Transition-metal-catalyzed methods for synthesis of fluorenes

Versatile C─H Alkylation and Alkylidenation via Catalytic Alkylidene Transfer of Enones

The alkylidene transfer reactions of alkenes are of particular significance but challenging. Here, we report that enones can serve as diverse alkylidene sources for catalyst-controlled selective C─H alkylation and/or alkylidenation of various nucleophiles. Treatment of a mixture of ketone (or lactam), enone, and diarylmethanol, with a catalytic amount of Y[N(TMS)2]3, gave the corresponding α-C─H bond alkylation products derived from the alkylidene transfer from enones to ketones/lactams, whereas the reaction of enones with various C-nucleophiles in the presence of KOH as a catalyst resulted in C─H alkylidenation. Moreover, the application of these strategies for the late-stage modification or structural simplification of some bioactive molecules is also presented. These alkylidene transfer reactions are characterized by operational simplicity, mild reaction conditions, and remarkable catalyst-controlled product outcomes. These results not only demonstrate a significant potential for easily accessible and recyclable enones to serve as versatile alkylidene sources in C─H alkylation and alkylidenation but also provide an attractive and concise method for hydrodealkylidenation of electron-deficient alkenes.

Divergent Aromatization of α-Halobenzyl γ-Butenolides Initiated by Selective Enol Protonation to Benzo[c]fluorenones and Naphthalenes

An unprecedented divergent aromatization reaction of α-halobenzyl γ-butenolides has been described for the selective and concise synthesis of highly substituted benzo and higher π-extended fluorenones, and 1,3-disubstituted naphthalenes depending on the migration ability of the quaternary α-substituent. This aromatization switch from Ag+-mediated planarization to ylidenebutenolides likely originates from selective protonation on the enolic double bond rather than the benzyl halides by TfOH.

The Biomimetic Synthesis of Polyarylated Fluorenes, Relevant to Selaginellaceae Polyphenols, Leading to the Spontaneous Formation of Stable Radicals

This work reports a biomimetic synthesis of polyarylated fluorene derivatives. The molecules are formed via intramolecular electrophilic aromatic substitution, resembling a cyclization leading towards the natural selaginpulvilins from selaginellins. The scope of the reaction was investigated, and the products were obtained in 60-95 % yields. Some of the compounds decompose to a stable radical. We investigated the nature and the origin of the radical using experimental methods, including EPR or electrochemical measurements, as well as theoretical methods, such as DFT calculations. Based on our observations, we hypothesize, that phenoxy radicals are formed in the first instance, which however undergo internal rearrangement to thermodynamically more stable carbon-centered radicals. The preliminary data also show the cytotoxic properties of some of the molecules.

6aH-Benzo[α]fluorene: NMR evidence of the unexpected product of the reaction of butyryl chloride with 1,2-diphenylacetylene

The reaction of butyryl chloride with ethynylbenzene in the presence of AlCl3 afforded a mixture of the Z/E-isomers of 1-chloro-2-phenylhex-1-en-3-one. 1,2-Diphenylethyne under these conditions gave a novel polycarbocycle core, 6aH-benzo[a]fluorene. The chemical structure of 11-chloro-5,6-diphenyl-6a-propyl-6aH-benzo[a]fluorene was established by means of IE-MS, 1 H, 13 C NMR, COSY, HSQC, HMBC, and 2D INADEQUATE technique.

Superacid Promoted Synthesis of 9,9'-Spirobifluorenes and Related Aza- and Diazaspirocycles

A variety of 9,9'-spirobifluorenes and related azaspirocycles and diazaspirocycles have been prepared in high yields by intramolecular Friedel-Crafts reaction using the Brønsted superacid, triflic acid (CF₃SO₃H). Compared to weaker Brønsted acids, the superacid consistently provides better yields along with reduced reaction times and it eliminates the need for heating. The superacid is particularly effective when dicationic electrophilic intermediates are generated in the conversions, for example with the diazaspirocycle syntheses.

Synthesis of 3-Hydroxy-9H-fluorene-2-carboxylates via Michael Reaction, Robinson Annulation, and Aromatization

A series of 3-hydroxy-fluorene-2-carboxylate compounds were prepared from Michael addition of acetoacetate to 2-benzylideneindan-l-one followed by Robinson annulation and aromatization. In this reaction, we were able to isolate two Robinson annulation products and characterize them. This sequential reaction could proceed without the isolation of intermediates to give the desired products directly in reasonable yields.

Rapid access to 9-arylfluorene and spirobifluorene through Pd-catalysed C-H arylation/deaminative annulation

We describe here a facile synthesis of 9-arylfluorenes and spirobifluorenes from readily available 1,1-diarylmethylamines and iodoarenes through Pd-cataylsed C(sp²)-H arylation and a sequential deaminative annulation. The reaction features high efficiency and simplicity of operation, constituting an interesting shortcut to access fluorene compounds.

