Facial Synthesis of o‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

1,3‐Dehydro‐o‐carborane is a useful synthon for selective cage boron functionalization of o‐carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)‐alkenyl/allenyl o‐carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage C?B multiple bond, which lowers the activation barriers of the ene reaction.     

Some aspects of the chemical behavior of icosahedral carboranes

Abatract  Important features of the chemical behavior of icosahedral carboranes are considered. The ways of introducing functional groups at the carbon and boron atoms of carboranes by conventional methods of organic chemistry and methods applicable only to carboranes are discussed. The latter methods include transformations of dicarbadodecaborate(14) dianions. Examples of the enantiomeric resolution of optically active carboranes, whose chirality is associated with the molecular asymmetry, are given. The diversity of chemical transformations of carboranes is exemplified by intramolecular nucleophilic rearrangements, cross-coupling reactions, and the formation of chelate metallacycles. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 801–822, April, 2008.     

Recent Progress in the Chemistry of Supercarboranes

The chemistry of boron clusters has been dominated by icosahedral carboranes for over half a century. Only in recent years has significant progress been made in the chemistry of supercarboranes (carboranes with more than 12 vertices). A number of CAd (carbon‐atoms‐adjacent) 13‐ and 14‐vertex carboranes, and CAp (carbon‐atoms‐apart) 13‐vertex carboranes as well as their corresponding 14‐ and 15‐vertex metallacarboranes have been successfully prepared and structurally characterized. This breakthrough relied on the use of CAd nido‐carborane dianions as starting materials. These supercarboranes can undergo single‐electron reduction to give stable supercarborane radical monoanions with [2n+3] framework electrons, and electrophilic substitution reaction to afford hexasubstituted supercarboranes. They can react with nucleophiles to offer monocarba‐closo‐dodecaborate monoanions from cage‐carbon extrusion reactions. Their unique chemical properties make the chemistry of supercarboranes distinct from that of their 12‐vertex analogues. These studies open up new possibilities for the development of polyhedral clusters of extraordinary size. This focus review offers an overview of recent advances in this growing research field.     

Competition between Halogen,Hydrogen and Dihydrogen Bonding in Brominated Carboranes

Halogen bonds are a subset of noncovalent interactions with rapidly expanding applications in materials and medicinal chemistry. While halogen bonding is well known in organic compounds, it is new in the field of boron cluster chemistry. We have synthesized and crystallized carboranes containing Br atoms in two different positions, namely, bound to C‐ and B‐vertices. The Br atoms bound to the C‐vertices have been found to form halogen bonds in the crystal structures. In contrast, Br atoms bound to B‐vertices formed hydrogen bonds. Quantum chemical calculations have revealed that halogen bonding in carboranes can be much stronger than in organic architectures. These findings open new possibilities for applications of carboranes, both in materials and medicinal chemistry.     

实时直接分析高分辨质谱技术研究碳硼烷类化合物

该文运用高分辨质谱技术对实时直接分析(Direct analysis in real time,DART)离子化条件下碳硼烷化合物的质谱行为进行了研究,对碳硼烷化合物DART高分辨质谱中所得到的同位素峰簇进行了表征与归属。研究结果表明,选取的碳硼烷化合物在DART负离子条件下均能得到较好的质谱信号,这可能与硼笼结构的“缺电性”有关。含10个B原子的碳硼烷化合物形成的离子同位素峰簇信号中,通常情况下相对丰度最高的同位素峰中含2个10B以及8个11B。将碳硼烷化合物高分辨质谱分析的精确m/z数据信息与图谱中同位素峰轮廓分析相结合,是碳硼烷化合物有效的质谱定性分析与表征策略。     

General Access to Aminobenzyl‐o‐carboranes as a New Class of Carborane Derivatives: Entry to Enantiopure Carborane–Amine Combinations

The convenient synthesis of original aminobenzyl‐o‐carboranes, which represent a new class of nitrogenated carborane derivatives, is described. These novel compounds have been efficiently prepared starting from commercially available aromatic aldehydes and monosubstituted o‐carboranes via carboranyl alcohols and chlorides as intermediates. The key step of this methodology is a selective nucleophilic amination under mild conditions that allows the formation of the expected amines while limiting the partial deboronation of the carborane cluster. This general strategy has been applied to the preparation of a wide variety of aminobenzyl‐o‐carboranes. The extension of this pathway to the synthesis of enantiopure carborane–amine combinations is also described.     

