Π-allylpalladium Intermediates Research Articles

Assembly of 2-Substituted Tetrahydroquinolines from ortho-Methylbenzenesulfamides and Dienes, Using a C(sp3)-H Activation/Annulation Sequence.

1,2,3,4-Tetrahydroquinolines (THQs) are essential structural cores in many natural products and pharmaceutical drugs. Especially relevant are those presenting substitutions at position 2, yet practical methods for their one-step assembly from acyclic precursors are very scarce. Herein, we present a straightforward approach to assembling these skeletons from ortho-methylanilines using a palladium-catalyzed C(sp3)-H activation/formal cycloaddition sequence. Key for the success of the approach is the use of dienes as partners, since they lead to stable π-allyl palladium intermediates that prevent β-hydride elimination processes and allow installation of versatile alkenyl handles at position 2. Moreover, installing a perfluorobenzenesulfonyl substituent at the amine not only facilitates the C-H activation but also allows for an easy recovery of the free amine.

Palladium(0) and Brønsted Acid Co-catalyzed Enantioselective Hydro-Cyclization of 2,4-Dienyl Hydrazones and Oximes.

The transition metal-catalyzed asymmetric hydro-functionalization of 1,3-dienes has been well explored, but most reactions focus on electron-neutral substrates in an intermolecular manner. Here we first demonstrate that readily available 2,4-dienyl hydrazones and oximes can be efficiently utilized in the hydro-cyclization reaction under co-catalysis of a Brønsted acid and a chiral palladium complex, furnishing multifunctional dihydropyrazones and dihydroisoxazoles, respectively. Diverse substitution patterns for both types of electron-deficient diene compounds are tolerated, and corresponding heterocycles were generally constructed with moderate to excellent enantioselectivity, which can be elaborated to access products with higher molecular complexity and diversity. Control experiments and density functional theory calculations support that α-regioselective protonation of dienyl substrates by acid and concurrent π-Lewis base activation of Pd0 complex is energetically favoured in the formation of active π-allylpalladium intermediates, and an outer-sphere allylic amination or etherification mode is adopted to deliver the observed cyclized products enantioselectively.

Stereodivergent Construction of 1,5/1,7-Nonadjacent Tetrasubstituted Stereocenters Enabled by Pd/Cu-Cocatalyzed Asymmetric Heck Cascade Reaction.

The construction of chiral motifs containing nonadjacent stereocenters in an enantio- and diastereoselective manner has long been a challenging task in synthetic chemistry, especially with respect to their stereodivergent synthesis. Herein, we describe a protocol that enables the enantio- and diastereoselective construction of 1,5/1,7-nonadjacent tetrasubstituted stereocenters through a Pd/Cu-cocatalyzed Heck cascade reaction. Notably, a C=C bond relay strategy involving the shift of the π-allyl palladium intermediate was successfully applied in the asymmetric construction of 1,7-nonadjacent stereocenters. The current method allows for the efficient preparation of chiral molecules bearing two privileged scaffolds, oxindoles and non-natural α-amino acids, with good functional group tolerance. The full complement of the four stereoisomers of products bearing 1,5/1,7-nonadjacent stereocenters could be readily accessed by a simple combination of two chiral metal catalysts with different enantiomers.

Merging of Light/Dark Palladium Catalytic Cycles Enables Multicomponent Tandem Alkyl Heck/Tsuji-Trost Homologative Amination Reaction toward Allylic Amines.

A visible light-induced palladium-catalyzed homologative three-component synthesis of allylic amines has been developed. This protocol proceeds via a unique mechanism involving two distinct cycles enabled by the same Pd(0) catalyst: a visible light-induced hybrid radical alkyl Heck reaction between 1,1-dielectrophile and styrene, followed by the "in dark" classical Tsuji-Trost-type allylic substitution reaction. This method works well with a broad range of primary and secondary amines, aryl alkenes, dielectrophiles, and in complex settings. The regiochemistry of the obtained products is primarily governed by the structure of 1,1-dielectrophile. Involvement of π-allyl palladium intermediates allowed for the control of stereoselectivity, which has been demonstrated with up to 95:5 er.

