Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes: A Mechanistic View

Metal π-Lewis base activation in palladium(0)-catalyzed trans-alkylative cyclization of alkynals

The Pd(0)-mediated umpolung reaction of an alkyne to achieve trans-difunctionalization is a potential synthetic methodology, but its insightful activation mechanism of Pd(0)-alkyne interaction has yet to be established. Here, a Pd(0)-π-Lewis base activation mode is proposed and investigated by combining theoretical and experimental studies. In this activation mode, the Pd(0) coordinates to the alkyne group and enhances its nucleophilicity through π-back-donation, facilitating the nucleophilic attack on the aldehyde to generate a trans-Pd(ii)-vinyl complex. Ligand-effect studies reveal that the more electron-donating one would accelerate the reaction, and the cyclization of the challenging flexible C- or O-tethered substrates has been realized. The origin of regioselectivities is also explicated by the newly proposed metal π-Lewis base activation mode.

Metal-Free, PPA-Mediated Fisher Indole Synthesis via Tandem Hydroamination-Cyclization Reaction between Simple Alkynes and Arylhydrazines

A new variant of Fisher indole synthesis involving Bronsted acid-catalyzed hydrohydrazination of unactivated terminal and internal acetylenes with arylhydrazines is reported. The use of polyphosphoric acid alone either as the reaction medium or in the presence of a co-solvent appears to provide the required balance for activating the C-C triple bond towards the nucleophilic attack of the hydrazine moiety without unrepairable reactivity loss of the latter due to competing amino group protonation. Additionally, the formal hydration of acetylenes to the corresponding ketones occurs under the same conditions, making it an alternative approach for generating carbonyl groups from alkynes.

Catalytic Asymmetric Aza-Electrophilic Additions of 1,1-Disubstituted Styrenes

Electrophilic addition of alkenes is a textbook reaction that plays a pivotal role in organic chemistry. In the past decades, catalytic asymmetric variants of this important type of reaction have witnessed great achievements by the development of novel catalytic systems. However, enantioselective aza-electrophilic additions of unactivated alkenes, which could provide a transformative strategy for the preparation of synthetically significant nitrogen-containing compounds, still remain a formidable challenge. Herein, we have developed unprecedented Au(I)/NHC-catalyzed asymmetric aza-electrophilic additions of unactivated 1,1-disubstituted styrenes by the utilization of readily available dialkyl azodicarboxylates as electrophilic nitrogen sources. Based on this approach, a series of transformations, including [2 + 2] cycloaddition, intermolecular 1,2-oxyamination, and several types of intramolecular hydrazination-induced cyclizations, have been realized. These transformations provide a previously unattainable platform for the divergent synthesis of hydrazine derivatives, which could also be converted to other nitrogen-containing chiral synthons. Experimental and computational studies support the idea that carbocation intermediates are involved in reaction pathways.

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

ConspectusChemists have long been inspired by biological photosynthesis, wherein a series of excited-state electron transfer (ET) events facilitate the conversion of low energy starting materials such as H2O and CO2 into higher energy products in the form of carbohydrates and O2. While this model for utilizing light-driven charge transfer to drive catalytic reactions thermodynamically "uphill" has been extensively adapted for small molecule activation, molecular machines, photoswitches, and solar fuel chemistry, its application in organic synthesis has been less systematically developed. However, the potential benefits of these approaches are significant, both in enabling transformations that cannot be readily achieved using conventional thermal chemistry and in accessing distinct selectivity regimes that are uniquely enabled by excited-state mechanisms. In this Account, we present work from our group that highlights the ability of visible light photoredox catalysis to drive useful organic transformations away from their equilibrium positions, addressing a number of long-standing synthetic challenges.We first discuss how excited-state ET enabled the first general methods for the catalytic anti-Markovnikov hydroamination of unactivated alkenes with alkyl amines. In these reactions, an excited-state iridium(III) photocatalyst reversibly oxidizes secondary amine substrates to their corresponding aminium radical cations (ARCs). These electrophilic N-centered radicals can then react with olefins to furnish valuable tertiary amine products with complete anti-Markovnikov regioselectivity. Notably, some of these products are less thermodynamically stable than their corresponding amine and alkene starting materials. We next present a strategy for light-driven C-C bond cleavage within various aliphatic alcohols mediated by homolytic activation of alcohol O-H bonds by excited-state proton-coupled electron transfer (PCET). The resulting alkoxy radical intermediates then undergo C-C β-scission to ultimately provide isomeric linear carbonyl products that are often higher in energy than their cyclic alcohol precursors. Applications of this chemistry for the light-driven depolymerization of lignin biomass, commercial phenoxy resin, hydroxylated polyolefin derivatives, and thermoset polymers are presented as well. We then describe a method for the contrathermodynamic positional isomerization of highly substituted olefins by means of cooperative photoredox and chromium(II) catalysis. In this work, generation of an allylchromium(III) species that can undergo highly regioselective in situ protodemetalation enables access to a less substituted and thermodynamically less stable positional isomer. Product selectivity in this reaction is determined by the large differential in oxidation potentials between differently substituted olefin isomers. Lastly, we discuss a light-driven deracemization reaction developed in collaboration with the Miller group, wherein a racemic urea substrate undergoes spontaneous optical enrichment upon visible light irradiation in the presence of an iridium(III) chromophore, a chiral Brønsted base, and a chiral peptide thiol. Excellent levels of enantioselectivity are achieved via sequential and synergistic proton transfer (PT) and H atom transfer (HAT) steps. Taken together, these examples highlight the ability of excited-state ET events to enable access to nonequilibrium product distributions across a wide range of catalytic, redox-neutral transformations in which photons are the only stoichiometric reagents.