Fluorescence of New o‐Carborane Compounds with Different Fluorophores: Can it be Tuned?

Two sets of o‐carborane derivatives incorporating fluorene and anthracene fragments as fluorophore groups have been successfully synthesized and characterized, and their photophysical properties studied. The first set, comprising fluorene‐containing carboranes 6 – 9 , was prepared by catalyzed hydrosilylation reactions of ethynylfluorene with appropriate carboranylsilanes. The compound 1‐[(9,9‐dioctyl‐fluorene‐2‐yl)ethynyl]carborane ( 11 ) was synthesized by the reaction of 9,9‐dioctyl‐2‐ethynylfluorene and decaborane (B₁₀H₁₄). Furthermore, reactions of the lithium salt of 11 with 1 equivalent of 4‐(chloromethyl)styrene or 9‐(chloromethyl)anthracene yielded compounds 12 and 13 . Members of the second set of derivatives, comprising anthracene‐containing carboranes, were synthesized by reactions of monolithium or dilithium salts of 1‐Me‐1,2‐C₂B₁₀H₁₁, 1‐Ph‐1,2‐C₂B₁₀H₁₁, and 1,2‐C₂B₁₀H₁₂ with 1 or 2 equivalents of 9‐(chloromethyl)anthracene, respectively, to produce compounds 14 – 16 . In addition, 2 equivalents of the monolithium salts of 1‐Me‐1,2‐C₂B₁₀H₁₁ (Me‐o‐carborane) and 1‐Ph‐1,2‐C₂B₁₀H₁₁ (Ph‐o‐carborane) were reacted with 9,10‐bis(chloromethyl)anthracene to produce compounds 17 and 18 , respectively. Fluorene derivatives 6 – 9 exhibit moderate fluorescence quantum yields (32–44 %), whereas 11 – 13 , in which the fluorophore is bonded to the C_cluster (C_c), show very low emission intensity (6 %) or complete fluorescence quenching. The anthracenyl derivatives containing the Me‐o‐carborane moiety exhibit notably high fluorescence emissions, with ?_F=82 and 94 %, whereas their Ph‐o‐carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the C_c?C_c bond length and the fluorescence intensity in CH₂Cl₂ solution, comparable to that observed for previously reported styrene‐containing carboranes. Thus, our hypothesis is that for systems of this type the fluorescence may be tuned and even predicted by changing the substituent on the adjacent C_c.     

Carboranes as a Tool to Tune Phosphorescence

Phosphorescent transition‐metal complexes (PTMCs) have attracted great interest because of their excellent properties which may lead to promising applications in optoelectronics. In recent years, carboranes have been demonstrated to be a novel and effective tool to tune phosphorescence of PTMCs. This Concept article deals with the advances of carborane‐functionalized PTMCs for potential optical applications. The discussions are focused on the design strategies and synthetic procedures leading to carborane‐functionalized PTMCs, the influence of carboranes on the optical properties of PTMCs, and the promising optical applications of this interesting class of phosphorescent materials.     

Metal-containing carboranes with antitumor activity

The review summarizes our results and data published in the literature on the synthesis and antitumor activity of metal-containing carboranes. Carboranyl derivatives of platinum and tin and some metallacarboranes exhibit activity against malignant tumors. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 620–635, April, 2007.     

