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ChemInform Abstract: Discovery and Development of Organic Super-Electron-Donors
Abstract
Based on simple ideas of electron-rich alkenes, exemplified by tetrakis(dimethylamino)ethene, TDAE, and on additional driving force associated with aromatisation, families of very powerful organic super-electron-donors (SEDs) have been developed. In the ground state, they carry out metal-free reductions of a range of functional groups. Iodoarenes are reduced either to aryl radicals or, with stronger donors, to aryl anions. Reduction to aryl radicals allows the initiation of very efficient transition metal-free coupling of haloarenes to arenes. The donors also reduce alkyl halides, arenesulfonamides, triflates and triflamides, Weinreb amides and acyloin derivatives. Under photoactivation at 365 nm, they are even more powerful and reductively cleave aryl chlorides. They reduce unactivated benzenes to the corresponding radical anions and display original selectivities in preferentially reducing benzenes over malonates or cyanoacetates. Additionally, they reductively cleave ArC-X, ArX-C (X = N or O) and ArC-C bonds, provided that the two resulting fragments are somewhat stabilised.
... The resulting framework, CORN-COF-1 (CORN = Cornell University), is the only C=C COF synthesized to date that does not require a condensation reaction to form, representing a key advance in the field. Furthermore, the neutral, highly electron-rich, and strongly reducing tetraazafulvalene linkages of this COF are akin to related organic "super-electron-donors." [25] When exposed to solutions of excess tetracyanoethylene (TCNE), CORN-COF-1 is oxidized and incorporates the radical anion TCNE *À and the dianion TCNE 2À as guest molecules. The facile introduction of TCNE *À radicals, which are rarely isolable in the solid state, engenders this COF with metal-free magnetism. ...
... Tetraazafulvalenes, the proposed linkages of CORN-COF-1, are neutral organic reductants termed "superelectron-donors" that stand out among other electron-rich olefins like tetrathiafulvalenes because they are significantly more reducing. [25] The closest molecular analog of CORN-COF-1, MA, undergoes a two-electron redox process (MA 2 + /MA) at E 1/2 = À 0.90 V vs SCE (SI Figure S104), [50] suggesting that CORN-COF-1 should be a similarly strong neutral electron-donor. There are no prior reports incorporating tetraazafulvalenes or any equally strong neutral organic reductants within COFs because they are incompatible with traditional COF-forming conditions. ...
Covalent organic frameworks linked by carbon‐carbon double bonds (C=C COFs) are an emerging class of crystalline, porous, and conjugated polymeric materials with potential applications in organic electronics, photocatalysis, and energy storage. Despite the rapidly growing interest in sp² carbon‐conjugated COFs, only a small number of closely related condensation reactions have been successfully employed for their synthesis to date. Herein, we report the first example of a C=C COF, CORN‐COF‐1 (CORN=Cornell University), prepared by N‐heterocyclic carbene (NHC) dimerization. In‐depth characterization reveals that CORN‐COF‐1 possesses a two‐dimensional layered structure and hexagonal guest‐accessible pores decorated with a high density of strongly reducing tetraazafulvalene linkages. Exposure of CORN‐COF‐1 to tetracyanoethylene (TCNE, E1/2=0.13 V and −0.87 V vs. SCE) oxidizes the COF and encapsulates the radical anion TCNE⋅⁻ and the dianion TCNE²⁻ as guest molecules, as confirmed by spectroscopic and magnetic analysis. Notably, the reactive TCNE⋅⁻ radical anion, which generally dimerizes in the solid state, is uniquely stabilized within the pores of CORN‐COF‐1. Overall, our findings broaden the toolbox of reactions available for the synthesis of redox‐active C=C COFs, paving the way for the design of novel materials.
... Electron-rich alkenes with neutral or anionic substituents have been used as strong reducing agents, and those that reduce aryl halides to aryl radicals or aryl anions are routinely called super electron donors. [1] These compounds were introduced as an alternative to metal-based reducing reagents. [2] A guiding idea for the preparation of superreductants involves the installation of strong electron donor groups (EDG) surrounding an alkene moiety, for example in tetrakis(dimethylamino)ethene, TDAE. ...
Excited states of radical anions derived from the photoreduction of stable organic molecules are suggested to serve as potent reductants. However, excited states of these species are too short‐lived to allow bimolecular quenching processes. Recently, the singlet excited state of Meisenheimer complexes, which possess a long‐lived excited state, was identified as the competent species for the reduction of challenging organic substrates (−2.63 V vs. SCE, saturated calomel electrode). To produce reasonably stable and simply accessible different Meisenheimer complexes, the addition of nBuLi to readily available aromatic heterocycles was investigated, and the photoreactivity of the generated species was studied. In this paper, we present the straightforward preparation of a family of powerful photoreductants (*Eox<−3 V vs. SCE in their excited states, determined by DFT and time‐dependent TD‐DFT calculations; DFT, density functional theory) that can induce dehalogenation of electron‐rich aryl chlorides and to form C−C bond through radical cyclization. Photophysical analyses and computational studies in combination with experimental mechanistic investigations demonstrate the ability of the adduct to act as a strong electron donor under visible light irradiation.
... In 2023, our group [17] developed an elegant procedure to realize the alkylation of aldehydes, utilizing simple unactivated secondary and even primary alkyl halides as alkyl precursors. In this method, deprotonated Breslow intermediates derived from mesoionic carbenes [18] served as super-electron-donors (SEDs), [19] which enable the single-electron reduction of unactivated alkyl halides. Noteworthy, this reaction system can not only have a good functional compatibility (including unprotected hydroxyl 15 b), but also can be applied to modification of latestage functionalization bearing bioactive molecule fragments, such as Epiandrosterone 15 e, Pregnenolone 15 f, Tocopherol 15 g, and so on. ...
This concept aims to summarize the direct coupling of aldehydes and organic halides (including activated and unactivated organic halides) catalyzed by N‐heterocyclic carbenes (NHCs). The key factor of this transformation is the formation of Breslow intermediates, which are either nucleophilic or reductive. Additionally, the activation of inert C−H bond also was reviewed in this performance via the processes of single electron transfer (SET) and subsequent intramolecular 1,n‐ hydrogen atom transfer (HAT) or intermolecular HAT. Finally, the key differences and reactivity of various Breslow intermediates, generated from different NHCs, have been discussed to give a deeper understanding of the ongoing popularity of NHC‐organocatalysis.
