Reverse Polarity Reductive Functionalization of Tertiary Amides via a Dual Iridium-Catalyzed Hydrosilylation and Single Electron Transfer Strategy

Visible Light-Induced, Nickel-Catalyzed Late-Stage 4-Alkylation of Hantzsch Esters with Alkyl Bromide

Herein, visible light-induced, nickel-catalyzed direct functionalization of the Hantzsch esters (HEs) with readily accessible alkyl bromides has been successfully achieved by taking advantage of HE as the reductant and substrate through an aromatization-dearomatization process. In this strategy, the single electron reduction of alkyl bromides by reactive Ni(I) species is essential for the success of this late-stage transformation. A wide range of 4-alkyl-1,4-dihydropyridines were rapidly assembled in moderate to good yields under mild conditions, rendering this photoinduced approach attractive for synthetic and medicinal chemistry.

Deoxy-Arylation of Amides via a Tandem Hydrosilylation/Radical- Radical Coupling Sequence

Vaska's complex is a prominent catalyst for the hydrosilylation of amides. The O-silyl hemiaminal intermediate formed in these processes has been demonstrated as an electrophile for nucleophilic additions. More recently, these intermediates have been shown to be suitable for single electron reduction to generate α-amino radicals. Leveraging the ability to generate α-amino radicals from these hemiaminals, we describe a two-step, one-pot, deoxy-arylation of amides utilizing iridium-catalyzed hydrosilylation and photoredox catalysis. This transformation can be tailored toward the late-stage functionalization of biologically relevant molecules, with drug discovery applications as shown in the streamlined synthesis of an NPY Y2 inhibitor.

Asymmetric Deoxygenative Functionalization of Secondary Amides with Vinylpyridines Enabled by a Triple Iridium-Photoredox-Chiral Phosphoric Acid System

An enantioselective deoxygenative functionalization of secondary amides with vinylpridines is developed by merging relay iridium catalysis and cooperative photoredox-chiral Brønsted acid catalysis, affording a series of valuable chiral amines in moderate to good yields with good enantioselectivities. The intriguing multiple catalytic system invoking triple-catalysis was found to be the key to the success of the current reactions, which may stimulate further development of catalytic methodologies for asymmetric deoxygenative transformations of amides.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Deoxygenative Nucleophilic Phosphonation and Electrophilic Alkylation of Secondary Amides: A Facile Access to Quaternary α-Aminophosphonates

The widespread occurrence and synthetic accessibility of amides render them valuable precursors for the synthesis of diverse nitrogen-containing compounds. Herein, we present a metal-free and streamlined synthetic strategy for the synthesis of quaternary α-aminophosphonates. This approach involves sequential deoxygenative nucleophilic phosphonation and versatile electrophilic alkylation of secondary amides in a one-pot fashion. Notably, this method enables the direct bis-functionalization of secondary amides with both nucleophiles and electrophiles for the first time, with simple derivatization leading to valuable free α-aminophosphonates by hydrolysis. The protocol has the advantages of operational simplicity, broad functional-group compatibility, environmental friendliness, and scalability to multigram quantities.

Iridium-Catalyzed Reductive (3+2) Annulation of Lactams Enabling the Rapid Total Synthesis of (±)-Eburnamonine

A reductive (3+2) annulation of lactams through iridium-catalyzed hydrosilylation and photoredox coupling with α-bromoacetic acid was developed. The iridium-catalyzed hydrosilylation of the lactam carbonyl group and subsequent elimination provide a transient cyclic enamine, which undergoes iridium-catalyzed photoredox coupling with α-bromoacetic acid in a one-pot process. The developed conditions show high functional-group tolerance and provide cyclic N,O-acetals containing a quaternary carbon center. The resulting N,O-acetals undergo a variety of acid-mediated nucleophilic addition reactions via iminium ions to give substituted cyclic amines. The developed sequence including reductive (3+2) annulation and acid-mediated nucleophilic addition was successfully applied to the four-step total synthesis of (±)-eburnamonine.

Enantioselective Total Synthesis of (+)-Incargranine A Enabled by Bifunctional Iminophosphorane and Iridium Catalysis

Herein we report the first enantioselective total synthesis of (+)-incargranine A, in nine steps. The total synthesis was enabled by an enantioselective intramolecular organocatalysed desymmetrising Michael addition of a malonamate ester to a linked dienone substrate that established pivotal stereocentres with excellent enantio- and complete diastereoselectivity. Furthermore, a key hemiaminal intermediate was accessed by developing an iridium-catalysed reductive cyclisation, and the scope of this transformation was explored to produce a range of bicyclic hemiaminal motifs. Once installed, the hemiaminal motif was used to initiate a biomimetic cascade to access the natural product directly in a single step.

