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In the last two decades, boron-based catalysis has been gaining increasing traction in the field of organic synthesis. The use of halogenated triarylboranes as main group Lewis acid catalysts is an attractive strategy. It has been applied in a growing number of transformations over the years, where they may perform comparably or even better than the gold standard catalysts. This review discusses methods of borane synthesis and cutting-edge boron-based Lewis acid catalysis, focusing especially on tris(pentafluorophenyl)-borane [B(C6F5)3], and other halogenated triarylboranes, highlighting how boron Lewis acids employed as catalysts can unlock a plethora of unprecedented chemical transformations or improve the efficiency of existing reactions.

The presented study depicts the synthesis of 11 carborane–thiazole conjugates with anticancer activity, as well as an evaluation of their biological activity as inhibitors of two enzymes: tyrosinase and 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). The overexpression of tyrosinase results in the intracellular accumulation of melanin and can be observed in melanoma. The overexpression of 11β-HSD1 results in an elevation of glucocorticoid levels and has been associated with the aggravation of metabolic disorders such as type II diabetes mellitus and obesity. Recently, as the comorbidity of melanomas and metabolic disorders is being recognized as an important issue, the search for new therapeutic options has intensified. This study demonstrates that carborane–thiazole derivatives inhibit both enzymes, exerting beneficial effects. The antiproliferative action of all newly synthesized compounds was evaluated using three cancer cell lines, namely A172 (human brain glioblastoma), B16F10 (murine melanoma) and MDA-MB-231 (human breast adenocarcinoma), as well as a healthy control cell line of HUVEC (human umbilical vein endothelial cells). The results show that 9 out of 11 newly synthesized compounds demonstrated similar antiproliferative action against the B16F10 cell line to the reference drug, and three of these compounds surpassed it. To the best of our knowledge, this study is the first to demonstrate dual inhibitory action of carborane–thiazole derivatives against both tyrosinase and 11β-HSD1. Therefore, it represents the first step towards the simultaneous treatment of melanoma and comorbid diseases such as type II diabetes mellitus.

Platinum and ruthenium nanoparticles stabilised by an amine modified polymer immobilised ionic liquid (MNP@NH 2 -PEGPIILS, M = Pt, Ru) catalyse the hydrolytic liberation of hydrogen from dimethylamine borane (DMAB), ammonia borane (AB) and NaBH 4 under mild conditions. While RuNP@NH 2 -PEGPIILS and PtNP@NH 2 -PEGPIILS catalyse the hydrolytic evolution of hydrogen from NaBH 4 with comparable initial TOFs of 6,250 molesH 2 .molcat −1 .h −1 and 5,900 molesH 2 .molcat −1 .h −1 , respectively, based on the total metal content, RuNP@NH 2 -PEGPIILS is a markedly more efficient catalyst for the dehydrogenation of DMAB and AB than its platinum counterpart, as RuNP@NH 2 -PEGPIILS gave initial TOFs of 8,300 molesH 2 .molcat −1 .h −1 and 21,200 molesH 2 .molcat −1 .h −1 , respectively, compared with 3,050 molesH 2 .molcat −1 .h −1 and 8,500 molesH 2 .molcat −1 .h −1 , respectively, for PtNP@NH 2 -PEGPIILS. Gratifyingly, for each substrate tested RuNP@NH 2 -PEGPIILS and PtNP@NH 2 -PEGPIILS were markedly more active than commercial 5wt % Ru/C and 5wt% Pt/C, respectively. The apparent activation energies of 55.7 kJ mol −1 and 27.9 kJ mol −1 for the catalytic hydrolysis of DMAB and AB, respectively, with RuNP@NH 2 -PEGPIILS are significantly lower than the respective activation energies of 74.6 kJ mol −1 and 35.7 kJ mol −1 for its platinum counterpart, commensurate with the markedly higher initial rates obtained with the RuNPs. In comparison, the apparent activation energies of 44.1 kJ mol −1 and 46.5 kJ mol −1 , for the hydrolysis NaBH 4 reflect the similar initial TOFs obtained for both catalysts. The difference in apparent activation energies for the hydrolysis of DMAB compared with AB also reflect the higher rates of hydrolysis for the latter. Stability and reuse studies revealed that RuNP@NH 2 -PEGPIILS recycled efficiently as high conversions for the hydrolysis of DMAB were maintained across five runs with the catalyst retaining 97% of its activity. Graphical Abstract

