Dedication
Dedicated to Prof. Dr. Paul Knochel on the occasion of his 70th birthday.
Abstract
Chiral aryl-(1R,2R)-N,N-dimethyldiamino-cyclohexane (DMDACH)-substituted cyclodiphosphazanes
are demonstrated to be efficient bifunctional hydrogen-bonding organocatalysts for
the enantioselective Michael additions of 2-hydroxy-1,4-naphthoquinone (<90% ee),
Meldrum’s acid (<55% ee), and kojic acid chloride (<80% ee) to β-nitrostyrene. The
parent, unsubstituted phenyl-cyclodiphosphazane with PO functions shows superior enantioselectivity
in most cases and even exceeds, with 55% ee, the enantioselectivity of Takemoto’s
thiourea catalyst for the challenging Meldrum’s acid substrate (46% ee). For Michael
additions of kojic acid chloride, the meta-CH3-substituted cyclodiphosphazane with PS functions is the most enantioselective (<
80% ee) catalyst. This highlights the benefits of modular cyclodiphosphazane structures,
which can be tailored for specific substrates.
Keywords
Asymmetric catalysis - Michael additions - Enols - Stereoselective synthesis - Phosphates
- Hydrogen bonding - Cyclodiphosphazanes - Organocatalysis - Enantioselectivity -
Hydroxyquinones - Meldrum’s acid - Kojic acid - Nitrostyrene
