Band 90, Heft 4

For the first time, carborane complexes containing a B(4)–S bond were obtained directly by heating mononuclear Pd-B(4)-bound carborane complexes. A possible mechanism involved in sulfur atom migration is presented in which the leaving group, pyridine, benzyl isocyanide or PPh 3 , is demonstrated to be the trigger of the reaction process. In this work, efficient routes are developed through one-pot reactions to prepare B(4)-S carborane derivatives.

Transition-metal free reactions of diazo compounds with organoboron compounds provide some unique approaches for the formation of C–C, C–B and C–Si bonds. With N -tosylhydrazones as the precursors for non-stabilized diazo compound, this type of reaction becomes practically useful in organic synthesis. Transition-metal-free synthetic methodologies for borylation, gem -diborylation, gem -silylborylation arylation, 2,2,2-trifluoroethylation and gem -difluorovinylation have been successfully developed.

The title compound, [(H 3 NCH 2 CH 2 NH 2 )Zn{κ 3 O , O ′, O ′′-B 12 O 18 (OH) 6 -κ 1 O ′′′}Zn(en)(NH 2 CH 2 CH 2 NH 3 )]·8H 2 O (en=1,2-diaminoethane) ( 1 ), was prepared as a crystalline solid in moderate yield from the reaction of B(OH) 3 with [Zn(en) 3 ][OH] 2 in aqueous solution (15:1) ratio. The structure contains a neutral bimetallic complex comprised of a unusual dodecaborate(6−) anion ligating two [H 3 NCH 2 CH 2 NH 2 Zn(en) n ] 3+ centers in a monodentate (n=1) or tridentate (n=0) manner.

A series of new closo - and nido -carborane based functional derivatives 1-X(CH 2 ) n S-1,2-C 2 B 10 H 11 (X=COOH, N 3 , CH(NH 2 )COOH) and [7-X(CH 2 ) n S-7,8-C 2 B 9 H 11 ] − (X=COOH, N 3 , NH 2 , CH(NH 2 )COOH) was prepared by alkylation of 1-mercapto- ortho -carborane. Dialkylsulfonium derivatives of nido -carborane 9-R(Me)S-7,8-C 2 B 9 H 11 and 10-R(Me)S-7,8-C 2 B 9 H 11 with boron-sulfur bond were prepared by alkylation of the corresponding methyl-carboranyl thioether. New types of intramolecular B–H···X and B–H···π(C≡C) interactions were found in nido -carborane alkylmethyl sulfonium derivatives 9-XCH 2 S(Me)S-7,8-C 2 B 9 H 11 and 10-RC≡CCH 2 S(Me)S-7,8-C 2 B 9 H 11 , respectively. Isomeric methylsulfide derivatives of transition metal bis(dicarbollide) complexes [X,Y-(MeS) 2 -3,3′-M(1,2-C 2 B 9 H 10 ) 2 ] − (M=Co, Fe) were prepared starting from the corresponding methylcarboranyl thioethers. The intramolecular CH carb ···S(Me)S hydrogen bonding between the dicarbollide ligands in cobalt bis(dicarbollide) complexes results in stabilization of definite rotational isomers – transoid in the case of the 8,8′-isomer and gauche in the case of the 4,4′- and 4,7′-isomers.

The skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally known oblatocloso dirhenaboranes Cp* 2 Re 2 B n −2 H n −2 (Cp*=η 5 Me 5 C 5 , n =8–12) suggest formal Re=Re double bonds through the center of a flattened Re 2 B n −2 deltahedron. Removal of a boron vertex from these oblatocloso structures leads to oblatonido structures such as Cp 2 W 2 B 5 H 9 and Cp 2 W 2 B 6 H 10 . Similar removal of two boron vertices from the Cp 2 Re 2 B n −2 H n −2 ( n =8–12) structures generates oblatoarachno structures such as Cp 2 Re 2 B 4 H 8 and Cp 2 Re 2 B 7 H 11 . Higher energy Cp 2 Re 2 B n −2 H n −2 (Cp=η 5 -C 5 H 5 , n =8–12) structures exhibit closo deltahedral structures similar to the deltahedral borane dianions B n H n 2− . The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re 2 Cl 8 2− by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe 2 B n −2 H n −2 structures (Pn=η 5 ,η 5 -pentalene) in which the pentalene ligand forces the rhenium atoms to occupy adjacent deltahedral vertices. The low-energy structures of the tritungstaboranes Cp 3 W 3 (H)B n −3 H n −3 ( n =5–12), related to the experimentally known Cp* 3 W 3 (H)B 8 H 8 , have central W 3 B n −3 deltahedra with imbedded bonded W 3 triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp 3 Re 3 B n −3 H n −3 . The metal atoms are located at degree 6 and 7 vertices in regions of relatively low surface curvature whereas the boron atoms are located at degree 3–5 vertices in regions of relatively high surface curvature. The five lowest-energy structures for the 11-vertex tritungstaborane Cp 3 W 3 (H)B 8 H 8 all have the same central W 3 B 8 deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W 3 triangles in these structures have two long ~3.0 Å W–W edges through the inside of the deltahedron with the third shorter W–W edge of ~2.7 to ~2.8 Å corresponding to a surface deltahedral edge.

