α-Functionalization of Imines via Visible Light Photoredox Catalysis

Direct Cyanoalkylation of Imines Driven by a Photoactive Electron Donor-Acceptor Complex

β-Amino nitriles are important molecular scaffolds. Cyanoalkylation of imines is the most straightforward method for the construction of these scaffolds. In this study, we report the novel cyanoalkylation of imines via radical coupling enabled by a photoactive electron donor-acceptor complex. This strategy is characterized by mild conditions, broad reaction scopes, and high atom economy. The scalability and practicality of this strategy are demonstrated by a 40 g continuous flow system from which a variety of important pharmaceutical-related molecules were obtained.

Synthesis, Theoretical and Experimental Investigation of Electronic Properties of New Fluoranthenyl-Based Compounds for OLEDs Applications

Three novel conjugated molecules have been synthesized: 2-(fluoranthen-3-yliminomethyl)-3,4-ethylenedioxythio-phene (EIF), 2-(fluoranthen-3-yliminomethyl) thiophene (TIF), and 2-(fluoranthen-3-yliminomethyl) fluorene (FIF). Optical properties were obtained from electronic absorption and emission spectra in DMSO solution. The solvatochromic properties of the molecules have been studied in different solvents. Electrochemical properties were studied by cyclic voltammetry in a Bu4NBF4 (0.1 M)/MeCN organic solution. As part of investigations to explain the nature of electronic transition process, we have performed geometry optimization of both the ground and the vertical excitation states, using the DFT B3LYP/6-311G (d, p) and TD-DFT (CPCM)/B3LYP/6-311G (d, p) approaches, respectively. Theoretical calculations closely match the experimental findings. Results show that EIF, TIF and FIF are potential candidates to be used as electron transport layer in Organic Light-Emitting Diodes (OLEDs).

Exploiting N-Centered Umpolung Reactivity of α-Iminomalonates for the Synthesis of N-Sulfenylimines and Sulfonamides

An efficient and interesting N-centered umpolung method has been disclosed to construct beneficial S-N bonds, furnishing N-sulfenylimines, which can readily be converted into the corresponding sulfonamide derivatives in a one-pot sequential operation. N-Sulfenylimines are potent intermediates in organic synthesis, whereas sulfonamides are of major molecular interest due to their rich biological activities and wide applicability in medicinal chemistry. Owing to the simple reaction conditions and setup, this protocol displays a broad and versatile substrate scope, resulting in excellent functional group tolerability toward the synthesis of both N-sulfenylimines and sulfonamides. A density functional theory (DFT) computed and experimentally supported convenient mechanism has been proposed for this unique method.

Visible-light-mediated α-amino alkylation of ketimines and aldimines for the synthesis of 1,2-diamines

A visible-light-mediated protocol to prepare 1,2-diamines has been successfully explored based on the photoredox/Brønsted acid co-catalyzed α-amino alkylations of imines with tertiary amines. Both ketimines and aldimines are applicable to this transformation. Various 1,2-diamines with different functional groups were produced in moderate to excellent yields. Moreover, this approach could be performed on a gram scale, showing its practicality.

Photoinduced Diastereoselective Aminoalkylation of Cubanes

The unique properties of rigid, nonconjugated hydrocarbons provide many opportunities to design molecular building blocks for a variety of applications, but the development of suitable conditions for alkylation of cubanes is quite challenging. Herein, a photoinduced method for aminoalkylation of cubanes is reported. The benign conditions reported allow the incorporation of a wide variety of (hetero)arylimine reaction partners with broad functional group tolerance and high diastereoselectivity.

Photoredox Promoted Barbier-Type Reaction of Alkyl Iodides with N-Alkyl and N-Aryl Imines

The reaction of organozinc reagents with unactivated imines is accelerated when performed in the presence of a photocatalyst under blue light irradiation. Coordination between Lewis acidic zinc iodide and the imine is a key factor responsible for the reaction efficiency. The method can be carried out using alkyl iodides under Barbier conditions.

