Formaldehyde N,N-Dialkylhydrazones as Neutral Formyl Anion Equivalents in Iridium-Catalyzed Asymmetric Allylic Substitution

Chemoselective and Stereoselective Allylation of Bis(alkenyl)boronates

Bis(alkenyl)boronates react with optically active Ir(π-allyl) species in a process that involves allylation of the more substituted olefin and 1,2-metalate shift of the less substituted olefin. The method constructs valuable enantioenriched tertiary allylic boronic esters with high chemoselectivity, enantioselectivity and diastereoselectivity. Allylic functionalization reactions transform the 1,3-stereodiad to 1,5- and 1,6-stereochemical relationships.

Development of Immobilized Carreira (Phosphoramidite, Olefin) Ligands and Application in Iridium-Catalyzed Asymmetric Allylic Amination

A family of polystyrene-supported (phosphoramidite, olefin) ligands L1-L4, based on the original design by Defieber and Carreira, has been developed and applied in iridium-catalyzed asymmetric allylic amination of unmasked allylic alcohols (27 examples, up to 99% ee). Among them, functional resins L1 and L4 exhibit important advantages such as easy preparation, ligand recyclability, and easy handling for sequential use. As a distinctive advantage, the catalytic use of the iridium complexes of L1 and L4 allows the straightforward reuse of a high percentage of the expensive iridium metal involved in the complexes, which is not achievable under homogeneous conditions with the corresponding monomeric complexes.

Green protocols for Tsuji–Trost allylation: an overview

Since its inception in 1960s, the Tsuji–Trost reaction, an allylic substitution reaction with diverse nucleophiles such as phenols, amines, thiols, and active methylene compounds, has remained as one of the most useful and widely used organic reactions for the construction of C–C and C–heteroatom bonds. Allylic compounds such as allylic acetates, alcohols, halides, and carbonates undergo this transformation which plays an important role in the total synthesis of various natural products. The competence to incorporate synthetically demanding allylic functionalities makes it a beneficial tool for the synthesis of complex molecules. Over the last two decades, major advancements for this unique and facile Tsuji–Trost allylation reaction have been made with special emphasis to develop greener and sustainable protocols. This chapter presents an update on the significant progress focusing on the newly designed catalytic systems with high efficiency, the use of eco-friendly solvents or solvent-free conditions, low or room temperature conditions and waste management, along with future outlook.

Photoinduced Copper-Catalyzed Asymmetric Acylation of Allylic Phosphates with Acylsilanes

We report a visible-light-induced copper-catalyzed highly enantioselective umpolung allylic acylation reaction with acylsilanes as acyl anion equivalents. Triplet-quenching experiments and DFT calculations supported our reaction design, which is based on copper-to-acyl metal-to-ligand charge transfer (MLCT) photoexcitation that generates a charge-separated triplet state as a highly reactive intermediate. According to the calculations, the allylic phosphate substrate in the excited state undergoes novel molecular activation into an allylic radical weakly bound to the copper complex. The allyl radical fragment undergoes copper-mediated regio- and stereocontrolled coupling with the acyl group under the influence of the chiral N-heterocyclic carbene ligand.

Asymmetric Brønsted base-catalyzed aza-Michael addition and [3 + 2] cycloaddition reactions of N-ester acylhydrazones and enones

We reported the first enantioselective Brønsted base-catalyzed aza-Michael addition and stepwise cascade [3+2] cycloaddition between acylhydrazones and unsaturated ketones. The enantio-enriched aza-Michael adducts can be transformed into 2-pyrazolines.

Palladium-catalyzed allylic alkylation of hydrazones with hydroxy-tethered allyl carbonates: synthesis of functionalized hydrazones

Pd-catalyzed allylic alkylation of hydroxy-tethered allyl carbonates and hydrazones worked well without an external base to afford various E configurations of functionalized hydrazones, which were successfully transformed into pyridazines.

Ruthenium- and Copper-Catalyzed Propargylic Substitution Reactions of Propargylic Alcohol Derivatives with Hydrazones

Ruthenium- and copper-catalyzed propargylic substitution reactions of propargylic alcohol derivatives with N-monosubstituted hydrazones as ambident nucleophiles are achieved in which N-monosubstituted hydrazones exhibit impressive different reactivities depending on different catalytic systems, behaving as carbon-centered nucleophiles to give the corresponding propargylic alkylated products in ruthenium catalysis, or as nitrogen-centered nucleophiles to afford the corresponding propargylic aminated products in copper catalysis. DFT calculations were carried out to investigate the detailed reaction pathways of these two systems. Further transformation of propargylic substituted products affords the corresponding multisubstituted pyrazoles as cyclization products in good to high yields.

