Polymerization of coordinated monomers. XXII. Ab initio molecular orbital study on the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride as a model of growing radical end

The geometrical and electronic structures of the binary radical complexes of 2-methoxycarbonyl propyl radical with boron trichloride and with boron trifluoride were determined by using an ab initio molecular orbital method. The 2-methoxycarbonyl propyl radical complex was a model of the growing radical end in the copolymerization of methyl methacrylate in the presence of boron halides. The most stable structure of the binary radical complex composed of 2-methoxycarbonyl propyl radical with boron trichloride was a twisted form in which the dihedral angle between the vinyl group and the ester group was 32°, while that of the binary radical complex composed of methyl methacrylate radical with boron trifluoride was a planar form as the free radical. The frontier orbital energy of 2-methoxycarbonyl propyl radical was lowered by 0.06 au by the coordination of boron trichloride, while that was lowered only by 0.02 au by the coordination of boron trifluoride. The polymerization mechanism was elucidated on the basis of these predictions. © 1993 John Wiley & Sons, Inc.     

Disclosing the Structure/Activity Correlation in Trivalent Boron‐Containing Compounds: A Tendency Map

Most trivalent boron reagents are electrophiles owing to the vacancy for two electrons to fill the outer orbital of boron; however, interestingly, trivalent boron compounds can change their electrophilic character to a nucleophilic character by only changing the nature of the substituents on the boron atoms. With the help of computational tools, we have analyzed the structural‐ and electronic properties of boryl fragments that were either bonded to main‐group metals or coordinated to transition‐metals/rare‐earth‐metals and we have designed a map that might help to identify certain trends. This trend map will be useful for selecting an appropriate trivalent boron compound, depending on the sought reactivity.     

Quantum chemical investigation on one- and two-photon absorption properties for a series of donor-π-acceptor-type compounds with trivalent boron as an acceptor

In this paper, one- and two-photon absorption properties as well as the transition nature of a series of donor-π-acceptor-type compounds with trivalent boron as an acceptor have been theoretically studied by using INDO/SDCI method. Our calculations indicate that the four o-methyl moieties on the two mesityl groups play an important part in protecting the trivalent boron from being attacked by oxygen in the air. The trivalent boron containing group can be an all-right electron-acceptor with some bulky groups attached to it. On the basis of geometry optimization and UV–vis spectra, the positions and strengths of two-photon absorption for these molecules were reported.     

Bonding and doping of simple icosahedral-boride semiconductors

A simple model of the bonding and doping of a series of icosahedral-boride insulators is presented. Icosahedral borides contain clusters of boron atoms that occupy the 12 vertices of icosahedra. This particular series of icosahedral borides share both the stoichiometry B₁₂X₂, where X denotes a group V element (P or As), and a common lattice structure. The inter-icosahedral bonding of these icosahedral borides is contrasted with that of B₁₂O₂ and with that of α-rhombohedral boron. Knowledge of the various types of inter-icosahedral bonding is used as a basis to address effects of inter-icosahedral atomic substitutions. The inter-icosahedral bonding is maintained when an atom of a group V element is replaced with an atom of a group IV element, thereby producing a p-type dopant. However, changes of inter-icosahedral bonding occur upon replacing an atom of a group V element with an atom of a group VI element or with a vacancy. As a result, these substitutions do not produce effective n-type dopants. Moreover, partial substitution of boron atoms for atoms of group V elements generally renders these materials p-type semiconductors.     

The Chemistry of Amino Imino Boranes

Dicoordinated boron compounds can no longer be considered as “exotic” species in boron chemistry: this is proved by the extensive chemistry of bis(dialkylamino)boron(1+) ions, alkylidene boranes, and imino boranes. The latter are characterized by high reactivity and chemical versatility. A special position is taken by amino imino boranes, whose chemistry is reported here. The presence of an amino group enhances the reactivity of imino boranes on the one hand and increases their kinetic stability on the other. New types of diamino boranes accessible via amino imino boranes often undergo intramolecular ring closure. These heterocycles are well suited to generate new types of cations of boron. Cycloaddition reactions using amino imino boranes open up a new heterocyclic chemistry of boron. The chemistry of N-functional amino imino boranes, which is still its infancy, shows much promise.     

