Nickel-Catalyzed Adaptive Migration-Enabled Asymmetric Cross-Hydrocarbonylation of Unactivated Alkenes

Transition-metal-catalyzed regio- and enantioselective cross-coupling of alkyl metallic species has emerged as a cornerstone in modern organic synthesis, which enables the construction of carbon-carbon and carbon-heteroatom bonds with high precision to facilitate rapid access to important organic targets with molecular complexity. However, the selective formation and utilization of different alkyl metallic intermediates from one precursor under identical conditions remain unknown and challenging. Herein, a Ni-catalyzed adaptive migratory asymmetric cross-hydroacylation of unactivated alkenes for the synthesis of enantioenriched α-arylated ketones has been developed. One alkene serves as a precursor for two different alkyl metallic intermediates by adaptive migration, providing one of the most straightforward pathways to access enantioenriched α-arylated ketones.

Iron-Catalyzed Regioselective C═C Bond Migration and Reductive Deuteration

We report a well-defined multifunctional iron catalyst, Cp*Fe(Ph2PN = C5H4N), that promotes the rearrangement of β,γ-unsaturated ketones, esters, and amides to their α,β-isomers, enabling regioselective C═C reductive deuteration. This iron-catalyzed protocol showcases the synthesis of a diverse range of α,β-dideuterated organic compounds, demonstrating broad substrate compatibility under mild conditions. Mechanism studies provide insights into the C═C bond relocation and cooperative D2 activation.

Co(II)-catalyzed isomerization of enals using hydrogen atom transfer

To develop a process for the synthesis of an enal acetate, the so called C5-acetate that is important in the industrial preparation of vitamin A, we show here that Co(II)-based hydrogen atom transfer catalysts under H2 promote such isomerizations in high yield with low catalyst loading (0.1 mol%). D-labelling studies suggest the enal isomerization process and catalyst activation by H2 to be reversible.

Nickel-Catalyzed 1,1-Carboboration of Polysubstituted Internal Alkenes

Herein, we report a nickel-catalyzed 1,1-carboboration of di- and trisubstituted alkenyl boronates through a chain-walking strategy. This reaction effectively addresses the polarity-mismatch problem via ligand control, enabling the coupling of various carbon-based electrophiles while accommodating a broad range of functional groups. The approach yields diverse tetrasubstituted carbon gem-diboronate derivatives with exceptional regioselectivity. The synthetic utility of this method is further demonstrated through the concise synthesis of high-value bioactive molecules.

Deconjugative Photoisomerization of Cyclic Enones

The deconjugative isomerization of α,β-unsaturated carbonyl compounds enables regioisomeric products to be forged with simultaneous Umpolung of alkene reactivity. Although highly enabling, the endergonic nature of the net process coupled with governing regioselectivity outcomes, renders it challenging. Innovations in the positional isomerization of linear species, often by light-triggered activation, have re-energized this area. However, the deconjugative isomerization of cyclic enones is underdeveloped and often associated with impractical reaction conditions, limited substrate scopes, and a lack of mechanistic clarity. Herein, we report an operationally simple photochemical isomerization of cyclic enones using near-UV (372 nm) irradiation with catalytic amounts of Brønsted acid (HCl). This platform enables exocyclic deconjugative isomerization of a diverse array of enones including α-isophorone (a key intermediate in a variety of industrial processes), terpenoids and steroids. Mechanistic studies reveal the pivotal role of the solvent as a key mediator in the isomerization, where sequential hydrogen atom transfer (HAT) and reverse-HAT (RHAT) are proposed to be operational.

Investigation of Dearomatizing Spirocyclizations and Spirocycle Functionalization En Route to Spirocalcaridines A and B-Some Trials and Tribulations

Spirocalcaridines A and B are among the most challenging members of the marine invertebrate-derived Leucetta alkaloids. Approaches to the construction and elaboration of the highly compact spirocyclic core are described. The synthesis of tricyclic guanidine via tandem oxidative amination dearomatizing spirocyclization (TOADS) using hypervalent iodine set the stage for total synthesis via the migration of the C4/C8 double bond to the C4/C5 position, followed by oxidation. The undesired but not surprising propensity of the spirocyclic cyclohexadienone to undergo rearrangement to the phenol hindered the desired olefin migration. Furthermore, initial efforts to install the oxidation sequentially, first at C5 and then at C4 in the complete carbon skeleton, were fraught with unforeseen challenges and unusual outcomes. In addition, the scope and limitations of hypervalent iodine-mediated tandem oxidative dearomatizing spirocyclization on various substrates were explored. Urethanes and thiourethanes underwent spirocyclization with an excellent yield, whereas the reaction with allylic substrates and species lacking the p-methoxy substituent did not proceed. Attempts to prepare other guanidine precursors are briefly discussed.

