Olefin Amine (OLA) Reagents for the Synthesis of Bridged Bicyclic and Spirocyclic Saturated N-Heterocycles by Catalytic Hydrogen Atom Transfer (HAT) Reactions

Synthesis of Primary Amines via Hydrogen Atom Transfer-Initiated Cyclization/Reduction Cascade of Unsaturated Nitriles

We report a hydrogen atom transfer-initiated cyclization/reduction cascade for the synthesis of primary amines from δ,ε- and ε,ζ-unsaturated nitriles. The HAT transformation employs Mn(acac)3 as a catalyst and utilizes air as an oxidant along with NaBH4 as a dual-purpose reductant toward the olefin and subsequently C═N. Aromatic and aliphatic nitriles incorporating mono-, di-, and trisubstituted olefins are substrates for the reaction. Starting materials bearing malonates are transformed into the corresponding bicyclic lactams, enabling the rapid buildup of structural complexity.

Intramolecular Carboamination of Aminodienes to N-Heterocycles via C-N Bond Activation

The catalytic transformation of ubiquitous but inert C-N bonds is highly appealing in synthetic chemistry, but the efficient cleaving inert C-N bond and simultaneous incorporation of both the cleaved C-moiety and N-moiety into the desired products has been a long-standing formidable challenge so far. Here, we developed a radical-addition triggered cyclization and C-N bond cleavage process enabled by the unique I2 /Ni or benzyl halide/Ni-catalytic system, allowing the formal insertion of diene into the inert C-N bond. This reaction features high atom economy and enables an expedient annulative carboamination of aminodienes to diverse pyrrolidines, piperidines, and tetrahydroisoquinolines. Mechanistic studies have revealed that the reaction is initiated via the generation of a benzyl radical and the formation of quaternary ammonium salt is key for the C-N bond cleavage.

Iron-Mediated Hydrogen Atom Transfer Radical Cyclization of Alkenyl Indoles and Pyrroles Gives Their Fused Derivatives: Total Synthesis of Bruceolline E and H

The iron-mediated hydrogen atom transfer (HAT) reaction is efficaciously employed for the synthesis of dihydropyrroloindoles and dihydropyrrolizines via 5-exo-trig radical cyclization where indoles and pyrroles are used as an acceptor. This radical approach has also been extended for the synthesis of tetrahydrocyclopenta[b]indolones via the Baldwin-disfavored 5-endo-trig cyclization pathway. The formal synthesis of bruceolline J and the total synthesis of bruceollines E and H have been expeditiously carried out by employing the former strategy.

Electrochemical synthesis of spirocyclic morpholines and tetrahydrofurans via an oxidative dearomatisation strategy

Simple and scalable electrochemical oxidation of the electron-rich benzene ring followed by intramolecular capture of reactive cation-radical intermediates opens access to spirocyclic morpholines and tetrahydrofurans. The obtained molecules can be readily modified to value-added building blocks.

Isocyanides as Acceptor Groups in MHAT Reactions with Unactivated Alkenes

The use of isocyanides as acceptor groups in metal-hydride hydrogen atom transfer (MHAT) coupling reactions with nonactivated alkenes to form heterocycles is described. Monosubstituted alkenes couple and cyclize directly, whereas more substituted alkenes proceed via a two-step, one-pot procedure involving MHAT reductive cyclization followed by a MHAT Minisci coupling upon the addition of acid. To highlight the utility of the methodology, a diverse variety of substituted heterocycles such as phenanthridines, indoles, and isoquinolines were prepared.

Stereospecific Cu(I)-Catalyzed C-O Cross-Coupling Synthesis of Acyclic 1,2-Di- and Trisubstituted Vinylic Ethers from Alcohols and Vinylic Halides

CuI and trans-N,N'-dimethylcyclohexyldiamine catalyze the single-step C-O bond cross-coupling between 1,2-di- and trisubstituted vinylic halides with functionalized alcohols, producing acyclic vinylic ethers. This stereospecific transformation selectively gives each of the (E)- and (Z)-vinylic ether products from the corresponding vinyl halide precursors. This method is compatible with carbohydrate-derived primary and secondary alcohols and several other functional groups. The conditions are mild enough to reliably generate vinylic allylic ethers without promoting Claisen rearrangements.

