Mechanistic Study and Development of Catalytic Reactions of Sm(II)

Abstract

Despite the broad utility and application of SmI 2 in synthesis, the reagent is used in stoichiometric amounts and has a high molecular weight, resulting in a large amount of material being used for reactions requiring one or more equivalents of electrons. We report mechanistic studies on catalytic reactions of Sm(II) employing a terminal magnesium reductant and trimethyl silyl chloride in concert with a non-coordinating proton donor source. Reactions using this approach permitted reductions with as little as 1 mol% Sm. The mechanistic approach enabled catalysis employing HMPA as a ligand, facilitating the development of catalytic Sm(II) 5-exo-trig ketyl olefin cyclization reactions. File list (3) download file view on ChemRxiv Sm(II) Catalysis (RAF).pdf (746.24 KiB) download file view on ChemRxiv SI_Sm(II) catalysis.pdf (3.10 MiB) download file view on ChemRxiv Sm(II) Catalysis (RAF).doc (574.50 KiB)

Development in SmI2 Catalyzed Reactions

“…Recently, Flowers reported a detailed mechanistic study on the development of catalytic reaction using a chemical approach [15] . Surprisingly, this study showed that presence of non‐coordinating alcohols (such as TFE, t‐BuOH) significantly enhances the rate of catalytic reaction as well as catalytic stability.…”

“…While SmI 2 has its unique ability to carry out reductive transformations, development of catalytic reactions of this reagent continues to be a challenging problem. Several factors such as i ) poor solubility of Sm 3+ , ii ) inertness of Sm 3+ towards reduction, iii ) ligand exchange reactions in Sm coordination sphere pose significant challenges in developing a sustainable and robust catalytic procedure [15] …”

“… Development of Sm(II) catalysis using a Lewis acid and Mg as terminal reductant. Reproduced from ref., [15] Copyright (2019), with permission from ACS.…”

“…The use of LiI was presumably due to convert Sm(OTf) 2 , which was produced in‐situ , to SmI 2 [16a] . Flowers group showed that when TMSCl was used as Lewis acid, SmI 2 turned into SmCl 2 during the course of the reaction [15] . Hence, it is quite possible that use of TMSOTf led to generation of Sm(OTf) 2 .…”

“…Addition of tetraglyme improved diastereoselectivity of pinacol product. Mechanistic investigations from Flowers group indicated that the presence of tetraglyme enhances the solubility of Sm 3+ , thus prevents catalyst deactivation through precipitation [15] . In both Endo and Greeves work the minimum catalyst loading was 10 mol % of SmI 2 with respect to substrate.…”