Palladium-catalyzed enantioselective carbonylation reactions

Carbonylation, one of the most powerful approaches to the preparation of carbonylated compounds, has received significant attention from researchers active in various fields. Indeed, impressive progress has been made on this subject over the past few decades. Among the various types of carbonylation reactions, asymmetric carbonylation is a straightforward methodology for constructing chiral compounds. Although rhodium-catalyzed enantioselective hydroformylations have been discussed in several elegant reviews, a general review on palladium-catalyzed asymmetric carbonylations is still missing. In this review, we summarize and discuss recent achievements in palladium-catalyzed asymmetric carbonylation reactions. Notably, this review’s contents are categorized by reaction type.

Stereoselective synthesis of functionalized azepines via gold and palladium relay catalysis

We developed a cycloisomerization/asymmetric [4 + 3] cycloaddition cascade reaction via gold/palladium relay catalysis, furnishing enantioenriched furan-fused azepines efficiently.

Development of Anthrazoline Photocatalyst for Promoting Amination and Amidation Reactions

In this work, we report the synthesis, optical and electrochemical properties of a series of organophotocatalysts bearing anthrazoline framework, as well as demonstrate their catalytic competencies in promoting C-N bond...

Cooperative Palladium-Catalyzed and P(NEt2)3-Mediated (4+1) Annulation of Isatins with 2-Hydroxymethylallylcarbonates

P(NR2)3-promoted transformations of isatins with electron-withdrawing alkenes have received consistently attention over the last decades. However, no example involving chemical transformations of nonactivated or weakly activated alkenes is discovered in...

Hydrogen-Catalyzed Acid Transformation for the Hydration of Alkenes and Epoxy Alkanes over Co-N Frustrated Lewis Pair Surfaces

Hydrogen (H₂) is widely used as a reductant for many hydrogenation reactions; however, it has not been recognized as a catalyst for the acid transformation of active sites on solid surface. Here, we report the H₂-promoted hydration of alkenes (such as styrenes and cyclic alkenes) and epoxy alkanes over single-atom Co-dispersed nitrogen-doped carbon (Co-NC) via a transformation mechanism of acid-base sites. Specifically, the specific catalytic activity and selectivity of Co-NC are superior to those of classical solid acids (acidic zeolites and resins) per micromole of acid, whereas the hydration catalysis does not take place under a nitrogen atmosphere. Detailed investigations indicate that H₂ can be heterolyzed on the Co-N bond to form H^δ--Co-N-H^δ+ and then be converted into OH^δ--Co-N-H^δ+ accompanied by H₂ generation via a H₂O-mediated path, which significantly reduces the activation energy for hydration reactions. This work not only provides a novel catalytic method for hydration reactions but also removes the conceptual barriers between hydrogenation and acid catalysis.

A Chemoenzymatic Cascade Combining a Hydration Catalyst with an Amine Dehydrogenase: Synthesis of Chiral Amines

An encapsulated gold carbene complex was combined with a free amine dehydrogenase (GkAmDH) as a co-catalyst, enabling a cascade synthetic route to directly access chiral amines from propargylethers. This process, combining an initial gold carbene catalyzed hydration of propargylethers to ketones followed by a subsequent reductive amination, produces a wide range of chiral amines in high yields and excellent enantioselectivities.An encapsulated gold carbene complex was combined with a free amine dehydrogenase (GkAmDH) as a co-catalyst, enabling a cascade synthetic route to directly access chiral amines from propargylethers. This process, combining an initial gold carbene catalyzed hydration of propargylethers to ketones followed by a subsequent reductive amination, produces a wide range of chiral amines in high yields and excellent enantioselectivities.

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ-enamide aldehydes bearing a fully substituted, benzylic stereogenic center - a structural motif common in many natural products and therapeutically active compounds - with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4- or 1,5- functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.

