Naokazu Kano

We report conjugated methine compounds generated from N-arylpyridiniums and amines; streptocyanines can be used as a new activation mode for amine catalysis and applied to the conversion of pyridine rings to benzene rings.

Abstract Ligand coupling on sulfur, as an alternative to transition-metal-catalyzed cross-coupling reactions, is a useful method for connecting sp2 carbons. Although pioneered more than 50 years ago, its synthetic utility has been overlooked until recently. This short review summarizes progress in C(sp2)–C(sp2) bond formation using ligand coupling on sulfur and discusses control of selectivity, expansion of the scope, and applications of the reaction. 1 Introduction 2 Cross-Coupling of Heteroaryl Reagents 3 Phenothiazinium-Mediated C(sp2)–C(sp2) Cross-Coupling 4 C(sp2)–C(sp2) Bond Formation Mediated by Sulfuranes Generated from Arynes and Sulfoxides 5 S-Alkenylbenzothiophenium-Mediated Alkenyl-Alkenyl Coupling 6 Conclusion

Trifluoromethylated thermally activated delayed fluorescent molecule 4[Cz(CF₃)₂]IPN has been demonstrated as a versatile photocatalyst that facilitates radical reactions via electron transfer and dearomative cycloaddition via energy transfer.

© 2020 Elsevier Ltd An anionic boron-substituted phosphorane, a phosphoranyl-triarylborate, was synthesised by stabilising the pentacoordinated phosphorus atom with a pair of N,O-bidentate ligands. Its structure was characterised by NMR spectroscopy and X-ray crystallographic analysis. The 11B and 31P NMR spectral data showed large s-character of the phosphorus–boron bond. DFT calculations revealed the charge distributions and molecular orbitals. The first oxidation potential of the phosphoranyl-borate was corresponded to the oxidation of the N,O-bidentate ligands from the distribution of the HOMO of the anion moiety.

Enzymes involved in the biosynthesis of ribosomally synthesized and post-translationally modified peptides (RiPPs) often have relaxed specificity profiles and are able to modify diverse substrates. When several such enzymes act together during precursor peptide maturation, a multitude of products can form, yet usually the biosynthesis converges on a single natural product. For the most part, the mechanisms controlling the integrity of RiPP assembly remain elusive. Here, we investigate the biosynthesis of lactazole A, a model thiopeptide produced by five promiscuous enzymes from a ribosomal precursor peptide. Using our in vitro thiopeptide production (FIT-Laz) system, we determine the order of biosynthetic events at the individual modification level and supplement this study with substrate scope analysis for participating enzymes. Our results reveal an unusual but well-defined assembly process where cyclodehydration, dehydroalanine formation, and azoline dehydrogenation events are intertwined due to minimal substrate recognition requirements characteristic of every lactazole enzyme. Additionally, each enzyme plays a role in directing LazBF-mediated dehydroalanine formation, which emerges as the central theme of the assembly process. Cyclodehydratase LazDE discriminates a single serine residue for azoline formation, leaving the remaining five as potential dehydratase substrates. Pyridine synthase LazC exerts kinetic control over LazBF to prevent the formation of overdehydrated thiopeptides, whereas the coupling of dehydrogenation to dehydroalanine installation impedes generation of underdehydrated products. Altogether, our results indicate that substrate-level cooperation between the biosynthetic enzymes maintains the integrity of lactazole assembly. This work advances our understanding of RiPP biosynthesis processes and facilitates thiopeptide bioengineering.

<p>Aryllithiums were arylated with <italic>S</italic>-arylphenothiazinium ions through selective ligand coupling of intermediary sulfuranes. Various unsymmetrical biaryls were obtained without transition-metal catalysis.</p>

The first trihydroborate bearing a pentacoordinated phosphorus atom was synthesized as a new P-B bonded compound. Hydride abstraction of the trihydroborate gave an intermediary dihydroborane, which showed hydroboration reactivity and was trapped with pyridine whilst maintaining the P-B bond. The dihydroborane underwent a rearrangement, which involved a double ring expansion to compensate for the unbalanced coordination states of the phosphorus and boron atoms, to give a new fused bicyclic phosphine-boronate.

