Victor Barba

New perspectives for boronic esters in macrocyclic chemistry

In the present contribution a tetrameric macrocyclic compound derived from 2,6-pyridinedimethanol and 3-nitrophenyl boronic acid, as well as 10 new dimeric boronates prepared from 2-salicylideneaminoethanol and different aryl boronic acids such as a 2-methylphenyl-, 3-methylphenyl-, 4-methylphenyl-, 3-methoxyphenyl-, 4-methoxyphenyl-, 3-chlorophenyl-, 4-chlorophenyl-, 3-nitrophenyl-, 3-trifluoromethylphenyl- and 4-fluorophenylboronic acid are described. The tetrameric and three of the dimeric structures have been analyzed by X-ray crystallography, and a series of parameters such as bond length, bond angles, deviation of the boron atom from the boronate mean plane and intermolecular interactions are discussed.

Boron-nitrogen macrocycles: a new generation of calix[3]arenes.

The simple condensation reaction of 3,5-di-tert-butyl salicylaldehyde and 3-aminophenylboronic acid leads to a trimeric macrocyclic compound. The ability of this molecule to include small organic molecules was in a first approximation analyzed by (1)H NMR spectroscopy and X-ray crystallography.

New boron macrocycles based on self-assembly of Schiff bases

Abstract In order to investigate the formation of macrocyclic boronates, we carried out the reaction of 3-aminophenyl boronic acid with salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxybenzophenone. In all cases, macrocyclic structures were formed and the X-ray crystallographic study of the trimeric complex derived from 2-hydroxyacetophenone showed that the macrocyclic system is asymmetric.

Synthesis and molecular structures of dimeric boron compounds

A series of eleven new dimeric boron chelates: [B(SAE)(m-NH2C6H4)]2 (3a), [B(SAE)(p-CF3C6H4)]2 (3b), [B(SAE)(pCHOC6H4)]2 (3c), [B(SAE)(p-BrC6H4)]2 (3d), [B(SAE)(p-EtOC6H4)]2 (3e), [B(SAE)(m-CH3COC6H4)]2 (3f), [B(SAE)(pCH3COC6H4)]2 (3g), [B(SAE)(o-F, p-FC6H3)]2 (3h), [B(SAE)(m-F, p-FC6H3)]2 (3i), [B(SAE)(m-F, m-FC6H3)]2 (3j) and [B(SAE)(o-F, o-FC6H3)]2 (3k), prepared by condensation of 2-salicylideneamino hydroxyethane (H2SAE) with arylboronic acids such as amino-, trifluoromethyl-, bromo-, formyl-, acetyl-, ethoxy- and difluoro-boronic acid allowed a study of the influence of different substituents in the B-phenyl moiety on coordination of the boron atom. In all cases, dimeric derivatives were formed and the molecular structures for [B(SAE)(p-CH3OC6H4)]2 (2e), 3b, 3d and 3h were established by X-ray diffraction analysis where the values of the intramolecular (NB) bond lengths confirmed a donor‐acceptor character.

Two new complexes of phenylboronic acid with N-salicylidene-4-aminobutanol

Abstract The tridentate ligand N-salicylidene-4-aminobutanol (6) was obtained by condensation of salicylaldehyde with 4-aminobutanol and reacted with phenylboronic acid to afford either the dimeric structure, bis[μ-[2-[[(4-hydroxy-κO)butyl]imino-κN]methyl-phenolato(2−)-κO]]diphenyldiboro (7) or the monomeric derivative, 2-phenylbenzo[j]-8-aza-1,3-dioxa-2-boracycloundeca-8-ene (8), depending on the reaction conditions. The monomeric structure is a [5.4.0] heterobicycle boronate and has been analyzed by X-ray crystallography.

Structural relationship between a host included chain of spirocyclic water hexamers and bulk water – the role of water clusters in self assembly and crystallization processes

Infinite chains of spirocyclic water hexamers are included in the crystal lattice of a tin complex with a curved, hydrophobic surface and only weak intermolecular bonding interactions between the host molecules, so that the enclosed water clusters might be reminiscent of the solvation sphere in solution.

A new oxo-bridged borate derived from dimeric boron complexes

Reaction of 2-salicylideneamino hydroxyethane (H2SAE) with B(NMe2)3, B(OH)3 and C6F5B(OH)2 leads to the dimeric boron compounds [B(SAE)(NMe2)]2 (2), [B(SAE)(OH)]2 (3) and [B(SAE)(C6F5)]2 (4) in moderate to good yields. Compound 2 hydrolyses slowly at room temperature (r.t.) to give the dimeric oxo-bridged boron compound m-oxo-[B(SAE)]2 (5). Dehydration of 3 or hydrolysis of 4 with elimination of the aryl group conduced to the same derivative. The X-ray crystallographic analysis confirmed the molecular structure for compound 5.

X-ray crystallographic study of three (N→B)-borinates prepared from 8-hydroxyquinoline and 2-hydroxypyridine

Abstract8-Hydroxyquinoline and 2-hydroxypyridine have been reacted with diphenylborinic acid or 9-BBN; the molecular structure of the resulting heterocycles has been studied by X-ray crystallography. A structural comparison of the so formed five- and six-membered heterocycles with similar complexes obtained from aliphatic amino alcohol and α-amino acid derivatives shows significant differences for the N→B, B−O and B−C bond lengths and some of the inner cycle bond angles. Other structural parameters discussed in this respect are the sum of bond lengths at the boron atom, the sum of bond angles in the heterocycle and the tetrahedral character of the boron atom. On the basis of these parameters a qualitative comparison of heterocycle stability is possible.

Designed synthesis of “L” shaped 17-halo-aryl-ethynyl steroids

Thirteen steroidal derivatives were synthesized through a Sonogashira cross-coupling reaction which has been found to be an excellent synthetic strategy to introduce halo-aromatic groups into C-17-ethynyl substituted steroidal frameworks. The structural modification was performed on ethynylestradiol, mestranol, norethisterone, ethisterone and 3-ethynyl-3-epi-sarsasapogenin. The solid state study by X-ray diffraction showed that most of them belong to the orthorhombic P212121 space group and the whole family has an “L” conformation, regardless of the nature of the steroid A-ring (3-hydroxy-aromatic or 3-oxo). Due to the presence of several moieties which are susceptible to forming secondary interactions, the crystalline packing is governed by O–H⋯O, C–H⋯O, and C–H⋯π interactions, and only 17α-(4′-iodophenylethynyl)-3-methoxy-estra-1,3,5(10)-trien-17-β-ol (mestranol derivative 3) showed an iodine–iodine interaction (dI⋯I = 4.116 A). The crystalline packing for ethynylestradiol derivatives 1, 2 and 4 showed the formation of holes with diameters greater than 5.2 A suggesting their potential application in host guest chemistry or as porous materials.

Dimeric boronates derived from the reaction of schiff bases and boronic acids

A sintese one-pot de complexos dimericos de boro e descrita. Os compostos foram obtidos pela reacao de bases de Schiff (ligantes tridentados) com acido trans-b-fenilvinilboronico, acido 3-tiofeninoboronico e acido metilboronico. A construcao das estruturas dimericas e favorecida pela presenca de ligacoes de coordenacao N® B intramoleculares, resultando na formacao de aneis heterociclicos de dez membros. A analise cristalografica de raio X de um deles confirmou a natureza dimerica desses compostos.