Hydrogen Peroxide Induced Activation of Gene Expression in Mammalian Cells using Boronate Estrone Derivatives

Keeping the boron out of the ER: A genetic switch was engineered that activates gene expression in the presence of H(2) O(2) . The use of a boronate group on an estrone molecule allows for activation of gene expression through binding of the estrogen receptor only when the boron group is oxidized by H(2) O(2) . This sensor is highly sensitive and specific for H(2) O(2) .     

Borate binding to siderophores: structure and stability

Well-known as specific iron chelating agents produced by bacteria, it is shown that some, but not all, siderophore classes have an unexpected binding affinity for boron. The relevant criterium is the availability of a vicinal dianionic oxygen containing binding group (i.e., citrate or catecholate). The resulting boron complexes have been characterized by ESI-MS, multinuclear NMR, and DFT calculations. Detailed boron binding constants have been measured for vibrioferrin, rhizoferrin, and petrobactin. The observed affinity of certain siderophores for borate, a common chemical species in the marine but not the terrestrial environment, allows for small, but potentially significant, concentrations of B-siderophores to exist at oceanic pH. We hypothesize that these concentrations could be sufficient for them to function as cell signaling molecules or as mediators of biological boron uptake. In addition, binding of the tetrahedral boron to these siderophores results in a conformation that is different from either the free siderophore or its iron complex and would thus allow a distinction to be made between its iron uptake and any putative cell signaling roles.     

Sulfonate pseudohalides of boron subphthalocyanine

The crystal structures of three sulfonate pseudohalide derivatives of boron subphthalocyanine (BsubPc) are described and compared with four structures of three published sulfonate derivatives. Benzenesulfonate boron subphthalocyanine [(benzenesulfonato)(subphthalocyaninato)boron, C₃₀H₁₇BN₆O₃S, (I)] crystallizes in the space group P with Z = 2. The structure contains two centrosymmetric π‐stacking interactions between the concave faces of the isoindoline units in the BsubPc ligands. 3‐Nitrobenzenesulfonate boron subphthalocyanine [(3‐nitrobenzenesulfonato)(subphthalocyaninato)boron, C₃₀H₁₆BN₇O₅S, (II)] crystallizes in the space group P2₁/c with Z = 4. The structure contains an intermolecular S—O...π interaction from the sulfonate group to a five‐membered N‐containing ring of an isoindoline unit on the concave side of a neighbouring BsubPc ligand, at a distance of 3.151 (3) Å. The crystal of methanesulfonate boron subphthalocyanine [(methanesulfonato)(subphthalocyaninato)boron, C₂₅H₁₅BN₆O₃S, (III)] was produced via sublimation and it is not a solvate, in contrast with two previously published structures of the same compound. Compound (III) crystallizes in the space group P2₁/n with Z = 2, and its structure is similar to that of the more common compound Cl‐BsubPc.     

Dative Bonding between Group 13 Elements Using a Boron‐Centered Lewis Base

An electron‐rich monovalent boron compound is used as a Lewis base to prepare adducts with Group 13 Lewis acids using both its boron and nitrogen sites. The hard Lewis acid AlCl₃ binds through a nitrogen atom of the Lewis base, while softer Lewis acids GaX₃ (Cl, Br, I) bind at the boron atom. The latter are the first noncluster Lewis adducts between a boron‐centered Lewis base and a main‐group Lewis acid.     

