Transition Metal-Free Aromatic C−H,C−N,C−S and C−O Borylation

Aromatic organoboron compounds are highly valuable building blocks in organic chemistry. They were mainly synthesized through aromatic C−H and C−Het borylation, in which transition metal-catalysis dominate. In the past decade, with increasing attention to sustainable chemistry, numerous transition metal-free C−H and C−Het borylation transformations have been developed and emerged as efficient methods towards the synthesis of aromatic organoboron compounds. This account mainly focuses on recent advances in transition metal-free aromatic C−H, C−N, C−S, and C−O borylation transformations and provides insights to where further developments are required.     

Not Carbon s–p Hybridization,but Coordination Number Determines C−H and C−C Bond Length

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp³ to sp² to sp.     

Spatial structure and electronic spectrum of TiSi n − clusters (n = 6–18)

Results from optimizing the spatial structure and calculated electronic spectra of anion clusters TiSi _n ^? (n = 6–18) are presented. Calculations are performed within the density functional theory. Spatial structures of clusters detected experimentally are established by comparing the calculated and experimental data. It is shown that prismatic and fullerene-like structures are the ones most energetically favorable for clusters TiSi _n ^? . It is concluded that these structures are basic when building clusters with close numbers of silicon atoms.     

Electron detachment and fragmentation in collisions between 50 keV C n − (1 ≤ n ≤ 86) clusters and H2

The destruction cross section for 50 keV negative carbon clusters C _n ^? (1 ≤ n ≤ 88) in collisions with n ₂ is reported. The dominant destruction channel is believed to be electron detachment. The measured cross section values are compared with theoretical values based on a simple geometrical model of the carbon cluster, and structural information is obtained. Fragment spectra of both positive and negative clusters are also recorded and fragmentation patterns are discussed in relation to fragmentation energies and ionization potentials.     

An ab Initio Study on the Chemical Bond and Reactivity of Molybdenum–Sulfur Clusters with a Mo2O n S4−n (n=1–3) Core

Using the ab initio method and natural bond analysis, the reactivity and chemical bond of dinuclear molybdenum sulfur clusters with Mo₂O _n S_4–n (n=1–3) core were studied. The results show that the Mo--Mo bonds in these clusters are significantly affected by the delocalization effects of the multicenter d--p and Mo--X ₁ bonds, besides the direct interactions between Mo atoms. The substitution reactions of the cluster core are also discussed, and it is indicated that sulfur atoms have a preference for the bridging sites and the oxygen atoms tend to attach to the terminal sites.     

Magnetische Kernresonanzuntersuchungen an Ti−C−H-Legierungen

Ohne ZusammenfassungMit 4 Abbildungen     

New methods of preparation of hydroxy-closo-decaborates [B10H10 − n (OH) n ]2− (n = 1, 2)

New methods of preparation of hydroxy-closo-decaborates [B₁₀H_{10 ? n} (OH) _n ]^2? (n = 1, 2) that are based on the reaction of anions [B₁₀H_{10 ? n} (OAc) _n ]^2? and alkoxyethylidenoxonio-closo-decaborates [2-B₁₀H₉OC(OR)CH₃]^? with aqueous solution of hydrazine are proposed. The obtained compounds were characterized by IR, ESI/MS, and NMR (¹H, ¹¹B, ¹³C) spectroscopy.     

Magneli phase Ti n O2n − 1 as corrosion-resistant PEM fuel cell catalyst support

Magneli phase titanium suboxide, Ti _n O_{2n ? 1}, with Brunauer–Emmett–Teller surface area up to 25 m² g^?1 was prepared using the heat treatment of titanium oxide (rutile) mixed with polyvinyl alcohol in ratios from 1:3 to 3:1. XRD patterns showed Ti₄O₇ as the major phase formed during the heat treatment process. The Ti _n O_{2n ? 1} showed excellent electrochemical stability in the potential range of ?0.25 to 2.75 V vs. standard hydrogen electrode. The Ti _n O_{2n ? 1} was employed as a polymer electrolyte membrane fuel cell catalyst support to prepare 20 wt% platinum (Pt)/Ti _n O_{2n ? 1} catalyst. A fuel cell membrane electrode assembly was fabricated using the 20 wt% Pt/Ti _n O_{2n ? 1} catalyst, and its performance was evaluated using H₂/O₂ at 80 °C. A current density of 0.125 A?cm^?2 at 0.6 V was obtained at 80 °C.     

