Rotational spectra of five cyano derivatives of fluorene

The recent interstellar detection of individual polycyclic aromatic hydrocarbons (PAHs) in the dense molecular cloud TMC-1 brings interest in related species that could be present in this astronomical environment. These detections, that include pure PAHs and their cyano-derivative counterparts, were performed through the interplay between laboratory rotational spectroscopy experiments and radioastronomical observations. Here, we present the laboratory rotational spectroscopic study of the five cyano-derivatives of the PAH fluorene (C13H10). The samples for these five species were synthetized in the laboratory and then characterized in the gas phase using a chirped-pulse Fourier-transform microwave spectrometer operating between 2 and 12 GHz. The analysis of the rotational spectra allowed us to derive accurate molecular constants for the five isomers used to obtain frequency predictions that enable astronomical searches of these molecules in the interstellar medium.

Direct C(sp3)-H Arylation of Unprotected Benzyl Anilines and Alkylarenes by Organocatalysis under Visible Light

Reported herein is direct C(sp3)-H arylation of unprotected benzyl anilines and alkylarenes via consecutive photoinduced electron transfer by visible light irradiation. Reductive quenching cycles and radical-radical cross-coupling were involved, and electron paramagnetic resonance experiments provide evidence for the formation of radical intermediates formed in situ. The protocol highlights transition metal free, external oxidant free, broad substrate scope, and high efficiency (>60 examples, up to 96%).

Dihydroindenofluorenes as building units in organic semiconductors for organic electronics

This review aims to discuss organic semiconductors constructed on dihydroindenofluorene positional isomers, which are key molecular scaffolds in organic electronics. Bridged oligophenylenes are key organic semiconductors that have allowed the development of organic electronic technologies. Dihydroindenofluorenes (DHIFs) belong to the family of bridged oligophenylenes constructed on a terphenyl backbone. They have proven to be very promising building blocks for the construction of highly efficient organic semiconductors for all OE devices, namely organic light emitting diodes (OLEDs), phosphorescent OLEDs, organic field-effect transistors (OFETs), solar cells, etc.

High-performance five-ring-fused organic semiconductors for field-effect transistors

Organic molecular semiconductors have been paid great attention due to their advantages of low-temperature processability, low fabrication cost, good flexibility, and excellent electronic properties. As a typical example of five-ring-fused organic semiconductors, a single crystal of pentacene shows a high mobility of up to 40 cm² V^-1 s^-1, indicating its potential application in organic electronics. However, the photo- and optical instabilities of pentacene make it unsuitable for commercial applications. But, molecular engineering, for both the five-ring-fused building block and side chains, has been performed to improve the stability of materials as well as maintain high mobility. Here, several groups (thiophenes, pyrroles, furans, etc.) are introduced to design and replace one or more benzene rings of pentacene and construct novel five-ring-fused organic semiconductors. In this review article, ∼500 five-ring-fused organic prototype molecules and their derivatives are summarized to provide a general understanding of this catalogue material for application in organic field-effect transistors. The results indicate that many five-ring-fused organic semiconductors can achieve high mobilities of more than 1 cm² V^-1 s^-1, and a hole mobility of up to 18.9 cm² V^-1 s^-1 can be obtained, while an electron mobility of 27.8 cm² V^-1 s^-1 can be achieved in five-ring-fused organic semiconductors. The HOMO-LUMO levels, the synthesis process, the molecular packing, and the side-chain engineering of five-ring-fused organic semiconductors are analyzed. The current problems, conclusions, and perspectives are also provided.

Synthesis of fluorenes and their related compounds from biaryls and Meldrum's acid derivatives

A new synthetic method for preparing fluorenes from amino group-containing biaryls and Meldrum's acid derivatives was developed. The reaction proceeded without a catalyst and loss of functional groups. The corresponding six- and seven-membered cyclic products were obtained using biaryl ether and ortho-terphenyl as substrates, respectively.

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

Conjugated molecular chains have the potential to act as "molecular wires" that can be employed in a variety of technologies, including catalysis, molecular electronics, and quantum information technologies. Their successful application relies on a detailed understanding of the factors governing the electronic energy landscape and the dynamics of electrons in such molecules. We can gain insights into the energetics and dynamics of charges in conjugated molecules using time-resolved infrared (TRIR) detection combined with pulse radiolysis. Nitrile ν(C[triple bond, length as m-dash]N) bands can act as IR probes for charges, based on IR frequency shifts, because of their exquisite sensitivity to the degree of electron delocalization and induced electric field. Here, we show that the IR intensity and linewidth can also provide unique and complementary information on the nature of charges. Quantifications of IR intensity and linewidth in a series of nitrile-functionalized oligophenylenes reveal that the C[triple bond, length as m-dash]N vibration is coupled to the nuclear and electronic structural changes, which become more prominent when an excess charge is present. We synthesized a new series of ladder-type oligophenylenes that possess planar aromatic structures, as revealed by X-ray crystallography. Using these, we demonstrate that C[triple bond, length as m-dash]N vibrations can report charge fluctuations associated with nuclear movements, namely those driven by motions of flexible dihedral angles. This happens only when a charge has room to fluctuate in space.

