Rosa Santillan

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.

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 Ccluster (Cc), 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 ϕF = 82 and 94 %, whereas their Ph-o-carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the Cc-Cc 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 Cc.

Synthesis, characterization and nonlinear optical (NLO) properties of a push–pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10−3 a.u. and leads to a β value equal to 49.2 × 10−30 cm5 esu−1. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

Anisochronous Dynamics in a Crystalline Array of Steroidal Molecular Rotors: Evidence of Correlated Motion within 1D Helical Domains

We describe the solid-state dynamics of a molecular rotator (2) consisting of a p-phenylene rotor flanked by two ethynyl steroidal moieties that act as a stator. Single-crystal X-ray diffraction analysis of polymorph I revealed a packing motif containing 1D columns of nested rotors arranged in helical arrays (space group P3(2)) with the central phenylenes disordered over two sites related by an 85° rotation about their 1,4-axes. Unexpected line shapes in quadrupolar-echo (2)H NMR measurements between 155 and 296 K for the same polymorph with a deuterated phenylene isotopologue (2-d(4)) were simulated by trajectories involving fast (>10(8) s(-1)) 180° rotation (twofold flips) in each of the two rotationally disordered sites and slower exchange (2 × 10(4) to 1.5 × 10(6) s(-1)) between them. A negative activation entropy and a low enthalpic barrier for the slower 85° exchange are interpreted in terms of highly correlated processes within the 1D helical domains.

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.

Regioselective cleavage of rings E and F in sarsasapogenin

Sapogenins from the 25R and 25S series show a marked difference on the E/F regioselectivity of the spiroketal cleavage with BF(3)/Ac(2)O. In contrast to the high yield of single E-ring cleavage products from diosgenin (3) and hecogenin (5), sapogenins of the 25R series (equatorial C-27 methyl), sarsasapogenin (1, 25S series, axial C-27 methyl) yields the corresponding acetyldihydropyran, (25S)-23-acetyl-22,26-epoxy-5beta-cholest-22-ene-3beta,16beta-diyl diacetate (8), two isomeric furostenes: (E)- and (Z)-(25S)-23-acetyl-5beta-furost-22-ene-3beta,26-diyl diacetate (9 and 10) and a third one bearing an additional acetyl group: (E)-(20S,25S)-20,23-diacetyl-5beta-furost-22-ene-3beta, 26-diyl diacetate (11). The structures of the compounds were unambiguously established using two dimensional NMR techniques. The lower E/F selectivity in the cleavage of 1 is attributed to steric hindrance resulting from the axial methyl in F ring on a beta elimination forming the dihydropyran double bond in the major product 8.

A Versatile Methodology for the Controlled Synthesis of Photoluminescent High‐Boron‐Content Dendrimers

Fluorescent star-shaped molecules and dendrimers with a 1,3,5-triphenylbenzene moiety as the core and 3 or 9 carborane derivatives at the periphery, have been prepared in very good yields by following different approaches. One procedure relies on the nucleophilic substitution of Br groups in 1,3,5-tris(4-(3-bromopropoxy)phenyl)benzene with the monolithium salts of methyl and phenyl-o-carborane. The second method is the hydrosilylation reactions on the peripheral allyl ether functions of 1,3,5-tris(4-allyloxy-phenyl)benzene and 1,3,5-tris(4-(3,4,5-trisallyloxybenzyloxy)phenyl)benzene with suitable carboranyl-silanes to produce different generations of dendrimers decorated with carboranyl fragments. This approach is very versatile and allows one to introduce long spacers between the fluorescent cores and the boron clusters, as well as to obtain a high loading of boron clusters. The removal of one boron atom from each cluster leads to high-boron-content water-soluble macromolecules. Thermogravimetric analyses show a higher thermal stability for the three-functionalized compounds than for those containing 9 clusters. All compounds exhibit photoluminescent properties at room temperature under ultraviolet irradiation with high quantum yields; these depend on the nature of the cluster and the substituent on the C(cluster). Cyclic voltammetry indicates that there is no electronic communication between the core and the peripheral carboranyl fragments. Due to the high boron content of these molecules, we currently focus our research on their biocompatibility, biodistribution in cells cultures, and potential applications for boron neutron capture therapy (BNCT).

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.

Facile preparation of [4.4]metacyclophane- and [5.5]paracyclophane-type macrocycles from arylboronic acids and salicylideneaminoaryl alcohols

Four different salicylideneaminoaryl alcohols have been treated with arylboronic acids in order to prepare air-stable cyclophane-type macrocyclic systems. In two cases, this objective could be realized with the high-yield formation of [4.4]metacyclophane and [5.5]paracyclophane derivatives. The skeleton in these macrocycles is held together by two chiral boron atoms. In the other two cases, monomeric boronates or polymeric material were obtained. The title structures were characterized by spectroscopic techniques and X-ray crystallography. They show transannular C-H...O hydrogen bonding, but no intramolecular pi-pi interactions. A synthetic strategy for the preparation of further boron macrocycles is presented.