Figure 2.

Elucidation of the origin of enantiocontrol. a) Evolution of the distribution of product 3 a during the progress of the model reaction in the absence of any catalyst (black line), or in the presence of 20 mol % of cinchona–thiourea 4 c (blue line), N,N′‐dicyclohexylthiourea (4 e; violet line), and quinuclidine (4 f; magenta line). b) Electron‐transfer (ET) quenching mechanism of the triplet state of 1 a, as mediated by geometrically constrained tertiary amines. c) Absorption at 450 nm of the transient E photoenol A (black line) generated upon 355 nm laser excitation of 2‐methylbenzophenone (1 a; [1 a]₀=5×10⁻³  m in benzene). A logarithmic scale is used for time. Absorption decay (red and blue lines) observed in the presence of increasing amounts of the cinchona–thiourea 4 c. Red line: ratio 1 a/4 c mimics the reaction conditions. ΔOD: optical‐density variation. d) Mutualistic relationship between the two chiral fragments in 4 c: The formation of a low amount of the photoenol, as controlled by the tertiary amino moiety, prevents the background reaction to compete with the enantioselective Diels–Alder trap with maleimide 2 a, which is guided by the thiourea moiety. Cy=cyclohexyl.