Lithiacarboranes and 1,2,4-triazine 4-oxides: S N H reactions and ring transformations

The reactions of 1-lithia-1,2- or 1,7-dicarba-closo-dodecaborane with 1,2,4-triazine 4-oxides can follow two competitive pathways: deoxygenative nucleophilic substitution of hydrogen to form 1-(1,2,4-triazin-5-yl)-1,2- or 1,7-dicarba-closo-dodecaboranes and the transformation of the 1,2,4-triazine ring into the triazoline ring giving rise to 1-(2-acetyl-1-aroyl-3-aryl-1,2,4-triazolin-5-yl)-1,2-dicarba-closo-dodecaboranes. Introduction of the electron-withdrawing triazine ring into the carborane cage substantially facilitates deboronation to give 1-(1,2,4-triazin-5-yl)-1,2-1,7-dicarba-nido-undecaboranes.     

Synthesis and structure of novel closo-dodecaborate-based glycerols

The reactions of oxonium derivatives of [B₁₂H₁₂]²⁻ with different glycerol-based nucleophiles were studied. A series of novel closo-dodecaborate-based glycerols with different net charges on the molecules were prepared. A structure of {2-[2-(4-(2, 3-dihydroxypropyl)-dipiperazinium-1-yl)-ethoxy]-ethoxy}-undecahydro-closo-dodecaborate was determined and the existence of different intermolecular H-bonds was shown.     

[4+2] Cycloaddition reactions of 1,2,4,5-tetrazines with allylcarboranes

The [4+2] cycloaddition reactions of 3,6-disubstituted 1,2,4,5-tetrazines with 9-allyl-1,7-,9-allyl-1,2-dicarba-closo-dodecaboranes and 1-allyl-2-isopropyl-1,2-dicarba-closo-dodecab-orane have been studied. The pyridazines containing carborane cage have been synthesized for the first time.     

Synthesis of Closo-1,7-Carboranyl Alkyl Amines

Of the three closo-carborane isomers (C₂B₁₀H₁₂), closo-1,2-carborane has been used most widely in the synthesis of carboranyl amines. However, closo-1,2-carboranes are prone to deboronation to nido-7,8-carborane under various conditions including attack by basic amino groups. In order to overcome this problem, closo-1,7-carboranyl ethyl-, propyl-, and butylamine were synthesized, which should be more stable towards basic deboronation than their closo-1,2-carboranyl counterparts. These closo-1,7-carboranyl amines (5, 18 and 19) were synthesized using two different methods, both starting from the corresponding closo-1,7-carboranyl alkyl iodides (3, 14 and 15). One of the carboranyl alkyl amine (5) was conjugated with folic acid to form a closo-1,7-carborane-folic acid bioconjugate (20).     

Synthesis of unsaturated 7,8- and 7,9-di(hydroxymethyl)decahydro-7,8- and-7,9-dicarba-nido-undecaborate(1−) diethers

Dicarba-nido-undecaborate(1–) anions were obtained by treatment of 1,2-di(hydroxymethyl)-1,2-dicarba-closo-dodecaborane(12) diallyl ether, 1,2-di(hydroxymethyl)-1,2-dicarba-closo-dodecaborane (12), and 1,7-di(hydroxymethyl)-1,7-dicarba-closo-dodecaborane(12) with ethanolic solutions of KOH and subsequent reaction of the products with cesium and tetramethylammonium chlorides.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1821–1823, September, 1995.     

Closo → nido cage degradation of 1-(substituted-phenyl)-1,2-dicarbadodecaborane(12)s in wet DMSO under neutral conditions

1-(2-Nitrophenyl)-1,2-dicarba-closo-dodecaborane(12) shows C-H?O intramolecular hydrogen bonding in chloroform. The reaction of isomeric 1-(nitrophenyl)-1,2-dicarba-closo-dodecaborane(12)s and of 1-(4-fluorophenyl)-1,2-dicarba-closo-dodecaborane(12) with wet DMSO causes the removal of 3-B or 6-B, leading cleanly to nido-carboranes. The rank order of rates of these deboronations is consistent with developing negative charge in the rate-determining step.     