1,2-Redox Transpositions of Tertiary Amides

Reactions capable of transposing the oxidation levels of adjacent carbon atoms enable rapid and fundamental alteration of a molecule's reactivity. Herein, we report the 1,2-transposition of the carbon atom oxidation level in cyclic and acyclic tertiary amides, resulting in the one-pot synthesis of 1,2- and 1,3-oxygenated tertiary amines. This oxidation level transfer was facilitated by the careful orchestration of an iridium-catalyzed reduction with the functionalization of transiently formed enamine intermediates. A novel 1,2-carbonyl transposition is described, and the breadth of this redox transposition strategy has been further explored by the development of aminoalcohol and enaminone syntheses. The diverse β-functionalized amine products were shown to be multifaceted and valuable synthetic intermediates, accessing challenging biologically relevant motifs.

Enabling Broader Adoption of Biocatalysis in Organic Chemistry

Biocatalysis is becoming an increasingly impactful method in contemporary synthetic chemistry for target molecule synthesis. The selectivity imparted by enzymes has been leveraged to complete previously intractable chemical transformations and improve synthetic routes toward complex molecules. However, the implementation of biocatalysis in mainstream organic chemistry has been gradual to this point. This is partly due to a set of historical and technological barriers that have prevented chemists from using biocatalysis as a synthetic tool with utility that parallels alternative modes of catalysis. In this Perspective, we discuss these barriers and how they have hindered the adoption of enzyme catalysts into synthetic strategies. We also summarize tools and resources that already enable organic chemists to use biocatalysts. Furthermore, we discuss ways to further lower the barriers for the adoption of biocatalysis by the broader synthetic organic chemistry community through the dissemination of resources, demystifying biocatalytic reactions, and increasing collaboration across the field.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Difluoroalkylation of Tertiary Amides and Lactams by an Iridium-Catalyzed Reductive Reformatsky Reaction

An iridium-catalyzed, reductive alkylation of abundant tertiary lactams and amides using 1-2 mol % of Vaska's complex (IrCl(CO)(PPh3)2), tetramethyldisiloxane (TMDS), and difluoro-Reformatsky reagents (BrZnCF2R) for the general synthesis of medicinally relevant α-difluoroalkylated tertiary amines is described. A broad scope (46 examples), including N-aryl- and N-heteroaryl-substituted lactams, demonstrated an excellent functional group tolerance. Furthermore, late-stage drug functionalizations, a gram-scale synthesis, and common downstream transformations proved the potential synthetic relevance of this new methodology.

Hydrosilane-Mediated Electrochemical Reduction of Amides

Electrochemical reduction of amides was achieved by using a hydrosilane without any toxic or expensive metals. The key reactive ketyl radical intermediate was generated by cathodic reduction. Continuous reaction with anodically generated silyl radicals or zinc bromide resulted in chemoselective deoxygenation to give the corresponding amines.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent-controlled stereodivergent carbocyclisation of aryl aldimine-derived, photocatalytically generated, α-amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α-amino radical species underwent a single stereoselective cyclisation to give trans-configured amino-indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis-fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good-to-excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Asymmetric Deoxygenative Alkynylation of Tertiary Amides Enabled by Iridium/Copper Bimetallic Relay Catalysis

A variety of inert tertiary amides have been successfully transformed into synthetically important chiral propargylamines in high yields with good to excellent enantioselectivities via a relayed sequence of Ir catalyzed partial reduction and Cu/GARPHOS catalyzed asymmetric alkynylation with terminal alkynes. The reaction was readily extended to some drug molecules and the transformations of representative products have been demonstrated, thus attesting the practical utilities and the robust nature of the protocol.