Copper-Catalyzed Domino Annulation of Isoselenocyanates: A Pathway to Structurally Diverse N, Se-Bis-Heterocyclic, and Fused Heterocyclic Compounds

A copper-catalyzed domino addition/cyclization reaction was developed to synthesize novel benzoselenazole-linked 1,2,3-triazole and tetracyclic fused 12H-benzo[4,5]selenazole[2,3-b]quinazolin-12-one derivatives from isoselenocyanates. This domino reaction efficiently constructed multiple new chemical bonds in a single step, forming either four (one C-Se and three C-N) or three (one C-Se and two C-N) bonds. The reaction offers several key advantages, including mild conditions, broad substrate compatibility, and straightforward and safe operation. It presents a new method for synthesizing N, Se-containing polycyclic compounds, which have potential interest in medicinal chemistry. Notably, some of the synthesized compounds exhibited anticancer activity with IC50 values below 20 μmol L-1 against T24 human bladder cancer cells.

Iterative synthesis of stereodefined polyacetals and their domino-Coates-Claisen rearrangement

We report a fully stereocontrolled synthesis of previously unknown unsaturated polyacetals through an iteration of two steps: (1) Pd-catalyzed hydroalkoxylation of alkoxyallenes; and (2) base-mediated isomerization of propargyl ethers into alkoxyallenes. Site- and stereoselective alkene isomerization of the capping allyl group initiates a domino-Coates-Claisen rearrangement, which completely rearranges the iteratively assembled backbone with the stereochemistry of the newly formed stereocentres controlled by the configuration of the acetal centres in the starting materials. The resulting products feature multiple stereocentres in a 1,3-relationship, which map onto a broad range of bioactive natural products, including deoxypropionates.

Stereospecific 3-Aza-Cope Rearrangement Interrupted Asymmetric Allylic Substitution-Isomerization

Transition-metal catalyzed asymmetric allylic substitution with alkyl and heteroaryl carbon nucleophiles has been well-established. However, the asymmetric allylic arylation of acyclic internal alkenes with aryl nucleophiles remains challenging and underdeveloped. Herein we report a stereospecific 3-aza-Cope rearrangement interrupted asymmetric allylic substitution-isomerization (Int-AASI) that enables asymmetric allylic arylation. By means of this stepwise strategy, both enantioenriched allylic arylation products and axially chiral alkenes could be readily obtained in high enantioselectivities. Experimental studies support a mechanism involving a cascade of asymmetric allylic amination, stereospecific 3-aza-Cope rearrangement and alkene isomerization. Density functional theory studies detailed the reasons of achieving the high chemoselectivity, regioselectivity, stereoselectivity and stereospecificity, respectively.

Stereoselective Synthesis of 1,n-Dicarbonyl Compounds Through Palladium-Catalyzed Ring Opening/Isomerization of Densely Substituted Cyclopropanols

We report a highly diastereoselective protocol for the synthesis of 1,4- and 1,5-dicarbonyl compounds from densely substituted cyclopropanols. The methodology involves a palladium-catalyzed ring opening reaction followed by a "metal-walk" and oxidation of a remote hydroxyl group. The methodology represents a new application of cyclopropanols as initiation sites for chain walking remote functionalization. Importantly, this approach provides a straightforward access to highly valuable succinaldehyde derivatives bearing vicinal quaternary and tertiary stereocenters as single diastereomers.

Tungsten-catalyzed stereodivergent isomerization of terminal olefins

Catalytic alkene isomerization is a powerful synthetic strategy for preparing valuable internal alkenes from simple feedstocks. The utility of olefin isomerization hinges on the ability to control both positional and stereoisomerism to access a single product among numerous potential isomers. Within base-metal catalysis, relatively little is known about how to modulate reactivity and selectivity with group 6 metal-catalyzed isomerization. Here, we describe a tungsten-catalyzed, positionally selective alkene isomerization reaction in which tuning the ligand environment grants access to either the E- or Z-stereoisomer. The reactions employ simple, commercially available precatalysts and ligands. Preliminary mechanistic studies suggest that the ligand environment around 7-coordinate tungsten is crucial for stereoselectivity, and that substrate directivity prevents over-isomerization to the conjugated alkene. These features allow for exclusive formation of β,γ-unsaturated carbonyl compounds that are otherwise difficult to prepare.

Engineered Polymeric Carbon Nitride for Photocatalytic Diverse Functionalization of Electronic-Rich Alkenes

Engineered polymeric carbon nitride represents a promising class of metal-free semiconductor photocatalysts for organic synthesis. Herein, we utilized engineered polymeric carbon nitride nanosheets, which exhibit an increased specific surface area and band gap due to enhanced quantum confinement from vacancy enrichment. These nanosheets serve as a heterogeneous organic semiconductor photocatalyst to facilitate diverse functionalizations of electron-rich alkenes, including arylsulfonylation, aminodifluoroalkylation, and oxytrifluoromethylation. This catalytic system operates under mild conditions, offering excellent functional group compatibility and high yields. Additionally, the catalyst demonstrates outstanding recyclability and efficiency in flow reactors, highlighting its significant potential for industrial applications.

Pd-Catalyzed Migratory 1,1-Cycloannulation Reaction of Alkenes

Here, we report a novel strategy for the preparation of diverse heterocycles via a Pd-catalyzed migratory 1,1-cycloannulation reaction (MCAR) of alkenes. Starting from readily available alkenyl amines and alkenyl alcohols, this approach allows the formation of a wide range of five- to seven-membered azaheterocycles and oxaheterocycles with high efficiency and good functional group tolerance. The key to the realization of this reaction is the use of 4-iodophenol or 2-iodophenol derivatives where the phenolic hydroxyl group plays a critical role in controlling the direction of migration and the ring-size of the heterocycles through the formation of a quinone methide intermediate.

