Α-amino Radicals Research Articles

Direct Acylation of C(sp(3))-H Bonds Enabled by Nickel and Photoredox Catalysis.

Using nickel and photoredox catalysis, the direct functionalization of C(sp(3))-H bonds of N-aryl amines by acyl electrophiles is described. The method affords a diverse range of α-amino ketones at room temperature and is amenable to late-stage coupling of complex and biologically relevant groups. C(sp(3))-H activation occurs by photoredox-mediated oxidation to generate α-amino radicals which are intercepted by nickel in catalytic C(sp(3))-C coupling. The merger of these two modes of catalysis leverages nickel's unique properties in alkyl cross-coupling while avoiding limitations commonly associated with transition-metal-mediated C(sp(3))-H activation, including requirements for chelating directing groups and high reaction temperatures.

Photoredox-Generated α-Amino Radicals Used in (Hetero)Arylation

Photoredox-Generated α-Amino Radicals Used in (Hetero)Arylation

Brønsted acid-catalysed conjugate addition of photochemically generated α-amino radicals to alkenylpyridines.

The conjugate addition of α-amino radicals to alkenylpyridines has been accomplished by the synergistic merger of Brønsted acid and visible light photoredox catalysis. Key to reaction development was the protonation of the alkenylpyridines that transiently generated a highly reactive, electrophilic pseudo-iminium ion intermediate. Initial investigations using chiral phosphoric acids provide clues on the feasibility of an enantioselective catalytic variant.

Umpolung synthesis of branched α-functionalized amines from imines via photocatalytic three-component reductive coupling reactions.

A three component reductive coupling reaction of a (hetero)aromatic amine, a (hetero)aromatic aldehyde and an electron deficient olefin catalysed by eosin Y under green LED light irradiation, for the direct generation of γ-amino acid derivatives, is described. This new umpolung synthesis of amines, which exploits the high nucleophilicity of a putative α-amino radical intermediate, generated via single electron reduction of the in situ generated imine from the Hantzsch ester terminal reductant, is efficient, operationally simple, broad in scope and offers a complementary strategy to existing synthetic approaches.

Dual Catalytic Decarboxylative Allylations of α-Amino Acids and Their Divergent Mechanisms.

The room temperature radical decarboxylative allylation of N-protected α-amino acids and esters has been accomplished via a combination of palladium and photoredox catalysis to provide homoallylic amines. Mechanistic investigations revealed that the stability of the α-amino radical, which is formed by decarboxylation, dictates the predominant reaction pathway between competing mechanisms.

Selective Radical-Radical Cross-Couplings: Design of a Formal β-Mannich Reaction.

A direct β-coupling of cyclic ketones with imines has been accomplished via the synergistic combination of photoredox catalysis and organocatalysis. Transient β-enaminyl radicals derived from ketones via enamine and oxidative photoredox catalysis readily combine with persistent α-amino radicals in a highly selective hetero radical-radical coupling. This novel pathway to γ-aminoketones is predicated upon the use of DABCO as both a base and an electron transfer agent. This protocol also formally allows for the direct synthesis of β-Mannich products via a chemoselective three-component coupling of aryl aldehydes, amines, and ketones.

Amine Functionalization via Oxidative Photoredox Catalysis: Methodology Development and Complex Molecule Synthesis.

