Mother Diamine: A Universal Building Block for Making Chiral Ligands from Daughter Diamines to Binol, Binap and Monophos Analogues

From imine to amine: an unexpected left turn. Cis-β iron(ii) PNNP' precatalysts for the asymmetric transfer hydrogenation of acetophenone

A slight change in the iron catalyst structure (amine arm with PEt₂ to imine arm with PPh₂) results in a complete reversal of the enantioselectivity toward ketone reduction.

A novel PNN ligand bearing an orthophenylene group and a primary amine was synthesized with the aid of a palladium-catalyzed amination and reacted with phosphonium dimers [–PR₂CH₂CH(OH)–]₂[Br]₂ R = Et, iPr, Cy, Ph, xylyl, and o-Tol, and [Fe(OH₂)₆]²⁺ to produce a new series of cis-β iron(ii) PNNP′ precatalysts cis-β-[Fe(Br)(CO)(PNNP′)]BPh₄ as a pair of diastereomers. The more stable orthophenylene amido group was chosen to imitate and replace the enamido moiety of a highly active iron precatalyst for the asymmetric transfer hydrogenation (ATH) of ketones in an attempt to prevent its deactivation caused by reduction of the enamido group. This objective was partially achieved using the complex with a PEt₂ group which catalyzed the transfer hydrogenation in isopropanol of 150 000 equivalents of acetophenone to racemic 1-phenylethanol. With a low acetophenone to catalyst ratio of 500 to 1, the catalytic activity was moderate and the enantiomeric excess (ee) of the product 1-phenylethanol ranged surprisingly from 94% (R) to 95% (S) depending on the nature of PR₂ and whether the precatalyst contained an imine or amine donor. The amine precatalyst with a PEt₂-group produced a more stable hydride species when activated, allowing the reaction mixture to be heated to 75 °C to obtain a TON of 8821 for acetophenone while retaining the high ee of 95% (S). The activation pathway in basic isopropanol (iPrOH) was studied for three precatalysts to elucidate that the cis-β precatalysts rearrange to form trans hydride complexes. The study suggests that the enantioselectivity of these complexes is determined by from which side of the PNNP′ plane the hydride transfer occurs.

Resonance-Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

Resonance-assisted hydrogen bonding (RAHB), a concept introduced by Gilli and co-workers in 1989, concerns a kind of intramolecular H-bonding strengthened by a conjugated π-system, usually in 6-, 8-, or 10-membered rings. This Review highlights the involvement of RAHB as a driving force in the synthesis of organic, coordination, and organometallic compounds, as a handy tool in the activation of covalent bonds, and in starting moieties for synthetic transformations. The unique roles of RAHB in molecular recognition and switches, E/Z isomeric resolution, racemization and epimerization of amino acids and chiral amino alcohols, solvatochromism, liquid-crystalline compounds, and in synthons for crystal engineering and polymer materials are also discussed. The Review can provide practical guidance for synthetic chemists that are interested in exploring and further developing RAHB-assisted synthesis and design of materials.