Lewis Acid Enables Ketone Phosphorylation to Form a C-P Bond and a C-C Bond: Synthesis of 9-Phosphoryl Fluorene Derivatives

An efficient method for the Lewis acid promotion of the synthesis 9-phosphoryl fluorenes has been reported. This method focuses on ketone phosphonylation to form a C-P bond and a C-C bond between diphenylmethanone and H-phosphinate esters, H-phosphites, and H-phosphine oxides via phospha-aldol elimination, in which a series of 9-phosphoryl fluorene derivatives were selectively obtained in moderate to excellent yields.

Intermolecular Three-Component Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Remote Nucleophilic Substitution Sequence

A three-component synthesis of multisubstituted fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/remote nucleophilic substitution sequence. A variety of nucleophiles can be installed in the second step including both heteroatom and carbon nucleophiles. An efficient synthesis of 5H-benzo[a]fluoren-5-ones has also been realized using N-(2-alkynyl)benzoylpyrrole as the reaction partner through a new reaction pathway.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

A denitrogenative palladium-catalyzed cascade for regioselective synthesis of fluorenes

We herein report a denitrogenative palladium-catalyzed cascade for the modular and regioselective synthesis of polysubstituted fluorenes. Hydrazone facilitates the Pd(ii) to Pd(iv) oxidative addition in a Catellani pathway and is also the methylene synthon in the proposed reaction. Aryl iodides and 2-bromoarylaldehyde hydrazones undergo a norbornene-controlled tandem reaction sequence to give a broad scope of fluorenes in the presence of a palladium catalyst. The method described is scalable and adaptable to a three-component reaction with in situ generation of the hydrazone group. Preliminary mechanistic investigations have been conducted.

Synthesis of Fluorenes Starting from 2-Iodobiphenyls and CH2Br2 through Palladium-Catalyzed Dual C-C Bond Formation

A facile and efficient approach is developed for the synthesis of fluorene and its derivatives starting from 2-iodobiphenyls and CH2Br2. A range of fluorene derivatives can be synthesized under relatively mild conditions. The reaction proceeds via a tandem palladium-catalyzed dual C-C bond formation sequence through the key dibenzopalladacyclopentadiene intermediates, which are obtained from 2-iodobiphenyls through palladium-catalyzed C-H activation.

Efficient palladium-catalyzed C(sp2)–H activation towards the synthesis of fluorenes

A newly developed palladium-catalyzed direct arylation via C–H activation adopting diphenylmethane type reactants for the synthesis of fluorene derivatives.

Transition-metal-free intramolecular carbene aromatic substitution/Büchner reaction: synthesis of fluorenes and [6,5,7]benzo-fused rings

Intramolecular aromatic substitution and Büchner reaction have been established as powerful methods for the construction of polycyclic compounds. These reactions are traditionally catalyzed by Rh(II) catalysts with α-diazocarbonyl compounds as the substrates. Herein a transition-metal-free intramolecular aromatic substitution/Büchner reaction is presented. These reactions use readily available N-tosylhydrazones as the diazo compound precursors and show wide substrate scope.

Aryl Bromides and Aryl Chlorides for the Direct Arylation of Benzylic Amines Mediated by Ruthenium(II)

The ruthenium(II)-catalyzed sp3 C-H bond arylation of benzylic amines with aryl halides is reported. In the present method, aryl iodides and, more importantly, also the cheaper aryl bromides and aryl chlorides can be applied as aryl sources. Additionally, the method does not require elaborate manipulations in a glove box and can be carried out in simple screw cap vials. Potassium pivalate proved to be beneficial for the transformation with aryl bromides or iodides as aryl source, but was not required for aryl chlorides. In the latter case, the addition of PPh3 led to high conversion. 3-Methyl and 3-phenyl pyridine were established as directing groups, and the substituent in the 3-position represents a key structural feature for high conversion. The directing group can be cleaved after the transformation, which allows access to diarylmethylamines. Mechanistic studies were carried out and critically compared to mechanistic reports of related transformations.

Indenofluorenes and derivatives: syntheses and emerging materials applications

The growing demand for flexible electronic devices and hydrogen storage materials has spurred a resurgence of interest in polyaryl hydrocarbons including graphene, acenes, fullerenes, polythiophenes, etc. Indenofluorenes are another polyaryl molecular scaffold that has shown utility in the organic and hybrid materials arena, with polymers incorporating the indeno[1,2-b]fluorene moiety being common in organic light emitting diodes. This review examines the syntheses and properties of the five distinct indenofluorene regioisomers, with a focus on small molecule applications in organic electronics of this intriguing and somewhat underexplored family of polyaryl hydrocarbons.

Transition metal-catalyzed arylation of unactivated C(sp3)-H bonds

Transition-metal-catalyzed C-H bond arylation has recently emerged as a powerful tool for the functionalization of organic molecules that may complement or even replace traditional catalytic cross-couplings. While many efforts have focused on the arylation of arenes and heteroarenes in the past two decades, less studies have been devoted to the arylation of nonacidic C-H bonds of alkyl groups. This tutorial review highlights recent work in this active area.