Versatile Self-Adapting Boronic Acids for H-Bond Recognition: From Discrete to Polymeric Supramolecules

Because of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest in using their aryl derivatives in materials science and supramolecular chemistry has risen. Nevertheless, their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of the first double-H-bonded DD·AA-type complexes of a series of aromatic boronic acids that adopt a syn-syn conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (K_a) of ortho-substituted boronic acids in solution showed that K_a for 1:1 association is in the range between 300 and 6900 M^-1. Crystallization of dimeric 1:1 and trimeric 1:2 and 2:1 complexes enabled an in-depth examination of these complexes in the solid state, proving the selection of the -B(OH)₂ syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho-substituted boronic acids result in "flat" complexes. On the other hand, sterically demanding analogues bearing ortho substituents strive to retain their recognition properties by rotation of the ArB(OH)₂ moiety, forming "T-shaped" complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. On the basis of the conformational dynamicity of the -B(OH)₂ functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double-H-bonding donor boronic acids could act as suitable organocatalysts or templates for the development of functional materials with tailored organizational properties.

Spectroscopic and Computational Investigations of The Thermodynamics of Boronate Ester and Diazaborole Self-Assembly

The solution phase self-assembly of boronate esters, diazaboroles, oxathiaboroles, and dithiaboroles from the condensation of arylboronic acids with aromatic diol, diamine, hydroxythiol, and dithiol compounds in chloroform has been investigated by (1)H NMR spectroscopy and computational methods. Six arylboronic acids were included in the investigations with each boronic acid varying in the substituent at its 4-position. Both computational and experimental results show that the para-substituent of the arylboronic acid does not significantly influence the favorability of forming a condensation product with a given organic donor. The type of donor, however, greatly influences the favorability of self-assembly. (1)H NMR spectroscopy indicates that condensation reactions between arylboronic acids and catechol to give boronate esters are the most favored thermodynamically, followed by diazaborole formation. Computational investigations support this conclusion. Neither oxathiaboroles nor dithiaboroles form spontaneously at equilibrium in chloroform at room temperature. Computational results suggest that the effect of borylation on the frontier orbitals of each donor helps to explain differences in the favorability of their condensation reactions with arylboronic acids. The results can inform the use of boronic acids as they are increasingly utilized in the dynamic self-assembly of organic materials and as components in dynamic combinatorial libraries.

Heats of Formation for the Boronic Acids R-B(OH)2 and Boroxines R3B3O3 (R=H, Li, HBe, H2B, H3C, H2N, HO, F, and Cl) Calculated at the G2, G3, and G4 Levels of Theory

Boronic acids (R-B(OH)₂) and their boroxine (R₃B₃O₃) dehydration products have emerged as important classes of compounds with a multitude of diverse applications. However, the available heats of formation for these compounds are not always as accurate as would be required for further use. In this study the heats of formation at 298.15 K of R-B(OH)₂ and R₃B₃O₃ (R = H, Li, HBe, H₂B, H₃C, H₂N, HO, F, and Cl) have been calculated at the G2, G3[G3B3], and G4 levels of theory and used to determine the enthalpy changes for the dehydration reactions: 3 R-B(OH)₂ → R₃B₃O₃ + 3 H₂O; comparisons are made with other rigorous levels of theory, e.g. CBS-Q[CBS-QB3] and W1U, as well as with experimental values wherever possible. Enthalpy changes for the dehydration reactions have also been calculated using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon correlation-consistent aug-cc-pVDZ and aug-cc-pVTZ basis sets, and B3LYP density functional theory with the 6-311++G(2df,2pd) basis set. With the exception of H₂N-B(OH)₂, the dehydration reactions are consistently predicted to be exothermic. Our results provide a cautionary note for the use of the B3LYP functional in the calculation of structures and energies of boronic acids and boroxines. Where comparisons could be made, the G4 and W1U predictions for the heats of formation of these boron compounds differ significantly.

Thermodynamics of boroxine formation from the aliphatic boronic acid monomers R-B(OH)2 (R = H, H3C, H2N, HO, and F): a computational investigation

Boroxines are the six-membered cyclotrimeric dehydration products of organoboronic acids, 3R–B(OH)2 → R3B3O3 + 3H2O, and in recent years have emerged as a useful class of organoboron molecules with applications in organic synthesis both as reagents and catalysts, as structural components in boronic-acid-derived pharmaceutical agents, and as anion acceptors and electrolyte additives for battery materials [Korich, A. L.; Iovine, P. M. Dalton Trans. 2010, 39, 1423−1431]. Second-order Møller–Plesset perturbation theory, in conjunction with the Dunning–Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets, was used to investigate the structures and relative energies of the endo–exo, anti, and syn conformers of the aliphatic boronic acids R–B(OH)2 (R = H, H3C, H2N, HO, and F), as well as the thermodynamics of their boroxine formation; single-point calculations at the MP2/aug-cc-pVQZ, MP2/aug-cc-pV5Z, and CCSD(T)/aug-cc-pVTZ levels using the MP2/aug-cc-pVTZ optimized geometries were also performed in selected cases. The endo–exo conformer was generally lowest in energy in vacuo, as well as in PCM and CPCM models of aqueous and carbon tetrachloride media. The values of ΔH(298)(0) for boroxine formation via dehydration from the endo–exo conformers of these aliphatic boronic acids ranged from −2.9 for (H2N)3B3O3 to +12.2 kcal/mol for H3B3O3 at the MP2/aug-cc-pVTZ level in vacuo; for H3B3O3, the corresponding values in PCM/UFF implicit carbon tetrachloride and aqueous media were +11.2 and +9.8 kcal/mol, respectively. On the basis of our calculations, we recommend that ΔHf(298K) for boroxine listed in the JANAF compilation needs to be revised from −290.0 to approximately −277.0 kcal/mol.

Reversible covalent chemistries compatible with the principles of constitutional dynamic chemistry: new reactions to create more diversity

An approach to make chemical diversity space more manageable is to search for smaller molecules, or fragments, and then combine or elaborate these fragments. Dynamic Combinatorial Chemistry (DCC) is a powerful approach whereby a number of molecular elements each with binding potential can be reversibly combined via covalent or noncovalent linkages to generate a dynamic library of products under thermodynamic equilibrium. Once a target molecule has been added, the distribution of products can be shifted to favor products that bind to the target. Thus the approach can be employed to identify products that selectively recognize the target. Although the size of the repertoire of reversible covalent reactions suitable for DCC has increased significantly over the past 5-10 years, the discovery of new reactions that satisfy all the criteria of reversibility and biocompatibility remains an exciting challenge for chemists. Increasing the number of chemical reactions will enable the engineering of larger and more diverse DCLs, which remains a key step toward a broader use of DCC. In this review, we aim to provide a nonexhaustive list of reversible covalent reactions that are compatible with the concept of DCC, focusing mainly on the most recent examples that were reported in the literature in the past 5 years.