Alessandro Bismuto

A novel and general method for the hydroboration of alkenes, including aryl alkenes, 1,1-disubstituted alkenes, aliphatic alkenes, and bio-derived alkenes, using BX₃ as the boration reagent in the presence of ⁱ Pr₂NEt is reported. The reaction was also applied to the synthesis of linear and branched 1,n-diborylalkanes. The reaction is thought to proceed by a frustrated Lewis pair type single-electron-transfer mechanism.

Abstract

An unprecedented and general hydroboration of alkenes with BX₃ (X=Br, Cl) as the boration reagent in the presence of ⁱ Pr₂NEt is reported. The addition of ⁱ Pr₂NEt not only suppresses alkene polymerization and haloboration side reactions but also provides an “H” source for hydroboration. More importantly, the site-fixed installation of a boryl group at the original position of the internal double bond is readily achieved in contrast to conventional transition-metal-catalyzed hydroboration processes. Further application to the synthesis of 1,n-diborylalkanes (n=3–10) is also demonstrated. Preliminary mechanistic studies reveal a major reaction pathway that involves radical species and operates through a frustrated Lewis pair type single-electron-transfer mechanism.

Bismuth complexes of acetate benzoazacrowns demonstrate various stability in model solutions and in biological media. The smaller macrocycle has to bend out to coordinate cation while the larger encapsulates cation inside the cavity and due to four pendant arms enables formation of highly stable chelate.

Abstract

The complexation properties towards Bi³⁺ of benzoazacrown ligands with five (H₃BA3A) and six (H₄BATA) heteroatoms and corresponding number of acetic arms have been investigated. Complexation constants were determined via potentiometric titration for BA3ABi complex (logβ=26.2) and refined for BATABi complex (logβ=31.7). According to NMR, X-ray and EXAFS results, chelation of cation by H₃BA3A appears to be outside the macrocyclic cavity both in the solid state and in the aqueous solution. This out-cage location despite the high stability constant causes serum and in vivo instability of complex of smaller azacrown H₃BA3A in contrast to H₄BATA. Whereas the larger macrocycle and four pendant arms of H₄BATA provide long-range coordination effectively shielding the cation thereby ensuring high inertness in vitro, low accumulation of the radionuclide in the organs, and fast clearance.

Abstract

A pincer ligand composed of a pyridine with ortho positions substituted by a bulky phosphine arm and a pyrazole arm (PNNH) is installed on nickel(II) to yield the diamagnetic planar complex [(PNNH)NiCl]Cl. The chloride anion can be replaced by BPh₄ ⁻ by a metathesis. The acidic pyrazole proton can be removed with LiN(SiMe₃)₂ to yield the square planar neutral molecule (PNN⁻)NiCl. The coordinated chloride can be metathetically replaced by azide to yield diamagnetic (PNN⁻)Ni(N₃). To evaluate changing the phosphine donor for a phosphine sulfide, the corresponding pincer ligand SPNNH was synthesized and installed on NiBr₂. The reduced steric bulk from the more distant phosphorous keeps both halides coordinated in the paramagnetic molecular species (SPNNH)NiBr₂. Several attempts to dehydrobrominate this species result in synthesis and characterization of two unexpected products. One effort revealed that the electrophilic character of P(V) leaves the phosphorus atom in (SPNNH)NiBr₂ vulnerable to nucleophilic attack, resulting in a P/C cleavage product which was characterized.

Al3+ sensor: A 3D porous anionic MOF displays superior turn-on sensing for Al(III) ions with ultralow sensitivity resulted from stronger interactions with free amines by employing strategic design principle. The detection approach is simple, fast, portable and economic as demonstrated by MOF filter paper tests for naked eye observation.

