Reconciling the stereochemical course of nucleopalladation with the development of enantioselective wacker-type cyclizations

Asymmetric 1,n-Remote Aminoacetoxylation of Unactivated Internal Alkenes Enabled by Palladium Catalysis

A palladium-catalyzed asymmetric 1,n-remote aminoacetoxylation of cis-alkenes has been developed using PhI(OAc)2 as an oxidant, providing the acetoxylated lactams with excellent enantioselectivities under mild reaction conditions. The sterically hindered pyridine-oxazoline (Pyox) L3 with a tert-butyl group in oxazoline ring and propyl group in C6 position of pyridinyl is vital for the reaction, where the former is good for asymmetric aminopalladation step and the latter for the chain walking process. The enantioenriched lactam products were proven to be good building blocks for the synthesis of azabicycles.

Pd/Cu-Cocatalyzed Enantio- and Diastereodivergent Wacker-Type Dicarbofunctionalization of Unactivated Alkenes

The Wacker and Wacker-type reactions are some of the most fundamental and powerful transformations in organic chemistry for their ability to efficiently produce valuable chemicals. Remarkable progress has been achieved in asymmetric oxy/aza-Wacker-type reactions; however, asymmetric Wacker-type dicarbofunctionalization remains underdeveloped, especially for the concurrent construction of two stereocenters. Herein, we report a Pd/Cu-cocatalyzed enantio- and diastereodivergent Wacker-type dicarbofunctionalization of alkene-tethered aryl triflates with imino esters. A series of 2-indanyl motifs bearing adjacent carbon stereocenters could be easily synthesized in moderate to excellent yields and with good to excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Density functional theory calculations revealed that the origin of diastereoselectivity in this Pd/Cu synergistic catalytic system is jointly determined by both the intermolecular anti-carbopalladation of alkenes and the reductive elimination processes, in accordance with the Curtin-Hammett principle.

Palladium-Catalyzed Oxidant Dependent Switchable Aza-Wacker Cyclization and Oxidative Dimerization of Benzimidates

An oxidant dependent switchable palladium-catalyzed synthesis of 1,3-oxazines and benzohydrazonates (azines) from O-homoallyl benzimidates has been developed. The reaction involved aza-Wacker-type intramolecular cyclization of O-homoallyl benzimidates with Cu(OAc)2 as oxidant under Pd-catalysis to deliver 4-methylene-1,3-oxazines, whereas dimerization of O-homoallyl benzimidates with K2S2O8 as oxidant resulted remarkably in benzohydrazonates (azines). The reaction is atom economic with an easily operational procedure for divergent synthesis of important scaffolds.

Synthesis of 3-Carbonyl Trisubstituted Furans via Pd-Catalyzed Aerobic Cycloisomerization Reaction: Development and Mechanistic Studies

Herein, we report the synthesis of 3-carbonyl-trisubstituted furans via Pd-catalyzed oxidative cycloisomerization reactions of 2-alkenyl-1,3-dicarbonyl scaffolds, using molecular oxygen as the sole oxidant to regenerate active palladium catalytic species, featuring good functional tolerance and mild reaction conditions. Deep investigation of intermediates and transition states of the reaction mechanism were conducted via experimental and DFT studies, providing a detailed mechanistical profile. The new developed methodology presents a greener alternative to Wacker-type cycloisomerizations and avoids the use of stoichiometric amounts of oxidants and strong acid additives.

Salient features of the aza-Wacker cyclization reaction

The intramolecular aza-Wacker reaction has unparalleled potential for the site-selective amination of olefins, but it is perhaps underappreciated relative to other alkene oxidations. The first part of this review makes the distinction between classical and tethered aza-Wacker cyclization reactions and summarizes examples of the latter. The second portion focuses on developments in asymmetric aza-Wacker cyclization technology. The final part of the review summarizes applications of all classes of aza-Wacker cyclization reactions to natural product assembly.

Ni(I)-Catalyzed β,δ-Vinylarylation of γ,δ-Alkenyl α-Cyanocarboxylic Esters via Contraction of Transient Nickellacycles

We disclose a transmetalation-initiated Ni(I)-catalyzed regioselective β,δ-vinylarylation of γ,δ-alkenyl α-cyanocarboxylic esters with vinyl triflates and arylzinc reagents. This reaction proceeds via contraction of six-membered nickellacycles to five-membered nickellacycles to form carbon-carbon bonds at the nonclassical homovicinal sites, and it provides expeditious access to a wide range of complex aliphatic α-cyanoesters, α-cyanocarboxylic acids, dicarboxylic acids, dicarboxylic acid monoamides, monocarboxylic acids, nitriles, and spirolactones. Control, deuterium labeling, and crossover experiments indicate that (i) the nickellacycle contraction occurs by β-H elimination, followed by hydronickellation on transiently formed alkenes, and (ii) the Ni species are stabilized as Ni-enolates.

