Synthesis of P-Containing Polycyclic Aromatic Hydrocarbons from Alkynyl-phosphonium Salts.

This work presents a straightforward method for synthesizing a series of phosphorus-containing polycyclic aromatic hydrocarbons (P-PAHs) featuring an internal ylidic bond. The method involves anion exchange, alkyne annulation, and deprotonation reactions, enabling the efficient production of cyclic phosphonium salts, which serve as pivotal intermediates in the synthesis of P-PAHs. The alkyne annulation reaction exhibits high regioselectivity, ensuring the successful synthesis of λ5-phosphaphenanthrene isomers. Additionally, the incorporation of electron-withdrawing groups effectively stabilizes the internal ylidic bond of P-PAHs.

Chiral Teropyrenes: Synthesis, Structure, and Spectroscopic Studies.

We present the inaugural synthesis of a chiral teropyrene achieved through a four-fold alkyne benzannulation catalyzed by InCl3, resulting in good yields. The product underwent thorough characterization using FT-Raman and FT-IR spectroscopies, demonstrating a close agreement with calculated spectra. X-ray crystallographic analysis unveiled a notable twist in the molecule's backbone, with an end-to-end twist angle of 51°, consistent with computational predictions. Experimentally determined enantiomeric inversion barriers revealed a significant energy barrier of 23 kcal/mol, facilitating the isolation of enantiomers for analysis via circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopies. These findings mark significant strides in the synthesis and characterization of chiral teropyrenes, offering insights into their structural and spectroscopic properties.

Dissymmetrical Chiral Peropyrenes: Synthesis via Iridium-Catalyzed C-H Activation/Alkyne Benzannulation and Study of Their Properties.

Dissymmetrical chiral peropyrenes with electron-rich and electron-deficient aryl substituents in the bay regions were synthesized via iridium-catalyzed C-H activation and alkyne benzannulation. The electronic properties were studied using cyclic and differential pulse voltammetry. The enantiomers were separated and exhibited high glum and gabs values in circularly polarized luminescence (CPL) and circular dichroism (CD), respectively. Variable-temperature NMR experiments were conducted on symmetrical and dissymmetrical chiral peropyrenes to compare the barrier to rotation of the aryl groups in the bay region.

Synthesis and Properties of Achiral and Chiral Dipyrenoheteroles and Related Compounds.

Achiral and chiral isomers of dipyrenoheteroles were synthesized via alkyne benzannulation. The electronic properties of these compounds were examined using cyclic voltammetry and differential pulse voltammetry. The enantiomers of the chiral isomers were separated, and their optical properties were examined in circular dichroism and circularly polarized luminescence studies. The chiral isomers exhibited a large bathochromic shift, relative to the achiral isomer, in both absorbance and fluorescence, resulting from decreased symmetry, rather than a change in the size of the backbone.

Pentacenones as Divergent Intermediates to Unsymmetrically Substituted Pentacenes: Synthesis and Crystallographic Analysis.

We report a general method for the desymmetrization of 6,13-pentacenequinone to access ethynylated pentacene ketones, namely, 13-hydroxy-13-(ethynylated)pentacene-6(13H)-ones. These pentacene ketones ("pentacenones") serve as divergent intermediates to unsymmetrically 6,13-disubstituted pentacenes, commonly used for studying singlet fission processes and charge transport phenomena in organic field effect transistors. We report a synthetic method to access pentacenones, which utilizes a precipitation/crystallization from the crude mixture to enable facile purification on a multigram scale. X-ray crystallographic analysis of the pentacenones reveals key noncovalent interactions that contribute to the crystallization, specifically, hydrogen bonding between the ketone and alcohol functional groups as well as π-π-stacking and dipole-dipole interactions.

Alkyne benzannulations in the preparation of contorted nanographenes.

Nanographenes are a popular area of research due to their promising properties for electronics. Over the last twenty years there has been a significant increase in interest in the development of contorted nanographenes. While many top-down techniques are employed in the synthesis of these planar nanographenes, the use of alkynes in bottom-up syntheses allows for easy functionalization and the development of contorted nanographenes. The syntheses of contorted nanographenes with a focus on utilizing alkynes is reviewed here.

Four-Fold Alkyne Benzannulation: Synthesis, Properties, and Structure of Pyreno[a]pyrene-Based Helicene Hybrids.

