[(RCN)2PdCl2]-catalyzed E/Z isomerization of alkenes: a non-hydride binuclear addition-elimination pathway

Stereodivergent, Kinetically Controlled Isomerization of Terminal Alkenes via Nickel Catalysis

Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal-catalyzed olefin mono-transposition (i.e., positional isomerization) approaches have emerged to afford valuable E- or Z- internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel-catalyzed platform for the stereodivergent E/Z-selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one-carbon transposition of terminal alkenes to valuable E- or Z-internal alkenes via a Ni-H-mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with the Z-selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and the E-selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over-isomerization.

Rh-coordinated histidyl bolaamphiphile assembly: a catalyst for the isomerization of cis-stilbene and cis-alkene

In this study, we present a colloidal assembly of histidyl bolaamphiphiles whose imidazoles coordinate with rhodium ions (HisC7[Rh]) to exhibit catalytic isomerization activity for cis-stilbene and cis-alkene molecules. The histidyl bolaamphiphiles self-assemble to form a soft scaffold that functions analogously to an apoenzyme. This scaffold exposes multiple histidyl imidazoles and carboxylates on its surface, to which rhodium ions bind, generating catalytically active sites. The Rh coordination with the biochemical functional groups was verified through comprehensive vibrational spectroscopy and calorimetry. The colloidal HisC7[Rh] demonstrated a significant catalytic effect on the isomerization of cis- to trans-stilbene under mild H2 conditions, resulting in 69% yield of trans-stilbene. In contrast, when Rh(cod)2BF4 was employed as a control catalyst, only the hydrogenated products of bibenzyl were obtained. These findings underscore the crucial role of histidyl motifs in exhibiting unique catalytic isomerization activity through the coordination with Rh. The catalytic activity of HisC7[Rh] is governed by several factors, such as rhodium content, solvent composition, temperature, and H2 pressure. Moreover, HisC7[Rh] displayed moderate isomerization activity towards not only stilbene but also unsaturated fatty acid isomers, highlighting its expansive potential as an isomerization catalyst.

PdII -Catalyzed C(alkenyl)-H Activation Facilitated by a Transient Directing Group

Palladium(II)‐catalyzed C(alkenyl)−H alkenylation enabled by a transient directing group (TDG) strategy is described. The dual catalytic process takes advantage of reversible condensation between an alkenyl aldehyde substrate and an amino acid TDG to facilitate coordination of the metal catalyst and subsequent C(alkenyl)−H activation by a tailored carboxylate base. The resulting palladacycle then engages an acceptor alkene, furnishing a 1,3‐diene with high regio‐ and E/Z‐selectivity. The reaction enables the synthesis of enantioenriched atropoisomeric 2‐aryl‐substituted 1,3‐dienes, which have seldom been examined in previous literature. Catalytically relevant alkenyl palladacycles were synthesized and characterized by X‐ray crystallography, and the energy profiles of the C(alkenyl)−H activation step and the stereoinduction model were elucidated by density functional theory (DFT) calculations.

On the Importance of Considering Multinuclear Metal Sites in Homogeneous Catalysis Modeling

In this short review, we provide an account of a number of computational studies of catalytic reaction mechanisms carried out in our groups. We focus in particular on studies in which we came to realize during the course of the investigation that the active catalytic species was a bimetallic complex, rather a monometallic one as previously assumed. In some cases, this realization was in part prompted by experimental observations, but careful exploration based on computation of the speciation of the metal precursor also provided a powerful guide: it is often possible to predict that bimetallic species (intermediates or transition states) lie lower in free energy than a priori competitive monometallic species. In this sense, we argue that in organometallic catalysis, the rule whereby “two is better than one” turns out to be relevant much more often than one might expect.

Mechanistic Studies of Pd(II)-Catalyzed E/Z Isomerization of Unactivated Alkenes: Evidence for a Monometallic Nucleopalladation Pathway

Pd(II)-catalyzed E/Z isomerization of alkenes is a common process-yet its mechanism remains largely uncharacterized, particularly with non-conjugated alkenes. In this work, the mechanism of Pd(II)-catalyzed E/Z isomerization of unactivated olefins containing an aminoquinoline-based amide directing group is probed using in situ kinetic analysis, spectroscopic studies, kinetic modeling, and DFT calculations. The directing group allows for stabilization and monitoring of previously undetectable intermediates. Collectively, the data are consistent with isomerization occurring through a monometallic nucleopalladation mechanism.

