Pinpoint-Fluorinated Phenacenes: New Synthesis and Solubility Enhancement Strategies

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Palladium-catalysed difluoroolefination of benzyl tosylates toward the synthesis of gem-difluoro-2-trifluromethyl styrene derivatives

We have presented an efficient method to access gem-difluoro-2-trifluromethyl styrene derivatives via palladium catalysis. This method features mild reaction conditions, broad substrate scope and good product yields. Moreover, gram–scale reactions demonstrated the robustness and potential of this method. Control experiments revealed that the –CF₃ group was essential to the success of this transformation. Finally, the practicality of this method was successfully proven by three synthetic applications.

We presented an efficient method to access gem-difluoro-2-trifluromethyl styrene derivatives via palladium catalysis, which features mild reaction conditions and broad substrate scope.

Fluorous phthalocyanines and subphthalocyanines

Incorporating fluorine atoms into a molecule can endow it with various unique properties that enable materials applications. Selective solubility in fluorous solvents is achieved by a high fluorine content and selective partitioning into perfluorinated liquids over organic and aqueous phases provides orthogonal opportunities for chemistry and materials assembly. Although there is a growing number of partially fluorinated molecules, there are insufficient structural design principles to produce diverse fluorous soluble dyes. Herein, we report the synthesis of six fluorous phthalocyanine and subphthalocyanine dyes, and study their properties in the fluorous phase. Phthalocyanines generally display limited solubility and we also observed apparent aggregation in the fluorous phase. However, the nonplanar subphthalocyanines showed greater solubility. Subphthalocyanines also displayed fluorescence in selected solvents, and their emissive properties were investigated. The materials described expand the library of fluorous dyes and provide insights for the design of new molecules with fluorous solubility.

Nonstationary Two-Dimensional Nuclear Magnetic Resonance: A Method for Studying Reaction Mechanisms in Situ

Nuclear magnetic resonance spectroscopy (NMR) is a versatile tool of chemical analysis allowing one to determine structures of molecules with atomic resolution. Particularly informative are two-dimensional (2D) experiments that directly identify atoms coupled by chemical bonds or a through-space interaction. Thus, NMR could potentially be powerful tool to study reactions in situ and explain their mechanisms. Unfortunately, 2D NMR is very time-consuming and thus often cannot serve as a "snapshot" technique for in situ reaction monitoring. Particularly difficult is the case of spectra, in which resonance frequencies vary in the course of reaction. This leads to resolution and sensitivity loss, often hindering the detection of transient products. In this paper we introduce a novel approach to correct such nonstationary 2D NMR signals and raise the detection limits over 10 times. We demonstrate success of its application for studying the mechanism of the reaction of AgSO₄-induced synthesis of diphenylmethane-type compounds. Several reactions occur in the studied mixture of benzene and toluene, all with rather low yield and leading to compounds with similar chemical shifts. Nevertheless, with the use of a proposed 2D NMR approach we were able to describe complex mechanisms of diphenylmethane formation involving AgSO₄-induced toluene deprotonation and formation of benzyl carbocation, followed by nucleophilic attacks.

Nickel-Catalyzed Defluorinative Reductive Cross-Coupling Reaction of gem-Difluoroalkenes with Thiosulfonate or Selenium Sulfonate

A nickel-catalyzed defluorinative reductive cross-coupling of gem-difluoroalkenes with thiosulfonate or selenosulfonates is described. The reaction involves the formation of thiolated or selenylated monofluoroolefins via regioselective C-F bond cleavage and C-S or C-Se bond formation and features easily available substrates, mild reaction conditions, and high E-selectivity. One of the derivatives by further cross coupling with PhMgBr exhibited an aggregation-induced emission enhancement effect.

Synthesis of (difluoromethyl)naphthalenes using the ring construction strategy: C-C bond formation on the central carbon of 1,1-difluoroallenes via Pd-catalyzed insertion

The insertion of 1,1-difluoroallenes was carried out to form a C-C bond exclusively on their central carbon. o-Bromophenyl-bearing 1,1-difluoroallenes underwent intramolecular insertion in the presence of a palladium catalyst. Regioselective C-C bond formation occurred to form a six-membered carbocycle, leading to pharmaceutically and agrochemically promising difluoromethylated naphthalenes.

Fluorine-Containing Dibenzoanthracene and Benzoperylene-Type Polycyclic Aromatic Hydrocarbons: Synthesis, Structure, and Basic Chemical Properties

Intramolecular photocyclization of stilbene derivatives (Mallory reaction) is one of the efficient methods for building polycyclic aromatic hydrocarbon (PAH) frameworks, and is also expected to be applicable to synthesis of fluorine-containing PAHs (F-PAHs). In this study, dibenzoanthracene-type (4a) and benzoperylene-type (4b) F-PAHs were synthesized using the Mallory reaction of the 1,4-distyrylbenzene-type π-conjugated molecule (3a), which was prepared by addition-defluorination of available octafluorocyclopentene (OFCP) and aryllithium in three steps. The structure of 4a originating from π⁻π interaction was characterized by X-ray crystallographic analysis. The absorption maxima of UV-Vis spectra and emission maxima of photoluminescence spectra of the PAHs were positioned at a longer wavelength compared to those of the corresponding unsubstituted PAHs, presumably due to the electron-withdrawing nature of perfluorocyclopentene (PFCP) units. The effect of PFCP units in F-PAHs was also studied by time-dependent density functional theory (TD-DFT) calculation.