Photoredox-Catalyzed Synthesis of β-Amino Alcohols: Hydroxymethylation of Imines with α-Silyl Ether as Hydroxymethyl Radical Precursor

Carbon-carbon bond formation is an efficient approach for the synthesis of amino alcohols using two simple starting materials. Herein, we present a novel method for a divergent synthesis of β-amino ethers and β-amino alcohols in a sequential one-pot protocol under high-efficiency, mild, and metal- or metal-free conditions. Especially, TMSCH₂OPMP was developed as a synthetic equivalent of α-hydroxymethyl radical in an in situ photocatalyzed oxidative PMP group deprotection strategy under air. A preliminary mechanistic investigation provides evidence for reaction mechanism involving a photoinduced α-alkoxy methyl radical and superoxide.

Photocatalytic Alkyl Addition to Access Quaternary Alkynyl α-Amino Esters

A regioselective alkylation of β,γ-alkynyl-α-imino esters by visible-light photocatalysis has been developed. This method enables 1,2-addition of methyl, primary, secondary, and tertiary alkyl radicals to the conjugated imines under mild conditions to produce a variety of quaternary alkynyl α-amino acid and cyclic amino acid motifs. Alkyl radicals are generated from alkyl bis(catecholato)silicates with an organic photocatalyst. This process is effective under an air atmosphere, providing operational benefits compared to conventional alkylation using organometallic reagents.

Blue Light-Mediated, Photocatalyst-Free Decarboxylative Alkylation of Heteroaryl Sulfinimines

We report a diastereoselective, photocatalyst-free decarboxylative alkylation of (hetero)aryl sulfinimines using redox-active esters under blue light. High yields and diastereoselectivities can be achieved under mild conditions, and we demonstrate its utility as a synthetic method, especially for medicinal chemists.

Visible-light-induced alkylation of 2-iminochromene

A metal-free, photochemical protocol for C-4 alkylation of 2-iminochromene has been developed by employing the naturally abundant feedstock carboxylic acid. Selective C-4 alkylation under photoredox conditions to access C-4 alkylated 2-iminochromene in up to 81% yield was achieved. In addition, biologically relevant chromophores can be easily incorporated under this protocol.

pH-Controlled Intramolecular Decarboxylative Cyclization of Biarylacetic Acids: Implication on Umpolung Reactivity of Aroyl Radicals

A simple approach for the intramolecular aroylation of electron-rich arenes under mild conditions has been developed. A pH-controlled polarity umpolung strategy can be used to synthesize different fluorenones, which are important building blocks for biological applications. Unlike previous acylation reactions involving nucleophilic aroyl radicals, this approach likely relies on in situ generated electrophilic aroyl radicals. Detailed mechanistic and computational investigations provide detailed insights into the reaction mechanism and support the hypothesis of a pH-mediated umpolung.

Tuning the Activity-Stability Balance of Photocatalytic Organic Materials for Oxidative Coupling Reactions

Three materials containing a photoactive unit, 10-phenyl phenothiazine (PTH), have been studied for the visible light-mediated oxidative coupling of amines. In particular, the materials considered are assembled through the condensation of extended polyimine, polyhydrazone, or polytriazine frameworks. These three materials present different stabilities in the presence of strong nucleophiles such as amines, which is a key factor for efficient catalytic performance. In the series of materials reported herein, the triazine-based material shows the optimal compromise between activity and stability when studied for the oxidative coupling of amines, achieving imine products. Accordingly, while significant leaching of molecular active fragments is ruled out for triazine-based polymers, other materials of the series show a significant chemical erosion as a result of the reaction with the amine substrates. Consequently, only a triazine-based material allows performing several catalytic cycles (up to seven) with yields higher than 80%. The applicability of this heterogeneous catalyst has been proven with a variety of substrates, confirming its stability and obtaining diverse imine coupling products with excellent yields.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Photoredox-Catalyzed Synthesis of α-Amino Acid Amides by Imine Carbamoylation

An operationally simple protocol for the photocatalytic carbamoylation of imines is reported. Easily available, bench-stable 4-amido Hantzsch ester derivatives serve as precursors to carbamoyl radicals that undergo rapid addition to N-aryl imines. The reaction proceeds under blue light irradiation in the presence of the photocatalyst 3DPAFIPN and Brønsted/Lewis acid additives. Mechanistic studies indicated a photoredox mechanism that involves carbamoyl radicals.