Fungicide-inspired precursors of π-allylpalladium intermediates for palladium-catalyzed decarboxylative cycloadditions.

Inspired by a fungicide, we designed 5-vinyloxazolidine-2,4-diones as new precursors of π-allylpalladium zwitterionic intermediates and developed palladium-catalyzed asymmetric (5 + 3) cycloaddition with azomethine imines and (3 + 2) cycloaddition with 1,1-dicyanoalkenes. Both reactions proceeded smoothly under mild reaction conditions to produce various chiral heterocyclic compounds in high yields with excellent enantioselectivities. These results revealed that 5-vinyloxazolidine-2,4-diones were a type of suitable precursor for palladium catalysis and will find extensive applications in Pd-catalyzed reactions such as cycloaddition and allylic alkylation.

Asymmetric Cascade Carbonylation/Annulation of Benzyl Bromides, CO, and Vinyl Benzoxazinanones Enabled by Pd/Chiral Lewis-Base Relay Catalysis.

A highly enantioselective cascade carbonylation/annulation of benzyl bromides, CO, and vinyl benzoxazinanones under mild conditions has been established by Pd/chiral Lewis base relay catalysis, providing an efficient method to assemble chiral quinolinones from readily available starting materials in good yields with excellent diastereo- and enantioselectivities. The palladium catalyst plays two roles in this reaction, enabling both the carbonylation process and the generation of the zwitterionic π-allyl palladium intermediate.

Palladium-Catalyzed Ring-Opening Reaction of Cyclopropenones with Vinyl Epoxides.

We report herein the Pd-catalyzed selective ring-opening reaction of cyclopropenones with vinyl epoxides. By using a commercially available Pd2(dba)3·CHCl3-BINAP catalyst, a wide range of conjugated alkadienyl carboxylates could be accessed in good yield and excellent regioselectivity. The new application of zwitterionic π-allyl palladium intermediates has been demonstrated in organic synthesis.

Enantioselective Intramolecular Allylic Substitution via Synergistic Palladium/Chiral Phosphoric Acid Catalysis: Insight into Stereoinduction through Statistical Modeling.

The mode of asymmetric induction in an enantioselective intramolecular allylic substitution reaction catalyzed by a combination of palladium and a chiral phosphoric acid was investigated by a combined experimental and statistical modeling approach. Experiments to probe nonlinear effects, the reactivity of deuterium-labeled substrates, and control experiments revealed that nucleophilic attack to the π-allylpalladium intermediate is the enantio-determining step, in which the chiral phosphate anion is involved in stereoinduction. Using multivariable linear regression analysis, we determined that multiple noncovalent interactions with the chiral environment of the phosphate anion are integral to enantiocontrol in the transition state. The synthetic protocol to form chiral pyrrolidines was further applied to the asymmetric construction of C-O bonds at fully substituted carbon centers in the synthesis of chiral 2,2-disubstituted benzomorpholines.

Pd-Catalyzed Decarboxylative Cycloaddition of Vinylethylene Carbonates with Isothiocyanates.

An efficient route for formal [3 + 2] cycloaddition of vinylethylene carbonates with isothiocyanates was developed for the synthesis of 1,3-oxazolidine-2-thione derivatives. The zwitterionic π-allyl palladium intermediates formed in situ by decarboxylation of VECs acted as the three-membered synthons. In this transformation, the C-N bond formation was selectively realized over the C-S bond formation.

Palladium(0)‐Catalyzed Rearrangement of Allylic Esters

AbstractThe palladium‐catalyzed transposition of allylic esters is usually performed with a rather high loading of a palladium(II) catalyst. In the present investigation, a palladium(0) catalyst is developed for rearranging methyl 2‐acyloxybut‐3‐enoates into the corresponding methyl 4‐acyloxycrotonates. The transformation is performed at room temperature with 2% of Pd(PPh3)4 and gives good yields with acetate, benzoate and propanoate esters. Methyl 2‐(methoxycarbonyloxy)but‐3‐enoate, on the other hand, is rearranged into methyl 4‐methoxycrotonate due to decarboxylation of the released methyl carbonate into methoxide. The mechanism for the transposition of methyl 2‐acetoxybut‐3‐enoate was investigated in two separate crossover experiments and shown to proceed by formation of the intermediate π‐allyl palladium(II) complex, which is then attacked by acetate in an intermolecular fashion.

