6-Aroylated Phenanthridines via Base Promoted Homolytic Aromatic Substitution (BHAS)

Iodide Ion-Promoted Highly Regioselective Triazolization of Aldehydes via Desulfonation-Associated Direct Radical Coupling

A highly efficient iodide ion (I-)-promoted method for direct α-C(sp2)-H triazolization of aldehydes has been developed for the regioselective synthesis of N2-substituted triazole derivatives. The developed method features the corresponding products in good yields (up to 99%) with an excellent functional group tolerance via an intermolecular oxidative radical coupling. Experimental mechanistic investigations indicate that the reaction proceeds via an I--promoted synergistic desulfonylation process, which provides a high regioselectivity. The developed method provides a direct, metal-free, operationally simple, and highly regioselective approach to C(sp2)-H triazolization from easily accessible aldehydes in air.

Tetraphenylethene-based mononuclear aggregation-induced emission (AIE)-active mechanofluorochromism gold(I) complexes with different auxiliary ligands

In this study, a series of tetraphenylethene-containing gold(I) complexes with different auxiliary ligands have been synthesized. These complexes were characterized using a variety of techniques including nuclear magnetic resonance spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Their aggregation-induced emission (AIE) behaviors were investigated through ultraviolet/visible and photoluminescence spectrum analyses, and dynamic light scattering measurements. Meanwhile, their mechanofluorochromic properties were also studied via solid-state photoluminescence spectroscopy. Intriguingly, all these mononuclear gold(I) molecules functionalized by tetraphenylethene group demonstrated AIE phenomena. Furthermore, five gold(I) complexes possessing diverse auxiliary ligands exhibited distinct fluorescence changes in response to mechanical grinding. For luminogens 2-5, their solids showed reversible mechanofluorochromic behaviors triggered by the mutual transformation of crystalline and amorphous states, while for luminogen 1, blue-green-cyan three-color solid fluorescence conversion was realized by sequential mechanical grinding and solvent fumigation. Based on this stimuli-responsive tricolored fluorescence feature of 1, an information encryption system was successfully constructed.

Thermal and Photoinduced Radical Cascade Annulation using Aryl Isonitriles: An Approach to Quinoline-Derived Benzophosphole Oxides

Herein, we present a radical cascade addition cyclization sequence to access quinoline-based benzophosphole oxides from ortho-alkynylated aromatic phosphine oxides using various aryl isonitriles as radical acceptors and inexpensive tert-butyl-hydroperoxide (TBHP) as a terminal oxidant in the presence of a catalytic amount of silver acetate. Alternatively, the same cascade can be realized through a sustainable photochemical approach utilizing 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as an organic photocatalyst at room temperature. The introduced modular approach shows broad functional group tolerance and offers straightforward access to complex P,N-containing polyheterocyclic arenes. These novel π-extended benzophosphole oxides exhibit interesting photophysical and electrochemical properties such as absorption in the visible region, emission and reversible reduction at low potentials, which makes them promising for potential materials science applications. The photophysical properties can further be tuned by the addition of external Lewis and Brønsted acids.

Redox-Neutral Cyclization of 2-Isocyanobiaryls through Photoredox/PPh3 Dual Catalysis

The photoredox/PPh3-mediated cyclization of 2-isocyanobiaryls has been developed. A substantial range of functional-group-rich phenanthridine derivatives were synthesized at room temperature in a highly selective and atom-economic manner. Mechanistic studies suggested that the cyclization process is probably mediated both by Ph3P radical cation with key 1,2-hydride transfer and hydrogen atom generated through O-H bond homolytic cleavage of Ph3P-OH radical intermediate.

NHC-Catalyzed Tandem Reaction: A Strategy for the Synthesis of 2-Pyrrolidinone-Functionalized Phenanthridines

Herein, an N-heterocyclic carbene (NHC)-catalyzed tandem cyclization/addition/cyclization reaction of 2-isocyanobiaryls and α-bromo-N-cinnamylamides for the synthesis of 2-pyrrolidinone-functionalized phenanthridines is developed. This protocol features a radical cascade process, broad substrate scope, and good functional group compatibility under metal- and oxidant-free reaction conditions.

