Catalyst‐Free Enantiospecific Olefination with In Situ Generated Organocerium Species

Described is the in situ formation of triorganocerium reagents and their application in catalyst‐free Zweifel olefinations. These unique cerium species were generated through novel exchange reactions of organohalides with n‐Bu₃Ce reagents. The adequate electronegativity of cerium allowed for compensating the disadvantages of both usually functional‐group‐sensitive organolithium species and less reactive organomagnesium reagents. Exchange reactions were performed on aryl and alkenyl bromides, enabling enantiospecific transformations of chiral boron pinacol esters. Finally, these new organocerium species were engaged in selective 1,2‐additions onto enolisable and sterically hindered ketones.     

Porphyrinylboranes Synthesized via Porphyrinyllithiums

As the most nucleophilic porphyrins, meso‐ or β‐lithiated porphyrins were generated by iodine–lithium exchange reactions of the corresponding iodoporphyrins with n‐butyllithium at ?98 °C. Porphyrinyllithiums thus prepared were used for synthesis of dimesitylporphyrinylboranes through reactions with fluorodimesitylborane. The boryl groups proved to serve as an electron‐accepting unit to alter the photophysical and electrochemical properties. In addition, 5‐diarylamino‐15‐dimesitylboryl‐substituted donor–accepter porphyrins showed increased intramolecular charge‐transfer character in the S₁ state. Furthermore, the reaction of β‐lithiated porphyrin with dichloromesitylborane provided a boron‐bridged porphyrin dimer, which exhibited a conjugative interaction between two porphyrin units through the vacant p‐orbital on the boron center.     

[4+2] Cycloaddition Reactions Between 1,8‐Disubstituted Cyclooctatetraenes and Diazo Dienophiles: Stereoelectronic Effects,Anticancer Properties and Application to the Synthesis of 7,8‐Substituted Bicyclo[4.2.0]octa‐2,4‐dienes

A detailed examination of [4+2] cycloaddition reactions between 1,8‐disubstituted cyclooctatetraenes and diazo compounds revealed that 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD) reacts to form either 2,3‐ or 3,4‐disubstituted adducts. The product distribution can be controlled by modulating the electron density of the cyclooctatetraene. Unprecedented [4+2] cycloadditions between diisopropyl azodicarboxylate (DIAD) and 1,8‐disubstituted cyclooctatetraenes are also described and further manipulation of a resulting cycloadduct uncovered a new pathway to the synthetically challenging bicyclo[4.2.0]octa‐2,4‐diene family. Variation of the substituents resulted in a range of compounds displaying selective action against different human tumour cell types.     

Silyl‐Phosphino‐Carbene Complexes of Uranium(IV)

Unprecedented silyl‐phosphino‐carbene complexes of uranium(IV) are presented, where before all covalent actinide–carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPM^TMS)(Cl)(μ‐Cl)₂Li(THF)₂] ( 1 , BIPM^TMS=C(PPh₂NSiMe₃)₂) into [U(BIPM^TMS)(Cl){CH(Ph)(SiMe₃)}] ( 2 ), and addition of [Li{CH(SiMe₃)(PPh₂)}(THF)]/Me₂NCH₂CH₂NMe₂ (TMEDA) gave [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)_0.5]₂ ( 3 ) by α‐hydrogen abstraction. Addition of 2,2,2‐cryptand or two equivalents of 4‐N,N‐dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(Cl)][Li(2,2,2‐cryptand)] ( 4 ) or [U{C(SiMe₃)(PPh₂)}(BIPM^TMS)(DMAP)₂] ( 5 ). The characterisation data for 3 – 5 suggest that whilst there is evidence for 3‐centre P?C?U π‐bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments.     

Universal Scaling of Intrinsic Resistivity in Two‐Dimensional Metallic Borophene

Two‐dimensional boron sheets (borophenes) have been successfully synthesized in experiments and are expected to exhibit intriguing transport properties. A comprehensive first‐principles study is reported of the intrinsic electrical resistivity of emerging borophene structures. The resistivity is highly dependent on different polymorphs and electron densities of borophene. Interestingly, a universal behavior of the intrinsic resistivity is well‐described using the Bloch–Grüneisen model. In contrast to graphene and conventional metals, the intrinsic resistivity of borophenes can be easily tuned by adjusting carrier densities, while the Bloch–Grüneisen temperature is nearly fixed at 100 K. This work suggests that monolayer boron can serve as intriguing platform for realizing tunable two‐dimensional electronic devices.     

