An Efficient and Convenient Synthesis of Ethyl 1‐(4‐Methoxyphenyl)‐5‐phenyl‐1H‐1,2,3‐triazole‐4‐carboxylate

The “click chemistry” of using organic azides and terminal alkynes is arguably the most efficient and straightforward route to the synthesis of 1,2,3‐triazoles. In this paper, an alternative and direct access to ethyl 1‐(4‐methoxyphenyl)‐5‐phenyl‐1H‐1,2,3‐triazole‐4‐carboxylate is described. Treatment of ethyl diazoacetate with 4‐methoxyaniline derived aryl imines in the presence of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene provided fully substituted 1,2,3‐triazoles in good to high chemical yields. The base‐mediated reaction tolerates various substituted phenyl imines as well as ethyl diazoacetate or the more bulky diazoacetamide. A reasonable mechanism is proposed that involves the addition of an imine nitrogen atom to the terminal nitrogen atom of the diazo compound, followed by aromatization to give the 1,2,3‐triazole. The presence of the 4‐carboxy group is advantageous as it can be easily transformed into other functional groups.     

Efficient Light‐Induced pKa Modulation Coupled to Base‐Catalyzed Photochromism

Photoswitchable acid–base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light‐induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light‐induced pK_a modulation is based on incorporating a 3‐H‐thiazol‐2‐one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground‐ and excited‐state potential energy surfaces, as indicated by quantum‐chemical calculations.     

N‐Heteroacenes

The synthesis and the property evaluation of several large N‐heteroacenes are discussed. Issues of stability and aromaticity are compared and investigated and a historical perspective of the field is given. Some of the larger heterocyclic materials that are evaluated in this concept article have been around for more than one hundred years, yet their chemistry and properties are not well known/understood.     

Redox‐Switchable 20π‐, 19π‐, and 18π‐Electron 5,10,15,20‐Tetraaryl‐5,15‐diazaporphyrinoid Nickel(II) Complexes

The first examples of air‐stable 20π‐electron 5,10,15,20‐tetraaryl‐5,15‐diaza‐5,15‐dihydroporphyrins, their 18π‐electron dications, and the 19π‐electron radical cation were prepared through metal‐templated annulation of nickel(II) bis(5‐arylamino‐3‐chloro‐8‐mesityldipyrrin) complexes followed by oxidation. The neutral 20π‐electron derivatives are antiaromatic and the cationic 18π‐electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20‐tetraarylporphyrins.     

[32]π Fused Core‐Modified Heptaphyrin with Möbius Aromaticity

A new fused core‐modified 32π heptaphyrin with Möbius aromatic character is reported. The ¹H NMR data indicated a weak Möbius aromaticity at 298 K; however, at 213–183 K, the molecule predominates [4n]π Möbius conformation with strong diatropic ring current, which was further confirmed by X‐ray analysis. The protonation experiment led to preservation of the Möbius aromaticity at 298 K. Nevertheless, the experimental results were further supported by theoretical studies. Overall, this study represents the first example of Möbius aromatic fused core‐modified expanded porphyrin.     

Ruthenocene‐Type Complexes of N‐Fused Porphyrins

Ruthenocene‐type hybrid complexes with N‐fused porphyrinato ligands, [Ru(NFp)Cp] (NFp=N‐fused porphyrin, Cp=cyclopentadienyl), have been prepared and characterized by NMR and UV/Vis/NIR spectroscopy, cyclovoltammetry, and X‐ray crystallography. [Ru(NFp)Cp] is a common low‐spin ruthenium(II) complex and shows strong aromaticity. The Ru–Cp distance (1.833 Å) in [Ru(NFp)Cp] is comparable to that in [RuCp₂] (1.840 Å). DFT calculations on [Ru(NFp)Cp] showed the unequivocal contribution of the RuCp moiety as well as the NFp moiety to both the HOMO and LUMO, constructing a three‐dimensional d–π conjugated system. The HOMO–LUMO gaps of [Ru(NFp)Cp] are insensitive to the substituents on the NFp ligand, which is illustrated spectroscopically as well as theoretically. This is in sharp contrast to the ligand precursor, the N‐fused porphyrin, in which the HOMO–LUMO gap is affected by substituents in a similar manner to standard porphyrins and related macrocycles.     

