Diastereo- and Enantioselective Iridium Catalyzed Coupling of Vinyl Aziridines with Alcohols: Site-Selective Modification of Unprotected Diols and Synthesis of Substituted Piperidines

The
chiral cyclometalated π-allyliridium ortho-C,O-benzoate complex (R)-Ir-VIb derived from [Ir­(cod)­Cl]2, allyl acetate, 4-cyano-3-nitro-benzoic
acid, and (R)-MeO-BIPHEP catalyzes the coupling of N-(p-nitrophenyl­sulfonyl) protected
vinyl aziridine 3a with primary alcohols 1a–1l to furnish branched products of C–C
bond formation 4a–4l with good levels
of anti-diastereo- and enantioselectivity. In the
presence of 2-propanol, but under otherwise identical conditions,
vinyl aziridine 3a and aldehydes 2a–2l engage in reductive coupling to furnish an equivalent set
of adducts 4a–4l with roughly equivalent
levels of anti-diastereo- and enantioselectivity.
Using enantiomeric iridium catalysts, vinyl aziridine 3a reacts with unprotected chiral 1,3-diols 1m–1o in a site-selective manner to deliver the diastereomeric
products of C-allylation syn-4m, -4n, -4o and anti-4m, -4n, -4o, respectively,
with good isolated yields and excellent levels of catalyst-directed
diastereoselectivity. These adducts were directly converted to the
diastereomeric 2,4,5-trisubstituted piperidines syn-5m, -5n, -5o and anti-5m, -5n, -5o.