What might you do with a hat that had so many decorations dangling from the brim that you couldn't put it on? Lewis acids and bases are the molecular versions of hats and heads. Stephan reviews the surprising chemistry of so-called frustrated Lewis pairs (FLPs), which cannot form their natural complex together. Over the past decade, such systems (most often comprising a borane with a nitrogen or phosphorus partner) have been used to catalyze hydrogenation reactions, activate a number of other small molecules, and generally promote a wide range of cooperative chemical reactivity. Science , this issue p. 10.1126/science.aaf7229 The revelation that combinations of Lewis acids and bases for which dative bonding is impeded can activate dihydrogen led to the concept of “frustrated Lewis pairs” (FLPs). Over the past decade, a range of FLP systems and substrate molecules have precipitated a paradigm change in main-group chemistry and metal-free catalysis. The FLP motif has also found application in a growing body of chemical problems in organic synthesis, transition metal and free radical chemistry, materials, enzymatic models, and surface chemistry. The current state of FLP chemistry is assessed herein, and the outlook for the future considered.

Ammonia borane (NH 3 BH 3 , AB) holds promise for chemical storage of hydrogen. However, designing superb and low-cost photocatalyst to drive hydrogen evolution from AB under visible light irradiation is highly desirable but remains a major challenge for promoting the practical utilization of AB. Herein, we demonstrated a heterostructure photocatalyst consisting of zero-dimensional (0D) CoP nanoparticles immobilized on two-dimensional (2D) Co 2 P nanosheets (CoP/Co 2 Ps) as a high-performance and low-cost catalyst for hydrogen evolution from AB hydrolysis, in which 0D/2D heterostructure was synthesized using the salt-induced phase transformation strategy. Interestingly, the optimized CoP/Co 2 Ps exhibit a robust H 2 evolution rate of 32.1 L·min −1 ·g Co −1 , corresponding to a turnover frequency (TOF) value of 64.1 min −1 , being among the highest TOF for non-noble-metal catalysts ever reported, even outperforming some precious metal catalysts. This work not only opens a new avenue to accelerate hydrogen evolution from AB by regulating the electronic structures of heterointerfaces, but also provides a novel strategy for the construction of precious-metal-free materials for hydrogen-related energy catalysis in the future.

Thermodynamic hydricity (HDAMeCN) determined as Gibbs free energy (ΔG°[H]−) of the H− detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [BnHn]2−, and their lithium salts Li2[BnHn] (n = 5–17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B2H6 (HDAMeCN = 82.1 kcal/mol) and its dianion [B2H6]2− (HDAMeCN = 40.9 kcal/mol for Li2[B2H6]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDAMeCN below 41 kcal/mol are strong hydride donors capable of reducing CO2 (HDAMeCN = 44 kcal/mol for HCO2−), whereas those with HDAMeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B2H6, B4H10, B5H11, etc.). The HDAMeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDAMeCN.

PEGylation is a reductive alkylation of a protein N-terminal/α-amine of protein with mPEG chain by reducing agent. To obtain quantitative and site-specific PEGylation, sodium cyanoborohydride is commonly used as a reducing agent. The reduction process of sodium cyanoborohydride produces highly poisonous hydrogen cyanide, which may render the final product toxic. Herein, we have studied various reducing agents such as dimethylamine borane, triethylamine borane, trimethylamine borane, pyridine borane, morpholine borane, 2-picoline borane, and 5-ethyl-2-methyl-pyridine borane were tested as alternatives to sodium cyanoborohydride for the PEGylation of L-asparaginase. The characterization of reacted pegaspargase was carried out by SDS-PAGE, Western blotting, SEC-HPLC, RP-HPLC, SEC-MALS, CD, enzyme activity, and cell proliferation assays using with lymphoblast cells and MTS/PMS as substrate. Pyridine borane was determined to be the best acceptable reducing agent for PEGylation in terms of purity and activity. As a result, instead of sodium cyanoborohydride, pyridine borane can be employed.

