The
design, synthesis, and development of a new class of modular,
strongly basic, and tunable bifunctional Brønsted base/H-bond-donor
organocatalysts are reported. These catalysts incorporate a triaryliminophosphorane
as the Brønsted basic moiety and are readily synthesized via
a last step Staudinger reaction of a chiral organoazide and a triarylphosphine.
Their application to the first general enantioselective organocatalytic
nitro-Mannich reaction of nitromethane to unactivated ketone-derived
imines allows the enantioselective construction of β-nitroamines
possessing a fully substituted carbon atom. The reaction is amenable
to multigram scale-up, and the products are useful for the synthesis
of enantiopure 1,2-diamine and α-amino acid derivatives.
An immobilized chiral bifunctional iminophosphorane superbase organocatalyst has been developed and applied in a range of challenging enantioselective reactions. A unique feature of this novel catalytic system is that the final step creation of the iminophosphorane occurs at the point of immobilization. The utility of the immobilized catalyst system was demonstrated in the nitro-Mannich reaction of ketimines as well as the conjugate addition of high pKa 1,3-dicarbonyl pro-nucleophiles to nitrostyrene. Catalyst recycling was also demonstrated.
A new family of bifunctional H-bond donor phase-transfer catalysts derived from cinchona alkaloids has been developed and evaluated in the enantio- and diastereoselective nitro-Mannich reaction of in situ generated N-Boc-protected imines of aliphatic, aromatic, and heteroaromatic aldehydes. Under optimal conditions, good reactivity and high diastereoselectivities (up to 24:1 dr) and enantioselectivities (up to 95% ee) were obtained using a 9-amino-9-deoxyepiquinidine-derived phase-transfer catalyst possessing a 3,5-bis(trifluoromethyl)phenylurea H-bond donor group at the 9-position.
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