Deeper insight into the mechanism of the reaction of photogenerated metallaketenes and imines.
The reaction mechanism between Fischer carbene complexes and imines to produce beta-lactams has been studied by a combination of computational (DFT) and experimental methodologies. After the photogeneration of metalla-cyclopropanone species, the reaction initiates by 1,3-migration of the metal fragment from the former carbene carbon atom to the oxygen in the S o hypersurface. In spite of the endothermic nature of this rearrangement, it is biased by the more favorable HOMO(imine)-LUMO+1 interaction in the oxygen-coordinated ketene. This species reacts with the imine to yield metalla-zwitterions, which form the final products through a four electron conrotatory ring closure. The presence of the metal moiety during the whole reaction coordinate is demonstrated by the use of a chiral chromium(0) alkoxycarbene complex, which produces low but significant ee in the reaction with imines. The cis-trans stereoselection derives from the nucleophilic addition of the nitrogen atom of the imine to the oxygen-coordinated ketene, since there are no significant differences between the classical and the metallated processes in the electrocyclation step leading to the final 2-azetidinones.
Fernández
Israel
I
Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, 28040-Madrid, Spain.
Sierra
Miguel A
MA
Mancheño
María J
MJ
Gómez-Gallego
Mar
M
Cossío
Fernando P
FP
D000475:Alkenes / Q000737:chemistry* / Q000528:radiation effects*; D001384:Azetidines / Q000138:chemical synthesis* / Q000737:chemistry; D002857:Chromium / Q000737:chemistry / Q000528:radiation effects*; D003198:Computer Simulation; D007097:Imines / Q000737:chemistry / Q000528:radiation effects*; D008956:Models, Chemical; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry / Q000528:radiation effects*; D010777:Photochemistry; D013237:Stereoisomerism; D014466:Ultraviolet Rays
C116379:2-azetidinone; D000475:Alkenes; D001384:Azetidines; D007097:Imines; D009942:Organometallic Compounds; D002857:Chromium