Isocyanides as Acceptor Groups in MHAT Reactions with Unactivated Alkenes

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

Access to Polyheterocyclic Compounds through Iron(II)-Mediated Radical Cascade Cyclization Utilizing 2-Ethynylbenzaldehydes and Aryl Isonitriles

An oxidative radical cascade addition cyclization approach for the synthesis of quinoline-based π-extended polyheterocyclic compounds is reported. Eco-friendly iron catalysis and inexpensive tert-butylhydroperoxide (TBHP) as the oxidant have been utilized in the transformation of various readily available ortho-alkynylated aromatic aldehydes as radical precursors with aryl isonitriles as radical acceptors. Indole and thiophene-based carbaldehydes allow the preparation of quinolines that are π-conjugated with an additional heteroarene moiety in a single sequence by applying the introduced method.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Electrochemical trifluoromethylation of 2-isocyanobiaryls using CF3SO2Na: synthesis of 6-(trifluoromethyl)phenanthridines

An efficient trifluoromethylation of 2-isocyanobiaryls was developed through the constant current electrolysis, employing sodium trifluoromethanesulfinate (CF₃SO₂Na) as the trifluoromethyl source. The method enabled the syntheses of a series of 6-(trifluoromethyl)phenanthridine derivatives in moderate to high yields under metal- and oxidant-free conditions. A gram-scale synthesis highlights the synthetic versatility of the reported protocol.

Photoinduced N-Heterocyclic Nitrenium-Catalyzed Single Electron Reduction of Acyl Fluorides for Phenanthridine Synthesis

Acyl fluorides are versatile reagents in organic synthesis. However, there is no precedent to employ acyl fluorides as acyl radical precursors. We herein report an N-heterocyclic nitrenium iodide salt-catalyzed photoreduction of acyl fluorides to produce acyl radicals, which could react with 2-isocyanobiaryls to afford various carbonyl phenanthridines.

Taming the stilbene radical anion

Radical anions appear as intermediates in a variety of organic reductions and have recently garnered interest for their role as mediators for electron-driven catalysis as well as for organic electron conductor materials. Due to their unstable nature, the isolation of such organic radical anions is usually only possible by using extended aromatic systems, whereas non-aromatic unsaturated hydrocarbons have so far only been observed in situ. We herein report the first isolation, structure and spectroscopic characterization of a simple aryl substituted alkene radical anion, namely that of stilbene (1,2-diphenyl ethylene), achieved by encapsulation between two [K{18c6}] cations. The formation of the radical anion is accompanied by Z → E isomerization of the involved double bond, also on a catalytic scale. Employing the linear iron(i) complex [Fe(NR2)2]- as a reductant and coordination site also allows for this transformation, via formation of an iron(ii) bound radical anion. The use of the iron complex now also allows for Z → E isomerization of electron richer, simple alkenes bearing either mixed alkyl/aryl or even bis(alkyl) substitution.

Visible Light-Promoted Radical-Mediated Ring-Opening/Cyclization of Vinyl Benzotriazoles: An Alternative Approach to Phenanthridines

A visible light-promoted radical-mediated ring-opening/cyclization of vinyl benzotriazoles has been developed. The method provides an efficient and practical approach to synthesize functionalized phenanthridines from vinyl benzotriazoles with alkyl bromides under mild conditions. Significantly, the readily available and bench-stable vinyl benzotriazoles can serve as valuable alternative radical acceptors during the synthesis of phenanthridines.

