Highly Efficient and Stereoselective Radical Addition of Tertiary Amines to Electron-Deficient Alkenes − Application to the Enantioselective Synthesis of Necine Bases

Site-Selective α-C-H Functionalization of Trialkylamines via Reversible Hydrogen Atom Transfer Catalysis

Trialkylamines are widely found in naturally occurring alkaloids, synthetic agrochemicals, biological probes, and especially pharmaceuticals agents and preclinical candidates. Despite the recent breakthrough of catalytic alkylation of dialkylamines, the selective α-C(sp³)-H bond functionalization of widely available trialkylamine scaffolds holds promise to streamline complex trialkylamine synthesis, accelerate drug discovery, and execute late-stage pharmaceutical modification with complementary reactivity. However, the canonical methods always result in functionalization at the less-crowded site. Herein, we describe a solution to switch the reaction site through fundamentally overcoming the steric control that dominates such processes. By rapidly establishing an equilibrium between α-amino C(sp³)-H bonds and a highly electrophilic thiol radical via reversible hydrogen atom transfer, we leverage a slower radical-trapping step with electron-deficient olefins to selectively forge a C(sp³)-C(sp³) bond with the more-crowded α-amino radical, with the overall selectivity guided by the Curtin-Hammett principle. This subtle reaction profile has unlocked a new strategic concept in direct C-H functionalization arena for forging C-C bonds from a diverse set of trialkylamines with high levels of site selectivity and preparative utility. Simple correlation of site selectivity and ¹³C NMR shift serves as a qualitative predictive guide. The broad consequences of this dynamic system, together with the ability to forge N-substituted quaternary carbon centers and implement late-stage functionalization techniques, hold potential to streamline complex trialkylamine synthesis and accelerate small-molecule drug discovery.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60 with α-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C₆₀ with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C₆₀ with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C₆₀. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of ³C₆₀* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C₆₀. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C₆₀ with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

Stereodivergent Synthesis of 1-Hydroxymethylpyrrolizidine Alkaloids

A first stereodivergent strategy for the asymmetric synthesis of all stereoisomers of 1-hydroxymethylpyrrolizidine alkaloids is developed using an asymmetric self-Mannich reaction as a key step. An anti-selective self-Mannich reaction of methyl 4-oxobutanoate with the PMP-amine catalyzed by a chiral secondary amine is successfully optimized for the asymmetric synthesis of (+)-isoretronecanol and (-)-isoretronecanol. A syn-selective self-Mannich reaction catalyzed by proline is utilized for the asymmetric synthesis of the diastereomer, (+)-laburnine, and its enantiomer, (-)-trachelanthamidine.

sp3 C-H Arylation and Alkylation Enabled by the Synergy of Triplet Excited Ketones and Nickel Catalysts

Triplet ketone sensitizers are of central importance within the realm of photochemical transformations. Although the radical-type character of triplet excited states of diaryl ketones suggests the viability for triggering hydrogen-atom transfer (HAT) and single-electron transfer (SET) processes, among others, their use as multifaceted catalysts in C-C bond-formation via sp³ C-H functionalization of alkane feedstocks still remains rather unexplored. Herein, we unlock a modular photochemical platform for forging C( sp³)-C( sp²) and C( sp³)-C( sp³) linkages from abundant alkane sp³ C-H bonds as functional handles using the synergy between nickel catalysts and simple, cheap and modular diaryl ketones. This method is distinguished by its wide scope that is obtained from cheap catalysts and starting precursors, thus complementing existing inner-sphere C-H functionalization protocols or recent photoredox scenarios based on iridium polypyridyl complexes. Additionally, such a platform provides a new strategy for streamlining the synthesis of complex molecules with high levels of predictable site-selectivity and preparative utility. Mechanistic experiments suggest that sp³ C-H abstraction occurs via HAT from the ketone triplet excited state. We believe this study will contribute to a more systematic utilization of triplet excited ketones as catalysts in metallaphotoredox scenarios.

Photoredox ketone catalysis for the direct C-H imidation and acyloxylation of arenes

The photoexcited aryl ketone-catalyzed C-H imidation of arenes and heteroarenes is reported. Using 3,6-dimethoxy-9H-thioxanthen-9-one as a catalyst in combination with a bench-stable imidating reagent, C-N bond formation proceeds with high efficiency and a broad substrate scope. A key part of this method is that the thioxanthone catalyst acts as an excited-state reductant, thus establishing an oxidative quenching cycle for radical aromatic substitution. The synthetic potential of this photoexcited ketone catalysis is further demonstrated by application to the direct C-H acyloxylation of arenes.

Studies in organic and physical photochemistry - an interdisciplinary approach

Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical chemistry. The aim of this review is to illustrate this very fruitful interdisciplinary approach and cooperation. A profound understanding of the photochemical reactivity and reaction mechanisms is particularly helpful for optimization and application of these reactions. Some typical reactions and particular aspects are reported such as the Norrish-Type II reaction and the Yang cyclization and related transformations, the [2 + 2] photocycloadditions, particularly the Paternò-Büchi reaction, photochemical electron transfer induced transformations, different kinds of catalytic reactions such as photoredox catalysis for organic synthesis and photooxygenation are discussed. Particular aspects such as the structure and reactivity of aryl cations, photochemical reactions in the crystalline state, chiral memory, different mechanisms of hydrogen transfer in photochemical reactions or fundamental aspects of stereoselectivity are discussed. Photochemical reactions are also investigated in the context of chemical engineering. Particularly, continuous flow reactors are of interest. Novel reactor systems are developed and modeling of photochemical transformations and different reactors play a key role in such studies. This research domain builds a bridge between fundamental studies of organic photochemical reactions and their industrial application.

