Acceleration of conjugated dienyne cycloaromatization

Copper(I)-Catalyzed Tandem Aminobenzannulation of Dienyne with Sulfonyl Azide

A general and efficient copper-catalyzed tandem aminobenzannulation of dienyne has been accomplished employing sulfonyl azides as a coupling partner. The reaction involves the formation of both C-C and C-N bonds via the initial generation of copper-catalyzed ketenimine from terminal alkyne and sulfonyl azide followed by electrocyclic ring closure and aromatization. The developed reaction is operationally simple and allows the synthesis of diverse substituted aminonaphthalenes and fused aminoarenes in good to excellent yields with high selectivities.

Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

Diversity-Oriented Synthesis of Benzo[f][1,4]oxazepine-, 2H-Chromene-, and 1,2-Dihydroquinoline-Fused Polycyclic Nitrogen Heterocycles under Microwave-Assisted Conditions

An efficient, diversity-oriented synthesis of oxazepino[5,4-b]quinazolin-9-ones, 6H-chromeno[4,3-b]quinolines, and dibenzo[b,h][1,6]naphthyridines was established involving a substrate-based approach under microwave-assisted and conventional heating conditions in high yields (up to 88%). The CuBr₂-catalyzed, chemoselective cascade annulation of O-propargylated 2-hydroxybenzaldehydes and 2-aminobenzamides delivered oxazepino[5,4-b]quinazolin-9-ones involving a 6-exo-trig cyclization-air oxidation-1,3-proton shift-7-exo-dig cyclization sequence. This one-pot process showed excellent atom economy (-H₂O) and constructed two new heterocyclic rings (six- and seven-membered) and three new C-N bonds in a single synthetic operation. On the other side of diversification, the reaction between O/N-propargylated 2-hydroxy/aminobenzaldehydes and 2-aminobenzyl alcohols delivered 6H-chromeno[4,3-b]quinolines and dibenzo[b,h][1,6]naphthyridines involving sequential imine formation-[4 + 2] hetero-Diels-Alder reaction-aromatization steps. The influence of microwave assistance was superior to conventional heating, where the reactions were clean, rapid, and completed in 15 min, and the conventional heating required a longer reaction time at a relatively elevated temperature.

Palladium-Catalyzed Formal (3 + 2) Cycloaddition Reactions of 2-Nitro-1,3-enynes with Vinylaziridines, -epoxides, and -cyclopropanes

A two-step Pd-catalyzed (3 + 2) cycloaddition/HNO₂ elimination reaction sequence has been developed to give novel cyclic 1,3-dien-5-yne systems from Pd-stabilized zwitterionic 1,3-dipoles and 2-nitro-1,3-enyne substrates. The process is highly atom-efficient and tolerates the reaction of 2-vinyloxirane, 1-tosyl-2-vinylaziridine, and diethyl 2-vinylcyclopropane-1,1-dicarboxylate derived 1,3-dipoles with a variety of 2-nitro-1,3-enyne substrates. The stereochemistry of the intermediate (3 + 2) cycloadducts was determined by single crystal X-ray analysis. Furthermore, a selective kinetic elimination of the cycloadduct with an antiperiplanar relationship between the NO₂ group and the participating hydrogen was demonstrated, allowing for efficient isolation of a single diastereoisomer of the cycloadduct.

Rhodium-Catalyzed Dehydrogenative Cycloisomerization of Dienylcyclopropane to Highly Substituted Toluene

A rhodium-catalyzed dehydrogenative cycloisomerization of dienylcyclopropane compounds is reported, which provides a straightforward approach to a variety of highly substituted toluene derivatives in 67-85% yields. The dienylcyclopropane-imides are produced by a single-step formal three-component olefination procedure. Preliminary mechanistic studies indicated that an electron-withdrawing group as R plays a critical role in completing this transformation.

Ruthenabenzene: A Robust Precatalyst

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Diels-Alder Cycloaddition to the Bay Region of Perylene and Its Derivatives as an Attractive Strategy for PAH Core Expansion: Theoretical and Practical Aspects

PAHs (polycyclic aromatics hydrocarbons), the compound group that contains perylene and its derivatives, including functionalized ones, have attracted a great deal of interest in many fields of science and modern technology. This review presents all of the research devoted to modifications of PAHs that are realized via the Diels–Alder (DA) cycloaddition of various dienophiles to the bay regions of PAHs, leading to the π-extension of the starting molecule. This type of annulative π-extension (APEX) strategy has emerged as a powerful and efficient synthetic method for the construction of polycyclic aromatic hydrocarbons and their functionalized derivatives, nanographenes, and π-extended fused heteroarenes. Then, [4 + 2] cycloadditions of ethylenic dienophiles, -N=N-, i.e., diazo-dienophiles and acetylenic dienophiles, are presented. This subject is discussed from the organic synthesis point of view but supported by theoretical calculations. The possible applications of DA cycloaddition to PAH bay regions in various science and technology areas, and the prospects for the development of this synthetic method, are also discussed.

