Unusually Short H⋠⋠⋠H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes.

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

Chemodivergent Synthesis of Polycyclic Aromatic Diarylamines and Carbazoles by Thermal/Photochemical Process-Controlled Dephosphinylative Functionalizations of Amino(phosphinyl)arenes.

A chemodivergent synthesis of polycyclic aromatic diarylamines and carbazoles was established by employing thermally or photochemically controlled processes using KOtBu/1,10-phenanthroline. The synthetic processes involved the dephosphinylation of 9-amino-10-(phosphinyl)phenanthrenes, which were obtained through a regioselective palladium-catalyzed direct [4 + 2] benzannulation of phosphinyl ynamines with 2-iodobiphenyls. When the dephosphinylation was conducted under heating conditions (∼100 °C), it proceeded to yield 9-aminophenanthrene. However, when the reaction was performed under the illumination of purple light (LEDs, λmax = ca. 390 nm), KOtBu/1,10-phenanthroline promoted single-electron-transfer-triggered dephosphinylation followed by cyclization, producing the corresponding π-expanded carbazoles. We successfully synthesized a highly π-expanded dicarbazole through a dual dephosphinylative cyclization. Additionally, we present the optical properties of a series of amino compounds produced through the dephosphinylative processes.

Synthesis, Structures, and Properties of Helically Fused Anthraquinones with Unusually Close Carbonyl-Carbonyl Contacts.

Electron-deficient aromatic ketones consisting of three fused anthraquinone units were synthesized by oxidation of the corresponding fused anthracenes. X-ray analysis revealed that these compounds had nonplanar helical structures with unusual contacts, C=O⋅⋅⋅C=O 2.467 Å, between the inner carbonyl groups. The role of n⋅⋅⋅π* interactions in the short contacts was evaluated using a noncovalent interaction plot and natural bond orbital analysis. The dynamic process involving helical inversion was observed by the variable temperature 1 H NMR measurement of a derivative with 2,4,6-trimethylphenyl groups, and the barrier was estimated to be 77 kJ mol-1 . DFT calculations indicated that the helical inversion proceeded via a multistep mechanism. The characteristic spectroscopic and electrochemical data due to the electron-deficient anthraquinone units and the sterically congested carbonyl groups are discussed with the aid of DFT calculations.

Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes.

Invited for the cover of this issue is Shinji Toyota and co-workers at Tokyo Institute of Technology and Okayama University of Science. The image depicts a spirally rising dragon to represent the helical molecular structures in the manuscript. Read the full text of the article at 10.1002/chem.202004720.

Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes.

Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl2 -catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the terminal anthracene moieties at 0.33 nm interlayer distance. In the UV-vis and fluorescence spectra, the absorption and emission bands were red-shifted as the number of fused anthracene units was increased. The characteristic broad and long-lived emission bands of the long analogues are explained by the excimer-like stabilization of the excited state. These photophysical data as well as their cyclic voltammetric data are discussed on the basis of the π-conjugation and interlayer π⋅⋅⋅π interactions in the molecular structures and the molecular orbitals. The barrier and mechanism of helical inversion are also reported.

Competitive Desulfonylative Reduction and Oxidation of α-Sulfonylketones Promoted by Photoinduced Electron Transfer with 2-Hydroxyaryl-1,3-dimethylbenzimidazolines under Air.

Desulfonylation reactions of α-sulfonylketones promoted by photoinduced electron transfer with 2-hydroxyarylbenzimidazolines (BIH-ArOH) were investigated. Under aerobic conditions, photoexcited 2-hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylketones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examination of the effects of solvents, radical trapping reagents, substituents of sulfonylketones, and a variety of hydroxyaryl- and aryl-benzimidazolines (BIH-ArOH and BIH-Ar) suggest that the oxidation products are produced by dissociation of α-ketoalkyl radicals from the initially formed solvent-caged radical ion pairs followed by reaction with molecular oxygen. In addition, the observations indicate that the reduction products are generated by proton or hydrogen atom transfer in solvent-caged radical ion pairs derived from benzimidazolines and sulfonylketones. The results also suggest that arylsulfinate anions arising by carbon-sulfur bond cleavage of sulfonylketone radical anions act as reductants in the oxidation pathway to convert initially formed α-hydroperoxyketones to α-hydroxyketones. Finally, density functional theory calculations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.

Visible Light and Hydroxynaphthylbenzimidazoline Promoted Transition-Metal-Catalyst-Free Desulfonylation of N-Sulfonylamides and N-Sulfonylamines.

