Enantioselective Brønsted Base Catalysis with Chiral Cyclopropenimines

Cyclopropenimines are shown to be a highly effective
new class
of enantioselective Brønsted base catalysts. A chiral 2,3-bis­(dialkylamino)­cyclopropenimine
catalyzes the rapid Michael reaction of a glycine imine substrate
with high levels of enantioselectivity. A preparative scale reaction
to deliver 25 g of product is demonstrated, and a trivial large scale
synthesis of the optimal catalyst is shown. In addition, the basicity
of a 2,3-bis­(dialkylamino)­cyclopropenimine is measured for the first
time and shown to be approximately equivalent to the P1-tBu phosphazene base. An X-ray crystal structure
of the protonated catalyst is shown along with a proposed mechanistic
and stereochemical rationale.