Chiral Phosphoramide-Catalyzed Aldol Additions of Ketone Enolates. Preparative Aspects

Moderation of the Electronic Structure of Phosphamides to Execute the Catalytic Appel Reaction Bypassing Phosphine

A set of structurally analogous, albeit electronically distinct, phosphamides (1aa-10aa) is prepared, and the effect of the electronic amendment due to p-substitution has been tested for the conversion of alcohols to halides via the Appel reaction. The -OMe-substituted diphosphamide (8aa) remains the most active, providing ∼96% conversion of alcohols to halides with a TON of 11 in moderate reaction conditions with a large substrate scope. Halide formation follows a pseudo-first-order rate with a constant rate (kobs) of 7.13 × 10-5 s-1. Temp-dependent kinetics and Eyring analyses reveal the activation parameters ΔH‡ of 28.95 (±1.6) kcal mol-1, ΔS‡ of -70.02 (±0.4) cal K-1 mol-1, and ΔG‡298 of 49.81 (±1.2) kcal mol-1. The deuterium labeling study highlights the O-H dissociation of the alcohol as the rate-determining step, while the Hammett analysis with p-substituted benzyl alcohols indicates a positive charge accumulation at the phosphorus center during the Appel reaction. The HOMO-LUMO energy and NPA analyses show that p-OMe substitutions in 8aa make the "P═O" bond more ionic and corresponding aminophosphine is nucleophilic, which are favorable for the Appel reaction. In situ detection of the Appel salt, [R3PX]CX3 and alkoxy phosphonium cation [R3POR]X, validates the reaction pathway mediated by the phosphamides.

Mechanistic Insights into the Appel Reaction Mediated by a Poly-Phosphamide Material as a Heterogeneous Catalyst

Due to notable thermochemical stability, polyphosphamides are often regarded as flame retardants, while molecular phosphamides can serve as versatile Lewis base to catalyze diverse organic transformations. Being chemically analogous to phosphine oxide, phosphamide can also be considered as a mediator for the phosphine-mediated reaction. Herein, an amorphous polymeric material consisting of phosphamide (-NH-P(O)) in the repeating unit (POP) has been prepared via condensation of tris(2-aminoethyl)amine (TREN) and phenyl phosphinic dichloride (PPDC). The POP is isolated as a metal-free and pure organic material which is made of a strong covalent bond and the phosphamide unit is deployed in the organic framework. The presence of phosphamide in the repeating unit of the isolated amorphous POP material can be confirmed by 31P CPMAS NMR, FTIR, and Raman studies. The core-level N 2p and P 2p X-ray photoelectron spectra are in accordance with the presence of tertiary amine nitrogen attached to carbon and secondary amine nitrogen attached to phosphorus. Elemental analyses have depicted approximately 19.7% of phosphorus content in the material, which is being utilized to study the catalytic Appel reaction with 76% conversion of alcohol to a corresponding halide and TON of 462. Quasi in situ Raman study has identified that amino phosphine formed via in situ reduction of the phosphamide unit of the POP catalyzes the halogenation of primary and secondary alcohols with wide substrate scope and functional group tolerance. Kinetic studies have established a first-order dependence with respect to alcohol, while deuterium labeling experiments emphasize that the deprotonation of alcohol is the rate-limiting step. High thermal stability of the material, scope of easy catalyst recyclability, and a cumulative TON of 1386 have led the POP as an emerging pure organic material to be explored further for other phosphine-mediated organocatalysis.

Blue LED-Mediated Syntheses of Arylazo Phosphine Oxides and Phosphonates via N-P Bond Formation

The synthesis of (E)-diphenyl(aryldiazenyl)phosphine oxides and dialkyl (E)-(aryldiazenyl)phosphonates via visible light-mediated N-P bond formation between diazo species and phosphine oxides and phosphite derivatives, respectively, is described. The diazo species were generated via the reaction of aniline with isoamyl nitrite, which upon reaction with phosphorus surrogates generated arylazophosphine oxides and arylazo phosphonates in good to excellent yields. This sustainable chemical process offers a broad substrate scope and reasonably viable product formation.

Redox-Neutral P(O)-N Coupling between P(O)-H Compounds and Azides via Dual Copper and Photoredox Catalysis

We report a redox-neutral P(O)-N coupling reaction of P(O)-H compounds with azides via photoredox and copper catalysis, providing new access to useful phosphinamides, phosphonamides, and phosphoramides. This transformation tolerates a wide range of nucleophilic functionalities including alcohol and amine nucleophiles, which makes up for the deficiency of classical nitrogen nucleophilic substitution reactions. As a demonstration of the broad potential applications of this new methodology, late-stage functionalization of a diverse array of azido-bearing natural products and drug molecules, a preliminary asymmetric reaction, and a continuous visible-light photoflow process have been developed.

