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

A comprehensive Review into Emission Sources, Formation Mechanisms, Ecological Effects, and Biotransformation Routes of Halogenated Polycyclic Aromatic Hydrocarbons (HPAHs)

Halogenated polycyclic aromatic hydrocarbons (HPAHs, H = F, Cl, Br) are a new class of PAHs derivatives that mainly originate from the incomplete combustion of halogen-laden materials and via metallurgical operations. These compounds circulate extensively in various environmental matrices. This survey provides a comprehensive review on governing synthesis routes of HPAHs, their environmental occurrence, and their health and ecological effects. The review comprehensively enlists and presents emission sources of these emerging organic pollutants into the air that serves as their main reservoir. The formation of HPAHs ensues through successive addition reactions of related precursors accompanied by ring cyclization steps; in addition to direct unimolecular fragmentation of parents halogenated. Halogenation of parent PAHs rapidly occurs in saline ecosystems, thus multiplying the availability of these notorious compounds in the environment. Certain HPAHs appear to be more carcinogenic than dioxins. Transmission routes of HPAHs from their emission sources to water bodies, soil, aquatic life, plants, terrestrial animals, and humans are well-documented. Later, the direct and indirect diffusion of HPAHs from air to the biotic (plants, animals, humans) and abiotic components (soil, water, sediments) are described in detail. The study concludes that HPAHs are permeable to the carbon matrices resulting in the alleviation of the source-to-sink interface. As a potential future perspective, understanding the transmission interfaces lays a foundation to intervene in the introduction of these toxicants into the food chain.

Through-space H-F coupling in a series of 4-substituted-1H-1,2,3-triazoles

In the 1H-NMR spectra of a series of N-1 substituted 4-substituted-1H-1,2,3-triazoles that have been prepared, the lone heterocyclic ring hydrogen (H-5) appears as a singlet in all cases except those compounds that contain a 2-fluorophenyl moiety at Position 4. In those cases, H-5 is a doublet with J ~3.7 Hz. Based on computational chemistry results and geometric considerations, we attribute this splitting to through-space H-F coupling.

Organic Binary and Ternary Cocrystal Engineering Based on Halogen Bonding Aimed at Room-Temperature Phosphorescence

Recently, there has been intense interest in pure organic room-temperature phosphorescence (ORTP) from cocrystals composed of 1,4-diiodotetrafluorobenzene (DITFB) and a variety of polycyclic aromatic hydrocarbons (PAHs) or their derivatives. To expand the possibility of halogen bonding-based cocrystals, the relationship between the crystal packing motifs and ORTP characteristics in binary cocrystals composed of DITFB and PAHs of phenanthrene (Phen), chrysene (Chry), and pyrene (Pyr), respectively, is investigated. The σ-hole···π and π-hole···π interactions determine not only the crystal packing motifs but also photoluminescence quantum yields (PLQYs). The Phen-DITFB and Chry-DITFB binary cocrystals with σ-hole···π interactions show higher PLQY compared with the Pyr-DITFB binary cocrystal with π-hole···π interaction. Further, to clarify the effect of crystal structures on PLQY, ternary cocrystals are prepared by partially doping Pyr into Phen-DITFB. The crystal packing motif of the ternary cocrystal originates from a Phen-DITFB cocrystal with σ-hole···π interaction, and some of the Phen sites are randomly replaced with Pyr molecules. The ORTP emission is derived from Pyr. The maximum PLQY is >20% due to suppressing nonradiative decay by changing the crystal packing motif.

Skeletal photoinduced rearrangement of diarylethenes: ethene bridge effects

A skeletal photorearrangement involving UV-induced 6π-electrocyclization of diarylethenes with various ethene bridges has been studied. It has been found that deprotonation is the predominant step among the three possible alternative reaction pathways (radical abstraction, deprotonation, or sigmatropic shift) following 6π-electrocyclization, and incorporation of an electronegative carbonyl group into the geminal position to the phenyl residue results in a reduction in the reaction time and an increase in the yield of the desired product. The significant increase in the reaction time in less polar solvents (toluene, TCM) also indicates a large contribution of the deprotonation step to the skeletal photorearrangement of diarylethenes. Performing the reaction in toluene in the presence of tertiary amines leads to a reduction in the reaction time and an increase in the yield of the desired product. The best results were achieved when the reaction was carried out in toluene in the presence of DIPEA. The experimental results are in good agreement with the DFT calculations.

Chrysene-Based Azahelicene π-Linker of D-π-D-Type Hole-Transporting Materials for Perovskite Solar Cells

Chrysene is a readily available material for exploring new polycyclic aromatic hydrocarbons (PAHs). In this study, two chrysene based azahelicenes, nine-membered BA7 and ten-membered DA6, are constructed by intermolecular oxidative annulation of 6-aminochrysene and intramolecular annulation of N⁶ ,N¹² -bis(1-chloronaphthalen-2-yl)chrysene-6,12-diamine, respectively. The hexylated BA7 and DA6 and their brominated products were undoubtedly characterized by single crystal XRD. Subsequent amination with bis(9-methyl-9H-carbazol-3-yl)amine (BMCA) electron donor afforded D-π-D-type semiconductors BA7-BMCA and DA6-BMCA with beneficial properties to act as hole transport materials for perovskite solar cell. Compared with 19.4 % champion power conversion efficiency (PCE) of BA7-BMCA based device, a higher PCE of 20.2 % for DA6-BMCA counterpart may be attributed to its S-shaped double helicene-like linker with extended π-conjugated system.

