Dimesitylborane monomer-dimer equilibrium in solution, and the solid-state structure of the dimer by single crystal neutron and X-ray diffraction

Bis(1-Methyl-ortho-Carboranyl)Borane

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B-H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.

Enantioselective Reductive Oligomerization of Carbon Dioxide into l-Erythrulose via a Chemoenzymatic Catalysis

A cell-free enantioselective transformation of the carbon atom of CO₂ has never been reported. In the urgent context of transforming CO₂ into products of high value, the enantiocontrolled synthesis of chiral compounds from CO₂ would be highly desirable. Using an original hybrid chemoenzymatic catalytic process, we report herein the reductive oligomerization of CO₂ into C₃ (dihydroxyacetone, DHA) and C₄ (l-erythrulose) carbohydrates, with perfect enantioselectivity of the latter chiral product. This was achieved with the key intermediacy of formaldehyde. CO₂ is first reduced selectively by 4e^- by an iron-catalyzed hydroboration reaction, leading to the isolation and complete characterization of a new bis(boryl)acetal compound derived from dimesitylborane. In an aqueous buffer solution at 30 °C, this compound readily releases formaldehyde, which is then involved in selective enzymatic transformations, giving rise either (i) to DHA using a formolase (FLS) catalysis or (ii) to l-erythrulose with a cascade reaction combining FLS and d-fructose-6-phosphate aldolase (FSA) A129S variant. Finally, the nature of the synthesized products is noteworthy, since carbohydrates are of high interest for the chemical and pharmaceutical industries. The present results prove that the cell-free de novo synthesis of carbohydrates from CO₂ as a sustainable carbon source is a possible alternative pathway in addition to the intensely studied biomass extraction and de novo syntheses from fossil resources.

Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene-A New Class of Lewis Superacids

The three-dimensional inorganic analogues of 9,10-diboraanthracene, B₂X₂(C₂B₁₀H₁₀)₂ (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li₂C₂B₁₀H₁₀ and trihaloboranes. The methyl- and phenyl-substituted compounds B₂Me₂(C₂B₁₀H₁₀)₂ (3) and B₂Ph₂(C₂B₁₀H₁₀)₂ (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compounds were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal X-ray diffraction analyses. Experimental (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF₅. Treatment of 1 or 2 with HSiEt₃ gave a rare neutral borane-silane adduct, (Et₃SiH)₂B₂H₂(C₂B₁₀H₁₀)₂ (5). The equilibrium of 5 in solution was thoroughly investigated by spectroscopy and quantum calculations.

Reactions of B2 (o-tolyl)4 with Boranes: Assembly of the Pentaborane(9), HB[B(o-tolyl)(μ-H)]4

Reactions of the diborane(4) B2 (o-tolyl)4 and monohydridoboranes are shown to give B(o-tolyl)3 and (o-tolyl)BR2 (R2 =(C8 H14 ) 3, cat 4, pin 5, (C6 F5 )2 6) as the major products. The corresponding reaction with BH3 -sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(μ-H)]4 8. The reaction pathways are probed experimentally and by computations.

An Organic Borate Salt with Superior p-Doping Capability for Organic Semiconductors

Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto-) electronics. Beyond single-component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes2B+; Mes: mesitylene) and the tetrakis(penta-fluorophenyl)borate anion [B(C6F5)4]- is demonstrated as p-type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes2B+, and the positive charge on the polymer is stabilized by [B(C6F5)4]-. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C6F5)4]- even enables the stabilization of polarons and bipolarons in poly(3-hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes2B+[B(C6F5)4]- is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.

Reactions of [(dmpe)2MnH(C2H4)]: synthesis and characterization of manganese(i) borohydride and hydride complexes