Vinyl-aziridines and cyclopropanes in Pd-catalyzed (3+2)-cycloaddition reactions with cyclic N-sulfonyl imines

Efficient palladium-catalyzed (3 + 2)-cycloaddition reactions of cyclic N-sulfonyl imines and vinyl-aziridines (or cyclopropanes) have been achieved. The reactions, with either vinylic substrate, proceed with excellent yields affording highly functionalized imidazolidine and pyrrolidine derivatives. The cycloadditions take place via the reaction of zwitterionic π-allyl palladium intermediates with cyclic N-sulfonyl imines through i) the formation of two NC bonds in the presence of vinylaziridines (synthesis of imidazolidines) and ii) one CC bond and one NC bond in the presence of vinylcyclopropanes (synthesis of pyrrolidines). Following on preliminary works on the diastereoselective synthesis of imidazolidines, herein we wish to give a broader view on the subject by describing derivatization reactions and attempts towards an enantioselective version. Moreover, we describe and discuss the behavior of each vinylic substrate (aziridine or cyclopropane) on the (3 + 2)-cycloaddition reactions. Mechanistic and (intriguing) selectivity outcomes are also going to be discussed.

Palladium(0)-catalyzed rearrangement of allyl enol ethers to form chiral quaternary carbon centers via asymmetric allylic alkylation

Herein we report the first palladium(0)-catalyzed asymmetric allylic alkylation (AAA) of allyl enol ether via π-allylpalladium intermediate using Trost chiral diphosphine. This unprecedented reaction produced very rare α-aryl quaternary aldehydes with multi-functional groups. The main novelty in the chemistry demonstrates that enol ethers can be used as precursors for π-allylpalladium intermediates, an observation that is certainly rare and to the best of our knowledge, perhaps without prior precedent. Chiral ligand (R,R)-L3 was found to be optimal in this Pd-AAA reaction and provided good to excellent yield (80–95%) and enantioselectivity (70–90%) with a range of analogs.

Synthesis of cis-5,5a,6,10b-Tetrahydroindeno[2,1-b]indoles through Palladium-Catalyzed Decarboxylative Coupling of Vinyl Benzoxazinanones with Arynes.

A novel palladium-catalyzed decarboxylative coupling reaction of vinyl benzoxazinanones with arynes which may feature an intramolecular nucleophilic attack of an amino group at the central carbon of π-allylpalladium intermediate has been developed. The cis-5,5a,6,10b-tetrahydroindeno[2,1-b]indoles were generated in moderate to good yields. One key to the success of the present reaction was to achieve comparable rates for the palladium-catalyzed decarboxylation and aryne formation steps.

Highly regio- and stereoselective palladium-catalyzed allene bifunctionalization cascade via π-allyl intermediate

We report a palladium-catalyzed allene bifunctionalization reaction that forms CC and either CO or CN bonds in one pot with excellent regio- and stereoselectivity. Carboxylic acids, amides, and hydroxide are all suitable nucleophiles. Organoboronic acid acts as hydroxide transfer reagent. An intermediate π-allyl palladium complex was isolated, which yields improved catalytic performance as well as evidence of the origin of stereoselectivity. A derivatization study emphasizes the utility of the functionalized allylic products.

ChemInform Abstract: Diastereoselective Synthesis of Trisubstituted Cyclopropanes by Palladium‐Catalyzed Intramolecular Allylic Alkylation of α‐Aryl Esters.

AbstractVarious trisubstituted cyclopropane derivatives are obtained by acid‐catalyzed intramolecular allylic carboxylation followed by a palladium‐catalyzed intramolecular allylic alkylation with an α‐aryl ester enolate.