Palladium-Catalyzed Enantioselective Alkenylation of Enelactams Using a Relay Heck Strategy

In this report, a palladium-catalyzed redox-relay Heck process to access optically active alkenylated α,β-unsaturated lactams is described. Under mild reaction conditions, electron-deficient alkenyl triflates and electron-rich alkenyl iodonium salts undergo enantioselective and site-selective coupling with enelactams to deliver the products in high yields and excellent enantioselectivities. Furthermore, the products allow facile access to natural products such as (+)-calvine and (+)-2-epicalvine in addition to the bioactive molecule aza-goniothalamin.

Enantioselective Markovnikov Addition of Carbamates to Allylic Alcohols for the Construction of α-Secondary and α-Tertiary Amines

Herein we describe the development of a Pd-catalyzed enantioselective Markovnikov addition of carbamates to allylic alcohols for the construction of α-tertiary and α-secondary amines. The reaction affords a range of β-amino alcohols, after reduction of the aldehyde in situ, which contain a variety of functional groups in moderate yields and moderate to good enantioselectivities. These products can be readily oxidized to β-amino acids, valuable building blocks for the synthesis of biologically active compounds. Mechanistic studies indicate that the C-N bond formation occurs via a syn amino-palladation mechanism, an insight which may guide future reaction development given the limited number of enantioselective syntheses of α-tertiary amines.

Enantioselective N-Alkylation of Indoles via an Intermolecular Aza-Wacker-Type Reaction

The development of an intermolecular and enantioselective aza-Wacker reaction is described. Using indoles as the N-source and a selection of alkenols as the coupling partners selective β-hydride elimination toward the alcohol was achieved. This strategy preserves the newly formed stereocenter by preventing the formation of traditionally observed enamine products. Allylic and homoallylic alcohols with a variety of functional groups are compatible with the reaction in high enantioselectivity. Isotopic-labeling experiments support a syn amino-palladation mechanism for this new class of aza-Wacker reactions.

Practical regio- and stereoselective azidation and amination of terminal alkenes

There is significant interest in developing more rapid and efficient production of nitrogen-containing allylic compounds, as widely used in various syntheses. This work reports a variety of allylic azides and allylic amines synthesized by an efficient, new one-pot protocol that employs readily available terminal alkenes as starting materials. This method is highly regio- and stereoselective, affording the linear (E)-isomer, under metal-free conditions. This process tolerates several functional groups including halogen-containing molecules; it is general for azides and amine nucleophiles; and, adducts were obtained in good yields.

Asymmetric Aza-Wacker-Type Cyclization of N-Ts Hydrazine-Tethered Tetrasubstituted Olefins: Synthesis of Pyrazolines Bearing One Quaternary or Two Vicinal Stereocenters

We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group, β-hydride elimination proceeds selectively at the methylene side, thus both diastereomers can be prepared via switching the configuration of the olefin. Furthermore, the product can be converted to a pharmaceutical compound in high yields over three steps.

Renaissance of pyridine-oxazolines as chiral ligands for asymmetric catalysis

Oxazoline-containing ligands have been widely employed in numerous asymmetric catalytic reactions. Pyridine-oxazoline-type ligands, a class of hybrid ligands, were designed earlier than bisoxazoline and phosphine-oxazoline ligands; however, their unique properties have only been discovered recently. Pyridine-oxazoline-type chiral ligands are rapidly becoming popular for use in asymmetric catalysis, especially for several new and efficient asymmetric methodologies. Several types of challenging asymmetric reactions have been discovered recently using pyridine-oxazoline-type ligands showing their special properties and potential for future application in a wide range of new catalytic methodologies. This review provides an overview of this field, with the aim of highlighting both ligand design and synthetic methodology development.

Palladium-Catalyzed Enantioselective Desymmetrizing Aza-Wacker Reaction: Development and Application to the Total Synthesis of (-)-Mesembrane and (+)-Crinane

Reported is an unprecedented catalytic enantioselective desymmetrizing aza-Wacker reaction. In the presence of a catalytic amount of a newly developed Pd(CPA)₂ (MeCN)₂ catalyst (CPA=chiral phosphoric acid), a pyrox ligand, and molecular oxygen, cyclization of properly functionalized prochiral 3,3-disubstituted cyclohexa-1,4-dienes afforded enantioenriched cis-3a-substituted tetrahydroindoles in good yields with excellent enantioselectivities. A cooperative effect between the phosphoric acid and the pyrox ligand ensured efficient transformation. This reaction was tailor-made for Amaryllidaceae and Sceletium alkaloids as illustrated by its application in the development of the concise and divergent total synthesis of (-)-mesembrane and (+)-crinane.

