Control of Stereoselectivity in the Ring-Opening Metathesis Polymerization of Norbornene by the Auxiliary Ligands Butadiene and o-Xylylene in Well-Defined Pentamethylcyclopentadiene Tantalum Carbene Complexes

cis-Dialkyl complexes of tantalum, Ta(CH₂Ph)₂Cp*(η⁴-C₄H₆) (2a) (Cp* = η⁵-C₅Me₅), TaMe(CH₂SiMe₃)Cp*(η⁴-butadiene) (6), and TaMe(CH₂CMe₃)Cp*(η⁴-butadiene) (7), were found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene to give poly(norbornene) with a high cis-vinylene double-bond (97−99%) content, while an o-xylylene complex Ta(CH₂Ph)₂(η⁴-o-(CH₂)₂C₆H₄)Cp* (9) was also an initiator to give poly(norbornene) with a high trans-vinylene double-bond (92−95%) content. When a Cp−butadiene complex Ta(CH₂Ph)₂Cp(η⁴-C₄H₆) (2b) was used as an initiator, we obtained poly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). The factors controlling these stereoselectivities have been investigated, and we obtained the following results:  (1) We isolated benzylidene complexes and found that the proportion of anti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalum center, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a in the presence of PMe₃ resulted in the formation of a benzylidene complex Ta(CHPh)Cp*(η⁴-C₄H₆)(PMe₃) (3a) as an anti-rotamer, which has been characterized by X-ray crystal structure analysis, while a similar treatment of 2b afforded Ta(CHPh)Cp(η⁴-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 with that of 3a. In sharp contrast to the anti-geometry, a benzylidene complex Ta(CHPh)(η⁴-o-(CH₂)₂C₆H₄)Cp* (12), bearing an o-xylylene ligand instead of the butadiene ligand, was obtained by thermolysis of 9. The X-ray crystal structure analysis of 12 revealed that it is a syn-rotamer and has three-legged piano stool geometry. (2) Metallacyclobutanes are considered as intermediates during the propagation step. We could not isolate any metallacyclobutane during the ROMP of norbornene; however, a tantalacyclobutane (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 with that of (14). (3) In contrast to high cis/trans stereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the ¹³C NMR spectra of the hydrogenated derivatives was found to be almost atactic. This suggests that the alkylidene species would be a resting state during the propagation, and thereby monomer could 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 congestion between the Cp* ligand and auxiliary ligands 1,3-butadiene or o-xylylene.