Abstract
The archetypal single electron transfer reductant, samarium(II) diiodide (SmI2, Kagan’s reagent), remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis. While the chemistry of SmI2 is very often unique, and thus the reagent is indispensable, it is almost invariably used in superstoichiometric amounts, thus raising issues of cost and waste. Of the few reports of the use of catalytic SmI2, all require the use of superstoichiometric amounts of a metal coreductant to regenerate Sm(II). Here, we describe a SmI2-catalyzed intermolecular radical coupling of aryl cyclopropyl ketones and alkynes. The process shows broad substrate scope and delivers a library of decorated cyclopentenes with loadings of SmI2 as low as 15 mol %. The radical relay strategy negates the need for a superstoichiometric coreductant and additives to regenerate SmI2. Crucially, our study uncovers an intriguing link between ketone conformation and efficient cross-coupling and thus provides an insight into the mechanism of radical relays involving SmI2. The study lays further groundwork for the future use of the classical reagent SmI2 in contemporary radical catalysis.
Highlights
The archetypal single electron transfer (SET)[1] reductant, samarium(II) diiodide (SmI2, Kagan’s reagent),[2] remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis.[3]Intramolecular processes using the commercially available reagent are popular, and SmI2-mediated radical cyclizations feature in the total synthesis of numerous high profile and complex natural products.[4]
Of the few reports of the use of catalytic SmI2, all require the use of superstoichiometric amounts of a metal coreductant to regenerate Sm(II).[5]
We recently reported a radical-relay approach to catalysis with SmI2 that negates the need for coreductants and additives: cyclopropyl ketones underwent catalytic radical cyclization to give complex bicyclic ketones.[6]
Summary
The archetypal single electron transfer (SET)[1] reductant, samarium(II) diiodide (SmI2, Kagan’s reagent),[2] remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis.[3]. Intramolecular processes using the commercially available reagent are popular, and SmI2-mediated radical cyclizations feature in the total synthesis of numerous high profile and complex natural products.[4] Intermolecular processes using SmI2 are inherently more challenging as intermolecular radical C−C bond formation must outrun the competing reduction of radicals to carbanions. While the cinhdeimspisetnrysaobfleS,2m−I42 is very it is often unique, and the reagent is almost invariably used in superstoichiometric amounts, raising issues of cost and waste. Corey described one of the very few SmI2-catalyzed intermolecular coupling processes:5b the catalytic system requires 15 equiv of Zn/Hg amalgam (Scheme 1A)
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