The synthesis of pyreno[a]pyrene-based helicene hybrids was achieved in good yield via a four-fold alkyne benzannulation reaction that was promoted by Brønsted acid. The molecules are configurationally locked, allowing the enantiomers to be separated using chiral HPLC so that their photophysical and chiroptical properties, including circular dichroism and circularly polarized luminescence, could be studied.

Synthesis, Structure, Photophysical Properties, and Photostability of Benzodipyrenes.

This work explores the syntheses, structures, photophysical properties, and photostability of benzodipyrenes (BDPs). BDPs were synthesized through an InCl3 -AgNTf2 -catalyzed, four-fold alkyne benzannulation reaction. The structures of BDP 4 a and its corresponding endoperoxide product were unambiguously confirmed by X-ray crystallography. The BDPs reported here can also be recognized as peri- and cata-benzannulated pentacenes with a non-functionalized central ring. Unlike the previous reported pentacene-based polycyclic aromatic hydrocarbons, the absorbances of the BDPs were blueshifted by ca. 40 nm relative to pentacene, even after extension of π-conjugation. The newly synthesized BDP products exhibit relatively good stability with half-lives as high as 4612 min in THF.

Highly Regioselective Domino Benzannulation Reaction of Buta-1,3-diynes To Construct Irregular Nanographenes.

The properties of nanographenes can be tuned by changing their shapes, therefore the development of new methods suitable for the synthesis of various nanographenes is highly desirable. Described herein is an intramolecular InCl3 /AgNTf2 -catalyzed regioselective domino benzannulation reaction of buta-1,3-diynes to build irregular nanographenes. Different nanographene compounds were easily obtained in moderate to high yields through careful design of the precursor compounds. This new domino reaction was successfully applied to a fourfold alkyne benzannulation of dimethoxy-1,1'-binaphthalene derivatives to arrive at novel chiral butterfly ligand precursors. The regioselectivity of the benzannulation reaction was unambiguously confirmed by X-ray crystallography. Moreover, this new method enables us to synthesize different nanographene isomers and study their optical properties as a function of shape.

One-Pot Domino Friedel-Crafts Acylation/Annulation between Alkynes and 2-Methoxybenzoyl Chlorides: Synthesis of 2,3-Disubstituted Chromen-4-one Derivatives.

A highly regioselective synthesis of 2,3-disubstituted chromen-4-one derivatives is accomplished from readily available internal alkynes and 2-methoxybenzoyl chlorides. The reaction proceeds via a domino intermolecular Friedel-Crafts acylation/intramolecular vinyl carbocation trapping (or oxa-Michael addition)/demethylation reaction sequence. This Lewis acid promoted method features relatively mild reaction conditions to synthesize a variety of 2,3-disubstituted chromen-4-one derivatives in one pot with up to 93% yield. The chromen-2-one (coumarin) product was obtained when 2,6-dimethoxybenzoyl chloride was used as a starting material via an electrophilic aromatic substitution/rearrangement process.

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.

One-Pot Synthesis of α-Carbonyl Bicyclic Furans via a Sequential Diels-Alder/5-Exo-Dig Cyclization/Oxidation Reaction.

A new one-pot approach to construct α-carbonyl bicyclic furans from easily accessible diynones is described. This reaction sequence proceeds via a Diels-Alder cycloaddition reaction catalyzed by dimethylaluminum chloride followed by a 5-exo-dig cyclization/oxidation reaction catalyzed by copper(II) chloride. This methodology generates α-carbonyl-functionalized dihydroisobenzofuran derivatives in good to excellent yields.

Chiral Peropyrene: Synthesis, Structure, and Properties.

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using p-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV-visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M-1 cm-1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations.

The Synthesis of Non-planar, Helically Coiled Graphene Nanoribbons.

Twisted system: A photochemical cyclodehydrochlorination (CDHC) reaction is a relatively mild and efficient way to access highly contorted, helically coiled graphene nanoribbons. The material is characterized by a number of techniques and boasts a relatively large optical band gap of 2.15 eV.

ß-Silyl-Assisted Tandem Diels-Alder/Nazarov Reaction of 1-Aryl-3-(trimethylsilyl) Ynones.