Homoleptic Zincate‐Promoted Room‐Temperature Halogen–Metal Exchange of Bromopyridines

Homoleptic lithium tri‐ and tetraalkyl zincates were reacted with a set of bromopyridines. Efficient and chemoselective bromine–metal exchanges were realized at room temperature with a substoichiometric amount of nBu₄ZnLi₂?TMEDA reagent (1/3 equiv; TMEDA=N,N,N′,N′‐tetramethylethylenediamine). This reactivity contrasted with that of tBu₄ZnLi₂?TMEDA, which was inefficient below one equivalent. DFT calculations allowed us to rationalize the formation of N???Li stabilized polypyridyl zincates in the reaction. The one‐pot difunctionalization of dibromopyridines was also realized using the reagent stoichiometrically. The direct creation of C? Zn bonds in bromopyridines enabled us to perform efficient Negishi‐type cross‐couplings.     

Luminescent Quadrupolar Borazine Oligomers: Synthesis,Photophysics, and Two‐Photon Absorption Properties

A set of monodisperse bent donor–acceptor–donor‐type conjugated borazine oligomers, BnNn+1 (n=1–4), incorporating electron‐rich triarylamine donor and electron‐deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon–boron and tin–boron exchange reactions. The effect of chain elongation on the electrochemical, one‐ and two‐photon properties and excited‐state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl‐centered acceptor sites results in emissions with high quantum yields (Φ_fl>0.5) in the range of 400–500 nm. Solvatochromic effects lead to solvent shifts as large as ～70 nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two‐photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410 GM (n=4), and broadband excited‐state absorption (ESA) attributed to long‐lived singlet–singlet and radical cation/anion absorption. The excited‐state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6‐31G*) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.     

(Dibenzoylmethanato)boron Difluoride Derivatives Containing Triphenylamine Moieties: A New Type of Electron‐Donor/π‐Acceptor System for Dye‐Sensitized Solar Cells

(Dibenzoylmethanato)boron difluoride derivatives containing triphenylamine moieties were synthesized as a new type of electron‐donor/π‐acceptor system. These new compounds exhibited long‐wavelength absorptions in the UV/Vis spectra, and reversible oxidation and reduction waves in cyclic voltammetry experiments. Their amphoteric redox properties are based on their resonance hybrid forms, in which a positive charge is delocalized on the triphenylamine moieties and a negative charge is localized on the boron atoms. Molecular orbital (MO) calculations indicate that their HOMO and LUMO energies vary with the number of phenylene rings connected to the difluoroboron‐chelating ring. This is useful for optimizing the HOMO and LUMO levels to an iodine redox (I^?/I₃^?) potential and a titanium dioxide conduction band, respectively. Dye‐sensitized solar cells fabricated by using these compounds as dye sensitizers exhibited solar‐to‐electric power conversion efficiencies of 2.7–4.4 % under AM 1.5 solar light.     

Synthesis of B/Ga and B/In heteronuclear bidentate Lewis acids: formation of intramolecular pi-arene-gallium(III) and -indium(III) complexes

Reaction of 1,8-dilithionaphthalene x TMEDA (TMEDA=tetramethylethylenediamine) with dimesitylboron fluoride in diethyl ether followed by treatment with THF affords the Li(THF)(4) salt of dimesityl-1,8-naphthalenediylborate (1). Compound 1-Li(THF)(4) undergoes a ring-opening reaction with trimethyltin chloride to afford high yields of 1-(dimesitylboron)-8-(trimethyltin)naphthalenediyl (2), a compound in which a boryl and a stannyl moiety coexist at the peri positions of a naphthalene core. Compound 2 has been characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray single-crystal analysis which reveal the existence of a sterically crowded structure. Compound 2 is a useful starting material for the synthesis of other heteronuclear naphthalene derivatives. Thus, the transmetalation reaction of 2 with GaCl(3) or InCl(3) in THF results in high yields of 1-(dichlorogallium)-8-(dimesitylboron)naphthalenediyl (3) and 1-(dichloroindium)-8-(dimesitylboron)naphthalenediyl (4), respectively. These unprecedented boron/gallium and boron/indium heteronuclear bidentate Lewis acids have been characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray single-crystal analysis. Owing to the presence of a short contact between the ipso-carbon atom of a mesityl group and the heavy Group 13 element (C(ipso)bondGa 2.279(4) A for 3, C(ipso)bondIn 2.442(6) A for 4), compounds 3 and 4 are best described as intramolecular pi-arene complexes and are the first examples of such complexes. As shown by (1)H and (13)C NMR spectroscopy, this pi interaction subsists in solution.     

Benzodipyrrole‐based Donor–Acceptor‐type Boron Complexes as Tunable Near‐infrared‐Absorbing Materials

Benzodipyrrole‐based donor–acceptor boron complexes were designed and synthesized as near‐infrared‐absorbing materials. The electron‐rich organic framework combined with the Lewis acidic boron co‐ordination enabled us to tune the LUMO energy level and the HOMO–LUMO gap (i.e.,the absorption wavelength) by changing the organic acceptor units, the number of boron atoms, and the substituents on the boron atoms.     

