Boramidine: A Versatile Structural Motif for the Design of Fluorescent Heterocycles

Removing Neighboring Ring Influence in Monocyclic B-OH Diazaborines: Properties and Reactivity as Phenolic Bioisosteres with Dynamic Hydroxy Exchange

The design of small molecules with unique geometric profiles or molecular connectivity represents an intriguing yet neglected challenge in modern organic synthesis. This challenge is compounded when emphasis is placed on the preparation of new chemotypes that have distinct and practical functions. To expand the structural diversity of boron-containing heterocycles, we report herein the preparation of novel monocyclic hemiboronic acids, diazaborines. These compounds have enabled the study of a pseudoaromatic boranol-containing (B-OH) ring free of influence from an appended aromatic system. Synthetic and spectroscopic studies have provided insight into the aromatic character, Lewis acidic nature, chemical reactivity, and unique ability of the exocyclic B-OH unit to participate in hydroxy exchange, suggesting their use in organocatalysis and as reversible covalent inhibitors. Moreover, density functional theory and nucleus-independent chemical shift calculations reveal that the aromatic character of the boroheterocyclic ring is increased significantly in comparison to known bicyclic benzodiazaborines (naphthoid congeners), consequently leading to attenuated Lewis acidity. Direct structural comparison to a well-established biaryl isostere, 2-phenylphenol, through X-ray crystallographic analysis reveals that N-aryl derivatives are strikingly similar in size and conformation, with attenuated logP values underscoring the value of the polar BNN unit. Their potential application as low-molecular-weight scaffolds in drug discovery is demonstrated through orthogonal diversification and preliminary antifungal evaluation (Candida albicans), which unveiled analogs with low micromolar inhibitory concentration.

Axially-chiral boramidine for detailed (chir)optical studies

The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence (Φ up to 94%), and ca. |10-3| gabs and glum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin-orbit coupling between the lowest singlet excited state and an upper triplet state.

Benchmarking the dynamic luminescent properties and UV stability of B18H22-based materials

The dynamic photoluminescent properties, and potential quenching mechanisms, of anti-B18H22, 4,4’-Br2-anti-B18H20, and 4,4’-I2-anti-B18H20 are investigated in solution and polymer films. UV stability studies of the neat powders show no decomposition...

Synthesis of Fluorinated Aminoalkylboronic Acids from Amphoteric α-Boryl Aldehydes

Our ongoing search for underdeveloped functional group combinations has brought to light α-fluorinated aminoalkylboronic acids, a new class of molecules featuring the B-CF linkage. These compounds can now be generated from secondary amines and α-boryl aldehydes through electrophilic fluorination of boryl enamines or enamides. Fluorinated β-aminoalkylboronic acids show no signs of degradation under ambient conditions. We present evidence for the involvement of chair-like motifs, favored over the acyclic forms by up to 1.7±0.1 kcal mol^-1 in water and held together by an amine-boronate hydrogen bond. Fluorinated β-aminoalkylboronic acids are stable over a wide pH range and are characterized by a pK_a of 3.4, which is the lowest of any alkylboronic acid.

Secondary ligands and the intramolecular hydrogen bonds drive photoluminescence quantum yields from aminopyrazine coordination polymers

Here the role of secondary ligands and their hydrogen bonding patterns in determining the photoluminescence quantum yields of aminopyrazine (ampyz) coordination polymers was probed.