Tachikawa Hiroto
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan.
ACS Omega. 2023 Mar 6;8(11):10600-10606. doi: 10.1021/acsomega.3c00520. eCollection 2023 Mar 21.
Transition-metal-catalyzed carbon-carbon (C-C) bond formation is an important reaction in pharmaceutical and organic chemistry. However, the reaction process is composed of multiple steps and is expensive owing to the presence of transition metals. This study proposes a lithium-catalyzed C-C coupling reaction of two benzene molecules (Bz) to form a biphenyl molecule, which is a transition-metal-free reaction, based on ab initio and direct ab initio molecular dynamics (AIMD) calculations. The static ab initio calculations indicate that the reaction of two Bz molecules with Li ions (reactant state, RC) can form a stable sandwiched complex (precomplex), where the Li ion is sandwiched by two Bz molecules. The complex formation reaction can be expressed as 2Bz + Li → Bz(Li )Bz, where the C-C distance between the Bz rings is 2.449 Å. This complex moves to the transition state (TS) via the structural deformation of Bz(Li)Bz, where the C-C distance is shortened to 2.118 Å. The barrier height was calculated to be -9.9 kcal/mol (relative to RC) at the MP2/6-311++G(d,p) level. After TS, the C(sp)-C(sp) single bond was completely formed between the Bz rings (the C-C bond distance was 1.635 Å) (late complex). After the dissociation of H from the late complex, a biphenyl molecule was formed: the C(sp)-C(sp) bond. The calculations suggest that the C-C bond coupling of Bz occurred spontaneously from 2Bz + Li, and biphenyl molecules were directly formed without an activation barrier. Direct AIMD calculations show that the C-C coupling reaction also takes place under electron attachment to Li(Bz): Li(Bz) + e → [Li(Bz)] → precomplex → TS → late complex, where [Li(Bz)] is the vertical electron capture species of Li(Bz). Namely, the C-C coupling reaction spontaneously occurred in Li(Bz) owing to electron attachment. Similar C-C coupling reactions were also observed for halogen-substituted benzene molecules (Bz-X, X = F and Cl). Furthermore, this study discusses the mechanism of C-C bond formation in electron capture based on the theoretical results.
过渡金属催化的碳-碳（C-C）键形成反应是药物化学和有机化学中的重要反应。然而，该反应过程由多个步骤组成，并且由于过渡金属的存在而成本高昂。本研究基于从头算和直接从头算分子动力学（AIMD）计算，提出了一种锂催化的两个苯分子（Bz）的C-C偶联反应以形成联苯分子，这是一种无过渡金属的反应。静态从头算计算表明，两个Bz分子与锂离子（反应物状态，RC）的反应可以形成一种稳定的夹心配合物（预配合物），其中锂离子被两个Bz分子夹住。配合物形成反应可以表示为2Bz + Li → Bz(Li)Bz，其中Bz环之间的C-C距离为2.449 Å。该配合物通过Bz(Li)Bz的结构变形移动到过渡态（TS），此时C-C距离缩短至2.118 Å。在MP2/6-311++G(d,p)水平下，计算得到的势垒高度为-9.9 kcal/mol（相对于RC）。经过TS后，Bz环之间完全形成了C(sp)-C(sp)单键（C-C键距离为1.635 Å）（后期配合物）。后期配合物中的H解离后，形成了联苯分子：C(sp)-C(sp)键。计算表明，Bz的C-C键偶联从2Bz + Li自发发生，并且直接形成联苯分子而没有活化势垒。直接AIMD计算表明，C-C偶联反应也在电子附着于Li(Bz)时发生：Li(Bz) + e → [Li(Bz)] → 预配合物 → TS → 后期配合物，其中[Li(Bz)]是Li(Bz)的垂直电子捕获物种。即，由于电子附着，C-C偶联反应在Li(Bz)中自发发生。对于卤素取代的苯分子（Bz-X，X = F和Cl）也观察到了类似的C-C偶联反应。此外，本研究基于理论结果讨论了电子捕获中C-C键形成的机理。