Photoinduced Pd-Catalyzed Direct Sulfonylation of Allylic C-H Bonds

Allylic sulfones are valuable motifs due to their medicinal and biological significance and their versatile chemical reactivities. While direct allylic C-H sulfonylation represents a straightforward and desirable approach, these methods are primarily restricted to terminal alkenes, leaving the engagement of the internal counterparts a formidable challenge. Herein we report a photocatalytic approach that accommodates both cyclic and acyclic internal alkenes with diverse substitution patterns and electronic properties. Importantly, the obtained allylic sulfones can be readily diversified into a wide range of products, thus enabling formal alkene transposition and all-carbon quaternary center formation through the sequential C-H functionalization.

Facile construction of distal and diversified tertiary and quaternary stereocenters

Exploration of novel chiral pharmaceutical candidates is motivation to immersive efforts among synthetic chemists. Achieving skeletal construction and chiral diversity in a highly efficient manner is a momentous goal in the chemical society. Unfortunately, current methods for chiral induction focus primarily on a specific site. Herein, we realized the asymmetric chain-walking arylation of alkenyl alcohols for the construction of multisite tertiary and quaternary stereocenters in high yields and enantioselectivity. This new operation-friendly strategy carries substantial potential for future industrialization.

Palladium-catalyzed remote internal C(sp3)-H bond chlorination of alkenes

C(sp3)-Cl bonds are present in numerous biologically active molecules and can also be used as a site for diversification by substitution or cross-coupling reactions. Herein, we report a remote internal site-selective C(sp3)-H bond chlorination of alkenes through sequential alkene isomerization and hydrochlorination, enabling the synthesis of both benzylic and tertiary chlorides with excellent site-selectivity. This transformation offers exciting possibilities for the late-stage chlorination of derivatives of natural products and pharmaceuticals. We also demonstrate the regioconvergent synthesis of a single alkyl chloride from unrefined mixtures of isomeric alkenes, which can be extracted directly from petrochemical sources.

Multifunctionalization of Alkenyl Alcohols via a Sequential Relay Process

Aryl-substituted aliphatic amines are widely recognized as immensely valuable molecules. Consequently, the development of practical strategies for the construction of these molecules becomes increasingly urgent and critical. Here, we have successfully achieved multifunctionalization reactions of alkenyl alcohols in a sequential relay process, which enables transformation patterns of arylamination, deuterated arylamination, and methylenated arylamination to the easy access of multifarious arylalkylamines. Notably, a novel functionalization mode for carbonyl groups has been developed to facilitate the processes of deuterium incorporation and methylene introduction, thereby providing new means for the diverse transformations of carbonyl groups. This methodology displays a wide tolerance toward functional groups, while also exhibiting good applicability across various skeletal structures of alkenols and amines.

Remote Catalytic C(sp3)-H Alkylation via Relayed Carbenoid Transfer upon Olefin Chain Walking

Transition metal carbenes have emerged as versatile intermediates for various types of alkylations. While reactions of metal carbene species with alkenes have been extensively studied, most examples focus on cyclopropanation and allylic C-H insertion. Herein, we present the first example of a catalytic strategy for the carbene-involved regioselective remote C-H alkylation of internal olefins by synergistically combining two iridium-mediated reactivities of olefin chain walking and carbenoid migratory insertion. The present method, utilizing sulfoxonium ylides as a bench-stable robust carbene precursor, was found to be effective for a series of olefins tethered with alkyl chains, heteroatom substituents, and complex biorelevant moieties. Combined experimental and computational studies revealed that reversible iridium hydride-mediated olefin chain walking proceeds to lead to a terminal alkyl-Ir intermediate, which then forms a carbenoid species for the final migratory insertion, resulting in regioselective terminal-alkylated products.

Ni-catalysed remote C(sp3)-H functionalization using chain-walking strategies

The dynamic translocation of a metal catalyst along an alkyl side chain - often coined as 'chain-walking' - has opened new retrosynthetic possibilities that enable functionalization at unactivated C(sp3)-H sites. The use of nickel complexes in chain-walking strategies has recently gained considerable momentum owing to their versatility for forging sp3 architectures and their redox promiscuity that facilitates both one-electron or two-electron reaction manifolds. This Review discusses the relevance and impact that these processes might have in synthetic endeavours, including mechanistic considerations when appropriate. Particular emphasis is given to the latest discoveries that leverage the potential of Ni-catalysed chain-walking scenarios for tackling transformations that would otherwise be difficult to accomplish, including the merger of chain-walking with other new approaches such as photoredox catalysis or electrochemical activation.

