Toshinobu Korenaga

Electron-Poor Chiral Diphosphine Ligands: High Performance for Rh-Catalyzed Asymmetric 1,4-Addition of Arylboronic Acids at Room Temperature

Electron-poor chiral diphosphine ligands, MeO-F(28)-BIPHEP (1a) and MeO-F(12)-BIPHEP (1b), were synthesized for controlling a transition-metal catalyst electronically. The 1b-ligated Rh catalyst showed excellent catalytic activity with high % ee for asymmetric 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyls at 20 degrees C. The strong pi-acceptor ability of 1b induces transmetalation of arylboronic acid to catalyst precursor [RhCl(1b)](2) directly in the first step of the catalytic cycle.

Highly enantioselective and efficient synthesis of flavanones including pinostrobin through the rhodium-catalyzed asymmetric 1,4-addition.

An efficient synthesis of bioactive chiral flavanones (1) was achieved through the Rh-catalyzed asymmetric 1,4-addition of arylboronic acid to chromone. The reaction in toluene proceeded smoothly at room temperature in the presence of 0.5% Rh catalyst with electron-poor chiral diphosphine MeO-F(12)-BIPHEP. In this reaction, the 1,2-addition to (S)-1 frequently occurred to yield (2S,4R)-2,4-diaryl-4-chromanol as a byproduct, which could be reduced by changing the reaction solvent to CH(2)Cl(2) to deactivate the Rh catalyst (3% required).

Transition-state models are useful for versatile biocatalysts: kinetics and thermodynamics of enantioselective acylations of secondary alcohols catalyzed by lipase and subtilisin

Abstract Lipases and subtilisins are versatile enzymes capable of showing high enantioselectivity and broad substrate specificity simultaneously. The transition-state models previously proposed to rationalize this important feature were intensively examined from kinetic and thermodynamic viewpoints. Kinetic measurements reaffirmed that chiral discrimination originates from the transition state and that the enantioselectivity results from the reduced activity of the enzymes for the slower-reacting enantiomer, but not from the enhanced activity for the faster-reacting enantiomer relative to a reference alcohol, cyclopentanol. The larger substituent of the slower-reacting enantiomers interacts repulsively with the protein in the transition state, and even the larger substituent of the faster-reacting enantiomers interacts unfavorably to some degree with the protein. A number of thermodynamic parameters, ΔΔ H ‡ and ΔΔ S ‡ , for the subtilisin-catalyzed acylations of secondary alcohols were determined. A linear compensation effect was found between the ΔΔ H ‡ and ΔΔ S ‡ values. As the ΔΔ H ‡ value becomes negatively large, the ΔΔ S ‡ value also becomes negatively large. This observation is explained in terms of the transition-state model. Because the widely accepted concepts such as the lock-and-key mechanism and the induced-fit mechanism cannot account for the peculiar behavior of these enzymes toward unnatural substrates, a new category, the non-lock-and-key mechanism, has been proposed.

Intermolecular oxygen atom⋯π interaction in the crystal packing of chiral amino alcohol bearing a pentafluorophenyl group

Abstract Presence of an unique atom-to-face alcoholic oxygen atom⋯π interaction between a pentafluorophenyl group and alcoholic oxygen (O⋯π) has been demonstrated by X-ray analysis of the novel chiral amino alcohol instead of the well-known interaction between usual aromatic ring and alcoholic hydrogen atom (OH⋯π).

Mechanistic Studies of Highly Enantio- and Diastereoselective Aza-Petasis–Ferrier Rearrangement Catalyzed by Chiral Phosphoric Acid

The precise mechanism of the highly anti- and enantioselective aza-Petasis-Ferrier (APF) rearrangement of hemiaminal vinyl ethers catalyzed by a chiral phosphoric acid was investigated by undertaking experimental and theoretical studies. The APF rearrangement is characterized by the following unique mechanistic features: (i) efficient optical kinetic resolution of the starting racemic hemiaminal vinyl ether, (ii) enantioconvergent process from racemic hemiaminal vinyl ethers to optically active β-amino aldehyde products, and (iii) anomalous temperature effects on the enantioselectivity (enantioselectivity increases as reaction temperature increases). The following experiments were conducted to elucidate the unique mechanistic features as well as to uncover the overall scheme of the present rearrangement: (A) X-ray crystallographic analysis of the recovered hemiaminal vinyl ether to determine its absolute configuration, (B) rearrangements of enantiomerically pure hemiaminal vinyl ethers to validate the stereochemical relationship between the hemiaminal vinyl ethers and β-amino aldehydes, (C) theoretical studies on the transition states of the C-O bond cleavage and C-C bond formation steps to gain an insight into the optical kinetic resolution of the hemiaminal vinyl ether and the origin of the stereoselectivity, as well as to elucidate the overall scheme of the present rearrangement, and (D) crossover experiments of two hemiaminal vinyl ethers having different vinyl ether and aliphatic substituents to comprehend the mechanism of the anomalous temperature effect and the enantioconvergent process. The results of experiments and theoretical studies fully support the proposed mechanism of the present anti- and enantioselective APF rearrangement.

