Ac hiral phosphoric acid with a2 ,2'-binaphthol core was prepared that displays two thioxanthone moieties at the 3,3'-position as light-harvesting antennas.D espite its relatively lowt riplet energy,t he phosphoric acid was found to be an efficient catalystf or the enantioselective intermolecular [2+ +2] photocycloaddition of b-carboxyl-substitutedc yclic enones (e.r.u pt o9 3:7). Binding of the carboxylic acid to the sensitizeri ss uggested by NMR studies and by DFT calculations to occurb ym eans of two hydrogen bonds. The binding event not only enables an enantioface differentiationb ut also modulates the triplet energy of the substrates.Recent interest in visible light-mediatedr eactions has triggered al arge number of studies towards the synthesis of new chromophores and chiral catalysts. [1] The long known thioxanthone chromophore [2] has been revisited in the context of triplet sensitization [3] and it has been attached to chiral scaffolds fora p-plications in enantioselective photochemistry. [4] The most frequently used chiral modification is represented by compound 1 (Figure 1) [4a] in which at hioxanthone is attached via an oxazole to positionC 7o f1 ,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one. The latter device serves as ah ydrogen bonding site [5] and allows to process lactamsi n[ 2 + +2] photocycloaddition [4a, 6] and deracemization [7] reactions. By attachment to a chiral bisoxazoline, thioxanthonec an be part of ab ifunctional chiral metal catalyst and ligand 2 has been successfully employedi nt he Ni-catalyzed oxygenation of b-ketoesters. [4b] Chiral imidazolinone-based organocatalysts have recently been reported for enamine catalysis in which the thioxanthone acts via singlee lectront ransfer. [4d] However,n ot any binding motif known from thermalr eactions will automatically lead to as uccessfulc hiral catalyst. Thiourea-linked thioxanthones such as 3, for example, did not display the expected enantioselectivity in photochemical reactions. [4c] Nonetheless, given the limited number of substrates that can yet be processed by chiral thioxanthones, it seems desirable to further investigate possible binding motifs to which a thioxanthone entity can be attached. In this communication, we describe the synthesis of a C 2 -symmetric chiral phosphoric acid with two thioxanthone units and report on preliminary studies as to its mode of action in photochemical [2+ +2] cycloaddition reactions.Since their initial use in organocatalysis, [8] chiral phosphoric acids have emerged as ah ighly efficient class of compounds for ap lethora of applications. [9] Most phosphoric acids display aryl groups in positionsC 3a nd C3' of the 2,2'-binaphthol core and we envisaged that these positions would serve as suitable pointso fattachment for at hioxanthone unit. However,t he direct linkage of a9 -oxo-9H-thioxanthen-3-yl group to posi-Figure 1. Structure of chiral thioxanthones 1-3 with as ubstrate or metal binding site (in gray).

N , O -Acetals derived from α,β-unsaturated β-aryl substituted aldehydes and (1-aminocyclohexyl)methanol were found to undergo a catalytic enantioselective [2 + 2] photocycloaddition to a variety of olefins (19 examples, 54–96% yield, 84–98% ee ). The reaction was performed by visible light irradiation (λ = 459 nm). A chiral phosphoric acid (10 mol %) with an ( R )-1,1′-bi-2-naphthol (binol) backbone served as the catalyst. The acid displays two thioxanthone groups attached to position 3 and 3′ of the binol core via a meta -substituted phenyl linker. NMR studies confirmed the formation of an iminium ion which is attached to the acid counterion in a hydrogen-bond assisted ion pair. The catalytic activity of the acid rests on the presence of the thioxanthone moieties which enable a facile triplet energy transfer and an efficient enantioface differentiation.

When irradiated with visible light, various cyclic, 2-alk-4-enyl-sub¬stituted 1,3-diketones underwent an intramolecular endo-addition (m = 1) onto the double bond resulting in spirocyclic products (11 examples, 62-92% yield). Both an organic (TXT – thioxanthone, 20 mol%) and an organometallic Ir-based photocatalyst (5 mol%) promoted the reaction. Addition of triisopropylthiophenol was required to secure high yields. The spirocyclization of a 2-alk-5-enyl-substituted substrate (m = 2) delivered a mixture of the seven-membered endo- and the six-membered exo-cyclization product.