Palladium-Catalyzed Isomerization/Hydrocarbonylation of Electron-Deficient Alkenes for the Construction of Lactones and Lactams

An efficient palladium-catalyzed isomerization/hydrocarbonylation of electron-deficient alkenes was developed, affording a variety of benzofuran-2(3H)-ones, dihydrofuran-2(3H)-ones, indolin-2-ones, and isochroman-3-ones in high yields. The reaction exhibits excellent regioselectivity, high atom economy, and a broad range of substrates. The late-stage modification of bioactive molecules and scaled-up reaction demonstrated the practicability of this methodology. Furthermore, the method was employed as the key step for the synthesis of an insulin degrading enzyme (IDE) activator.

Structural engineering in hierarchical nanoarchitectures of metal-organic frameworks and their derivatives

Metal-organic frameworks (MOFs) have attracted much attention owing to their tuneable structures, high surface areas, and good functionalization. Nanoreactors derived from various MOFs are now widely used in heterogeneous catalysis, electrocatalysis and photocatalysis. The nanoarchitectures of MOFs and their derivatives have a great impact on mass and energy transfer pathways, thus affecting the activity and selectivity of the catalysts. In this review, we intend to provide a universal survey of reported methods to synthesize MOF-based core-satellite, core-shell, yolk-shell and hollow-shell structures or their derivatives in recent years and present a continuous evolution among them. We hope that this review could provide some perspectives for exploring new facile methods to prepare different hierarchical nanoarchitectures of MOFs or their derivatives.

Linear Free Energy Relationships Associated with Hydride Transfer From [(6,6'-R2-bpy)Re(CO)3H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects in the Secondary Coordination Sphere

Six rhenium hydride complexes, [(6,6'-R2-bpy)Re(CO)3H] (bpy = 2,2'-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO2 to form rhenium formate complexes of the type [(6,6'-R2-bpy)Re(CO)3{OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted. A solid-state structure of [(6,6'-Me2-bpy)Re(CO)3H] determined using MicroED also featured a tilted bpy ligand. The kinetics of CO2 insertion into complexes of the type [(6,6'-R2-bpy)Re(CO)3H] were measured experimentally and the thermodynamic hydricities of [(6,6'-R2-bpy)Re(CO)3H] species were determined using theoretical calculations. A Brønsted plot constructed using the experimentally determined rate constants for CO2 insertion and the calculated thermodynamic hydricities for [(6,6'-R2-bpy)Re(CO)3H] revealed a linear free energy relationship (LFER) between thermodynamic and kinetic hydricity. This LFER is different to the previously determined relationship for CO2 insertion into complexes of the type [(4,4'-R2-bpy)Re(CO)3H]. At a given thermodynamic hydricity, CO2 insertion is faster for complexes containing a 6,6'-substituted bpy ligand. This is likely in part due to the tilting observed for systems with 6,6'-substituted bpy ligands. Notably, the 6,6'-(NHMe)2-bpy ligand could in principle stabilize the transition state for CO2 insertion via hydrogen bonding. This work shows that if only the rate of CO2 insertion into [(6,6'-(NHMe)2-bpy)Re(CO)3H] is compared to [(4,4'-R2-bpy)Re(CO)3H] systems, the increase in rate could be easily attributed to hydrogen bonding, but in fact all 6,6'-substituted systems lead to faster than expected rates.

Alkene Isomerisation Catalysed by a Superbasic Sodium Amide

Typically catalysed by transition metals, alkene isomerisation is a powerful methodology for preparation of internal olefins. In contrast, the use of more earth abundant main group reagents is limited to activated substrates, requiring high temperatures and excess stoichiometric amounts. Opening a new avenue for progressing this field, here we report applications of bulky sodium amide NaTMP (TMP=2,2,6,6-tetramethylpiperidide) when partnered with tridentate Lewis donor PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine) in catalytic alkene isomerisation of terminal olefins under mild reaction conditions. An array of distinct olefins could successfully be isomerised, including unactivated olefins, allylamines, and allylethers, showing the high activity of this partnership. In-depth mechanistic insights provided by X-ray crystallography, real-time nuclear magnetic resonance (NMR) monitoring, and density functional theory (DFT) calculations have unveiled the crucial role of in situ-generated TMP(H) in facilitating efficient isomerisation, and the choice of alkali-metal. Additionally, theoretical studies shed light on the observed E/Z selectivity, particularly accounting for the selective formation of Z-vinyl ethers. The versatility of our method is further demonstrated through the isomerisation of unactivated cycloalkenes, which undergo hydrogen isotope exchange to produce deuterated compounds.

Alkene Isomerization Catalyzed by a Mn(I) Bisphosphine Borohydride Complex

An additive-free manganese-catalyzed isomerization of terminal alkenes to internal alkenes is described. This reaction is implementing an inexpensive nonprecious metal catalyst. The most efficient catalyst is the borohydride complex cis-[Mn(dippe)(CO)2(κ2-BH4)]. This catalyst operates at room temperature, with a catalyst loading of 2.5 mol %. A variety of terminal alkenes is effectively and selectively transformed into the respective internal E-alkenes. Preliminary results show chain-walking isomerization at an elevated temperature. Mechanistic studies were carried out, including stoichiometric reactions and in situ NMR analysis. These experiments are flanked by computational studies. Based on these, the catalytic process is initiated by the liberation of "BH3" as a THF adduct. The catalytic process is initiated by double bond insertion into an M-H species, leading to an alkyl metal intermediate, followed by β-hydride elimination at the opposite position to afford the isomerization product.