ConspectusWhile the use of visible light to drive chemical reactivity is of high importance to the development of environmentally benign chemical transformations, the concomitant use of a stoichiometric electron donor or acceptor is often required to steer the desired redox behavior of these systems. The low-cost and ubiquity of tertiary amine bases has led to their widespread use as reductive additives in photoredox catalysis. Early use of trialkylamines in this context was focused on their role as reductive excited state quenchers of the photocatalyst, which in turn provides a more highly reducing catalytic intermediate.In this Account, we discuss some of the observations and thought processes that have led from our use of amines as reductive additives to their use as complex substrates and intermediates for natural product synthesis. Early attempts by our group to construct key carbon–carbon bonds via free-radical intermediates led to the observation that some trialkylamines readily behave as efficient hydrogen atom donors under redox-active photochemical conditions. In the wake of in-depth mechanistic studies published in the 1970s, 1980s and 1990s, this understanding has in turn allowed for a systematic approach to the design of a number of photochemical methodologies through rational tuning of the amine component. Minimization of the C–H donicity of the amine additive was found to promote desired C–C bond formation in a number of contexts, and subsequent elucidation of the amine’s redox fate has sparked a reevaluation of the amine’s role from that of reagent to that of substrate.The reactivity of tertiary amines in these photochemical systems is complex, and allows for a number of mechanistic possibilities that are not necessarily mutually exclusive. A variety of combinations of single-electron oxidation, C–H abstraction, deprotonation, and β-scission result in the formation of reactive intermediates such as α-amino radicals and iminium ions. These processes have been explored in depth in the photochemical literature and have resulted in a firm mechanistic grasp of the behavior of amine radical cations in fundamental systems. Harnessing the synthetic potential of these transient species represents an ongoing challenge for the controlled functionalization of amine substrates, because these mechanistic possibilities may result in undesired byproduct formation or substrate decomposition. The presence of tertiary amines in numerous alkaloids, pharmaceuticals, and agrochemicals lends credence to the potential utility of this chemistry in natural product synthesis, and herein we will discuss how these transformations might be controlled for synthetic purposes.

Enantioselective conjugate additions of α-amino radicals via cooperative photoredox and Lewis acid catalysis.

We report the highly enantioselective addition of photogenerated α-amino radicals to Michael acceptors. This method features a dual-catalyst protocol that combines transition metal photoredox catalysis with chiral Lewis acid catalysis. The combination of these two powerful modes of catalysis provides an effective, general strategy to generate and control the reactivity of photogenerated reactive intermediates.

SET-promoted photoaddition reactions of N-α-trimethylsilylmethyl-N,N-dibenzylamines with fullerene C60. Electronic factors that govern photoaddition efficiencies

Single electron transfer (SET) promoted photoaddition reactions of N-α-trimethylsilylmethyl-N,N-dibenzylamines to fullerene C60 were investigated as part of an effort aimed at developing a general method to prepare various aryl ring containing aminomethylfullerenes and exploring factors that govern photoaddition efficiencies. The results show that the photoaddition reactions take place highly efficiently to form 1,2-adducts. The mechanism for this process involves generation of aminium radicals and C60 anion radical intermediates by a pathway initiated by excited state SET. SET is followed by desilylation of the aminium radicals to produce α-amino radicals that couple with either C60 or its radical anion to form precursors of the 1,2-adducts. The electronic nature of para-substituents on the aryl ring of N-α-trimethylsilylmethyl-N,N-dibenzylamines has a pronounced effect on the efficiency of the photoaddition reaction, with electron donating groups causing a greater than 2-fold enhancement compared to that brought about by electron withdrawing groups.

Photoredox α-vinylation of α-amino acids and N-aryl amines.

A new coupling protocol has been developed that allows the union of vinyl sulfones with photoredox-generated α-amino radicals to provide allylic amines of broad diversity. Direct C–H vinylations of N-aryl tertiary amines, as well as decarboxylative vinylations of N-Boc α-amino acids, proceed in high yield and with excellent olefin geometry control. The utility of this new allyl amine forming reaction has been demonstrated via the syntheses of several natural products and a number of established pharmacophores.

Method for the synthesis of amine-functionalized fullerenes involving SET-promoted photoaddition reactions of α-silylamines.

