Mashima, Kazushi Kaidzu, Michitaka Tanaka, Yoshiyuki Nakayama, Yuushou Nakamura, Akira Hamilton, James G. Rooney, John J. Control of Stereoselectivity in the Ring-Opening Metathesis Polymerization of Norbornene by the Auxiliary Ligands Butadiene and <i>o</i>-Xylylene in Well-Defined Pentamethylcyclopentadiene Tantalum Carbene Complexes <i>cis</i>-Dialkyl complexes of tantalum, Ta(CH<sub>2</sub>Ph)<sub>2</sub>Cp*(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>) (<b>2a</b>) (Cp* = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>), TaMe(CH<sub>2</sub>SiMe<sub>3</sub>)Cp*(η<sup>4</sup>-butadiene) (<b>6</b>), and TaMe(CH<sub>2</sub>CMe<sub>3</sub>)Cp*(η<sup>4</sup>-butadiene) (<b>7</b>), were found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene to give poly(norbornene) with a high <i>cis</i>-vinylene double-bond (97−99%) content, while an <i>o</i>-xylylene complex Ta(CH<sub>2</sub>Ph)<sub>2</sub>(η<sup>4</sup>-<i>o</i>-(CH<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)Cp* (<b>9</b>) was also an initiator to give poly(norbornene) with a high <i>trans</i>-vinylene double-bond (92−95%) content. When a Cp−butadiene complex Ta(CH<sub>2</sub>Ph)<sub>2</sub>Cp(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>) (<b>2b</b>) was used as an initiator, we obtained poly(norbornene) with no selectivity (1:1 mixture of <i>trans</i>- and <i>cis</i>-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 <i>anti</i>- and <i>syn</i>-rotamers obtained depended on the kind of auxiliary ligands on the tantalum center, i.e., 1,3-butadiene or <i>o</i>-xylylene. Thermolysis of <b>2a</b> in the presence of PMe<sub>3</sub> resulted in the formation of a benzylidene complex Ta(CHPh)Cp*(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>)(PMe<sub>3</sub>) (<b>3a</b>) as an <i>anti</i>-rotamer, which has been characterized by X-ray crystal structure analysis, while a similar treatment of <b>2b</b> afforded Ta(CHPh)Cp(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>)(PMe<sub>3</sub>) (<b>3b</b>), also in the <i>anti</i>-rotamer form, as revealed by a comparison of the chemical shift value of the benzylic proton of <b>3b</b> with that of <b>3a</b>. In sharp contrast to the <i>anti</i>-geometry, a benzylidene complex Ta(CHPh)(η<sup>4</sup>-<i>o</i>-(CH<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)Cp* (<b>12</b>), bearing an <i>o</i>-xylylene ligand instead of the butadiene ligand, was obtained by thermolysis of <b>9</b>. The X-ray crystal structure analysis of <b>12</b> revealed that it is a <i>syn</i>-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 (<b>15</b>) was isolated in 18% yield when acenaphthylene, instead of the norbornene, was added to <b>12</b>. The <i>trans</i>-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 <b>15</b>, whose structural features were compared with that of (<b>14</b>). (3) In contrast to high <i>cis</i>/<i>trans</i> stereoselectivity, the stereoregularity or tacticity of the cyclopentane ring sequences in poly(norbornene) obtained by the tantalum complexes estimated from the <sup>13</sup>C NMR spectra of the hydrogenated derivatives was found to be almost <i>atactic</i>. 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 <i>atactic</i> polymer. Consequently, the high <i>cis</i> and <i>trans</i> stereoselectivity thus demonstrated is ascribed to the huge congestion between the Cp* ligand and auxiliary ligands 1,3-butadiene or <i>o</i>-xylylene. rotamer;2 b;cyclopentane ring sequences;2 C 6 H 4;13 C NMR spectra;tran;ligand;chemical shift value;metallacyclobutane ring system;C 4 H 6;ci;poly;ROMP;CH;xylylene;Cp;butadiene;Ta;Auxiliary Ligands Butadiene 1998-08-27
    https://acs.figshare.com/articles/journal_contribution/Control_of_Stereoselectivity_in_the_Ring-Opening_Metathesis_Polymerization_of_Norbornene_by_the_Auxiliary_Ligands_Butadiene_and_i_o_i_-Xylylene_in_Well-Defined_Pentamethylcyclopentadiene_Tantalum_Carbene_Complexes/3782439
10.1021/om9802966.s003