Asymmetric 1,4-Conjugate Addition of Diarylphosphines to α,β,γ,δ-Unsaturated Ketones Catalyzed by Transition-Metal Pincer Complexes

Regio- and Stereoselective Synthesis of 3-Selenylazaflavanones and 3-Selenylflavanones via Electrochemically Facilitated Selenylation Cascade

Herein, an oxidant- and metal-free electrochemical selenylation reaction of chalcones with diselenides for the synthesis of 3-selenylazaflavanones and 3-selenylflavanones at room temperature was reported. The method proceeded under mild conditions, exhibited a broad substrate scope, and provided the selenylated products in moderate to excellent yields with high regio- and stereoselectivity. The reaction could also be readily scaled up with high efficiency. Detailed mechanistic studies through control experiments disclosed that a selenium-based radical might participate in this electrochemical transformation.

Palladium-Catalyzed Regio- and Enantioselective Hydrophosphination of gem-Difluoroallenes

Chiral allylic phosphines and gem-difluoroalkenes are both important structural motifs in various bioactive molecules, chiral ligands, and natural products. These two motifs are now integrated, and we herein report a straightforward and atom-economical enantioselective hydrophosphination of gem-difluoroallenes using disubstituted phosphines. A wide array of enantioenriched fluorinated allylic phosphines has been accessed with excellent regio- and enantioselectivity and high efficiency. Synthetic and catalytic applications of phosphine products have been demonstrated.

Enantioselective copper-catalyzed hydrophosphination of alkenyl isoquinolines

An enantioselective Cu-catalyzed hydrophosphination of alkenyl quinolines was developed to access a variety of potential chiral bidentate P,N-ligands.

Ni-Catalyzed Enantioselective Hydrofunctionalizations of 1,3-Dienes

Ni-catalyzed enantioselective hydrofunctionalizations of conjugated dienes are particularly demanding reactions to devise because they require not only addressing the inherent challenges associated with the development of an enantioselective transformation but also overcoming all other aspects of selective catalysis (chemoselectivity, regioselectivity, diastereoselectivity, etc.). However, the value-added nature of the chiral allylic and homoallylic derivatives obtained by these methods, the lack of efficient alternatives, and the use of an earth-abundant first-row transition metal have led to renewed interest over the past decade. In this Perspective, we give an overview of the developments in this field, from the original findings (often dating back to the last century) to the most recent contributions. Emphasis is placed on the nature of the hydrofunctionalization agent (C(sp), C(sp2), C(sp3), N, P, or O).

Broken Promises? On the Continued Challenges Faced in Catalytic Hydrophosphination

In this Perspective, we discuss what we perceive to be the continued challenges faced in catalytic hydrophosphination chemistry. Currently the literature is dominated by catalysts, many of which are highly effective, that generate the same phosphorus architectures, e.g., anti-Markovnikov products from the reaction of activated alkenes and alkynes with diarylphosphines. We highlight the state of the art in stereoselective hydrophosphination and the scope and limitations of chemoselective hydrophosphination with primary phosphines and PH₃. We also highlight the progress in the chemistry of the heavier homologues. In general, we have tried to emphasize what is missing from our hydrophosphination armament, with the aim of guiding future research targets.

Nickel/Brønsted acid dual-catalyzed regio- and enantioselective hydrophosphinylation of 1,3-dienes: access to chiral allylic phosphine oxides

While chiral allylic organophosphorus compounds are widely utilized in asymmetric catalysis and for accessing bioactive molecules, their synthetic methods are still very limited. We report the development of asymmetric nickel/Brønsted acid dual-catalyzed hydrophosphinylation of 1,3-dienes with phosphine oxides. This reaction is characterized by an inexpensive chiral catalyst, broad substrate scope, and high regio- and enantioselectivity. This study allows the construction of chiral allylic phosphine oxides in a highly economic and efficient manner. Preliminary mechanistic investigations suggest that the 1,3-diene insertion into the chiral Ni-H species is a highly regioselective process and the formation of the chiral C-P bond is an irreversible step.

Chiral (phosphine)-(imidazoline) PCN pincer palladium(II) complexes: synthesis and application in asymmetric hydrophosphination of 2-alkenoylpyridines with diphenylphosphine

A series of new chiral PCN pincer Pd(II) complexes 3a−l with aryl-based (phosphine)-(imidazoline) ligands were conveniently synthesized from readily available starting materials with the key step being phosphination/C−H palladation reaction....

