α-Halocarbonyls as a Valuable Functionalized Tertiary Alkyl Source

This review introduces the synthetic organic chemical value of α-bromocarbonyl compounds with tertiary carbons. This α-bromocarbonyl compound with a tertiary carbon has been used primarily only as a radical initiator in atom transfer radical polymerization (ATRP) reactions. However, with the recent development of photo-radical reactions (around 2010), research on the use of α-bromocarbonyl compounds as tertiary alkyl radical precursors became popular (around 2012). As more examples were reported, α-bromocarbonyl compounds were studied not only as radicals but also for their applications in organometallic and ionic reactions. That is, α-bromocarbonyl compounds act as nucleophiles as well as electrophiles. The carbonyl group of α-bromocarbonyl compounds is also attractive because it allows the skeleton to be converted after the reaction, and it is being applied to total synthesis. In our survey until 2022, α-bromocarbonyl compounds can be used to perform a full range of reactions necessary for organic synthesis, including multi-component reactions, cross-coupling, substitution, cyclization, rearrangement, stereospecific reactions, asymmetric reactions. α-Bromocarbonyl compounds have created a new trend in tertiary alkylation, which until then had limited reaction patterns in organic synthesis. This review focuses on how α-bromocarbonyl compounds can be used in synthetic organic chemistry.

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.

I2 Catalyzed and TBHP/Ammonium-Promoted Conversion of Arylethanone to Nitriles via β-Scission of Iminyl Radicals

Herein, we report a general I2-catalyzed and TBHP/ammonium-promoted conversion of arylethanone to aromatic nitriles under air. This procedure proceeded with the β-scission of iminyl radical, which was facilitated via quenching the released alkyl radical by tert-butyl peroxyl radical leading to peroxide followed with Kornblum-DeLaMare rearrangement. A series of aryl methyl ketone and alkyl aryl ketone worked well with good functional group tolerance in high yields. As such, this metal-free procedure represents a facile, safe, green, and practical procedure in conversion of arylethanone to aromatic nitriles.

Copper-Catalyzed Bifunctionalization/Annulation of Unactivated Alkene with Alkyl Bromides

β-Lactam is a ubiquitous scaffold in bioactive compounds and pharmaceuticals. Herein, we disclose a streamlined method for the construction of β-lactams starting from unactivated alkenes and alkyl bromides via a Cu-catalyzed inter-/intramolecular carboamidation. This reaction proceeded smoothly under mild reaction conditions and exhibited a broad substrate scope and various functional groups. This protocol is not only compatible with 1, 2, and 3° alkyl bromides but also suitable for α-bromo nitrile as well as various benzyl bromides. The mechanism exploration indicated that sequential radical addition/reductive elimination was involved.

Visible-Light-Promoted Cascade Cyclization of 3-Ethynyl-[1,1'-biphenyl]-2-Carbonitriles with Unsaturated α-Bromocarbonyls

A visible-light-promoted cascade cyclization of 3-ethynyl-[1,1'-biphenyl]-2-carbonitriles with unsaturated α-bromocarbonyls for the synthesis of tetrahydrobenzo[mn]cyclopenta[b]acridines is described. Three C(sp3)-C(sp2) bonds, one C(sp2)-N bond, and three cycles can be formed in a single reaction through the addition of a C-centered radical to the carbon-carbon triple bond and three radical cyclizations. This reaction features mild conditions, wide substrate scope, and high bond-forming efficiency.

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.

The Renaissance of Organo Nitriles in Organic Synthesis

In the arena of functional group-oriented organic synthesis, the nitrile or cyano functionality is of immense importance. The presence of nucleophilic N -atom, π-coordinating ability of the triple bond, and electrophilic C-center imparts unique and interesting reactivity. Owing to the ability of the nitrile to transform into various other functional groups or intermediates, the chemistry is very rich and diverse. In particular, the involvement of nitrile in numerous organic reactions such as inter- or intramolecular alkyne insertion, [2 + 2 + 2] cycloaddition with alkynes, [3 + 2] cycloaddition with azides, [4 + 2] cycloaddition with diene allow the synthesis of many important carbocycles and heterocycles. Furthermore, the nitrile serves as a directing group in many C-H bond functionalization reactions to introduce diverse functionality and participate as a radical acceptor in radical cascade strategies to obtain a large variety of functional molecules. This review mainly focuses on the reactivity and diverse synthetic application of the nitrile including C-H bond functionalization, alkyne insertion, cycloaddition, and thermal or photochemical cascade strategy. The objective of the current review aims at bringing out the striking collection of various nitrile-triggered organic transformations.