A novel method for the preparation of structurally diverse fullerene derivatives, which relies on the use of single electron transfer (SET)-promoted photochemical reactions between fullerene C60 and α-trimethylsilylamines, has been developed. Photoirradiation of 10% EtOH-toluene solutions containing C60 and α-silylamines leads to high-yielding, regioselective formation of 1,2-adducts that arise through a pathway in which sequential SET-desilylation occurs to generate α-amino and C60 anion radical pair intermediates, which undergo C-C bond formation. Protonation of generated α-aminofullerene anions gives rise to formation of monoaddition products that possess functionalized α-aminomethyl-substituted 1,2-dihydrofullerene structures. Observations made in this effort show that the use of EtOH in the solvent mixture is critical for efficient photoproduct formation. In contrast to typical thermal and photochemical strategies devised previously for the preparation of fullerene derivatives, the new photochemical approach takes place under mild conditions and does not require the use of excess amounts of substrates. Thus, the method developed in this study could broaden the scope of fullerene chemistry by providing a simple photochemical strategy for large-scale preparation of highly substituted fullerene derivatives. Finally, the α-aminomethyl-substituted 1,2-dihydrofullerene photoadducts are observed to undergo photoinduced fragmentation reactions to produce C60 and the corresponding N-methylamines.

Electro-catalytic degradation mechanism of nitenpyram in synthetic wastewater using Ti-based SnO2–Sb with rare earth-doped anode

Nitenpyram (NIT), a kind of pyridine-based neonicotinoid insecticide, presents a high threat to water system. This work firstly examined the electro-catalytic (EC) degrading the NIT in synthetic wastewater by using the modified Ti/SnO2–Sb anode, which was characterized by scanning electronic microscopy, X-ray energy dispersive spectroscopy, X-ray diffraction, and electrochemical measurement. The NIT removal and total organic carbon removal are monitored. The NIT EC treatment was found to obey the pseudo-first-order reaction kinetics within 60 min electrolysis and was controlled by the mass transport, the chemical reaction, and the oxygen evolution. Stable degradation intermediates were detected through electrospray ionization quadrupole time-of-flight tandem mass spectrometry. Experiments indicated that several pyridine derivatives form during the EC process. Furthermore, the cleavage of pyridine ring could be demonstrated, which took place after the hydroxylation of the ring, leading to the complete mineralization of a NIT molecule. HO⋅ and H2O2 played the dominant role in the EC degradation of NIT. The in situ-generating H2O2 was attributed to the reaction between dioxygen and α-amino radical.

ChemInform Abstract: The Chemistry of Amine Radical Cations Produced by Visible Light Photoredox Catalysis

Amine radical cations are highly useful reactive intermediates in amine synthesis. They have displayed several modes of reactivity leading to some highly sought-after synthetic intermediates including iminium ions, α-amino radicals, and distonic ions. One appealing method to access amine radical cations is through one-electron oxidation of the corresponding amines under visible light photoredox conditions. This approach and subsequent chemistries are emerging as a powerful tool in amine synthesis. This article reviews synthetic applications of amine radical cations produced by visible light photocatalysis.

Amine α-heteroarylation via photoredox catalysis: a homolytic aromatic substitution pathway.

The direct α-heteroarylation of tertiary amines has been accomplished via photoredox catalysis to generate valuable benzylic amine pharmacophores. A variety of five-and six-membered chloroheteroarenes are shown to function as viable coupling partners for the α-arylation of a diverse range of cyclic and acyclic amines. Evidence is provided for a homolytic aromatic substitution mechanism, in which a catalyticallygenerated α-amino radical undergoes direct addition to an electrophilic chloroarene.

The chemistry of amine radical cations produced by visible light photoredox catalysis

Amine radical cations are highly useful reactive intermediates in amine synthesis. They have displayed several modes of reactivity leading to some highly sought-after synthetic intermediates including iminium ions, α-amino radicals, and distonic ions. One appealing method to access amine radical cations is through one-electron oxidation of the corresponding amines under visible light photoredox conditions. This approach and subsequent chemistries are emerging as a powerful tool in amine synthesis. This article reviews synthetic applications of amine radical cations produced by visible light photocatalysis.

