Asymmetric 1,3‐Dipolar Cycloaddition Reaction between α,β‐Unsaturated Aldehydes and Nitrones Catalyzed by Well‐Defined Iridium or Rhodium Catalysts

Reaction of the complexes ( S M , R C )‐[(η 5 ‐C 5 Me 5 )M{( R )‐Prophos}(H 2 O)](SbF 6 ) 2 (M=Rh, Ir) with α,β‐unsaturated aldehydes diastereoselectively gave complexes ( S M , R C )‐[(η 5 ‐C 5 Me 5 )M{( R )‐Prophos}(enal)](SbF 6 ) 2 which have been fully characterized, including an X‐ray molecular structure determination of the complex ( S Rh , R C )‐[(η 5 ‐C 5 Me 5 )Rh{( R )‐Prophos}( trans ‐2‐methyl‐2‐pentenal)](SbF 6 ) 2 . These enal complexes efficiently catalyze the enantioselective 1,3‐dipolar cycloaddition of the nitrones N ‐benzylideneaniline N ‐oxide and 3,4‐dihydroisoquinoline N ‐oxide to the corresponding enals. Reactions occur with excellent regioselectivity, perfect endo selectivity and with enantiomeric excesses up to 94 %. The absolute configuration of the adduct 5‐methyl‐2,3‐diphenylisoxazolidine‐4‐carboxaldehyde was determined through its ( R )‐(−)‐α‐methylbenzylamine derivative.