Synthesis of a 1-boratabenzene-(2,3,4,5-tetramethylphosphole) : towards a planar monophosphole

Novel boratabenzene–phosphole complexes have been prepared and structurally characterized. The electronic communication between the two heterocyclic rings linked by a P–B bond and the aromaticity of these systems were probed using crystallographic and density functional studies

Dehydrohalogenation of halobenzenes and C(sp3)-X (X = F, OPh) bond activation by a molecular calcium hydride

International audienceThe molecular calcium hydride, [(BDI)CaF](2) (BDI = HC{(Me)CN-2,6-i-Pr2C6H3}(2)), effects the slow hydrodehalogenation of C6H5X (X = I, Br) to provide benzene and the respective dimeric calcium halides, [(BDI)CaX](2). Although a similar reaction with fluorobenzene was non-discriminating, the analogous hydrogenation of chlorobenzene was observed, albeit this process yields the calcium hydride-chloride as the alkaline earth-containing product. Assessment of the bromide- and chloride-based processes by density functional theory (DFT) calculations, imply that the reactions take place with the retention of the dimeric calcium structures throughout. Both systems invoke an S N Ar-type displacement of the halide, via barriers (in the range 32-34 kcal mol(-1) for C6H5Br and 31.1-32.9 kcal mol(-1) for C6H5Cl), which vary only marginally during the transformation of the initial hydride and halide-hydride intermediates. The isolation of the calcium hydride-chloride is ascribed, therefore, to its more rapid crystallisation and depletion from solution. Also reported is the reactivity of [(BDI)CaH](2) with alpha,alpha,alpha-trifluorotoluene and anisole, which yield the corresponding dicalcium hydride-fluoride and phenoxide derivatives, respectively, rather than the products of directed ortho metalation. (C) 2021 Elsevier Ltd. All rights reserved

Organocalcium-mediated nucleophilic alkylation of benzene

The electrophilic aromatic substitution of a C–H bond of benzene is one of the archetypal transformations of organic chemistry. In contrast, the electron-rich p-system of benzene is highly resistant to reactions with electron-rich and negatively charged organic nucleophiles. Here, we report that this previously insurmountable electronic repulsion may be overcome through the use of sufficiently potent organocalcium nucleophiles. Calcium n-alkyl derivatives—synthesized by reaction of ethene, but-1-ene, and hex-1-ene with a dimeric calcium hydride— react with protio and deutero benzene at 60°C through nucleophilic substitution of an aromatic C–D/H bond. These reactions produce the n-alkyl benzenes with regeneration of the calcium hydride. Density functional theory calculations implicate an unstabilized Meisenheimer complex in the C–H activation transition state

Reducing CO2 to methanol using frustrated Lewis pairs : on the mechanism of phosphine-borane mediated hydroboration of CO2

The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2-C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step

Reactivity of a Cl-boratabenzene Pt(II) complex with Lewis bases : generation of the kinetically favoured Cl-boratabenzene anion

Complex [(IMes)2Pt(H)(ClBC5H4SiMe3)] (IMes = 1,3-di(2,4,6-trimethylphenyl)imidazolin-2-ylidene) reacts with Lewis bases (L = pyridine, trimethylphosphine, acetonitrile, tert-butylisocyanide) to generate the kinetically favoured ion pairs [(IMes)2Pt(H)(L)][ClBC5H4SiMe3]. Over time, the formation of the thermodynamically favoured borabenzene-L adducts is observed with L = pyridine and trimethylphosphine

A highly active phosphine-borane organocatalyst for the reduction of CO2 to methanol using hydroboranes

In this work, we report that organocatalyst 1-Bcat-2-PPh2-C6H4 ((1); cat = catechol) acts as an ambiphilic metal-free system for the reduction of carbon dioxide in presence of hydroboranes (HBR2 = HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo[3.3.1]nonane), BH3·SMe2 and BH3·THF) to generate CH3OBR2 or (CH3OBO)3, products that can be readily hydrolyzed to methanol. The yields can be as high as 99% with exclusive formation of CH3OBR2 or (CH3OBO)3 with TON (turnover numbers) and TOF (turnover frequencies) reaching >2950 and 853 h(-1), respectively. Furthermore, the catalyst exhibits "living" behavior: once the first loading is consumed, it resumes its activity on adding another loading of reagents