A Computational NICS and 13C NMR Characterization of C60−n Si n Heterofullerenes (n = 1, 2, 6, 12, 20, 24, 30)

Density functional theory (DFT) calculations are performed for a representative set of low-energy structures of C_60-n Si _n heterofullerenes (n = 1, 2, 6, 12, 20, 24, 30) to investigate the effect of silicon doping on the electron structure of fullerene. The results show that chemical shielding (CS) parameters are so sensitive to the structural distortion made by outwardly relaxing silicon doped atoms from the fullerene surface which results in puckered Si-doped rings. As a result, the chemical shifts of the nearest carbon sites of silicon atoms considerably shift to downfield. Our survey shows that those first neighbors of silicon atoms which have minor ¹³C chemical shift belong to normal (un-puckered) rings. Meanwhile, the chemical shielding anisotropy (Δσ) parameter detects the effects of dopant so that Δσ values of the carbon atoms which are contributed to the Si–C bond are mainly larger than the others. Compensation between diatropic and paratropic ring currents lead to less negative NICS values at cage centers of Si-doped fullerenes than that of C₆₀ except C₅₈Si₂-b and C₅₄Si₆-b in which more negative NICS values may be attributed to more spherical geometries of their carbon cages.     

Theoretical Study of Electronic Structure and Stability of Mixed AlmBn−mH n 2− and CmBn−mH n 2−m (n = 6, 10, 12 and m = 1, 2) Clusters

Density functional theory (DFT) method with B3LYP functional and 6-311++G(d,p) basis set has been used to predict the geometries, relative stabilities, electronic structures and bonding analysis of Mixed Al_mB_n?mH _n ^2? and C_mB_n?mH _n ^2?m (n = 6, 10, 12 and m = 1, 2) clusters; being compared to the B_nH _n ^2? ones. Therefore, the DFT results suggest that the replacing of boron by aluminium or carbon is governed by Natural net charges following Gimar’s and Williams’s rules. The Al_mB_n?mH _n ^2? structures are relatively distorted compared to those of B_nH _n ^2? and C_mB_n?mH _n ^2?m . In Al_mB_n?mH _n ^2? structures Al atoms prefer the adjacent sites, however for the C₂B_n?2H_n cluster cages, the carbon atoms are positioned at diametrically opposed sites. The large HOMO–LUMO gaps show that the predicted clusters have chemical stabilities, principally, those of Al_mB_n?mH _n ^2? ones, which are not experimentally isolated. The optimized geometries obtained through boron substitution by Al and C lead to compactness and to contracted structures, respectively, where B–B bonds are the shortest in mono- and di-carbaboranes.     

Divergent Coupling of Benzocyclobutenones with Indoles via C−H and C−C Activations

Highly selective divergent coupling reactions of benzocyclobutenones and indoles, in which the chemoselectivity is controlled by catalysts, are reported herein. The substrates undergo C2(indole)–C8(benzocyclobutenone) coupling to produce benzylated indoles and benzo[b]carbazoles in the Ni- and Ru-catalyzed reactions. A completely different selectivity pattern C2(indole)–C2(benzocyclobutenone) coupling to form arylated indoles is observed in the Rh-catalyzed reaction. Preliminary mechanistic studies suggest C−H and C−C activations in the reaction pathway. Synthetic utility of this protocol is demonstrated by the selective synthesis of three different types of carbazoles from the representative products.     

Electronic structures of C36 fulleride anions: C36− and C362−

Electronic structures for mono- and dianionic species of two promising C₃₆ fullerene isomers, 14 and 15, are investigated by means of the hybrid Hartree–Fock (HF)/density functional (DF) method. Structural deformations, charge distributions, and spin densities upon one- or two-electron reduction are explained in light of the lowest unoccupied molecular orbitals (LUMOs) of each neutral isomer. First electron affinities for the neutral isomers 14 and 15 are predicted to be 2.3 and 2.5 eV, respectively, facilitating n-type doping for C₃₆ solids. The degrees of local aromaticity of the isomers 14 and 15 tend to decrease with reduction in contrast with C₆₀.     

