Air- and Water-Tolerant Rare Earth Guanidinium BINOLate Complexes as Practical Precatalysts in Multifunctional Asymmetric Catalysis

Shibasaki’s REMB catalysts (REMB; RE = Sc, Y, La–Lu;
M = Li, Na, K; B = 1,1′-bi-2-naphtholate;
RE/M/B = 1/3/3) are among the most enantioselective asymmetric catalysts
across a broad range of mechanistically diverse reactions. However,
their widespread use has been hampered by the challenges associated
with their synthesis and manipulation. We report here the self-assembly
of novel hydrogen-bonded rare earth metal BINOLate complexes that
serve as bench-stable precatalysts for Shibasaki’s REMB catalysts. Incorporation of hydrogen-bonded guanidinium cations
in the secondary coordination sphere leads to unique properties, most
notably, improved stability toward moisture in solution and in the
solid state. We have exploited these properties to develop straightforward,
high-yielding, and scalable open-air syntheses that provide rapid
access to crystalline, nonhygroscopic complexes from inexpensive hydrated
RE starting materials. These compounds can be used as precatalysts
for Shibasaki’s REMB frameworks, where we have
demonstrated that our system performs with comparable or improved
levels of stereoselectivity in several mechanistically diverse reactions
including Michael additions, aza-Michael additions, and direct Aldol
reactions.