A visible light promoted process for desulfonylation of N-sulfonylamides and -amines has been developed, in which 1,3-dimethyl-2-hydroxynaphthylbenzimidazoline (HONap-BIH) serves as a light absorbing, electron and hydrogen atom donor, and a household white light-emitting diode serves as a light source. The process transforms various N-sulfonylamide and -amine substrates to desulfonylated products in moderate to excellent yields. The observation that the fluorescence of 1-methyl-2-naphthoxy anion is efficiently quenched by the substrates suggests that the mechanism for the photoinduced desulfonylation reaction begins with photoexcitation of the naphthoxide chromophore in HONap-BIH, which generates an excited species via intramolecular proton transfer between the HONap and BIH moieties. This process triggers single electron transfer to the substrate, which promotes loss of the sulfonyl group to form the free amide or amine. The results of studies employing radical probe substrates as well as DFT calculations suggest that selective nitrogen-sulfur bond cleavage of the substrate radical anion generates either a pair of an amide or amine anion and a sulfonyl radical or that of an amidyl or aminyl radical and sulfinate anion, depending on the nature of the N-substituent on the substrate. An intermolecular version of this protocol, in which 1-methyl-2-naphthol and 1,3-dimethyl-2-phenylbenzimidazoline are used concomitantly, was also examined.

Nano-Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C60.

An anthracene cyclic hexamer was synthesized by the coupling reaction as a macrocyclic hydrocarbon host. This disk-shaped host included a C60 guest in 1:1 ratio to form a Saturn-type supramolecular complex in solution and in crystals. X-ray analysis unambiguously revealed that the guest molecule was accommodated in the middle of the host cavity with several CH⋅⋅⋅π contacts. The association constant Ka determined by NMR titration measurements was 2.3×103  L mol-1 at 298 K in toluene. The structural features and the role of CH⋅⋅⋅π interactions are discussed with the aid of DFT calculations.

Phenothiazine cyclic hexamers: synthesis, properties, and complexation behavior with C60.

Phenothiazine cyclic hexamers linked at the 3,7-positions were synthesized. Effects of the cyclic structure as well as substituent effects of the nitrogen atom were disclosed. Furthermore, the cyclic hexamer encapsulated C60 inside the ring in a 1 : 1 ratio giving a Saturn-type complex in solution. The structure and interactions were discussed based on DFT calculations.

Synthesis, optical properties and crystal structure of (E,E)-1,3-(3,4:9,10-dibenzododeca-1,11-diene-5,7-diyne-1,12-di-yl)benzene.

The de-hydro-benzannulene (E,E)-1,3-(3,4:9,10-dibenzododeca-1,11-diene-5,7-diyne-1,12-di-yl)benzene, C26H16, was successfully synthesized via photocatalyst-assisted stereoselective reductive de-sulfonyl-ation of 1,3-bis-{1-phenyl-sulfonyl-2-[2-(tri-methyl-silylethyn-yl)phen-yl]ethen-yl}benzene, C44H42O4S2Si2, and subsequent desilylative cyclization of the resulting (E,E)-bis-silyl-protected dienyne, C32H34Si2. The structure of the de-hydro-benzannulene thus obtained was confirmed by single-crystal X-ray analysis; three benzene rings are connected to one another by a 1,3-butadiynylene and a pair of ethenylene arrays. Although the π-system expanded efficiently in the de-hydro-benzannulene, it was observed that the butadiynylene and ethenylene arrays were strained, showing smaller [171.3 (2)-172.6 (2) °] and larger bond angles [122.5 (2)-131.9 (2)°] than the conventional bond angles, respectively. In CHCl3, the de-hydro-benzannulene showed the longest absorption band at 377 nm. When irradiated by UV light, it emitted fluorescence at 468 nm (ΦF = 0.26) and 504 nm (ΦF = 0.24) in CHCl3 and in the powdered state, respectively.

Degradation of three ß-O-4 lignin model compounds via organic electrolysis and elucidation of the degradation mechanisms.

Woody biomass comprising cellulose, hemicellulose, and lignin has been the focus of considerable attention as an alternative energy source to fossil fuel for various applications. However, lignin has a complex structure, which is difficult to degrade. Typically, lignin degradation is studied using ß-O-4 lignin model compounds as lignin contains a large number of ß-O-4 bonds. In this study, we investigated the degradation of the following lignin model compounds via organic electrolysis: 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)ethanol 1a, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol 2a, and 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol 3a. The electrolysis was conducted for 2.5 h at a constant current of 0.2 A using a carbon electrode. Various degradation products such as 1-phenylethane-1,2-diol, vanillin, and guaiacol were identified upon separation via silica-gel column chromatography. The degradation reaction mechanisms were elucidated using electrochemical results as well as density functional theory calculations. The results suggest that the organic electrolytic reaction can be used for the degradation reaction of a lignin model with ß-O-4 bonds.

