the highest barriers of reaction may be about found that the Sm(II) carbenoid CH3SmCH2I is with 40.00 kJ/mol if a bigger basis set is employed. As C1−I bond lengths noticeably elongated comparing to there are low barriers and high reaction activity, the the C1−Cl and C1−Br bond lengths in Fig.1 and Fig.2. reaction could occur via two different reaction mecha- In transition state TS5, the Sm−I bond length is 0.0405 nisms. However, the reaction for the methylene trans- and 0.0216 nm longer, respectively, than the Sm−Cl fer pathway take place more easily, especially that the bond length in TS1 and Sm−Br bond length in TS3. CH3SmCH2Br and CH3SmCH2I carbenoid could be one Meanwhile, the C1−I bond length is 0.0353 and 0.0185 of the most efficient and highly stereoselective cyclo- nm longer, respectively, than the C1−Cl and C1−Br propanating reagents. Recently we are studying the role bond length. In transition state TS6, the Sm−I bond of THF solvent on the Sm(II) carbenoid cyclopropana- length is 0.0193−0.0322 nm longer than the Sm−Cl tion reactions. Chin. J. Chem. Phys., Vol. 19, No. 1 Theoretical Studies on Sm(II) Carbenoid 75

IV. CONCLUSION [3] G. A. Molander and J. B. Etter, J. Org. Chem. 52, 3944 (1987). A theoretical investigation of the cyclopropanation [4] G. A. Molander, J. B. Etter and P. W. Zinke, J. Am. Chem. Soc. 109, 453(1987). reactions for CH3SmCH2X (where X=Cl, Br, I) carbenoid with ethylene is given via two different reac- [5] G. A. Molander and L. S. Harring, J. Org. Chem. 54, tion pathways. A π-complex RC is formed when the 3525 (1989). [6] J. M. Concellón,S. H. Rod´rguezand C. Gómez,Angew. two molecules(CH3SmCH2X (where X=Cl, Br, I) and Chem. Int. Ed. 41, 1917 (2002). C2H4) move towards one another and it can be re- [7] C. Y. Zhao, D. Q. Wang and D. L. Phillips, J. Am. garded as the reactant complex for these two reac- Chem.Soc. 125, 15200 (2003). tion pathways. They reaction proceeds with the three- [8] C. Y. Zhao, D. Q. Wang and D. L. Phillips, J. Am. centered transition states or the four-centered transition Chem. Soc. 124,12903 (2002). states for reactions with ethylene through two differ- [9] C. Y. Zhao, D. Q. Wang and D. L. Phillips, Journal ent pathways to produce cyclopropane (c-C3H6) and of Theoretical and Computational Chemistry. 2, 357 CH3SmX (where X=Cl, Br, I). The barriers for the (2003). Sm(II) carbenoid methylene transfer pathway reactions [10] D. Y. Tang, W. Shen and M. Li, Journal of South- become lower than the carbometalation pathway, and west China Normal University (Natural Science) 28, 766 (2003). the methylene transfer pathway is strong exothermic. [11] D. L. Phillips, W. H. Fang and X. Zheng, J. Am. Chem. The reaction of the Sm(II) carbenoid CH3SmCH2X Soc. 123, 4197 (2001). (where X=Br, I) occur easily with CH3SmCH2Cl car- [12] Z. Y. Yang, J. Org. Chem. 69, 2394 (2004). benoid. A brief comparison of the Sm(II) carbenoid [13] J. Zhou, M. C. Ye, Z. Z. Huang and Y. Tang, J. Org. CH3SmCH2X and the others carbenoid species has Chem. 69, 1309 (2004). shown that the CH3SmCH2Br and CH3SmCH2I car- [14] K. Suenobu ,M. Itagaki and E. Nakamura, J. Am. benoid could be one of the most efficient and highly Chem. Soc. 126, 7271 (2004). stereoselective cyclopropanating reagents. [15] S. I. M. Jonathan and I. H. Hillier, J. Am. Chem. Soc. 125, 628 (2003). [16] S. J. Jeffrey and A. E. David Acc. Chem. Res. 33, 325 V. ACKNOWLEDGMENT (2000). [17] A. Harai, M. Nakamura and E. Nakamura, Chem. Lett. This work was supported by the Northwest Normal 9, 927 (1998). [18] H. Hermann, J. C. W. Lohrenz, A. Kuhn and G. Boche, University Science Foundation of Gansu Province (No. Tetrahedron. 56, 4109 (2000). NWNU-QN-2001-08). [19] D. Wang, D. L. Phillips and W. H. Fang, Organomet- allics. 21, 5901 (2002). [20] M. Dolg, H. Stoll, A. Savin and H. Preuss, Theor. Chim. Acta 75, 173 (1989). [21] M. Dolg, H. Stoll, A. Savin and H. Preuss, Theor. Chim. [1] D. L. Phillips and W. H. Fang, J. Org. Chem. 124, 5890 Acta 85, 441 (1993). (2001). [22] M. Dolg, H. Stoll, A. Savin and H. Preuss, J. Chem. [2] H. E. Simmons and R. D. Smith, J. Am. Chem. Soc. Phys. 90, 1730 (1989). 80, 5323 (1958).