Brønsted Acid Cocatalysts in Photocatalytic Radical Addition of α-Amino C–H Bonds across Michael Acceptors

In marked contrast to the variety of strategies available for oxidation and nucleophilic functionalization of methylene groups adjacent to amines, relatively few approaches for modification of this position with electrophilic reaction partners have been reported. In the course of an investigation of the reactions of photogenerated α-amino radicals with electrophiles, we made the surprising observation that the efficiency of radical photoredox functionalization of N-aryl tetrahydroisoquinolines is dramatically increased in the presence of a Brønsted acid cocatalyst. Optimized conditions provide high yields and efficient conversion to radical addition products for a range of structurally modified tetrahydroisoquinolines and enones using convenient household light sources and commercially available Ru(bpy)3Cl2 as a photocatalyst. Our investigations into the origins of this unexpected additive effect have demonstrated that the carbon-carbon bond-forming step is accelerated by TFA and is a rare example of Brønsted acid catalysis in radical addition reactions. Moreover, a significant conclusion arising from these studies is the finding that product formation is dominated by radical chain processes and not by photocatalyst turnover. Together, these findings have important implications for the future design and mechanistic evaluation of photocatalytic radical processses.

ChemInform Abstract: Direct α‐Functionalization of Saturated Cyclic Amines

AbstractReview: 126 refs.

Direct α‐Functionalization of Saturated Cyclic Amines

Recent advances in synthetic methods for the direct α-functionalization of saturated cyclic amines are described. Methods are categorized according to the in situ formed reactive intermediate (α-amino cation, α-amino anion, and α-amino radical). Transition-metal-catalyzed reactions involving other intermediates have been treated as a separate and fourth class.

On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation

It is well known that endogenous daylight-absorbing compounds produce the sensitized photodegradation of biologically relevant substrates. In this context the photostability of a mixture of the indole neurotransmitter serotonin (Sero) and vitamin B2 (riboflavin, Rf) upon visible-light irradiation and the possible role of Sero and related compounds as generators or deactivators of reactive oxygen species (ROS) was investigated through a kinetic and mechanistic study. The work was done at pH 7 and under experimental conditions in which only the vitamin absorbs photoirradiation. Tryptamine (Trpa) and 5-hydroxyindole (OHIn) were included in the study as model compounds for the neurotransmitter. The visible light irradiation of aqueous Rf in the individual presence of Sero, Trpa and 5-OHIn, under aerobic conditions, induce degradative processes on the indole derivatives (In-der). At least two different mechanisms operate. Our analysis shows that the main reaction pathway is an electron-transfer-mediated quenching of triplet excited Rf (3Rf*) by the In-der. It produces the species Rf−/RfH and the In-der radical cation that could react to form phenoxy and α-amino radicals. In a further reaction step the species O2- and OH could be produced. In parallel, energy transfer from 3Rf* to dissolved oxygen would generate O2(1Δg).Within the frame of the proposed mechanism, results suggest that Rf-sensitized degradation of Sero occurs via the mentioned ROS and non-oxygenated radical-mediated processes.The indole compound quenches O2(1Δg) in a dominant physical fashion. This fact constitutes a desirable property in antioxidants, provided that the quenching process practically does not eliminate the scavenger.Sero exerts a photoprotective effect towards tryptophan through the combined quenching of O2(1Δg) and 3Rf*, the latter excited species responsible for the generation of ROS. The amino acid can be taken as a target model of oxidizable biological substrates, particularly proteins.

Visible Light Photoredox Catalysis: Generation and Addition of N-Aryltetrahydroisoquinoline-Derived α-Amino Radicals to Michael Acceptors

The photoredox-catalyzed coupling of N-aryltetrahydroisoquinoline and Michael acceptors was achieved using Ru(bpy)(3)Cl(2) or [Ir(ppy)(2)(dtb-bpy)]PF(6) in combination with irradiation at 455 nm generated by a blue LED, demonstrating the trapping of visible light generated α-amino radicals. While intermolecular reactions lead to products formed by a conjugate addition, in intramolecular variants further dehydrogenation occurs, leading directly to 5,6-dihydroindolo[2,1-a]tetrahydroisoquinolines, which are relevant as potential immunosuppressive agents.