Hydroximoyl fluorides as the precursors of nitrile oxides: synthesis, stability and [3 + 2]-cycloaddition with alkynes

The transformation of hydroximoyl fluorides to nitrile oxides for [3 + 2]-cycloaddition with alkynes has been achieved for the first time. The hydroximoyl fluorides used in this work appeared to be not stable, which was proved by a series of experiments. A DFT calculation was performed to better understand the properties of hydroximoyl fluorides. Although not stable, the hydroximoyl fluorides could be successfully converted to the corresponding nitrile oxides for in situ [3 + 2]-cycloaddition with alkynes to yield the isoxazoles. Furthermore, it was feasible to conduct [3 + 2]-cycloaddition reaction without purification after the synthesis of hydroximoyl fluorides from gem-difluoroalkenes. By investigating a class of interesting yet previously rarely explored fluorinated compounds, this work sheds new light on the stability and reactivity of a C-F bond on a C[double bond, length as m-dash]N double bond.

Ring-size-selective construction of fluorine-containing carbocycles via intramolecular iodoarylation of 1,1-difluoro-1-alkenes

1,1-Difluoro-1-alkenes bearing a biaryl-2-yl group effectively underwent site-selective intramolecular iodoarylation by the appropriate cationic iodine species. Iodoarylation of 2-(2-aryl-3,3-difluoroallyl)biaryls proceeded via regioselective carbon–carbon bond formation at the carbon atoms in β-position to the fluorine substituents, thereby constructing dibenzo-fused six-membered carbocycles bearing a difluoroiodomethyl group. In contrast, 2-(3,3-difluoroallyl)biaryls underwent a similar cyclization at the α-carbon atoms to afford ring-difluorinated seven-membered carbocycles.

Enhancing charge mobilities in organic semiconductors by selective fluorination: a design approach based on a quantum mechanical perspective

Selective fluorination of organic semiconducting molecules is proposed as a means to achieving enhanced hole mobility. Naphthalene is examined here as a root molecular system with fluorination performed at various sites. Our quantum chemical calculations show that selective fluorination can enhance attractive intermolecular interactions while reducing charge trapping. Those observations suggest a design principle whereby fluorination is utilized for achieving high charge mobilities in the crystalline form. The utility of this design principle is demonstrated through an application to perylene, which is an important building block of organic semiconducting materials. We also show that a quantum mechanical perspective of nuclear degrees of freedom is crucial for a reliable description of charge transport.

Reconnaissance of reactivity of an Ag(ii)SO4 one-electron oxidizer towards naphthalene derivatives

Ag(ii)SO₄ – a powerful oxidizer – allows for single-pot oxidative aromatic coupling of naphthalene and its 1- and 2-substituted derivatives.

Palladium-Catalyzed Defluorinative Coupling of 1-Aryl-2,2-Difluoroalkenes and Boronic Acids: Stereoselective Synthesis of Monofluorostilbenes

The palladium-catalyzed defluorinative coupling of 1-aryl-2,2-difluoroalkenes with boronic acids is described. Broad functional-group tolerance arises from a redox-neutral process by a palladium(II) active species which is proposed to undergo a β-fluoride elimination to afford the products. The monofluorostilbene products were formed with excellent diastereoselectivity (≥50:1) in all cases, and it is critical, as traditional chromatographic techniques often fail to separate monofluoroalkene isomers. As a demonstration of this method's unique combination of reactivity and functional-group tolerance, a Gleevec® analogue, using a monofluorostilbene as an amide isostere, was synthesized.

Regiospecifically Fluorinated Polycyclic Aromatic Hydrocarbons via Julia-Kocienski Olefination and Oxidative Photocyclization. Effect of Fluorine Atom Substitution on Molecular Shape

A modular synthesis of regiospecifically fluorinated polycyclic aromatic hydrocarbons (PAHs) is described. 1,2-Diarylfluoroalkenes, synthesized via Julia-Kocienski olefination (70-99% yields), were converted to isomeric 5- and 6-fluorobenzo[c]phenanthrene, 5-and 6-fluorochrysene, and 9- and 10-benzo[g]chrysene (66-83% yields) by oxidative photocyclization. Photocyclization to 6-fluorochrysene proceeded more slowly than conversion of 1-styrylnaphthalene to chrysene. Higher fluoroalkene dilution led to a more rapid cyclization. Therefore, photocyclizations were performed at higher dilutions. To evaluate the effect of fluorine atom on molecular shapes, X-ray data for 5- and 6-fluorobenzo[c]phenanthrene, 6-fluorochrysene, 9- and 10-fluorobenzo[g]chrysene, and unfluorinated chrysene as well as benzo[g]chrysene were obtained and compared. The fluorine atom caused a small deviation from planarity in the chrysene series and decreased nonplanarity in the benzo[c]phenanthrene derivatives, but its influence was most pronounced in the benzo[g]chrysene series. A remarkable flattening of the molecule was observed in 9-fluorobenzo[g]chrysene, where the short 2.055 Å interatomic distance between bay-region F-9 and H-8, downfield shift of H-8, and a 26.1 Hz coupling between F-9 and C-8 indicate a possible F-9···H-8 hydrogen bond. In addition, in 9-fluorobenzo[g]chrysene, the stacking distance is short at 3.365 Å and there is an additional interaction between the C-11-H and C-10a of a nearby molecule that is almost perpendicular.

Aluminium-mediated aromatic C–F bond activation: regioswitchable construction of benzene-fused triphenylene frameworks

Aluminium-mediated selective synthesis of benzo[f]tetraphenes or benzo[g]chrysenes was achieved via aromatic C–F bond cleavage and regioselective C–C bond formation.

Catalytic defluorinative [3 + 2] cycloaddition of trifluoromethylalkenes with alkynes via reduction of nickel(ii) fluoride species

A catalytic synthesis of 2-fluoro-1,3-cyclopentadienes was achieved via [3 + 2] cycloaddition of 2-trifluoromethyl-1-alkenes and alkynes using a nickel catalyst and a diboron-based ternary additive system.