Formal anti-carbopalladation reactions of non-activated alkynes: requirements, mechanistic insights, and applications

Enantioselective Synthesis of Bicyclo[3.2.1]octadienes via Palladium-Catalyzed Intramolecular Alkene-Alkyne Coupling Reaction

Bicyclo[3.2.1]octadiene compounds and derivatives exist in a number of natural products and bioactive compounds. Nevertheless, catalytic enantioselective protocols for the synthesis of these skeletons have not been disclosed. Herein we reported a palladium-catalyzed asymmetric intramolecular alkene-alkyne coupling of alkyne-tethered cyclopentenes, affording a library of enantionenriched bicyclo[3.2.1]octadienes in excellent yields and enantioselectivities (mostly >99 % ee). Moreover, the products could undergo an unusual iodination-induced 1,2-acyl migration, forming iodinated bicyclo[3.2.1]octadienes with three vicinal stereocenters. The enone and isolated olefin motifs embedded in the products provide useful handles for downstream elaboration.

Gold-Catalyzed 1,2-Dicarbofunctionalization of Alkynes with Organohalides

Herein, we report the first gold-catalyzed 1,2-dicarbofunctionalization of alkynes using organohalides as non-prefunctionalized coupling partners. The mechanism of the reaction involves an oxidative addition/π-activation mechanism in contrast to the migratory insertion/cis-trans isomerization pathway that is predominantly observed with other transition metals yielding products with anti-selectivity. Mechanistic insights include several control experiments, NMR studies, HR-MSMS analyses, and DFT calculations that strongly support the proposed mechanism.

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

Palladium(0)-Catalyzed Anti-Selective Addition-Cyclizations of Alkynyl Electrophiles

Palladium(0)/monophosphine complexes catalyze anti-selective alkylative, arylative, and alkynylative cyclizations of alkynyl electrophiles with organometallic reagents. The remarkable anti-selectivity results from novel oxidative addition, that is, the nucleophilic attack of electron-rich palladium(0) on the electrophile across the alkyne followed by transmetalation and reductive elimination ("anti-Wacker"-type cyclization). With regard to 5-alkynals, triphenylphosphine (PPh₃ )-ligated palladium(0) catalyzes the cyclization of terminal alkynes and conjugated alkenyl- or alkynyl-substituted ones to afford 2-cyclohexen-1-ol and 2-alkylidene-cyclopentanol derivatives, respectively. For 6-alkyl- or 6-aryl-5-alkynals, the cyclization does not proceed with the palladium/PPh₃ catalyst; however, it does proceed with palladium/tricyclohexylphosphine (PCy₃ ), to yield the former products predominantly. Remarkably, the latter catalyst completely switches the regioselectivity in the cyclization of the conjugated diyne-aldehydes. Notably, palladium/PPh₃ -catalyzed cyclizations also proceed with other organometallics or even without them.

Pd(0)-Catalyzed Asymmetric 7-Endo Hydroacyloxylative Cyclization of 1,6-Enyne Enabled by an Anion Ligand-Directed Strategy

Despite diversity in reaction mechanisms, the palladium-catalyzed cyclization of 1,6-enyne generally proceeds in a 5-exo manner. Herein, we report the development of a Pd(0)-catalyzed hydroacyloxylative cyclization of 1,6-enyne in either 7-endo-trig or 6-exo-trig fashion when paired with an appropriate dihaloacetic acid reactant, such as F₂HCCO₂H and Cl₂HCCO₂H. Using the combination of Pd₂(dba)₃ and a chiral phosphine ligand, the hydroacyloxylative cyclization of 1,6-enyne bearing a 1,1-disubstituted alkene moiety readily gives highly enantiopure seven-membered heterocycles while the reaction of those having a 1,2-disubstituted alkene affords six-membered rings with moderate enantioselectivity. Preliminary experimental studies suggest a reaction mechanism featuring an unusual E-to-Z vinyl-Pd(II) isomerization and alkene trans-oxypalladation, which is proven to be governed by the rationally selected carboxylate.

DFT Study of Mechanism and Stereochemistry of Nickel-Catalyzed trans-Arylative Desymmetrizing Cyclization of Alkyne-Tethered Malononitriles

Present here is a density functional theory (DFT) study of the mechanism and origin of enantioselectivity of Ni-catalyzed desymmetric cyclization of alkyne-tethered malononitriles and aryl boronic acids. The reaction starts from transmetalation and arylnickel addition, followed by trans to cis isomerization to give cis-alkenyl nickel species. The stereodetermining step is the CN insertion, which prefers a transition state with the bystander CN group staying away from the ligand to reduce steric repulsion, and gives the final (R)-product.

Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C–H Bond Formation

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehyde-derived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45–87% yields and 84–94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

α,β-Unsaturated Carbonyls for One-Pot Transition-Metal-Free Access to 3,6-Dihydro-2H-pyrans

An efficient protocol has been developed for accessing mono-, di-, and trisubstituted 3,6-dihydro-2H-pyran derivatives by simply subjecting α,β-unsaturated carbonyls to the carefully optimized Corey-Chaykovsky reaction conditions. The strategy provides selectively substituted dihydropyran derivatives in good to excellent yields with a broad substrate scope under very mild reaction conditions. Easy transformation of the final 3,6-dihydro-2H-pyran to the valued 5,6-dihydro-2H-pyran-2-one and tetrahydro-2H-pyran derivatives expanded the scope of this methodology to diverse oxacycles. Further, the developed strategy also found application in a two-step route to racemic goniothalamin, which is widely studied for its cytotoxic behavior.

Nickel-Catalyzed Arylative Cyclizations of Alkyne- and Allene-Tethered Electrophiles using Arylboron Reagents

The use of arylboron reagents in metal‐catalyzed domino addition–cyclization reactions is a well‐established strategy for the preparation of diverse, highly functionalized carbo‐ and heterocyclic products. Although rhodium‐ and palladium‐based catalysts have been commonly used for these reactions, more recent work has demonstrated nickel catalysis is also highly effective, in many cases offering unique reactivity and access to products that might otherwise not be readily available. This review gives an overview of nickel‐catalyzed arylative cyclizations of alkyne‐ and allene‐tethered electrophiles using arylboron reagents. The scope of the reactions is discussed in detail, and general mechanistic concepts underpinning the processes are described.

Mapping Ambiphile Reactivity Trends in the Anti-(Hetero)annulation of Non-Conjugated Alkenes via PdII /PdIV Catalysis

In this study, we systematically evaluate different ambiphilic organohalides for their ability to participate in anti-selective carbo- or heteroannulation with non-conjugated alkenyl amides under Pd^II /Pd^IV catalysis. Detailed optimization of the reaction conditions has led to protocols for synthesizing tetrahydropyridines, tetralins, pyrrolidines, and other carbo/heterocyclic cores via [n+2] (n=3-5) (hetero)annulation. Expansion of scope to otherwise unreactive ambiphilic haloketones through Pd^II /amine co-catalysis is also demonstrated. Compared to other annulation processes, this method proceeds via a distinct Pd^II /Pd^IV mechanism involving Wacker-type directed nucleopalladation. This difference results in unique reactivity and selectivity patterns, as revealed through assessment of reaction scope and competition experiments.

Cobalt-Catalyzed Sequential Site- and Stereoselective Hydrosilylation of 1,3- and 1,4-Enynes

Catalytic sequential hydrosilylation of 1,3-enynes and 1,4-enynes promoted by cobalt complexes derived from bisphosphines are presented. Site- and stereoselective Si-H addition of primary silanes to 1,3-enynes followed by sequential intramolecular diastereo- and enantioselective Si-H addition afforded enantioenriched cyclic alkenylsilanes with simultaneous construction of a carbon-stereogenic center and a silicon-stereogenic center. Reactions of 1,4-enynes proceeded through sequential isomerization of the alkene moiety followed by site- and stereoselective hydrosilylation. A wide range of alkenylsilanes were afforded in high efficiency and selectivity. Functionalization of the enantioenriched silanes containing a stereogenic center at silicon delivered a variety of chiral building blocks that are otherwise difficult to access.