Enantioselective acyl transfer catalysis by a combination of common catalytic motifs and electrostatic interactions

Catalysts that can promote acyl transfer processes are important to enantioselective synthesis and their development has received significant attention in recent years. Despite noteworthy advances, discovery of small-molecule catalysts that are robust, efficient, recyclable and promote reactions with high enantioselectivity can be easily and cost-effectively prepared in significant quantities (that is, >10 g) has remained elusive. Here, we demonstrate that by attaching a binaphthyl moiety, appropriately modified to establish H-bonding interactions within the key intermediates in the catalytic cycle, and a 4-aminopyridyl unit, exceptionally efficient organic molecules can be prepared that facilitate enantioselective acyl transfer reactions. As little as 0.5 mol% of a member of the new catalyst class is sufficient to generate acyl-substituted all-carbon quaternary stereogenic centres in quantitative yield and in up to 98:2 enantiomeric ratio (er) in 5 h. Kinetic resolution or desymmetrization of 1,2-diol can be performed with high efficiency and enantioselectivity as well.

Enantioselective Aza Michael-Type Addition to Alkenyl Benzimidazoles Catalyzed by a Chiral Phosphoric Acid.

Highly enantioselective Michael-type addition (MTA) reactions between N-protected alkenyl benzimidazoles and either pyrazoles or indazoles as nitrogen nucleophiles are accomplished for the first time using chiral phosphoric acid catalyst. Theoretical studies elucidated the reaction pathway and the origin of the stereochemical outcomes, where the catalyst substituent and the N-protecting group of benzimidazole contributed to the resulting high enantioselectivity.

Mechanism-based enzymatic method for reliable determination of absolute configuration of chiral 1-substituted ethanols: Combination with NMR method

It has been demonstrated that lipase is useful not only for kinetic resolution but also for the rapid determination of absolute configurations. We have previously proposed a mechanism represented by transition-state models to rationalize the enantioselectivity in the lipase- and subtilisin-catalyzed kinetic resolutions of secondary alcohols. The mechanism indicates that the enzyme-catalyzed reactions can be used as a tool for determining the absolute stereochemistry of secondary alcohols. In order to increase reliability, the enzymatic method was combined with Moshers method using MTPA, to give a protocol which is named the double-confirmation method. The absolute configurations of six 1-substituted ethanols were determined consistently by this new procedure. The enzymatic method is quick, easy, economical, and reliable. An interesting similarity in conformation between the transition-state models and MTPA esters is also described.

Highly active rhodium catalyst with electron-poor diphosphine enables efficient synthesis of chiral 4-aryl-δ-lactones

Chiral 4-aryl-δ-lactones could be synthesized efficiently with high enantioselectivity through asymmetric 1,4-addition of arylboronic acid to α,β-unsaturated lactones using Rh catalyst including electron-poor diphosphine (MeO-F 12 -BIPHEP) at room temperature for 1 h. In particular, our catalytic system proved to be applicable to relatively large coumarin analogues, giving optically pure 4-phenylchroman-2-one analogues in a short time.

Enantioselective Steglich Rearrangement of Oxindole Derivatives by Easily Accessible Chiral N,N-4-(Dimethylamino)pyridine Derivatives

Chiral N,N-4-(dimethylamino)pyridine (DMAP) derivatives, which can be readily prepared by the Ugi multicomponent reaction in a one-pot manner, have been efficiently applied to the enantioselective Steglich rearrangement of oxindole derivatives to give the desired products bearing a quaternary carbon center in high yield (>98% yield) and with high enantioselectivity (up to 99:1 er).