Hussein Aqeel A, Jafar Nadhir N A, Ma Yumiao
Department of Biology, College of Science, Al-Qasim Green University, Al-Qasim, Babylon 51013, Iraq.
Al-Zahraa Center for Medical and Pharmaceutical Research Sciences (ZCMRS), Al-Zahraa University for Women, Karbala 56001, Iraq.
J Org Chem. 2024 May 17;89(10):6892-6902. doi: 10.1021/acs.joc.4c00268. Epub 2024 May 3.
A computational study is reported here on the mechanism of tetrahydrofuran (THF)-diol formation from the Os(VI)-catalyzed oxidative cyclization of 5,6-dihydroxyalkene ligated with citric acid and in the presence of Bro̷nsted acid. Initiated by Os(VI) dioxo citrate formation, coordination of co-oxidant pyridine-N-oxide (PNO) and protonation of its oxo group generate the active catalyst. The catalytic cycle commences through successive steps, including dihydroxyalkene addition to the active catalyst in a concerted mechanism to form hexacoordinated alkoxy-protonated PNO-complexed Os(VI) bisglycolate as a turnover-limiting step (TLS), cyclization to Os(IV) THF-diolate, reoxidation to Os(VI) THF-diolate, and hydrolysis via a dissociative mechanism to furnish the THF-diol and regenerate the active species, sustaining the catalytic cycle through an Os(VI)/Os(IV) cycle. Despite the overall exergonic nature of catalytic cycle (Δ = -45.0 kcal/mol), the TLS is accelerated by the formation of an open-valence 16-electron Os(VI) intermediate but decelerated by the undesired formation of a saturated/hexacoordinate 18-electron Os(VI) intermediate. Bro̷nsted acid plays crucial roles in the formation of Os(VI) citrate and the active catalyst, impediment of the second cycle, and the cyclization step. Additionally, besides its role as a co-oxidant, and in the presence of acid, PNO is found to assist the insertion of dihydroxyalkene and, importantly, in releasing the THF-diol to regenerate the active intermediate.
本文报道了一项关于在布朗斯特酸存在下，由柠檬酸连接的5,6 -二羟基烯烃经锇（VI）催化氧化环化形成四氢呋喃（THF）二醇的机理的计算研究。由锇（VI）柠檬酸二氧配合物引发，共氧化剂吡啶 - N -氧化物（PNO）的配位及其氧代基团的质子化产生活性催化剂。催化循环通过连续步骤开始，包括二羟基烯烃以协同机理加成到活性催化剂上，形成六配位的烷氧基 - 质子化的PNO - 络合锇（VI）双乙醇酸酯作为周转限制步骤（TLS），环化生成锇（IV）四氢呋喃二醇盐，再氧化为锇（VI）四氢呋喃二醇盐，以及通过解离机理水解以提供四氢呋喃二醇并再生活性物种，通过锇（VI）/锇（IV）循环维持催化循环。尽管催化循环总体上是放能的（Δ = -45.0千卡/摩尔），但TLS通过形成开壳层16电子的锇（VI）中间体而加速，但由于不期望的饱和/六配位18电子锇（VI）中间体的形成而减速。布朗斯特酸在锇（VI）柠檬酸盐和活性催化剂的形成、第二个循环的阻碍以及环化步骤中起关键作用。此外，除了作为共氧化剂的作用外，在酸存在下，发现PNO有助于二羟基烯烃的插入，并且重要的是，有助于释放四氢呋喃二醇以再生活性中间体。