Cationic-palladium catalyzed regio- and stereoselective syn-1,2-dicarbofunctionalization of unsymmetrical internal alkynes

π-Extended tetrasubstituted olefins are widely found motifs in natural products, leading drugs, and agrochemicals. Thus, development of modular strategies for the synthesis of complex all-carbon-substituted olefins always draws attention. The difunctionalization of unsymmetrical alkynes is an attractive approach but it has remained faced with regioselectivity issues. Here we report the discovery of a regio- and stereoselective syn-1,2-dicarbofunctionalization of unsymmetrical internal alkynes. A cationic Pd-catalyzed three-component coupling of aryl diazonium salts, aryl boronic acids (or olefins) and yne-acetates enables access to all-carbon substituted unsymmetrical olefins. The transformation features broad scope with labile functional group tolerance, building broad chemical space of structural diversity (94 molecules). The value of this synthetic method is demonstrated by the direct transformation of natural products and drug candidates containing yne-acetates, to enable highly substituted structurally complex allyl acetate analogues of biologically important compounds. Synthetic versatility of the carboxylate bearing highly substituted olefins is also presented. The reaction outcome is attributed to the in situ formation of stabilized cationic aryl-Pd species, which regulates regioselective aryl-palladation of unsymmetrical yne-acetates. Control experiments reveal the synergy between the carboxylate protecting group and the cationic Pd-intermediate in the regioselectivity and reaction productivity; density functional theory (DFT) studies rationalize the selectivity of the reaction.

Synthesis of Benzofused O- and N-Heterocycles through Cascade Carbopalladation/Cross-Alkylation of Alkynes Involving the C–C Cleavage of Cyclobutanols

We report a Pd-catalyzed route to heterocycles bearing a tetrasubstituted alkene fragment. Our approach merges the intramolecular carbopalladation of tethered alkynes with an alkylation step produced by the C–C cleavage of cyclobutanol derivatives. An alkenyl-Pd(II) intermediate has been isolated and characterized by X-ray diffraction studies. Interestingly, the nature of the tethering alkynyl chain influences the E/Z stereochemistry of the alkenyl fragment in the functionalized heterocycles.

Origin of Ligand Effects on Stereoinversion in Pd-Catalyzed Synthesis of Tetrasubstituted Olefins

The mechanism and origin of ligand effects on stereoinversion of Pd-catalyzed synthesis of tetrasubstituted olefins were investigated using DFT calculations and the approach of energy decomposition analysis (EDA). The results reveal that the stereoselectivity-determining steps are different when employing different phosphine ligands. This is mainly due to the steric properties of ligands. With the bulkier Xantphos ligand, the syn/anti-to-Pd 1,2-migrations determine the stereoselectivity. While using the less hindered P(o-tol)₃ ligand, the 1,3-migration is the stereoselectivity-determining step. The EDA results demonstrate that Pauli repulsion and polarization are the dominant factors for controlling the stereochemistry in 1,2- and 1,3-migrations, respectively. The origins of differences of Pauli repulsion and polarization between the two stereoselective transition states are further identified.

Alkyne Aminopalladation/Heck and Suzuki Cascades: An Approach to Tetrasubstituted Enamines

Alkyne aminopalladation reactions starting from tosylamides are reported. The emerging vinylic Pd species are converted either in an intramolecular Heck reaction with olefinic units or in an intermolecular Suzuki reaction by using boronic acids exhibiting broad functional group tolerance. Tetra(hetero)substituted tosylated enamines are obtained in a simple one-pot process.

Palladium-Catalyzed anti-Carbosilylation of Alkynes to Access Isoquinolinone-Containing Exocyclic Vinylsilanes

A Pd-catalyzed trans-selective carbosilylation reaction of alkynes has been developed. The trans-vinylpalladium species, generated through intramolecular syn-carbopalladation of alkynes and subsequent cis-trans isomerization, were captured by hexamethyldisilane to form multisubstituted vinylsilanes. This reaction provides a useful strategy for the stereoselective synthesis of isoquinolinone-containing exocyclic tetrasubstituted vinylsilanes.

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.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Palladium Nanoparticle-Catalyzed Stereoselective Domino Synthesis of 3-Allylidene-2(3H)-oxindoles and 3-Allylidene-2(3H)-benzofuranones

A single-step, stereoselective protocol for the synthesis of unsymmetrically substituted (E)-3-allylideneoxindole and (E)-3-allylidenebenzofuran from readily accessible starting materials using palladium binaphthyl nanoparticles (Pd-BNPs) has been developed. Pd-BNP showing a wide range of functional group tolerance and an immense array of substrate scope have been explored with the successful synthesis of the drug molecule "tubulin polymerization inhibitor" free from trace metal impurities. The model reaction is extended to a gram-scale synthesis, and one of the products is utilized for derivatization. The Pd-BNP has been recycled up to 5 catalytic cycles without any loss in reaction yields and particle size of nanoparticles.

