No abstract

Despite Nature's longstanding ability to use a proton, the most prevalent Lewis acid, to both activate and orient a substrate during an enantioselective reaction, this work represents the first example of this phenomenon outside of a protein. A chiral, nonracemic BisAMidine (BAM) ligand was designed, synthesized, and complexed to the proton of a Brønsted acid. The resulting coordination compound catalyzed the production of enantioenriched product from the combination of a Schiff base and nitroalkane (the aza-Henry reaction). This particular reaction is also considered a model for many analogous carbon-carbon bond-forming reactions catalyzed by enzymes (e.g., the Mannich reaction). This discovery suggests the use of ionic hydrogen bonds in asymmetric catalysis may not only be more general than previously thought, but also a viable "green" approach to single-enantiomer organic compounds.

An unsymmetrical, bifunctional chiral proton catalyst (2) has been developed to enable the highly diastereo- and enantioselective synthesis of epimerizable addition products from nitroacetic acid esters. The strategy is analogous to O'Donnell's glycine Schiff base alkylations that have been broadly applied to the synthesis of α-amino acids. In this regard, nitroacetic acid esters provide complementary products, namely, anti-α,β-diamino acids, derived from addition reactions. Products are obtained with high diastereoselection (generally 6−12:1) and enantioselection (generally >90% ee) using 5 mol% catalyst loading.

A general synthesis of β-diketimines derived from Isoquinoline and AminoNaphthalene components (IAN amines) is reported. Beginning from inexpensive 2-naphthol and isoquinoline, the sequence of reactions leading to R-IAN amines is convergent, short, and high-yielding. Several new derivatives are reported (e.g., R = Bn, i Pr, t Bu, Ph, 2,6-Me2Ph, 2,6- i Pr2Ph, 2-Np). All of these were complexed to zirconium(IV) by transamination with Zr(NMe2)4, and in all cases 1:1 R-IAN:Zr complexes immediately formed at 25 °C. Except in cases of severe steric hindrance ( i Pr-IAN and 2,6-Me2Ph), the corresponding 2:1 IAN:Zr complexes formed at temperatures ranging from 25 to 100 °C in toluene. A feature conserved among all 2:1 IAN:Zr complexes was a remarkable degree of diastereoselectivity favoring a C 2-symmetric bis(β-diketiminate) isomer bearing cis-NMe2 and cis-pyridyl ligands. Although all complexations were performed from rac-IAN, the resulting complexes are composed solely of homochiral ligands. The configurational integrity of Me-IAN and its ability to transfer asymmetry upon metal coordination was demonstrated in the catalyzed enantioselective addition of diethylzinc to benzaldehyde. Given the topographical similarity between these and metallocene complexes, several derivatives were preliminarily evaluated in ethylene polymerization.

Chiral diamine-derived hydrogen-bond donors were evaluated for their ability to effect stereocontrol in an intramolecular hetero-Diels–Alder (HDA) reaction hypothesized in the biosynthesis of brevianamides A and B. Collectively, these results provide proof of principle that small-molecule hydrogen-bond catalysis, if even based on a hypothetical biosynthesis construct, holds significant potential within enantioselective natural product synthesis.