... nucleophilic handles for forging sp 3 linkages. [14] Among these, significant contributions from Walsh have illustrated the potential of super-electron donors [15] as vehicles for accessing new molecular scaffolds via single-electron transfer. [16] In line with our ongoing interest in sp 3 CÀ H functionalization, [17] we wondered whether an α-difluoroalkylation of benzyl amines with trifluoromethylarenes could be designed via the intermediacy of I by using readily accessible benzophenone imine as activating agent (Scheme 2, bottom). ...
A highly modular technique that streamlines the synthesis of α‐difluoroalkylated amines from benzyl amines and trifluoromethylarenes has been developed. This protocol demonstrates remarkable operational simplicity, excellent chemoselectivity, and wide applicability, including with advanced synthetic intermediates, thus providing a novel pathway to access medicinally‐relevant α‐difluoroalkylated amines from readily available and simple precursors. image
... [22] Under metal-and light-free conditions, direct single electron reduction of alkyl halides by organic super electron donors is also viable. [23] In this paper, we have shown that alkyl halides are readily reduced by deprotonated Breslow intermediates derived from mesoionic carbenes. Thus, MIC-catalyzed formyl alkylation of aldehydes has been achieved with broad substrate scope and high functional group tolerance. ...
Ketones are among the most useful functional groups in organic synthesis, and they are commonly encountered in a broad range of compounds with various applications. Herein, we describe the mesoionic carbene‐catalyzed coupling reaction of aldehydes with non‐activated secondary and even primary alkyl halides. This metal‐free method utilizes deprotonated Breslow intermediates derived from mesoionic carbenes (MICs), which act as super electron donors and induce the single‐electron reduction of alkyl halides. This mild coupling reaction has a broad substrate scope and tolerates many functional groups, which allows to prepare a diversity of simple ketones as well as bio‐active molecules by late‐stage functionalization.
Electron donors that can be excited to higher energy states through light absorption can achieve oxidation potentials as low as −3.0 V (vs. SCE). However, ground‐state organic electron transfer reagents operating at such potentials remain underdeveloped, often necessitating multi‐step syntheses and elevated reaction temperatures for activation. The longer lifetime of ground‐state reagents is an advantage compared to most photoexcited single‐electron reductants, which typically have relatively short lifetimes. In this study, catalytically generated phosphine oxide radical anions derived from phosphines and water applying redox catalysis are introduced as highly efficient single‐electron reductants. The in situ generated radical anions are capable of reducing electron‐rich aryl chlorides at potentials as low as −3.3 V (vs. SCE). Cyclic voltammetry studies and DFT calculations provide valuable insights into the behavior of these phosphorus‐based ground‐state electron donors. These findings do not only expand the chemistry of phosphoranyl radicals but also unlock the potential of in situ generated organic ground state electron donors that reach potentials comparable to elemental potassium.
Zusammenfassung
Elektronendonoren, die durch Lichtabsorption in höhere Energiezustände angeregt werden können, erreichen Oxidationspotenziale von bis zu −3,0 V (vs. SCE ). Allerdings sind organische Elektrontransferreagenzien im Grundzustand, die bei derart niedrigen Potenzialen agieren, bislang nur unzureichend entwickelt. Ihre Aktivierung erfordert häufig mehrstufige Synthesen und erhöhte Reaktionstemperaturen. Ein Vorteil von Grundzustand‐Elektronendonoren besteht jedoch in ihrer längeren Lebensdauer im Vergleich zu den meisten photoangeregten Einelektron‐Reduktionsmitteln, die typischerweise eine relativ kurze Lebensdauer aufweisen. In dieser Studie werden katalytisch erzeugte Phosphanoxid‐Radikalanionen, die aus Phosphanen und Wasser durch Redoxkatalyse generiert werden, als hocheffiziente Einelektron‐Reduktionsmittel vorgestellt. Die in situ erzeugten Radikalanionen sind in der Lage, elektronenreiche Arylchloride bei Reduktionspotenzialen von bis zu −3,3 V (vs. SCE ) zu reduzieren. Cyclovoltammetriestudien sowie DFT‐Berechnungen liefern wertvolle Einblicke in die Eigenschaften dieser phosphorbasierten Grundzustand‐Elektronendonoren. Diese Erkenntnisse erweitern nicht nur die Chemie der Phosphoranylradikale, sondern eröffnen auch das Potenzial von in situ erzeugten organischen Grundzustand‐Elektronendonoren, deren Oxidationspotenziale mit denen von elementarem Kalium vergleichbar sind.
Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl–halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl–I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl–I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C–C and C–heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.
In the quest for powerful, safe, and storable photoinduced-electron transfer (PET) donors, the attention is turned to the α-trihalomethylated amine moiety that is not studied in the context of PET-reductants. The thermal and photophysical properties of α-trifluoromethylated quinolines are thus studied and their reducing abilities evaluated as initiators of polymerization reactions. Polymers of high molecular weights are obtained through a radical polymerization process and the PET-donor can be stored within the monomer for several months without losing its efficiency. Mechanistic investigations, combining spectroscopic analysis and theoretical calculations, confirm the mode of activation of these electron donors and the generation of radical intermediates through single electron transfer.
