Demystifying Cp2Ti(H)Cl and Its Enigmatic Role in the Reactions of Epoxides with Cp2TiCl
Demystifying Cp2Ti(H)Cl and Its Enigmatic Role in the Reactions of Epoxides with Cp2TiCl
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DOI：
10.1021/acs.organomet.8b00793
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发表时间：
2018-12-24
期刊：
影响因子：
2.8
通讯作者：
Nugent, William A.
中科院分区：
文献类型：
--
作者：
Gordon, Jonathan;Hildebrandt, Sven;Nugent, William A.
用实验和计算相结合的方法研究了Cp2Ti(H)Cl在Cp2TiCl与三取代环氧化物反应中的作用。虽然Cp2Ti(H)Cl通常被认为是一种强大的物质，但其分解为Cp2TiCl和分子氢被发现是放热的（当考虑THF溶剂化效应时δ G = -11 kcal/mol）。在实验室研究中，以1,2-环氧-1-甲基环己烷与Cp2TiCl反应为模型生成Cp2Ti(H)Cl。结果表明，Cp2Ti(H)Cl确实是一种热不稳定分子，在反应条件下发生了氢的分子间还原消除。反应的化学计量（Cp2TiCl：环氧化物= 1:1）和产生的氢的数量（每2mol环氧化物产生1mol）与这一断言一致。在质子催化和非质子催化条件下，这些反应中烯丙醇的产率降低可以根据溶液中主要存在的钛（III）来理解。在质子催化条件下，Cp2TiCl配合物与盐酸collidine和钛（III）中心发生“交叉歧化”的可能性较小；这导致氢原子转移自由基捕获的副产物，产生饱和醇。
The role of Cp2Ti(H)Cl in the reactions of Cp2TiCl with trisubstituted epoxides has been investigated in a combined experimental and computational study. Although Cp2Ti(H)Cl has generally been regarded as a robust species, its decomposition to Cp2TiCl and molecular hydrogen was found to be exothermic (Delta G = -11 kcal/mol when the effects of THF solvation are considered). In laboratory studies, Cp2Ti(H)Cl was generated using the reaction of 1,2-epoxy-1-methylcyclohexane with Cp2TiCl as a model. Rapid evolution of hydrogen gas was demonstrated, indicating that Cp2Ti(H)Cl is indeed a thermally unstable molecule, which undergoes intermolecular reductive elimination of hydrogen under the reaction conditions. The stoichiometry of the reaction (Cp2TiCl:epoxide = 1:1) and the quantity of hydrogen produced (1 mol per 2 mol of epoxide) is consistent with this assertion. The diminished yield of allylic alcohol from these reactions under the conditions of protic versus aprotic catalysis can be understood in terms of the predominant titanium(III) present in solution. Under the conditions of protic catalysis, Cp2TiCl complexes with collidine hydrochloride and the titanium(III) center is less available for "cross-disproportionation" with carbon-centered radicals; this leads to byproducts from radical capture by hydrogen atom transfer, resulting in a saturated alcohol.