A Dual-Catalysis Approach to Enantioselective [2 + 2] Photocycloadditions Using Visible Light

Abstract

In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here we describe a strategy for eliminating the racemic background reaction in asymmetric [2+2] photocycloadditions of α,β-unsaturated ketones to the corresponding cyclobutanes by employing a dual-catalyst system consisting of a visible light-absorbing transition metal photocatalyst and a stereocontrolling Lewis acid co-catalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

Enantioselektive Katalyse photochemischer Reaktionen

“…[105] Für die Kombination mit einer Photooxygenierung wurde das Ammoniumsalz 87 eingesetzt, [106] während das Amin 88 a sowie das Imin 88 b als Liganden für ein Metallkation fungierten (Abbildung 15). [107] Von 1-Indanon abgeleitete Ketoester dienten in einer 2012 von Meng, Gao und Mitarbeitern publizierten Studie [106] als Vorstufen für Enolate, die unter Verwendung des Ammoniumsalzes 87 enantioselektiv mit Singulett-Sauerstoff umgesetzt werden kçnnen (Schema 26). Unter a-Oxygenierung entstanden die Produkte 89 mit Enantioselektivitäten von 39-75 % ee.…”

“…[107] Dabei war die Beobachtung, dass die Ru-vermittelte Reduktion der Arylenon-Komponente durch Koordination an eine Lewis Die oxidative photochemische Herstellung von 3,4-Dihydroisochinoliniumionen wurde kürzlich durch die Gruppe von Li erfolgreich mit der enantioselektiven Kupfer-katalysierten Addition von Alkinen kombiniert. [109] Unter den getesteten Kupfer-Liganden (unter anderem Bisoxazoline, [110] Diphosphane) war 1-(2-Diphenylphosphanyl-1-naphthyl)isochinolin (QUINAP) am erfolgreichsten.…”

Catalytic Supramolecular Photochirogenesis

“…[68] Yoon and coworkers have recently reported a dual catalyst approach to the enantiodifferentiating [2+2] photocycloaddition of a,b-unsaturated ketones 46 and 47 (Scheme 12) using a Ru(II) tris(2,2'-bipyridine) complex as a visible light-absorbing PET sensitizer, and a chiral Lewis acid (Gd(III) or Eu(III) triflate) as a stereocontrolling cocatalyst to give the corresponding cyclobutane 48 in 89-93% ee with chiral ligand 50a and epimeric 49 in 84-97% ee with 50b. [69] Meggers and coworkers have reported the enantiodifferentiating photoalkylation of acylimidazole 51 with benzyl bromide 52 catalyzed by a photoredox transition metal complex (Scheme 13). [70] …”

Recent developments in enantioselective lanthanide-catalyzed transformations

“…In this context, Yoon et al have developed asymmetric [2+2] photocycloadditions of a,b-unsaturated ketones using visible light [37]. These processes were based on a dual-catalyst system consisting of a visible light-absorbing transition metal photocatalyst, such as Ru(bpy) 3 Cl 2 , and a stereocontrolling Lewis acid co-catalyst, such as Eu(OTf) 3 combined with chiral Schiff base ligand 64.…”

“…[2+2] Photocycloadditions of a,b-unsaturated ketones promoted by europium and ruthenium catalysts [37]. more than 10 g of products in a minimized cooling volume with more than 200 TON.…”