Diels-Alder cycloaddition of acetylene gas to a polycyclic aromatic hydrocarbon bay region

Intramolecular Diels-Alder Reaction of a Biphenyl Group in a Strained meta-Quaterphenylene Acetylene

At elevated temperatures, a strained, cyclic meta-quaterphenylene acetylene undergoes an intramolecular cyclization reaction to form benz[e]indeno[1,2,3-hi]acephenanthrylene. This reaction represents an example of a Diels-Alder reaction at the 2-, 1-, 1'-, and 2'-positions of a biphenyl derivative, a region analogous to the bay regions of perylene and other periacenes. The reaction proceeds cleanly with high conversion. Kinetics studies of a methylated derivative reveal that the ΔG‡ for the reaction is ∼40-41 kcal/mol, and computational models predict a similar value of Grel for the transition state of a concerted [4 + 2]-cycloaddition.

Bay-Region Annulative π-Extension of o-Iodobiphenyls with Aliphatic Anhydrides Catalyzed by Pd(OAc)2

Bay-region annulative π-extension of o-iodobiphenyls with aliphatic anhydrides was developed. Many o-iodobiphenyls and aliphatic anhydrides can react well under the optimized conditions. A lot of phenanthrol derivatives can be efficiently prepared by this approach. The control experiments support that dibenzopalladacyclopentadienes may be the reaction intermediates.

Acetylene and Ethylene: Universal C2 Molecular Units in Cycloaddition Reactions

Acetylene and ethylene are the smallest molecules that contain an unsaturated carbon–carbon bond and can be efficiently utilized in a large variety of cycloaddition reactions. In this review, we summarize the application of these C2 molecular units in cycloaddition chemistry and highlight their amazing synthetic opportunities.

1 Introduction

2 Fundamental Features and Differences of Cycloaddition Reactions Involving Acetylene and Ethylene

3 (2+1) Cycloaddition

4 [2+2] Cycloaddition

5 (3+2) Cycloaddition

6 [4+2] Cycloaddition

7 (2+2+1) Cycloaddition

8 [2+2+2] Cycloaddition

9 The Use of Acetylene and Ethylene Cycloaddition for Deuterium and 13C Labeling

10 Conclusions

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.

Reversal of Clar's Aromatic-Sextet Rule in Ultrashort Single-End-Capped [5,5] Carbon Nanotubes

The three-fold HOMO-LUMO gap oscillation, typical of finite length armchair carbon nanotubes (CNT), has a major effect on the magnetic response of ultrashort, single-end-capped [5,5] carbon nanotubes to a perturbing magnetic field parallel to the main symmetry axis. For the CNT's containing 40, 70, and 100 carbon atoms, for which 100 % of the C=C double bonds can be grouped into aromatic-sextets, i. e., fully or complete Clar networks, large paratropic (antiaromatic) global circulations around the cylindrical axis are predicted at the DFT level of calculation. Local and semi-global diatropic (aromatic) currents of strengths not larger than that of the benzene molecule are determined for a perpendicular perturbing magnetic field. CNTs of intermediate lengths do not display this enhanced antiaromatic response. The paratropic current flow clearly shows that these complete Clar networks can be viewed as stacked cycloparaphenylene belts, each providing a double 4 n annulene circuit as a consequence of the quinoidal resonance structure that results from their closure. Paradoxically, the fully aromatic Clar structure itself is responsible for the enhanced global antiaromaticity.

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

Facile and versatile access to substituted hexabenzoovalene derivatives: characterization and optoelectronic properties

We describe the design and modular synthesis of a library of substituted hexabenzoovalene derivatives (SHBO), along with the key precursor dinaphthopyrenes (3), highlighting the influence of a wide array of substituent variation on the photophysical properties via UV-vis absorption, fluorescence spectra and electrochemical methods. The results show that the cyclized hexabenzoovalenes present a stronger spectroscopic red-shift than the corresponding dinaphthopyrenes. X-ray diffraction analysis suggests that intermediate 3hx containing two nitro groups forms a trans-configuration with twisted structures. Our systematic investigation might provide a realistic design strategy to construct large one-dimensional and two-dimensional materials via bottom-up approaches.