Synthesis,Characterisation, and Biodistribution of Radioiodinated C‐Hydroxy‐Carboranes

The synthesis, radiolabelling and biodistribution of iodinated C‐hydroxy‐nido‐carborane ligands is described. Microwave heating by using NaF in aqueous ethanol was used to prepare {sodium [7‐hydroxy‐7,8‐dicarba‐nido‐undecaborate], nido‐carboranol} and {sodium [7‐hydroxy‐7,8‐dicarba‐nido‐undecaborate‐8‐carboxylic acid], nido‐salborin} in 97 and 90 % yield, respectively. Radioiodination of these nido‐carboranes was completed by using both ¹²⁵I and ¹²³I, and the products were obtained in high radiochemical purity (>99 %) and yield (72 to 87 %). The structures of the radiolabelled products were validated through comparison to authentic standards. Biodistribution studies in BALB/c mice showed low accumulation of the labelled compounds in the liver and intestines, which are sites where labelled carboranes typically localise. The labelled cluster bearing hydroxy and carboxylic acid groups on the two carbon vertices demonstrated preferential clearance through the kidneys and low thyroid uptake. This compound had substantially reduced non‐specific binding than the deshydroxy analogue making it an attractive bifunctional ligand for preparing targeted molecular imaging and therapy agents.     

Preparation of synthons for carborane containing macromolecules

The modification of para-carborane with appropriate functionalities for incorporation within a dendrimer framework was accomplished by functionalizing the carbon centers with protected alcohol and free acid groups. These compounds are excellent candidates for utilization as functional linkers between two generations of an aliphatic polyester dendrimer structure. Future assembly of these structures will result in dendritic macromolecules containing carboranes within their interior and will be enveloped by hydrophilic groups (hydroxyls) to maintain their water solubility and biocompatibility. These structures have potential applications in Boron Neutron Capture Therapy and Synovectomy. Additionally, carboranes were coupled to polymerizable acrylate structures, and it was shown that the resulting carborane monomers could be polymerized using living free-radical polymerization techniques.     

Aggregation‐Induced Delayed Fluorescence Based on Donor/Acceptor‐Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs

Luminescent materials consisting of boron clusters, such as carboranes, have attracted immense interest in recent years. In this study, luminescent organic–inorganic conjugated systems based on o‐carboranes directly bonded to electron‐donating and electron‐accepting π‐conjugated units were elaborated as novel optoelectronic materials. These o‐carborane derivatives simultaneously possessed aggregation‐induced emission (AIE) and thermally activated delayed fluorescence (TADF) capabilities, and showed strong yellow‐to‐red emissions with high photoluminescence quantum efficiencies of up to 97 % in their aggregated states or in solid neat films. Organic light‐emitting diodes utilizing these o‐carborane derivatives as a nondoped emission layer exhibited maximum external electroluminescence quantum efficiencies as high as 11 %, originating from TADF.     

Mercuration ofB(9)-substitutedmeta- andortho-carboranes

The mercuration of substituted R₂C₂B₁₀H₉X-9 type carboranes (where R=m-H, X= Cl, Br, I, Me; R=o-H, X=Me) was studied. It was found that mercury atoms add to the boron atoms in position 10 ofmeta-carboranes and in position 12 ofortho-carboranes,i.e., to the boron atoms adjacent to the boron atom bonded to the X substituent. Symmetrical (R₂C₂B₁₀H₈X)₂Hg type derivatives were obtained. It was shown that they can be used as starting materials in transmetallation reactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 743–747, April, 1995.This work was carried out with financial support from the Russian Foundation for Basic Research (Project No 93-03-18654) and the International Science Foundation (Grant NDM 000).     

From Mono‐ to Poly‐Substituted Frameworks: A Way of Tuning the Acidic Character of Cc?H in o‐Carborane Derivatives

The incorporation of iodine atoms onto the boron vertices of the o‐carborane framework causes, according to spectroscopic data, a uniform increase in the acidic character of the C_c? H (C_c= cluster carbon) vertices, whereas the incorporation of methyl groups onto the boron vertices of the o‐carborane framework reduces their acidity. Methyl groups when attached to boron are electron‐withdrawing in boron clusters, whereas iodine atoms bonded to boron act as electron donors. This has been proven on B‐methyl and B‐iodinated o‐carboranes with NMR spectroscopy measurements and DFT calculations of natural bond orbital (NBO) charges, which show a cumulative buildup of positive cluster‐only total charge (CTC) on B‐methyl o‐carboranes and a cumulative buildup of negative cluster‐only total charge for B‐iodinated o‐carboranes.     

