BOROX Catalysis: Self-assembled <scp>amino</scp>-BOROX and <scp>imino</scp>-BOROX Chiral Brønsted Acids in a Five Component Catalyst Assembly/Catalytic Asymmetric Aziridination

A five-component catalyst assembly/aziridination reaction is described starting from an aldehyde, an amine, ethyl diazoacetate, B(OPh)(3), and a molecule of a vaulted biaryl ligand (VAPOL or VANOL). A remarkable level of chemo-selectivity was observed since, while 10 different products could have resulted from various reactions between the five components, an aziridine was formed in 85% yield and 98% ee and only two other products could be detected in 3% yield. Studies reveal that the first in a sequence of three reactions is an exceedingly rapid amine-induced assembly of an amino-BOROX chiral Br&#248;nsted acid species from VAPOL and B(OPh)(3), which is followed by imine formation from the amine and aldehyde and the concomitant formation of an imino-BOROX chiral Br&#248;nsted acid and finally the reaction of the imine with ethyl diazoacetate mediated by the imino-BOROX catalyst to give aziridine-2-carboxylic esters with very high diastereo- and enantioselectivity.