Covalent nanosynthesis of fluorene-based macrocycles and organic nanogrids

Gridization is an alternative way to create macromolecules of various sizes in addition to linear and dendritic polymerization as well as cyclization. Organic nanogrids are an expanding family of macrocycle-like closed structures at the nanoscale, but with a series of well-defined extension edges and vertices. Cyclic nanogrids can be used as nanoscale building blocks for the fabrication of not only rotaxanes, catenanes, knots, 3D cages, but also nanopolymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and complex molecular cross-scale architectures. In this review, the history of fluorene-based macrocycles has first been explored, followed by the development of the synthetic methodologies; in particular, fluorene-based nanogrids are highlighted owing to their features and applications. Typically, fluorenes are fused arenes with a hybrid entity between tetrahedral Csp³ and Csp². Four ingenious connection modes of fluorene-based macrocycles, including 2,7-, 3,6-, 9,9-, and 2,9-linkages, fully demonstrate the geometric possibilities of the macrocycles and nanogrids. Such fluorene-based nanogrids will give birth to organic intelligence.

Palladium Catalyzed Regioselective Cyclization of Arylcarboxylic Acids via Radical Intermediates with Diaryliodonium Salts

Palladium-catalyzed C₂-arylation/intramolecular acylation with arylcarboxylic acids was developed by using diaryliodonium salts. The protocol has the advantage of good step-economy by two chemical bonds formation in one pot.

Synthesis of highly substituted fluorenones via metal-free TBHP-promoted oxidative cyclization of 2-(aminomethyl)biphenyls. Application to the total synthesis of nobilone

Highly substituted fluorenones are readily prepared in mostly fair to good yields via metal- and additive-free TBHP-promoted cross-dehydrogenative coupling (CDC) of readily accessible N-methyl-2-(aminomethyl)biphenyls and 2-(aminomethyl)biphenyls. This methodology is compatible with numerous functional groups (methoxy, cyano, nitro, chloro, and SEM and TBS-protective groups for phenols) and was further utilized in the first total synthesis of the natural product nobilone.

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.

Iridium(III)-Catalyzed Diarylation/Annulation of Benzoic Acids: Facile Access to Multi-Aryl Spirobifluorenes as Pure Hydrocarbon Hosts for High-Performance OLEDs

Herein disclosed is the first example of diarylation/annulation of benzoic acids via an iridium catalyst system. This protocol provides a step-economic and highly efficient pathway to 1-aryl, 1,3-diaryl, 1,7-diaryl and 1,3,7-triaryl spirobifluorenes from readily available starting materials. The applications of multi-aryl spirobifluorenes as pure hydrocarbon (PHC) hosts for red, green, and blue (RGB) phosphorescent organic light-emitting diodes (PhOLEDs) were explored. Due to high triplet energies, 1,3-diaryl spirobifluorenes exhibit the potential as the host material of blue PhOLEDs. 1,7-Diaryl spirobifluorene can serve as the host of green PhOLEDs. 1,3,7-Triaryl spirobifluorene is a high-performance host for red PhOLEDs, which exhibits a high external quantum efficiency (EQE) up to 27.3 %. This work not only exemplifies the great potential of multi-aryl spirobifluorenes as PHC hosts, but also offers a new approach for the synthesis of these PHC hosts.

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.

Comprehensive review of polycyclic aromatic hydrocarbons in water sources, their effects and treatments

Polycyclic aromatic hydrocarbons (PAHs) are principally derived from the incomplete combustion of fossil fuels. This study investigated the occurrence of PAHs in aquatic environments around the world, their effects on the environment and humans, and methods for their removal. Polycyclic aromatic hydrocarbons have a great negative impact on the humans and environment, and can even cause cancer in humans. Use of good methods and equipment are essential to monitoring PAHs, and GC/MS and HPLC are usually used for their analysis in aqueous solutions. In aquatic environments, the PAHs concentrations range widely from 0.03 ng/L (seawater; Southeastern Japan Sea, Japan) to 8,310,000 ng/L (Domestic Wastewater Treatment Plant, Siloam, South Africa). Moreover, bioaccumulation of ∑16PAHs in fish has been reported to range from 11.2 ng/L (Cynoscion guatucupa, South Africa) to 4207.5 ng/L (Saurida undosquamis, Egypt). Several biological, physical and chemical and biological techniques have been reported to treat water contaminated by PAHs, but adsorption and combined treatment methods have shown better removal performance, with some methods removing up to 99.99% of PAHs.

Synthesis of Tri- and Disubstituted Fluorenols and Derivatives Thereof Using Catalytic [2+2+2] Cyclotrimerization

A method for regioselective synthesis of 2,4-disubstituted and more highly substituted fluorenols using catalytic [2+2+2]cyclotrimerization of mono- and disubstituted diynes with terminal alkynes was explored. In the former case, the preferential formation of the 2,4-regioisomers was achieved in the presence of Cp*Ru(cod)Cl, whereas Rh-based catalysts tended to provide 3,4-regioisomers as the major products. The 2,4-disubstituted fluorenols were converted into the corresponding 9,9′-spirobifluorene derivatives and their structural and photophysical properties were evaluated.