Synthesis,molecular structure and reactivity of the dimethyl sulfide adduct of a 1,3,2-diselenaborolane with an annelated dicarba-closo-dodecaborane(12) unit

The reaction of 1,2-diselenolato-1,2-dicarba-closo-dodecaborane(12) dianion [1,2-(1,2-C₂B₁₀H₁₀)Se₂]²⁻ with HBBr₂–SMe₂ affords the dimethyl sulfide adduct of 4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-diselena-2-borolane in good yield. The molecular structure was determined by X-ray diffraction, and the solution-state structure was established by NMR spectroscopy (¹H, ¹¹B, ¹³C, ⁷⁷Se NMR). ¹¹B and ⁷⁷Se chemical shifts were reproduced by DFT calculations. Attempts were made to abstract dimethyl sulfide, and the parent donor-free compound could be detected in solution. The reactivity of the title compound was studied towards pyridine, Me₃SnF, [Pt(PPh₃)₂(C₂H₄)], tert-Bu-OH, AlMe₃ and AlH₃-N(Et)Me₂ as well as a hydroborating reagent.     

Ethynylmonocarba-closo-dodecaborates: M[12-HCC-closo-1-CB11H11] and M[7,12-(HCC)2-closo-1-CB11H10] (M = Cs, [Et4N])

Cesium and tetraethylammonium salts of the ethynyl functionalized monocarba-closo-dodecaborate anions [12-HCC-closo-1-CB₁₁H₁₁]⁻ and [7,12-(HCC)₂-closo-1-CB₁₁H₁₀]⁻ were obtained by desilylation of [Et₄N][12-Me₃SiCC-closo-1-CB₁₁H₁₁] and [Et₄N][7,12-(Me₃SiCC)₂-closo-1-CB₁₁H₁₀], respectively. Their thermal properties were examined by differential scanning calorimetry. The compounds were characterized by multi-NMR, IR, and Raman spectroscopy, (−)-MALDI mass spectrometry, and elemental analysis. Single-crystals of Cs[12-HCC-closo-1-CB₁₁H₁₁] and [Et₄N][7,12-(HCC)₂-closo-1-CB₁₁H₁₀] were studied by X-ray diffraction. The discussion of the spectroscopic and structural properties is supported by data derived from theoretical calculations using density functional theory as well as perturbation theory.     

N-arylammonio- and N-pyridinium-substituted derivatives of dodecahydro-closo-dodecaborate(2-)

We report two methods for preparing N-arylammonio, N-pyridyl and N-arylamino dodecaborates: heating of the tetrabutylammonium salt of dodecahydro-closo-dodecaborate(2-) with aryl and pyridyl amines, or nucleophilic attack of [closo-B₁₂H₁₁NH₂]²⁻ on a strongly deactivated aromatic system. With aryl amines we obtained [1-closo-B₁₂H₁₁N(R¹)₂C₆H₅]⁻ (R¹ = H, CH₃). With 4-(dimethylamino)pyridine, [1-closo-(B₁₂H₁₁NC₅H₄)-4-N(CH₃)₂]⁻, with a bond between the boron and the pyridinium nitrogen, was obtained. A presumable mechanism for this kind of reactions is reported. By nucleophilic substitution, two products, [1-closo-(B₁₂H₁₁NHC₆H₃)-3,4-(CN)₂]²⁻ and [1-closo-(B₁₂H₁₁NHC₆H₂)-2-(NO₂)-4,5-(CN)₂]²⁻, were formed with 4-nitrophthalonitrile and 1-chloro-2,4-dinitrobenzene gave [1-closo-(B₁₂H₁₁NHC₆H₃)-2,4-(NO₂)₂]²⁻. For [1-closo-B₁₂H₁₁N(CH₃)₂C₆H₅]⁻ and [1-closo-(B₁₂H₁₁NHC₆H₃)-2,4-(NO₂)₂]²⁻ single crystal X-ray structures were obtained.     

Acidic hydrolysis of N-acyl-1-substituted 3-amino-1,2-dicarba-closo-dodecaboranes

Acidic hydrolysis of N-acyl 1-methyl- and 1-phenyl-3-amino-1,2-dicarba-closo-dodecaboranes has been studied. It has been shown that acidic hydrolysis of diastereomeric amides of 1-methyl-3-amino-1,2-dicarba-closo-dodecaborane results in the partial racemization of the target 3-amino-1-methylcarborane. Under the similar conditions, the hydrolysis of N-acyl-3-amino-1-phenyl-1,2-dicarba-closo-dodecaboranes resulted in amide bond cleavage accompanied by simultaneous deboronation with the removal of boron atom at position 6 of carborane cage and formation of 7,8-dicarba-nido-undecaborane derivative according to ¹¹B and ¹H NMR spectroscopy.     