Abstract

Accumulation of high concentrations of Al(III) in body has a direct impact on health and therefore, the trace detection of Al(III) has been a matter for substantial concern. An anionic metal organic framework ({[Me₂NH₂]_0.5[Co(DATRz)_0.5(NH₂BDC)] ⋅ xG} _n ; 1; HDATRz=3,5-diamino-1,2,4-triazole, H₂NH₂-BDC=2-amino-1,4-benzenedicarboxylic acid, G=guest molecule) composed of two types of secondary building units (SBU) and channels of varying sizes was synthesized by employing a rational design mixed ligand synthesis approach. Free −NH₂ groups on both the ligands are immobilized onto the pore surface of the MOF which acts as a superior luminescent sensor for turn-on Al(III) detection. Furthermore, the large channels could allow the counter-ions to pass through and get exchanged to selectively detect Al(III) in presence of other seventeen metal ions with magnificent luminescence enhancement. The observed limit of detection is as low as 17.5 ppb, which is the lowest among the MOF-based sensors achieved so far. To make this detection approach simple, portable and economic, we demonstrate MOF filter paper test for real time naked eye observation.

Copper catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands exhibit remarkably high Markovnikov selectivity in the protoboration and -silylation of terminal alkynes. This work demonstrates the ability of this strongly σ-donating family of ligands to perturb regioselectivity in the addition of copper−boryl species to C−C π-bonds.

Abstract

Regioselective hydrofunctionalization of alkynes represents a straightforward route to access alkenyl boronate and silane building blocks. In previously reported catalytic systems, high selectivity is achieved with a limited scope of substrates and/or reagents, with general solutions lacking. Herein, we describe a selective copper-catalyzed Markovnikov hydrofunctionalization of terminal alkynes that is facilitated by strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands. Using this method, both alkyl- and aryl-substituted alkynes are coupled with a variety of boryl and silyl reagents with high α-selectivity. The reaction is scalable, and the products are versatile intermediates that can participate in various downstream transformations. Preliminary mechanistic experiments shed light on the role of CAAC ligands in this process.

A new type of π-backbonding is discussed, which was observed between the π-acceptors CO/N₂ and the closo-dodecaborate fragments [B₁₂X₁₁]⁻ (X=F, Cl, Br, I, CN). The highly reactive [B₁₂X₁₁]⁻ anions transfer electron density from their delocalized σ-electron systems into antibonding orbitals of the respective acceptor molecules. Moreover, the comprehensive set of data provided with this study gives insights into the reactivity of [B₁₂X₁₁]⁻ anions depending on the substituent X.

Abstract

Electrophilic anions of type [B₁₂X₁₁]⁻ posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B₁₂X₁₁]⁻ with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N₂ as σ-donor-π-acceptors. Temperature-dependent formation of [B₁₂X₁₁NG]⁻ indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B₁₂X₁₁]⁻ and steric effects. The binding of CO and N₂ to [B₁₂X₁₁]⁻ is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B₁₂X₁₁CO]⁻ and [B₁₂X₁₁N₂]⁻ were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N₂ stretching frequencies and , respectively. Observed shifts of and are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B₁₂X₁₁]⁻ reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.

The metal-catalyzed cross-coupling of organozinc reagents has been studied since its discovery in 1977, but the dramatic dependence on salt additives was only realized some 30 years later. LiCl and LiBr have been found not only to improve coupling outcomes, but their absence can shut down the reaction completely. This Review summarizes all reports outlining the ways that salt additives impact the formation of organozinc reagents and their use in Negishi cross-coupling.

Abstract

The first cross-coupling of organozinc nucleophiles with aryl halides was reported in 1977 by Negishi. Unknown to all at the time was the importance of salt additives that were often present as a byproduct from the organozinc preparation. For decades, these salt additives were overlooked until 2006 when it was discovered that two different, yet effective methods for preparing organozinc solutions (i.e. one with salt and one without) provided drastically different results. Since this finding, the exact role of salt additives in cross-coupling has been debated in the catalysis community. In this Review we highlight all the major discoveries regarding the influence of salt additives on the formation of organozinc reagents and their use in the Negishi reaction. These effects include solubilizing key intermediates, the formation of higher-order zincates, product inhibition, catalyst protection, and solvent effects.