The first dianionic compound bearing a bond between two pentacoordinated tin atoms, a distannate, was synthesized in a stable form by using two sets of an electron-withdrawing C,O-bidentate ligand on each tin atom. The structure of the distannate was determined by NMR spectroscopy and X-ray crystallographic analysis. The Sn-Sn bond of the distannate was shown to be a single bond featuring high s-character. The (1)J(Sn-Sn) coupling constant was larger than that of Sn(sp(3))-Sn(sp(3)) bonds found in most hexaorganodistannanes. This bond feature was also supported by computational studies. The Sn-Sn bond was cleaved by treatment with hydrochloric acid, which shows a different reactivity to the homonuclear bonds of pentacoordinated disilicates and digermanates.

The SiSi bond of a pentacoordinated disilicate was readily cleaved by treatment with 2,3-dichloro-5,6-dicyano-p-benzoquinone in the presence of sodium carbonate under mild conditions. The bond cleavage did not proceed under the same conditions after conversion of the disilicate into the corresponding monoanionic silylsilicate and neutral disilane by protonation. The difference in the charges of the SiSi bond compounds affects the reactivity toward an oxidant, resulting in the SiSi bond cleavage, considering that all of these compounds have a bond between pentacoordinated silicon atoms.

The first dianionic compounds bearing a bond between two pentacoordinated germanium atoms have been synthesized in a stable form as candidates for a core unit to construct unprecedented structures in group 14 element chemistry. X-ray crystallographic analysis and computational study indicated the single-bond character of the GeGe bond. They were reversibly converted to the anionic and neutral GeGe-bonded compounds while maintaining the coordination number of the germanium atoms. The interconversion features changes in structures around the germanium atoms, including changes from a staggered conformation of the GeO bonds to a parallel conformation.

(E)-4,4'-Dimethyl-2,2'-disilylazobenzenes were synthesized. Double intramolecular N center dot center dot center dot Si coordination in the bis(fluorodimethylsilyl) and bis(trifluorosilyl) derivatives was confirmed using X-ray crystallographic analysis and Si-29 NMR spectroscopy. In the absorption bands, due to the pi,pi* transitions, introduction of silyl groups was found to cause a bathochromic shift. In contrast to most azobenzenes, which do not fluoresce at all, the (E)-2,2'-bis(trifluorosilyl)azobenzene derivative with the N center dot center dot center dot Si coordination fluoresced a yellow-green colour at room temperature. Methyl and trifluorosilyl groups lowered the n and p* orbitals, as revealed by DFT calculations. As a result, the lowest singlet excitation energy state is found to be the allowed pi,pi* transition, different from the forbidden n, pi* transition in general azobenzenes, as revealed by TD-DFT calculations. The allowed transition character of the lowest singlet excited state and moderately rigid conformation of the azo moiety, provided by the double N center dot center dot center dot Si coordination, account for the fluorescence emission. Nevertheless, the N center dot center dot center dot Si coordination is weak enough to be cleaved upon photoexcitation, and thus the (E)-2,2'-disilylazobenzenes undergo photoisomerization to the (Z)-isomers. Both the photoisomerization and fluorescence emission properties of the azobenzene moiety have been achieved for the first time. After photoisomerization of the (E)-2,2'-disilylazobenzenes to the corresponding (Z)-isomer, they do not fluoresce. This change in the fluorescence intensity upon photoisomerization is useful for the regulation of fluorescence properties. Therefore, this compound can be recognized as a unique photoisomerizable fluorophore to regulate the fluorescence intensity using a single light source.

The title trans and cis isomers (trans-3 and cis-3) were successfully synthesized as stable compounds by taking advantage of the Martin ligand. The X-ray crystallographic analysis of trans-3 showed that it has a distorted pseudotrigonal bipyramidal structure, and that the two phenyl groups at the 3- and 4-positions are cis to each other. The thermolysis of trans-3 in the presence of excess dimethyl acetylenedicarboxylate (DMAD) gave a mixture of the corresponding aziridines (trans-7 and cis-7) and cycloadducts (11a and 11b) of the azomethine ylides formed by ring opening of the aziridines with DMAD. Heating of cis-7 and excess DMAD under the same conditions gave diastereomeric mixtures of both the aziridines and the cycloadducts, which means that the stereochemistry of the aziridine formation from the 1,2-selenazetidine cannot be determined in this system.