Machine-Learning-Assisted Discovery of Mechanosynthesized Lead-Free Metal Halide Perovskites for the Oxidative Photocatalytic Cleavage of Alkenes

Lead-free metal halide perovskites can potentially be air- and water-stable photocatalysts for organic synthesis, but there are limited studies on them for this application. Separately, machine learning (ML), a critical subfield of artificial intelligence, has played a pivotal role in identifying correlations and formulating predictions based on extensive datasets. Herein, an iterative workflow by incorporating high-throughput experimental data with ML to discover new lead-free metal halide perovskite photocatalysts for the aerobic oxidation of styrene is described. Through six rounds of ML optimization guided by SHapley Additive exPlanations (SHAP) analysis, BA2CsAg0.95Na0.05BiBr7 as a photocatalyst that afforded an 80% yield of benzoic acid under the standard conditions is identified, which is a 13-fold improvement compared to the 6% with when using Cs2AgBiBr6 as the initial photocatalyst benchmark that is started. BA2CsAg0.95Na0.05BiBr7 can tolerate various functional groups with 22 styrene derivatives, highlighting the generality of the photocatalytic properties demonstrated. Radical scavenging studies and density functional theory calculations revealed that the formation of the reactive oxygen species superoxide and singlet oxygen in the presence of BA2CsAg0.95Na0.05BiBr7 are critical for photocatalysis.

Remote Functionalization by Pd-Catalyzed Isomerization of Alkynyl Alcohols

In recent years, progress has been made in the development of catalytic methods that allow remote functionalizations based on alkene isomerization. In contrast, protocols based on alkyne isomerization are comparatively rare. Herein, we report a general Pd-catalyzed long-range isomerization of alkynyl alcohols. Starting from aryl-, heteroaryl-, or alkyl-substituted precursors, the optimized system provides access preferentially to the thermodynamically more stable α,β-unsaturated aldehydes and is compatible with potentially sensitive functional groups. We showed that the migration of both π-components of the carbon-carbon triple bond can be sustained over several methylene units. Computational investigations served to shed light on the key elementary steps responsible for the reactivity and selectivity. These include an unorthodox phosphine-assisted deprotonation rather than a more conventional β-hydride elimination in the final tautomerization event.

Catalytic Olefin Transpositions Facilitated by Ruthenium N,N,N-Pincer Complexes

In this report, we demonstrate olefin transposition/isomerization reactions catalyzed by a series of N,N,N-pincer (1,3-bis(2-pyridylimino)isoindoline) Ru-hydride complexes. The protocol proceeds at room temperature for most substrates, achieving excellent yields, regioselectivity, and diastereoselectivity in short reaction times. The air-stable Ru-chloride derivatives of these complexes exhibit comparable reactivity enabling benchtop setup and synthetic versatility. Furthermore, we demonstrate the potential for one-pot cascade sequences of the products derived from the transposition reactions.

Isomerization reactions of metal vinylidene units

Isomerization reactions of unsaturated molecules offer an efficient strategy in atom-economical synthesis. Although isomerization reactions of unsaturated organic and organometallic compounds, such as alkenes, alkynes, and metal carbynes, have been achieved, those of metal vinylidene units that contain cumulated double bonds have never been reported. Herein, we inaugurally discovered isomerization reactions of metal vinylidene units via protonation and deprotonation reactions of metal carbenes. Experimental and theoretical investigations indicate that the electrical characteristics of substituents on the rings play a crucial role in controlling the formation of metal vinylidene units. The isomerization reactions of metal vinylidene units were driven by thermodynamic forces. Moreover, one of the angles at metal vinylidenes was found as 126.9°, representing the smallest angle in metal vinylidenes and the first cyclic 4d transition metal (Ru) vinylidene complex was successfully isolated. These investigations unveil novel structures and reactivity for metal vinylidenes, offering a fresh perspective on the isomerization reactions of unsaturated molecules containing cumulative unsaturated bonds.

Amide-Directed Rhodium-Catalyzed Chain-Walking Hydrothiolation of Internal Alkenes

We developed a rhodium-catalyzed chain-walking hydrothiolation process for internal alkenes, which offers a novel and efficient alternative for C(sp3)-H bond cleavage, while focusing on thiol incorporation. This method exclusively affords N,S-acetals at 36-90% yields. Regioconvergent hydrothiolation significantly improved the effectiveness of this transformation. Preliminary mechanistic investigations revealed that an amide-directing group is essential for regioselective synthesis, underlining its significance in this process.