Transition-Metal-Catalyzed Regioselective Functionalization of Monophosphino-o-Carboranes

A facile approach to the synthesis of diaryl- and dialkyl-substituted monophosphino-o-carboranes by rhodium(I)-catalyzed phosphine-directed B^3,6−H activation has been developed for the first time. Upon switching rhodium(I) to palladium(II), C-arylated and B⁶-halogenated products were obtained by using tBuOLi and Li₂CO₃ as base, respectively. These discoveries provide some simple and efficient approaches to the modification of monophosphino-o-carboranes.     

Photoarylation of Iodocarboranes with Unactivated (Hetero)Arenes: Facile Synthesis of 1,2-[(Hetero)Aryl]n-o-Carboranes (n=1,2) and o-Carborane-Fused Cyclics

Photoarylation of iodocarboranes with unactivated arenes/heteroarenes at room temperature has been achieved, for the first time, thus leading to the facile synthesis of a large variety of cage carbon mono(hetero)arylated and di(hetero)arylated o-carboranes. This work represents a clean, efficient, transition-metal-free, and cheap synthesis of functionalized carboranes, which has significant advantages over the known methods.     

Rhodium‐Catalyzed Regioselective Hydroxylation of Cage B−H Bonds of o‐Carboranes with O2 or Air

A rhodium‐catalyzed hydroxylation of a cage B4?H bond in o‐carboranes with either O₂ or air as the oxygen source is described, and serves as a new methodology for the regioselective generation of a series of 4‐OH‐o‐carboranes in a one‐pot process. The use of either O₂ or air as both the oxidant and the oxygen source makes this protocol very environmentally friendly and practical.     

Simple Electrochemical Characterization of ortho‐Carborane and some of its exo‐Skeletal Derivatives

Electrochemical behaviour of 12 icosahedral carboranes – ortho‐, meta‐ and para‐carborane with different exo‐skeletal substituents was investigated. The study was done using phosphate buffers, in some cases with 20 % and 30 % dimethyl sulfoxide (DMSO) addition to increase the solubility of studied compounds. Commercial glassy carbon electrode or home‐made screen printed electrodes were used. Distinct electrochemical responses were observed only for the ortho‐carborane and its exo‐skeletal derivatives. The study of different exo‐skeletal substituents on the electrochemical behaviour of the carboranes is crucial for the intended use of these compounds as electrochemical labels of biomolecules.     

High yield [I]iodide-labeling of iodinated carboranes by palladium-catalyzed isotopic exchange

We have recently shown the feasibility of palladium-catalyzed isotopic exchange between [¹²⁵I]iodide and 2-iodo-para-carborane. In this work we have modified the methodology and extended its application to a wider range of iodinated carboranes. Thus, by using Herrmann's catalyst in toluene at 100 °C, 2-I-p-, 3-I-o-, 9-I-o-, 9-I-m-carborane, 1-phenyl-3-I-o-carborane and 1,2-diphenyl-3-I-o-carborane have been radiolabeled with ¹²⁵iodine in high to excellent yields.     

Nickel‐Catalyzed Cross‐Coupling Reactions of o‐Carboranyl with Aryl Iodides: Facile Synthesis of 1‐Aryl‐o‐Carboranes and 1,2‐Diaryl‐o‐Carboranes

A nickel‐catalyzed arylation at the carbon center of o‐carborane cages has been developed, thus leading to the preparation of a series of 1‐aryl‐o‐carboranes and 1,2‐diaryl‐o‐carboranes in high yields upon isolation. This method represents the first example of transition metal catalyzed C,C′‐diarylation by cross‐coupling reactions of o‐carboranyl with aryl iodides.