Tetracoordinated and pentacoordinated 1,2-thiazetidines bearing the Martin ligand, trans-5 and trans-6, were synthesized as stable compounds. The X-ray crystallographic analyses showed that trans-5 and trans-6 have distorted pseudotrigonal bipyramidal (pseudo-TBP) and TBP structures, respectively, and that the two phenyl groups at the 3- and 4-positions of both compounds are cis to each other. Thermal isomerization of the aziridine cis-11, which is considered a thermolysis product of trans-6, occurred even at relatively low temperature (100 degrees C) in solution and made it difficult to determine the stereochemistry of the aziridine formation from trans-6 in solution. Flash vacuum pyrolysis, which potentially prevents the isomerization of the product, was applied to trans-6 to give eventually the stereochemistry-retained aziridine cis-11. This result indicates a ligand coupling pathway in the aziridine formation process from trans-6.

A silylsilicate, which is an intermediate in double protonation of a disilicate bearing two sets of a C,O-bidentate ligand at two pentacoordinated silicon atoms to give a disilane, was synthesized and isolated. The X-ray crystallographic analysis of both the silylsilicate and the disilane revealed intramolecular O center dot center dot center dot Si coordination, giving the silicon atoms pentacoordinated states. In the silylsilicate, the Si1-O-2 and Si2-O-3 bonds are almost in parallel. Deprotonation of the silylsilicate, which was also prepared by monodeprotonation of the disilane, gave the disilicate. The oxidation potential and UV/vis absorption maximum are both intermediate between those of the disilicate and the disilane, reflecting the changes in the charges.

Tetracoordinated and pentacoordinated 1,2-thiazetidines bearing the Martin ligand, trans-5 and trans-6, were synthesized as stable compounds. The X-ray crystallographic analyses showed that trans-5 and trans-6 have distorted pseudotrigonal bipyramidal (pseudo-TBP) and TBP structures, respectively, and that the two phenyl groups at the 3- and 4-positions of both compounds are cis to each other. Thermal isomerization of the aziridine cis-11, which is considered a thermolysis product of trans-6, occurred even at relatively low temperature (100°C) in solution and made it difficult to determine the stereochemistry of the aziridine formation from trans-6 in solution. Flash vacuum pyrolysis, which potentially prevents the isomerization of the product, was applied to trans-6 to give eventually the stereochemistry-retained aziridine cis-11. This result indicates a ligand coupling pathway in the aziridine formation process from trans-6.

Azobenzenes are constituents of the commonly and widely used azo dyes. Many dyes, except for the azo dyes, have been utilized for fluorescent materials. However, there are only a few fluorescent azobenzene derivatives and their fluorescence efficiencies are quite low. The current perspective provides an account of the fluorescent azobenzenes and aromatic aldimines featuring an N-B interaction. Incorporation of the intramolecular N-B interaction by using the bis(pentafluorophenyl)boryl group makes the azobenzenes and aromatic aldimines fluorescent with a range of colours. Some of them fluoresce with extraordinarily high fluorescence quantum yields. Their synthesis, structures, fluorescence properties, and applications are discussed. © 2013 The Royal Society of Chemistry.

Organosilanes bearing a phosphine imide moiety were synthesized and crystallographically characterized. Reaction of the pentacoordinated hydrodiphenylsilyl derivative with water gave [2-(diphenylphosphino)phenyl] diphenylsilanol accompanied by both reduction of the phosphine imide moiety and hydrolytic oxidation of the Si-H moiety. © 2013 The Royal Society of Chemistry.

The first stable anionic iron(0) complexes bearing an ate-type pentacoordinated germanium(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge-Fe bonds which can be cleaved by oxidation.

2,2'-Bis[bis(pentafluorophenyl)boryl]azobenzenes were synthesized. X-ray crystallographic analysis exhibits a planar core structure of the azobenzene moiety, double intramolecular NB coordination, and two tetracoordinate boron atoms. The 2,2'-diborylazobenzenes show fluorescence emission with orange and red colors upon irradiation. The double NB coordination causes redshifts in both the absorption and emission maxima and a decrease in the Stokes shifts relative to those of the 2-borylazobenzene derivative. On the basis of the density functional theory calculations of the molecular orbitals of 2,2'-diborylazobenzene, the p* orbital (LUMO) was found at a much lower energy level (4.67 eV) than that in 2-borylazobenzene (3.70 eV). A reversible reduction wave was observed at a low reduction potential in the cyclic voltammograms of the 2,2'-diborylazobenzenes. Single-electron reduction of one of the 2,2'-diborylazobenzenes generates an azobenzene radical anion, which was confirmed by an active ESR signal. The fluorescence was quenched by the reduction and recovered by air oxidation in this azobenzene.

