Tobisch Sven
Institut für Anorganische Chemie der Martin-Luther-Universität Halle-Wittenberg, Fachbereich Chemie, Kurt-Mothes-Strasse 2, 06210 Halle, Germany.
Chemistry. 2005 May 6;11(10):3113-26. doi: 10.1002/chem.200401102.
A detailed theoretical investigation of alternative mechanisms for chain initiation of the organolanthanide-promoted ring-opening polymerization of 2-phenyl-1-methylenecyclopropane (PhMCP) with an archetypical [Cp2SmH] model catalyst is presented. Several conceivable pathways for important elementary steps, which also included ring-opening isomerization of PhMCP to phenylbutadienes, were critically scrutinized for a tentative course of the catalytic reaction. The operative mechanism starts with the first exo-methylene C=C insertion into the Sm-H bond in a 1,2 fashion and is followed by shift-based beta-alkyl eliminative cyclopropyl ring opening by cleavage of a proximal bond, while the alternative mechanism that commences with 2,1-insertion and subsequent ring opening by distal bond scission is revealed to be almost entirely precluded. The facile and irreversible insertion process is not found to occur in a regioselective fashion. The ring-opening process is analyzed as the critical step that discriminates between the two conceivable mechanisms. Opening of the cyclopropyl ring is kinetically easy and proceeds readily for the 1,2-insertion species, while a prohibitively large barrier must be overcome for ring opening of 2,1-insertion species. The isomerization of PhMCP in a ring-opened fashion, which would afford phenylbutadienes as possible products, is predicted to be a less likely process, owing to both kinetic and thermodynamic factors. The phenyl functionality has been demonstrated to distinguish between the regioisomeric ring-opening pathways, both kinetically and thermodynamically, thereby rendering this process selective with regard to the regiochemistry. Overall, chain initiation of the samarocene-mediated ring-opening polymerization of PhMCP is predicted to be a smooth, kinetically facile process.
本文对典型的[Cp2SmH]模型催化剂促进2-苯基-1-亚甲基环丙烷（PhMCP）开环聚合的链引发替代机制进行了详细的理论研究。对重要基本步骤的几种可能途径进行了严格审查，这些途径还包括PhMCP开环异构化为苯基丁二烯，以推测催化反应的初步过程。操作机制始于第一个外亚甲基C=C以1,2方式插入Sm-H键，随后通过近端键的断裂进行基于移位的β-烷基消除环丙基开环，而始于2,1-插入并随后通过远端键断裂开环的替代机制几乎完全被排除。未发现容易且不可逆的插入过程以区域选择性方式发生。开环过程被分析为区分两种可能机制的关键步骤。对于1,2-插入物种，环丙基环的开环在动力学上很容易且容易进行，而对于2,1-插入物种的开环必须克服极高大小的势垒。由于动力学和热力学因素，PhMCP以开环方式异构化生成苯基丁二烯作为可能产物的过程预计不太可能发生。苯基官能团已被证明在动力学和热力学上都能区分区域异构的开环途径，从而使该过程在区域化学方面具有选择性。总体而言，预测二茂钐介导的PhMCP开环聚合的链引发是一个顺利、动力学上容易的过程。