Chemo-, regio- and stereoselective synthesis of monofluoroalkenes via a tandem fluorination-desulfonation sequence

A widely applicable approach for the synthesis of Z-monofluoroalkenes from readily available alkyl triflones and NFSI has been reported. The reaction proceeded under mild conditions, affording mono-fluorinated alkenes in good to excellent yields with excellent chemo- regio- and stereoselectivity. The mechanism may involve electrophilic fluorination of triflones followed by the highly stereoselective concerted bimolecular elimination (E2) of CF3SO2H.

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

The intramolecular Diels-Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F₄ -[7]helicene) undergoes a photoinduced Diels-Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F₃ -[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels-Alder reactions, but the whole domino process became slow. F₂ -[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F₂ -[7]helicene was less compressed than F₄ -[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene-fluoroarene stacking interactions and thus promoted the transformations.

Exploration of Kobayashi's aryne precursor: a potent reactive platform for the synthesis of polycyclic aromatic hydrocarbons

Arynes due to their transient nature leads to remarkable and versatile applications in the synthetic world. Apparently, researchers have focused on the construction of simple to complex π-conjugated systems using arynes as the reactive platform. In this regard, Kobayashi's aryne precursor has shown a great extent of reactivity and afforded significant advancement in the synthesis of polycyclic aromatic systems with wide practical utility. This review emphasizes the extensive utilization of Kobayashi's aryne intermediates and their derivatives for the synthesis of different classes of polycyclic aromatic hydrocarbons (PAHs).

Selanyl and tellanyl electrophiles as a driving force in the construction of sophisticated polyaromatic hydrocarbons

Herein, we report the first examples of N-polyaromatic compounds bearing up to 13 fused aromatic rings, including 23H-benzo[12,1]tetrapheno[8,9-b]benzo[12,1]tetrapheno[9,8-h]carbazole derivatives.

Synthesis of Phenacene-Helicene Hybrids by Directed Remote Metalation

Polycyclic aromatic hydrocarbons (PAHs) with six and seven rings were synthesized via directed metalation and cross-coupling of chrysenyl N,N-diethyl carboxamides with o-tolyl and methylnaphthalenyl derivatives. In the presence of competing ortho sites, the site selectivity in iodination of chrysenyl amides by directed ortho metalation (DoM) was influenced by the lithium base. The catalyst ligand bite angle was presumably important in the cross-coupling of sterically hindered bulky PAHs. Subsequent directed remote metalation of biaryls under standard conditions and at elevated temperatures afforded various fused six- and seven-ring PAHs, all in good yields and with fluorescent properties.

A General Synthetic Approach and Photophysical Properties of Regioselectively Fluorinated [5]- and [6]-Helical Bispiroindenofluorenes

A first series of fluorinated [n]helical compounds (n=5 and 6) with the dihydroindenofluorene scaffold was prepared in 5 or 9 (octafluorinated dihydroindenofluorene) steps and their photophysical properties were determined. Rh-catalyzed intramolecular [2+2+2] cyclotrimerization of triyndiols, which were prepared in a modular fashion from simple starting material such as fluorinated haloarylcarbaldehydes, to the intermediate [n]helical dihydroindeno[2,1-c]fluorene-5,8-diols was the crucial synthetic step and proceeded with high efficacy. Their further transformation gave the desired selectively fluorinated bispirodihydroindeno[2,1-c]fluorenes. Their absorption and emission spectra were recorded. The fluorescence quantum yields were up to 92 % and the emission maxima were red-shifted in comparison with their non-fluorinated counterparts (386-413 nm).

Columnar Stacking of Partially Fluorinated [4]Helicenes: C-H⋅⋅⋅F Interactions Change the Stacking Orientation

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F₆ -[4]Helicene forms head-to-tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene-fluoroarene (Ar^H -Ar^F ) hetero-stacking to Ar^H -Ar^H /Ar^F -Ar^F homo-stacking with the help of intermolecular C-H⋅⋅⋅F contacts in the fjord region. As a result, head-to-head columnar stacks appear. Therefore, the conventional Ar^H -Ar^F stacking motif is not always applicable to F_n -[4]helicenes with twisted π-surfaces.

Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings

The first racemization-stable helicene derivatives fluorinated at terminal rings, 1,2,3,4-tetrafluoro[6]helicene (6) and 1,2,3,4,13,14,15,16-octafluoro[6]helicene (15), were synthesized via the Wittig reaction followed by oxidative photocyclization in an overall yield of 41% of 6 and 76% of 15. The changed electronic structure in fluorinated helicenes was reflected in a slight shift of UV-vis absorption, fluorescence excitation, and emission spectra maxima when compared to unsubstituted [6]helicene. Cyclic voltammetry revealed a moderate decrease in the HOMO-LUMO gap with increasing fluorination. The specific rotation of tetrafluoro[6]helicene 6 enantiomers was found to be approximately 25% lower than that of unsubstituted [6]helicene. The theoretical study of the racemization barrier suggested a reasonable shift toward higher energy with increasing fluorination. The increasing fluorination also significantly affected the intermolecular interactions in the crystal lattice. The observed CH···F interactions led to the formation of 1D-molecular chains in the crystal structures of both fluorinated helicenes.

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.

Generating Stereodiversity: Diastereoselective Fluorination and Highly Diastereoselective Epimerization of α-Amino Acid Building Blocks

Diastereoselective fluorination of N-Boc ( R)- and ( S)-2,2-dimethyl-4-((arylsulfonyl)methyl)oxazolidines and a previously unknown diastereoselective epimerization at the fluorine-bearing carbon atom α to the sulfone was realized. Diastereoselectivities of both reactions were excellent for benzothiazolyl sulfones, allowing access to two enantiomerically pure diastereomers from one chiral precursor. To demonstrate synthetic utility, the benzothiazolyl sulfones were converted to diastereomerically pure ( S, S)- and ( R, S)-benzyl sulfones via sulfinate salts and to amino acids. To understand the diastereoselectivities, DFT analysis was performed.