Enantioselective Visible-Light-Induced Radical-Addition Reactions to 3-Alkylidene Indolin-2-ones

The title compounds underwent a facile and high-yielding addition reaction (19 examples, 66-99% yield) with various N-(trimethylsilyl)methyl-substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non-symmetric and if the reaction is performed in the presence of a chiral hydrogen-bonding template, products are obtained with significant enantioselectivity (58-72% ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.

Photoorganocatalysed and visible light photoredox catalysed trifluoromethylation of olefins and (hetero)aromatics in batch and continuous flow

Trifluoromethylation of olefins and (hetero)aromatics with sodium triflinate as CF₃source and readily accessible benzophenone derivatives as photosensitisers has been developed in batch and flow.

Asymmetric synthesis of functionalized pyrrolizidines by an organocatalytic and pot-economy strategy

An enantioselective synthesis of indolizidines was achieved with a one-step purification by sequential asymmetric Michael–oxidative esterification–Michael–reduction–reductive Mannich–amidation reactions.

Photochemically-induced C-C bond formation between tertiary amines and nitrones

Photoexcited nitrones serve as excellent electron acceptors as well as radical acceptors in the presence of tertiary amines to give β-amino hydroxylamines via photochemically-induced direct sp(3) C-H functionalization of the tertiary amines. The combined use of an organophotosensitizer and photoirradiation was highly effective in accelerating addition reactions. Several nitrones and tertiary amines were successfully utilized to give β-amino hydroxylamines in good yield. Highly regioselective generation of primary α-aminoalkyl radicals based on Lewis's stereoelectronic rule and diastereoselective addition reactions of primary α-aminoalkyl radicals with nitrones were successfully achieved. Furthermore, a highly diastereoselective reaction of an α-aminoalkyl radical with a chiral (E)-geometry-fixed α-alkoxycarbonylnitrone was performed.

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.

Photochemically induced radical reactions with furanones

Radicals are easily generated via hydrogen transfer form secondary alcohols or tertiary amines using photochemical sensitization with ketones. They can subsequently add to the electron deficient double bond of furanones. The addition of the alcohols is particularly efficient. Therefore, this reaction was used to characterize and to compare the efficiency of different photochemical continuous flow microreactors. A range of micro-structured reactors were tested and their performances evaluated. The enclosed microchip enabled high space-time-yields but its microscopic dimensions limited its productivity. In contrast, the open microcapillary model showed a greater potential for scale-up and reactor optimization. A 10-microcapillary reactor was therefore constructed and utilized for typical R&D applications. Compared to the corresponding batch processes, the microreactor systems gave faster conversions, improved product qualities and higher yields. Similar reactions have also been carried out with electronically excited furanones and other α,β-unsaturated ketones. In this case, hydrogen is transferred directly to the excited olefin. This reaction part may occur either in one step, i.e., electron and proton are transferred simultaneously, or it may occur in two steps, i.e., the electron is transferred first and the proton follows. In the first case, a C–C bond is formed in the α position of the α,β-unsaturated carbonyl compound and in the second case this bond is formed in the β position. For the first reaction, the influence of stereochemical elements of the substrate on the regioselectivity of the hydrogen abstraction on the side chain has been studied.

Visible-light-induced, Ir-catalyzed reactions of N-methyl-N-((trimethylsilyl)methyl)aniline with cyclic α,β-unsaturated carbonyl compounds

N-Methyl-N-((trimethylsilyl)methyl)aniline was employed as reagent in visible-light-induced, iridium-catalyzed addition reactions to cyclic α,β-unsaturated carbonyl compounds. Typical reaction conditions included the use of one equivalent of the reaction substrate, 1.5 equivalents of the aniline and 2.5 mol % (in MeOH) or 1.0 mol % (in CH₂Cl₂) [Ir(ppy)₂(dtbbpy)]BF₄ as the catalyst. Two major reaction products were obtained in combined yields of 30–67%. One product resulted from aminomethyl radical addition, the other product was a tricyclic compound, which is likely formed by attack of the intermediately formed α-carbonyl radical at the phenyl ring. For five-membered α,β-unsaturated lactone and lactam substrates, the latter products were the only products isolated. For the six-membered lactones and lactams and for cyclopentenone the simple addition products prevailed.

From 'Lab & Light on a Chip' to Parallel Microflow Photochemistry

Continuous-flow microreactors offer major advantages for photochemical applications. This mini-review summarizes the technological development of microflow devices in the Applied and Green Photochemistry Group at James Cook University, and its associates, from fixed microchips for microscale synthesis to flexible multicapillary systems for parallel photochemistry. Whereas the enclosed microchip offered high space–time-yields, the open capillary-type reactor showed a greater potential for further modifications. Consequently, a 10-microcapillary reactor was constructed and used successfully for process optimization, reproducibility studies, scale-up, and library synthesis. To demonstrate the superiority of microflow photochemistry over conventional batch processes, the reactors were systematically evaluated using alcohol additions to furanones as model reactions. In all cases, the microreactor systems furnished faster conversions, improved product qualities, and higher yields. UVC-induced [2+2] cycloadditions of furanone with alkenes were exemplarily examined in a capillary reactor, thus proving the broad applicability of this reactor type.

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.

Microphotochemistry: 4,4'-Dimethoxybenzophenone mediated photodecarboxylation reactions involving phthalimides

A series of 4,4'-dimethoxybenzophenone mediated intra- and intermolecular photodecarboxylation reactions involving phthalimides have been examined under microflow conditions. Conversion rates, isolated yields and chemoselectivities were compared to analogous reactions in a batch photoreactor. In all cases investigated, the microreactions gave superior results thus proving the superiority of microphotochemistry over conventional technologies.