Reactivity of Bridged and Nonbridged Zirconocenes toward Biorenewable Itaconic Esters and Anhydride

2017-07-12T12:07:27Z (GMT) by Fernando Vidal Eugene Y.-X. Chen
This work investigates the reactivity of neutral and cationic complexes of both bridged <i>ansa</i>-zirconocenes, <i>rac-</i>[C<sub>2</sub>H<sub>4</sub>(Ind)<sub>2</sub>]­ZrMe­[OC­(O<sup><i>i</i></sup>Pr)CMe<sub>2</sub>] (<b>1</b>) and <i>rac-</i>[C<sub>2</sub>H<sub>4</sub>(Ind)<sub>2</sub>]­Zr<sup>+</sup>(THF)­[OC­(O<sup><i>i</i></sup>Pr)CMe<sub>2</sub>]­[MeB­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup> (<b>1</b><sup><b>+</b></sup>), and nonbridged zirconocenes, Cp*­(<sup><i>n</i></sup>PrCp)­ZrMe­[OC­(O<sup><i>i</i></sup>Pr)CMe<sub>2</sub>] (<b>13</b>) and Cp*­(<sup><i>n</i></sup>PrCp)­Zr­(THF)­[OC­(O<sup><i>i</i></sup>Pr)CMe<sub>2</sub>]<sup>+</sup>[MeB­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup> (<b>13</b><sup><b>+</b></sup>), toward biorenewable itaconic dialkyl esters (itaconates) and anhydride (IA). Behaving <i>similarly</i>, both cationic complexes <b>1</b><sup><b>+</b></sup> and <b>13</b><sup><b>+</b></sup> react readily with itaconates to form cleanly single monomer addition products, eight-membered-ring metallacycles <b>2</b> and <b>15</b>, respectively, and neutral enolate complexes <b>1</b> and <b>13</b> insert 1 equiv of IA to afford single-IA-addition products <b>5</b> and <b>17</b>. Behaving <i>differently</i>, eight-membered-ring chelates <b>2</b> derived from the bridged metallocene framework undergo slow isomerization at room temperature via ligand exchange between the coordinated and uncoordinated ester groups to form thermodynamically favored seven-membered-ring chelates <b>4</b>, while eight-membered chelates <b>15</b> derived from the sterically more crowded unbridged metallocene framework are stable at room temperature and do not undergo such isomerization. The above cationic complexes exhibit no reactivity toward further additions of itaconates. Replacing itaconates with more basic monomers such as <i>N</i>,<i>N</i>-dimethylacrylamide that can ring open the chelating resting intermediate, however, brings about effective and controlled polymerization by eight-membered Zr-itaconate metallacycles <b>2</b> and <b>15</b>, but not seven-membered <b>4</b>, producing either highly isotactic polymers (>99% <i>mm</i>, by <b>2</b>) or polymers with narrow molecular weight distributions (<i><i>Đ</i></i> < 1.19, by <b>15</b>). These results further highlight the <i>ansa</i> effects in the metallocene polymerization chemistry and the importance of the formation and ring opening of the eight-membered chelating intermediates involved in the metallocene-mediated conjugate-addition polymerization.