Reactions of trans-[(dmpe)₂MnH(C₂H₄)] (1) with BH₃(NMe₃), 9-BBN, and HBMes₂ yielded the manganese(i) borohydride complexes [(dmpe)₂Mn(μ-H)₂BR₂] (3: R = H, 4: R₂ = C₈H₁₄, 5: R = Mes). The reaction of 1 with BH₃(NMe₃) proceeds via ethylene substitution. By contrast, a detuerium labelling study indicates that the reaction of 1 with HBMes₂ involves initial isomerization of 1 to an unobserved 5-coordinate ethyl intermediate, [(dmpe)₂MnEt], which reacts with the hydroborane to afford EtBR₂ and [(dmpe)₂MnH], followed by reaction with a second equivalent of hydroborane to generate 5 (an analogous pathway is likely followed for other base-free hydroboranes such as 9-BBN). Identification of 3-5 as κ²-borohydride complexes, as opposed to boryl dihydride or hydroborane hydride isomers, is supported by ¹¹B NMR spectroscopy, X-ray diffraction, and Atoms in Molecules calculations. Two byproducts were observed in the syntheses of 3-5: [{(dmpe)₂MnH}₂(μ-dmpe)] (6) and [(dmpe)₂MnH(κ¹-dmpe)] (7). These complexes were independently prepared by exposure of 1 to free dmpe under an atmosphere of Ar or H₂, and the generality of this synthetic route was demonstrated by the reaction of 1 with PMe₃ (under H₂) to form [(dmpe)₂MnH(PMe₃)] (8). Complexes 6-8 can exist as isomers with either a trans or a cis relationship between the hydride and κ¹-coordinated phosphine ligands on manganese. trans to cis isomerization of 6-8 is photochemically induced, whereas the reverse reaction occurs under thermal conditions. X-ray crystal structures were obtained for 3-5, trans,trans-6, cis,cis-6, trans-7, and trans-8.

The B(OH)-NH Analog Is a Surrogate for the Amide Bond (CO-NH) in Peptides: An ab Initio Study

The conformational preferences of N-methyl-methylboronamide (NMB), a B(OH)-NH analog of the amide CO-NH in natural peptides, have been investigated at the Hartree-Fock; Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Parr; and second-order Møller-Plesset levels of theory with the 6-31+G* basis set. The minima, saddle points, and rotation barriers on the potential energy surface of NMB have been located and the energy barriers estimated. Besides the global minimum, there are three local minima within 2.0 kcal mol(-)(1) of the global minimum characterized by specific ω and τ torsion values. The energy barriers for rotation about the "ω angle" are 16.4-18.8 kcal mol(-)(1) and are a consequence of the double-bond character of the B-N bond as revealed by natural bond orbitals calculations. The "ω angle" and the ω rotation barrier are nearly the same as those seen in natural peptides. The τ rotation barriers (B-O bond) are relatively low because of the single-bond character of the B-O bond. Ala-BON, the Ala-dipeptide derived from NMB, has been constructed as a model peptide to study the conformational preferences about the φ and ψ torsion angles. The study reveals a strong preference for α-helix, type-II β-turn, 2.27 ribbon, and antiparallel β-sheet conformations, and mirror images of both type-II β-turn and 2.27 ribbon motifs whose φ and ψ values fall in the "disfavored regions" of the Ramachandran map. Thus, the replacement of the carbonyl group by B-OH retains the geometry and barrier around the "ω angle" and induces a strong preference for regular secondary structure motifs and also structures with positive φ values. This makes the B(OH)-NH analog an important surrogate for the peptide bond, with the additional advantage of stability to proteolytic enzymes.

The ω, φ, and ψ Space of N-Hydroxy-N-methylacetamide and N-Acetyl-N '-hydroxy-N '-methylamide of Alanine and Their Boron Isosteres

The conformational space of N-hydroxy-N-methylacetamide [CH3-CO-N(OH)CH3, NMAOH] and its boron isostere [CH3-CO-B(OH)CH3, BMAOH] has been studied by quantum chemical methods. The potential energy surface of NMAOH and BMAOH has been built at the HF, B3LYP, and MP2 levels of theory with the 6-31+G* basis set. The minima and transition states for rotations about various torsional angles have been located, and the energy barriers have been estimated. The global minimum energy structure of both peptides exhibits an intramolecular hydrogen bond between the carbonyl oxygen and the hydroxyl group, imparting a conformational rigidity to the peptides. The omega rotation barrier is lower in the boron isostere than in NMAOH. The difference in the rotation barrier has been attributed to second-order orbital interactions, like negative hyperconjugation, as revealed by NBO calculations. In contrast, the rotation barrier around the torsion angle tau (torsion governing rotation about the N-O and B-O bonds) is relatively higher in the boron analogue. This difference is due to the double bond character in the B-O bond as opposed to the N-O bond which has the character of a single bond. As an extension, N-acetyl-N'-hydroxy-N'-methylamide of alanine (Ala-NOH) and its boron isostere (Ala-BOH) have been adopted as model peptides to study the conformational preferences about the φ and ψ torsion angles. The study reveals a strong preference for a Type I beta turn as well as inclinations for a left-handed alpha helix, for positive phi torsions, and for extended psi conformations for Ala-NOH; Ala-BOH, on the other hand, shows a leaning toward positive phi and extended psi, with no preference for any regular secondary structure motifs. The replacement of nitrogen by boron changes the electronic and conformational properties of the peptide, extending greater flexibility around the omega angle, a strong preference for positive phi values, and a shift in the site of nucleophilic attack from the carbonyl group to boron.

