Control of Stereoselectivity in the Ring-Opening Metathesis Polymerization of Norbornene by the Auxiliary Ligands Butadiene and o-Xylylene in Well-Defined Pentamethylcyclopentadiene Tantalum Ca
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- 作者：Kazushi Mashima ; Michitaka Kaidzu ; Yoshiyuki Tanaka ; Yuushou Nakayama ; Akira Nakamura ; James G. Hamilton ; and John J. Rooney
- 刊名：Organometallics
- 出版年：1998
- 出版时间：September 14, 1998
- 年：1998
- 卷：17
- 期：19
- 页码：4183 - 4195
- 全文大小：270K
- 年卷期：v.17,no.19(September 14, 1998)
- ISSN：1520-6041
文摘
cis-Dialkyl complexes of tantalum, Ta(CH2Ph)2Cp*(
4-C4H6) (2a) (Cp* = 5-C5Me5),TaMe(CH2SiMe3)Cp*(4-butadiene) (6), and TaMe(CH2CMe3)Cp*(4-butadiene) (7), werefound to be catalyst precursors for ring-opening metathesis polymerization (ROMP) ofnorbornene to give poly(norbornene) with a high cis-vinylene double-bond (97-99%) content,while an o-xylylene complex Ta(CH2Ph)2(4-o-(CH2)2C6H4)Cp* (9) was also an initiator togive poly(norbornene) with a high trans-vinylene double-bond (92-95%) content. When aCp-butadiene complex Ta(CH2Ph)2Cp(4-C4H6) (2b) was used as an initiator, we obtainedpoly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). Thefactors controlling these stereoselectivities have been investigated, and we obtained thefollowing results: (1) We isolated benzylidene complexes and found that the proportion ofanti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalumcenter, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe3 resultedin the formation of a benzylidene complex Ta(=CHPh)Cp*(4-C4H6)(PMe3) (3a) as an anti-rotamer, which has been characterized by X-ray crystal structure analysis, while a similartreatment of 2b afforded Ta(=CHPh)Cp(4-C4H6)(PMe3) (3b), also in the anti-rotamer form,as revealed by a comparison of the chemical shift value of the benzylic proton of 3b withthat of 3a. In sharp contrast to the anti-geometry, a benzylidene complex Ta(=CHPh)(4-o-(CH2)2C6H4)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, wasobtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it isa syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes areconsidered as intermediates during the propagation step. We could not isolate anymetallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane Cp*(4-o-xylylene)Ta[CH(Ph)CH(C10H6) CH] (15) was isolated in 18% yield when acenaphthylene,instead of the norbornene, was added to 12. The trans-phenyl geometry of the metallacyclobutane ring system, which indicated the retention of the Ta=C bond stereochemistry,was determined by the X-ray analysis of 15, whose structural features were compared withthat of Cp*(4-butadiene)Nb[CH2CH(C10H6) CH] (14). (3) In contrast to high cis/transstereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the 13C NMR spectra ofthe hydrogenated derivatives was found to be almost atactic. This suggests that thealkylidene species would be a resting state during the propagation, and thereby monomercould be added to both faces of the Ta=C bond, leading to the atactic polymer. Consequently,the high cis and trans stereoselectivity thus demonstrated is ascribed to the huge congestionbetween the Cp* ligand and auxiliary ligands 1,3-butadiene or o-xylylene.
4-C4H6) (2a) (Cp* = 5-C5Me5),TaMe(CH2SiMe3)Cp*(4-butadiene) (6), and TaMe(CH2CMe3)Cp*(4-butadiene) (7), werefound to be catalyst precursors for ring-opening metathesis polymerization (ROMP) ofnorbornene to give poly(norbornene) with a high cis-vinylene double-bond (97-99%) content,while an o-xylylene complex Ta(CH2Ph)2(4-o-(CH2)2C6H4)Cp* (9) was also an initiator togive poly(norbornene) with a high trans-vinylene double-bond (92-95%) content. When aCp-butadiene complex Ta(CH2Ph)2Cp(4-C4H6) (2b) was used as an initiator, we obtainedpoly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). Thefactors controlling these stereoselectivities have been investigated, and we obtained thefollowing results: (1) We isolated benzylidene complexes and found that the proportion ofanti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalumcenter, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe3 resultedin the formation of a benzylidene complex Ta(=CHPh)Cp*(4-C4H6)(PMe3) (3a) as an anti-rotamer, which has been characterized by X-ray crystal structure analysis, while a similartreatment of 2b afforded Ta(=CHPh)Cp(4-C4H6)(PMe3) (3b), also in the anti-rotamer form,as revealed by a comparison of the chemical shift value of the benzylic proton of 3b withthat of 3a. In sharp contrast to the anti-geometry, a benzylidene complex Ta(=CHPh)(4-o-(CH2)2C6H4)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, wasobtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it isa syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes areconsidered as intermediates during the propagation step. We could not isolate anymetallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane 4-o-xylylene)4-butadiene)