Iridium-Catalyzed Aerobic α,β-Dehydrogenation of γ,δ-Unsaturated Amides and Acids: Activation of Both α- and β-C-H bonds through an Allyl-Iridium Intermediate

Direct aerobic α,β-dehydrogenation of γ,δ-unsaturated amides and acids using a simple iridium/copper relay catalysis system is described. We developed a new strategy that overcomes the challenging issue associated with the low α-acidity of amides and acids. Instead of α-C-H metalation, this reaction proceeds by β-C-H activation, which results in enhanced α-acidity. Conjugated dienamides and dienoic acids were synthesized in excellent yield with this reaction, which uses a simple reaction protocol. Mechanistic experiments suggest a catalyst resting state mechanism in which both α-C-H and β-C-H cleavage is accelerated.

Mechanistic studies: enantioselective palladium(ii)-catalyzed intramolecular aminoarylation of alkenes by dual N–H and aryl C–H bond cleavage

The role of PGA and the origin of the enantioselectivity in the palladium(ii)-catalyzed intramolecular aminoarylation of alkenes.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Recent developments in the use of aza-Heck cyclizations for the synthesis of chiral N-heterocycles

Aza-Heck cyclizations are an emerging method for the construction of chiral N-heterocyclic systems. In these processes, an activated N-O bond replaces the C-X bond (X = halide, OTf) used in conventional Heck reactions, with the associated aza-Pd(ii)-intermediate engaging pendant alkenes in a Heck-like manner. This perspective article commences with an historical overview of the area, which stems from Narasaka's seminal studies using oxime esters as the initiating motif. The scope and mechanism of associated chiral N-heterocyclic methodologies are then outlined, including cascade processes that enable diverse alkene 1,2-carboaminations. The recent emergence of new N-O donors and the realization of highly enantioselective aza-Heck cyclizations are then discussed. Collectively, these studies suggest that the aza-Heck approach can underpin a broad family of redox-neutral and enantioselective C-N bond forming processes.

Enantioselective Narasaka-Heck cyclizations: synthesis of tetrasubstituted nitrogen-bearing stereocenters

The first examples of highly enantioselective Narasaka-Heck cyclizations are described. A SPINOL-derived P,N-ligand system enables Pd-catalyzed 5-exo cyclization of a range of oxime esters with sterically diverse trisubstituted alkenes to generate dihydropyrroles containing tetrasubstituted nitrogen-bearing stereocenters in 56 to 86% yield and 90 : 10 to 95 : 5 e.r. These processes are rare examples of reactions that proceed via enantioselective migratory insertion of alkenes into Pd-N bonds, and the first where trisubstituted alkenes are used to generate tetrasubstituted stereocenters with high enantioselectivity.

Metal- and Reagent-Free Intramolecular Oxidative Amination of Tri- and Tetrasubstituted Alkenes

A metal- and reagent-free, electrochemical intramolecular oxidative amination reaction of tri- and tetrasubstituted alkenes has been developed. The electrosynthetic method proceeds through radical cyclization to form the key C-N bond, allowing a variety of hindered tri- and tetrasubstituted olefins to participate in the amination reaction. The result is the efficient synthesis of a host of alkene-bearing cyclic carbamates and ureas and lactams.

Synthesis of Secondary Unsaturated Lactams via an Aza-Heck Reaction

The preparation of unsaturated secondary lactams via the palladium-catalyzed cyclization of O-phenyl hydroxamates onto a pendent alkene is reported. This method provides rapid access to a broad range of lactams that are widely useful building blocks in alkaloid synthesis. Mechanistic studies support an aza-Heck-type pathway.

Synthesis of 2,3-Dihydrobenzofurans via the Palladium Catalyzed Carboalkoxylation of 2-Allylphenols

A new Pd-catalyzed alkene carboalkoxylation strategy for the preparation of 2,3-dihydrobenzofurans is described. This method effects the coupling of readily available 2-allylphenol derivatives with aryl triflates to generate a wide range of functionalized 2,3-dihydrobenzofurans in good yields and diastereoselectivities (up to >20:1). Use of newly developed reaction conditions that promote anti-heteropalladation of the alkene is essential in order to generate products in high yield.