Methods for the chemical synthesis of carbon nanotubes: an approach based on hemispherical polyarene templates

Hemispherical polyarenes represent attractive templates from which carbon nanotubes of the same diameter and rim structure (chirality) might be grown by repetitive annulation reactions. The resulting single-index (n,m) nanotubes would have one end open and the other end capped by the original template. Efforts in the author’s laboratory to synthesize (5,5) and (6,6) nanotube end-caps are described. Nitroethylene is shown to serve well as a “masked acetylene” for the conversion of polyarene bay regions into new unsubstituted benzene rings by a Diels-Alder cycloaddition/aromatization process. Benzyne reacts similarly, both in solution and in the gas phase. These annulation reactions are proposed as methods for elongating large-diameter templates that have bay regions on their rims into structurally uniform, single-walled carbon nanotubes. Unfortunately, the bay regions on the strongly curved rim of the small-diameter (5,5) nanotube end-cap 3 resist Diels-Alder cycloadditions with both nitroethylene and benzyne. Pentabenzocorannulene (14) is proposed as a promising candidate for surface-catalyzed cyclodehydrogenation to a surface-bound hemispherical polyarene that could serve as a template for synthesis of pure (5,5) carbon nanotubes.

Implications of the final ring closure to 10b-aza-10c-borapyrene for aryl–alkyne ring-closing mechanisms

Through a combined computational and isotopic labeling study, it has been observed that the activation energy for the aryl–alkyne ring closure of the azaborine containing 4-ethynyl-4a-aza-4b-boraphenanthrene is dramatically lower, and it appears to proceed via an alternate mechanism than that of its hydrocarbon analog. This catalyst-free reaction proceeds at modest reaction conditions compared with traditional pyrolytic synthetic methods and holds promise for the efficient construction of fused ring systems containing azaborine functional groups that may not be accessible in the purely hydrocarbon form.

A mechanistic study on cationic Li prompted Diels–Alder cycloaddition of cycloparaphenylene

Reaction barriers for Diels–Alder cycloadditions of cycloparaphenylenes are reduced by using cationic Li as a Lewis acid catalyst.

Towards pi-extended cycloparaphenylenes as seeds for CNT growth: investigating strain relieving ring-openings and rearrangements

Upon exposure to Scholl reaction conditions, cycloparaphenylenes undergo facile strain-relieving rearrangements and ring-openings.

Despite significant multidisciplinary effort over many years, the preparation of uniform carbon nanotubes (CNTs) is still an unsolved problem in the scientific community. This inaccessibility hampers the commercial use of CNTs in electronic devices due to the sensitive connection between their electronic properties and molecular structure. The [n]cycloparaphenylenes ([n]CPPs), the smallest horizontal segment of an armchair CNT, hold great promise as “seeds”, or templates, for the preparation of homogenous batches of CNTs. Initial reports towards this goal, however, suggest that it would be advantageous to pi-extend these structures through traditional organic synthesis before their use in CNT growth. Towards this, several strategies have been reported attempting to utilize the Scholl reaction on aryl-substituted cycloparaphenylenes to yield a small CNT for use as a template for larger tubes. Prominently used in polyaromatic hydrocarbon chemistry, the Scholl reaction has afforded numerous extraordinary targets, such as graphene nanoribbons and graphene propellors. In this work, both experimental and computational studies are provided to unravel the complex cationic rearrangements and ring-openings associated with the Scholl reaction in the context of the cycloparaphenylenes—systems that are thermodynamically and kinetically different from flat graphene fragments. Additionally, this work demonstrates the unique reactivity of cycloparaphenylenes in the context of cationic or radical cationic intermediates, which are common reaction pathways for numerous transformations.