Covalent organic frameworks linked by carbon‐carbon double bonds (C=C COFs) are an emerging class of crystalline, porous, and conjugated polymeric materials with potential applications in organic electronics, photocatalysis, and energy storage. Despite the rapidly growing interest in sp ² carbon‐conjugated COFs, only a small number of closely related condensation reactions have been successfully employed for their synthesis to date. Herein, we report the first example of a C=C COF, CORN‐COF‐1 (CORN=Cornell University), prepared by N ‐heterocyclic carbene (NHC) dimerization. In‐depth characterization reveals that CORN‐COF‐1 possesses a two‐dimensional layered structure and hexagonal guest‐accessible pores decorated with a high density of strongly reducing tetraazafulvalene linkages. Exposure of CORN‐COF‐1 to tetracyanoethylene (TCNE, E 1/2 =0.13 V and −0.87 V vs. SCE) oxidizes the COF and encapsulates the radical anion TCNE⋅ ⁻ and the dianion TCNE ²⁻ as guest molecules, as confirmed by spectroscopic and magnetic analysis. Notably, the reactive TCNE⋅ ⁻ radical anion, which generally dimerizes in the solid state, is uniquely stabilized within the pores of CORN‐COF‐1. Overall, our findings broaden the toolbox of reactions available for the synthesis of redox‐active C=C COFs, paving the way for the design of novel materials.
Over the past few decades, organic transformations facilitated by small organic molecules have witnessed considerable advancements. Small organic molecules have been widely applied to the construction of various skeletons. However, reduction as one of the fundamental reactions is generally initiated by metal‐containing reducing agents, although the initial progress of organic electron donors (OEDs) enabled reductive cleavage of some chemical bonds had been achieved in 1990s. Due to their structural diversity and tunability, OEDs can overcome the limitations of metal‐based reducing agents, such as low selectivity and poor functional group tolerance as well as the environmental problems caused by substantial inorganic waste after reactions. Building on a brief historical overview of OED research, this review focuses on the rencent advancements of the OEDs promoted radical reactions. It covers the reduction of C−X (X=C, O, N etc.) single bonds, radical cyclization, intermolecular radical addition to unsaturated bonds, radical coupling, functionalization of aromatics, and C−H bond activation. Additionally, the mechanistic insights of the typical reactions are highlighted for well understanding of the reaction mode involving OEDs.
Catalytic amounts of 1,3-di(methyl)imidazole-2-ylidene, one of the simplest and most prototypical N-heterocyclic carbenes, can up-convert aldehydes into powerful stoichiometric sources of electrons (Super Electron Donors) for reductive transformations of iodoaryls (Ered < −2 V). In particular, the hydroarylation of 1,1′-diarylethylenes, which may require high temperatures and inherently generate stoichiometric amounts of oxidized waste, was performed at room temperature, with the concomitant formation of esters as oxidized co-products.
This review describes recent advances in the generation of aryl radicals using light and electricity. Such modern techniques allow for efficient energy and resource utilization, thus providing more sustainable radical arylation methods.
Excited states of radical anions derived from the photoreduction of stable organic molecules are suggested to serve as potent reductants. However, excited states of these species are too short‐lived to allow bimolecular quenching processes. Recently, the singlet excited state of Meisenheimer complexes, which possess a long‐lived excited state, was identified as the competent species for the reduction of challenging organic substrates (–2.63 V vs. SCE, saturated calomel electrode). To produce reasonably stable and simply accessible different Meisenheimer complexes, the addition of nBuLi to readily available aromatic heterocycles was investigated, and the photoreactivity of the generated species was studied. In this paper, we present the straightforward preparation of a family of powerful photoreductants (*Eox<–3 V vs. SCE in their excited states, determined by DFT and time‐dependent TD‐DFT calculations; (DFT, density functional theory) that can induce dehalogenation of electron‐rich aryl chlorides and to form C−C bond through radical cyclization. Photophysical analyses and computational studies in combination with experimental mechanistic investigations demonstrate the ability of the adduct to act as a strong electron donor under visible light irradiation.
Bispyridinylidenes are neutral organic molecules capable of two‐electron oxidation at a range of redox potentials that are widely tunable by choice of substituent, making them attractive as homogeneous organic reductants and active materials in redox flow batteries. In an effort to readily predict the redox potentials of this important class of compounds, we have developed correlations between the experimental redox potentials and both experimental and theoretical predictors. On the experimental side, we show that multinuclear NMR chemical shifts of related pyridinium ions correlate well with the redox potentials of bispyridinylidenes, with R² and standard errors (S) reaching 0.9810 and 0.048 V, respectively, when the ¹³C (N‐CH3) and ¹H (ortho) chemical shifts are used together. Theoretical studies of the bispyridinylidenes and their doubly oxidized bipyridinium ions gave a range of predictively valuable equations at various levels of computational cost. This ranged from a simple model using only the EHOMO of the bispyridinylidenes (R²=0.9689; S=0.060 V), to a more computationally intensive model which include solvation effects for both redox states which gave the highest predictive value for all methods (R²=0.9958; S=0.022 V). This work will guide further studies of this important class of molecules.
α-Silyl amines are unique organosilicon compounds containing a geminal NCH2Si fragment. The introduction of a triorganylsilyl group at α-position to the nitrogen atom decreases the ionization potential and promotes an electron transfer with the formation of the corresponding radical cation. The review discusses methods for generating α-silyl amine radical cations (electrochemical, chemical, and photochemical) and also generalizes and systematizes reactions of α-silyl amines which include the stage of single-electron transfer. Single-electron oxidation of neutral α-silyl amines is a simple and efficient approach to highly reactive C-centered α-aminoalkyl radicals, which are very attractive from the point of view of synthetic organic chemistry in terms of the formation of new carbon—carbon bonds.
Due to amine derivatives widely existing in numerous clinical medicines and bioactive compounds, their synthesis has received considerable attention over the past few decades. Traditional methods for the synthesis of amine derivatives largely relied on the reduction of nitroarenes, amides, hydrazines, nitriles, and azides. Recently, the discovery of 2‐azaallyl anions as super‐electron‐donors (SEDs) has opened up new possibilities for the construction of diverse carbon‐carbon and carbon‐heteroatom bonds through radical coupling strategies. This breakthrough highlights the potential of generating 2‐azaallyl radicals as versatile intermediates in organic synthesis. Then, hydrolysis of the coupling product can easily separate the corresponding amine derivatives. Thus, tremendous attention has been paid to the C−H functionalization of ketimines as an alternative strategy for the synthesis of amine derivatives. Herein, we comprehensively summarize the recent progress in radical coupling strategies enabled by 2‐azaallyl anions as SEDs. Their proposed mechanistic pathways, advantages, and limitations are also discussed in detail. 1. Introduction 2. C−C Bond Formation 2.1. Vinylation of Ketimines 2.2. Arylation of Ketimines 2.3. Alkylation of Ketimines 2.3.1. Methylation of Ketimines 2.3.2. Other Alkylation of Ketimines 3. C−X Bond Formation 3.1. C−P and C−S Bonds Formation 3.2. C−N and C−O Bonds Formation 4. Conclusions
Pyrroloindolines are important structural units in nature and the pharmaceutical industry, however, most approaches to such structures involve transition-metal or photoredox catalysts. Herein, we describe the first tandem SET/radical cyclization/intermolecular coupling between 2-azaallyl anions and indole acetamides. This method enables the transition-metal-free synthesis of C3a-substituted pyrroloindolines under mild and convenient conditions. The synthetic utility of this transformation is demonstrated by the construction of an array of C3a-methylamine pyrroloindolines with good functional group tolerance and yields. Gram-scale sequential one-pot synthesis and hydrolysis reactions demonstrate the potential synthetic utility and scalability of this approach.