Ternary Catalysis Enabled Three-Component Asymmetric Allylic Alkylation as a Concise Track to Chiral α,α-Disubstituted Ketones

Multicomponent reactions that involve interception of onium ylides through Aldol, Mannich, and Michael addition with corresponding bench-stable acceptors have demonstrated broad applications in synthetic chemistry. However, because of the high reactivity and transient survival of these in situ generated intermediates, the substitution-type interception process, especially the asymmetric catalytic version, remains hitherto unknown. Herein, a three-component asymmetric allylation of α-diazo carbonyl compounds with alcohols and allyl carbonates is disclosed by employing a ternary cooperative catalysis of achiral Pd-complex, Rh₂(OAc)₄, and chiral phosphoric acid CPA. This method represents the first example of three-component asymmetric allylic alkylation through an S_N1-type trapping process, which involves a convergent assembly of two active intermediates, Pd-allyl species, and enol derived from onium ylides, providing an expeditious access to chiral α,α-disubstituted ketones in good to high yields with high to excellent enantioselectivity. Combined experimental and computational studies have shed light on the mechanism of this novel three-component reaction, including the critical role of Xantphos ligand and the origin of enantioselectivity.

TfOH catalysed domino-double annulation of arenes with propargylic alcohols: a unified approach to indene polycyclic systems

The design and development of a TfOH catalysed domino strategy for the double annulation of arenes with propargylic alcohols for the rapid generation of indene based polycyclic systems is reported. The dehydration, intramolecular 6-endo-dig hydroarylation, and cationic cyclization were consecutively promoted by TfOH. The key features of this strategy are the formation of two C-C bonds, unified access to indene polycyclic systems, excellent yields (up to 95%), high atom economy (>90%), an operationally simple procedure, and water being the only byproduct. By extending this strategy, a two-step synthesis of the pentacyclic systems of hypoxylonol A (43% overall yield from α-tetralone), daldinone A (63% overall yield from β-tetralone) and spiro-tetracyclic framework of incarviatone A has also been achieved.

Chiral Indoline-2-carboxylic Acid Enables Highly Enantioselective Catellani-type Annulation with 4-(Bromomethyl)cyclohexanone

Chiral indoline-2-carboxylic acid has been identified to enable a highly enantioselective Catellani-type annulation of (hetero)aryl, alkenyl triflate and conjugated vinyl iodides with 4-(bromomethyl)cyclohexanone, directly assembling a diverse range of chiral all-carbon bridged ring systems. Control experiments and DFT calculations suggest that the coordinating orientation of the chiral amino acid to the arylpalladium(II) center allows for high levels of stereochemical control.

C 1-Symmetric diphosphorus ligands in metal-catalyzed asymmetric hydrogenation to prepare chiral compounds

Asymmetric hydrogenation has remained an important and challenging research area in industry as well as academia due to its high atom economy and ability to induce chirality. Among several types of ligands, chiral bidentate phosphine ligands have played a pivotal role in developing asymmetric hydrogenation. Although C₂-symmetric chiral bidentate phosphine ligands have dominated the field, it has been found that several C₁-symmetric ligands are equally effective and, in many cases, have outperformed their C₂-symmetric counterparts. This review evaluates the possibility of the use of C₁-symmetric diphosphorus ligands in asymmetric hydrogenation to produce chiral compounds. The recent strategies and advances in the application of C₁-symmetric diphosphorus ligands in the metal-catalyzed asymmetric hydrogenation of a variety of CC bonds have been summarized. The potential of diphosphorus ligands in asymmetric hydrogenation to produce pharmaceutical intermediates, bioactive molecules, drug molecules, agrochemicals, and fragrances is discussed. Although asymmetric hydrogenation appears to be a problem that has been resolved, a deep dive into the recent literature reveals that there are several challenges that are yet to be addressed. The current asymmetric hydrogenation methods mostly employ precious metals, which are depleting at a fast pace. Therefore, scientific interventions to perform asymmetric hydrogenation using base metals or earth-abundant metals that can compete with established precious metals hold significant potential.

A Bifunctional Copper Catalyst Enables Ester Reduction with H2: Expanding the Reactivity Space of Nucleophilic Copper Hydrides

Employing a bifunctional catalyst based on a copper(I)/NHC complex and a guanidine organocatalyst, catalytic ester reductions to alcohols with H₂ as terminal reducing agent are facilitated. The approach taken here enables the simultaneous activation of esters through hydrogen bonding and formation of nucleophilic copper(I) hydrides from H₂, resulting in a catalytic hydride transfer to esters. The reduction step is further facilitated by a proton shuttle mediated by the guanidinium subunit. This bifunctional approach to ester reductions for the first time shifts the reactivity of generally considered "soft" copper(I) hydrides to previously unreactive "hard" ester electrophiles and paves the way for a replacement of stoichiometric reducing agents by a catalyst and H₂.

Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations.

Catalytic asymmetric [3+2] cycloaddition of isomünchnones with methyleneindolinones

An efficient enantioselective [3+2] cycloaddition of isomünchnones with methyleneindolinones that are generated by an in situ intramolecular addition of the carbonyl group to rhodium carbenes is realized with a chiral N,N'-dioxide/Zn(II) complex as a Lewis acid. A series of chiral oxa-bridged 3-spiropiperidines are obtained in high yields with excellent dr and excellent ee values.

Copper/Lewis base cooperatively catalyzed asymmetric allylic alkylation of Morita-Baylis-Hillman carbonates with azomethine ylides

In this paper, an asymmetric allylic alkylation of easily available azomethine ylides with Morita-Baylis-Hillman (MBH) carbonates through a copper (i)/Lewis base cooperative catalysis strategy has been realized. The co-catalyzed asymmetric allylic alkylation provided the corresponding amino acid derivatives in up to 90% yields with up to 99% ee as well as good to excellent regioselectivity.

Enantioselective Friedel-Crafts reaction of hydroxyarenes with nitroenynes to access chiral heterocycles via sequential catalysis

Naphthols, hydroxyindoles and an activated phenol are reacted with differently substituted (E)-nitrobut-1-en-3-ynes using the commercially available Rawal's chiral squaramide. The corresponding β-nitroalkynes were obtained with good yields and excellent enantioselectivities. Moreover, dihydronaphthofurans can be accessed via silver catalysed cyclization in a tandem one-pot procedure, with high preservation of the optical purity.

Synergistic Pd/Amine-Catalyzed Stereodivergent Hydroalkylation of 1,3-Dienes with Aldehydes: Reaction Development, Mechanism, and Stereochemical Origins

Metal-hydride-catalyzed hydroalkylation of 1,3-dienes with enolizable carbonyl compounds is an atom- and step-economical method for preparing chiral molecules with allylic stereocenters. Although high diastereo- and enantioselectivities have been achieved for many coupling partners, aldehydes have not yet been used for this purpose because they are less stable than other carbonyl compounds under basic conditions and they have the potential to rapidly epimerize at the α-position. Moreover, stereodivergent hydroalkylation reactions of 1,3-dienes to access complementary diastereomers with vicinal stereocenters is challenging. Herein, we describe a synergistic palladium/amine catalyst system that allowed us to achieve the first stereodivergent hydroalkylation reactions of 1,3-dienes with aldehydes. By choosing an appropriate combination of chiral palladium and amine catalysts, we could obtain either syn or anti coupling products, and this method therefore provides highly diastereo- and enantioselective access to complementary diastereomers of chiral aldehydes with α,β-vicinal stereocenters. Density functional theory calculations revealed a mechanism involving PdH formation and migratory insertion into the alkene, followed by C-C bond formation. The origin of the stereoselectivities was investigated by means of distortion/interaction analysis.

Organocatalysis emerging as a technology

During the last 20 years, organocatalysis has significantly advanced as a field. Thanks to contributions from hundreds of groups and companies around the world, the area has risen from a few mechanistically ill-defined niche reactions, to one of the most vibrant and innovative fields in chemistry, providing several well-defined generic activation modes for selective catalysis. Organocatalysis is also on the rise in industrial settings, especially for the production of enantiomers, which are of use in fine chemistry, pharma, crop-protection, and fragrance chemistry. Here we will look at some of the specific elements of organocatalysis that we think are particularly attractive and contribute to this successful development.

Synergistic Cu/Pd-catalyzed asymmetric allylation: a facile access to α-quaternary cysteine derivatives

An efficient synthetic methodology to access biologically important and synthetically useful α-quaternary cysteine derivatives via asymmetric catalytic α-allylation of readily available 2-thiazoline-4-carboxylates was successfully developed through a synergistic Cu/Pd catalytic system. A wide array of α-quaternary cysteine derivatives were obtained in moderate to high yields with good to excellent enantioselectivities (45-98% yields and 69->99% ee). Gram-scale asymmetric allylation was performed to obtain high yields maintaining the enantioselectivity. Moreover, some synthetic transformations to access chiral spirocyclic compounds proceeded smoothly, which exhibited the important utility of this methodology.