Synthesis and spectral properties of cholesterol- and FTY720-containing boron dipyrromethene dyes

Two analogues (1, 2) of free cholesterol and one analogue (3) of the immunosuppressive sphingolipid FTY720 containing a boron dipyrromethene chromophore (BODIPY) were synthesized. The synthetic routes involved preparation of boron dipyrromethene moieties (5, 11), bearing a phenylethynyl group at different positions of the chromophore, and lipids (13, 20) bearing an azido group. The dye was tethered to the lipid via a 1,2,3-triazole in the linker by the click reaction. Analogues derived from 11 [in which an (E)-styrylethynyl moiety is bonded to C-5 of BODIPY] exhibited a marked red shift (approximately 70-80 nm) compared with those derived from 5 (in which a phenylethynyl moiety is bonded to C-8 of BODIPY).     

Synthesis of organoboron compounds bearing an azo group and substituent effects on their structures and photoisomerization

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. ¹¹B 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.     

Boron rings enclosing planar hypercoordinate group 14 elements

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six pi and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A-B bonds is compensated by their unusually large number. Although a C7v pyramidal SiB8 structure is more stable than the D8h isomer, Born-Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization.     

Structural Characteristics of Hydrogenated Carbon and Boron Nitride Nanotubes: Impact of H?H Interactions

The structural characteristics of perhydrogenated carbon and boron nitride nanotubes are determined by means of quantum chemical calculations. Two families of nanotubes are systematically studied for both carbon and boron nitride, the nanotubes being derived from the perhydrogenated (110) and (111) sheets of diamond and cubic boron nitride. Single‐walled perhydrogenated carbon nanotubes prefer structures analogous to the (111) sheet. In clear contrast, the single‐walled perhydrogenated boron nitride nanotubes prefer structures analogous to the (110) sheet. The significantly different structural characteristics are due to the polarization of hydrogen atoms in the perhydrogenated boron nitride nanotubes. The presence of attractive electrostatic H? H interactions leads to a strong preference for multilayering of the boron nitride sheets and nanotubes. The results are expected to provide new insights into the structural characteristics of main‐group binary hydrides.     

Boron‐Cluster‐Enhanced Ultralong Organic Phosphorescence

Although carborane‐based luminescent materials have been studied for years, no persistent phosphor has been reported so far. Herein, we describe boron‐cluster‐based persistent phosphors obtained by linking a σ‐aromatic carboranyl cage to the π system of a carbazolyl group. The carboranes were found to promote intersystem crossing from a singlet to a triplet state. The rigid boron cluster was able to stabilize the ultralong triplet excitons through multiple nonclassical hydrogen bonds, such as B?H???π interactions, thus leading to a long lifetime of up to 0.666 s and an absolute phosphorescence quantum yield of 7.1 %, which is outstanding for an organic phosphor without heavy atoms. These phosphors can be excited by visible light and show dynamic emission behavior, including thermochromism and mechanochromism. This study demonstrates that non‐metal/heavy‐atom boron clusters can be used to develop multifunctional high‐performance phosphors for potential applications.     

α‐Amino boronates as cyanoborane complexes: crystal structure and inhibition properties for the serine proteases: α‐chymotrypsin and trypsin

The first examples of α‐amino boronate complexes stabilized by amino cyanoborane complexation were tested as trypsin and chymotrypsin inhibitors, and they showed moderate inhibition. The structure of compound 1 that contains two different boron atoms reveals that the geometry around the boron atom in the cyano group is tetrahedral, whereas a trigonal planar geometry exists around the boron atom attached to two oxygen atoms and a carbon atom. Copyright © 2006 John Wiley & Sons, Ltd.     

Ternary magnesium rhodium boride Mg2Rh1-xB6+2x with a modified Y2ReB6-type crystal structure

The new ternary magnesium rhodium boride Mg2Rh1-xB6+2x has been prepared by the reaction of the mixture of Mg powder, RhB, and crystalline boron in a Ta container sealed under argon. The crystal structure of Mg2Rh0.75(1)B6.50(4) is determined using single-crystal X-ray diffraction, electron diffraction, and high-resolution electron microscopy (space group Pbam, a=8.795(2) A, b=11.060(2) A, c=3.5279(5) A, Z=4, 630 reflections, RF=0.045). It represents a modified Y2ReB6 structure type with an unusual replacement of part of the Rh atoms by boron pairs located in the pentagonal channels parallel to the c axis. The pairs interconnect the neighboring planar boron nets into the 3D framework. The variation of the lattice parameters reveals a homogeneity range Mg2Rh1-xB6+2x. The random distribution of the Rh atoms and boron pairs and the stabilizing effect of the boron pairs on the Y2ReB6 type structure motif are discussed using electronic band structure calculations and chemical bonding analysis with the electron localization function (ELF).     