Photocatalyzed alkylative cyclization of 2-isocyanobiphenyls with unactivated alkyl iodides

We herein report the first photocatalyzed radical cascade cyclization of 2-isocyanobiaryls with unactivated alkyl iodides. This simple protocol operates under mild reaction conditions and affords 6-alkyl phenanthridines in good yields. To elucidate the reaction mechanism, Stern-Volmer quenching studies were carried out and these studies revealed that the photocatalyst is not directly involved in a single electron transfer process with the alkyl iodide.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

2,3-Difunctionalized Indoles via Radical Acylation or Trifluoromethylation of ortho-Alkynylphenyl Isonitriles

A radical cascade to 2,3-disubstituted indoles proceeding via acylation or trifluoromethylation of ortho-alkynylphenyl isonitriles is presented. In these cascades, two C-C bonds and one C-O bond are formed using an inexpensive oxidant and a catalytic copper or iron salt. The starting isonitriles are easily accessible, and commercially available aldehydes and fluoromethylation reagents serve as reaction partners. Functional group tolerance is high, as documented by the successful preparation of a series of 2,3-disubstituted indoles.

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

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

Photocatalytic xanthate-based radical addition/cyclization reaction sequence toward 2-biphenyl isocyanides: synthesis of 6-alkylated phenanthridines

A photocatalytic xanthate-based radical addition/cyclization reaction cascade toward 2-biphenylisocyanides is described as a practical and modular approach to 6-alkylated phenanthridines. The use of xanthates as radical precursors allowed the synthesis of diversely 6-substituted phenanthridines. Electrophilic radicals derived from nitriles, aromatic and aliphatic ketones, malonates, and amide derivatives, as well as radicals derived from phthalimidomethyl and benzylic derivatives were successfully introduced. The reaction proceeds under mild conditions without a stoichiometric amount of oxidant. Thirty novel phenanthridine scaffolds were synthesized with yields ranging from 24 to 76%.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Additive-Free Radical Cascade Reaction of Oxime Esters: Synthesis of Pyrroline-Functionalized Phenanthridines

A variety of dihydropyrrole-functionalized phenanthridines were efficiently synthesized by the metal-free, radical cascade cyclization reaction of 2-isocyanobiphenyls with γ,δ-unsaturated oxime esters. The C-N/C-C/C-C bonds were formed via the oil bath method in a one-pot procedure with broad substrate applicability. The radical process was supported by kinetic isotope effect studies and radical inhibition studies.

Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs

The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The ¹H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.

PPh3/NaI driven photocatalytic decarboxylative radical cascade alkylarylation reaction of 2-isocyanobiaryls

The first triphenylphosphine/sodium iodide driven photocatalytic decarboxylative cascade cyclization of 2-isocyano-biaryls with alkyl N-hydroxyphthalimide (NHP) esters was developed. This operationally simple protocol results in multiple carbon-carbon bond formation under transition metal free conditions, affording a novel and environmentally benign entry to producing 6-alkyl phenanthridines with moderate to good yields.

Revealing the Iron-Catalyzed β-Methyl Scission of tert-Butoxyl Radicals via the Mechanistic Studies of Carboazidation of Alkenes

We describe here a mechanistic study of the iron-catalyzed carboazidation of alkenes involving an intriguing metal-assisted β-methyl scission process. Although t-BuO radical has frequently been observed in experiments, the β-methyl scission from a t-BuO radical into a methyl radical and acetone is still broadly believed to be thermodynamically spontaneous and difficult to control. An iron-catalyzed β-methyl scission of t-BuO is investigated in this work. Compared to a free t-BuO radical, the coordination at the iron atom reduces the activation energy for the scission from 9.3 to 3.9 ~ 5.2 kcal/mol. The low activation energy makes the iron-catalyzed β-methyl scission of t-BuO radicals almost an incomparably facile process and explains the selective formation of methyl radicals at low temperature in the presence of some iron catalysts. In addition, a radical relay process and an outer-sphere radical azidation process in the iron-catalyzed carboazidation of alkenes are suggested by density functional theory (DFT) calculations.

