Aryl Trihydroxyborate Salts: Thermally Unstable Species with Unusual Gelation Abilities

Protodeboronation of (Hetero)Arylboronic Esters: Direct versus Prehydrolytic Pathways and Self-/Auto-Catalysis

The kinetics and mechanism of the base-catalyzed hydrolysis (ArB(OR)₂ → ArB(OH)₂) and protodeboronation (ArB(OR)₂ → ArH) of a series of boronic esters, encompassing eight different polyols and 10 polyfluoroaryl and heteroaryl moieties, have been investigated by in situ and stopped-flow NMR spectroscopy (¹⁹F, ¹H, and ¹¹B), pH-rate dependence, isotope entrainment, ²H KIEs, and KS-DFT computations. The study reveals the phenomenological stability of boronic esters under basic aqueous-organic conditions to be highly nuanced. In contrast to common assumption, esterification does not necessarily impart greater stability compared to the corresponding boronic acid. Moreover, hydrolysis of the ester to the boronic acid can be a dominant component of the overall protodeboronation process, augmented by self-, auto-, and oxidative (phenolic) catalysis when the pH is close to the pK_a of the boronic acid/ester.

A novel symmetrically multifunctionalized dodecamethoxy-cycloparaphenylene: synthesis, photophysical, and supramolecular properties

In the present study, we report the synthesis of a novel symmetrically multifunctionalized cycloparaphenylene (CPP), dodecamethoxy-[9]CPP, through nickel-mediated macrocyclization and subsequent reductive aromatization reactions.

Base-Catalyzed Aryl-B(OH)2 Protodeboronation Revisited: From Concerted Proton Transfer to Liberation of a Transient Aryl Anion

Pioneering studies by Kuivila, published more than 50 years ago, suggested ipso protonation of the boronate as the mechanism for base-catalyzed protodeboronation of arylboronic acids. However, the study was limited to UV spectrophotometric analysis under acidic conditions, and the aqueous association constants (K_a) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C₆H_nF_(5-n)B(OH)₂ with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH-rate profiles, pK_a and ΔS^⧧ values, kinetic isotope effects (²H, ¹⁰B, ¹³C), linear free-energy relationships, and density functional theory calculations, we have identified a mechanistic regime involving unimolecular heterolysis of the boronate competing with concerted ipso protonation/C-B cleavage. The relative Lewis acidities of arylboronic acids do not correlate with their protodeboronation rates, especially when ortho substituents are present. Notably, 3,5-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar pK_a.

Synthesis of new dithia[3.3]parapara- and metapara-cyclophane based tectons: toward an universal surface-confined 2D/3D molecular binding motif

A series of new paraphenylene-based 2D and 3D tectons has been designed for supramolecular self-assembly on both HOPG and Au. Several versatile chemical strategies have been developed to reach the target tectons, bearing either metaparacyclophane or paraparacyclophane cores, functionalized or not, allowing the obtention soon of more complex 3D tectons bearing functional unit such as chromophores. Moreover, preliminary STM results show that these compounds can successfully self-assemble both on HOPG and Au(111) substrates at liquid-solid interface, encouraging us in finding an universal surface-confined 2D/3D molecular binding motif. This feature combined with the preliminary results of the self-assembly on the plasmonic substrate Au(111) open-up opportunities in the field of Nanoscience.

Effect of pH and oxygen atom of the hydrophobic chain on the self-assembly property and morphology of the pyridyl boronic acid based amphiphiles

The surface activity and aggregation behavior of two synthesized boronic acid based anionic surfactants, sodium salt of 2-dodecyl pyridine-5-boronic acid (SDDPB) and sodium salt of 2-oxydodecyl pyridine-5-boronic acid (SODDPB), were studied in buffer solution at pH 9 and 13 containing carbohydrates. The self-assembly formation was investigated by use of a number of techniques including surface tension, conductivity, fluorescence spectroscopy, dynamic light scattering, X-ray diffraction, and transmission electron microscopy (TEM). Both of the amphiphiles exhibit a single break in the surface tension vs log(concentration) plots, indicating existence of one critical aggregation concentration. Steady state fluorescence spectroscopy was used to determine the polarity indexes using pyrene and the rigidity of the microenvironments of the aggregates using 1,6-diphenyl-1,3,5-hexatriene (DPH) as fluorescence probe molecules. The pKa's of both amphiphiles were determined in buffer solutions of different pH's. XRD studies were performed to shed light on the morphology of the self-assemblies. TEM micrographs revealed the existence of vesicles for both the amphiphiles in buffer solution of pH 9, but at pH 13, TEM pictures indicate the existence of closed vesicles in SDDPB solution and at concentrated solution the vesicles are fused to form sponge-like micelles. After aging the vesicular solution of pH 13 of SDDPB, the closed vesicles are destroyed. In contrast, for SODDPB at pH 13, TEM pictures suggest the existence of spherical and complex micelles in solution which were further transformed into crystal-like structure upon aging. The average hydrodynamic radii were determined by dynamic light scattering measurement. Therefore, for the first time, we have successfully synthesized two new surfactants containing pyridyl-boronic acid as a headgroup which shows remarkable tuning of morphology in two different pH's and in the presence of two different carbohydrates.

Geminal imidazolium salts: a new class of gelators

The gelling behavior of some geminal diimidazolium salts was investigated in solvents differing in polarity and hydrogen bond donor ability. The used salts, namely the 3,3'-di-n-decyl-1,1'(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(decim)(2)][Br](2) (1), the 3,3'-di-n-dodecyl-1,1'(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(dodecim)(2)][Br](2) (2), and the 3,3'-di-n-dodecyl-1,1'(1,4-phenylenedimethylene)diimidazolium ditetrafluoroborate [p-Xyl-(dodecim)(2)][BF(4)](2) (3), differ in the alkyl chain length and in the anion properties, such as size, shape, and coordination ability. In all cases in which gelation process was observed, the obtained gels were characterized by gel melting temperature determination, resonance light scattering, and UV-vis measurements. On the whole, the investigation allowed to get information about both the thermodynamic stability and the features of the aggregates characterizing the soft materials at the equilibrium. Data collected by us point out that the used organic salts are able to behave as both hydro- and organogelators. In particular, bromide salts formed hydrogels in the presence of α-cyclodextrin allowing to hypothesize that the gelation process is favored by the formation of supramolecular assemblies. To verify this hypothesis, 1D and 2D (1)H NMR measurements were carried out. Both the alkyl chain length and the anion ability to reticulate the three-dimensional network proved to be determinant factors in affecting the gelation process as well as the features of the gel phases. Finally, with the future aim to use the obtained gels as reaction media, the effect of a guest molecule such as the UV-vis active probe Nile Red was studied.