Yasunori Toda

Double Bond Isomerization/Enantioselective Aza-Petasis−Ferrier Rearrangement Sequence as an Efficient Entry to Anti- and Enantioenriched β-Amino Aldehydes

Highly anti- and enantioselective synthesis of beta-amino aldehydes having an aliphatic substituent at the beta-position was accomplished by a combination of two catalytic reactions, that is, an initial Ni(II) complex-catalyzed isomerization of a double bond followed by a chiral phosphoric acid catalyzed aza-Petasis-Ferrier rearrangement, using hemiaminal allyl ethers as the initial substrate. The chiral phosphoric acid also functioned as an efficient resolving catalyst of racemic hemiaminal vinyl ethers.

Chiral Silver Phosphate Catalyzed Transformation of ortho‐Alkynylaryl Ketones into 1H‐Isochromene Derivatives through an Intramolecular‐Cyclization/Enantioselective‐Reduction Sequence

The transformation of ortho-alkynylaryl ketones through a cyclization/enantioselective-reduction sequence in the presence of a chiral silver phosphate catalyst afforded 1H-isochromene derivatives in high yield with fairly good to high enantioselectivity. An asymmetric synthesis of the 9-oxabicyclo[3.3.1]nona-2,6-diene framework, which has been found in some biologically active molecules, is presented as a demonstration of the synthetic utility of this method.

Relay Catalysis Using a Rhodium Complex/Chiral Brønsted Acid Binary System: Enantioselective Reduction of a Carbonyl Ylide as the Reactive Intermediate†

Carbonyl ylides are generally non-isolable reactive intermediates and have been extensively utilized as the dipole in 1,3-dipolar cycloaddition reactions with electron-deficient and electron-rich dipolarophiles to afford polycyclic compounds including five-membered oxacycles. The most efficient method for generating the carbonyl ylide is the interaction of a metal carbene complex with the oxygen atom of a carbonyl group. One practical way of generating metal carbene complexes as a precursor of carbonyl ylides is the decomposition of a-diazocarbonyl compounds by rhodium catalysts, and significant progress has been made in developing stepwise sequences for rhodium carbene/carbonyl ylide formation. However, this sequential process with carbonyl ylides has been applied only to 1,3-dipolar cycloaddition reactions and, to the best of our knowledge, further utilization of this attractive intermediate has never been demonstrated in any range of organic transformations. To broaden the scope of the synthetic applicability of the carbonyl ylide, we envisioned the combination of an organocatalytic method with a rhodium-catalyst-initiated reaction sequence in one pot. Combined use of a transition-metal catalyst and an organocatalyst has stimulated intensive interest in recent years, as it could potentially enable highly efficient and/or unprecedented transformations in a one-pot operation. Indeed, excellent transformations have been established by taking advantage of both of these catalytic approaches, where two types of catalyst combinations have been developed in the binary catalytic system. One is that each reactant is activated simultaneously by one type of catalyst; for instance, a metal catalyst is used to activate the nucleophile while an organocatalyst is used to activate the electrophile in a cooperative manner. The other is the consecutive transformation using a binary catalytic system, that is, relay catalysis for a multistep sequence in which each catalyst promotes one type of reaction in the sequence in one pot. Herein, we report an unprecedented relay catalysis for a carbonyl ylide formation/enantioselective reduction sequence using a binary catalytic system that consisting of the dirhodium(II) tetracarboxylate 1 and chiral phosphoric acid 2 as a chiral Bronsted acid catalyst (Scheme 1). The proposed relay catalysis involves a four-step transformation: a) decomposition of the a-diazocarbonyl compound 3 by 1 to generate

Chiral anion catalysis in the enantioselective 1,4-reduction of the 1-benzopyrylium ion as a reactive intermediate.

Anionic! Novel chiral anion catalysis of the enantioselective 1,4-reduction of the 1-benzopyrylium ion by a chiral phosphoric acid was accomplished with a Hantzsch ester as the reducing agent. The enantioselective reduction established is composed of a two-step consecutive transformation involving stereoablative loss of the hydroxy group from racemic 2H-chromen-2-ol derivatives to generate the achiral 1-benzopyrylium ion as a reactive key intermediate.

