Enantioselective [2 + 2] Photocycloaddition via Iminium Ions: Catalysis by a Sensitizing Chiral Brønsted Acid

Abstract

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.

Controllable multiple-step configuration transformations in a thermal/photoinduced reaction

“…However, cyclobutanes have four stereocenters, and their control suffers from relatively poor regio- and stereoselectivity 3 – 9 . Various supramolecular templates 10 – 12 , host-guest assemblies 13 , 14 , quantum dots 15 , chiral molecular catalysts 16 – 18 , and Lewis acid cocatalysts 19 , 20 have been developed to control the enantioselectivity and diastereoselectivity of [2 + 2] photocycloaddition transformations. Although it is possible to capture cyclic intermediates at low temperature 21 , the control of their stereochemistry during the reaction has not yet been documented.…”

Construction of Chiral Cyclic Compounds Enabled by Enantioselective Photocatalysis

“…Another chiral oxazaborolidine Lewis acid (23) catalyzed photocycloaddition was reported by Bach et al for the cooperation of olefins (24) with phenanthrene-9-carboxaldehydes (25) in 2018 (Scheme 6) [16]. The photocyclization products (26) were afforded with good enantioselectivity (46-93% yield, 82-98% ee) upon the irradiation of visible light (λ = 457 nm). Substituents at 3-and 6-positions of phenanthrene-9-carboxaldehydes were well tolerated, but the substituents at 2-and 5-positions could decrease the enantioselectivity somehow.…”

“…Very recently, Bach et al reported another enantioselective [2+2] photocycloaddition catalyzed by chiral phosphoric acid (66). Under the irradiation of visible light (λ = 459 nm), the reaction could combine N,O-acetals (67) with olefins (68) to form cyclobutanecarbaldehydes (69) in good yields and enantioselectivity (54-96% yield, 84-98% ee) (Scheme 16) [26]. The para-substituents on the phenyl ring, such as methyl, bromine, chloro, methoxy, and borate group, were well tolerated.…”

Syntheses of new chiral chimeric photo-organocatalysts

“…showed that C2-symmetric catalyst E containing a phenyl linker between BINOL and thioxanthone unit was competent catalyst for enantioselective [2 + 2] cycloadditions. 16 Nearly at the same time, our group found that chiral BINOL phosphoric acid F with one thioxanthone unit in the 3-position ( Fig. 1 ) could promote a tandem three-component electrophilic amination of enecarbamates with dibenzyl azodicarboxylate and pyrazoles with high enantioselectivity.…”