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.