Chiral 1,1′-Binaphthyl-2,2′-Disulfonic Acid (BINSA) and Its Derivatives for Asymmetric Catalysis

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

2,2'-Tethered Binaphthyl-Embedded One-Handed Helical Ladder Polymers: Impact of the Tether Length on Helical Geometry and Chiroptical Property

Synthetic breakthroughs diversify the molecules and polymers available to chemists. We now report the first successful synthesis of a series of optically-pure 2,2'-tethered binaphthyl-embedded helical ladder polymers based on quantitative and chemoselective ladderization by the modified alkyne benzannulations using the 4-alkoxy-2,6-dimethylphenylethynyl group as the alkyne source, inaccessible by the conventional approach lacking the 2,6-dimethyl substituents. Due to the defect-free helix formation, the circular dichroism signal increased by more than 6 times the previously reported value. The resulting helical secondary structure can be fine-tuned by controlling the binaphthyl dihedral angle in the repeating unit with variations in the 2,2'-alkylenedioxy tethering groups by one carbon atom at a time. The optimization of the helical ladder structures led to a strong circularly polarized luminescence with a high fluorescence quantum yield (28 %) and luminescence dissymmetry factor (2.6×10-3 ).

A Chiral Iron Disulfonate Catalyst for the Enantioselective Synthesis of 2-Amino-2'-hydroxy-1,1'-binaphthyls (NOBINs)

A novel type of chiral redox disulfonate iron complex for asymmetric catalysis is reported. The [Fe((R_a)-BINSate)]⁺ (BINSate = 1,1'-binaphthalene-2,2'-disulfonate) complex effectively promotes the enantioselective oxidative cross-coupling between 2-naphthols (1) and 2-aminonaphthalene derivatives (2), affording optically enriched (R_a)-2-amino-2'-hydroxy-1,1'-binaphthyls (NOBINs) with exceptional yields and enantioselective ratios (up to 99% yield and 96:4 er). The [Fe((R_a)-BINSate)]⁺ catalyst was designed as a chiral version of FeCl₃ with multicoordination sites available for binding the two coupling partners 1 and 2 as well as the oxidant. Our structure-selectivity and activity study, which covered most of the important positions in the NOBIN scaffold, revealed the effect of different substitution patterns on the coupling efficiency and stereoselectivity.

Ternary complexes of chiral disulfonimides in transfer-hydrogenation of imines: the relevance of late intermediates in ion pair catalysis

In ion pairing catalysis, the structures of late intermediates and transition states are key to understanding and further development of the field. Typically, a plethora of transition states is explored computationally. However, especially for ion pairs the access to energetics via computational chemistry is difficult and experimental data is rare. Here, we present for the first time extensive NMR spectroscopic insights about the ternary complex of a catalyst, substrate, and reagent in ion pair catalysis exemplified by chiral Brønsted acid-catalyzed transfer hydrogenation. Quantum chemistry calculations were validated by a large amount of NMR data for the structural and energetic assessment of binary and ternary complexes. In the ternary complexes, the expected catalyst/imine H-bond switches to an unexpected O-H-N structure, not yet observed in the multiple hydrogen-bond donor-acceptor situation such as disulfonimides (DSIs). This arrangement facilitates the hydride transfer from the Hantzsch ester in the transition states. In these reactions with very high isomerization barriers preventing fast pre-equilibration, the reaction barriers from the ternary complex to the transition states determine the enantioselectivity, which deviates from the relative transition state energies. Overall, the weak hydrogen bonding, the hydrogen bond switching and the special geometrical adaptation of substrates in disulfonimide catalyst complexes explain the robustness towards more challenging substrates and show that DSIs have the potential to combine high flexibility and high stereoselectivity.