Water-soluble fluorescent azobenzenes have been synthesized by quaternization of 2-[bis(pentafluorophenyl) boryl] azobenzenes bearing dimethylamino groups with iodomethane or propanesultone. The 2-borylazobenzenes both dissolve and fluoresce in water. The double introduction of ionic moieties provides the azobenzenes with higher water solubility. Dynamic light scattering and scanning electron microscopy measurements indicate that the 2-borylazobenzenes aggregate in water. They are expected to be applied in aqueous environments. (C) 2011 Elsevier B.V. All rights reserved.

Phosphine imides with a boryl substituent 35 were synthesized. Their structures were revealed to have a tetracoordinated boron and a phosphorus atom, featuring the NB coordination by X-ray crystallographic analysis and NMR spectroscopy. The phosphine imide moiety was persistent to hydrolysis, methylation, and the aza-Wittig reaction. The NB coordination remained intact upon treating with pyridine or fluoride ion. (C) 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:429434, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.21033

Several 2-phosphinoazobenzenes, which are in equilibrium with inner phosphonium salts, were synthesized. Effects of substituents, solvents, and acidic additives on their equilibria are described. Thermodynamic parameters of the equilibria in various solvents suggest that the acceptor character of the solvents is mainly responsible for the solvent effects. Addition of phenols changed the equilibria depending on their acidity. Substituents at the 4- and 4'-positions of the azobenzene also affected the equilibrium constants, which shifted the equilibrium toward the phosphonium salt in the order of their electron-withdrawing ability. Photoisomerization of the 2-phosphinoazobenzenes bearing electron-donating substituents at the 4- and 4'-positions, which shifted the equilibrium toward the 2-phosphinoazobenzene, proceeded successfully. While the phosphonium salt in equilibrium with the (E)-isomer of the 2-phosphinoazobenzene was protonated by perchlorophenol, the (Z)-isomer did not react with a proton source because it could not take on the form of an inner phosphonium salt. Thus, the properties and reactivity of the inner phosphonium salts in equilibrium with the phosphines can be successfully controlled by photoirradiation.

Control of coordination modes of a ligand in metal complexes is significant because the coordination modes influence catalytic properties of transition metal catalysts. Reactions of 2-diphenyl-phosphinoazobenzenes, which are in equilibrium with the inner phosphonium salts, with ZnCl2, W(CO)(5)(THF), and PtCl2(cod) gave three different coordination types of metal complexes with distinctive UV-vis absorptions. All the complexes were characterized by X-ray crystallographic analyses. In the zinc and tungsten complexes, the source molecule functions as an amide ligand and a phosphine ligand, respectively. In the platinum complex, the phosphorus molecule works as a tridentate ligand with formation of a carbon-platinum bond.

Two of five conceivable geometrical isomers of a hexacoordinated dihydrophosphate bearing two sets of a bidentate ligand were investigated. X-ray crystallographic analysis of both of isomers, 1a-TPP and 1b-TEA, revealed their octahedral geometries of C(2) and C(1) symmetry, respectively, which were consistent with the NMR spectra. The isomer 1b-TEA underwent both hydride reduction of an aldehyde and proton exchange with water at room temperature in DMSO without any additive. A one-pot reaction of both of the reactions of 1b-TEA with D(2)O and an aldehyde or a ketone under the above conditions proceeded successfully to give the deuterated alcohol. Thus, umpolung of a hydrogen atom of water with 1b-TEA was achieved under much milder conditions than those used in the reaction with another isomer, la-TEA. Quantitative isomerization of 1b-TEA to la-TEA occurred in methanol at room temperature. Calculations on the five conceivable geometrical isomers of the anionic part of the dihydrophosphate revealed their relative stability, which reasonably explained the isomerization, and the larger negative charge at the atoms located at the trans positions of the oxygen atoms. The smaller coupling constants of the P H and P C bonds located at the rear of an oxygen atom in the NMR spectra resulted in the smaller s character of these bonds. The differences in both hydride-donation and proton-exchange reactivities between la-TEA and 1b-TEA could be explained by the differences in the atomic charge of the hydrogen atom and the stability difference of the initially formed phosphorane intermediates, respectively.