Visible-light-driven photocatalyst-free deoxygenative alkylation of imines with alcohols

Upon easy access and direct photoexcitation of xanthate anions, visible-light-driven deoxygenative alkylation of imines with a wide variety of alcohols has been achieved via a phosphine-assisted one-pot protocol, without any photocatalysts.

Radical umpolung chemistry enabled by dual catalysis: concept and recent advances

We present a perspective on recent advances in radical umpolung chemistry; some selected examples in this area have been highlighted.

Solvent-Free Visible Light Photocatalytic Oxidation Processes Mediated by Transparent Films of an Imine-Based Organic Polymer

Reaction between concentrated solutions of phenotiazine containing trialdehyde building block 4,4′,4″-(10-phenothiazine-3,7,10-triyl)tribenzaldehyde and (1R,2R)-cyclohexane-1,2-diamine results in the formation of a yellow transparent film. Exhaustive characterization of this material indicates that it is the result of the assembly of a linear polymer resulting from the linking of imine-based macrocycles. Phenotiazine units confer to this plastic the optical properties characteristic of photocatalytic materials. The transparency of the obtained material enabled the performance of solvent-free photocatalytic processes. This concept is illustrated by the oxidation of liquid organic sulfides, which can be performed in a recyclable manner. According to selective quenching experiments, such processes are the result of the energy transfer to oxygen molecule, generating singlet oxygen that is able to activate the sulfide molecules directly.

Boron Chelates Derived from N-Acylhydrazones as Radical Acceptors: Photocatalyzed Coupling of Hydrazones with Carboxylic Acids

Difluoroboryl complexes obtained from N-acyl hydrazones upon brief treatment with boron trifluoride and allylic silane serve as efficient acceptors of alkyl radicals. The reaction of the boryl chelates with carboxylic acids in the presence of an acridine-type photocatalyst leading to N-acyl hydrazides is described. The efficiency of addition at the C═N bond of the chelates is determined by the formation of a nitrogen-centered radical stabilized by the boron-containing heterocyclic ring.

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.

Photoredox-catalyzed multicomponent Petasis reaction in batch and continuous flow with alkyl boronic acids

Multicomponent reactions (MCRs) are ideal platforms for the generation of a wide variety of organic scaffolds in a convergent and atom-economical manner. Many strategies for the generation of highly substituted and diverse structures have been developed and among these, the Petasis reaction represents a viable reaction manifold for the synthesis of substituted amines via coupling of an amine, an aldehyde and a boronic acid (BA). Despite its synthetic utility, the inherent drawbacks associated with the traditional two-electron Petasis reaction have stimulated continuous research towards more facile and tolerant methodologies. In this regard, we present the use of free alkyl BAs as effective radical precursors in this MCR through a single-electron transfer mechanism under mild reaction conditions. We have further demonstrated its applicability to photo-flow reactors, facilitating the reaction scale-up for the rapid assembly of complex molecular structures.

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Multicomponent photocatalyzed Petasis reaction

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Boronic acids as alkyl radical precursors under mild reaction conditions

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Photo-flow chemistry application for Petasis reaction

Direct α-Alkenylation of Cyclic Amines with Maleimides through Fe(III)-Catalyzed C(sp3)-H/C(sp2)-H Cross Dehydrogenative Coupling

Presented herein is a novel and efficient α-C(sp³)-H alkenylation of cyclic amines with maleimides. Mechanistically, this C(sp³)-H/C(sp²)-H cross dehydrogenative coupling (CDC) reaction involves a cascade procedure including oxidative α-amino radical formation from the cyclic amine substrate and nucleophilic addition of the in situ formed α-amino radical onto the electron-deficient carbon-carbon double bond of maleimide followed by oxidation and β-elimination. Notably, this direct α-functionalization provides an effective alternative to the conventional ionic reaction mode, in which an imine or iminium intermediate is formed to react with electron-rich coupling partners other than electron-deficient ones. In general, this method features readily available and structurally diverse substrates, a green and economical catalyst, a unique reaction pathway, mild reaction conditions, high efficiency, and excellent atom economy. This new reaction enriches the application of Fe(III)-catalyzed C(sp³)-H activation and functionalization.