Diastereoselective Synthesis of Trisubstituted Cyclopropanes by Palladium-Catalyzed Intramolecular Allylic Alkylation of α-Aryl Esters­

We have developed a novel method for synthesizing trisubstituted cyclopropane derivatives by a palladium-catalyzed intramolecular allylic alkylation of α-aryl esters. By using α-aryl α-(methoxycarbonyl) γ-vinyl γ-lactones as substrates, decarboxylative formation of π-allylpalladium(II) intermediates followed by an intramolecular allylic alkylation of the ester enolates proceeded in the presence of 5 mol% of a palladium catalyst, producing 1-aryl-1-(methoxycarbonyl)-2-vinylcyclopropanes in good to excellent yields and high diastereoselectivities. The relative configuration of the major isomer was determined by transforming the product into a known intermediate of milnacipran synthesis. When we extended our method to asymmetric catalysis, we obtained methyl (1<i>S</i>,2<i>S</i>)-1-phenyl-2-vinylcyclopropanecarboxylate in up to 55% ee by using (<i>S</i>)-[2′-(diphenylmethoxy)-1,1′-binaphthalen-2-yl](diphenyl)phosphine as a chiral monodentate phosphorus ligand.

Access to Spirocyclized Oxindoles and Indolenines via Palladium-Catalyzed Cascade Reactions of Propargyl Carbonates with 2-Oxotryptamines and Tryptamines.

Reported here are methods for the direct construction of a range of spirocyclized oxindoles and indolenines in good to excellent yields. Specifically, we report the palladium-catalyzed reactions of oxindoles and indoles, both functioning as bis-nucleophiles, with propargyl carbonates to afford spirocyclic products having an exocyclic double bond on the newly formed ring. The reaction proceeds through a process wherein the first nucleophilic unit on the oxindole or indole reacts with an allenyl-palladium species, formed from oxidative addition of Pd(0) to propargyl carbonates, to generate a π-allyl palladium intermediate that then reacts further with the second nucleophilic component of the oxindole or indole. The cascade process forges two bonds en route to spirocyclized oxindole and indolenine products. The use of chiral phosphines renders the cyclization sequence enantioselective, providing spirocyclic products with modest to good enantioselectivities.

Regio- and stereoselective synthesis of functionalized allylic fluorides via palladium-catalyzed three-component tandem carbofluorination of allenes

We developed a highly efficient palladium-catalyzed three-component tandem carbofluorination of allenes with ArI and AgF. Nucleophilic trapping of π-allyl palladium intermediates generated from carbopalladation of allenes with AgF delivers functionalized allylic fluorides with highly branched selectivity. This transformation features simultaneous construction of C–C and C–F bond in one step, allowing for the facile synthesis of functionalized allylic fluorides, valuable synthetic building blocks. Moreover, a good correlation between regioselectivity and substituent Hammett constants was observed in the reactions of mono-alkyl substituted allene substrates, corroborating with the proposed outer-sphere fluorination pathway. This study uncovered the regioselectivity trends being controlled by both electronic and steric factors, which paved the way for further developments in selective synthesis of branched functionalized allylic fluorides difficult to access from established methodologies.

Redox-neutral α-allylation of amines by combining palladium catalysis and visible-light photoredox catalysis.

An unprecedented α-allylation of amines was achieved by combining palladium catalysis and visible-light photoredox catalysis. In this dual catalysis process, the catalytic generation of allyl radical from the corresponding π-allylpalladium intermediate was achieved without additional metal reducing reagents (redox-neutral). Various allylation products of amines were obtained in high yields through radical cross-coupling under mild reaction conditions. Moreover, the transformation was applied to the formal synthesis of 8-oxoprotoberberine derivatives which show potential anticancer properties.

Activation of alcohols with carbon dioxide: intermolecular allylation of weakly acidic pronucleophiles.

The direct coupling of allyl alcohols with nitroalkanes, nitriles, and aldehydes using catalytic Pd(PPh3)4 has been accomplished via activation of C–OH bonds with CO2. The in situ formation of carbonates from alcohols and CO2 facilitates oxidative addition to Pd to form reactive π-allylpalladium intermediates. In addition, the formation of a strong base activates nucleophiles toward the reaction with the π-allylpalladium electrophile. Overall, this atom economical reaction provides a new C–C bond without the use of an external base and generates water as the only byproduct.