Rhodium-catalyzed diastereoselective synthesis of highly substituted morpholines from nitrogen-tethered allenols

Rhodium-catalyzed intramolecular cyclization of nitrogen-tethered allenols was investigated for the synthesis of functionalized morpholines. By using this strategy, various N-protected 2,5- and 2,6-disubstituted as well as 2,3,5- and 2,5,6-trisubstituted morpholines were obtained via an atom-economic pathway with high to excellent yields, diastereo- and enantioselectivities (up to 99% yield, up to >99 : 1 dr and up to >99.9 ee). The utilities of the synthesized morpholines in ozonolysis, hydration, metathesis and epoxidation reactions were also investigated.

Construction of oxygenated 2-azabicyclo[2.2.1]heptanes via palladium-catalyzed 1,2-aminoacyloxylation of cyclopentenes

Herein, we describe a palladium-catalyzed 1,2-aminoacyloxylation of cyclopentenes to synthesize oxygenated 2-azabicyclo[2.2.1]heptanes. This reaction proceeds efficiently with a broad array of substrates. The products could be further functionalized to build up a library of bridged aza-bicyclic structures.

Aldehydes as O-Nucleophiles in Cobalt Hydride Hydrogen Atom Transfer Catalysis: Overriding the Innate Somophilicity

In metal hydride-catalyzed alkene hydrofunctionalization reactions via hydrogen atom transfer, simple carbonyl groups have been well-recognized as good somophiles at the carbon for C-C bond formation. Here we report an alternative pathway exploring the carbonyl as an O-nucleophile to make new C-O bonds during the CoH-catalyzed oxidative cyclization of alkenyl aldehydes. This reaction provides a rapid, mild, modular, and stereoselective (up to >20:1) entry to saturated O-heterocycles via nucleophilic trapping of an in situ-formed oxocarbenium intermediate. The key to overriding the carbonyl's innate somophilicity was found to be promoting the formation of organocobalt species and suppressing the radical exchange.

Enantioselective Syntheses of 2-Azabicyclo[2.2.1]heptanes via Brønsted Acid Catalyzed Ring-Opening of meso-Epoxides

A chiral phosphoric acid-catalyzed ring-opening of meso-epoxides was developed. A range of 2-azabicyclo[2.2.1]heptanes were obtained in high yields with excellent enantioselectivities. In addition, the hydroxyl and amide groups in the products provided handles for further derivatization.

Multicomponent Direct Assembly of N-Heterospirocycles Facilitated by Visible-Light-Driven Photocatalysis

N-heterospirocycles are interesting structural units found in both natural products and medicinal compounds but have relatively few reliable methods for their synthesis. Here, we enlist the photocatalytic generation of N-centered radicals to construct β-spirocyclic pyrrolidines from N-allylsulfonamides and alkenes. A variety of β-spirocyclic pyrrolidines have been constructed, including drug derivatives, in moderate to very good yields. Further derivatization of the products has also been demonstrated as has a viable scale-up procedure, making use of flow chemistry techniques.

2-Oxa-5-azabicyclo[2.2.1]heptane as a Platform for Functional Diversity: Synthesis of Backbone-Constrained γ-Amino Acid Analogues

We communicate a versatile synthetic approach to C-3 disubstituted 2-oxa-5-azabicyclo[2.2.1]heptanes as carbon-atom bridged morpholines, starting with 4R-hydroxy-l-proline as a chiron. Attaching an acetic acid moiety on the C-3 carbon of the 2-oxa-5-azabicyclo[2.2.1]heptane core reveals the framework of an embedded γ-amino butyric acid (GABA). Variations in the nature of the substituent on the tertiary C-3 atom with different alkyls or aryls led to backbone-constrained analogues of the U.S. Food and Drug Administration-approved drugs baclofen and pregabalin.

CoH-catalyzed radical hydroalkylation of alkenes with 1,3-dicarbonyls

Metal-hydride hydrogen atom transfer (MHAT) catalysis has emerged as a useful reaction platform for alkene hydrofunctionalization with high chemoselectivity and predictable branched selectivity. However, MHAT-mediated hydrofunctionalization involves carbon-carbon bond formation still confined to carbon electrophiles. Here, we describe a mild, general, scalable, and functional group tolerant CoH-catalyzed intermolecular hydroalkylation of alkenes with 1,3-dicarbonyls. This kind of CoH-catalyzed coupling of alkenes with carbon nucleophiles represents an important complement to the arsenal of MHAT-initiated hydrofunctionalization of alkenes.