Synthesis of novel boron‐containing epoxy–novolac resins and properties of cured products

Epoxy–novolac resins were synthesized by modifying a commercial novolac resin with epichlorohydrin. These epoxy–novolac resins were characterized and further modified with different contents of bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide or bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxa‐borolanyl)oxide. The boron‐containing epoxy–novolac resins were autocatalytically crosslinked or crosslinked with BF₃MEA and their thermal stability and flame retardancy were determined by thermogravimetric analysis and limiting oxygen index (LOI) values. These LOI values for the bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide derivatives were higher than the boron‐free novolac resins, which shows the benefit of the presence of boron. To test the role of boron in the enhancement of flammability, scanning electronic microscopy and energy‐dispersive X‐ray spectroscopy images were made. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6332–6344, 2006     

Directly Diphenylborane‐Fused Porphyrins

Mono‐ and bis(diphenylborane)‐fused porphyrins were synthesized from the corresponding β‐(2‐trimethylsilylphenyl)‐substituted porphyrins through the sequence of Si–B exchange reaction, intramolecular bora‐Friedel–Crafts reaction, and ring‐closing Si–B exchange reaction. Effective electronic interactions of the empty p‐orbital of the boron atom with the porphyrin π‐circuit lead to red‐shifted absorption spectra and substantially decreased LUMO energy levels. Pyridine adds at the boron center to cause disruption of the electronic interaction of the boron atom with large association constants (1.9–17×10⁴ m ^?1) depending on the central metal at the porphyrin. The Zn^II complex behaved as a hetero‐dinuclear Lewis acid, exhibiting regioselective binding of pyridines at the boron or the zinc center.     

Simple Bipolar Hosts with High Glass Transition Temperatures Based on 1,8‐Disubstituted Carbazole for Efficient Blue and Green Electrophosphorescent Devices with “Ideal” Turn‐on Voltage

Two hybrids based on 1,8‐disubstituted carbazole, 1,8‐OXDCz and 1,8‐mBICz , have been designed and synthesized through a facile process. The incorporation of oxadiazole or N‐phenylbenzimidazole moieties at the 1,8‐positions of carbazole greatly improves its morphological stability, giving glass transition temperatures (T_g) as high as 138 and 154 °C, respectively. Blue phosphorescent organic light‐emitting devices (PhOLEDs) with 1,8‐mBICz exhibit almost the same performance as a similarly structured device based on the mCP host, and green PhOLEDs employing the new host material 1,8‐OXDCz exhibit an ideal turn‐on voltage (2.5 V at 1.58 cd m^?2), a maximum current efficiency (η_c,max) of 73.9 cd A^?1, and a power efficiency (η_p,max) of 89.7 lm W^?1. These results are among the best performances of [Ir(ppy)₃]‐based devices with simple device configurations.     

β‐Iminoenamine‐BF2 Complexes: Aggregation‐Induced Emission and Pronounced Effects of Aliphatic Rings on Radiationless Deactivation

The synthesis, photophysical, and electrochemical attributes of a novel class of boron difluorides containing an aromatic‐fused alicyclic/hetero‐alicyclic ring built on a β‐iminoenamine chromophoric backbone are reported. The compounds displayed large Stokes shifts (86–121 nm), and were emissive in the solid state. The quantum yields obtained in solution at room temperature were unusually lower by an order of magnitude compared to those in the solid state. Some of the tested compounds displayed aggregation‐induced emission (AIE). Single crystal XRD analyses revealed a lack of interplanar π–π interactions, which are presumed to be absent owing to non‐planarity of the alicyclic component in the molecule. For most of the studied compounds, time‐dependent DFT (TD‐DFT) calculations invariably reveal intramolecular charge transfer (π–π*) characteristics with the frontier orbitals concentrated on the boron–nitrogen heterocycle. The participation of boron and fluorine atoms was found to be negligible.     

A novel and facile one‐pot synthesis of pyridylselenium compounds through selective bromine–magnesium exchange with isopropylmagnesium halide

One‐pot synthesis of various unsymmetrical 2‐bromo‐5‐pyridylselenium compounds has been carried out under non‐cryogenic conditions by selective single bromine–magnesium exchange of 2,5‐dibromopyridine using isopropylmagnesium chloride. This exchange gives 2‐bromo‐5‐pyridylmagnesium chloride, which upon the insertion of elemental selenium followed by the treatment with alkyl halide gives the title compounds in good yield. This exchange has also been extended towards bromine–magnesium exchange of 2‐bromopyridine for the improved synthesis of 2‐pyridylselenium compounds. The molecular structure of 2‐bromo‐5‐selenopyridyltribromomethane has been examined by single crystal X‐ray diffraction. This compound crystallizes in the monoclinic space group P2₁/n. From the molecular structure it was found that intermolecular BrBr, NSe and SeBr interactions control its crystal packing. Copyright © 2005 John Wiley & Sons, Ltd.     