Selective E to Z isomerization of 1,3-Dienes Enabled by A Dinuclear Mechanism

The E/Z stereocontrol in a C=C bond is a fundamental issue in olefin synthesis. Although the thermodynamically more stable E geometry is readily addressable by thermal Z to E geometric isomerization through equilibrium, it has remained difficult to undergo thermal geometric isomerization to the reverse E to Z direction in a selective manner, because it requires kinetic trapping of Z-isomer with injection of chemical energy. Here we report that a dinuclear PdI-PdI complex mediates selective isomerization of E-1,3-diene to its Z-isomer without photoirradiation, where kinetic trapping is achieved through rational sequences of dinuclear elementary steps. The chemical energy required for the E to Z isomerization can be injected from an organic conjugate reaction through sharing of common Pd species.

Reduction Chemistry of Natural Pyrethrins and Preliminary Insecticidal Activity of Reduced Pyrethrins

The natural extract pyrethrum is an insecticidal oil derived from Tanacetum cinerariifolium that is commonly used in domestic and agricultural pesticides. The major constituents of the extract are the Pyrethrins, six esters that provide pyrethrum with its insecticidal properties. These Pyrethrins readily degrade through several environmental means and as such, there can be significant Pyrethrin losses during processing and long-term storage of pyrethrum-based insecticides. This work attempts to alleviate the effect of these degradative processes through the pursuit of stabilised Pyrethrins by chemically removing oxidatively sensitive functionality. Several reduced Pyrethrin analogues were produced and a method to convert the more sensitive Pyrethrins present in the pyrethrum concentrate into their respective more stable jasmolin counterparts, as a mixture with the over-reduced tetrahydropyrethrins, was developed. All other reduction processes abolished insecticidal activity against Lucilia cuprina larvae, whereas some isomerised analogues showed comparable potency with the individual natural pyrethrin esters. This work has revealed new insights into the structure–activity relationships in this unique class of insecticide.

Organo-photoredox-Catalyzed Atom-Transfer Radical Substitution of Alkenes with α-Carbonyl Alkyl Halides

A light-driven atom-transfer radical substitution (ATRS) and carboesterification reaction of alkenes with alkyl halides has been developed using PTH as the organo-photoredox catalyst. Two types of products were obtained, depending on the additive and solvent used during the reaction. Primary, secondary, and tertiary alkyl halides reacted to give the ATRS products. This protocol has several advantages: it requires mild reaction conditions and a low catalyst loading and exhibits a broad substrate scope and good functional group tolerance. Mechanistic studies indicate that alkyl radicals might be generated as the key intermediates via photocatalysis, providing a new direction for ATRS reactions.

The ambiguous behaviour of diphosphines towards the quasilinear iron(i) complex [Fe(N(SiMe3)2)2]− – between inertness, P–C bond cleavage and C–C double bond isomerisation

A quasilinear iron(i) complex facilitates C–C bond isomerisation or P-aryl bond cleavage of diphosphines but is otherwise inert to simple phosphine coordination.

Highlights of aliphatic C(sp3 )-H hydrogen isotope exchange reactions

This review summarizes the highlights of aliphatic C (sp³ )-H carbon hydrogen isotope exchange (HIE) methods developed in the last 10 years. In particular, new highly selective and reactive protocols in the areas of nanoparticle and metal-catalyzed homogeneous catalysis are reported.

Catalytic synthesis of 9-cis-retinoids: mechanistic insights

The regioselective Z-isomerization of thermodynamically stable all-trans retinoids remains challenging, and ultimately limits the availability of much needed therapeutics for the treatment of human diseases. We present here a novel, straightforward approach for the catalytic Z-isomerization of retinoids using conventional heat treatment or microwave irradiation. A screen of 20 transition metal-based catalysts identified an optimal approach for the regioselective production of Z-retinoids. The most effective catalytic system was comprised of a palladium complex with labile ligands. Several mechanistic studies, including isotopic H/D exchange and state-of-the-art quantum chemical calculations using coupled cluster methods indicate that the isomerization is initiated by catalyst dimerization followed by the formation of a cyclic, six-membered chloropalladate catalyst-substrate adduct, which eventually opens to produce the desired Z-isomer. The synthetic development described here, combined with thorough mechanistic analysis of the underlying chemistry, highlights the use of readily available transition metal-based catalysts in straightforward formats for gram-scale drug synthesis.

Directed, Palladium(II)-Catalyzed Enantioselective anti-Carboboration of Alkenyl Carbonyl Compounds

A substrate-directed enantioselective anti-carboboration reaction of alkenes has been developed, wherein a carbon-based nucleophile and a boron moiety are installed across the C=C bond through a 5-membered palladacycle intermediate. A preliminary result also shows it is possible to extend this reaction to alkenes that are more distal from the directing group and react via a 6-membered palladacycle. The reaction is promoted by a palladium(II) catalyst and a monodentate oxazoline ligand. A range of enantioenriched secondary alkylboronate products were obtained with moderate to high enantioselectivity that could be further upgraded by recrystallization. This work represents an efficient method to synthesize versatile and valuable alkylboronate building blocks. Building on an earlier mechanistic proposal by Peng, He, and Chen, a revised model is proposed to account for the stereoconvergent nature of this transformation.