An eight-member cyclic his (iminophosphorane) was synthesized by treatment of 2,2'-bis(phosphino)azobenzene with pentachlorophenol via N=N bond cleavage of the azobenzene. The crystal structure of the cyclic his (iminophosphorane) was determined by the X-ray ciystallographic analysis. A study on the reaction mechanism showed that both protonation of the azo moiety and intramolecular nucleophilic attacks of the phosphino groups played a key role in the formation of the bis(iminophosphorane) from the azo compound bearing two phosphino groups. (C) 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:301-306, 2011; View this article online at wileyonlinelibrary.com. DOT 10.1002/hc.20680

N-(2-Anthry1)-2-[bis(pentafluorophenyl)boryl]benzyl-ideneamine changed its fluorescence color from yellow to green upon addition of an equimolar amount of cyanide ion, in contrast to the N-phenyl derivatives, which showed quenching of the emission. Theoretical calculations suggested that switching of the imine-based emission to anthryl-based emission by adduct formation provides the N-anthrylimine derivative with the fluorescence color change.

Silicon can form bonds to other tetracoordinated silicon atoms and these bonds form the framework of many organosilicon compounds and crystalline silicon. Silicon can also form a pentacoordinated anionic structure-a so-called &apos;silicate&apos;. No compounds containing a direct bond between two silicate moieties-&apos;disilicates&apos; where two silicate structures are combined in one species-have been reported because of the electronic repulsion between the anionic halves and difficulty preventing the release of anions. Here we report the synthesis of thermally stable and isolable disilicates by the reductive coupling reaction of a silane bearing two electron-withdrawing bidentate ligands. Two pentacoordinated silicons, positively charged despite the formal negative charge, constitute a single sigma-bond and bind eight negatively charged atoms. They can be reversibly protonated, cleaving two Si-O bonds, to afford a tetracoordinated disilane. Their unique electronic properties could be promising for the construction of functional materials with silicon wire made up of silicate chains.

Transition metal complexes are known to react with disulfides and insert into the disulfide bond, but there has been no report on similar reactivity towards the hypervalent sulfur(IV)-sulfur(II) bond compound, a sulfur-substituted sulfurane, because of its instability. In this paper, we report the reactions of the sulfur-substituted sulfurane 1 with platinum and palladium complexes, in which the corresponding complexes 2 and 3 with an O,S,S&apos;-ligand were obtained by bond cleavage of both the sulfur(IV)-sulfur(II) bond and the sulfur(IV)-oxygen bond. The structures of the complexes were characterized by NMR spectroscopic studies, elemental analysis, and X-ray crystallographic analysis. The palladium complex 3 underwent elimination of triphenylethylene episulfide when treated with dppe to give the dppe palladium complex 4 with an O,S-ligand, which was also obtained by the reaction of Pd(dba)(2) and dppe with the sulfurane 1. X-ray crystallographic analysis of complexes 2 and 4 revealed a square-planar structure around the platinum or palladium. The formation mechanisms of these complexes 2-4 are discussed.

2-[Bis(pentafluorophenyl)-boryl]azobenzenes bearing hydrogen, methoxy, dimethylamino, trifluoromethyl, fluoro, n-butyl, and tert-butyldimethylsiloxy groups at the 4'-position or methoxy and bromo groups at the 4-position have been synthesized. The 4-bromo group of the 2-bory1-4-bromoazobenzene derivative was converted to phenyl and diphenylamino groups by palladium-catalyzed reactions. The absorption and fluorescence properties have been investigated using UV/Vis and fluorescence spectroscopy. The 2- borylazobenzenes emitted an intense green, yellow, and orange fluorescence, in marked contrast to the usual azobenzene fluorescence. The 4'-siloxy derivative showed the highest fluorescence quantum yield (0.90) among those reported for azobenzenes to date. The correlation between the substituent and the fluorescence properties was elucidated by studying the effect of the substituent on the relaxation process and from DFT and TD-DFT calculations. An electron-donating group at the 4'-position was found to be important for an intense emission. Application of fluorescent azobenzenes as a fluorescent vital stain for the visualization of living tissues was also investigated by microinjection into Xenopus embryos, suggesting these compounds are nontoxic towards embryos.

The polarity of a chemical bond is an important factor that determines the reactivity because a reaction of a positively charged atom with a negatively charged atom easily occurs. We report here a method for reversing the polarity of a hydrogen atom of water, which meant conversion of a protic hydrogen (H(delta+)) to a hydridic hydrogen (H(delta-)), by using a hexacoordinated phosphorus species. The method enables a reductive deuteration of carbonyl compounds utilizing inexpensive and easily usable D(2)O under mild conditions. The umpolung of a hydrogen atom was achieved by utilizing tautomerization of phosphorus compounds. Classical metal hydrides never have this function, and these results demonstrated the potential utility of nonmetallic compounds for umpolung of a hydrogen atom.