Brønsted Acid Catalyzed Phosphoramidic Acid Additions to Alkenes: Diastereo- and Enantioselective Halogenative Cyclizations for the Synthesis of C- and P-Chiral Phosphoramidates

The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

Secondary stereocontrolling interactions in chiral Brønsted acid catalysis: study of a Petasis–Ferrier-type rearrangement catalyzed by chiral phosphoric acids

Chiral phosphoric acids have emerged as promising asymmetric Bronsted acid catalysts that harness hydrogen bonding interactions as key stereocontrolling elements. A new approach to chiral phosphoric acid catalysis through ion-pairing interactions between the anionic conjugate base of the catalyst and a cationic electrophile has recently attracted attention. However, the mechanism of stereocontrol through ion-pairing interactions is still elusive. As a probe reaction for studying the stereocontrolling element involved in such catalytic reactions, we investigated the Petasis–Ferrier-type rearrangement of a 7-membered cyclic vinyl acetal catalyzed by chiral phosphoric acids. DFT calculations suggested that non-classical C–H⋯O hydrogen bonds between the catalyst and the substrate play an important role in determining the stereoselectivity. In addition, π–π stacking interactions were found to be a key factor for stereocontrol when using a 9-anthryl group-bearing catalyst.

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.

Chiral Lewis Acid-Catalyzed Enantioselective Cycloadditions between Indoles and Cyclic Carbonyl Ylides Derived from Diazodiketone or Diazoketoester Derivatives.

Asymmetric 1,3-dipolar cycloaddition reactions between N-methylindoles and several cyclic carbonyl ylides that were derived from diazodiketone or diazoketoester precursors in the presence of both achiral Rh and chiral lanthanoid metal catalysts are described. For the six-membered cyclic carbonyl ylides derived from 1-diazo-5-aryl-2,5-pentanedione precursors, the cycloaddition reactions were carried out using Rh2(OAc)4 (2 mol %) and the chiral Pybox-Ph2-Lu(OTf)3 complex (10 mol %) as catalysts, resulting in high enantioselectivities (83% to >98% ee (exo)) along with relatively good exo-selectivities (exo:endo = 65:35 to 94:6) and yields (63-85%). For the five-membered cyclic carbonyl ylide derived from 1-diazo-2,4-pentandione precursor, the cycloaddition reaction with 5-bromo-1-methylindole was carried out in the presence of Rh2(OAc)4 (2 mol %) and the chiral Pybox-Ph2-Er(OTf)3 complex (30 mol %) as catalysts, resulting in relatively good enantioselectivity (78% ee) and endo-selectivity (endo:exo = 81:19).

Enantioconvergent Nucleophilic Substitution Reaction of Racemic Alkyne–Dicobalt Complex (Nicholas Reaction) Catalyzed by Chiral Brønsted Acid

Catalytic enantioselective syntheses enable a practical approach to enantioenriched molecules. While most of these syntheses have been accomplished by reaction at the prochiral sp(2)-hybridized carbon atom, little attention has been paid to enantioselective nucleophilic substitution at the sp(3)-hybridized carbon atom. In particular, substitution at the chiral sp(3)-hybridized carbon atom of racemic electrophiles has been rarely exploited. To establish an unprecedented enantioselective substitution reaction of racemic electrophiles, enantioconvergent Nicholas reaction of an alkyne-dicobalt complex derived from racemic propargylic alcohol was developed using a chiral phosphoric acid catalyst. In the present enantioconvergent process, both enantiomers of the racemic alcohol were transformed efficiently to a variety of thioethers with high enantioselectivity. The key to achieving success is dynamic kinetic asymmetric transformation (DYKAT) of enantiomeric cationic intermediates generated via dehydroxylation of the starting racemic alcohol under the influence of the chiral phosphoric acid catalyst. The present fascinating DYKAT involves the efficient racemization of these enantiomeric intermediates and effective resolution of these enantiomers through utilization of the chiral conjugate base of the phosphoric acid.

Tetraarylphosphonium Salt-Catalyzed Synthesis of Oxazolidinones from Isocyanates and Epoxides

Preparation of a range of oxazolidinones, including enantioenriched N-aryl-substituted oxazolidinones, in which tetraarylphosphonium salts (TAPS) catalyze the [3 + 2] coupling reaction of isocyanates and epoxides effectively, is described. The key finding is a Brønsted acid/halide ion bifunctional catalyst that can accelerate epoxide ring opening with high regioselectivity. Mechanistic studies disclosed that the ylide generated from TAPS, along with the formation of halohydrins, plays a crucial role in the reaction with isocyanates.