APEX Strategy Represented by Diels-Alder Cycloadditions-New Opportunities for the Syntheses of Functionalised PAHs

Diels-Alder cycloaddition of various dienophiles to the bay region of polycyclic aromatic hydrocarbons (PAHs) is a particularly effective and useful tool for the modification of the structure of PAHs and thereby their final properties. The Diels-Alder cycloaddition belongs to the single-step annulative π-extension (APEX) reactions and represents the maximum in synthetic efficiency for the constructions of π-extended PAHs including functionalised ones, nanographenes, and π-extended fused heteroarenes. Herein we report new applications of the APEX strategy for the synthesis of derivatives of 1,2-diarylbenzo[ghi]perylene, 1,2-diarylbenzo[ghi]perylenebisimide and 1,2-disubstituted-benzo[j]coronene. Namely, the so far unknown cycloaddition of 1,2-diarylacetylenes into the perylene and perylenebisimide bay regions was used. 1,2-Disubstituted-benzo[j]coronenes were obtained via cycloaddition of benzyne into 1,2-diarylbenzo[ghi]perylenes by using a new highly effective system for benzyne generation and/or high pressure conditions. Moreover, we report an unprecedented Diels-Alder cycloaddition-cycloaromatisation domino-type reaction between 1,4-(9,9-dialkylfluoren-3-yl)-1,3-butadiynes and perylene. The obtained diaryl-substituted core-extended PAHs were characterised by DFT calculation as well as electrochemical and spectroscopic measurements.

Cyclization of ortho-ethynylbiaryls as an emerging versatile tool for the construction of polycyclic arenes

Cyclization and cycloisomerization of ortho-aryl(ethynyl)arenes provide an easy direct access to fused polycyclic aromatic carbo- and heterocycles. This methodology has demonstrated an impressive progress in the recent years. The goal of this review is to give a comprehensive outlook on the synthetic potential, scope, limitations, and mechanistic aspects of the cyclization reactions. The material is arranged according to the activation method that can be used to induce cyclization: pyrolysis, metal catalysis, electrophilic activation, radical induction, base catalysis. Particular attention is paid to the specificity of ortho-ethynylbiaryls with a heterocyclic central core.

The bibliography includes 257 references.

Nanographene and Graphene Nanoribbon Synthesis via Alkyne Benzannulations

The extension of π-conjugation of polycyclic aromatic hydrocarbons (PAHs) via alkyne benzannulation reactions has become an increasingly utilized tool over the past few years. This short review will highlight recent work of alkyne benzannulations in the context of large nanographene as well as graphene nanoribbon synthesis along with a brief discussion of the interesting physical properties these molecules display.

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

1,3-Dien-5-ynes have been extensively used as starting materials for the synthesis of a wide number of different carbo- and heterocycles. The aim of this review is to give an overview of their utility in organic synthesis, highlighting the variety of compounds that can be directly accessed from single reactions over these systems. Thus, cycloaromatization processes are initially commented, followed by reactions directed toward the syntheses of five-membered rings, other carbocycles and, finally, heterocycles. The diverse methodologies that have been developed for the synthesis of each of these types of compounds from 1,3-dien-5-ynes are presented, emphasizing the influence of the reaction conditions and the use of additional reagents in the outcome of the transformations.

(Hetero)aromatics from dienynes, enediynes and enyne–allenes

Recent advances in the synthesis of (hetero)aromatics starting from polyenes, such as dienynes, enediynes and enyne–allenes, are discussed.

The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.

New advances in nanographene chemistry

This review discusses recent advancements in nanographene chemistry, focusing on the bottom-up synthesis of graphene molecules and graphene nanoribbons.

α-Diketone formation accompanied by oxidation of sulfur functional group by the reaction of o-alkynylarenesulfoxide with iodine

The reaction of o-alkynylarenesulfoxide with iodine was investigated in detail, revealing functionalities of the formation of α-diketones with sulfenyl, sulfinyl, and sulfonyl. Additives can change the ratio of products to give medium-to-excellent yields. Results show that water is taken into only sulfonyl compound and that other oxygen atoms constructed in the products are presumably derived from sulfoxide of the starting material and molecular oxygen.

Synthesis of symmetric 1,4-diamino-2-butynes via a Cu(I)-catalyzed one-pot A3-coupling/decarboxylative coupling of a propiolic acid, an aldehyde, and an amine

A novel microwave-assisted approach for the one-pot Cu(I)-catalyzed A(3)-coupling/decarboxylative coupling (PA(2)-coupling) of a propiolic acid, an aldehyde, and an amine, resulting in the formation of diversely substituted 1,4-diamino-2-butynes,is described. It is noteworthy that this new multicomponent coupling provides an efficient access to introduce alkyl and aryl group at the 1,4-position of the 1,4-diamino-2-butynes.