E-Olefins through intramolecular radical relocation

Full control over the selectivity of carbon-carbon double-bond migrations would enable access to stereochemically defined olefins that are central to the pharmaceutical, food, fragrance, materials, and petrochemical arenas. The vast majority of double-bond migrations investigated over the past 60 years capitalize on precious-metal hydrides that are frequently associated with reversible equilibria, hydrogen scrambling, incomplete E/Z stereoselection, and/or high cost. Here, we report a fundamentally different, radical-based approach. We showcase a nonprecious, reductant-free, and atom-economical nickel (Ni)^(I)-catalyzed intramolecular 1,3-hydrogen atom relocation to yield E-olefins within 3 hours at room temperature. Remote installations of E-olefins over extended distances are also demonstrated.

A Supramolecular Palladium Catalyst Displaying Substrate Selectivity by Remote Control

Inspired by enzymes such as cytochrome P-450, the study of the reactivity of metalloporphyrins continues to attract major interest in the field of homogeneous catalysis. However, little is known about benefitting from the substrate-recognition properties of porphyrins containing additional, catalytically relevant active sites. Herein, such an approach is introduced by using supramolecular ligands derived from metalloporphyrins customized with rigid, palladium-coordinating nitrile groups. According to different studies (NMR and UV/Vis spectroscopy, XRD, control experiments), the supramolecular ligands are able to accommodate pyridine derivatives as substrates inside the porphyrin pocket while the reactivity occurs at the peripheral side. By simply tuning a remote metal center, different binding events result in different catalyst reactivity, and this enzyme-like feature leads to high degrees of substrate selectivity in representative palladium-catalyzed Suzuki-Miyaura reactions.

The critical role played by water in controlling Pd catalyst speciation in arylcyanation reactions

Different ‘L_nPd(0)’ species play a role in arylcyanation processes, depending on H₂O content.

Diastereodivergent Reductive Cross Coupling of Alkynes through Tandem Catalysis: Z- and E-Selective Hydroarylation of Terminal Alkynes

A diastereodivergent hydroarylation of terminal alkynes is accomplished using tandem catalysis. The hydroarylation allows highly selective synthesis of both E and Z diastereoisomers of aryl alkenes, from the same set of starting materials, using the same combination of palladium and copper catalysts. The selectivity is controlled by simple changes in the stoichiometry of the alcohol additive. The hydroarylation has excellent substrate scope and can be accomplished in the presence of various classes of compounds, including esters, nitriles, alkyl halides, epoxides, carbamates, acetals, ethers, silyl ethers, and thioethers. The Z-selective hydroarylation is accomplished using a new approach based on tandem Sonogashira coupling and catalytic semireduction. The E-selective hydroarylation involves an additional catalytic isomerization of the Z-alkene. Our explorations of the reaction mechanism explain the role of individual reaction components and how the subtle changes in the reaction conditions influence the rates of specific steps of the hydroarylation. Our studies also show that, although the Z- and E-selective hydroarylation reactions are mechanistically closely related, the roles of the palladium and copper catalysts in the two reactions are different.

E-Z isomerization in Suzuki cross-couplings of haloenones: ligand effects and evidence for a separate catalytic cycle

Suzuki cross-coupling of haloalkenes is generally assumed to occur with retention of the alkene stereochemistry. While studying Suzuki cross-couplings on E-1,2-dichlorovinyl phenyl ketone, we were surprised to observe extensive isomerization. More surprisingly, the ligand employed strongly influenced the degree of isomerization: DPEphos and Xantphos led to 96% isomerized cross-coupled product whereas reactions in the absence of a phosphine ligand, or reactions employing t-BuXantphos, gave 94% retention of stereochemistry. While E-Z isomerization in Pd-catalyzed vinylic couplings has previously been attributed to events within the cross-coupling catalytic cycle, we present experimental and computational evidence for a separate Pd-catalyzed isomerization process in these reactions.