Nanostructured boron compounds have emerged as one of the promising frontiers in boron chemistry. These species possess unique physical and chemical properties in comparison with classical small boron compounds. The nanostructured boron composites generally have large amounts of boron contents and thus have the potential to deliver significant amount of boron to the tumor cells, that is crucial for boron neutron capture therapy (BNCT). In theory, BNCT is based on a nuclear capture reaction with the 10 B isotope absorbing a slow neutron to initiate a nuclear fission reaction with the release of energetic particles, such as lithium and helium (α particles), which travel the distance of around nine microns within the cell DNA or RNA to destroy it. The recent studies have demonstrated that the nanostructured boron composites can be combined with the advanced targeted drug delivery system and drug detection technology. The successful combination of these three areas should significantly improve the BNCT in cancer treatment. This mini review summarizes the latest developments in this unique area of cancer therapy.

A number of heterometallic boride clusters have been synthesized and structurally characterized using various spectroscopic and crystallographic analyses. Thermolysis of [Ru 3 (CO) 12 ] with [Cp*WH 3 (B 4 H 8 )] ( 1 ) yielded [{Cp*W(CO) 2 } 2 ( μ 4 -B){Ru(CO) 3 } 2 ( μ -H)] ( 2 ), [{Cp*W(CO) 2 } 2 ( μ 5 -B){Ru(CO) 3 } 2 {Ru(CO) 2 }( μ -H)] ( 3 ), [{Cp*W(CO) 2 }( μ 5 -B){Ru(CO) 3 } 4 ] ( 4 ) and a ditungstaborane cluster [(Cp*W) 2 B 4 H 8 Ru(CO) 3 ] ( 5 ) (Cp*= η 5 -C 5 Me 5 ). Compound 2 contains 62 cluster valence-electrons, in which the boron atom occupies the semi-interstitial position of a M 4 -butterfly core, composed of two tungsten and two ruthenium atoms. Compounds 3 and 4 can be described as hetero-metallic boride clusters that contain 74-cluster valence electrons (cve), in which the boron atom is at the basal position of the M 5 -square pyramidal geometry. Cluster 5 is analogous to known [(Cp*W) 2 B 5 H 9 ] where one of the BH vertices has been replaced by isolobal {Ru(CO) 3 } fragment. Computational studies with density functional theory (DFT) methods at the B3LYP level have been used to analyze the bonding of the synthesized molecules. The optimized geometries and computed 11 B NMR chemical shifts satisfactorily corroborate with the experimental data. All the compounds have been characterized by mass spectrometry, IR, 1 H, 11 B and 13 C NMR spectroscopy, and the structural architectures were unequivocally established by crystallographic analyses of clusters 2–5 .

Rapid development of research on the chemistry of boronic acids is connected with their applications in organic synthesis, analytical chemistry, materials’ chemistry, biology and medicine. In many applications Lewis acidity of boron atoms plays an important role. Special group of arylboronic acids are fluoro-substituted compounds, in which the electron withdrawing character of fluorine atoms influences their properties. The present paper deals with fluoro-substituted boronic acids and their derivatives: esters, benzoxaboroles and boroxines. Properties of these compounds, i.e. acidity, hydrolytic stability, structures in crystals and in solution as well as spectroscopic properties are discussed. In the next part examples of important applications are given.

Chiral N -heterocyclic organoboronates represent promising intermediates for the preparation of various bioactive and pharmaceutical compounds. We recently reported the first asymmetric dearomative borylation of indoles by copper-catalyzed borylation. Then we further developed dearomatization/enantioselective borylation sequence. Chiral 3-boryl-tetrahydropyridines and chiral boryl-tetrahydroquinolines via the copper(I)-catalyzed regio-, diastereo- and enantioselective borylation of 1,2-dihydropyridines and 1,2-dihydroquinilines, which were prepared by the partial reduction of the corresponding pyridine or quinoline derivatives. This dearomatization/enantioselective borylation procedures provide a direct access to chiral piperidines and tetrahydroquinolines from readily available pyridines or quinolines in combination with the stereospecific transformation of the stereogenic C–B bond.