From 1,2-difunctionalisation to cyanide-transfer cascades - Pd-catalysed cyanosulfenylation of internal (oligo)alkynes

Internal alkynes substituted by aliphatic or aromatic moieties or by heteroatoms were converted into sulphur-substituted acrylonitrile derivatives. Key is the use of Pd catalysis, which allows the addition of aromatic and aliphatic thiocyanates in an intra- and intermolecular manner. Substrates with several alkyne units underwent further carbopalladation steps after the initial thiopalladation step, thus generating in a cascade-like fashion an oligoene unit with sulphur at one terminus and the cyano group at the other.

The intra- and intermolecular Pd-catalysed cyanosulfenylation of internal alkynes enables the formation of tetrasubstituted thioacrylonitriles and is extended to oligoyne systems leading to oligoenes and a cyanide transfer over four or six atoms.

Stereoselective synthesis of alkyl-, aryl-, vinyl- and alkynyl-substituted Z-enamides and enol ethers

Enamides and enol ethers are valuable building blocks in synthetic chemistry, yet their stereoselective synthesis can be challenging. Herein, we report a new stereoselective synthesis of vinyl, aryl, alkynyl, alkyl and thio-substituted Z-enamides and enol ethers based on the use of vinylbenziodoxolone (VBX) reagents. The stable VBX reagents were synthesized by stereoselective addition of N- or O-nucleophiles on the corresponding alkynyl reagents in the presence of a catalytic amount of cesium carbonate. The VBX reagents were used in palladium-catalyzed cross-couplings at room temperature to access Z-enamides and enol ethers.

Cascade reaction based synthetic strategies targeting biologically intriguing indole polycycles

In this review, cascade reaction based synthesis strategies delivering biologically intriguing indole polycycles are presented.

Intramolecular trans-Carbocarbonation of Internal Alkynes by a Cascade of Formal anti-Carbopalladation/Cyclopropanol Opening

An intramolecular cascade reaction consisting of a formal anti-carbopalladation terminated by the ring-opening of a cyclopropanol unit is presented. The products, which involve tetrasubstituted alkene units embedded in an oligocyclic ring system, are generated in moderate to excellent yield. The opening of the cyclopropanol unit leads to a keto group in the γ-position to the emerging double bond.

Palladium-Catalyzed Hydride Addition/C-H Bond Activation Cascade: Cycloisomerization of 1,6-Diynes

The use of ammonium halide salts as metal hydride precursors in a new Pd-catalyzed cycloisomerization of 1,6-diynes, which affords unexplored silylated 2-azafluorenes, is reported. This cascade process includes the addition of a Pd-hydride species to a π-system, intramolecular carbopalladation, and C(sp²)-H bond activation. A variety of functional groups are tolerated, and the synthetic utility of the resulting products has been demonstrated by a series of derivatizations.

Intramolecular Pd-Catalyzed Formal anti-Carboalkoxylation of Alkynes: Access to Tetrasubstituted Enol Ethers

An intramolecular Pd-catalyzed formal anti-carboalkoxylation reaction is presented that provides access to tetrasubstituted enol ethers. The key to success is a cascade consisting of a formal anti-carbopalladation of a carbon-carbon triple bond followed by a nucleophilic attack of a hydroxy group at the emerging vinyl organopalladium species. The desired transformation proceeded smoothly with primary, secondary, and tertiary alcohols, and even with phenols. Depending on the substitution pattern of the enol ethers, a further Tsuji-Trost-type step may occur resulting in oligocyclic ketals.

trans-Carbocarbonation of Internal Alkynes through a Formal anti-Carbopalladation/C-H Activation Cascade

An intramolecular Pd-catalyzed cascade reaction is presented that consists of a formal anti-carbopalladation of a C-C triple bond followed by C-H activation. As a result, oligocyclic ring systems with an embedded tetrasubstituted double bond are formed. The key to success in affording the trans geometry of the emerging double bond are alkyne units with residues that must not undergo β-hydride elimination (e.g., t-butyl or silyl groups). Silyl groups proved to be a perfect handle to further convert the tetrasubstituted alkenes. The evaluation of kinetic data with a deuterium-labeled compound and X-ray analyses of trapped intermediates provided additional insight into the catalytic cycle.