Highly enantioselective tandem cycloisomerization/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals: dual catalysis with platinum and amines

Herein, we disclosed a highly efficient strategy of enantioselective synthesis of 2,3-furan-fused carbocycles bearing three-contiguous stereocenters. This transformation is catalyzed by dual catalysis of PtCl4/chiral amines via tandem dehydrogenative annulation/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals. The in situ generation of the furan-based ortho-quinodimethane intermediates and the iminium activation of enals are crucial to this transformation.

Asymmetric Total Synthesis of Sarpagine and Koumine Alkaloids

We report here a concise, collective, and asymmetric total synthesis of sarpagine alkaloids and biogenetically related koumine alkaloids, which structurally feature a rigid cage scaffold, with L-tryptophan as the starting material. Two key bridged skeleton-forming reactions, namely tandem sequential oxidative cyclopropanol ring-opening cyclization and ketone α-allenylation, ensure concurrent assembly of the caged sarpagine scaffold and installation of requisite derivative handles. With a common caged intermediate as the branch point, by taking advantage of ketone and allene groups therein, total synthesis of five sarpagine alkaloids (affinisine, normacusine B, trinervine, N_a -methyl-16-epipericyclivine, and vellosimine) with various substituents and three koumine alkaloids (koumine, koumimine, and N-demethylkoumine) with more complex cage scaffolds has been accomplished.

Copper Catalyzed Decarboxylative Functionalization of Ketoacids

Selective copper catalyzed activation of ketoacids and notably bio-sourced 1,3-acetonedicarboxylic acid, represents an attractive strategy to solve key synthetic challenges. Condensation with aldehydes under exceedingly mild conditions can create more rapidly known natural products scaffolds such as 1,3 polyols. In this account, the recent progress in this field, notably through multicatalytic combination with organocatalysis is described. In addition to the rapid preparation of natural product fragments, cascade incorporation of fluorine also provided new type of synthetic analogues of improved properties in a broad range of applications.

Asymmetric synthesis of dihydro-1,3-dioxepines by Rh(ii)/Sm(iii) relay catalytic three-component tandem [4 + 3]-cycloaddition

Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N'-dioxide-Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.

Enantioselective Reductive Cyanation and Phosphonylation of Secondary Amides by Iridium and Chiral Thiourea Sequential Catalysis

The combination of transition-metal catalysis and organocatalysis increasingly offers chemists opportunities to realize diverse unprecedented chemical transformations. By combining iridium with chiral thiourea catalysis, direct enantioselective reductive cyanation and phosphonylation of secondary amides have been accomplished for the first time for the synthesis of enantioenriched chiral α-aminonitriles and α-aminophosphonates. The protocol is highly efficient and enantioselective, providing a novel route to the synthesis of optically active α-functionalized amines from the simple, readily available feedstocks. In addition, the reactions are scalable and the thiourea catalyst can be recycled and reused.

Regioselective Ru(II)/Pd(0) Dual Catalysis: One-Pot C-H Diarylation of Five-Membered Heterocyclic Derivatives

Herein, we report a one-pot site-selective dual metal catalyzed C-H diarylation reaction for the synthesis of multiarylated thiophene and furan derivatives in yields up to 92%. The regioselectivity of the developed methodology was achieved with the sequential use of two metal catalysts within a single vessel, starting with a Ru(II)-catalyzed C3 arylation assisted by an azine directing group, followed by a Pd(0)-catalyzed C-H functionalization on the C5-position of the five-membered heterocycle. Furthermore, the kinetic studies support that the position of the nitrogen atom within the azine moiety exhibits an evident effect on the efficiency of the ruthenium-catalyzed arylation step.