HIV protease (PR) represents a prime target for rational drug design, and protease inhibitors (PI) are powerful antiviral drugs. Most of the current PIs are pseudopeptide compounds with limited bioavailability and stability, and their use is compromised by high costs, side effects, and development of resistant strains. In our search for novel PI structures, we have identified a group of inorganic compounds, icosahedral metallacarboranes, as candidates for a novel class of nonpeptidic PIs. Here, we report the potent, specific, and selective competitive inhibition of HIV PR by substituted metallacarboranes. The most active compound, sodium hydrogen butylimino bis-8,8-[5-(3-oxa-pentoxy)-3-cobalt bis(1,2-dicarbollide)]di-ate, exhibited a Ki value of 2.2 nM and a submi-cromolar EC50 in antiviral tests, showed no toxicity in tissue culture, weakly inhibited human cathepsin D and pepsin, and was inactive against trypsin, papain, and amylase. The structure of the parent cobalt bis(1,2-dicarbollide) in complex with HIV PR was determined at 2.15 $\\ring{A}$ resolution by protein crystallography and represents the first carborane-protein complex structure determined. It shows the following mode of PR inhibition: two molecules of the parent compound bind to the hydrophobic pockets in the flap-proximal region of the S3 and S3' subsites of PR. We suggest, therefore, that these compounds block flap closure in addition to filling the corresponding binding pockets as conventional PIs. This type of binding and inhibition, chemical and biological stability, low toxicity, and the possibility to introduce various modifications make boron clusters attractive pharmacophores for potent and specific enzyme inhibition.

The chemistry of some reactive frustrated Lewis pairs (FLPs) is reported. This includes intramolecular P/B and N/B FLPs, some of which were used as catalysts for the hydrogenation of electron-rich olefin substrates. Some advanced intermolecular FLPs are reported, which includes systems derived from very bulky alkenyl boranes obtained from 1,1-carboboration reactions of 1-alkynes with tris(pentafluorophenyl)borane. Some such systems activate dihydrogen and transfer the resulting proton/hydride pair even to some electron-poor alkynes. Eventually, we report on the reaction of our intramolecular ethylene-bridged P/B FLP with nitric oxide (NO). N,B-addition of the P-Lewis base/B-Lewis acid combination is observed to form a new type of a persistent aminoxyl radical. Some of the chemistry of the new FLP-NO radicals is presented and discussed.

Boron functional groups are often introduced in place of aromatic carbon–hydrogen bonds to expedite small-molecule diversification through coupling of molecular fragments 1 – 3 . Current approaches based on transition-metal-catalysed activation of carbon–hydrogen bonds are effective for the borylation of many (hetero)aromatic derivatives 4 , 5 but show narrow applicability to azines (nitrogen-containing aromatic heterocycles), which are key components of many pharmaceutical and agrochemical products 6 . Here we report an azine borylation strategy using stable and inexpensive amine-borane 7 reagents. Photocatalysis converts these low-molecular-weight materials into highly reactive boryl radicals 8 that undergo efficient addition to azine building blocks. This reactivity provides a mechanistically alternative tactic for sp 2 carbon–boron bond assembly, where the elementary steps of transition-metal-mediated carbon–hydrogen bond activation and reductive elimination from azine-organometallic intermediates are replaced by a direct, Minisci 9 -style, radical addition. The strongly nucleophilic character of the amine-boryl radicals enables predictable and site-selective carbon–boron bond formation by targeting the azine’s most activated position, including the challenging sites adjacent to the basic nitrogen atom. This approach enables access to aromatic sites that elude current strategies based on carbon–hydrogen bond activation, and has led to borylated materials that would otherwise be difficult to prepare. We have applied this process to the introduction of amine-borane functionalities to complex and industrially relevant products. The diversification of the borylated azine products by mainstream cross-coupling technologies establishes aromatic amino-boranes as a powerful class of building blocks for chemical synthesis. Selective borylation of azines—nitrogen-containing aromatic heterocycles used in the synthesis of many pharmaceuticals—is made possible by forming a radical from an aminoborane using a photocatalyst.