Diversity-Oriented Approaches to Polycyclics and Bioinspired Molecules via the Diels-Alder Strategy: Green Chemistry, Synthetic Economy, and Beyond

We describe diverse approaches to various dienes and their utilization in the Diels-Alder reaction to produce a variety of polycycles. The dienes covered here are prepared by simple alkylation reaction or via the Claisen rearrangement or by enyne metathesis of alkyne or enyne building blocks. Here, we have also included the Diels-Alder chemistry of dendralenes, a higher analog of cross-conjugated dienes. The present article is inclusive of o-xylylene derivatives that are generated in situ starting with benzosultine or benzosulfone derivatives. The Diels-Alder reaction of these dienes with various dienophiles gave diverse polycyclic systems and biologically important targets.

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.

Synthesis and photophysical properties of biphenyl and terphenyl arylene-ethynylene macrocycles

A series of single-walled carbon nanotube precursors, C3h-symmetric cyclotri(ethynylene)(biphenyl-2,4'-diyl) and cyclotri(ethynylene)(p-terphenyl-2,4″-diyl), have been prepared by a linear stepwise oligomerization-cyclization route and by statistical intermolecular cyclooligomerization. In addition to producing these members of a novel class of arylene ethynylene macrocycles, 1 and 2, the latter statistical process produces the smaller cyclic dimer, cyclodi(ethynylene)(p-terphenyl-2,4″-diyl) and the larger cyclic tetramer cyclotetra(ethynylene)(biphenyl-2,4'-diyl). These macrocycles display large Stokes shifts in their fluorescence spectra. Their biphenyl or terphenyl connectivity prevents these macrocycles from achieving full planarity in the ground state, and the ethynylene moieties could provide synthetic access to cyclic arylene oligomers and discrete carbon nanotube segments.

1,1,n,n-Tetramethyl[n](2,11)teropyrenophanes (n = 7-9): a series of armchair SWCNT segments

A new iterative bridge formation strategy has been employed in the synthesis of a series of [n](2,11)teropyrenophanes (n = 7-9). The generation of the nonplanar teropyrene system, which is calculated to be bent through 178.7° for the smallest homologue (n = 7), is accomplished using a VID reaction of a cyclophanemonoene precursor for the first time.

Diels-Alder reactions of graphene: computational predictions of products and sites of reaction

The cycloaddition reactions and noncovalent π interactions of 2,3-dimethoxybutadiene (DMBD), 9-methylanthracene (MeA), tetracyanoethylene (TCNE), and maleic anhydride (MA) with graphene models have been investigated using density functional theory (DFT) calculations. Reaction enthalpies have been obtained to assess the reactivity and selectivity of covalent and noncovalent functionalization. Results indicate that graphene edges may be functionalized by the four reagents through cycloaddition reactions, while the interior regions cannot react. Noncovalent complexation is much more favorable than cycloaddition reactions on interior bonds of graphene. The relative reactivities of different sites in graphene are related to loss of aromaticity and can be predicted using Hückel molecular orbital (HMO) localization energy calculations.

Initiation of carbon nanotube growth by well-defined carbon nanorings

Carbon nanotubes (CNTs), tubular molecular entities that consist of sp(2)-hybridized carbon atoms, are currently produced as mixtures that contain tubes of various diameters and different sidewall structures. The electronic and optical properties of CNTs are determined by their diameters and sidewall structures and so a controlled synthesis of uniform-diameter, single-chirality CNTs-a significant chemical challenge-would provide access to pure samples with predictable properties. Here we report a rational bottom-up approach to synthesize structurally uniform CNTs using carbon nanorings (cycloparaphenylenes) as templates and ethanol as the carbon source. The average diameter of the CNTs formed is close to that of the carbon nanorings used, which supports the operation of a 'growth-from-template' mechanism in CNT formation. This bottom-up organic chemistry approach is intrinsically different from other conventional approaches to making CNTs and, if it can be optimized sufficiently, offers a route to the programmable synthesis of structurally uniform CNTs.