Recent advances in C(sp3)-N bond formation via metallaphoto-redox catalysis

The C(sp3)-N bond is ubiquitous in natural products, pharmaceuticals, biologically active molecules and functional materials. Consequently, the development of practical and efficient methods for C(sp3)-N bond formation has attracted more and more attention. Compared to the conventional ionic pathway-based thermal methods, photochemical processes that proceed through radical mechanisms by merging photoredox and transition-metal catalyses have emerged as powerful and alternative tools for C(sp3)-N bond formation. In this review, recent advances in the burgeoning field of C(sp3)-N bond formation via metallaphotoredox catalysis have been highlighted. The contents of this review are categorized according to the transition metals used (copper, nickel, cobalt, palladium, and iron) together with photocatalysis. Emphasis is placed on methodology achievements and mechanistic insight, aiming to inspire chemists to invent more efficient radical-involved C(sp3)-N bond-forming reactions.

Two-step synthesis of vicinal trifluoromethyl primary amines from α-(trifluoromethyl)styrenes and phthalimide

A novel two-step synthesis of β-trifluoromethyl primary amines from readily available α-(trifluoromethyl)styrenes and phthalimide is developed. The first step involves a hydroamination between α-(trifluoromethyl)styrenes and phthalimide (PhthNH) with the assistance of a base. Next, the hydrazinolysis of the resulting N-(β-trifluoromethyl-β-arylethyl)phthalimides with hydrazine hydrate affords the desired N-(β-trifluoromethyl-β-arylethyl)amines.

Acetate Assistance in Regioselective Hydroamination of Allenamides: A Combined Experimental and Density Functional Theory Study

A key factor in the development of selective nucleophilic addition to allenamides is controlling the reactivity of electrophilic intermediates, which is generally achieved using an electrophilic activator via conjugated iminium intermediates. In this combined experimental and computational study, we show that a general and highly chemoselective hydroamination of allenamides can be accomplished using a combination of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and NaOAc. Experimental mechanistic studies revealed that HFIP mediates proton transfer to activate the allenamide, while the acetate additive significantly contributes to N-selective interception. This strategy enables a general hydroamination of allenamides without the use of metals. We demonstrated that various functionalized 1,3-diamines could be readily synthesized and diversified into value-added structural motifs. Detailed mechanistic investigations using the density functional theory revealed the role of NaOAc in the formation of reactive electrophilic intermediates, which ultimately governed the selective formation of 1,3-diamine products. Critically, calculations of the potential energy surface around the proton-transfer transition state revealed that two different reactive electrophilic intermediates were formed when NaOAc was added.