A Torquoselective Thermal 6π-Electrocyclization Approach to 1,4-Cyclohexadienes via Solvent-Aided Proton Transfer: Experimental and Theoretical Studies

The thermal 6π-electrocyclization of hexatriene typically delivers 1,3-cyclohexadiene (1,3-CHD). However, there is only limited success in directly synthesizing 1,4-cyclohexadiene (1,4-CHD) using such an approach, probably due to the difficulty in realizing thermally-forbidden 1,3-hydride shift after electrocyclic ring closure. The present study shows that by heating (2E,4E,6E)-hexatrienes bearing ester or ketone substituents at the C1-position in a mixture of toluene/MeOH or EtOH (2 : 1) solvents at 90-100 °C, 1,4-CHDs can be selectively synthesized. This is achieved through a torquoselective disrotatory 6π-electrocyclic ring closure followed by a proton-transfer process. The success of this method depends on the polar protic solvent-assisted intramolecular proton transfer from 1,3-CHD to 1,4-CHD, which has been confirmed by deuterium-labeling experiments. There are no reports to date for such a solvent-assisted isomerization. Density functional theory (DFT) studies have suggested that forming 1,3-CHD and subsequent isomerization is a thermodynamically feasible process, regardless of the functional groups involved. Two possible successive polar solvent-assisted proton-transfer pathways have been identified for isomerization.

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking

Herein, we report the direct carboxylation of unactivated secondary alkyl bromides enabled by the merger of photoredox and nickel catalysis, a previously inaccessible endeavor in the carboxylation arena. Site-selectivity is dictated by a kinetically controlled insertion of CO2 at the initial C(sp3)-Br site by the rapid formation of Ni(I)-alkyl species, thus avoiding undesired β-hydride elimination and chain-walking processes. Preliminary mechanistic experiments reveal the subtleties of stereoelectronic effects for guiding the reactivity and site-selectivity.

Chemo- and Regioselective Catalytic Cross-Coupling Reaction of Ketones for the Synthesis of β, γ-Disubstituted β, γ-Unsaturated Ketones

C-C bond forming reaction of ketone with aldehyde is well-studied for the synthesis of α, β-unsaturated ketones, however, the reaction with two different ketones to unsaturated carbonyl compound has not yet been systematically studied. Probably due to the relatively low reactivity of ketones as electrophiles (aldol acceptors), its propensity for retro-aldol reaction. The reactions often suffer from unsatisfactory chemoselectivity (self- vs. crossed aldol products) and regioselectivity (thermodynamic vs. kinetic enolate). In this quest, we report here for the first time selective cross-coupling reaction of ketones to β-branched β, γ-unsaturated ketones by using ruthenium catalysis. Interestingly, single crossed aldol condensation products are formed even in reactions where a mixture of products is possible. Reaction is highly chemoselective, regioselective and produces H2 O as the only byproducts making the protocol environmentally benign. Method is compatible with a wide variety of sensitive functional group and applicable for even problematic aliphatic ketones as substrates. Notably, acetone was found as a three-carbon feedstock for the syntheses of simple β, γ-unsaturated ketone compounds. The process can further be extended to the gram-scale reaction and late-stage functionalization of natural products. With the help of DFT calculations, several control experiments, and deuterium-labeling experiments, the mechanistic finding demonstrated that initial aldol-condensation of ketones to a β, β-disubstituted α, β-unsaturated ketone, which further isomerizes to a β, γ- unsaturated ketone via η3 -allyl ruthenium complex.

Transition-metal-free and additive-free intermolecular hydroarylation of alkenes with indoles in hexafluoroisopropanol

Hydroarylation of alkenes is one of the most straightforward and atom-economical strategy for the construction of multi-aryl-substituted alkanes, but systematic studies have been limited to transition metal catalysis. Here we report a hexafluoroisopropanol (HFIP)-promoted hydroarylation of alkenes with indoles without the presence of transition metal catalysts or any additive. HFIP was the only reagent used in this work, and could be easily removed via evaporation, and recovered via distillation in industry settings. This reaction was shown to provide an efficient, clean and operationally simple procedure with a remarkable substrate scope and versatile transformations, delivering a variety of multi-aryl alkanes incorporating the indole motif. In preliminary studies, several of these products showed biologically activity against cells from an array of human cancer cell lines. A mechanistic study was also carried out and suggested that the quinone methide might be the key intermediate. And in contrast to the conclusions of a previous report, the current work suggested that protonation by HFIP might not be the rate-determining step.

Efficient one-pot syntheses of secondary amines from nitro aromatics and benzyl alcohols over Pd/NiTi-LDH under visible light

Solar energy-induced cascade/tandem reactions in one-pot are sustainable and green. Herein, the Pd/NiTi-LDH nanocomposite, with Pd nanoparticles (NPs) (∼3-6 nm) deposited on NiTi-LDH nanosheets, was obtained and was applied in the reaction between nitro aromatics and alcohols to synthesize secondary amines under visible light. The superior performance observed over the as-obtained Pd/NiTi-LDH nanocomposite for this reaction can be attributed to a successful merging of Pd-based hydrogenation and LDH-based photocatalysis, in which consecutive light-induced hydrogenation of nitro compounds to amines, dehydrogenation of alcohols to aldehydes, condensation between the in situ formed aldehydes and amines to imines and the hydrogenation of final imines to generate the desired secondary amines were realized in one pot over Pd/NiTi-LDH under visible light. This work shows an effective and green strategy in the synthesis of secondary amines. This study also demonstrates the high potential of using metal/LDH nanocomposites for light-initiated organic syntheses.

