A Chiral Thioxanthone as an Organocatalyst for Enantioselective [2+2] Photocycloaddition Reactions Induced by Visible Light

Abstract

Thioxanthone 1, which was synthesized in a concise fashion from methyl thiosalicylate, exhibits a significant absorption in the visible light region. It allows for an efficient enantioselective catalysis of intramolecular [2+2] photocycloaddition reactions presumably by triplet energy transfer.

Enantioselective Excited-State Photoreactions Controlled by a Chiral Hydrogen-Bonding Iridium Sensitizer

“…Complex 6b is an exceptionally effective asymmetric photocatalyst; it provides high ee’s at catalyst loadings considerably lower than the optimal conditions reported for chiral organic photosensitizers. 9,10 As a demonstration of this point, we conducted a [2+2] cycloaddition using only 0.1 mol% of 6b. Although this led to the formation of 4 at diminished rate (38% yield at 24 h), there was negligible effect on enantioselectivity (88% ee).…”

“…Instead, Meggers showed that the rhodium center forms an association complex with the substrate, in a manner analogous to the Lewis acid catalyzed photocycloaddition methods described by Bach. 9 The key enabling feature of this reaction is the appearance of a strong, long-wavelength feature in the UV-vis spectrum of the Rh–substrate complex that is significantly enhanced relative to the sum of the individual spectra of the catalyst and substrate. We conducted an analogous UV-vis absorption experiment using Ir catalyst 6 and quinonlone 3 (Figure 2).…”

“…6,7 To date, only a handful of systems have been able to deliver high ee’s in primary photoreactions at reasonably low concentrations of chiral catalyst (e.g., >80% ee at <10 mol%). 8 Arguably the most well-established of these are chiral hydrogen-bonding organic photosensitizers developed by Bach 9 and Sivaguru, 10 both of which feature photosensitizing chromophores functionalized with a hydrogen-bonding moiety that orients a polar, achiral organic substrate within the stereocontrolling environment of the chiral photosensitizer (Scheme 1). Notably, the photocatalytic moieties in both systems are organic chromophores.…”

Enantioselektive Katalyse photochemischer Reaktionen

“…Überdies legten die tabellierten TriplettEnergien [48] für die Stammverbindungen Xanthon (E T = 310 kJ mol À1 ) und Benzophenon (E T = 287 kJ mol À1 ) eine energetische Bevorzugung einer Sensibilisierung durch 34 im Vergleich zu 25 nahe. Im Jahr 2009 berichtete unsere Gruppe über die erste Anwendung des Xanthons 34, [49] im Jahr 2014 folgte das Thioxanthon 35 (Abbildung 6), [50] das eine deutliche längerwellige Absorption aufweist.…”

Mechanism of the Enantioselective Intramolecular [2 + 2] Photocycloaddition Reaction of Coumarin Catalyzed by a Chiral Lewis Acid: Comparison with Enone Substrates

“…[17][18][19] In contrast to the two-step approach, the notable strategy is to employ noncovalent interactions between the catalyst and its substrate to provide a chiral environment. By using several entities of this type including cyclodextrins [20][21][22][23] and hydrogen-bonding templates, [24][25][26][27][28][29] the direct approach shows an excellent performance in both enantioselectivity and yield, and has the advantage that it is not necessary to first add and then remove the auxiliary. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 As a promising solution，the Lewis acid catalysts have been considered as the potential chiral templates to carry out enantioselective photochemical reactions.…”