Structure and Photophysical Properties of 1,1,2,2-Tetra(1-anthryl)ethane: A C(sp3 )-C(sp3 ) Bond Substituted with Four Anthracene Units.

The title aromatic compound comprising four anthracene units was synthesized by the McMurry coupling of di(1-anthryl) ketone as a hydrogenated product in 65 % yield. The molecule forms a C2 symmetric structure with the ap conformation about the C(sp3 )-C(sp3 ) single bond, as revealed by X-ray analysis and DFT calculations. The UV/vis and fluorescence spectra of this compound were compared with those of anthracene, di(1-anthryl)methane, and 1,2-di(1-anthryl)ethane. The fluorescence spectrum showed a broad emission band at 450 nm having a long lifetime at 21 ns, which was assignable to an excimer-type emission, in contrast to the other reference compounds. The characteristic photophysical property is discussed in terms of the molecular structure with the aid of the noncovalent interaction plots and the conformational analysis.

Complexation of an Anthracene-Triptycene Nanocage Host with Fullerene Guests through CH⋠⋠⋠π Contacts.

A bicyclic anthracene macrocycle containing two triptycene units at the bridgehead positions was synthesized by Ni-mediated coupling of the corresponding precursor as a cage-shaped aromatic hydrocarbon host. This cage host formed an inclusion complex with C60 or C70 guest in 1 : 1 ratio in solution. The association constants (Ka ) determined by the fluorescence titration method were 1.3×104 and 3.3×105  L mol -1 for the C60 and C70 complexes, respectively, at 298 K in toluene. DFT calculations revealed that the guest molecules were included in the middle of the cavity with several CH⋅⋅⋅π contacts. The strong affinity of the cage host for the fullerene guests and the high selectivity toward C70 are discussed on the basis of spectroscopic and structural data.

Sterically Regulated α-Oxygenation of α-Bromocarbonyl Compounds Promoted Using 2-Aryl-1,3-dimethylbenzimidazolines and Air.

A debrominative oxygenation protocol has been developed for the conversion of α-bromo-α,α-dialkyl-substituted carbonyl compounds to their corresponding α-hydroxy analogues. For example, stirring a solution of α-bromoisobutyrophenone and 2-aryl-1,3-dimethylbenzimidazoline (BIH-Ar) at room temperature under an air atmosphere leads to the efficient formation of α-hydroperoxyisobutyrophenone, which can be converted to α-hydroxyisobutyrophenone using Me2S reduction. In contrast, reaction of α-bromoacetophenone under the same conditions produces the α-hydrogenated product acetophenone. α-Keto-alkyl and benzimidazolyl radicals (BI•-Ar), generated via dissociative electron transfer from BIH-Ar to α-bromoketone substrates, serve as key intermediates in the oxidation and reduction processes. The dramatic switch from hydrogenation to oxygenation is attributed to a steric effect of α-alkyl substituents, which causes hydrogen atom abstraction from sterically crowded BIH-Ar to α-keto-alkyl radicals to be slow and enable preferential reaction with molecular oxygen. Generation of the α-keto-alkyl radical and BI•-Ar intermediates in these process and their sterically governed hydrogen atom transfer reactions are supported by results arising from DFT calculations. Moreover, an electron spin resonance study showed that visible light irradiation of phenyl benzimidazoline (BIH-Ph) in the presence of molecular oxygen produces the benzimidazolyl radical (BI•-Ph). The addition of thiophenol into the reaction of α-bromoisobutyrophenone and BIH-Ph predominantly produced α-phenylthiolated isobutyrophenone even if a high concentration of molecular oxygen exists. Furthermore, the developed protocol was applied to other α-bromo-α,α-dialkylated carbonyl compounds.

Macrocyclic 2,7-Anthrylene Oligomers.

A macrocyclic compound consisting of six 2,7-anthrylene units was successfully synthesized by Ni-mediated coupling of the corresponding dibromo precursor as a novel π-conjugated compound. This compound was sufficiently stable and soluble in organic solvents due to the presence of mesityl groups. X-ray analysis showed that the molecule had a nonplanar and hexagonal wheel-shaped framework of approximately S6 symmetry. The dynamic process between two S6 structures was observed by using the dynamic NMR technique, the barrier being 58 kJ mol(-1) . The spectroscopic properties of the hexamer were compared with those of analogous linear oligomers.