The use of visible light as a means of promoting chemical transformations offers a more sustainable and efficient alternative to traditional methods in organic synthesis. This reaction bias represents an...
We describe herein easily oxidizable 2-substituted-1,3-imidazolidines for the photocatalyzed generation of (substituted) alkyl radicals for the forging of C(sp ³ )–C(sp ³ ) bonds under metal-free conditions.
We report the preparation of a new organic σ‐donor with a phenylene linker between an N‐heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N‐heterocyclic quinodimethane (NHQ). The aromatization of the phenylene linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N‐heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head‐to‐head dimerization by C−C coupling of the methylidene groups. DFT computations indicate that this reaction precedes via an open‐shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N‐heterocyclic bis‐quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2 = −1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet‐triplet gap of ΔES→T = 4.4 kcal mol−1.
We report the preparation of a new organic σ‐donor with a C6H4‐linker between an N‐heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N‐heterocyclic quinodimethane (NHQ). The aromatization of the C6H4‐linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N‐heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head‐to‐head dimerization by C−C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open‐shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N‐heterocyclic bis‐quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=−1.71 V vs. Fc/Fc⁺) and exhibits a remarkable small singlet–triplet gap of ΔES→T=4.4 kcal mol⁻¹.
Herein, we report diphenylthiourea (DPTU) as a potent photocatalyst at its deprotonated stage that can promote reductive bond cleavage chemistry on aryl bromide substrates. An easy deprotonation of the DPTU...
In the last two decades, chiral hydrogen bonding catalysis has been enormously utilized to trigger several valuable synthetic transformations. Similarly, modern day radical chemistry has witnessed the potency of visible light photoredox catalysis to generate radicals via single electron transfer pathway. Recently, their cooperative effect has proved to be a key strategy to access many elegant transformations that are only feasible under the synergistic action of these two catalytic systems. This dual catalytic system demonstrates high efficiency for introducing molecular complexity in an enantioselective fashion. In this mini review, we have compiled the most recent development of this dual catalytic system that surfaced in the last three years with particular attention paid to the mechanistic details and its substrate scope. We hope that this mini review will provide readers with a brief overview of the advancement and application of photoredox catalysts and chiral hydrogen bond donors for synergistic dual catalytic reactions.
An α‐difluoroalkylation of benzyl amines with trifluoro‐methylarenes is disclosed herein. This protocol is characterized by its operational simplicity, excellent chemoselectivity and broad scope – even with advanced synthetic intermediates –, thus offering a new entry point to medicinally‐relevant α‐difluoroalkylated amines from simple, yet readily accessible, precursors.
Hydrogen atom transfer (HAT) processes are among the most useful approaches for the selective construction of C(sp3)-C(sp3) bonds. 1,5-HAT with heteroatom-centered radicals (O•, N•) have been well established and are favored relative to other 1,n-HAT processes. In comparison, net 1,2-HAT processes have been observed infrequently. Herein, the first amidyl radicalls are reported that preferentially undergo a net 1,2-HAT over 1,5-HAT. Beginning with single electron transfer from 2-azaallyl anions to N-alkyl N-aryloxy amides, the latter generate amidyl radicals. The amidyl radical undergoes a net-1,2-HAT to generate a C-centered radical that participates in an intermolecular radical-radical coupling with the 2-azaallyl radical to generate 1,2-diamine derivatives. Mechanistic and EPR experiments point to radical intermediates. Density functional theory calculations provide support for a base-assisted, stepwise-1,2-HAT process. It is proposed that the generation of amidyl radicals under basic conditions can be greatly expanded to access α-amino C-centered radicals that will serve as valuable synthetic intermediates.
Chalcogen-substituted carbenes are examined computationally using density functional theory. Several approaches are used to assess the stability and reactivity of chalcogenazol-2-ylidene carbenes (NEHCs; E = O, S, Se, Te). The known unsaturated species 1,3-dimethylimidazol-2-ylidene is studied at the same level of theory as the NEHC molecules, as a reference. Electronic structures, stability towards dimerization, and ligand properties are discussed. The results highlight the NEHCs as potentially valuable ancillary ligands for stabilizing low-valent metals or paramagnetic main group molecules. A simple, effective computational method for evaluating σ donor ability and π acidity of carbenes is presented.
[(Dpp-bian)B(DMAP)]2 (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene, DMAP = 4-(dimethylamino)pyridine), a bottleable super electron donor (SED), was demonstrated to serve as a SED catalyst in the borylation of aryl iodides, bromides, and the more challenging chlorides. Apart from installing Bpin from B2pin2, the SED-catalyzed borylation protocol is also applicable for installing Bdan from Bpin-Bdan. The radical mechanism has been confirmed by the radical trapping and radical clock experiments.
