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.