Jenseits von Samariumdiiodid: Perspektiven für Lanthanoid(II)‐vermittelte Reduktionen

Abstract

Reaktionen, die über offenschalige Einelektronenmechanismen ablaufen, führen zu interessanten alternativen Resultaten gegenüber solchen Reaktionen, die Zweielektronenmechanismen mit geschlossenen Schalen gehorchen. In diesem Zusammenhang entwickelte sich Samariumdiiodid zu einem der wichtigsten Elektronentransferreagentien. In letzter Zeit wurde ein signifikanter Fortschritt auf dem Gebiet der reduktiven Chemie anderer zweiwertiger Lanthanoide erzielt, die viele Jahre lang als zu reaktiv erachtet worden waren, um von Nutzen für Synthesechemiker zu sein. Wir beschreiben hier, wie neue Samarium(II)‐Komplexe und nichtklassische Lanthanoid(II)‐Reagentien die moderne reduktive Chemie verändern.

Uncovering the Importance of Proton Donors in TmI2‐Promoted Electron Transfer: Facile CN Bond Cleavage in Unactivated Amides

“… 9 Recently, nonclassical lanthanide(II) iodides (TmI 2 , thulium diiodide; DyI 2 , dysprosium diiodide; NdI 2 , neodymium diiodide) have emerged as an attractive solution to the problem of insufficient redox potential of SmI 2 (Figure 1 b ). 10 In analogy to SmI 2 , these extremely reducing lanthanide iodides ( E ° (Ln III/II )=−2.2, −2.5, −2.6 V vs. NHE, 8 respectively) have been fully characterized in ethereal solvents 11 and can be easily obtained in multigram quantities. 12 Seminal work by Evans et al provided the first evidence that TmI 2 , DyI 2 , and NdI 2 mediate challenging cross-coupling reactions beyond the scope of SmI 2 .…”

Selective Reduction of Barbituric Acids Using SmI2/H2O: Synthesis, Reactivity, and Structural Analysis of Tetrahedral Adducts

“…[5] Single-electron-transfer reactions open up unexplored reaction space charted with chemoselectivity and reactivity levels difficult to access by ionic reaction mechanisms. [6] The generation of ketyl-type radicals with SmI 2 is particularly valuable in this regard because of the excellent chemoselectivity imparted by the reagent and the potential to effect polarity reversal of the carbonyl group through a singleelectron-reduction event (Figure 1 b). [7,8] However, the selective reduction of amide carbonyls with SmI 2 is challenging and no general method to achieve this highly desirable transformation is currently available.…”

“…hemiacetals) by a formal amide polarity reversal event. [6] We hypothesized that single-electron reduction of barbituric acids (cyclic 1,3-diimides) to their respective radical anions could provide a benchmark for the development of a general system for the reduction of a wide range of amide functional groups. We considered that 1) in the barbituric acid system the reduction of one of the imide carbonyls would be enhanced because of its lower energy p* CO orbital, 2) the reduction would be favored by anomeric stabilization of the radical anion intermediate, and 3) the n N !p* CO delocalization into the remaining carbonyl in a conformationally locked system would provide access to stable, and unusual, hemiaminal products.…”