Unlocking the Chain-Walking Process in Gold Catalysis

The successful realization of gold-catalyzed chain-walking reactions, facilitated by ligand-enabled Au(I)/Au(III) redox catalysis, has been reported for the first time. This breakthrough has led to the development of gold-catalyzed annulation reaction of alkenes with iodoarenes by leveraging the interplay of chain-walking and π-activation reactivity mode. The reaction mechanism has been elucidated through comprehensive experimental and computational studies.

Thermally Activated Geometrical Regioselective E→Z Isomerization-Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

The geometrical regioselective E→Z isomerization of a conjugated alkene under thermal activation pose a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E→Z isomerization with subsequent cyclization cascade, particularly in the absence of commonly employed light, acids, or metal-catalysts. Thus, linearly conjugated dienals in a mixture of toluene-alcohol (2 : 1) solvents or only with alcohol at 60-70 °C can be converted to γ-alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E-isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E→2Z,4E isomerization of dienal is a thermodynamically facile (ΔG <0) process. Structural elucidation further reveals that the presence of a certain charge transfer and a non-covalent interaction may be the primary reasons for the enhanced stability of the 2Z,4E-isomer. The thermodynamic plausibility of the subsequent cascade reaction from the Z-isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the "conjugate addition pathway".

Iridium-Catalyzed Migratory Terminal C(sp3)-H Amidation of Heteroatom-Substituted Internal Alkenes via Olefin Chain Walking

Hydroamination facilitated by metal hydride catalysis is an appealing synthetic approach to access valuable nitrogen-containing compounds from readily available unsaturated hydrocarbons. While high regioselectivity can be achieved usually for substrates bearing polar chelation groups, the reaction involving simple alkenes frequently provides nonselective outcomes. Herein, we report an iridium-catalyzed highly regioselective terminal C(sp3)-H amidation of internal alkenes utilizing dioxazolones as an amino source via olefin chain walking. Most notably, this mechanistic motif of double bond migration to the terminal position operates not only with dialkyl-substituted simple alkenes including styrenes but also with heteroatom-substituted olefins such as enol ethers, vinyl silanes, and vinyl borons, thus representing the first example of the terminal methyl amidation of the latter type of alkenes through a nondissociative chain walking process.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Enantioselective Palladium-Catalyzed Directed Migratory Allylation of Remote Dienes

Chain walking has been an efficient route to realize the functionalization of inert C(sp3 )-H bonds, but this strategy is limited to mono-olefin migration and functionalization. Herein, we demonstrate the feasibility of tandem directed simultaneous migrations of remote olefins and stereoselective allylation for the first time. The adoption of palladium hydride catalysis and secondary amine morpholine as solvent is critical for achieving high substrate compatibility and stereochemical control with this method. The protocol is also applicable to the functionalization of three vicinal C(sp3 )-H bonds and thus construct three continuous stereocenters along a propylidene moiety via a short synthetic process. Preliminary mechanistic experiments corroborated the design of simultaneous walking of remote dienes.

Making Persistent Plastics Degradable

The vastness of the scale of the plastic waste problem will require a variety of strategies and technologies to move toward sustainable and circular materials. One of these strategies to address the challenge of persistent fossil-based plastics is new catalytic processes that are being developed to convert recalcitrant waste such as polyethylene to produce propylene, which can be an important precursor of high-performance polymers that can be designed to biodegrade or to degrade on demand. Remarkably, this process also enables the production of biodegradable polymers using renewable raw materials. In this Perspective, current catalyst systems and strategies that enable the catalytic degradation of polyethylene to propylene are presented. In addition, concepts for using "green" propylene as a raw material to produce compostable polymers is also discussed.

Ir-Catalyzed Distal Branch-Selective Hydroarylation of Unactivated Internal Alkenes with Benzanilides via C-H Activation along with Consecutive Isomerization

We herein report a synergistic strategy of C-H activation and consecutive isomerization catalyzed by an Ir catalyst to selectively obtain branched isomers as C-H alkylated products of benzanilide derivatives. A well-tuned ligand and a directing group are crucial to achieve this selectivity. The scope of this reaction is demonstrated by the use of a variety of substituents and complex molecules.

Multi-site programmable functionalization of alkenes via controllable alkene isomerization

Direct and selective functionalization of hydrocarbon chains is a fundamental problem in synthetic chemistry. Conventional functionalization of C=C double bonds and C(sp3)-H bonds provides some solutions, but site diversity remains an issue. The merging of alkene isomerization with (oxidative) functionalization provides an ideal method for remote functionalization, which would provide more opportunities for site diversity. However, the reported functionalized sites are still limited and focus on a specific terminal position and internal site; new site-selective functionalization, including multi-functionalization, remains a largely unmet challenge. Here we describe a palladium-catalysed aerobic oxidative method for the multi-site programmable functionalization, involving the C=C double bond and multiple C(sp3)-H bonds, of terminal olefins via a strategy that controls the reaction sequence between alkene isomerization and oxidative functionalization. Specifically, 1-acetoxylation (anti-Markovnikov), 2-acetoxylation, 1,2-diacetoxylation and 1,2,3-triacetoxylation have been realized, accompanied by controllable remote alkenylation. This method enables available terminal olefins from petrochemical feedstocks to be readily converted into unsaturated alcohols and polyalcohols and particularly into different monosaccharides and C-glycosides.