General α-Amino 1,3,4-Oxadiazole Synthesis via Late-Stage Reductive Functionalization of Tertiary Amides and Lactams*

An iridium-catalyzed reductive three-component coupling reaction for the synthesis of medicinally relevant α-amino 1,3,4-oxadiazoles from abundant tertiary amides or lactams, carboxylic acids, and (N-isocyanimino) triphenylphosphorane, is described. Proceeding under mild conditions using (<1 mol %) Vaska's complex (IrCl(CO)(PPh3 )2 ) and tetramethyldisiloxane to access the key reactive iminium ion intermediates, a broad range of α-amino 1,3,4-oxadiazole architectures were accessed from carboxylic acid feedstock coupling partners. Extension to α-amino heterodiazole synthesis was readily achieved by exchanging the carboxylic acid coupling partner for C-, S-, or N-centered Brønsted acids, and provided rapid and modular access to these desirable, yet difficult-to-access, heterocycles. The high chemoselectivity of the catalytic reductive activation step allowed late-stage functionalization of 10 drug molecules, including the synthesis of heterodiazole-fused drug-drug conjugates.

A General Iridium-Catalyzed Reductive Dienamine Synthesis Allows a Five-Step Synthesis of Catharanthine via the Elusive Dehydrosecodine

A new reductive strategy for the stereo- and regioselective synthesis of functionalized isoquinuclidines has been developed. Pivoting on the chemoselective iridium(I)-catalyzed reductive activation of β,γ-unsaturated δ-lactams, the efficiently produced reactive dienamine intermediates readily undergo [4 + 2] cycloaddition reactions with a wide range of dienophiles, resulting in the formation of bridged bicyclic amine products. This new synthetic approach was extended to aliphatic starting materials, resulting in the efficient formation of cyclohexenamine products, and readily applied as the key step in the shortest (five-step) total synthesis of vinca alkaloid catharanthine to date, proceeding via its elusive biosynthetic precursor, dehydrosecodine.

General Pyrrolidine Synthesis via Iridium-Catalyzed Reductive Azomethine Ylide Generation from Tertiary Amides and Lactams

An iridium-catalyzed reductive generation of both stabilized and unstabilized azomethine ylides and their application to functionalized pyrrolidine synthesis via [3 + 2] dipolar cycloaddition reactions is described. Proceeding under mild reaction conditions from both amide and lactam precursors possessing a suitably positioned electron-withdrawing or a trimethylsilyl group, using 1 mol% Vaska’s complex [IrCl(CO)(PPh₃)₂] and tetramethyldisiloxane (TMDS) as a terminal reductant, a broad range of (un)stabilized azomethine ylides were accessible. Subsequent regio- and diastereoselective, inter- and intramolecular dipolar cycloaddition reactions with variously substituted electron-deficient alkenes enabled ready and efficient access to structurally complex pyrrolidine architectures. Density functional theory (DFT) calculations of the dipolar cycloaddition reactions uncovered an intimate balance between asynchronicity and interaction energies of transition structures, which ultimately control the unusual selectivities observed in certain cases.

Regio- and Stereoselective (SN2) N-, O-, C- and S-Alkylation Using Trialkyl Phosphates

Bimolecular nucleophilic substitution (SN2) is one of the most well-known fundamental reactions in organic chemistry to generate new molecules from two molecules. In principle, a nucleophile attacks from the back side of an alkylating agent having a suitable leaving group, most commonly a halide. However, alkyl halides are expensive, very harmful, toxic and not so stable, which makes them problematic for laboratory use. In contrast, trialkyl phosphates are inexpensive, readily accessible and stable at room temperature, under air, and are easy to handle, but rarely used as alkylating agents in organic synthesis. Here, we describe a mild, straightforward and powerful method for nucleophilic alkylation of various N-, O-, C- and S-nucleophiles using readily available trialkyl phosphates. The reaction proceeds smoothly in excellent yield, and quantitative yield in many cases, and covers a wide range of substrates. Further, the rare stereoselective transfer of secondary alkyl groups has been achieved with inversion of configuration of chiral centers (up to 98% ee).

Reductive Arylation of Amides via a Nickel-Catalyzed Suzuki-Miyaura-Coupling and Transfer-Hydrogenation Cascade

We report a means to achieve the addition of two disparate nucleophiles to the amide carbonyl carbon in a single operational step. Our method takes advantage of non-precious-metal catalysis and allows for the facile conversion of amides to chiral alcohols via a one-pot Suzuki-Miyaura cross-coupling/transfer-hydrogenation process. This study is anticipated to promote the development of new transformations that allow for the conversion of carboxylic acid derivatives to functional groups bearing stereogenic centers via cascade processes.

K2S2O8 promoted dehydrative cross-coupling between α,α-disubstituted allylic alcohols and thiophenols/thiols

K₂S₂O₈ promoted dehydrative cross-coupling between α,α-disubstituted allylic alcohols and thiophenols/thiols is demonstrated for the first time, leading to a wide range of allyl sulfides in good to high yields.