Scalable Total Synthesis of (-)-Triptonide: Serendipitous Discovery of a Visible-Light-Promoted Olefin Coupling Initiated by Metal-Catalyzed Hydrogen Atom Transfer (MHAT)

An efficient and scalable total synthesis of (-)-triptonide is accomplished based on a metal-catalyzed hydrogen atom transfer (MHAT)-initiated radical cyclization. During the optimization of the key step, we discovered that blue LEDs significantly promoted the efficiency of reaction initiated by Co(TPP)-catalyzed MHAT. Further exploration and optimization of this catalytic system led to development of a dehydrogenative MHAT-initiated Giese reaction.

SOMOphilic Alkynylation of Unreactive Alkenes Enabled by Iron-Catalyzed Hydrogen Atom Transfer

We report an efficient and practical iron-catalyzed hydrogen atom transfer protocol for assembling acetylenic motifs into functional alkenes. Diversities of internal alkynes could be obtained from readily available alkenes and acetylenic sulfones with excellent Markovnikov selectivity. An iron hydride hydrogen atom transfer catalytic cycle was described to clarify the mechanism of this reaction.

Fe-catalyzed Fukuyama-type indole synthesis triggered by hydrogen atom transfer

Fe, Co, and Mn hydride-initiated radical olefin additions have enjoyed great success in modern synthesis, yet the extension of other hydrogen radicalophiles instead of olefins remains largely elusive. Herein, we report an efficient Fe-catalyzed intramolecular isonitrile-olefin coupling reaction delivering 3-substituted indoles, in which isonitrile was firstly applied as the hydrogen atom acceptor in the radical generation step by MHAT. The protocol features low catalyst loading, mild reaction conditions, and excellent functional group tolerance.

Palladium-Catalyzed Ring-Closing Reaction for the Synthesis of Saturated N-Heterocycles with Aminodienes and N,O-Acetals

An efficient palladium-catalyzed ring-closing reaction of aminodienes with N,O-acetals for the synthesis of saturated N-heterocycles is described. The reaction is consistently operated at room temperature and tolerates a wide range of functional groups with volatile MeOH as the sole byproduct. This method provides rapid and practical access to a broad range of saturated N-heterocycles with diverse structural backbones that are useful building blocks in natural product synthesis and drug discovery.

Concise Unified Access to (-)-8-Deoxy-13-dehydroserratinine, (+)-Fawcettimine, (+)-Fawcettidine, and (-)-8-Deoxyserratinine Using a Direct Intramolecular Reductive Coupling

A short, scalable, and collective total synthesis of four fawcettimine-type Lycopodium alkaloids in eight or nine steps is disclosed. A dense multi-small-ring spiro-α-aminocyclopentanone successfully served as the key intermediate, which was directly accessed by a LiDBB-mediated intramolecular reductive coupling of the aliphatic imine and an ester-carbonyl. Compared to those that employ classical Heathcock intermediate(s) containing a nine-membered ring, the new strategy shows the significant improvement of the synthetic step and redox economies as well as excellent stereochemical control.

Cobalt-Catalyzed Markovnikov-Selective Radical Hydroacylation of Unactivated Alkenes with Acylphosphonates

Acylphosphonates having the 5,5-dimethyl-1,3,2-dioxophosphinanyl skeleton are developed as efficient intermolecular radical acylation reagents, which enable the cobalt-catalyzed Markovnikov hydroacylation of unactivated alkenes at room temperature under mild conditions. The protocol exhibits broad substrate scope and wide functional group compatibility, providing branched ketones in satisfactory yields. A mechanism involving the Co-H mediated hydrogen atom transfer and subsequent trapping of alkyl radicals by acylphosphonates is proposed.

Iron-catalyzed hydrogen atom transfer induced cyclization of 1,6-enynes for the synthesis of ketoximes: a combined experimental and computational study

An iron-catalyzed reductive radical cyclization/hydro-oximation of 1,6-enynes with tBuONO was developed, leading to functionalized benzofuran, benzothiophene, and cyclopentenyl-based ketoximes.

SLAP reagents for the photocatalytic synthesis of C3/C5-substituted, N-unprotected selenomorpholines and 1,4-selenazepanes

Herein, we disclose the first set of unique selenium-containing SLAP (SiLicon Amine Protocol) reagents for the direct synthesis of C3/C5-substituted selenomorpholines and 1,4-selenazepanes from diverse (hetero)aldehydes under mild photocatalytic conditions. Enantiomerically pure 1,2-amino alcohol/α-amino acid versions of these heterocycles were also synthesized. Further, we have shown the late-stage modification of certain biologically active agents using the developed seleno-SLAP reagents.

