Highly Enantioselective Ruthenium/PNNP-Catalyzed Imine Aziridination: Evidence of Carbene Transfer from a Diazoester Complex

The ruthenium/PNNP complexes [RuCl(Et2O)(PNNP)]Y (Y = PF6, 4PF6; BF4, 4BF4; or SbF6, 4SbF6) (10 mol %) catalyze the enantioselective aziridination of imines with ethyl diazoacetate (EDA) as carbene source (PNNP = (1S,2S)-N,N′-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine). The highest enantioselectivity was obtained with 4SbF6, which aziridinated N-benzylidene-1,1-diphenylmethanamine (5a) to cis-ethyl 1-benzhydryl-3-phenylaziridine-2-carboxylate (cis-6a) with 93% ee at 0 °C. To the best of our knowledge, this is the highest enantioselectivity ever obtained in transition metal-catalyzed asymmetric aziridination. Aziridine yields were overall moderate to low (up to 33% isolated yield of the cis isomer) because of the competitive formation of diethyl maleate (7). The scope of the catalyst was studied with p- and m-substituted imines. NMR spectroscopic studies with 13C- and 15N-labeled EDA indicate that aziridine 6a is formed by carbene transfer from an EDA complex, [RuCl(EDA)(PNNP)]PF6 (8), to the imine. The observation of a dinitrogen complex (9) gives further support to this mechanism. The EDA adduct 8 decomposes to the carbene complex [RuCl(CHCO2Et)(PNNP)]+ (10), whose reaction with EDA gives diethyl maleate. This unprecedented mechanism is rationalized on the basis of the nucleophilic nature of diazoalkanes, which is enhanced by coordination to a π-back-donating metal such as ruthenium(II).