Enantioselective [4+2]-Cycloaddition Reaction of a Photochemically Generated o-Quinodimethane: Mechanistic Details, Association Studies, and Pressure Effects

1,2,3,4-Tetrahydro-2-oxoquinoline-5-aldehyde (2) was prepared from m-aminobenzoic acid and 3-ethoxyacryloyl chloride (4) in 19% overall yield. Compound 2 underwent a photochemically induced [4+2]-cyclo-addition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10 a was obtained with acrylonitrile (9a) as the dienophile, whereas methyl acrylate (9b) and dimethyl fumarate (9c) furnished the endo products 11b and 11c (69-77% yield). The reactions proceeded at -60°C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91-94% ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)-dienol 8 was spectroscopically detected at -196°C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (KA) was 589 M^-1 at room temperature (25°C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of K_A was 703 M^-1. Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25°C the enantiomeric excess for the enantioselective reaction 2 + 9a→10a decreased from 68% ee at 0.1 MPa to 58% ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10a and ent-10a.