Asymmetric "Clip-Cycle" Synthesis of Pyrrolidines and Spiropyrrolidines

The development of an asymmetric "clip-cycle" synthesis of 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines, which are increasingly important scaffolds in drug discovery programs, is reported. Cbz-protected bis-homoallylic amines were activated by "clipping" them to thioacrylate via an alkene metathesis reaction. Enantioselective intramolecular aza-Michael cyclization onto the activated alkene, catalyzed by a chiral phosphoric acid, formed a pyrrolidine. The reaction accommodated a range of substitutions to form 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines with high enantioselectivities. The importance of the thioester activating group was demonstrated by comparison to ketone and oxoester-containing substrates. DFT studies supported the aza-Michael cyclization as the rate- and stereochemistry-determining step and correctly predicted the formation of the major enantiomer. The catalytic asymmetric syntheses of N-methylpyrrolidine alkaloids (R)-irnidine and (R)-bgugaine, which possess DNA binding and antibacterial properties, were achieved using the "clip-cycle" methodology.

A modular and divergent approach to spirocyclic pyrrolidines

An efficient three-step sequence to afford a valuable class of spirocyclic pyrrolidines is reported. A reductive cleavage/Horner-Wadsworth-Emmons cascade facilitates the spirocyclisation of a range of isoxazolines bearing a distal β-ketophosphonate. The spirocyclisation precursors are elaborated in a facile and modular fashion, via a [3 + 2]-cycloaddition followed by the condensation of a phosphonate ester, introducing multiple points of divergence. The synthetic utility of this protocol has been demonstrated in the synthesis of a broad family of 1-azaspiro[4,4]nonanes and in a concise formal synthesis of the natural product (±)-cephalotaxine.

Catalyst- and Silane-Controlled Enantioselective Hydrofunctionalization of Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover

The catalytic enantioselective synthesis of tetrahydrofurans, which are found in the structures of many biologically active natural products, via a transition-metal-catalyzed hydrogen atom transfer (TM-HAT) and radical-polar crossover (RPC) mechanism is described herein. Hydroalkoxylation of nonconjugated alkenes proceeded efficiently with excellent enantioselectivity (up to 94% ee) using a suitable chiral cobalt catalyst, N-fluoro-2,4,6-collidinium tetrafluoroborate, and diethylsilane. Surprisingly, the absolute configuration of the product was highly dependent on the steric hindrance of the silane. Slow addition of the silane, the dioxygen effect on the solvent, thermal dependence, and DFT calculation results supported the unprecedented scenario of two competing selective mechanisms. For the less-hindered diethylsilane, a high concentration of diffused carbon-centered radicals invoked diastereoenrichment of an alkylcobalt(III) intermediate by a radical chain reaction, which eventually determined the absolute configuration of the product. On the other hand, a more hindered silane resulted in less opportunity for a radical chain reaction, instead facilitating enantioselective kinetic resolution during the late-stage nucleophilic displacement of the alkylcobalt(IV) intermediate.

Catalytic Dealkylative Synthesis of Cyclic Carbamates and Ureas via Hydrogen Atom Transfer and Radical-Polar Crossover

Guided by the transition-metal hydrogen atom transfer and radical-polar crossover concepts, we developed a functional-group-tolerant and scalable method for the synthesis of cyclic carbamates and ureas, which are found in the structures of bioactive compounds. This method provides not only a common five-membered ring but also six-to-eight-membered ring products. The reaction proceeds through the intramolecular displacement of an alkylcobalt(IV) intermediate and dealkylation by 2,4,6-collidine; the activation energies of these steps were calculated by DFT.

Recent Developments in C–C Bond Formation Using Catalytic Reductive Coupling Strategies

Metal-catalyzed reductive coupling processes have emerged as a powerful methodology for the introduction of molecular complexity from simple starting materials. These methods allow for an orthogonal approach to that of redox-neutral strategies for the formation of C–C bonds by enabling cross-coupling of starting materials not applicable to redox-neutral chemistry. This short review summarizes the most recent developments in the area of metal-catalyzed reductive coupling utilizing catalyst turnover by a stoichiometric reductant that becomes incorporated in the final product.

1 Introduction

2 Ni Catalysis

3 Cu Catalysis

4 Ru, Rh, and Ir Catalysis

4.1 Alkenes

4.2 1,3-Dienes

4.3 Allenes

4.4 Alkynes

4.5 Enynes

5 Fe, Co, and Mn Catalysis

6 Conclusion and Outlook

A Cascade Reaction of Cinnamyl Azides with Acrylates Directly Generates Tetrahydro-Pyrrolo-Pyrazole Heterocycles

Developing reactions to generate complex and modular building blocks in a concise and direct fashion remains a contemporary synthetic challenge. This work describes a stereoselective cascade reaction between allylic azides and acrylates that directly generates tetrahydro-pyrrolo-pyrazole ring systems. These products contain up to four contiguous stereocenters, two of which may be tetrasubstituted carbon atoms attached to a nitrogen atom. Over 30 examples are provided with an average isolated yield of 71% (ranging from 40% to 94%). The reaction was easily scaled to use more than one gram of starting material, and the products can be readily diversified.

