Ligand effects in the hydrogenation of methacycline to doxycycline and epi-doxycycline catalysed by rhodium complexes molecular structure of the key catalyst [closo-3,3-(η2,3-C7H7CH2)-3,1,2-RhC2B9H11]

Synthesis and Structural Characterization of p-Carboranylamidine Derivatives

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = ⁱPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C₂H₁₀B₁₀[C(NⁱPr)₂Li(DME)]₂ (2) and p-C₂H₁₀B₁₀[C(NCy)₂Li(THF)₂]₂ (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me₃SiCl) provided the silylated neutral bis(amidine) derivatives p-C₂H₁₀B₁₀[C{ⁱPrN(SiMe₃)}(=NⁱPr)]₂ (4) and p-C₂H₁₀B₁₀[C{CyN(SiMe₃)}(=NCy)]₂ (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Microwave-Assisted, One-Pot Synthesis of Doxycycline under Heterogeneous Catalysis in Water

The selective synthesis of active pharmaceutical molecules is a challenging issue, particularly when attempting to make the reactions even more sustainable. The present work focuses on the microwave-assisted hydrogenolysis of oxytetracycline to selectively produce α-doxycycline. Although the combination of microwave irradiation and a heterogeneous rhodium catalyst provided good conversions, the selective synthesis of active α-doxycycline was only achieved when an oxytetracycline-cyclodextrin complex was used as the starting material, giving the desired product at 34.0% yield in a one-step reaction under very mild conditions.

Synthesis and Structural Characterization of Two New Main Group Element Carboranylamidinates

Two new main group element carboranylamidinates were synthesized using a bottom-up approach starting from o-carborane, ortho-C2B10H12 (1, = 1,2-dicarba-closo-dodecaborane). The first divalent germanium carboranylamidinate, GeCl[HLCy] (3, [HLCy]− = [o-C2B10H10C(NCy)(NHCy)]−, Cy = cyclohexyl), was synthesized by treatment of GeCl2(dioxane) with 1 equiv. of in situ-prepared Li[HLCy] (2a) in THF and isolated in 47% yield. In a similar manner, the first antimony(III) carboranylamidinate, SbCl2[HLiPr] (4, [HLiPr]− = [o-C2B10H10C(NiPr)(NHiPr)]−), was obtained from a reaction of SbCl3 with 1 equiv. of Li[HLiPr] in THF (56% yield). The title compounds were fully characterized by analytical and spectroscopic methods as well as single-crystal X-ray diffraction. Both compounds 3 and 4 are monomeric species in the solid state, and the molecular geometries are governed by a stereo-active lone pair at the metal centers.

Three new types of transition metal carboranylamidinate complexes

Three new types of transition metal carboranylamidinate complexes are reported.

Synthesis and crystal structures of two new tin bis-(carboranylamidinate) complexes

Reaction of 2 equiv. of the lithium carboranylamidinate Li[o-(C2H10B10)C(NCy)(NHCy)] with SnCl2 in THF afforded the stannylene compound bis(N,N'-dicyclohexylamidinatocarboranate)tin(II), SnII[o-(C2H10B10)C(NCy)(NHCy)]2 (1). A similar reaction of SnCl4 with 2 equiv. of Li[o-(C2H10B10)C(N i Pr)(NH i Pr)] unexpectedly afforded the known solvated penta-chlorido-stannate(IV) salt [Li(THF)4][SnCl5(THF)] as the main reaction product. Small amounts of the new chlorido-tin(IV) bis-(carboranylamidinate) bis(N,N'-diiso-propylamidinatocarboranate)chloridotin(IV), SnIVCl[o-(C2H10B10)C(N i Pr)(NH i Pr)][o-(C2H10B10)C(N i Pr)2] (2), were isolated as a by-product. Single-crystal X-ray structure analysis revealed a κC,κN-chelating coordination of the carboranylamidinate ligands in both 1 and 2. The Sn atom in 1 adopts a pseudo-trigonal-bipyramidal coordination under participation of a stereoactive lone pair. In 2, a trigonal-bipyramidal coordination of Sn is completed by a chlorido ligand.

