om400735p_si_001.pdf (7.29 MB)
Highly Enantioselective Ruthenium/PNNP-Catalyzed Imine Aziridination: Evidence of Carbene Transfer from a Diazoester Complex
journal contribution
posted on 2013-08-26, 00:00 authored by Joël Egloff, Marco Ranocchiari, Amata Schira, Christoph Schotes, Antonio MezzettiThe
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).