Ni-Catalyzed Asymmetric Hydrophosphinylation of Conjugated Enynes and Mechanistic Studies

The catalytic asymmetric synthesis of P-stereogenic phosphines is an efficient strategy to access structurally diverse chiral phosphines that could serve as organocatalysts and ligands to transition metals and motifs of antiviral drugs. Herein, we describe a Ni catalyzed highly regio and enantioselective hydrophosphinylation reaction of secondary phosphine oxides and enynes. This method afforded a plethora of alkenyl phosphine oxides which could serve as valuable precursors to bidentate ligands. A new type of mechanism was discovered by combined kinetic studies and density functional theory (DFT) calculations, which was opposed to the widely accepted Chalk–Harrod type mechanism. Notably, the alkene moiety which could serve as a directing group by coordinating with the Ni catalyst in the transition state, plays a vital role in determining the reactivity, regio and enantioselectivity.

A Ni-catalyzed hydrophosphinylation reaction of enynes was reported with excellent regio and enantioselectivity. A protonation mechanism was uncovered by combined kinetic studies and DFT calculations, which may lead to the discovery of other hydrofunctionalization reactions.

Organometallic chemistry and application of palladacycles in asymmetric hydrophosphination reactions

A number of palladacycles containing chiral chelating auxiliaries have been utilized as efficient catalysts for asymmetric hydrophosphination reactions. In all cases, the chiral auxiliaries remained coordinated to the palladium centres throughout the course of the reactions. Despite the presence of a large quantity of powerful tertiary phosphines, which are known to be strong metal ion sequesters, the expected catalyst poisoning was rarely observed in these palladacycle catalyzed processes. This review highlights the unique stereoelectronic features and the important organometallic chemistry of palladacycle catalysts which are essential to their synthetic operations.

Carbene-Catalyzed Enantioselective Hydrophosphination of α-Bromoenals to Prepare Phosphine-Containing Chiral Molecules

Disclosed herein is the first carbene-organocatalyzed asymmetric addition of phosphine nucleophiles to the in situ generated α,β-unsaturated acyl azolium intermediates. Our reaction enantioselectively constructs carbon-phosphine bonds and prepares chiral phosphines with high optical purities. The phosphine products are suitable for transforming to chiral ligands or catalysts with applications in asymmetric catalysis. The diarylalkyl or trialkyl phosphine products from our catalytic reactions, air-sensitive and reactive in nature, can be trapped (and stored) in their sulfur-oxidized form for operational simplicities.

Enantioselective Palladium-Catalyzed Hydrophosphinylation of Allenes with Phosphine Oxides: Access to Chiral Allylic Phosphine Oxides

A Pd-catalyzed hydrophosphinylation of alkyl and aryl-oxyallenes with phosphine oxides has been developed for the efficient and rapid construction of a family of chiral allylic phosphine oxides with a diverse range of functional groups. This methodology was further applied in the facile construction of chiral 2H-chromene and later stage functionalization of cholesterol.

Organocatalytic 1,6-hydrophosphination of para-quinone methides: enantioselective access to chiral 3-phosphoxindoles bearing phosphorus-substituted quaternary carbon stereocenters

An efficient organocatalytic enantioselective 1,6-hydrophosphonylation of para-quinone methides has been achieved for the construction of phosphorus-substituted quaternary carbon stereocenters.

Catalytic and Mechanistic Developments of the Nickel(II) Pincer Complex-Catalyzed Hydroarsination Reaction

Synthetic challenges have significantly slowed the development of the catalytic asymmetric hydroarsination reaction despite it being a highly attractive C-As bond formation methodology. In addition, there is a poor understanding of the main reaction steps in such reactions which limit further development in the field. Herein, key intermediates of the hydroarsination reaction catalyzed by a PCP Ni^II -Cl pincer complex are presented upon investigating the reaction with DFT calculations, conductivity measurements, NMR spectroscopy, and catalytic screening. The novel Ni-Cl-As interaction proposed was then contrasted against known Ni^II -catalyzed hydrophosphination reactions to highlight dissimilarities between them even though P and As share a close group relationship. Lastly, the asymmetric hydroarsination of nitroolefins was further developed to furnish a library of chiral organoarsines in up to 99 % yield and 80 % ee under mild conditions (-20 °C to RT) between 5 to 210 mins.