Novel inorganic heterocycles from dimetalated carboranylamidinates

Mono- and dianionic carboranylamidinates are readily available in one-pot reactions directly from o-carborane (1). In situ-monolithiation of 1 followed by treatment with N,N'-diisopropylcarbodiimide, (i)PrN=C=N(i)Pr, or N,N'-dicyclohexylcarbodiimide, CyN=C=NCy, provided the lithium carboranylamidinates (o-C2B10H10C(NH(i)Pr)(=N(i)Pr)-κ(2)C,N)Li(DME) (2a) and (o-C2B10H10C(NH(i)Cy)(=N(i)Cy)-κ(2)C,N)Li(THF)2 (2b). Controlled hydrolysis of 2a,b afforded the free carboranylamidines o-C2B10H11C(NH(i)R)(=N(i)R) (3a: R = (i)Pr, 3b: R = Cy). The first dimetalated carboranylamidinates, o-C2B10H10C(N(i)Pr)(=N(i)Pr)Li2(DME)2 (4a) (DME = 1,2-dimethoxyethane) and o-C2B10H10C(N(i)Pr)(=N(i)Pr)Li2(THF)4 (4b), were prepared in high yield (83% yield) directly from 1 using a simple one-pot synthetic protocol. Treatment of 4b with 2 equiv. of Me3SiCl afforded the disilylated derivative o-C2B10H10-κ(2)C,N-[C(N(i)PrSiMe3)(=N(i)Pr)]SiMe3 (5). Dianionic 4b also served as an excellent precursor for novel inorganic heterocycles incorporating the closo-1,2-C2B10H10 cage, including the unsymmetrical distannene [o-C2B10H10C(N(i)Pr)(=N(i)Pr)-κ(2)C,N]Sn=Sn[((i)PrN)2C(n)Bu]2 (6) and the azaphosphole derivative [o-C2B10H10C(N(i)Pr)(=N(i)Pr)-κ(2)C,N]PPh (7). Surprisingly, it was found that the synthesis of new inorganic ring systems from dianionic carboranylamidinates can also be achieved by employing only 1 equiv. of n-butyllithium in the generation of the anionic carboranylamidinate intermediates. Using this straightforward one-pot synthetic protocol, the Group 14 metallacycles [o-C2B10H10C(NCy)(=NCy)-κ(2)C,N]SiR2 (R = Cl (8), Me (9), Ph (10)) and [o-C2B10H10C(NCy)([=NCy)-κ(2)C,N]GeCl2 (11) have become accessible. The same synthetic strategy could be successfully adapted to prepare the corresponding Group 4 metallocene derivatives Cp2Ti[o-C2B10H10C(NCy)(=NCy)-κ(2)C,N] (12) and Cp2Zr[o-C2B10H10C(NCy)(=NCy)-κ(2)C,N] (13). The molecular structures of 2b, 3b, 4b, 5, 6, 7, 10, 12, and 13 were confirmed by single-crystal X-ray diffraction.

Synthesis and characterization of new fluorescent styrene-containing carborane derivatives: the singular quenching role of a phenyl substituent

A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1-Ph-1,2-C(2)B(10)H(11), 1-Me-1,2-C(2)B(10)H(11) and 1,2-C(2)B(10)H(12) with one equivalent of 4-vinylbenzyl chloride leads to the formation of compounds 1-3, whereas the reaction of the dilithium salt of 1,2-C(2)B(10)H(12) with two equivalents of 4-vinylbenzyl chloride gives disubstituted compound 4. The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1-4 were analyzed by X-ray diffraction. All compounds, except 1, display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second C(cluster) atom. In general, closo compounds 2-4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the C(cluster) results in an excellent electron-acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge-separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.

Synthesis and fluorescence emission of neutral and anionic di- and tetra-carboranyl compounds

A new family of photoluminescent neutral and anionic di-carboranyl and tetra-carboranyl derivatives have been synthesized and characterized. The reaction of α,α'-bis(3,5-bis(bromomethyl)phenoxy-m-xylene with 4 equiv. of the monolithium salt of 1-Ph-1,2-C(2)B(10)H(11) or 1-Me-1,2-C(2)B(10)H(11) gives the neutral tetracarboranyl-functionalized aryl ether derivatives closo-1 and closo-2, respectively. The addition of the monolithium salt of 1-Ph-1,2-closo-C(2)B(10)H(11) to α,α,'-dibromo-m-xylene or 2,6-dibromomethyl-pyridine gives the corresponding di-carboranyl derivatives closo-3 and closo-4. These compounds, which contain four or two closo clusters, were degraded using the classical method, KOH in EtOH, affording the corresponding nido species, which were isolated as potassium or tetramethylammonium salts. All the compounds were characterized by IR, (1)H, (11)B and (13)C NMR spectroscopy, and the crystal structure of closo-3 was analysed by X-ray diffraction. The carboranyl fragments are bonded through CH(2) units to different organic moieties, and their influence on the photoluminescent properties of the final molecules has been studied. All the closo- and nido-carborane derivatives exhibit a blue emission under ultraviolet excitation at room temperature in different solvents. The fluorescence properties of these closo and nido-derivatives depend on the substituent (Ph or Me) bonded to the C(cluster), the solvent polarity, and the organic unit bearing the carborane clusters (benzene or pyridine). In the case of nido-derivatives, an important effect of the cation is also observed.