Growth of Au nanoparticles on phosphorylated zein protein particles for use as biomimetic catalysts for cascade reactions at the oil-water interface

Chemo-enzymatic cascade processes are invaluable due to their ability to rapidly construct high-value products from available feedstock chemicals in a one-pot relay manner. However, they have proven to be challenging because of the mutual inactivation of both catalysts. A conceptually novel strategy based on Pickering interfacial catalysis (PIC) is proposed here to address this challenge. This study aimed to construct a protein-stabilized Pickering system for biphasic cascade catalysis, enabled by phosphorylated zein nanoparticles (ZCPOPs) immobilized in gold nanoparticles (Au NCs). Ultra-small Au NCs, 1-2 nm in diameter, were integrated into ZCPOPs at room temperature. Then, the as-synthesized ZCPOPs-Au NCs were used to stabilize the oil-in-water (o/w) Pickering emulsion. Besides their excellent catalytic activity and recycling ability in a variety of oil phases, ZCPOPs-Au NCs possess unpredictable catalytic activity and exhibit mimicking properties of horseradish peroxidase. Particularly, the cascade reaction is well achieved using a metal catalyst and a biocatalyst at the oil-water interface. The result showed that such a combination of chemo- and biocatalysis improved the catalytic yield more than two times compared with that of sole metal catalysis. This study opened a new avenue to design nanomaterials using the combination of chemo- and biocatalysis in a Pickering emulsion system for multistep syntheses.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Iridium-catalyzed enantioselective olefinic C(sp2)-H allylic alkylation

The first iridium-catalyzed enantioselective olefinic C(sp2)-H allylic alkylation is developed in cooperation with Lewis base catalysis. This reaction, catalyzed by cinchonidine and an in situ generated cyclometalated Ir(i)/phosphoramidite complex, makes use of the latent enolate character of an α,β-unsaturated carbonyl compound, namely coumalate ester, to introduce an allyl group at its α-position in a branched-selective manner in moderate to good yield with good to excellent enantioselectivities (up to 98 : 2 er).

Dual-Metal N-Heterocyclic Carbene Complex (M = Au and Pd)-Functionalized UiO-67 MOF for Alkyne Hydration-Suzuki Coupling Tandem Reaction

Metal N-heterocyclic carbene complexes (NHC-M) have been recognized as an important class of organometallic catalysts. Herein, we demonstrate that different NHC-M (M = Au and Pd) species can be simultaneously introduced into a single metal organic framework (MOF) by direct assembly of NHC-M-decorated ligands and metal ions under solvothermal conditions. The obtained UiO-67-Au/Pd-NHBC MOF with different organometallic NHC-M species can be a highly reusable dual catalyst to sequentially promote alkyne hydration-Suzuki coupling reaction. The potential utility of this strategy is highlighted by the preparation of many more new multicatalysts of this type for various organic transformations in a sequential way.

Collective synthesis of acetylenic pharmaceuticals via enantioselective Nickel/Lewis acid-catalyzed propargylic alkylation

Chiral acetylenic derivatives are found in many bioactive compounds and are versatile functional groups in organic chemistry. Here, we describe an enantioselective nickel/Lewis acid-catalyzed asymmetric propargylic substitution reaction from simple achiral materials under mild condition. The introduction of a Lewis acid cocatalyst is crucial to the efficiency of the transformation. Notably, we investigate this asymmetric propargylic substitution reaction for the development of a range of structurally diverse natural products. The power of this strategy is highlighted by the collective synthesis of seven biologically active compounds: (-)-Thiohexital, (+)-Thiopental, (+)-Pentobarbital, (-)-AMG 837, (+)-Phenoxanol, (+)-Citralis, and (-)-Citralis Nitrile.

Asymmetric cycloisomerization/[3 + 2] cycloaddition for the synthesis of chiral spiroisobenzofuran-1,3′-pyrrolidine derivatives

An asymmetric tandem cycloisomerization/[3 + 2] cycloaddition reaction of 2,2′-diester aziridine and 2-ethynyl benzyl alcohol with Au(i)/chiral N,N′-dioxide−Dy(iii) as a relay catalyst system was developed.

Umpolung coupling of pyridine-2-carboxaldehydes and propargylic carbonates via N-heterocyclic carbene/palladium synergetic catalysis

The umpolung cross-coupling reaction of pyridine-2-carboxaldehydes and propargylic carbonates has been developed for the first time through N-heterocyclic carbene/palladium cooperative catalysis with the judicious selection of the palladium catalyst, ligand and N-heterocyclic carbene, giving the propargylic ketones regioselectively.