The CH functionalization reactions are of utmost importance in synthetic chemistry and multiple approaches have been adopted to discover easy and mild processes. This article focuses on CH functionalization reactions that are predominantly promoted by radical species. Many prior processes often rely on prefunctionalization of a particular CH bond, hence a direct method without such prior activation would be very much appreciable. In a radical‐driven CH functionalization process, an alkyl or aryl radical can be generated from an appropriate precursor by single electron transfer event. Beyond that, a significant effort is devoted to discover new precursor molecules from which a variety of alkyl and aryl radicals will be efficiently generated. The mildness of the process is often demonstrated by the chemoselectivity, and a wide functional group tolerance. The applicability of the radical‐mediated CH functionalization processes stretches further to the realm of medicinally and pharmaceutically important molecules. Additionally, many photochemical processes are recent addition to the repertoire of radical generation that facilitate the methods without any heating. Intriguing mechanistic scenarios for radical generation and further reactions toward CH functionalization is described and discussed.
A unique transition-metal-free radical thiolation of 2-azaallyl anions has been developed. Easily accessible thiosulfonates and 2-azaallyls undergo the tandem process of single-electron transfer and radical-radical coupling to construct C(sp3)-S bonds. This robust protocol enables a mild and chemoselective coupling between 2-azaallyl anions and thiosulfonates to access α-amino sulfides in 50-92% yields (25 examples). The scalability of this protocol was demonstrated by telescopic gram-scale experiments. Mechanistic studies provide significant evidence for this radical thiolation reaction.
Ketones are among the most useful functional groups in organic synthesis, and they are commonly encountered in a broad range of compounds with various applications. Herein, we describe the mesoionic carbene‐catalyzed coupling reaction of aldehydes with non‐activated secondary and even primary alkyl halides. This metal‐free method utilizes deprotonated Breslow intermediates derived from mesoionic carbenes (MICs), which act as super electron donors and induce the single‐electron reduction of alkyl halides. This mild coupling reaction has a broad substrate scope and tolerates many functional groups, which allows to prepare a diversity of simple ketones as well as bio‐active molecules by late‐stage functionalization.
The head‐to‐tail dimerization of N‐heterocyclic diazoolefins is described. The products of these formal (3+3) cycloaddition reactions are strongly reducing quinoidal tetrazines. Oxidation of the tetrazines occurs in a stepwise fashion, and we were able to isolate a stable radical cation and diamagnetic dications. The latter are also accessible by oxidative dimerization of diazoolefins.
The head‐to‐tail dimerization of N‐heterocyclic diazoolefins is described. The products of these formal (3+3) cycloaddition reactions are strongly reducing quinoidal tetrazines. Oxidation of the tetrazines occurs in a stepwise fashion, and we were able to isolate a stable radical cation and diamagnetic dications. The latter are also accessible by oxidative dimerization of diazoolefins.
Recent papers report transition metal-free couplings of haloarenes to arenes to form biaryls, triggered by alkali metal tert-butoxides in the presence of various additives. These reactions proceed through radical intermediates, but understanding the origin of the radicals has been problematic. Electron transfer from a complex formed from potassium tert-butoxide with additives, such as phenanthroline, has been suggested to initiate the radical process. However, our computational results encouraged us to search for alternatives. We report that heterocycle-derived organic electron donors achieve the coupling reactions and these donors can form in situ in the above cases. We show that an electron transfer route can operate either with phenanthrolines as additives or using pyridine as solvent, and we propose new heterocyclic structures for the respective electron donors involved in these cases. In the absence of additives, the coupling reactions are still successful, although more sluggish, and in those cases benzynes are proposed to play crucial roles in the initiation process.
A photoactivated neutral organic super electron donor cleaves challenging arenesulfonamides derived from dialkylamines at room temperature. It also cleaves a)ArC-NR and b)ArN-C bonds. This study also highlights the assistance given to these cleavage reactions by the groups attached to N in (a) and to C in (b), by lowering LUMO energies and by stabilizing the products of fragmentation. Radical fragmentations: Electron transfer from the photoactivated neutral electron donor 1 delivers high yields of S-N and C-N cleavage products for a range of nitrogen-containing species. These reactions proceed at room temperature and under mild reaction conditions in the absence of any metal reagents. DMF=N,N-dimethylformamide, Ts=4-toluenesulfonyl. © 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Neutral organic electron donors, featuring pyridinylidene-imidazolylidene, pyridinylidene-benzimidazolylidene and imidazolylidene-benzimidazolylidene linkages are reported. The pyridinylidene-benzimidazolylidene and imidazolylidene-benzimidazolylidene hybrid systems were designed to be the first super electron donors to convert iodoarenes to aryl radicals at room temperature, and indeed both show evidence for significant aryl radical formation at room temperature. The stronger pyridinylidene-imidazolylidene donor converts iodoarenes to aryl anions efficiently under appropriate conditions (3 equiv of donor). The presence of excess sodium hydride base has a very important and selective effect on some of these electron-transfer reactions, and a rationale for this is proposed.
Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-σ-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.
Treatment of the arenediazonium tetrafluoroborates 4a and 4b in Me2SO solution with copper(II) bromide or chloride gave the cyclized halo compounds 5a, 5b, and 5c in good yield. Copper(I) cyanide/pyridine effected ring closure of 4b to afford the nitrile 5d. Dihydrobenzofuran derivatives were also formed on treatment of 4a or 4b with benzenethiolate or butanethiolate ion in Me2SO. Chain mechanisms involving cyclization of an intermediate aryl radical are suggested.
The scope of neutral organic super-electron donors as reducing agents has been extended to include the reductive cleavage of N-O bonds in Weinreb amides. This methodology proved to be applicable to a large array of substrates to afford their reduced counterparts in good to excellent yields. The variation in reactivity within the set of tested amides is rationalised.
One-electron reduction is commonly used in organic chemistry for the formation of radicals by the stepwise transfer of one or two electrons from a donor to an organic substrate. Besides metallic reagents, single-electron reducers based on neutral organic molecules have emerged as an attractive novel source of reducing electrons. The past 20 years have seen the blossoming of a particular class of organic reducing agents, the electron-rich olefins, and their application in organic synthesis. This Review gives an overview of the different types of organic donors and their specific characteristics in organic transformations.
Radical reactions are a powerful class of chemical transformations.
