Asymmetric Hetero-Diels–Alder Reaction of Diazenes Catalyzed by Chiral Silver Phosphate: Water Participates in the Catalysis and Stereocontrol

The chiral silver phosphate was confirmed
to efficiently catalyze
a highly regio- and enantioselective hetero-Diels–Alder reaction
of diazenes to furnish piperazine derivatives in high yields and excellent
ee values. DFT calculations revealed that the water molecule participates
in the catalysis by coordination to silver phosphate and also found
that the hydroxy group of 1-hydroxy-2,3-hexadiene not only formed
a hydrogen bond with the oxygen of phosphate but also coordinated
to the Ag­(I) to simultaneously stabilize the transition states and
control the regioselectivity.