UV-Photoelectron Spectroscopy of 1,2- and 1,3-Azaborines: A Combined Experimental and Computational Electronic Structure Analysis

Abstract

We present a comprehensive electronic structure analysis of structurally simple BN heterocycles using a combined UV-photoelectron spectroscopy (UV-PES) / computational chemistry approach. Gas-phase He I photoelectron spectra of 1,2-dihydro-1,2-azaborine 1, N-Me-1,2-BN-toluene 2, and N-Me-1,3-BN-toluene 3 have been recorded, assessed by density functional theory calculations, and compared with their corresponding carbonaceous analogues benzene and toluene. The first ionization energies of these BN heterocycles are in the order N-Me-1,3-BN-toluene 3 (8.0 eV) < N-Me-1,2-BN-toluene 2 (8.45 eV) < 1,2-dihydro-1,2-azaborine 1 (8.6 eV) < toluene (8.83 eV) < benzene (9.25 eV). The computationally determined molecular dipole moments are in the order 3 (4.577 Debye) > 2 (2.209 Debye) > 1 (2.154 Debye) > toluene (0.349 Debye) > benzene (0 Debye) and are consistent with experimental observations. The λmax in the UV-Vis absorption spectra are in the order 3 (297 nm) > 2 (278 nm) > 1 (269 nm) > toluene (262 nm) > benzene (255 nm). We also establish that the measured anodic peak potentials and electrophilic aromatic substitution (EAS) reactivity of BN heterocycles 1–3 are consistent with the electronic structure description determined by the combined UV-PES/computational chemistry approach.

UV-Photoelectron Spectroscopy of BN Indoles: Experimental and Computational Electronic Structure Analysis

“…35 On the other hand, the HOMOs of BN indoles I and II in this study do not appear to have higher energy levels than the natural indole. In fact, the HOMO of fused BN indole II is somewhat stabilized relative to natural indole according to the experimental gas-phase UV-PES analysis (Figure 1).…”

Boron‐Substituted 1,3‐Dihydro‐1,3‐azaborines: Synthesis, Structure, and Evaluation of Aromaticity

“…In a more direct comparison of diphenylether structures, we prepared 1,2-azaborine 9 [10] and determined its corresponding B-O distance, which is 1.392(1)Å (Figure 2). The weaker B-O bonding in 1,3-azaborines in comparison to 1,2-azaborines is consistent with the electrostatic potential map calculations [6] in which a highly delocalized intra-ring electron system renders the boron atom relatively less capable of accepting electrons from exocyclic substituents.Another striking feature of the solid-state structure of 8f is the short B-C(2) distance of 1.476(5) Å. [13,14] However, the crystallographically determined short B-C(2) distance is not predicted by gas-phase density functional theory (DFT) calculations at B3LYP/DZVP2 [15] level (B-C(2) = [1.516 Å]).…”

“…[6] In order to exploit the unique properties of this family of heterocycles in materials and biomedical applications, new synthetic tools need to be developed. Late-stage functionalization strategies are arguably the most efficient approach to generate an array of derivatives from an assembled BN heterocyclic core.…”

“…[5] On the other hand, we showed that treatment of 1 with acetic acid readily furnished the B-OAc-substituted product. [6] Encouraged by this preliminary finding, we envisioned that 1,3-azaborine 1 could be converted to B-substituted derivatives under acidic conditions. Scheme 2 illustrates that reaction of precursor 1 with 2 equiv.…”

“…With this strategy in mind, we focused our initial attention on developing B-OAc-substituted 1,3-azaborine 6 as a precursor for nucleophilic substitution reactions based on our prior success in using 6 and LiAlH 4 to produce N-Me-1,3-BN toluene. [6] However, treatment of 6 with the stronger base n-BuLi gave only a trace amount of desired substitution product (Scheme 4, top). For compound 6, we realized that in addition to the desired nucleophilic attack at boron (path a) two possible side reactions may be competitive with a strong nucleophile/base such as n-BuLi: 1) attack at the carbonyl carbon (path b) and 2) deprotonation (path c).…”