Enantioselective Hetero-Diels–Alder Reaction Catalyzed by Dirhodium(II) Carboxamidates

Abstract

The enantioselectivity in dirhodium(II) complexes-catalyzed hetero-Diels–Alder reaction of 1-methoxy-3-triethylsiloxy-1,3-butadiene with various aldehydes was investigated. The use of Rh₂(S-BPTPI)₄, a new dirhodium(II) carboxamidate incorporating (S)-3-(benzene-fused-phthalimido)-2-piperidonate as the bridging ligand, was found to produce 2-substituted-2,3-dihydropyran-4-ones in up to 96% ee. It is noteworthy that the present protocol was also successful for enantio- and endo-selective hetero-Diels–Alder reactions between 1,3-dienes activated by one oxygen atom and substituted propargylaldehydes. Formyl C–H hydrogen-bonding model proposed by Corey coupled with the solid-state structure of Rh₂(S-PTPI)₄, the parent complex of Rh₂(S-BPTPI)₄, confirms the observed stereochemical preferences.