Synthesis of new charge-compensated cobalt bis(1,2-dicarbollide) derivatives

New hetero-substituted charge-compensated cobalt bis(1,2-dicarbollide) derivatives were synthesized by the reaction of 8,8′-μ-iodo-3-commo-3-cobalta-bis(1,2-dicarba-closo-dodecaborane) [8,8′-μ-I-3,3′-Co(1,2-C₂B₉H₁₀)₂] with 1,4-thioxane, pyridine N-oxide, and tetrahydropyran. X-ray diffraction studies showed that the 8′-iodo-8-(pyridiniumoxy)eucosahydro-1,1′,2,2′-tetracarba-3-commo-cobalta-closo-tricosaborate molecule has the gauche-conformation (the substituents are turned with respect to each other by 69.2°). The positive charge is predominantly localized on the N(Py) atom.     

The crystal and molecular structures of three germatrane derivatives

The molecular and crystal structure of three germatrane compounds: 1-(1-bromobicyclo[4.1.0]hept-7-yl)germatrane, l-(1-trimethylsilanyl-cyclopropyl)-germatrane and 1-(1-Methyl-1,2-dicarba-closo-dodecaboranyl)germatrane has been determined by X-ray diffraction method in order to investigate main geometrical regularities of the germatrane molecules. There were established all values of the main structural parameters above-mentioned molecules.     

Reaction of closo-dodecaborate anion B12H 12 2− with iminium salts

The reaction of the closo-dodecaborate anion with iminium salts was studied. The iminium salts were either formed in situ or introduced into the reaction in an individual state. Mono-and disubstituted products with an exopolyhedral boron-nitrogen bond were found to form. The structures of compounds 1,7-[NMe₂(CH₂Cl)]₂B₁₂H₁₀ and 1,7-[MeNPh(CH₂Cl)]₂B₁₂H₁₀ were determined using X-ray crystallography.     

Structure and properties of 1,2-, 1,7-, and 1,12-dicarba-closo-dodecaboranes(12): A quantum chemical study

The molecular and electronic structures and thermodynamic parameters of 1,2-, 1,7-, and 1,12-dicarba-closo-dodecaborane(12) molecules in the singlet ground state were calculated by the Hartree-Fock, DFT, and MP2 (including B3LYP/6-311G(2d,2p) and MP2/6-311+G(d,2p) methods). The energies and character of spatial localization of the frontier MOs in o-, m-, p-carboranes(12) correlate with the changes in the configuration stabilities and reactivities in the reactions of carboranes with electrophilic and nucleophilic agents and bases. The electrostatic potential distributions in the molecules and the atomic charge distribution of hydrogen atoms correlate with the known chemical properties of carboranes(12). The thermodynamic parameters of two isomerization reactions, o-C₂B₁₀H₁₂ ⇌ m-C₂B₁₀H₁₂ and m-C₂B₁₀H₁₂ ⇌ p-C₂B₁₀H₁₂, calculated for the temperature range 298–1000 K agree with experimental data within the limits of measuring error. The values of the electron density and the Laplacian of the electron density at the (3, −1) critical points of the B-H and C-H bonds correlate with the reactivities of the title compounds. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2074–2080, December, 2006.     

NMR study of 1-aryl-1,2-dicarba-closo-dodecaboranes: intramolecular C-H?O hydrogen bonding in solution

Dicarba-closo-dodecaborane(12) (carborane) has recently received much attention as a building block for supramolecular assemblies and bioactive compounds. Among carborane isomers, 1,2-dicarba-closo-dodecaborane(12) (o-carborane) has unique chemical properties, including the ability of the o-carborane C-H hydrogens to form hydrogen bonds. To evaluate intramolecular hydrogen bond formation between the o-carborane C-H hydrogen and various hydrogen bond acceptors in solution, we have designed and synthesized 1-aryl-o-carboranes 2. Intramolecular hydrogen bonding ability was evaluated by means of ¹H NMR measurement of the o-carborane C-H hydrogen signal of 2. The 1-(2-methoxyphenyl)-o-carborane derivative 2m appeared to form an intramolecular hydrogen bond between o-carborane C-H hydrogen and the oxygen atom acting as a hydrogen bonding acceptor. In this study, we present evidence for hydrogen bond formation in solution between the o-carborane C-H and hydrogen bond acceptors positioned with appropriate geometry.     

The synthesis of functional derivatives of the [1-CB9H10] anion by Brellochs reaction

Reactions of decaborane with various aldehydes in alkaline media were studied. The reactions with HCOH and 2-MeOC₆H₄CHO give the corresponding arachno-carboranes [6-R-arachno-CB₉H₁₃]⁻ (R = H, C₆H₄-2-OMe), whereas the reactions with C₆H₅CHO, 4-BrC₆H₄CHO, 4-MeCONHC₆H₄CHO, and 2-SC₄H₃CHO result in the nido-carboranes [6-R-nido-CB₉H₁₁]⁻ (R = C₆H₅, C₆H₄-4-Br, C₆H₄-4-NHCOMe, 2-SC₄H₃). Both the arachno- and nido-carboranes can be easily oxidized with elemental iodine in an alkaline aqueous solution giving the corresponding closo-derivatives [2-R-closo-2-CB₉H₉]⁻. These closo-2-isomers, under heating in solution, undergo rearrangement to more thermodynamically favorable closo-1-isomers [1-R-closo-1-CB₉H₉]⁻. The structure of (Bu₄N)[1-(4-BrC₆H₄)-1-CB₉H₉] was determined using single crystal X-ray diffraction.     