Vibrational spectra and electronic structure of 1-germatranol, 1,1-quasi-germatrandiole,and 1,1,1-hypogermatrantriole (HO)4−n Ge(OCH2CH2) n NR3−n (R = H,Me; n = 1–3)

IR spectra of 1-germatranol, 1,1-quasi-germatrandiole, 1,1,1-hypogermatrantriole with a general formula (HO)_4?n Ge(OCH₂CH₂) _n NR_3?n (n = 1–3) are obtained. At the B3LYP/aug-cc-pVDZ density functional level the equilibrium structures and vibrational spectra of these compounds along with their hydrogen-bonded dimers are calculated. Based on the calculations the band assignment is performed in the IR spectra of 1-germatranol, 1,1-quasi-germatrandiole, and 1,1,1-hypogermatrantriole. The existence of dimers is manifested in the IR spectra as the absence of bands in the frequency ranges characteristic of the bending vibrations of Ge-OH groups and the presence of bands in the vibrational range of hydrogen-bonded germatranyl groups.     

C(spn)−X (n=1–3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C(spⁿ)−X bonds (n = 1–3 and X = H, CH₃, Cl) in archetypal model substrates H₃C−CH₂−X, H₂C=CH−X and HC≡C−X by catalysts PdL_n with L_n = no ligand, Cl⁻, and (PH₃)₂, using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp³)−X, to C(sp²)−X, to C(sp)−X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp³, to sp², to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spⁿ)−X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst–substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp³)−X to C(sp)−X.     

Ca4Fe3−xMnxO8−δCl2: A new n=3 Ruddlesden–Popper oxychloride

Solid state solutions of Ca₄Fe_3?xMn_xO_8?δCl₂ (0.92≤x≤1.79 (δ～0.1) single crystals were synthesized in CaCl₂-flux in air. The structure, determined by single-crystal X-ray diffraction, is related to the n=3 Ruddlesden–Popper phase in space group I4/mmm with strong deviations from the ideal structure. Mn and Fe are disordered over two transition metal sites. Due to the positional disordering of the equatorial oxygen atoms in the MO₆ octahedra in Ca₄Fe_3?xMn_xO_8?δCl₂ both tilting (～9°) along the c-axis and rotation (～10.5°) within the ab-plane are observed. All the Fe ions are trivalent, as confirmed by ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption near edge spectroscopy (XAS), while the formal valence state of Mn varies from very close to 4+ in the x=0.92 to mix-valent 3+/4+ in the x=1.79 member, as indicated by XAS. Magnetic investigations evidence short-range antiferromagnetic ordering already at room temperature and spin-glass behavior at low temperature due to the structural disordering of Mn/Fe.     

Reversible Boron-Insertion into Aromatic C−C Bonds

Formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron-insertion into aromatic C−C bonds in the photo-promoted skeletal rearrangement reaction of triarylboranes bearing an ortho-phosphino substituent (ambiphilic phosphine-boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X-ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron-doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z-isomerization of an alkene moiety proceed via a highly strained trans-borepin intermediate.     

Why edge inversion? Theoretical characterization of the bonding in the transition states for inversion in F n NH(3−n) and FnPH(3−n) (n = 0–3)

As first noted by Dixon et al. (J Am Chem Soc 108:2461–2462, 1986), heavily fluorinated pyramidal phosphorus compounds, e.g., F _n PH_(3?n) with n > 1, invert through a T-shaped transition state (edge inversion) rather than the D_3h-like transition states (vertex inversion) found in the corresponding nitrogen compounds and less fluorinated phosphorus compounds. Subsequent studies by Dixon and coworkers established that this is a general phenomenon and has important chemical consequences. But what is the reason for the change in the structure of the transition state? Recent theoretical investigations have resulted in the discovery of a new type of chemical bond, the recoupled pair bond. In particular, it was found that recoupled pair bond dyads account for the hypervalency of the elements beyond the first row. In this paper, we show that recoupled pair bond dyads also account for the existence of the edge inversion pathway in heavily fluorinated phosphorus compounds and likely account for the presence of the lower energy inversion pathways in pyramidal compounds of other elements beyond the first row.     

Rhoda-Electrocatalyzed C−H Methylation and Paired Electrocatalyzed C−H Ethylation and Propylation

The use of electricity over traditional stoichiometric oxidants is a promising strategy for sustainable molecular assembly. Herein, we describe the rhoda-electrocatalyzed C−H activation/alkylation of several N-heteroarenes. This catalytic approach has been successfully applied to several arenes, including biologically relevant purines, diazepam, and amino acids. The versatile C−H alkylation featured water as a co-solvent and user-friendly trifluoroborates as alkylating agents. Finally, the rhoda-electrocatalysis with unsaturated organotrifluoroborates proceeded by paired electrolysis.