Metal-Free [2+2+1] Annulation of Alkynes, Nitriles and Nitrogen Atoms from Iminoiodanes for Synthesis of Highly Substituted Imidazoles

Silver-Mediated [2 + 2 + 1] Cyclization of ortho-Propioloylbenzonitriles with Elemental Selenium: Synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones

An efficient silver-mediated [2 + 2 + 1] cyclization protocol of ortho-propioloylbenzonitriles with elemental selenium for the synthesis of 4H-indeno[1,2-c][1,2]selenazol-4-ones has been developed. One C-Se bond, one N-Se bond, and one C-C bond were rapidly constructed in one step. The reaction might proceed via the formation of a highly reactive selenoketene intermediate, followed by intramolecular cyclization.

In Situ Generation of N-Triflylimino-λ3-iodanes: Application to Imidation of Phosphines and Catalytic α-Amidation of 1,3-Dicarbonyl Compounds

We describe the imidation of phosphines and α-amidation of 1,3-dicarbonyl compounds using N-triflylimino-λ³-iodane, which is generated in situ from iodosylarene and triflylamide without any other additives. Furthermore, the imino-λ³-iodane catalytically generated from an iodoarene precatalyst with oxone and triflylamide promotes α-amidation of 1,3-dicarbonyl compounds, representing the first method catalyzed by imino-λ³-iodane.

Iron-catalyzed stereoselective haloamidation of amide-tethered alkynes

In this work, by using N-methoxybenzamides as efficient acyl nitrene precursors, an iron-catalyzed formal cis-haloamidation of alkyne is reported. Without assistance of additives, the reaction worked well in the presence of 50 mol% FeCl3 or FeBr3, leading to a series of chloro/bromo-containing isoindolin-5-ones with high efficiency and wide reaction scope. In the reaction, the iron-facilitated haloamidation proceeds through a halo anion-participating concerted [3+2] cyclization to release the final products. The key intermediate ferric acyl nitrene A is generated in situ from a formal removal of MeOH.

Unravelling nitrene chemistry from acyclic precursors: recent advances and challenges

Recent advances in nitrene chemistry from acyclic precursors are reviewed in this paper.

Iron-catalyzed intramolecular acyl nitrene/alkyne metalation for the synthesis of pyrrolo[2,1-a]isoindol-5-ones

In this work, by using N-methoxybenzamides as efficient acyl nitrene precursors, an iron-catalyzed acyl nitrene/alkyne metalation is reported for the synthesis of pyrrolo[2,1-a]isoindol-5-ones.

A versatile iodo(III)etherification of terminal ethynylsilanes using BF3-OiPr2 and alkyl benzyl ethers

A series of (E)-α-silyl-β-alkoxyvinyl-λ3-iodanes was synthesized from iodosylbenzene, BF3-ether complexes, and terminal ethynylsilanes. The combined use of BF3-OiPr2 and benzyl ethers of primary alcohols (ROBn) allows the chemoselective transfer of primary alkoxy groups (RO) onto the β-position of the terminal ethynylsilanes.

Intermolecular scandium triflate-promoted nitrene-transfer [5 + 1] cycloadditions of vinylcyclopropanes

Sc(OTf)3-promoted [5 + 1] cycloaddition of vinylcyclopropanes with PhINTs is reported, enabling the regioselective preparation of a range of 1,2,3,6-tetrahydropyridine scaffolds under mild conditions. This represents the second example of a [5 + 1] nitrene-transfer cycloaddition and exhibits complementary substrate scope to the antecedent work, expanding the range of N-heterocycles accessible via this strategy.

Rh(II)-Catalyzed Nitrene-Transfer [5 + 1] Cycloadditions of Aryl-Substituted Vinylcyclopropanes

Formal [5 + 1] cycloadditions between aryl-substituted vinylcyclopropanes and nitrenoid precursors are reported. The method, which employs Rh₂(esp)₂ as a catalyst, leads to the highly regioselective formation of substituted tetrahydropyridines. Preliminary mechanistic studies support a stepwise, polar mechanism enabled by the previously observed Lewis acidity of Rh-nitrenoids. Overall, this work expands the application of nitrene-transfer cycloaddition, a relatively underexplored approach to heterocycle synthesis, to the formation of six-membered rings.

2-Picoline catalyst-triggered [2 + 2 + 2] cycloaddition-type reaction of acetylenedicarboxylates, aldehydes and alkenes

2-Picoline efficiently catalyzes the formation of α,β-enones from acetylenedicarboxylates and aldehydes in the presence of alkenes, thereby leading to pyrans in moderate to good yields with complete regioselectivities. This synthetic method of pyrans is represented as a first example of catalytic and direct [2 + 2 + 2] cycloaddition of three different components under the metal-free conditions.

Regio- and Stereoselective Chlorocyanation of Alkynes

A variety of terminal and internal alkynes were converted regio- and stereoselectively into (Z)-3-chloroacrylonitriles by treatment with BCl₃ in the presence of stoichiometric amounts of imidazolium thiocyanates. These products could be readily functionalized to provide useful building blocks, thus demonstrating the synthetic value of the method. Preliminary mechanistic studies suggest initial activation of the cationic thiocyanate by the Lewis acid, followed by electrophilic attack of the alkyne. The syn addition of a chloride to the vinyl cation intermediate and final elimination of the thiourea unit afford the desired chloroacrylonitriles.

One-Pot Synthesis of 2,4,5-Trisubstituted Imidazoles via [2 + 2 + 1] Cycloannulation of 1,3-Bishet(aryl)-monothio-1,3-diketones, α-Substituted Methylamines and Sodium Nitrite through α-Nitrosation of Enaminones

An efficient one-pot synthesis of a series of diversely functionalized trisubstituted 4(5)het(aroyl)-2,5(4)-het(aryl)/alkylimidazoles from readily available 1,3-bishet(aryl)monothio-1,3-diketones has been reported. This novel sequential one-pot, three step protocol, wherein three new carbon nitrogen bonds are formed in contiguous fashion, involves in situ generation of enaminones by reaction of monothio-1,3-diketones with α-substituted methylamines, followed by their α-nitrosation with sodium nitrite and subsequent base mediated intramolecular heterocyclization of the resulting α-hydroxyiminoimines to trisubstituted imidazoles in high yields under mild conditions. These newly prepared 4(5)-het(aroyl)-5(4)-het(aryl)/alkylimidazoles are shown to exist as tautomeric mixture, however, their subsequent alkylation with methyl iodide in the presence of potassium carbonate affords 1-N-methy-2,5-bishet(aryl)-4-het(aroyl)imidazoles in highly regioselective fashion in most of the cases. Synthesis of few 4(5)-(2-hydroxyphenyl)-2,5(4)-substituted imidazoles, which are known to be good coordinating ligands, has also been reported. A probable mechanism for the formation of these imidazoles from hydroxyiminoimine intermediates has also been suggested.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Oxidative Heterocycle Formation Using Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) reagents have been widely exploited in a diverse array of synthetic transformations. This chapter focuses on the general application of hypervalent iodine(III) reagents in the de novo synthesis and in the late stage functionalization of heterocyclic compounds.