High boron content carboranyl-functionalized aryl ether derivatives displaying photoluminescent properties

The reaction of alpha,alpha'-bis(3,5-bis(bromomethyl)phenoxy-p-xylene (3) with 4 equiv of the monolithium salt of 1-Ph-1,2-C2B10H11 or 1-Me-1,2-C2B10H11 gave the corresponding neutral carboranyl-functionalized aryl ether derivatives closo-4 and closo-5, respectively. These compounds contain four closo clusters that were degraded using basic conditions with KOH in EtOH, affording the corresponding nido-6 and nido-7 as potassium salts. Nido species were also isolated with tetramethylammonium as cation giving compounds nido-8 and nido-9 in good yield. The potassium salts showed good solubility in water and polar solvents. All these compounds were characterized by 1H, 11B and 13C NMR spectroscopy and UV-vis. The electronic data in different solvents indicated a solvatochromic shift for all compounds and a red shift of the absorption maxima for the nido species with respect to the closo derivatives. These neutral and anionic carboranyl-functionalized aryl ether derivatives represent a new family of high boron content luminescent compounds that show strong fluorescence emission in different solvents at room temperature. This phenomenon is very interesting considering the fact that none of the precursors have such a property. The fluorescence emission depends on the cluster substituent (Ph or Me) and the solvent polarity. Additionally, the fluorescence emission intensity was clearly dependent on the solvent polarity; the closo species showed strongest fluorescence intensities in the non-polar solvents, while anionic species were highly emissive in polar solvents.

Synthesis and investigation of the boron cluster anion [7-(2′-pyridyl)-7,8-nido-dicarbaundecaborate] and its protonated form

The structurally chiral [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-), [](-), anion was prepared by a partial degradation reaction of 1-(2'-pyridyl)-1,2-closo-C(2)B(10)H(11). From this anion a protonated specie, H[7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)] , and a tetramethylammonium salt, [NMe(4)][7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)], [NMe(4)][] can be obtained. The (1)H{(11)B} DNMR study on in the temperature range from 298 to 203 K identified the weakly basic nitrogen atom in the pyridine ring as the proton accepting site in solid state and low temperature and revealed pronounced weakening of the nitrogen-proton interaction while the temperature increases. Capillary electrophoresis and X-ray diffraction confirmed the pyridine nitrogen atom as the proton binding site. Separation of the electrophoretically pure racemic [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-) ion into two peaks by the chiral selector beta-cyclodextrine has been achieved.

Synthesis, NMR, and X-ray crystallographic analysis of C-hydrazino-C-carboxycarboranes: versatile ligands for the preparation of BNCT and BNCS agents and (99m)Tc radiopharmaceuticals

Protected hydrazine derivatives of ortho-, meta-, and para-carboranes were synthesized in good to excellent yields by reacting the mono-lithio salts of the respective carboranes with di-tert-butyl azodicarboxylate (DBAD). Subsequent deprotonation of the remaining carborane CH group, followed by the addition of CO(2)(g), resulted in the formation of bifunctional C-hydrazino-C-carboxycarboranes in good to excellent overall yields. Crystal structures of the monosubstituted ortho-carborane, 1-[(N,N'((tert-butyloxy)carbonyl)hydrazino)]-1,2-dicarba-closo-dodecaborane (8) [a = 21.213(6) A, b = 10.498(3) A, c = 9.866(2) A, alpha = gamma = 90 degrees, beta = 90.529(4) degrees ] and the bifunctional para-carborane 1-[(N,N'((tert-butyloxy)carbonyl)hydrazino)]-1,12-dicarba-closo-dodecaborane-12-carboxylic acid (3) [a = 12.744(10) A, b = 12.875(9) A, c = 14.767(9) A, alpha = beta = gamma = 90 degrees ] were obtained. Intermolecular hydrogen bonding was a dominant packing feature in both structures. The reported compounds represent a unique class of bifunctional carboranes that can be used in peptidomimetic research and as synthons to prepare novel radiopharmaceuticals and boron neutron capture therapy/boron neutron capture synovectomy (BNCT/BNCS) agents.