6-Chloro-5,6-dihydro-dibenzo[c,e][1,2]azaborinine ( 1 ) reacts with pyridine derivatives to borenium or boronium ions. The boronium ion obtained from reaction with an excess of pyridine could be characterized structurally and transforms into the borenium ion in solution. The reaction of 1 with 2,2′-bipyridine ( a ) and derivatives ( b : 6,6′-dimethyl; c : 4,4′-di- tert -butyl) results in spirocyclic boronium ions 8a–c of which 8b could be characterized by X-ray crystallography. Despite the chelate effect, the spirocyclic boronium ions readily undergo hydrolysis or alcoholysis. Treatment of the spirocyclic boronium ion 8c with potassium hexamethyl disilazide (KHMDS) results in neutral products that are monomers, dimers, or trimers of dibenzo[c,e][1,2]azaborinine (“BN-phenanthryne”) trapped with 4,4′-di- tert -butyl-2,2′-bipyridine by formation of B–C, B–N, or dative B–N bonds, indicative of deprotonation of NH and CH bonds of the boronium ion by KHMDS.

Several approaches toward electrophilic C–H silylation of electron-rich arenes are discussed, comprising transition-metal-catalyzed processes as well as Lewis-acid- and Brønsted-acid-induced protocols. These methods differ in the catalytic generation of the silicon electrophile but share proton removal in form of dihydrogen. With slight modifications, these methods are often also applicable to the related electrophilic C–H borylation.

Significant progress in the functionalization of icosahedral boron clusters has been made in the past years, leading to an increasing number of applications in various fields of research. The direct conversion of B–H bonds to substituted vertices constitutes an attractive strategy to synthesize cage compounds with desired properties. In this report, recent advances in the transition metal-catalyzed B–H activation of neutral and anionic boron clusters are presented.

Maleimide-functionalized closo -dodecaborate (MID) and isothiocyanate-functionalized closo -dodecaborate (ISD) were synthesized from closo -dodecaborate via ring opening reaction of 1,4-dioxane- closo -dedecaborate complex 1 with ammonia. MID was found to possess highest conjugation efficacy to bovine serum albumin among three closo -dodecaborate derivatives, MID, ISD, and 1 . The conjugation reaction of MID to human serum albumin (HSA) proceeded under PBS buffer conditions (pH 7.4). Boron distribution studies in colon 26 tumor-bearing mice revealed that HSA-MID was highly accumulated in tumor (23 ppm B), whereas boron concentrations in other organs such as liver, kidney and spleen were low (3~8 ppm B).

Chemistry and Human Health, Division VII of the International Union on Pure and Applied Chemistry (IUPAC), provides guidance on risk assessment methodology and, as appropriate, assessment of risks to human health from chemicals of exceptional toxicity. The aim of this document is to describe dose-response relationships for the health effects of low-level exposure to cadmium, in particular, with an emphasis on causation. The term “cadmium” in this document includes all chemical species of cadmium, as well as those in cadmium compounds. Diet is the main source of cadmium exposure in the general population. Smokers and workers in cadmium industries have additional exposure. Adverse effects have been shown in populations with high industrial or environmental exposures. Epidemiological studies in general populations have also reported statistically significant associations with a number of adverse health effects at low exposures. Cadmium is recognized as a human carcinogen, a classification mainly based on occupational studies of lung cancer. Other cancers have been reported, but dose-response relationships cannot be defined. Cardiovascular disease has been associated with cadmium exposure in recent epidemiological studies, but more evidence is needed in order to establish causality. Adequate evidence of dose-response relationships is available for kidney effects. There is a relationship between cadmium exposure and kidney effects in terms of low molecular mass (LMM) proteinuria. Long-term cadmium exposures with urine cadmium of 2 nmol mmol −1 creatinine cause such effects in a susceptible part of the population. Higher exposures result in increases in the size of these effects. This assessment is supported by toxicokinetic and toxicodynamic (TKTD) modelling. Associations between urine cadmium lower than 2 nmol mmol −1 creatinine and LMM proteinuria are influenced by confounding by co-excretion of cadmium with protein. A number of epidemiological studies, including some on low exposures, have reported statistically significant associations between cadmium exposure and bone demineralization and fracture risk. Exposures leading to urine cadmium of 5 nmol mmol −1 creatinine and more increase the risk of bone effects. Similar associations at much lower urine cadmium levels have been reported. However, complexities in the cause and effect relationship mean that a no-effect level cannot be defined. LMM proteinuria was selected as the critical effect for cadmium, thus identifying the kidney cortex as the critical organ, although bone effects may occur at exposure levels similar to those giving rise to kidney effects. To avoid these effects, population exposures should not exceed that resulting in cadmium values in urine of more than 2 nmol mmol −1 creatinine. As cadmium is carcinogenic, a ‘safe’ exposure level cannot be defined. We therefore recommend that cadmium exposures be kept as low as possible. Because the safety margin for toxic effects in kidney and bone is small, or non-existent, in many populations around the world, there is a need to reduce cadmium pollution globally.