Nitriles as radical acceptors in radical cascade reactions

The application of the cyano group as a radical acceptor in the cascade reactions for the construction of various important heterocycles and carbocycles was summarized.

Radical Transformations towards the Synthesis of Quinoline: A Review

Quinoline is considered one of the most ubiquitous heterocycles due to its engaging biological activities and synthetic utility over organic transformations. Over the past few decades, numerous reports have been documented in the synthesis of quinolines. The classical methods including, Skraup, Friedlander, Doebner-von-Miller, Conrad-Limpach, Pfitzinger quinoline synthesis, and so forth, these are the well-known methods to construct principal quinoline scaffold with several advantages and limitations. Recently, radical insertion or catalyzed reactions have emerged as a powerful and efficient tool to construct heterocycles with high atom efficiency and step economy. In this concern, this minireview mainly focused on the developments of Quinoline synthesis via radical reactions. In addition, a brief description of the preparation procedure, reactivity, and mechanisms is also included, where as possible. Respectively, the synthesis of quinolines is classified and summarized based on its reactivity, so it will help the researchers to grab the information in this exploration area, as Quinolines are promising pharmacophores.

Visible-light-induced radical cascade cyclization of oxime esters and aryl isonitriles: synthesis of cyclopenta[b]quinoxalines

The in situ-formed nitrile produced by oxime esters was taken into a visible-light-induced radical cascade cyclization with aryl isonitriles.

Cascade Annulation of 2-Alkynylthioanisoles with Unsaturated α-Bromocarbonyls Leading to Thio-Benzobicyclic Skeletons

A protocol of Cu-catalyzed annulation of phenylethynylsulfanes with unsaturated α-bromocarbonyls for the construction of thio-benzobicyclic skeletons is described. In this single reaction, three new bonds and two new rings can be established, highlighting the step-economics and high efficiency of this protocol.

Visible-Light-Induced Radical Cascade Cyclization: Synthesis of the ABCD Ring Cores of Camptothecins

A new strategy for constructing indolizino[1,2-b]quinolin-9(11H)-ones (ring cores of camptothecins) from readily available isocyanoarenes and N-(alkyl-2-yn-1-yl)pyridin-2(1H)-ones has been developed through a visible-light-induced radical cascade cyclization process. The reaction proceeds under mild conditions with fair to excellent yields. The easy introduction of substituents for both reactants and the broad functional group tolerance of the reaction make it a straightforward route to the cores of the marketed camptothecins and their derivatives.

Visible-light-promoted iminyl radical formation from vinyl azides: synthesis of 6-(fluoro)alkylated phenanthridines

An efficient strategy assisted by visible-light-promoted iminyl radical formation has been developed for the synthesis of 6-(fluoro)alkylated phenanthridine derivatives. In the reactions, addition of alkyl and trifluoromethyl radicals onto vinyl azides gives iminyl radicals, which then undergo intramolecular homolytic aromatic substitution leading to phenanthridines. These reactions can be carried out under mild conditions with high chemical yields and broad substrate scope.

Synthetic Strategies for 5- and 6-Membered Ring Azaheterocycles Facilitated by Iminyl Radicals

The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole, indole, pyridine, quinoline, quinazoline and related 6-membered ring-containing aza-arenes figure prominently. This review focuses on the search for fast, efficient and environmentally friendly preparative methods for these rings with specific emphasis on iminyl radical-mediated procedures. Oxime derivatives, particularly oxime esters and oxime ethers, are attractive precursors for these radicals. Their use is described in conventional thermolytic, microwave-assisted and UV-vis based preparative procedures. Photoredox-catalyzed protocols involving designer oxime ethers are also covered. Choice can be made amongst these synthetic strategies for a wide variety of 5- and 6-membered ring heterocycles including phenanthridine and related aza-arenes. Applications to selected natural products and bioactive molecules, including trispheridine, vasconine, luotonin A and rutaecarpine, are included.

Recent progress in imidoyl radical-involved reactions

This review discusses the recent progress in imidoyl radical-involved reactions including the synthesis of functionalized heterocycles, nitriles, imines, amines,etc.

Recent advances in the synthesis of nitrogen heterocycles via radical cascade reactions using isonitriles as radical acceptors

Radical isonitrile insertion reactions are highly valuable for the construction of N-containing heterocycles such as phenanthridines, quinolines, isoquinolines, quinoxalines and indoles.