A one-pot tandem Diels-Alder/Nazarov reaction of 1-aryl-3-(trimethylsilyl) ynones has been achieved to generate carbo- and heterocyclic fused ring systems in good to excellent yields. The ß-silyl effect is instrumental in accessing this otherwise challenging cascade annulation reaction. The tandem reaction proceeds in the presence of BCl3 to generate three new carbon-carbon bonds, a quaternary carbon, and two stereogenic centers with excellent diastereocontrol. A variety of substituted arenes, and even heteroaromatics, are tolerated to provide tricyclic products that are of interest as advanced intermediates toward biologically relevant compounds.

Pyrenes, Peropyrenes, and Teropyrenes: Synthesis, Structures, and Photophysical Properties.

The design of a relatively simple and efficient method to extend the π-conjugation of readily available aromatics in one-dimension is of significant value. In this paper, pyrenes, peropyrenes, and teropyrenes were synthesized through a double or quadruple benzannulation reaction of alkynes promoted by Brønsted acid. This novel method does not involve cyclodehydrogenation (oxidative aryl-aryl coupling) to arrive at the newly incorporated large arene moieties. All of the target compounds were synthesized in moderate to good yields and were fully characterized with the structures unambiguously confirmed by X-ray crystallography. As expected, photophysical characterization clearly shows increasing red-shifts as a function of extended conjugation within the fused ring systems.

Bottom-Up Synthesis of Soluble and Narrow Graphene Nanoribbons Using Alkyne Benzannulations.

Since the discovery of graphene, there is an increasing amount of research devoted to graphene materials, namely, graphene nanoribbons (GNRs). The "top-down" production of narrow (<10 nm wide), unoxidized, and easily processable GNRs with atomically precise edges is challenging, and therefore, new methods need to be developed. We have designed a "bottom-up" approach for the synthesis of very narrow (ca. 0.5 nm) and soluble GNRs using a nonoxidative alkyne benzannulation strategy promoted by Brønsted acid. Suzuki polymerization was used to produce the GNR precursor, a poly(2,6-dialkynyl-p-phenylene) (PDAPP), with a weight-average molecular weight of 37.6 kg mol(-1). Cyclization of the ethynylaryl side chains on PDAPP was efficiently achieved using Brønsted acids to ultimately produce the GNRs. Infrared and Raman spectroscopic characterization of the GNRs matches very well with calculated results. The formation of the GNRs was also supported by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM).

Multicomponent Double Diels-Alder/Nazarov Tandem Cyclization of Symmetric Cross-Conjugated Diynones to Generate [6-5-6] Tricyclic Products.

The construction of complex polycyclic terpenoid products in an efficient and step-economical manner using multicomponent and tandem processes is highly valuable. Herein, we report a tandem cyclization sequence that initiates with a multicomponent double Diels-Alder reaction of cross-conjugated diynones, followed by a Nazarov cyclization to efficiently produce [6-5-6] tricyclic products with excellent regio- and diastereoselectivity. This methodology generates five new carbon-carbon bonds, three rings, quaternary or vicinal quaternary carbons, and stereogenic centers in a one-pot reaction.

Highly Regio- and Diastereoselective Formation of Tetrasubstituted (Z)-1,2-Dihaloalkenes from the Halogenation of Trimethylsilyl Alkynes with ICl.

The stereoselective anti addition of diatomic halogens to alkynes has been well studied. A method is reported that utilizes the ß-silyl effect to override this typically observed anti selectivity and provides halogenation products that result from syn addition. The reaction involves the addition of iodine monochloride to trialkylsilyl-substituted alkynes to produce tetrasubstituted (Z)-dihaloalkenes in good to excellent yields and with excellent regio- and diastereoselectivity.

Direct and highly regioselective and enantioselective allylation of ß-diketones.

The enantioselective allylation of ketones is a problem of fundamental importance in asymmetric reaction design, especially given that only a very small number of methods can generate tertiary carbinols. Despite the vast amount of attention that synthetic chemists have given to this problem, success has generally been limited to just a few simple ketone types. A method for the selective allylation of functionally complex ketones would greatly increase the utility of ketone allylation methods in the chemical synthesis of important targets. Here we describe the operationally simple, direct, regioselective and enantioselective allylation of ß-diketones. The strong tendency of ß-diketones to act as nucleophilic species was overcome by using their enol form to provide the necessary Brønsted-acid activation. This reaction significantly expands the pool of enantiomerically enriched and functionally complex tertiary carbinols that may be easily accessed. It also overturns more than a century of received wisdom regarding the reactivity of ß-diketones.