Coordination to a di-tert-butylphosphidoboratabenzene ligand with electronically unsaturated group 10 transition metals

A new boratabenzene-phosphine ligand, di-tert-butylphosphidoboratabenzene, [DTBB]−, has successfully been synthesized by reduction of the corresponding di-tert-butylchlorophosphidoborabenzene compound (2). The species was structurally characterized with both K+ (3) and 18-crown-6·K+ (4) as counterions. Reactions of two equivalents of di-tert-butylphosphidoboratabenzene with NiBr2(PPh3)2, PtCl2, and PtCl2(COD) were undertaken and were successful in yielding three new organometallic boratabenzene species, (μ-κ-η6-C5H5BP(tBu)2)2Ni2 (5), (η3-(C,B,P)-C5H5BP(tBu)2)2Pt (6), and (η3-(C,B,P)-C5H5BP(tBu)2)(κ-C8H12(P(tBu)2BC5H5)Pt (7), respectively. The di-tert-butylphosphidoboratabenzene species displays a remarkable tendency to coordinate to transition metal species in two distinct modes closely associated with other reported boratabenzene and allyl-like interactions. Also of interest is the ability for di-tert-butylphosphidoboratabenzene to be able to coordinate within monomeric as well as dimeric transition metal compounds. The synthesis and characterization will be discussed in detail along with DFT calculations in order to validate these research findings

On the interaction of phosphines with high surface area mesoporous silica

To increase the efficiency and selectivity of homogeneous catalysts, particularly useful in the synthesis of fine chemicals and drugs, fine tuning of the steric and electronic properties of the complexes can be achieved by modification of the ligands in the coordination sphere of the metal center. Considerable efforts have been devoted in order to immobilize such well-defined catalysts on solid substrates, e.g., silica, to facilitate catalysts’ recovery and to reduce contamination of desired products by metallic impurities. However, the presence of the silica surface can play a very important role in tuning the electronic properties of the metal, its steric environment, or in participating in the reactivity of the complex. In this context, several moieties have been used to anchor metallic catalysts on surfaces, but one of the most interesting is phosphine. Herein, we report on the addition of PPh2Cl which leads to the grafting and the oxidation of the phosphine species, even in absence of oxygen, and that the nature of the surface plays an important role in secondary interactions, e.g., hydrogen bonding, and modifies the spectroscopic properties of the functional groups on the surface. In particular, the chemical shift of the phosphorous resonance in the P NMR spectra is altered by hydrogen bonding between available silanol or water molecules present on the silica surface and the phosphorous oxide. The DFT models developed for this process are in direct accordance with the experimental results and demonstrate firmly that the oxidation of the phosphine after grafting of ClPR2 is highly favored thermodynamically and occurs with the formation of Si-Cl bonds on the surface. Passivation of the surface with hexamethyldisilazane limits the extent of the H-bonding between the surface and the oxide, but also leads to some substitution reaction between bound phosphorous species and the trimethylsilyl (TMS) moieties. These findings offer new knowledge critical to fully ascertain the environment and the stability of immobilized phosphine-containing catalytic systems and, thus, further broaden the range of their reactivity

Role of the Meso Substituent in Defining the Reduction of Uranyl Dipyrrin Complexes

The uranyl complex UVIO2Cl(LMes) of the redox-active, acyclic dipyrrin–diimine anion LMes– [HLMes = 1,9-di-tert-butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported UVIO2Cl(LF) [HLF = 1,9-di-tert-butyl-imine-5-(pentafluorophenyl)dipyrrin] to understand the influence of the meso substituent. Cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory studies show that the alteration from an electron-withdrawing meso substituent to an electron-donating meso substituent on the dipyrrin ligand significantly modifies the stability of the products formed after reduction. For UVIO2Cl(LMes), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [UVO2(LMes)]2 is seen, whereas in contrast, for UVIO2Cl(LF), only ligand reduction occurs. Computational modeling of these reactions shows that while ligand reduction followed by chloride dissociation occurs in both cases, ligand-to-metal electron transfer is favorable for UVIO2Cl(LMes) only, which subsequently facilitates uranyl(V) dimerization