Synthesis and structure of 1-iodo-7-dioxonium-decahydro-closo-dodecaborate

Reaction of [B₁₂H₁₁I]^2? with 1,4-dioxane in presence of HBF₄ resulted in 1-iodo-7-(tetramethylene-(3-oxa)-oxonium)-decahydro-closo-dodecaborate regioselectively with good yield. Its structure, especially 1,7-isomery, was definitely proved by NMR and XRD methods. This novel compound has two reactive centers, perspective either for nucleophlic cleavage of dioxonium ring and/or for substitution of iodine in various cross-coupling reactions.     

Reactions of oxonium derivatives of [B12H12] with sulphur nucleophiles. Synthesis of novel B12-based mercaptanes, sulfides and nucleosides

Cleavage of oxonium derivatives of [B₁₂H₁₂]²⁻ by various S-nucleophiles was studied. Reactions of tetrabutylammonium 1-tetramethylene-(3-oxa)-oxonium-closo-undecahydrododecaborate with thiourea and a series of thiols resulted in novel B₁₂-based mercaptane, aliphatic, aromatic and heteroaromatic sulfides with high yields under mild conditions. This approach was also used for the preparation of novel guanosine derivative of [B₁₂H₁₂]²⁻.     

Syntheses of C-substituted icosahedral dicarbaboranes bearing the 8-dioxane-cobalt bisdicarbollide moiety

The treatment of 1,2-, 1,7- and 1,12-carbaborane lithiated isomers with [3,3′-Co-8-(CH₂CH₂O)₂-(1,2-C₂B₉H₁₀)-(1′,2′-C₂B₉H₁₁)] (1) at molar ratios 1:1 or 1:2 at room temperature in THF leads generally to the formation of a series of orange double-cluster mono and dianions. These were characterized by NMR and MS methods as [1′′-X-1′′,2′′-closo-C₂B₁₀H₁₁]⁻, [2]⁻; [1′′-X-1′′,7′′-closo-C₂B₁₀H₁₁]⁻, [3]⁻ and [1′′-X-1′′,12′′-closo-C₂B₁₀H₁₁], [4]⁻ for the monoanions, whereas [1′′,2′′-X₂-1′′,2′′-closo-C₂B₁₀H₁₀]²⁻, [2]²⁻; [1′′,7′′-X₂-1′′,7′′-closo-C₂B₁₀H₁₀]²⁻, [3]²⁻; and [1′′,12′′-X₂-1′′,12′′-closo-C₂B₁₀H₁₀]²⁻, [4]²⁻ for the dianions (where X = 3,3′-Co-8-(CH₂CH₂O)₂-(1,2-C₂B₉H₁₀)-1′,2′-(C₂B₉H₁₁)). Moreover, these borane-cage subunits can be easily modified via attaching variable substituents onto cage carbon and boron vertices, which makes these compounds structurally flexible potential candidates for BNCT of cancer and HIV-PR inhibition.     

3,3-Bis(3,5-dimethylpyrazol-1-yl)propionic acid: A tripodal N,N,O ligand for manganese and rhenium complexes - Syntheses and structures

The tripodal N,N,O ligands 3,3-bis(3,5-dimethylpyrazol-1-yl)propionic acid (Hbdmpzp) (1) and 3,3-bis(pyrazol-1-yl)propionic acid (Hbpzp) (2) form the “missing link” between the well-known bis(pyrazol-1-yl)acetic acids and related ligands with a longer “carboxylate arm”. To illustrate the reactivity of this ligand, manganese and rhenium complexes bearing the ligand bdmpzp are reported. The complexes are compared to related compounds bearing other tripod ligands such as bis(3,5-dimethylpyrazol-1-yl)acetate (bdmpza) and 3,3-bis(1-methylimidazol-2-yl)propionate (bmip). Spectroscopic and structural data are used as a basis for comparison, as well as DFT calculations. Both ligands 1 and 2 and the complexes fac-[Mn(bdmpzp)(CO)₃] (3) and fac-[Re(bdmpzp)(CO)₃] (4) were characterised by X-ray crystallography.