Chiral palladium pincer complexes for asymmetric catalytic reactions

Palladium pincer complexes, containing a monoanionic terdentate ligand composed of an anionic aryl carbon atom and two mutually compatible donor sites, have aroused considerable interest since their first reports in the late 1970s. The high stability of the Pd pincer complexes and particularly their high modularity make these species ideal candidates for catalysis. Furthermore, the nature of the meridional coordination of the pincer ligands, and along with this their ability to enforce a stereo-specific environment around the Pd center, provide a good opportunity for developing chiral Pd pincer catalysts. Thus, a broad variety of chiral Pd pincer complexes have been prepared by the introduction of various stereochemical centers in the pincer skeletons. These chiral Pd pincer complexes have been successfully applied to many asymmetric catalytic reactions such as hydrophosphination reactions, allylation of aldehydes and imines, Michael and aldol reactions, Suzuki-Miyaura reactions as well as reactions of nitrile compounds with imines. This review focuses on the synthetic methods and the applications of chiral Pd pincer complexes in asymmetric catalysis.

Investigating palladium pincer complexes in catalytic asymmetric hydrophosphination and hydroarsination

Given the periodic relationship of phosphines and arsines, is remodeling the catalytic asymmetric hydrophosphination reaction an efficient manner to develop the corresponding hydroarsination reaction? Herein, a chiral PCP-Pd(ii) pincer complex adept at generating enantioenriched phosphines was examined in the asymmetric hydroarsination reaction. Under distinct conditions, tertiary phosphines and arsines were generated in excellent yields (P: 96%, As: 91%) and ees (P: 90%, As: 85%). While secondary arsine reagents were not direct substitutes for the analogous phosphines, important parameters were identified which increased yield and ee of the hydroarsination reaction. Unlike the PCP-PdOAc pincer complex commonly used for hydrophosphinations, hydroarsination reactions involved a PCP-PdCl catalyst with 10 equiv. of CsF for optimal performance. Notable differences between the two reactions and their workup procedures were highlighted to guide further developments in the field. Lastly, respective mechanisms were proposed and contrasted for the activation of HEPh2 (E = P, As).

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Nickel catalyzed enantioselective hydroarsination of nitrostyrene

A catalytic asymmetric hydroarsination reaction of an activated alkene viz. (E)-nitrostyrene was developed using chiral PCP Pt-, Pd- and Ni-pincer complexes as catalysts. The corresponding chiral tertiary arsine adduct was obtained in ees of up to 80% under mild reaction conditions using the PCP Ni-Cl pincer catalyst. The arsine adduct was furnished with catalyst loadings of 1-5 mol% and the reaction duration ranging from <5 min to 180 min. The subsequent coordination of the hydroarsination product to gold(i) chloride allowed for the confirmation of the stereochemistry of the arsine adduct via crystallographic analysis.

Efficient access to a designed phosphapalladacycle catalyst via enantioselective catalytic asymmetric hydrophosphination

A chiral phosphine auxiliary was generated with excellent ee via catalytic asymmetric hydrophosphination of 3-(naphthalen-1-ylmethylene)pentane-2,4-dione. The subsequent metal complexation of the monophosphine yielded two different coordination complexes depending on the reaction conditions. The ortho-palladation of both coordination complexes resulted in the formation of a single dimeric phosphapalladacycle complex that could be further converted to the monomeric bisacetonitrile derivative. Moreover, the palladium complex exhibits interesting oxophilicity as the stable bisaquo derivative could be isolated and characterized crystallographically. The catalytic potential of the phosphapalladacycle was also demonstrated.

Mechanistic insights into the role of PC- and PCP-type palladium catalysts in asymmetric hydrophosphination of activated alkenes incorporating potential coordinating heteroatoms

The impact of the structural attributes of chiral PC- and PCP-palladium catalysts was investigated in the asymmetric hydrophosphination of various heterocycle-functionalized enone substrates. Due to the architecture of the catalysts, they are confronted with potential catalyst deactivation arising from the coordination of the electron-rich heteroatoms (P, O, N and S) to the metal center. A systematic variation of the location and identity of the heteroatoms demonstrated the impact of structural modifications on the substrates, which have a significant influence on both yields (16-99%) and enantioselectivities (0-99%). A detailed discussion on the distinct catalytic mechanisms (intra- vs. inter-molecular addition) provides important information to explain the results obtained.

The synthesis and efficient one-pot catalytic “self-breeding” of asymmetrical NC(sp3)E-hybridised pincer complexes

Asymmetric NC(sp³)E (E = S, O) pincer complexes were examined as catalysts in asymmetric hydrophosphination.