Visible-Light Radical–Radical Coupling vs. Radical Addition: Disentangling a Mechanistic Knot

A highly enantioselective protocol has been recently described as allowing the synthesis of five-membered cyclic imines harnessing the selective generation of a β-Csp3-centered radical of acyl heterocyclic derivatives and its subsequent interaction with diverse NH-ketimines. The overall transformation represents a novel cascade process strategy crafted by individual well-known steps; however, the construction of the new C-C bond highlights a crucial knot from a mechanistically perspective. We believe that the full understanding of this enigmatic step may enrich the current literature and expand latent future ideas. Therefore, a detailed mechanistic study of the protocol has been conducted. Here, we provide theoretical insight into the mechanism using quantum chemistry calculations. Two possible pathways have been investigated: (a) imine reduction followed by radical–radical coupling and (b) radical addition followed by product reduction. In addition, investigations to unveil the origin behind the enantioselectivity of the 1-pyrroline derivatives have been conducted as well.

Catalytic Generation of Carbanions through Carbonyl Umpolung

Carbonyl Umpolung is a powerful strategy in organic chemistry to construct complex molecules. Over the last few years, versatile catalytic approaches for the generation of acyl anion equivalents from carbonyl compounds have been developed, but methods to obtain alkyl carbanions from carbonyl compounds in a catalytic fashion are still at an early stage. This Minireview summarizes recent progress in the generation of alkyl carbanions through catalytic carbonyl Umpolung. Two different catalytic approaches can be utilized to enable the generation of alkyl carbanions from carbonyl compounds: the catalytic Wolff–Kishner reaction and the catalytic single‐electron reduction of carbonyl compounds and imines. We discuss the reaction scope, mechanistic insights, and synthetic applications of the methods as well as potential future developments.

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions

When aiming to synthesize molecules with elevated molecular complexity starting from relatively simple starting materials, photochemical transformations represent an open avenue to circumvent analogous multistep procedures. Specifically, light-mediated cycloadditions remain as powerful tools to generate new bonds begotten from non-very intuitive disconnections, that alternative thermal protocols would not offer. In response to the current trend in both industrial and academic research pointing towards green and sustainable processes, several strategies that meet these requirements are currently available in the literature. This Minireview summarizes [2+2] and [4+2] photocycloadditions that do not require the use of metal photocatalysts by means of alternative strategies. It is segmented according to the cycloaddition type in order to give the reader a friendly approach and we primarily focus on the most recent developments in the field carried out using visible light, a general overview of the mechanism in each case is offered as well.

Visible Light-Induced Pd-Catalyzed Alkyl-Heck Reaction of Oximes

A visible light-induced palladium-catalyzed oxidative C-H alkylation of oximes has been developed. This mild protocol allows for an efficient atom economical C-C bond construction of alkyl-substituted oximes. A broad range of primary, secondary, and tertiary alkyl bromides and iodides, as well as a range of different formaldoximes, can efficiently undergo this transformation. The method features visible light-induced generation of nucleophilic hybrid alkyl Pd radical intermediates, which upon radical addition at the imine moiety and a subsequent β-hydrogen elimination deliver substituted imines.

New Trends in Asymmetric Catalysis

As far back as the mid-nineteenth century, the studies of Louis Pasteur brought to light the essential role of molecular chirality in biology [...]

Visible-Light-Mediated α-Amino Alkylation of Azomethine Imines: An Approach to N-(β-Aminoalkyl)pyrazolidinones

Herein, a mild and robust photocatalytic protocol for the combination of amino and pyrazolidinone functionalities through a radical α-amino alkylation of azomethine iminium ions is demonstrated. This method presents a high functional group tolerance providing direct access to a large family of N-(β-aminoalkyl)pyrazolidinones in good to excellent yields, including the late-stage incorporation of the pyrazolidinone moiety to pharmaceutical ingredients. We propose a plausible scenario for the C-C bond-forming step which involves radical addition followed by a spin-center-shift event.