Asymmetric Palladium-Catalyzed Alkene Carboamination Reactions for the Synthesis of Cyclic Sulfamides

The synthesis of cyclic sulfamides by enantioselective Pd-catalyzed alkene carboamination reactions between N-allylsulfamides and aryl or alkenyl bromides is described. High levels of asymmetric induction (up to 95:5 e.r.) are achieved using a catalyst composed of [Pd2 (dba)3 ] and (S)-Siphos-PE. Deuterium-labelling studies indicate the reactions proceed through syn-aminopalladation of the alkene and suggest that the control of syn- versus anti-aminopalladation pathways is important for asymmetric induction.

Diazafluorenone-Promoted Oxidation Catalysis: Insights into the Role of Bidentate Ligands in Pd-Catalyzed Aerobic Aza-Wacker Reactions

2,2'-Bipyridine (bpy), 1,10-phenanthroline (phen) and related bidentate ligands often inhibit homogeneous Pd-catalyzed aerobic oxidation reactions; however, certain derivatives, such as 4,5-diazafluoren-9-one (DAF), can promote catalysis. In order to gain insight into this divergent ligand behavior, eight different bpy- and phen-derived chelating ligands have been evaluated in Pd(OAc)₂-catalyzed oxidative cyclization of (E)-4-hexenyltosylamide. Two of the ligands, DAF and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me₂bpy), support efficient catalytic turnover, while the others strongly inhibit the reaction. DAF is especially effective and is the only ligand that exhibits "ligand-accelerated catalysis". Evidence suggests that the utility of DAF and 6,6'-Me₂bpy originates from the ability of these ligands to access κ¹-coordination modes via dissociation of one of the pyridyl rings. This hemilabile character is directly observed by NMR spectroscopy upon adding one equivalent of pyridine to solutions of 1:1 L/Pd(OAc)₂ (L = DAF and 6,6'-Me₂bpy), and is further supported by an X-ray crystal structure of Pd(py)(κ¹-DAF)OAc₂. DFT computational studies illuminate the influence of three different chelating ligands [DAF, 6,6'-Me₂bpy, and 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂phen)] on the energetics of the aza-Wacker reaction pathway. The results show that DAF and 6,6'-Me₂bpy destabilize the corresponding ground-state Pd(N~N)(OAc)₂ complexes, while stabilizing the rate-limiting transition state for alkene insertion into a Pd-N bond. Interconversion between κ²- and κ¹-coordination modes facilitate access to open coordination sites at the Pd^II center. The insights from these studies introduce new ligand concepts that could promote numerous other classes of Pd-catalyzed aerobic oxidation reaction.

Copper-Catalyzed Intramolecular Oxidative Amination of Unactivated Internal Alkenes

A copper-catalyzed oxidative amination of unactivated internal alkenes has been developed. The Wacker-type oxidative alkene amination reaction is traditionally catalyzed by a palladium through a mechanism involving aminopalladation and β-hydride elimination. Replacing the precious and scarce palladium with a cheap and abundant copper for this transformation has been challenging because of the difficulty associated with the aminocupration of internal alkenes. The combination of a simple copper salt, without additional ligand, as the catalyst and Dess-Martin periodinane as the oxidant, promotes efficiently the oxidative amination of allylic carbamates and ureas bearing di- and trisubstituted alkenes leading to oxazolidinones and imidazolidinones. Preliminary mechanistic studies suggested a hybrid radical-organometallic mechanism involving an amidyl radical cyclization to form the key C-N bond.

Enantioselective Dehydrogenative Heck Arylations of Trisubstituted Alkenes with Indoles to Construct Quaternary Stereocenters

An enantioselective, intermolecular dehydrogenative Heck arylation of trisubstituted alkenes to construct remote quaternary stereocenters has been developed. Using a new chiral pyridine oxazoline ligand, good to high enantioselectivity is achieved for various combinations of indole derivatives and trisubstituted alkenes. However, some combinations of substrates led to lower enantioselectivity, which provided the impetus to use structure enantioselectivity correlations to design a better performing ligand.

Regioselective aerobic oxidative Heck reactions with electronically unbiased alkenes: efficient access to α-alkyl vinylarenes

Branched-selective oxidative Heck coupling reactions have been developed between arylboronic acids and electronically unbiased terminal alkenes. The reactions exhibit high catalyst-controlled regioselectivity favoring the less common branched isomer. The reactions employ a catalyst composed of Pd(TFA)2/dmphen (TFA = trifluoroacetate, dmphen = 2,9-dimethyl-1,10-phenanthroline) and proceed efficiently at 45-60 °C under 1 atm of O2 without requiring other additives. A broad array of functional groups, including aryl halide, allyl silane and carboxylic acids are tolerated.