l-Threonine-Derived Novel Bifunctional Phosphine−Sulfonamide Catalyst-Promoted Enantioselective Aza-Morita−Baylis−Hillman Reaction

Effects of Anchimeric Assistance in Phosphonium Enolates Chemistry

Abstract

Phosphonium enolates are key intermediates of phosphine-catalyzed reactions extensively used in current organic synthesis for construction of carbon–carbon and carbon–heteroatom bonds. Herein, general methods for increasing the rates, chemo- and stereoselectivity on the basis of effects of anchimeric assistance for the reactions involving formation of phosphonium enolates are discussed.

Phosphine-catalyzed Divergent Domino Processes between γ-Substituted Allenoates and Carbonyl-Activated Alkenes

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed. In the presence of NUSIOC-Phos, triketone enone substrates smoothly reacted with γ-substituted allenoates to form bicyclic furofurans in good yields with high stereoselectivities. Alternatively, the reaction between diester-activated enone substrates and γ-substituted allenoates formed chiral conjugated 1,3-dienes in good yields with excellent enantioselectivities. Notably, by employing substrates with subtle structural difference, under virtually identical reaction conditions, we were able to access two types of chiral products, which are of biological relevance and synthetic importance.

Highly enantioselective and chemodivergent domino reactions between γ-substituted allenoates and activated alkenes have been developed.

Multifunctional chiral aminophosphines for enantiodivergent catalysis in a palladium-catalyzed allylic alkylation reaction

Trifunctional MAP-based chiral phosphines were tested as new ligands in a Pd-catalyzed asymmetric allylic alkylation, demonstrating fast and enantiodivergent catalysis. The palladium complexes of representative ligands by X-ray analysis revealed a novel mode of P,N-coordination of the ligand to the palladium center, which may contribute to switching the sense of the asymmetric induction via combined steric and tunable H-bonding interactions between the metal complex and the substrates.

Recent advances in reactions promoted by amino acids and oligopeptides

During the last 20 years, Organocatalysis has become one of the major fields of Catalysis. Herein, we provide a recent overview on reactions where the use of amino acids and peptides as the organocatalysts was employed. All aspects regarding aldol reactions, Michael reactions, epoxidation, Henry reactions and many others that are crucial for the reaction conditions and reaction mechanisms are discussed.

Organocatalytic Synthesis of Highly Functionalized Heterocycles by Enantioselective aza-Morita-Baylis-Hillman-Type Domino Reactions

Organocatalytic enantioselective domino reactions are an extremely attractive methodology, as their use enables the construction of complex chiral skeletons from readily available starting materials in two or more steps by a single operation under mild reaction conditions. Thus, these reactions can save both the quantity of chemicals and length of time typically required for the isolation and/or purification of synthetic intermediates. Additionally, no metal contamination of the products occurs, given that organocatalysts include no expensive or toxic metals. The aza-Morita-Baylis-Hillman (aza-MBH) reaction is an atom-economical carbon-carbon bond-forming reaction between α,β-unsaturated carbonyl compounds and imines mediated by Lewis base (LB) catalysts, such as nucleophilic phosphines and amines. aza-MBH products are functionalized chiral β-amino acid derivatives that are highly valuable as pharmaceutical raw materials. Although various enantioselective aza-MBH processes have been investigated, very few studies of aza-MBH-type domino reactions have been reported due to the complexity of the aza-MBH process, which involves a Michael/Mannich/H-transfer/β-elimination sequence. Accordingly, in this review article, our recent efforts in the development of enantioselective domino reactions initiated by MBH processes are described. In the domino reactions, chiral organocatalysts bearing Brønsted acid (BA) and/or LB units impart synergistic activation to substrates, leading to the easy synthesis of highly functionalized heterocycles (some of which have tetrasubstituted and/or quaternary carbon stereocenters) in high yield and enantioselectivity.

One-Pot Synthesis of Aza-Morita-Baylis-Hillman Adducts via Zinc-Mediated Allylation of 4-Bromocrotonates and Imines

An approach to the synthetically valuable β-substituted aza-Morita-Baylis-Hillman (MBH) type products, through zinc-mediated allylation of imines with 4-bromocrotonates followed by subsequent isomerization of the double bond, was developed. Complementary to classical MBH reaction, this reaction utilizes easily available 4-bromocrotonates as the reaction partners, providing a rapid alternative to the traditional use of α,β-unsaturated carbonyl compounds. The short reaction time, one-pot operation, broad substrate scopes, gram-scale synthesis, and synthetic application exemplified the utility and practicability of this method.

