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(CH 2 Ph) 2 Cp*(η 4 -C 4 H 6 ) ( 2a ) (Cp* = η 5 -C 5 Me 5 ), TaMe(CH 2 SiMe 3 )Cp*(η 4 -butadiene) ( 6 ), and TaMe(CH 2 CMe 3 )Cp*(η 4 -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(CH 2 Ph) 2 (η 4 - o -(CH 2 ) 2 C 6 H 4 )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(CH 2 Ph) 2 Cp(η 4 -C 4 H 6 ) ( 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 PMe 3 resulted in the formation of a benzylidene complex Ta( CHPh)Cp*(η 4 -C 4 H 6 )(PMe 3 ) ( 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(η 4 -C 4 H 6 )(PMe 3 ) ( 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)(η 4 - o -(CH 2 ) 2 C 6 H 4 )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 13 C 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.