Transition-metal-free C(sp3)-H/C(sp3)-H dehydrogenative coupling of saturated heterocycles with N-benzyl imines

A unique C(sp3)-H/C(sp3)-H dehydrocoupling of N-benzylimines with saturated heterocycles is described. Using super electron donor (SED) 2-azaallyl anions and aryl iodides as electron acceptors, single-electron-transfer (SET) generates an aryl radical. Hydrogen atom transfer (HAT) from saturated heterocycles or toluenes to the aryl radical generates alkyl radicals or benzylic radicals, respectively. The newly formed alkyl radicals and benzylic radicals couple with the 2-azaallyl radicals with formation of new C-C bonds. Experimental evidence supports the key hydrogen-abstraction by the aryl radical, which determines the chemoselectivity of the radical-radical coupling reaction. It is noteworthy that this procedure avoids the use of traditional strong oxidants and transition metals.

SnAP reagents for the synthesis of selenomorpholines and 1,4-selenazepanes and their biological evaluation

Herein, we disclose the first set of unique selenium-containing SnAP reagents for the direct synthesis of C-substituted selenomorpholines and 1,4-selenazepanes, including their amino acid derivatives from commercially available aldehydes under mild conditions. These elusive N-unprotected heterocycles are not accessible by classical routes. Biological evaluation of these compounds revealed promising activities against clinically relevant fungal strains.

Designing the recognition of Sn2+ ions and antioxidants: N-heterocyclic organosilatranes and their magnetic nanocomposites

The present work involves synthesis of new series of N-heterocyclic unsymmetrical organosilatranes and the synthesized compounds were explored for the detection of Sn²⁺, cytotoxicity and antioxidant activity.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.

Synthesis of Unprotected Spirocyclic β-Prolines and β-Homoprolines by Rh-Catalyzed C-H Insertion

A series of unprotected spirocyclic β-prolines and β-homoprolines are prepared by Rh-catalyzed C-H insertion. The key intermediate, a Rh nitrenoid, is generated by the N-O bond cleavage of a substituted isoxazolidin-5-one. The reaction proceeds on a gram scale with a catalyst loading of as little as 0.1 mol %, affording spirocyclic β-amino acids that are otherwise difficult to obtain. The building blocks prepared in this work will likely find applications in medicinal chemistry.

Alkyl Radical Addition to Aliphatic and Aromatic N-Acylhydrazones Using an Organic Photoredox Catalyst

Increased versatility of intermolecular radical addition to imino acceptors via photoredox catalysis is reported. Primary and secondary radicals, generated via visible-light photocatalysis from alkyl biscatecholatosilicates with organocatalyst 4CzIPN, add successfully to both aromatic and aliphatic N-acylhydrazones in the presence of MgCl2. With N-benzoylhydrazones, a simple reductive cleavage of the N-N bond of the hydrazine adduct furnishes the free amine. Synthetic utility is exemplified in a synthetic application toward repaglinide, a clinically important hypoglycemic agent.

Cycloaddition Strategies for the Synthesis of Diverse Heterocyclic Spirocycles for Fragment-Based Drug Discovery

In recent years the pharmaceutical industry has benefited from the advances made in fragment-based drug discovery (FBDD) with more than 30 fragment-derived drugs currently marketed or progressing through clinical trials. The success of fragment-based drug discovery is entirely dependent upon the composition of the fragment screening libraries used. Heterocycles are prevalent within marketed drugs due to the role they play in providing binding interactions; consequently, heterocyclic fragments are important components of FBDD libraries. Current screening libraries are dominated by flat, sp2-rich compounds, primarily owing to their synthetic tractability, despite the superior physicochemical properties displayed by more three-dimensional scaffolds. Herein, we report step-efficient routes to a number of biologically relevant, fragment-like heterocyclic spirocycles. The use of both electron-deficient and electron-rich 2-atom donors was explored in complexity-generating [3+2]-cycloadditions to furnish products in 3 steps from commercially available starting materials. The resulting compounds were primed for further fragment elaboration through the inclusion of synthetic handles from the outset of the syntheses.