Dual Emission and Mechanofluorochromism of a V-Shaped π-System Composed of Disulfonyl-Substituted Dibenzocyclooctatetraenes.

A series of dibenzocyclooctatetraenes 6 bearing phenylethynyl and phenylsulfonyl groups were synthesized from bromo-substituted formylbenzyl sulfone 4 via cyclic dimerization of 4 and Sonogashira coupling of the resulting dibromocyclooctatetraene 3 with terminal acetylenes. The diamino derivative 6b exhibited dual emission with emission maxima at 436 and 547 nm. Furthermore, in the fluorescence of 6b, solvatofluorochromism was observed in response to solvent polarity, whereas in the solid states, mechanofluorochromism was observed.

One-Shot Double Amination of Sondheimer-Wong Diynes: Synthesis of Photoluminescent Dinaphthopentalenes.

Photoluminescent diamino-substituted dinaphthopentalenes were synthesized successfully by the treatment of in situ prepared dinaphthocyclooctadiyne with lithium amide. This reaction involves a series of transformations including the nucleophilic addition of the lithium amide to a triple bond of the cyclooctadiyne moiety, transannulation, protonation of the resulting pentalene anion, and the nucleophilic substitution of the pentalene core with the lithium amide. In this procedure, a novel double amination step plays a key role. When the diamino-substituted dinaphthopentalenes were irradiated with UV light in toluene, fluorescence was observed at around 580 nm (ΦF < 0.03).

Stereogenic motif consisting of rigid ring and intraannular chains: isolation and structures of stereoisomers of 9-alkyl-1,8-anthrylene-butadiynylene cyclic dimers.

Stereoisomers of the title pi-conjugated compounds with intraannular propyl and butyl groups were isolated by chromatography. The high barrier to isomerization is attributed to the steric hindrance between the alkyl chains and the rigid framework.

Synthesis and spectroscopic studies of arylethynylsilanes.

A variety of arylethynylsilanes (Ar-C[triple bond]C-C6H4-C[triple bond]C)(n)SiMe(4-n) were prepared successfully by reaction of the corresponding chlorosilanes Me(4-n)SiCl(n) with Ar-C[triple bond]C-C6H4-C[triple bond]CM (M = Li, MgBr), which was prepared by treatment of ethynyl(diarylethyne)s Ar-C[triple bond]C-C6H4-C[triple bond]CH with BuLi or MeMgBr. The ethynyl(diarylethyne)s were readily prepared in good yields by the double-elimination method: addition of lithium hexamethyldisilazide to a mixture of ArCH2SO2Ph, TMS-C[triple bond]C-C6H4-CHO, and ClP(O)(OEt)2, followed by desilylation. In the tetrakis(arylethynyl)silanes (Ar-C[triple bond]C-C6H4-C[triple bond]C)4Si thus prepared, through-space conjugation of four triple bonds on the silicon atom emerges as a result of participation of the silicon orbitals in the acetylenic pi orbitals. This participation enhances the emissive quantum yields of arylethynylsilanes with an increase in the number of arylethynyl moieties on silicon: quantum yields of emission (phiF) of 0.72 for (MeOC6H4-C[triple bond]C-C6H4-C[triple bond]C)4Si, 0.53 for (MeOC6H4-C[triple bond]C-C6H4-C[triple bond]C)2SiMe2, and 0.47 for MeO-C6H4-C[triple bond]C-C6H4-C[triple bond]CSiMe3 were obtained. Although this enhancement effect was also observed in the phenylethynylarylsilane (MeOC6H4-C[triple bond]C-C6H4)2SiMe2, the bis(arylethynyl)disilane (MeOC6H4-C[triple bond]C-C6H4-C[triple bond]C-SiMe2)2 exhibited non-enhanced emission.

Metal-assisted assembly of pyridine-containing arylene ethynylene strands to enantiopure double helicates.

Pyridine-containing arylene ethynylene strands were connected to the 2- and 2'-positions of (R)- and (S)-1,1'-binaphthyl templates. The arylene ethynylene moieties underwent intramolecular coordination with Ag(I) or Cu(I) ion to afford enantiopure double helicates. The double-helical structure was elucidated on the basis of circular dichroic (CD) spectra. The importance of intramolecular complexation of the double strands for the helicate formation was confirmed by comparison with a ligand bearing a single strand. Connection of the strands through an ether linkage enabled a sorting out of the Cotton effect induced by double-helical arylene ethynylene moieties. The CD exciton chirality method unambiguously proved that the termini of the strands approach each other upon complexation and that the sense of the induced helicity is the same as predicted by molecular modeling.