N-Aryl, A-alkyl, N-alkoxy, and N-amino derivatives of 2-[bis(pentafluorophenyl)boryl]benzylideneamine were synthesized by the condensation reactions of 2-[bis(pentanuorophenyl)boryl]benzaldehyde with the corresponding amines. Their structures were investigated by NMR and Xray crystallographic analysis. Their properties were investigated by UV-vis and fluorescence spectroscopy. The boryl-substituted N-arylimines show blue or green fluorescence in hexane at room temperature, and their fluorescence efficiency is Much higher than that of N-benzylideneaniline. In particular, the boryl-substituted N-(4-dimethylaiminophenyl)imine showed strong green emissions with at least 7000 times higher fluorescence quantum yield (0.73) compared with that of N-benzylideneaniline. The boryl-substituted N-(1-indolyl)- and N-(9-carbazolyl)imines showed dual emissions, one of which was assignable as arising from the lowest singlet excited state and the other from the local excited state of the substituent on the imine nitrogen. The fluorescent properties of the boryl-substituted N-butyl- and N-methoxyimines were also investigated. Reactions of the N-arylimine derivatives with cyanide ion gave the corresponding cyanide adducts and quenched the fluorescence, indicating that these 2-[bis(pentafluorophenyl)boryl]benzylideneamine derivatives have a potential as a cyanide ion sensor.

A tetracoordinated 1,2-thiazetidine gave a 2-amino-4H-3,1-benzoxathiin under thermal conditions. The product was characterized by X-ray crystallographic analysis. The heterocyclic product was considered to be formed via the aza-Pummerer type rearrangement.

Coordination states of trihydrosilane (E)-1, chlorodihydrosilane (E)-2, and tetrahydrodisiloxane (E,E)-4 bearing 2(phenylazo)phenyl groups were revealed by X-ray crystallographic analysis and NMR spectroscopy. Trihydrosilane (E)-1 with a tetracoordinate silicon showed higher reactivity with CuCl(2) than trihydrophenylsilane. Photoirradiation of (E)-2 and (E,E)-4, which have pentacoordinate silicon atom(s), successfully gave the corresponding (Z)-isomers. Each reaction of trihydrosilane (E)-1 and tetrahydrodisiloxane (E,E)-4 with fluoride ion gave 1,2-diphenylhydrazine through an intramolecular hydrosilylation.

2.2&apos;-Bis(fluorodiphensylsilyl)azobenzenes bearing two methyl and butyl groups at the 4- and 4&apos;-positions were synthesized, and their X-ray crystallographic analyses revealed that they have tetracoordinated and pentacoordinated silicon atoms, respectively. Although the two compounds showed almost the same spectra in solution, the reflectance spectra of the solid revealed that the pentacoordinated state had a weaker absorbance in the long-wavelength region compared with that of the tetracoordinated state, which is ascribed to the shift of the n-pi* transition. This difference in the absorptions due to the n-pi* transition drastically affects the color of the azobenzenes: the color of the pentacoordinated state becomes apparently paler than that of the tetracoordinated state.

2-[Bis(pentafluorophenyl)boryl]-4'-(dimethylamino)azobenzene was synthesized as a hybrid of the intensely fluorescent 2-borylazobenzene and the pH-responsive 4-(dimethylamino)-azobenzene. Quenching and recovery of fluorescence of this azobenzene were reversibly performed by addition of trifluoroacetic acid and triethylamine, respectively. The reason of fluorescence quenching was investigated by theoretical calculations.

2-(Phenylazo)phenylboranes bearing several substituents were synthesized and substituent effects on their structures and photoisomerization behaviors were investigated to reveal the scope of the photoswitching of the coordination number of the boron by using an azobenzene-based photoresponsive ligand, 2-(phenylazo)phenyl group. (11)B NMR, X-ray crystallographic analysis, and UV-vis spectra revealed that electron-donating ability of the substituents at both the boron atom and the azobenzene moiety determined the strength of the interaction between the boron and the nitrogen of the azo group. Photoisomerization behaviors of 2-(phenylazo)phenylboranes are largely affected by the B-N interaction. (C) 2008 Elsevier Ltd. All rights reserved.