Toward product control in ring‐opening oligomerization of 9H‐9‐borafluorenes

To get deep insights into the structure–reactivity relationship for ring‐opening oligomerization reactions toward targeted design of novel main‐chain boron‐containing materials, detailed DFT B97D/TZVP calculations are carried out to compare the ring‐opening oligomerization of both unsubstituted and tert‐butyl (tBu)‐substituted 9H?9‐borafluorenes. In contrast to substituent exchange between normal boranes, such reactions are initiated by substituent exchanges involving double B? C? B bridged intermediates. On tBu‐substitution, the B? C? B, and B? H? B bridged dimer intermediate is stabilized mainly due to enhanced barrier of 18.1 kcal/mol toward further trimerization channel and higher isomerization barrier of 22.5 kcal/mol toward the double B? H? B bridged dimer. In good agreement with available experiments, it is clearly shown that various product channels can be efficiently controlled by bulky substitution and by reaction temperatures, pointing out the way toward desired higher oligomers with improved thermal stability. © 2014 Wiley Periodicals, Inc.     

2,3,17,18‐Tetrahalohexaphyrins and the First Phlorin‐type Hexaphyrins

1‐(Triisopropylsilyl)‐3,4‐dichloropyrrole and 1‐(triisopropylsilyl)‐3,4‐difluoropyrrole were conveniently prepared from the corresponding 3,4‐dibromopyrrole by lithiation followed by halogenation. 2,3,17,18‐Tetrahalogeno [26]‐ and [28]hexaphyrins have been prepared by condensation of 3,4‐dihalopyrroles and a dipyrromethane‐dicarbinol. 2,3,17,18‐Tetrahalogenated hexaphyrins display variable structural and electronic properties depending upon the halogen atom and the number of π‐electrons. Tetrabromo[28]hexaphyrin and tetrachloro[28]hexaphyrin were further reduced with excess NaBH₄ to furnish meso‐reduced hexaphyrins as the first example of phlorin‐type meso‐aryl‐substituted hexaphyrins.     

Cross‐Conjugated Hexaphyrins and Their Bis‐Rhodium Complexes

A cross‐conjugated hexaphyrin that carries two meso‐oxacyclohexadienylidenyl (OCH) groups 9 was synthesized from the condensation of 5,10‐bis(pentafluorophenyl)tripyrrane with 3,5‐di‐tert‐butyl‐4‐hydroxybenzaldehyde. The reduction of 9 with NaBH₄ afforded the Möbius aromatic [28]hexaphyrin 10 . Bis‐rhodium complex 11 , prepared from the reaction of 10 with [{RhCl(CO)₂}₂], displays strong Hückel antiaromatic character because of the 28 π electrons that occupy the conjugated circuit on the enforced planar structure. The oxidation of 11 with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) yielded complexes 12 and 13 depending upon the reaction conditions. Both 12 and 13 are planar owing to bis‐rhodium metalation. Although complex 12 bears two meso‐OCH groups at the long sides and is quinonoidal and nonaromatic in nature, complex 13 bears 3,5‐di‐tert‐butyl‐4‐hydroxyphenyl and OCH groups and exhibits a moderate diatropic ring current despite its cross‐conjugated electronic circuit. The diatropic ring current increases upon increasing the solvent polarity, most likely due to an increased contribution of an aromatic zwitterionic resonance hybrid.     

Synthesis,Coordination Behavior,and Reduction Chemistry of Cymantrenyl‐1,3‐bis(2,3,4,5‐tetraphenyl)borole

We describe the synthesis of base‐free bisborole [Cym^?(BC₄Ph₄)₂]—Cym^?=(OC)₃Mn(η⁵‐C₅H₃)—and its transformation into two fully characterized Lewis acid–base adducts with pyridine bases of the type 4‐R? NC₅H₄ (R=tBu, NMe₂). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym^?(BC₄Ph₄)₂] and the related monoborole derivative [Cym(BC₄Ph₄)]—Cym=(OC)₃Mn(η⁵‐C₅H₄)—provided conclusive evidence for 1) the enhanced Lewis acidity of the two boron centers that result from conjugation of two borole fragments, and 2) the fact that Mn? B bonding interactions between the Lewis acidic borole moieties and the Mn center are considerably less pronounced for bisborole [Cym^?(BC₄Ph₄)₂]. In addition, the reduction chemistry of [Cym^?(BC₄Ph₄)₂] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym^?(BC₄Ph₄)₂] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg? OC bonding mode in the solid state.     