Nickel-Catalyzed Isomerization of Homoallylic Alcohols

Nickel-catalyzed isomerizations of homoallylic alcohols and a bishomoallylic alcohol are presented. These isomerization reactions occur in the presence of a simple nickel catalyst that does not require addition of an exogenous ligand. The corresponding ketone products are generated in ≤98% yield.

Nickel-Catalyzed Remote Asymmetric Hydroalkylation of Alkenyl Ethers to Access Ethers of Chiral Dialkyl Carbinols

Site- and enantio-selective alkyl-alkyl bond formation is privileged in the retrosynthetic analysis due to the universality of sp³-hybridized carbon atoms in organic molecules. Herein, we report a nickel-catalyzed remote asymmetric hydroalkylation of alkenyl ethers via synchronous implementation of alkene isomerization and enantioselective C(sp³)-C(sp³) bond formation. Regression analysis of catalyst structure-activity relationships accelerates the rational ligand modification through modular regulation. This reaction has several advantages for synthesizing chiral dialkyl carbinols and their ether derivatives, including the broad substrate scope, good functional group tolerance, excellent regioselectivity (>20:1 regioisomeric ratio), and high enantioselectivity (up to 95% enantiomeric excess).

Dual Ruthenium-Catalyzed Alkene Isomerization-Hydrogen Auto-Transfer Unlocks Skipped Dienes as Pronucleophiles for Enantioselective Alcohol C-H Allylation

The first use of 1,4-pentadiene and 1,5-hexadiene as allylmetal pronucleophiles in regio-, anti-diastereo-, and enantioselective carbonyl addition from alcohol proelectrophiles is described. As corroborated by deuterium labeling experiments, primary alcohol dehydrogenation delivers a ruthenium hydride that affects alkene isomerization to furnish a conjugated diene, followed by transfer hydrogenative carbonyl addition. Hydrometalation appears to be assisted by the formation of a fluxional olefin-chelated homoallylic alkylruthenium complex II, which exists in equilibrium with its pentacoordinate η1 form to enable β-hydride elimination. This effect confers remarkable chemoselectivity: while 1,4-pentadiene and 1,5-hexadiene are competent pronucleophiles, higher 1,n-dienes are not, and the olefinic functional groups of the products remain intact under conditions in which the 1,4- and 1,5-dienes isomerize. A survey of halide counterions reveals iodide-bound ruthenium-JOSIPHOS catalysts are uniquely effective in these processes. This method was used to prepare a previously reported C1-C7 substructure of (-)-pironetin in 4 vs 12 steps.

Mechanistic Insight into Palladium/Brønsted Acid Catalyzed Methoxycarbonylation and Hydromethoxylation of Internal Alkene: A Computational Study

Density functional theory (DFT) calculations were performed to study the palladium/Brønsted acid-catalyzed methoxycarbonylation and hydromethoxylation reactions of internal alkene. The calculated results show that the pyridyl group (N atom) in bidentate phosphine ligand with built-in base (L1) plays a crucial role in controlling the selectivity. With the help of the pyridyl group, the methanolysis steps in the methoxycarbonylation reaction and the hydromethoxylation reaction become easy, and both the linear ester methyl 3,4-dimethylpentanoate (P1) and the hydromethoxylation product 2-methoxy-2,3-dimethylbutane (P2) could be obtained. In contrast, the possibility of leading to branched ester P1' was ruled out according to our calculations. The steric effect could account for the observed selectivity. In the presence of the DPEphos ligand (L2) that does not bear the pyridyl group, the methanolysis step in the methoxycarbonylation reaction becomes the rate-determining step with a high overall energy barrier. Neither linear nor branched methoxycarbonylation product could be generated. The palladium/Brønsted acid co-catalyzed hydromethoxylation also become difficult without the assistance of the pyridyl group in the presence of the L2 ligand. Instead, TsOH-catalyzed hydromethoxylation reaction could take place to generate the ether product P2.

Palladium-Catalyzed Hydroboration/Cyclization of 1,n-Dienes

While the hydroboration of alkenes is well established, the corresponding cyclization reaction of dienes remains challenging. Here, we report a new method for hydroboration/cyclization applicable to various 1,n-dienes and hydroboranes. The method features the direct synthesis of borylalkyl cyclopentanes from common 1,6-dienes, which is highlighted by syntheses of elaborated pyrrolidine cores from easily accessible diallylamines. Notably, 1,n-dienes (n > 6) also undergo five-membered ring formation, offering "remote" hydroboration/cyclization that would be otherwise difficult to achieve.

Merging Alkene Isomerization Enables Difunctionalization of Cyclic Enamines toward Ring-Fused Aminal Synthesis

Here we report a Pd-catalyzed isomerization of alicyclic allyl amine to achieve the unprecedented α,β-difunctionalization of synthetically inaccessible trisubstituted cyclic enamine. The dual role of in situ formed enamine intermediate allows for the intermolecular formal [4 + 2] reaction with acrylamide or isatoic anhydride to simultaneously construct the C-C bond and C-N bond, thus realizing the expedient construction of [4.3.0]-aminal with excellent diastereoselectivity and high atom economy.