Chiral Amphiphilic Secondary Amine-Porphyrin Hybrids for Aqueous Organocatalysis

Two chiral proline-derived amphiphilic 5-substituted-10,15,20-tris(4-sulfonatophenyl)porphyrins were prepared, and their pH-dependent supramolecular behavior was studied. In neutral aqueous solutions, the free-base form of the hybrids is highly soluble, allowing enamine-based organocatalysis to take place, whereas under acidic conditions, the porphyrinic protonated core of the hybrid leads to the formation of self-assembled structures, so that the hybrids flocculate and their catalytic activity is fully suppressed. The low degree of chirality transfer observed for aqueous Michael and aldol reactions strongly suggests that these reactions take place under true "in water" organocatalytic conditions. The highly insoluble catalyst aggregates can easily be separated from the reaction products by centrifugation of the acidic reaction mixtures, and after neutralization and desalting, the sodium salts of the sulfonated amine-porphyrin hybrids, retaining their full catalytic activity, can be recovered in high yield.

Multilayer 3D Chirality and Its Synthetic Assembly

3D chirality of sandwich type of organic molecules has been discovered. The key element of this chirality is characterized by three layers of structures that are arranged nearly in parallel fashion with one on top and one down from the center plane. Individual enantiomers of these molecules have been fully characterized by spectroscopies with their enantiomeric purity measured by chiral HPLC. The absolute configuration was unambiguously assigned by X-ray diffraction analysis. This is the first multilayer 3D chirality reported and is anticipated to lead to a new research area of asymmetric synthesis and catalysis and to have a broad impact on chemical, medicinal, and material sciences in future.

Multifunctional phosphoramide-(S)-prolinamide derivatives as efficient organocatalysts in asymmetric aldol and Michael reactions

Knowledge accumulated in the field of organocatalysis led to the design and synthesis of three novel and efficient organocatalysts for the stereoselective aldol and Michael reactions in the presence of water.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Temperature-controlled Mukaiyama aldol reaction of cyclododecanone (CDD) with aromatic aldehydes promoted by TMSCl via the (TMS)3Si intermediate generated in situ

A temperature controlled chemo-, regio- and diastereoselective synthesis of enones and Mukaiyama aldol reaction have been developed using sterically hindered CDD with organosilane as a catalyst.

Lewis base catalysis of the Mukaiyama directed aldol reaction: 40 years of inspiration and advances

Since the landmark publications of the first directed aldol addition reaction in 1973, the site, diastereo-, and enantioselective aldol reaction has been elevated to the rarefied status of being both a named and a strategy-level reaction (the Mukaiyama directed aldol reaction). The importance of this reaction in the stereoselective synthesis of untold numbers of organic compounds, both natural and unnatural, cannot be overstated. However, its impact on the field extends beyond the impressive applications in synthesis. The directed aldol reaction has served as a fertile proving ground for new concepts and new methods for stereocontrol and catalysis. This Minireview provides a case history of how the challenges of merging site selectivity, diastereoselectivity, enantioselectivity, and catalysis into a unified reaction manifold stimulated the development of Lewis base catalyzed aldol addition reactions. The evolution of this process is chronicled from the authors' laboratories as well as in those of Professor Teruaki Mukaiyama.

Lewis acid organocatalysts

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 degrees C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of their cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis.

From Allylic Alcohols to Aldols through a New Nickel-Mediated Tandem Reaction: Synthetic and Mechanistic Studies

Nickel hydride type complexes have been successfully developed as catalysts for the tandem isomerization-aldolization reaction of allylic alcohols with aldehydes. Optimization of the reaction conditions has shown that a cocatalyst, such as MgBr2, has a very positive effect on the kinetics of the reaction and in the yields of aldols. Under such optimized conditions {[NiHCl(dppe)] + MgBr(2) at 3-5 mol %)}, this reaction affords the aldols in good to excellent yields. It is a full-atom-economy-type reaction that occurs under mild conditions. Furthermore, it has a broad scope for the allylic alcohols and it is compatible with a wide range of aldehydes, including very bulky derivatives. The reaction is completely regioselective, but it exhibits a low stereoselectivity, except for allylic alcohols with a bulky substituent at the carbinol center. The use of chiral nonracemic catalysts was not successful, affording only racemic compounds. However, it was possible to use asymmetric synthesis for the preparation of optically active aldols. Various mechanistic studies have been performed using, for instance, a deuterated alcohol or a deuterated catalyst. They gave strong support to a mechanism involving first a transition-metal-mediated isomerization of the allylic alcohol into the free enol, followed by the addition of the latter intermediate onto the aldehyde in an "hydroxyl-carbonyl-ene" type reaction. These results confirm that allylic alcohols can be considered as new and useful partners in the development of the aldol reaction.

Palladium-Catalyzed Cross-Coupling of Baylis−Hillman Acetate Adducts with Organosilanes

[Reaction: see text]. A cross-coupling reaction between acetates of Baylis-Hillman adducts and organosilanes is described. A nonconventional solvent poly(ethylene glycol) (PEG) is used as the reaction medium.

A Novel Pyrrolidine Imide Catalyzed Direct Formation of α,β-Unsaturated Ketones from Unmodified Ketones and Aldehydes

A method for direct, stereoselective preparation of (E)-alpha,beta-unsaturated ketones from ketones and aldehydes, promoted by a novel pyrrolidine imide organocatalyst, has been developed in moderate to high yields. Unlike the Claisen-Schmidt condensation and Lewis acid catalyzed tandem aldol-dehydration processes, this method provides mild reaction conditions to access alpha,beta-unsaturated ketones from simple, unmodified ketones. [reaction: see text]