Boron(III) Carbazosubphthalocyanines: Core‐Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues

Condensation of 1,8‐diamino‐3,6‐dichlorocarbazole with a series of disubstituted 1,3‐diiminoisoindolines, followed by treatment with BF₃?OEt₂ led to the formation of the corresponding core‐expanded boron(III) subphthalocyanine analogues. These air‐stable π‐conjugated boron(III) carbazosubphthalocyanines possess two boron‐containing seven‐membered‐ring units and a 16 π‐electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single‐crystal X‐ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone‐shaped structure, the π skeleton of this compound is almost planar.     

Amphiphilic Inclusion Spaces for Various Guests and Regulation of Fluorescence Intensity of 1,8‐Bis(4‐aminophenyl)anthracene Crystals

A host framework for inclusion of various guest molecules was investigated by preparation of inclusion crystals of 1,8‐bis(4‐aminophenyl)anthracene (1,8‐BAPA) with organic solvents. X‐ray crystallographic analysis revealed construction of the same inclusion space incorporating 1,8‐BAPA and eight guest molecules including both non‐polar (benzene) and polar guests (N,N‐dimethylformamide, DMF). Fluorescence efficiencies varied depending on guest molecule polarity; DMF inclusion crystals exhibited the highest fluorescence intensity (Φ_F=0.40), four times as high as that of a benzene inclusion crystal (Φ_F=0.10). According to systematic investigations of inclusion phenomena, strong host–guest interactions and filling of the inclusion space led to a high fluorescence intensity. Temperature‐dependent fluorescence spectral measurements revealed these factors effectively immobilised the host framework. Although hydrogen bonding commonly decreases fluorescence intensity, the present study demonstrated that such strong interactions provide excellent conditions for fluorescence enhancement. Thus, this remarkable behaviour has potential application toward sensing of highly polar molecules, such as biogenic compounds.     

Anion‐Exchange Properties of Trifluoroacetate and Triflate Salts of N‐Alkylammonium Resorcinarenes

The synthesis of N‐benzyl‐ and N‐cyclohexylammonium resorcinarene trifluoroacetate (TFA) and triflate (OTf) salt receptors was investigated. Solid‐state analysis by single‐crystal X‐ray diffraction revealed that the N‐alkylammonium resorcinarene salts (NARSs) with different upper substituents had different cavity sizes and different affinities for anions. Anion‐exchange experiments by mixing equimolar amounts of N‐benzylammonium resorcinarene trifluoroacetate and N‐cyclohexylammonium resorcinarene triflate, as well as N‐benzylammonium resorcinarene triflate and N‐cyclohexylammonium resorcinarene trifluoroacetate showed that the NARS with flexible benzyl groups preferred the larger OTf anion, whereas the rigid cyclohexyl groups preferred the smaller TFA anions. The anion‐exchange processes were confirmed in the solid state by single‐crystal and powder X‐ray diffraction experiments and in the gas phase by electrospray ionization mass spectrometry.     

Organic–inorganic hybrid silica as supporting matrices for selective recognition of bovine hemoglobin via covalent immobilization

The synthesis of poly‐aminophenylboronic acid (APBA) imprinted hybrid silica‐based polymers for selective recognition of bovine hemoglobin (BHb) was described, where the mesoporous hybrid silica supporting matrices were prepared by a mild sol–gel process with tetraethoxysilane and 3‐aminopropyltriethoxysilane as two precursors. Covalent immobilization of BHb was adopted in order to create homogeneous recognition sites. After removal of the template, the resulting imprinted polymers showed high binding affinity toward BHb and the imprinting factor (α) reached 2.12. The specificity of the BHb recognition was evaluated with competitive experiments, indicating the imprinted polymers have a higher selectivity for the template BHb. The easy preparation protocol and good protein recognition properties made the approach an attractive solution to depletion of high‐abundance protein from bovine blood.     

Selective Co‐Encapsulation Inside an M6L4 Cage

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy‐transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh‐Cp‐type metal complexes can be encapsulated inside a self‐assembled M₆L₄ metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co‐encapsulation is observed. This principle is demonstrated by co‐encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge‐transfer interaction may also contribute to this effect. Charge‐transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge‐transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge‐transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.