Mechanism of Ketone Allylation with Allylboronates as Catalyzed by Zinc Compounds: A DFT Study

The mechanism of the allylation reaction between 4‐chloroacetophenone and pinacol allylboronates catalyzed by ZnEt₂ with alcohols was investigated using density functional theory (DFT) at the M05‐2X/6‐311++G(d,p) level. The calculations reveal that the reaction prefers to proceed through a double γ‐addition stepwise reaction mechanism rather than a Lewis acid‐catalyzed concerted one. The intermediate with a four‐coordinated boron center, which is formed through proton transfer from EtOH to the ethyl group of ZnEt₂ mediated by the boron center, is the active species and an entrance for the catalytic cycle. The latter is composed of three elementary steps: 1) boron to zinc transmetalation leading to the formation of allylzincate species, 2) electrophilic addition of ketone to allylzincate species, and 3) generation of the final product with recovery of the catalyst. The boron to zinc transmetalation step has the largest energy barrier of 61.0 kJ mol^?1 and is predicted to be the rate‐determining step. The calculations indicate that the additive EtOH plays important roles both in lowering the activation free energy for the formation of the four‐coordinated boron active intermediate and in transforming the low catalytic activity ZnEt₂ into high activity zinc alkoxide species. The alcohols with a less sterically encumbering R group might be the effective additives. The substituted groups on the allylboronates might primarily affect the boron to zinc transmetalation, and the allylboronates with substituents on the C_γ atom is poor in reactivity. The comparison of the catalytic effect between the zinc compounds investigated suggest that Zn(OEt)₂, Zn(OH)₂, and ZnF₂ exhibit higher catalytic efficiency for the boron to zinc transmetalation due to the activation of the B? C_α bond through orbital interactions between the p orbitals of the EtO, OH, F groups and the empty p orbital of the boron center.     

Alkynyl Moiety for Triggering 1,2‐Metallate Shifts: Enantiospecific sp2–sp3 Coupling of Boronic Esters with p‐Arylacetylenes

The enantiospecific coupling of secondary and tertiary boronic esters to aromatics has been investigated. Using p‐lithiated phenylacetylenes and a range of boronic esters coupling has been achieved by the addition of N‐bromosuccinimide (NBS). The alkyne functionality of the intermediate boronate complex reacts with NBS triggering the 1,2‐migration of the group on boron to carbon giving a dearomatized bromoallene intermediate. At this point elimination and rearomatization occurs with neopentyl boronic esters, giving the coupled products. However, using pinacol boronic esters, the boron moiety migrates to the adjacent carbon resulting in formation of ortho boron‐incorporated coupled products. The synthetic utility of the boron incorporated product has been demonstrated by orthogonal transformation of both the alkyne and boronic ester functionalities.     

Migratory Arylboration of Unactivated Alkenes Enabled by Nickel Catalysis

An unprecedented arylboration of unactivated terminal alkenes, featuring 1,n‐regioselectivity, has been achieved by nickel catalysis. The nitrogen‐based ligand plays an essential role in the success of this three‐component reaction. This transformation displays good regioselectivity and excellent functional‐group tolerance. In addition, the incorporation of a boron group into the products provides substantial opportunities for further transformations. Also demonstrated is that the products can be readily transformed into pharmaceutically relevant molecules. Unexpectedly, preliminary mechanistic studies indicate that although the metal migration favors the α‐position of boron, selective and decisive bond formation is favored at the benzylic position.