Iridium-Catalyzed Asymmetric Allylic Alkylation of Deconjugated Butyrolactams

Compared with the ever-growing list of nonprochiral nucleophiles in Ir-catalyzed asymmetric allylic substitution reactions, prochiral nucleophiles are less studied. We present a new prochiral nucleophile, namely, deconjugated butyrolactam, for Ir-catalyzed asymmetric allylic alkylation (AAA). This reaction provides access to α-allyl deconjugated butyrolactams with a moderate to good dr and an excellent er. This is the first AAA reaction of deconjugated butyrolactams. Allyl transposition through Cope rearrangement appears to proceed stereospecifically to form γ-allyl conjugated butyrolactams.

Nickel-catalyzed enantioselective vinylation of aryl 2-azaallyl anions

A unique enantioselective nickel-catalyzed vinylation of 2-azaallyl anions is advanced for the first time. This method affords diverse vinyl aryl methyl amines with high enantioselectivities, which are frequently occurring scaffolds in natural products and medications. This C-H functionalization method can also be extended to the synthesis of enantioenriched 1,3-diamine derivatives by employing suitably elaborated vinyl bromides. Key to the success of this process is the identification of a Ni/chiraphos catalyst system and a less reducing 2-azaallyl anion, all of which favor an anionic vinylation route over a background radical reaction. A telescoped gram scale synthesis and a product derivatization study confirmed the scalability and synthetic potential of this method.

Enantioselective Allylation of Alkenyl Boronates Promotes a 1,2-Metalate Rearrangement with 1,3-Diastereocontrol

Alkenyl boronates add to Ir(π-allyl) intermediates with high enantioselectivity. A 1,2-metalate shift forms a second C-C bond and sets a 1,3-stereochemical relationship. The three-component coupling provides tertiary boronic esters that can undergo multiple additional functionalizations. An extension to trisubstituted olefins sets three contiguous stereocenters.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

N,N-Dialkylhydrazones as Versatile Umpolung Reagents in Enantioselective Anion-Binding Catalysis

An enantioselective anion-binding organocatalytic approach with versatile N,N-dialkylhydrazones (DAHs) as polarity-reversed (umpolung) nucleophiles is presented. For the application of this concept, a highly ordered hydrogen-bond (HB) network between a carefully selected CF3 -substituted triazole-based multidentate HB-donor catalyst, the ionic substrate and the hydrazone in a supramolecular chiral ion-pair complex was envisioned. The formation of such a network was further supported by both experimental and computational studies, which showed the crucial role of the anion as a template unit. The asymmetric Reissert-type reaction of quinolines as a model test reaction chemoselectively delivered highly enantiomerically enriched hydrazones (up 95:5 e.r.) that could be further derivatized to value-added compounds with up to three stereocenters.

Stereodivergent Carbon-Carbon Bond Formation between Iminium and Enolate Intermediates by Synergistic Organocatalysis

We report here a stereodivergent method for the Michael addition of aryl acetic acid esters to α,β-unsaturated aldehydes catalyzed by a combination of a chiral pyrrolidine and a chiral Lewis base. This reaction proceeds through a synergistic catalytic cycle which consists of one cycle leading to a chiral iminium electrophile and a second cycle generating a nucleophilic chiral enolate for the construction of a carbon-carbon bond. By varying the combinations of catalyst enantiomers, all four stereoisomers of the products with two vicinal stereocenters are accessible with high enantio- and diastereoselectivity. The products of the Michael addition, 1,5-aldehyde esters, can be readily transformed into a variety of other valuable enantioenriched structures, including those bearing three contiguous stereocenters in an acyclic system, thus providing an efficient route to an array of structural and stereochemical diversity.

Formaldehyde tert-butyl hydrazone as a formyl anion equivalent: asymmetric addition to carbonyl compounds

The asymmetric 1,2-addition of formyl anion equivalents to carbonyl compounds is a powerful synthetic tool that ideally provide access to highly functionalizable α-hydroxy aldehydes in an enantioselective fashion. In this context, the nucleophilic character of formaldehyde hydrazones, together with their remarkable stability as monomeric species, has been exploited for the functionalization of diverse carbonyl compounds, using initially auxiliary-based methodologies and, more recently, catalytic enantioselective versions. This feature article highlights our research progress employing formaldehyde tert-butyl hydrazone as a versatile formyl anion equivalent, in combination with bifunctional H-bonding organocatalysis. The design and optimization of different catalytic systems, focusing on a dual activation of both reagents, is reviewed, as well as the racemization free unmasking of the formyl group and representative product transformations for the construction of valuable, densely functionalyzed chiral building blocks.

Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected Allenylic Amines Employing Ammonia Surrogates

The first iridium catalyzed, enantioconvergent amination of allenylic carbonates is reported. This process utilizes various commercially available carbamates and sulfonamides to generate allenylic amines including commonly employed protected groups (Boc, Fmoc, Cbz, Ts, Ns) in 62-82 % yield and 87-98 % ee. The products generated through this scalable procedure serve as effective linchpins for the rapid, enantiospecific synthesis of a wide range of complex structures.

Catalytic Asymmetric Addition Reactions of Formaldehyde N,N-Dialkylhydrazone to Synthesize Chiral Nitrile Derivatives

A number of nitrile-containing chiral molecules were synthesized via asymmetric nucleophilic addition of formaldehyde N,N-dialkylhydrazone as the nitrile equivalent. Chiral N,N'-dioxide/metal salt complexes enabled the asymmetric addition reactions to both isatin-derived imines and α,β-unsaturated ketones, generating amino nitriles and 4-oxobutanenitrile derivatives in good yields with high enantioselectivities. This protocol was highlighted by avoiding the use of toxic nitrile reagents, wide substrate scope, and versatile transformations of chiral hydrazone adducts into other valuable molecules.

Iridium-Catalyzed Asymmetric Synthesis of Functionally Rich Molecules Enabled by (Phosphoramidite,Olefin) Ligands

The catalytic, asymmetric synthesis of complex molecules has been a core focus of our research program for some time because developments in the area can have an immediate impact on the identification of novel strategies for the synthesis of value-added molecules. In concert with this central interest, we have emphasized the design of ligand scaffolds as a tactic to discover and develop novel chemistry and overcome well-recognized synthetic challenges. Based on our group's work on chiral pool-derived diolefin ligands, we designed and implemented a class of hybrid (phosphoramidite,olefin) ligands, which combines the properties of both phosphoramidite and olefin motifs to impact, fine-tune, and even override the inherent reactivity of the metal center. Specifically, we have utilized these unique modifying ligands to address several recognized limitations in the field of iridium-catalyzed, asymmetric allylic substitution. The methods we have documented typically employ branched, unprotected allylic alcohols as substrates and obviate the need for rigorous exclusion of air and moisture. Following Takeuchi's seminal report demonstrating the high aptitude of Ir(I)-phosphite catalysts for  branch-selective allylic substitution, concerted efforts from numerous research laboratories have led to a broadening of the synthetic utility of this reaction class. The first section of this Account outlines the process leading to our discovery of an unprecedented (phosphoramidite,olefin) ligand and its validation in the first iridium-catalyzed amination of branched, unprotected allylic alcohols. This section continues with our work involving heteroatom-based nucleophiles within inter- and intramolecular etherification, thioetherification and spiroketalization processes. The second section highlights the use of readily available carbon nucleophiles possessing sp, sp², and sp³ hybridization in a series of enantioselective carbon-carbon bond-forming reactions. We describe how alkylzinc, allylsilane, and several classes of organotrifluoroborate nucleophiles can be coupled enantioselectively to enable construction of several key motifs including 1,5-dienes, 1,4-dienes, and 1,4-enynes. Since the unique electronic and steric properties of this class of ligands renders the (η³-allyl)-Ir(III) intermediate highly electrophilic, even weak nucleophiles such as alkyl olefins can be used. We also show that more nucleophilic alkene motifs such as enamines and in situ generated ketene acetals smoothly participate in substitution reactions with allylic alcohols to yield valuable piperidines and γ,δ-unsaturated esters, respectively. The concept of stereodivergent dual catalysis, which synergistically combines chiral amine catalysis with iridium catalysis to furnish α-allylated aldehydes containing two independently controllable stereocenters is then discussed. This process has enabled the independent, stereoselective synthesis of all four possible product stereoisomers from a single set of starting materials, and was highlighted in the stereodivergent synthesis of Δ⁹-tetrahydrocannabinol. This Account concludes with an overview of our organometallic mechanistic studies regarding relevant intermediates within the catalytic cycle of this class of allylic substitution. These studies have allowed us to better understand the origin of the unique characteristics exhibited by this catalyst in comparison to related systems.

Photoredox Decarboxylative C(sp3)-N Coupling of α-Diazoacetates with Alkyl N-Hydroxyphthalimide Esters for Diversified Synthesis of Functionalized N-Alkyl Hydrazones

Herein we report a metal-free photocatalytic coupling reaction for the synthesis of structurally and functionally diverse N-alkyl hydrazones from α-diazoacetates and N-alkyl hydroxyphthalimide esters. By employing Rose Bengal as a photocatalyst with yellow LEDs irradiation, over 60 N-alkyl hydrazones were synthesized. Fluorescence quenching analysis and deuterium incorporation experiments reveal that Hantzsch ester serves as both an electron donor and proton source for the reaction. This strategy offers a simple retrosynthetic disconnection for conventionally inaccessible C(sp³)-rich N-alkyl hydrazones.