CRYSTALSenhancements: dealing with hydrogen atoms in refinement

Because they scatter X-rays weakly, H atoms are often abused or neglected during structure refinement. The reasons why the H atoms should be included in the refinement and some of the consequences of mistreatment are discussed along with selected real examples demonstrating some of the features for hydrogen treatment that can be found in the software suiteCRYSTALS.

Kinetics and mechanism of hydroboration of oct-1-and-4-ene by dimeric dialkylboranes

The kinetics and mechanism of hydroboration of oct-1-and-4-ene with a series of dimeric dialkylboranes was investigated. The kinetic results showed that the hydroboration of terminal olefins proceeds via a three-halves-order mechanism, first-order with respect to the olefin and one-half-order with respect to the dimer. Using dicyclohexylborane, diisopinocamphenylborane, and 3,6-dimethylborepane the observed rate constants for the hydroboration of oct-4-ene were approximately 6 times smaller than those for oct-1-ene. Supporting computations showed that both steric and electronic effects influence the rate of hydroboration of both internal and terminal olefins. A model computational study of the isomerization of oct-4-ene with di(prop-2-yl)borane showed that formation of the terminal hydroborated complex is thermodynamically favored over the internal complex.

Fluorescent Ethenyl- and Ethynyl-dimesitylboranes Derived from 5-(Dimethylamino)-N-(prop-2-ynyl)naphthalene-1-sulfonamide

Two new fluorescent ethenyl- and ethynyl-dimesitylboranes functionalized with a dansyl group 1 and 2 have been synthesized in good yields. Compound 1 was prepared by the hydroboration of 5-(dimethylamino)-N-(prop-2-ynyl)naphthalene-1-sulfonamide I with dimesitylborane (HB(Mes)2) in dry tetrahydrofuran at room temperature, and compound 2 was synthesized by Pd-catalyzed cross-coupling of I with 4-I-C6H4B(Mes)2. Both organoborane compounds 1 and 2 have been characterized by infrared spectroscopy, NMR spectroscopy, and mass spectrometry. The molecular structures of I and 1 were confirmed by X-ray crystallography. The electronic absorption and redox properties of 1 and 2 were investigated. They both exhibit large positive solvatochromism and their emission spectra have been recorded in a range of organic solvents with the fluorescence maxima exhibiting large bathochromic shifts in highly polar solvents, indicative of charge transfer which leads to large dipole moments in the excited state. The application of 1 as a blue fluorescent dopant in doped guest–host organic light-emitting diodes is also described.

Synthesis, Structures, and Luminescent Properties of Phenol−Pyridyl Boron Complexes

Syntheses of the four mixed phenol-pyridine derivatives 1,6-bis(2-hydroxyphenyl)pyridyl boron naphthalene (1), 1,6-bis(2-hydroxy-5-methylphenyl)pyridyl boron naphthalene (2), 1,6-bis(2-hydroxyphenyl)pyridyl boron 2-methoxylbenzene (3), and 1,6-bis(2-hydroxy-5-methylphenyl)pyridyl boron 2-methoxylbenzene (4) are reported. The structures of the boron compounds 1, 3, and 4 were determined by single-crystal X-ray diffraction. The molecular packing is characterized by intermolecular pi...pi and hydrogen-bonding interactions. DSC analysis demonstrates that 1 and 2 have good thermal stability with higher glass transition temperatures (Tg) and melting points (Tm) than 3 and 4. Boron complexes 1-4 display bright blue luminescence in solution and the solid state. White and blue electroluminescent (EL) devices were fabricated successfully using these boron compounds.

Gas-phase electron diffraction studies of the icosahedral carbaboranes, ortho-, meta- and para-C2B10H12

The molecular structures of the three closo-carbaboranes, ortho-, meta- and para-C2B10H12, were experimentally determined using gas-phase electron diffraction (GED). All unique bond distances for ortho and meta carbaboranes were determined experimentally for the first time. For ortho-carbaborane (RG= 0.046), a model with C2v symmetry refined to give bond distances of 1.624(8) A for C-C, 1.093(8)A for C-H and 1.192(3)-1.196(3) A for B-H. For meta-carbaborane (RG= 0.040) a model with C2v symmetry refined to give a CC distance of 2.575(9) A. For para-carbaborane (RG= 0.062) a model with D5d symmetry refined to give a C-B bond distance of 1.698(3) A, B2-B3 of 1.785(1), B2-B7 of 1.774(4) and CC of 3.029(5)A. These GED structures are compared with geometries from other experimental diffraction methods (neutron, X-ray) and ab initio calculations.