Diverse Approaches for Enantioselective C-H Functionalization Reactions Using Group 9 Cpx MIII Catalysts

Transition-metal-catalyzed C-H functionalization reactions with Cp*M^III catalysts (M=Co, Rh, Ir) have found a wide variety of applications in organic synthesis. Albeit the intrinsic difficulties in achieving catalytic stereocontrol using these catalysts due to their lack of additional coordination sites for external chiral ligands and the conformational flexibility of the Cp ligand, catalytic enantioselective C-H functionalization reactions using the Group 9 metal triad with Cp-type ligands have been intensively studied since 2012. In this minireview, the progress in these reactions according to the type of the chiral catalyst used are summarized and discussed. The development of chiral Cp^x ligands the metal complexes thereof, artificial metalloenzymes, chiral carboxylate-assisted enantioselective C-H activations, enantioselective alkylations assisted by chiral carboxylic acids or chiral sulfonates, and chiral transient directing groups are discussed.

Enantioselective Iron/Bisquinolyldiamine Ligand-Catalyzed Oxidative Coupling Reaction of 2-Naphthols

An iron-catalyzed asymmetric oxidative homo-coupling of 2-naphthols for the synthesis of 1,1'-Bi-2-naphthol (BINOL) derivatives is reported. The coupling reaction provides enantioenriched BINOLs in good yields (up to 99%) and moderate enantioselectivities (up to 81:19 er) using an iron-complex generated in situ from Fe(ClO4)2 and a bisquinolyldiamine ligand [(1R,2R)-N1,N2-di(quinolin-8-yl)cyclohexane-1,2-diamine, L1]. A number of ligands (L2-L8) and the analogs of L1, with various substituents and chiral backbones, were synthesized and examined in the oxidative coupling reactions.

Total synthesis of (±)-fumimycin and analogues for biological evaluation as peptide deformylase inhibitors

A concise 7-step total synthesis of (±)-fumimycin in 11.6 % overall yield is reported. An acid-catalyzed intramolecular aza-Friedel-Crafts cyclization was developed to construct the benzofuranone skeleton of the natural product bearing an α,α-disubstituted amino acid moiety in a single step. Regioselective chlorination followed by a Suzuki-Miyaura cross-coupling rapidly enabled the preparation of a library of analogues which were evaluated against peptide deformylase for antibacterial activity.

Enantioselective Cycloaddition of Styrenes with Aldimines Catalyzed by a Chiral Magnesium Potassium Binaphthyldisulfonate Cluster as a Chiral Brønsted Acid Catalyst

A chiral magnesium potassium binaphthyldisulfonate cluster, as a chiral Brønsted acid catalyst, was shown to catalyze an enantioselective cycloaddition of styrenes with aldimines for the first time. The strong Brønsted acidity of the catalyst precursors, which might dissolve drying agents and take up the leached Mg²⁺ and K⁺, serendipitously led to good enantioselectivity. Mechanistic aspects were supported by X-ray and ESI-MS analysis of the catalyst and a kinetics study of the reaction. Useful transformations to optically active 1,3-amino alcohols on a gram scale were also demonstrated.

Amine-Catalyzed Highly Regioselective and Stereoselective C(sp(2) )-C(sp(2) ) Cross-Coupling of Naphthols with trans-α,β-Unsaturated Aldehydes

A metal-free C(sp(2) )-C(sp(2) ) cross-coupling approach to highly congested (E)-α-naphtholylenals from simple naphthols and enals is described. The mild reaction conditions with pyridine hydrobromideperbromide (PHBP) as the bromination reagent in the presence of piperidine or diphenylprolinol trimethylsilyl (TMS) ether as promoters enable the process in good yields and with high chemoselectivity, regioselectivity, and stereoselectivity. The process involves an unprecedented pathway of in situ regioselective 4-bromination of 1-naphthols and the subsequent unusual aromatic nucleophilic substitution of the resulting 4-bromo-1-naphthols with the α-C(sp(2) ) of enals through a Michael-type Friedel-Crafts alkylation-dearomatization followed by a cyclopropanation ring-opening cascade process. The noteworthy features of this strategy are highlighted by the highly efficient creation of a C(sp(2) )-C(sp(2) ) bond from readily available unfunctionalized naphthols and enals catalyzed by non-metal, readily available cyclic secondary amines under mild reaction conditions.