Iridium-Catalyzed Enantioselective Allylation of Aryl Enamides and Enecarbamates

Aromatic enamide and enecarbamate as a novel type of nucleophile in the asymmetric allylation of branched, racemic allylic alcohols to give homoallylic ketones has been described. Enabled by Carreira's chiral Ir (P, olefin) complex, the reactions proceed in good yields with excellent enantioselectivities.

Highly Diastereo- and Enantioselective Synthesis of Quinuclidine Derivatives by an Iridium-Catalyzed Intramolecular Allylic Dearomatization Reaction

Asymmetric construction of quinuclidine derivatives has been realized by an iridium-catalyzed allylic dearomatization reaction. The catalytic system, derived from [Ir(cod)Cl] 2 and the Feringa ligand, tolerates a broad range of substrates. A large array of quinuclidine derivatives can be obtained under mild conditions in good to excellent yields (68%–96%), diastereoselectivity (up to >20/1 dr), and enantioselectivity (up to >99% ee). These products feature versatile functional group diversity and can undergo diverse transformations. A model that accounts for the origin of the stereoselectivity has been proposed based on density functional theory (DFT) calculations.

Iridium-catalyzed intramolecular enantioselective allylation of quinazolin-4(3H)-one derivatives

An efficient chiral cyclic phosphoramidite ligand-enabled iridium-catalyzed intramolecular allylation of quinazolin-4(3H)-one derivatives has been developed with high reactivity and high to excellent enantioselectivity.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Intermolecular Stereoselective Iridium-Catalyzed Allylic Alkylation: An Evolutionary Account

Our lab has long been interested in the development of methods for the creation of enantioenriched all-carbon quaternary stereocenters. Historically, our interest has centered on palladium-catalyzed allylic alkylation, though recent efforts have moved to include the study of iridium catalysts. Whereas palladium catalysts enable the preparation of isolated stereocenters, the use of iridium catalysts allows for the direct construction of vicinal stereocenters via an enantio-, diastereo-, and regioselective allylic alkylation. This account details the evolution of our research program from inception, which focused on the first iridium-catalyzed allylic alkylation to prepare stereodyads containing a single quaternary center, to our most recent discovery that allows for the synthesis of vicinal quaternary centers.

Iridium-catalyzed enantioselective direct vinylogous allylic alkylation of coumarins

The first iridium-catalyzed enantioselective vinylogous allylic alkylation of coumarins is presented. Using easily accessible linear allylic carbonates as the allylic electrophile, this reaction installs unfunctionalized allyl groups at the γ-position of 4-methylcoumarins in an exclusively branched-selective manner generally in high yields with an excellent level of enantioselectivity (up to 99 : 1 er).

Cu-catalyzed enantioselective synthesis of tertiary benzylic copper complexes and their in situ addition to carbonyl compounds

Catalytic chemo- and enantioselective generation of tertiary benzylic copper complexes from Cu-B(pin) (pin = pinacolato) additions to 1,1-disubstituted alkenes followed by in situ reactions with ketones and carboxylic acid phenol esters to construct multifunctional alkylboron compounds that contain quaternary stereogenic centers is presented. The method is distinguished by the unprecedented reaction mode of tertiary benzylic Cu complexes, allowing reaction with a wide range of carbonyl electrophiles in good yields and with high chemo-, site-, diastereo- and enantioselectivity. The catalytic protocol was performed with easily accessible chiral ligands and copper salts at ambient temperature. Functionalization of multifunctional alkylboron products provides useful building blocks that are otherwise difficult to access.

Bifunctional Squaramide Organocatalysts for the Asymmetric Addition of Formaldehyde tert-Butylhydrazone to Simple Aldehydes

The nucleophilic addition of formaldehyde tert-butylhydrazone to simple aldehydes (a formal hetero-carbonyl-ene reaction) can be performed with good reactivity and excellent enantioselectivity by virtue of the dual hydrogen-bonding activation exerted by amide-squaramide organocatalysts. The resulting hydroxydiazenes (azo alcohols) were isolated in high yields as enantiomerically enriched azoxy compounds after a regioselective azo-to-azoxy transformation. Subsequent derivatization provides an entry to relevant amino alcohols, oxazolidinones, and derivatives thereof.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.