However, the formation of radical species to initiate these reactions
has often required the use of stoichiometric amounts of toxic reagents,
such as tributyltin hydride. Recently, the use of visible-light-mediated
photoredox catalysis to generate radical species has become popular, but
the scope of these radical precursors has been limited. Here, we
describe the identification of reaction conditions under which
photocatalysts such as fac-Ir(ppy)3 can be utilized to form
radicals from unactivated alkyl, alkenyl and aryl iodides. The generated
radicals undergo reduction via hydrogen atom abstraction or reductive
cyclization. The reaction protocol utilizes only inexpensive reagents,
occurs under mild reaction conditions, and shows exceptional functional
group tolerance. Reaction efficiency is maintained upon scale-up and
decreased catalyst loading, and the reaction time can be significantly
shortened when the reaction is performed in a flow reactor.
Tricyclic donor has been prepared and is the most reducing neutral ground-state organic molecule known, with an oxidation potential 260 mV more negative than the previous record. Cyclic voltammetry shows that a 2-electron reversible redox process occurs in DMF as solvent at -1.46 V vs. Ag/AgCl.
Recently, samarium(ii) iodide reductants have emerged as powerful single electron donors for the highly chemoselective reduction of common functional groups. Complete control of the product formation can be achieved on the basis of a judicious choice of a Sm(ii) complex/proton donor couple, even in the presence of extremely sensitive functionalities (iodides, aldehydes). In most cases, the reductions are governed by thermodynamic control of the first electron transfer, which opens up new prospects for unprecedented transformations via radical intermediates under mild regio-, chemo- and diastereoselective conditions that are fully orthogonal to hydrogenation or metal-hydride mediated processes.
Previous efforts to prepare tetraazafulvalenes derived from imidazolium salt precursors have met with little success (one anomalously favourable example is known), and this is in line with the predicted reactivity of these compounds. However, we now report the preparation of a series of these tetraazafulvalenes formed either by deprotonation of 1,3-dialkylimidazolium salts or by Birch reduction of biimidazolium salts. The tetraazafulvalenes are highly reactive; for example, they act as Super-Electron-Donors towards iodoarenes. The two most reactive examples are formed more efficiently by Birch reduction than by the deprotonation route. Nevertheless, even in cases where the deprotonation approach affords a low stationary concentration, the mixture of precursor salt and base still produces the same powerful reductive chemistry that is the hallmark of tetraazafulvalene electron donors.
The tetrathiafulvalene-induced ‘radical–polar crossover’ reaction has been applied to the total synthesis of the pentacyclic Aspidosperma alkaloid aspidospermidine.
Tetrathiafulvalene behaves as a catalyst in mediating reaction cycles which feature electron transfer reactions, radical cyclisations and nucleophilic displacements.
Treatment of bis-1,3-dithiole with 1 equiv. of chlorine affords a very stable radical cation which is amenable to full chemical and spectroscopic characterization.
1,4-Diketones were prepared by the reaction of α-bromo ketones with tetrakis(dimethylamino)ethylene (TDAE) in moderate to good yields. Similarly, α-bromo esters were reductively coupled using TDAE to give the 1,4-diesters in moderate yields.
A new method for the reductive debromination of 1,2-bis(bromomethyl)arenes has been developed. The treatment of 1,2-bis(bromomethyl)benzene with tetrakis(dimethylamino)ethylene (TDAE) (1) in the presence of olefins gave 1,2,3,4-tetrahydronaphthalenes in moderate to good yields.
The prevalence of metal-based reducing reagents, including metals, metal complexes and metal salts, has produced an empirical order of reactivity that governs our approach to chemical synthesis. However, this reactivity may be influenced by stabilization of transition states, intermediates and products through substrate-metal bonding. This article reports that, in the absence of such stabilizing interactions, established chemoselectivities can be overthrown. Thus photoactivation of recently developed neutral organic super-electron-donor 5 selectively reduces alkyl-substituted benzene rings in the presence of activated esters and nitriles, in direct contrast to metal-based reductions, opening a new perspective on reactivity. The altered outcomes arising from the organic electron donors are attributed to selective interaction between the neutral organic donors and the arene rings of the substrates.
ortho-Substituted aryl radicals have been generated by reduction of arenediazonium salts in aqueous solution by use of a flow cell. The e.s.r. spectra indicate that radicals containing olefinic bonds in the 5,6- or 6,7-positions with respect to the radical centre undergo rapid cyclization by addition to the 5- and 6-positions respectively to give the thermodynamically less stable products. No evidence was obtained for addition to the remote termini of the double bonds. The direction of cyclization is not determined primarily by the case of approach of the radical centre to the olefinic carbon atoms. Aryl radicals containing saturated ortho-substituents undergo rearrangement by 1,5-intramolecular hydrogen atom transfer. The conclusions based on e.s.r. spectroscopic studies have been supported by the isolation of rearranged products from the reduction of suitable arenediazonium salts.
A novel route to advanced synthetic precursors of Aspidosperma alkaloids is described, utilising radical-polar crossover reactions.
Cyclic ethers result from intramolecular trapping of cations formed through the radical-polar crossover process.
Diazadithiafulvalenes act as excellent electron donors to arenediazonium salts. The diazadithiafulvalenium radical cations trap primary carbon radicals successfully. However, the diazadithiafulvalenium salts which form, undergo rapid ring fragmentation in contrast to their tetrathia counterparts.
The oligomers 2, 4 and 6, 6a, derived from tetrafluoroethene, octafluorocyclopentene, and hexafluorocyclobutene respectively, are converted into their corresponding dienes 7–9 by defluorination, using sodium amalgam. An alternative route using tetrakis(dimethylamino)ethene (TDAE) is also described and forms a fluoride salt of TDAE. The diene 7 shows a remarkably low extinction coefficient for UV absorption.
We report herein the reaction of 2,2-dibromomethylquinoxaline 2 with aromatic aldehydes 3a–g in the presence of TDAE. These reactions lead to a mixture of cis/trans-isomers of corresponding oxiranes 4a–g in good yields. The stereoselectivity of the reaction was sensitive to steric hindrance.
Aspidospermidine has been prepared by a novel route featuring TTF-induced cyclisation of a diazonium salt.