Selective Dehydrogenative Acylation of Enamides with Aldehydes Leading to Valuable β-Ketoenamides

We have presented a unique example of dehydrogenative acylation of enamides with aldehydes enabled by an earth-abundant iron catalyst. The protocol provides the straightforward access to valuable β-ketoenamides with ample substrate scope and excellent functional group tolerance. Notably, distinct C-H acylation of enamide rather than at N-H moiety site occurs with absolute Z-selectivity was observed. Late-stage modifications of complex molecules and versatile synthetic utility of β-ketoenamides further highlight the practicability of this transformation.

Mn-mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides with carbazate: access to quinazoline-2-carboxylates

Mn-TBHP mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides using methyl carbazate has been described.

Energy transfer-driven regioselective synthesis of functionalized phenanthridines by visible-light Ir photocatalysis

A photocatalytic strategy for selective synthesis of 2-substituted phenanthridines from N-iminylpyridinium salts has been developed.

Photocatalytic reverse polarity Povarov reaction

A visible light mediated iridium photocatalysed reverse polarity Povarov reaction of aryl imines and electron deficient alkenes is described. Operating via a putative nucleophilic α-amino radical, generated by a proton coupled electron transfer process, addition to a range of conjugated electron deficient alkene substrates affords substituted tetrahydroquinoline products in high yields and with typically good to excellent diastereoselectivity in favor of the trans diastereoisomer. Sub-stoichiometric quantities of Hantzsch ester were found to be key to initiate the overall redox-neutral, free radical cyclization cascade. This new reaction complements existing two electron Lewis acid mediated variants and expands the capabilities of imine umpolung chemistry to synthetically relevant cyclisation methodology.

Radical Cyanotrifluoromethylation of Isocyanides: Step-Economical Access to CF3-Containing Nitriles, Amines, and Imines

A novel copper-catalyzed radical cyanotrifluoromethylation has been achieved through a multicomponent reaction of isocyanides, Togni's reagent, and trimethylsilyl cyanides, affording trifluoroacetimidoyl nitriles in good yields. This reaction demonstrates a unique feature of merging two valuable functional groups-trifluoromethyl (CF₃) and cyan (CN)-onto the same C atom. The transformation proceeds by the initial addition of the CF₃ radical to isocyanide and the subsequent intermolecular C-CN formation. The products can be successfully transformed to a series of CF₃-containing amines and imines that may serve in the synthesis of valuable pharmaceuticals and agrochemicals.

To each his own: isonitriles for all flavors. Functionalized isocyanides as valuable tools in organic synthesis

The term functionalized isocyanides refers to all those isocyanides in which a neighbouring functional group can finely tune the reactivity of the isocyano group or can be exploited in post-functionalization processes. In this manuscript, we have reviewed all the isocyanides in which the pendant functional group causes either deviation from or reinforces the normal reactivity of the isocyano group and categorized them to highlight their common features and differences. An analysis of their synthetic potential and the possible unexplored directions for future research studies is also addressed.

Convenient synthesis of 6-alkyl phenanthridines and 1-alkyl isoquinolines via silver-catalyzed oxidative radical decarboxylation

A convenient and efficient protocol for the synthesis of 6-alkyl phenanthridines and 1-alkyl isoquinolines has been developed. The reaction relies on the coupling of 2-isocyanobiphenyls and vinyl isonitriles with alkyl radicals formed by the silver-catalyzed decarboxylation of stoichiometric aliphatic carboxylic acids, and affords diverse phenanthridine and isoquinoline derivatives under mild reaction conditions. The experiment of β-scission of cyclobutylcarbinyl radicals is used to shed light on the reaction mechanism.

Metal-free oxidative isocyanides insertion with aromatic aldehydes to aroylated N-heterocycles

A metal-free, mild and efficient protocol for the construction of aroylated N-heterocycles via radical cascade aroylation of phenyl or vinyl isocyanides with aromatic aldehydes has been developed. Both phenanthridine and isoquinoline derivatives are delivered quickly in moderate to good yields with high regioselectivity.