Au-Pd alloy nanoparticles supported on layered double hydroxide for heterogeneously catalyzed aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones to phenols

Phenol, an important industrial chemical, is widely produced using the well-developed cumene process. However, demand for the development of a novel alternative method for synthesizing phenol from benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone-alcohol (KA) oil, catalyzed by Mg-Al-layered double hydroxide (LDH)-supported Au-Pd alloy nanoparticles (Au-Pd/LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation. Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity, Au-Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production. Mechanistic studies showed that β-H elimination from Pd-enolate species is accelerated by Au species, likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic performance of Au-Pd/LDH, Au-Pd bimetallic nanoparticles supported on Al2O3, TiO2, MgO, and CeO2 were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation possibly through its assistance in the deprotonation steps. The synthetic scope of the Au-Pd/LDH-catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently converted into the corresponding phenols, and N-substituted anilines were synthesized from cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au-Pd/LDH could be reused without substantial loss of its high performance. The present transformation is scalable, utilizes O2 in air as the terminal oxidant, and generates water as the only by-product, highlighting the potential practical utility and environmentally benign nature of the present transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3) disproportionation of cyclohexenones to afford the desired phenols. In the present Au-Pd/LDH-catalyzed transformation, the oxidation of the Pd-H species is included in the rate-determining step.

Regioselective Intermolecular Diamination and Aminooxygenation of Alkenes with Saccharin

Palladium catalysis enables the regioselective difunctionalization of alkenes using saccharin as the nitrogen source in the initial step of aminopalladation. Depending on the reaction conditions, diamination or aminooxygenation pathways can be accessed using hypervalent iodine reagents as the terminal oxidants. The aminooxygenation of allylic ethers originates from an unprecedented ambident behavior of saccharin. The participating palladium catalysts contain a palladium-saccharide unit. Two representative complexes of this type could be isolated and characterized.

Palladium(II)-Catalyzed Annulation between ortho-Alkenylphenols and Allenes. Key Role of the Metal Geometry in Determining the Reaction Outcome

2-Alkenylphenols react with allenes, upon treatment with catalytic amounts of Pd(II) and Cu(II), to give benzoxepine products in high yields and with very good regio- and diastereoselectivities. This contrasts with the results obtained with Rh catalysts, which provided chromene-like products through a pathway involving a β-hydrogen elimination step. Computational studies suggest that the square planar geometry of the palladium is critical to favor the reductive elimination process required for the formation of the oxepine products.

Defined Palladium-Phthalimidato Catalysts for Improved Oxidative Amination

New palladium(II)-phthalimidato complexes have been synthesized, isolated, and structurally characterized. As demonstrated from over 30 examples, they constitute superior catalysts for oxidative amination reactions of alkenes with phthalimide as the nitrogen source. This work streamlines vicinal difunctionalization of alkenes and provides access to significantly improved and experimentally simplified synthetic protocols.

(Pyridyl)benzoazole palladium(ii) complexes as homogeneous catalysts in hydrogenation of alkenes and alkynes

Complexes 1–6 form “cocktail” catalysts in the hydrogenation and isomerization of alkenes and alkynes where the active species are homogeneous.

Well-defined transition metal hydrides in catalytic isomerizations

This Feature Article intends to provide an overview of a variety of catalytic isomerization reactions that have been performed using well-defined transition metal hydride precatalysts. A particular emphasis is placed on the underlying mechanistic features of the transformations discussed. These have been categorized depending upon the nature of the substrate and in most cases discussed following a chronological order.

N–H⋯π induced configurational isomerism and the role of temperature in the Z to E isomerization of 2-fluoro-N′-(3-fluorophenyl)benzimidamide

The role of temperature in polymorphism and the isolation of configurational isomers of 2-fluoro-N′-(3-fluorophenyl)benzimidamide have been investigated.

Palladium hydride promoted stereoselective isomerization of unactivated di(exo)methylenes to endocyclic dienes

The exomethylenes of 2,6-disubstituted bicyclo[3.3.1]nonan-9-ones 2 are readily isomerized over a palladium catalyst under an atmosphere of hydrogen to predominantly form the isomer 3 with C2 symmetry with very little formation of the analogous product with C(s) symmetry. A hydrogen source is essential to effect the rearrangement.

An air-stable cationic iridium hydride as a highly active and general catalyst for the isomerization of terminal epoxides

We describe the use of an air-stable iridium hydride catalyst for the isomerization of terminal epoxides into aldehydes with perfect regioselectivity.

Pd-catalyzed cross-coupling of potassium alkenyltrifluoroborates with 2-chloroacetates and 2-chloroacetamides

A protocol for the stereocontrolled synthesis of (E)- and (Z)-β,γ-unsaturated esters and amides is reported. 2-Chloroacetates as well as secondary and tertiary 2-chloroacetamides were successfully employed as electrophiles in the Suzuki-Miyaura cross-coupling reaction with potassium (E)- and (Z)-alkenyltrifluoroborates, affording the corresponding products in high yield.