Decyanation of geminal dinitriles and α-alkoxycarbonyl substituted nitriles promoted by samarium(II) iodide has been efficiently achieved. This method has advantages over the previously known radical route using tin hydride with respect to applicable substrates and the reaction temperature employed. This decyanation could broaden the synthetic applicability of the nitrile derivatives.
An efficient one-pot procedure is described for the reduction of aryl iodides to aryl anions using a structurally simple bis-pyridinylidene electron donor, prepared in situ by treating 4-DMAP methiodide salt with base. The results show (i) that pyridinylidene carbenes can be easily used for intermolecular C-C bond formation, (ii) that bis-pyridinylidenes demonstrate superior robustness compared to electron-donor systems based on bis-imidazolylidenes, and (iii) that electron-donor strength is enhanced in the simplified DMAP-based donor. Deuterated analogues of this donor also provide mechanistic information on the source of protons when the aryl anions are quenched in situ.
Preparation of bisannulated salts of 2,2′-biimidazole, 2-(2′-imidazolyl)benzimidazole, 2,2′-bibenzimidazole, and annulated salts of 1,1′-dimethyl-2,2′-biimidazole, 1,1′-dimethyl-2-(2′-imidazolyl)benzimidazole, and 1,1′-dimethyl-2,2′-bibenzimidazole is accomplished by direct alkylation of the parent azaheterocycle with excess 1,n-dihaloalkane. Discussions of the product conformations use electronic absorption spectra and NMR. The redox potentials of these salts are measured in DMF and CH3CN, and become increasingly more negative and less reversible as the systems become less planar. Keywords: electrochemistry, bridged azabiaryl salts.
The use of diazonium salts for aryl radical generation and CH arylation processes has been known since 1896 when Pschorr first used the reaction for intramolecular cyclizations. Meerwein developed it further in the early 1900s into a general arylation method. However, this reaction could not compete with the transition-metal-mediated formation of C(sp(2) )C(sp(2) ) bonds. The replacement of the copper catalyst with iron and titanium compounds improved the situation, but the use of photocatalysis to induce the one-electron reduction and activation of the diazonium salts is even more advantageous. The first photocatalyzed Pschorr cyclization was published in 1984, and just last year a series of papers described applications of photocatalytic Meerwein arylations leading to aryl-alkene coupling products. In this Minireview we summarize the origins of this reaction and its scope and applications.
The chemical review highlights the early work on the use of transition metal complexes as photoredox catalysts to promote reactions of organic compounds. Studies on the use of transition metal complexes as visible light photocatalysts for organic synthesis have significantly benefited from advances in the related fields of organic and semiconductor photocatalysis. It is better to consider the photochemistry of the prototypical photoredox catalyst Ru(bpy)2+3 before discussing organic transformations enabled by photoredox catalysis. An electron in one of the photocatalyst's metal-centered t2g orbitals is excited to a ligand-centered π* orbital on absorption of a photon in the visible region. This transition is termed a metal to ligand charge transfer (MLCT) and results in a species in which the metal has been effectively oxidized to a Ru(III) oxidation state and the ligand framework has undergone a single-electron reduction.
The mechanisms for the reductive cleavage of benzylic esters and ethers by neutral organic electron donor 1 are different. Products isolated from the cleavage of benzylic ethers result from the transfer of two electrons, without the intermediacy of benzyl radicals, which are believed to be intermediates in the reductive cleavage of benzylic esters.
We report herein the selective C–C bond formation by the reaction of nitrobenzyl carbanions, formed via the TDAE strategy, with α-haloesters and α-haloamides. This reaction, extended in benzodioxole and dimethoxybenzene series provides new potentially CNS active agents.Graphical abstract
Electrochemical reduction of a wide variety of aromatic diazonium salts on carbon electrodes (glassy carbon, highly oriented pyrolytic graphite) leads to the covalent attachment of the corresponding aromatic radicals. The films thus deposited on glassy carbon surfaces require mechanical abrasion to be removed. Cyclic voltammetry, X-ray photoelectron spectroscopy, polarization modulation IR reflection absorption spectroscopy, Auger spectroscopy, and Rutherford backscattering spectroscopy allow the characterization of the overlayer and an estimate of the surface coverage. The latter can be controlled through diazonium concentration and electrolysis duration. The mechanism of derivatization is discussed on the basis of the kinetic data obtained from cyclic voltammetry and preparative electrolysis. This versatile method of surface modification may find applications in the field of carbon−epoxy composites as attested by the successful binding of grafted p-aminophenyl groups with epichlorhydrin.
The reactions of chlorotrifluoroethylene and hexafluoiocyclobutene with primary and secondary amines have been investigated. With aliphatic amines these reactions occurred readily and gave a variety of products. In the case of primary aliphatic amines, the products were imino- or imido-type compounds, while secondary amines gave substituted tertiary amines. Primary aromatic amines reacted slowly to yield products of the imino-type structure, but diphenylamine was inert toward both polyfluoro olefins.
Several dithiadiazafulvalenes, which are potent electron donors, were isolated as pure compounds for the first time. Solid charge-transfer complexes with TCNQ and also cation-radical and dication salts with perchlorate gegenions were obtained. Two of the TCNQ complexes are moderately conductive at room temperature (0.083 and 0.011 S/cm). A solution EPR study, combined with theoretical calculations, allowed the determination of the relative equilibrium cation-radical and anion-radical concentrations.
The equilibrium acidity for the C2-H bond in the 3,4-dimethylthiazolium cation (TZCH+), a model for thiamin, was estimated to be higher than 16 by direct titrations with indicators in DMSO solution. This result is in agreement with several earlier indirect estimates based on kinetic acidities and Bronsted plots, which place the acidity in the 16-20 pK(HA+) region in aqueous solution, but not with a direct titration in aqueous medium made under stopped-flow conditions, which placed the acidity in the region of pK(HA+) almost-equal-to 13. 3-Methylbenzothiazolium cation (BZCH+), which was found to be considerably more acidic, reacted with Et3N in DMSO to give a dimer BZC=CZB. Evidence is presented to show that this and similar dimerizations occur by addition of the conjugate base of BZCH+ to the H-C=N+ bond to the BZCH+ thiazolium cation, followed by deprotonation. The conjugate base of BZCH+ adds to BZCH+ in preference to reacting with excess of electrophiles such as t-buOH, PhCHO, or PhCH=CH2. Amines, such as piperidine, add rapidly to less acidic thiazolium cations under conditions where little or no deprotonation occurs. These observations exclude a carbene mechanism for dimerization and amine adduct formation.