Porphyrin/Quinoidal‐Bithiophene‐Based Macrocycles and Their Dications: Template‐Free Synthesis and Global Aromaticity

We report the template‐free synthesis and characterization of a new type of porphyrin/quinoidal‐bithiophene‐based conjugated macrocycle. X‐ray crystallographic analysis of the dimer ( 2MC ) revealed a cyclophane‐like geometry with large dihedral angles between the porphyrin and the neighboring thiophene rings, and NMR measurements and theoretical calculations confirmed a localized aromatic character of the porphyrin/thiophene rings and quinoidal character of the bithiophene linkers. Restricted rotation of the thiophene rings linked to the porphyrin unit was observed by variable‐temperature NMR measurements. The dication ( 2MC²⁺ ) adopts a chair‐shaped conformation to facilitate π‐electron delocalization around the whole macrocycle. As a result, the molecule is globally aromatic, with a dominant 54 π conjugation pathway. The trimer ( 3MC ) also shows localized aromatic character of porphyrin rings and conformational flexibility, but its dication ( 3MC²⁺ ) is rigid and globally aromatic with a dominant 82 π conjugation pathway.     

Crystallographic study of self‐organization in the solid state including quasi‐aromatic pseudo‐ring stacking interactions in 1‐benzoyl‐3‐(3,4‐dimethoxyphenyl)thiourea and 1‐benzoyl‐3‐(2‐hydroxypropyl)thiourea

1‐Benzoylthioureas contain both carbonyl and thiocarbonyl functional groups and are of interest for their biological activity, metal coordination ability and involvement in hydrogen‐bond formation. Two novel 1‐benzoylthiourea derivatives, namely 1‐benzoyl‐3‐(3,4‐dimethoxyphenyl)thiourea, C₁₆H₁₆N₂O₃S, (I), and 1‐benzoyl‐3‐(2‐hydroxypropyl)thiourea, C₁₁H₁₄N₂O₂S, (II), have been synthesized and characterized. Compound (I) crystallizes in the space group P , while (II) crystallizes in the space group P 2₁/c . In both structures, intramolecular N—H…O hydrogen bonding is present. The resulting six‐membered pseudo‐rings are quasi‐aromatic and, in each case, interact with phenyl rings via stacking‐type interactions. C—H…O, C—H…S and C—H…π interactions are also present. In (I), there is one molecule in the asymmetric unit. Pairs of molecules are connected via two intermolecular N—H…S hydrogen bonds, forming centrosymmetric dimers. In (II), there are two symmetry‐independent molecules that differ mainly in the relative orientations of the phenyl rings with respect to the thiourea cores. Additional strong hydrogen‐bond donor and acceptor –OH groups participate in the formation of intermolecular N—H…O and O—H…S hydrogen bonds that join molecules into chains extending in the [001] direction.     

Carbo‐Quinoids: Stability and Reversible Redox‐Proaromatic Character towards Carbo‐Benzenes

The carbo‐mer of the para‐quinodimethane core is stable within in a bis(9‐fluorenylidene) derivative. Oxidation of this carbo‐quinoid with MnO₂ in the presence of SnCl₂ and ethanol affords the corresponding p‐bis(9‐ethoxy‐fluoren‐9‐yl)‐carbo‐benzene. The latter can be in turn converted back into the carbo‐quinoid by reduction with SnCl₂, thus evidencing a chemical reversibility of the interconversion between a pro‐aromatic carbo‐quinoid and an aromatic carbo‐benzene, and is reminiscent of the behavior of the benzoquinone/hydroquinone redox couple (in the red–ox opposite sense).