Rhodium-Catalyzed Asymmetric Macrocyclization towards Crown Ethers Using Hydroamination of Bis(allenes)

Enantiomerically enriched crown ethers (CE) exhibit strong asymmetric induction in phase transfer catalysis, supramolecular catalysis and molecular recognition processes. Traditional methods have often been used to access these valuable compounds, which limit their diversity and consequently their applicability. Herein, a practical catalytic method is described for the gram scale synthesis of a class of chiral CEs (aza-crown ethers; ACEs) using Rh-catalyzed hydroamination of bis(allenes) with diamines. Using this approach, a wide range of chiral vinyl functionalized CEs with ring sizes ranging from 12 to 36 have been successfully prepared in high yields of up to 92 %, dr of up to >20 : 1 and er of up to >99 : 1. These vinyl substituted CEs allow for further diversification giving facile access to various CE derivatives as well as to their three-dimensional analogues using ring-closing metathesis. Some of these chiral CEs themselves display high potential for use in asymmetric catalysis.

Aromatic Amine and Chiral Phosphoric Acid Synergistic Catalyzed Cascade Reaction of Alkynylnaphthols with Aldehydes

A novel approach using aromatic amines and chiral phosphoric acids in a synergistic catalytic cascade reaction of 2-alkynylnaphthols with aldehydes has been established. This method offers a direct route to preparing flavanone analogues with excellent stereoselectivity. Mechanistic studies reveal a sequential process involving addition, elimination, cyclization, and hydrolysis in which aromatic amines and chiral phosphoric acids play key roles via imine-enamine and hydrogen bonding models.

Cobalt-catalyzed synthesis of amides from alkenes and amines promoted by light

Catalytic methods to couple alkene and amine feedstocks are valuable in synthetic chemistry. The direct carbonylative coupling of alkenes and amines holds promise as a perfectly atom-economical approach to amide synthesis, but general methods remain underdeveloped. Herein, we report an alkene hydroaminocarbonylation catalyzed by unmodified, inexpensive cobalt carbonyl under mild conditions and low pressure promoted by light. Silane addition after the reaction enables sequential cobalt-catalyzed amide reduction, constituting a formal alkene hydroaminomethylation. These methods exhibit exceptional scope across both alkene and amine components with high chemo- and regioselectivity and proceed efficiently even in the absence of solvent. The formation of a hydridocobalt through photodissociation of a carbonyl ligand is proposed to enable catalytic activity under mild conditions, which addresses a long-standing challenge in catalysis.

Generation of Aromatic N-Heterocyclic Radicals for Functionalization of Unactivated Alkenes

Nitrogen-centered radicals (NCRs) have been widely recognized as versatile synthetic intermediates for the construction of nitrogen containing molecules of high value. As such, there has been a long-standing interest in the field of organic synthesis to develop novel nitrogen-based radicals and explore their inherent reactivity. In this study, we present the generation of aromatic N-heterocyclic radicals and their application in a novel and diverse functionalization of unactivated alkenes. Bench-stable aromatic N-heterocyclic pyridinium salts were employed as crucial NCR precursors, which enabled the efficient conversion of various unactivated alkenes into medicinally relevant alkylated N-heterocyclic amines. This approach offers an unexplored retrosynthetic disconnection for the synthesis of related molecules that commonly possess therapeutic value. Furthermore, this platform can be extended to the synthesis of densely functionalized heterocyclic amines by utilizing disulfides and diethyl bromomalonate as radical quenchers. Mechanistic investigations indicate an energy transfer (EnT) pathway involving the formation of a transient aromatic N-heterocyclic radical, radical addition to unactivated alkenes, and subsequent generation of the amination product through either hydrogen atom transfer (HAT) or radical addition processes.

Photocatalytic Anti-Markovnikov Hydroamination of Alkenes with Primary Heteroaryl Amines

We report a light-driven method for the intermolecular anti-Markovnikov hydroamination of alkenes with primary heteroaryl amines. In this protocol, electron transfer between an amine substrate and an excited-state iridium photocatalyst affords an aminium radical cation (ARC) intermediate that undergoes C-N bond formation with a nucleophilic alkene. Integral to reaction success is the electronic character of the amine, wherein increasingly electron-deficient heteroaryl amines generate increasingly reactive ARCs. Counteranion-dependent reactivity is observed, and iridium triflate photocatalysts are employed in place of conventional iridium hexafluorophosphate complexes. This method exhibits broad functional group tolerance across 55 examples of N-alkylated products derived from pharmaceutically relevant heteroaryl amines.

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.