Herein we report on the synthesis of the new strong N-base and electron donor tdmegb [1,2,4,5-tetrakis(N,N′-dimethylethyleneguanidino)benzene]. Compared to the previously synthesized ttmgb [1,2,4,5-tetrakis(tetramethylguanidino)benzene], this compound turned out to be a slightly better electron donor and a slightly weaker base. In experiments in which [AuCl(PPH3)] was dissolved in CH3CN together with tdmegb, we observed the formation of the first cyanomethyl complex of Au, namely [Au(CH2CN)(PPh3)] in good yield. This reaction does not take place for ttmgb. Moreover, in CH2Cl2 solutions containing the three components [AuCl(PPh3)], tdmegb and a nitrile (in large excess), only AuI reduction leading to a [Au11Cl3(PPh3)7] cluster is observed. Possible reaction mechanisms for this unusual reaction are discussed.
Chemoselective bis-trifluoromethylation of acyl chlorides using the CF3I/TDAE-derived nucleophilic trifluoromethyl anion reagent is reported. Very high yields are obtained of an ester product formed by sequential nucleophilic bis-trifluoromethylation, followed by acylation of the resultant alcoholate.
A series of disubstituted diarylethanols was prepared in moderate to good yields by reaction of p-nitrobenzyl chloride (1) with various aromatic aldehydes (2-12) in presence of tetrakis(dimethylamino)ethylene (TDAE).
The cyclic voltammetry of the reductive cleavage of some bromodifluoromethyl heterocycles and of the oxidation of the tetrakis(dimethylamino)ethylene was investigated in N,N-dimethylformamide and acetonitrile, at an inert electrode. The systematic investigation of the kinetics of the electrochemical reduction of this series of bromodifluoromethyl compounds provides clear evidence of a concerted electron-transfer-bond-breaking mechanism. Application of the theory of the dissociative electron transfer allowed the estimation of the carbon-halogen bond dissociation energy and the standard potential of the reaction. On the basis of the electrochemical experiments, the tetrakis(dimethylamino)ethylene (TDAE) was found to be an effective reductant of the 2-(bromodifluoromethyl)benzoxazole (1) and of the 5-(bromodifluoromethyl)-3-phenyl-1,2,4-oxadiazole (4). A stepwise electron transfer with a difluoromethyl radical as intermediate is assumed to take place in this reaction. Under mild conditions, the generated difluoromethyl heterocyclic anion was efficiently trapped with aromatic and heterocyclic aldehydes 7-17 and ketones 18 and 19. In this way, the corresponding beta,beta-difluoro-alpha-heteroarylated alcohols 20-38 were obtained in moderate to good yields and the compounds 20, 21, and 23-27 were tested against the HIV-1 virus.
Tetrakis(dimethylamino)ethylene (TDAE) was found to be an effective reductant of the 2-(bromodifluoromethyl)benzoxazole 1. The generated 2-(difluorormethyl) benzoxazole anion was trapped with several aldehydes 2-9, under mild conditions, to give the corresponding 2-(difluoromethyl)benzoxazole alcohols 10-17, in moderate to good yields. (C) 1997, Published by Elsevier Science Ltd.
An amino-linked nitrogen heterocyclic carbene (amino-NHC), 1-tBu, has been shown to mediate carbon-carbon coupling through the direct C-H functionalization of benzene and pyridine in the absence of a metal catalyst. Using EPR, the first spectroscopic evidence corroborating the single electron transfer mechanism for the metal-free carbon-carbon coupling manifold, as reported by others, is introduced.
Powerful reduction reactions: Simple organic electron donors, composed solely of the elements carbon, hydrogen, and nitrogen, upon photoactivation, reduce ground-state benzene rings to their radical anions by electron transfer (DMF=dimethylformamide).
The bis-pyridinylidene 13 converts aliphatic and aryl triflate esters to the corresponding alcohols and phenols respectively, using DMF as solvent, generally in excellent yields. While the deprotection of aryl triflates has been seen with other reagents and by more than one mechanism, the deprotection of alkyl triflates is a new reaction. Studies with (18)O labelled DMF indicate that the C-O bond stays intact and hence it is the S-O bond that cleaves, underlining that the cleavage results from the extraordinary electron donor capability of 13. Trifluoromethanesulfonamides are converted to the parent amines in like manner, representing the first cleavage of such substrates by a ground-state organic reducing reagent.
The molecule (C═C)TTP (TTP = tetra-p-tolylporphyrin) and the triflate salt of its dication, [(C═C)TTP][OTf](2), have been synthesized and characterized. NMR spectroscopy, nucleus-independent chemical shift calculations, and the crystal structure of (C═C)TTP indicate that (C═C)TTP is antiaromatic and (C═C)TTP(2+) is aromatic.
Potassium tert-butoxide mediated intramolecular cyclization of aryl ethers, amines, and amides was efficiently performed under microwave irradiation to provide the corresponding products in high regioisomeric ratios. The reaction proceeds via single-electron transfer to initiate the formation of an aryl radical, followed by a kinetically favored 5-exo-trig and subsequent ring expansion.
A radical outlook: Recently published "organocatalytic C-H activation reactions" have now been interpreted as base-promoted homolytic substitutions. The addition of an aryl radical to an arene followed by deprotonation (see above) and electron transfer form part of the chain reaction. Although these new results are not conceptual breakthroughs, they could be experimental breakthroughs because they presage new transformations in radical (anion) chemistry.
Reactions of super-electron-donors (SEDs) derived from 4-dimethylaminopyridine and from N-methylbenzimidazole with α-methoxy-γ-alkoxyalkyl iodides lead to liberation of the γ-alkoxy groups as their alcohols. This is consistent with generation of alkyl radicals from the alkyl halide precursors, and trapping of these radicals by the radical-cation of the SED, followed by a heterolytic fragmentation.