Catalyst-controlled regioselectivity in phosphine catalysis: the synthesis of spirocyclic benzofuranones via regiodivergent [3 + 2] annulations of aurones and an allenoate

Catalyst-controlled regiodivergent [3 + 2] annulations of aurones and allenoates have been developed. When a dipeptide phosphine catalyst with an l-d- configuration was employed, α-selective [3 + 2] annulation products could be obtained with good regioselectivities and enantioselectivities. With the employment of l-l- dipeptide phosphines, γ-selective annulation products could be selectively obtained with excellent enantioselectivities. By simply tuning the catalyst configurations, a wide range of α-selective or γ-selective spirocyclic benzofuranones with either aryl or alkyl substitutions could be readily prepared. DFT calculations suggest that the conformation of the dipeptide phosphines influences the hydrogen bonding interactions or the distortion energy, resulting in delicate energy differentiation in the transition states, and accounting for the observed regioselectivity.

P-Stereogenic β-Aminophosphines: Preparation and Applications in Enantioselective Organocatalysis

The synthesis of stereodefined β-aminophosphines having both carbon- and phosphorus-based chirality centers is described. The method involves resolution of a mixture of β-aminophosphine oxide diastereomers accessed by ring-opening of an amino alcohol-derived cyclic sulfamidate. A stereospecific, borane-promoted reduction of β-aminophosphine oxides, which occurs under mild conditions and with inversion of configuration at phosphorus, is a key step in this process. The products obtained are new building blocks for the synthesis of P-chiral ligands and organocatalysts. In a proof-of-concept application in organocatalysis, the diastereomeric P-chiral β-aminophosphine-based bifunctional thioureas displayed significantly different activities in the Morita-Baylis-Hillman reaction of methyl acrylate with benzaldehyde derivatives.

Amino Acid-Derived Bifunctional Phosphines for Enantioselective Transformations

Even though seminal reports on phosphine catalysis appeared in the 1960s, in the last few decades of the past century trivalent phosphines were viewed primarily as useful ligands for transition-metal-mediated processes. The 1990s saw revived interest in using phosphines in organic catalysis, but the key advances in asymmetric phosphine catalysis have all come within the past decade. The uniqueness of phosphine catalysis can be attributed to the high nucleophilicity of the phosphorus atom. In typical phosphine-catalyzed reactions, nucleophilic attacks of the phosphorus atom on electron-deficient multiple bonds create different reactive ylide-type intermediates. When such structurally diverse zwitterionic species react with a variety of suitable substrates, new reaction patterns are often discovered and a diverse array of reactions can be developed. In recent years, substantial progress has been made in the field of asymmetric phosphine catalysis; many new reactions have been discovered, and numerous enantioselective processes have been reported. However, we felt that powerful and versatile phosphine catalysts that can work for a wide range of asymmetric reactions are still lacking. We therefore set our goal to develop a family of easily derived phosphine catalysts that are efficient in asymmetric induction for a broad range of phosphine-mediated transformations. This Account describes our efforts in the past few years on the development of amino acid-based bifunctional phosphines and their applications to enantioselective processes. Building upon our previous success in primary-amine-mediated enamine catalysis, we first established that bifunctional phosphines could be readily prepared from amino acids. In most of our studies, we chose threonine as the key backbone for catalyst development, and threonine-based monoamino acid or dipeptide bifunctional phosphines have displayed remarkable stereochemical control. We began our investigations by demonstrating the usefulness of our phosphine catalysts in aza-Morita-Baylis-Hillman (aza-MBH) and MBH reactions. We then showed the great power of amino acid/dipeptide phosphines in a wide range of [3 + 2] annulation processes, including [3 + 2] cycloaddition of allenoates to acrylates/acrylamides, [3 + 2] annulation of imines with allenoates, and [3 + 2] cyclization employing MBH carbonates and activated alkenes. By utilizing α-substituted allenoates and activated alkenes, we developed an enantioselective [4 + 2] annulation to access functionalized cyclohexenes. We also devised a novel enantioselective [4 + 2] annulation process by using α-substituted allenones for the construction of 3,4-dihydropyrans. With the use of β'-acetate allenoate, a [4 + 1] annulation process has been designed to access chiral spiropyrazolones. Another array of reactions that make use of the basicity of zwitterionic phosphonium enolate intermediates have been successfully attained, including the first phosphine-catalyzed asymmetric Michael addition, enantioselective allylic substitution of MBH carbonates by phthalides, and enantioselective γ-additions of prochiral 3-substituted oxindoles, 5H-thiazol-4-ones, 5H-oxazol-4-ones, and oxazol-5-(4H)-ones to 2,3-butadienoates. Bifunctional modes of action in our reported reactions have been supported by experimental results and theoretical studies. With the establishment of the new families of powerful amino acid-derived bifunctional phosphines, the discovery of new modes of phosphine activation, unknown reactions, and more enantioselective processes are well-anticipated.

Enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines catalyzed by a bifunctional dipeptide phosphine

The first enantioselective [3 + 2] annulation of α-substituted allenoates with β,γ-unsaturated N-sulfonylimines is described. In the presence of a dipeptide phosphine catalyst, a wide range of highly functionalized cyclopentenes bearing an all-carbon quaternary center were obtained in moderate to good yields and with good to excellent enantioselectivities.

A versatile synthesis of chiral β-aminophosphines

A protocol for the synthesis of chiral β-aminophosphines by ring-opening of cyclic sulfamidates with metallated secondary phosphine oxides is described. It allows access to previously unknown derivatives having electron-deficient, electron-rich and sterically hindered P-aryl substituents.

Recent developments in the synthesis and utilization of chiral β-aminophosphine derivatives as catalysts or ligands

In the last few years, the research area of chiral β-aminophosphines capable of promoting a wide range of diverse organic transformations has attracted more attention.

Aza-Morita–Baylis–Hillman reactions catalyzed by a cyclopropenylidene

Catalysis using a bis(dialkylamino)cyclopropenylidene (BAC) has been developed, which relies on a formal umpolung activation of Michael acceptor pro-nucleophiles.

Morita-Baylis-Hillman Reaction of β,β-Disubstituted Enones: An Enantioselective Organocatalytic Approach for the Synthesis of Cyclopenta[b]annulated Arenes and Heteroarenes

The first enantioselective organocatalytic intramolecular Morita-Baylis-Hillman (MBH) reaction of sterically highly demanding β,β-disubstituted enones is presented. The MBH reaction of β,β-disubstituted-α,β-unsaturated electron-withdrawing systems was previously considered to be unfeasible. Towards this end, designer substrates, which under simple and practical reaction conditions generate a variety of cyclopenta[b]annulated arenes and heteroarenes in excellent enantiopurities and near-quantitative yields in remarkably short reaction times, are described. The reason for the unusually facile nature of this reaction is attributed to the synergy guided and entropically favored intramolecular reaction. Further, this strategy provides easy access to a substantial number of bioactive natural products and pharmaceutically significant compounds.

Chiral phosphine-catalyzed tunable cycloaddition reactions of allenoates with benzofuranone-derived olefins for a highly regio-, diastereo- and enantioselective synthesis of spiro-benzofuranones

The first regioselective catalytic asymmetric [3 + 2] cycloaddition of benzofuranone-derived olefins with allenoates and substituted allenoates has been developed in the presence of (R)-SITCP, affording different functionalized 3-spirocyclopentene benzofuran-2-ones in good yields with high enantioselectivities under mild conditions. The substrate scope has also been examined. The regioselective outcomes for this phosphine-catalyzed [3 + 2] cycloaddition reaction can be rationalized using DFT calculations.

Highly enantioselective construction of tertiary thioethers and alcohols via phosphine-catalyzed asymmetric γ-addition reactions of 5H-thiazol-4-ones and 5H-oxazol-4-ones: scope and mechanistic understandings

Phosphine-catalyzed highly enantioselective γ-additions of 5H-thiazol-4-ones and 5H-oxazol-4-ones to allenoates have been developed for the first time. With the employment of amino-acid derived bifunctional phosphines, a wide range of substituted 5H-thiazol-4-one and 5H-oxazol-4-one derivatives bearing heteroatom (S or O)-containing tertiary chiral centers were constructed in high yields and excellent enantioselectivities. The reported method provides facile access to enantioenriched tertiary thioethers/alcohols. The mechanism of the γ-addition reaction was investigated by performing DFT calculations, and the hydrogen bonding interactions between the Brønsted acid moiety of the phosphine catalysts and the "C[double bond, length as m-dash]O" unit of the donor molecules were shown to be crucial in asymmetric induction.

Catalytic enantioselective synthesis of quaternary 3,3′-indolyloxindoles by combination of Rh(ii) complexes and chiral phosphines

Rh(ii)/chiral phosphine combined catalysis has been developed for a highly efficient sequential C–H functionalization/asymmetric allylation or Michael addition reaction to afford a diverse range of quaternary 3,3′-indolyloxindole derivatives.

Phosphine-catalyzed regioselective Michael addition to allenoates

The first phosphine catalysed Michael addition of arylcyanoacetates to allenoates has been developed, and the β-selective products with a quaternary center were obtained in excellent yields.