Synthesis of a borepin

Assessment of the aromaticity of borepin rings by spectroscopic, crystallographic and computational methods: a historical overview

“…in 1955 to describe B ‐ethanolamino‐10,11‐dihydrodibenzo[ b , f ]borepin ( 9 ), the first example of a heterocyclic ring involving only boron and carbon atoms, but lacking the third pair of π‐electrons required for aromaticity . To synthesize the first formally aromatic borepin, the anhydride of 9 was treated with N ‐bromosuccinimide and then sodium methoxide in methanol leading to the intermediate B ‐hydroxydibenzo[ b , f ]borepin ( 10 ), which was isolated as B ‐ethanolaminodibenzo[ b , f ]borepin ( 11 ) by van Tamelen et al . They also suggest that they may have synthesized the B ―H dibenzoborepin as an unstable pyridine complex on the basis of an absorption band in the infrared region, at 2326 cm −1 assigned to the B―H stretch .…”

“…In aromatic systems like benzene, the diamagnetic anisotropy deshields the protons on the exterior of the ring, and the relative difference in chemical shift can be used as a tool to evaluate the strength of this ring current in carbocyclic systems. Fusion with other aromatic rings was initially required to stabilize the borepin moiety, but recently larger fused polycyclic aromatics are being revisited for their intriguing properties as π‐electron materials . This report will survey the historical milestones leading to our current understanding of the formally aromatic seven‐membered boron‐containing ring.…”

3-Phenyl-3-benzoborepin, a carbon-boron heterocycle with aromatic character

“…The dibenzo derivative (III) has been synthesized by Van Tamelen et a1. (4), but these authors do not make clear the extent of aromatic stabilization in the central ring.…”

Prospecting in Hückel‐space: From Hinokitiol to Non‐benzenoid Organic Electronics

“…Our rationale for this fairly specific substructure was that the flanking benzo fragments would be easier to modify with substituents necessary for extended molecular manipulation in such a way as to promote extended intramolecular electronic conjugation. At the onset of this work, we only had the synthetic guidance of Eugene van Tamelen's 1960 synthesis of oxidized and presumably solvent coordinated B‐ hydroxy dibenzoborepins (Figure a: the learnéd chemists in Baltimore might refer to this as a “natty B ‐OH”) . As we were completing our initial studies, Warren Piers reported the synthesis of benzo‐ and naphtho‐fused borepins in the context of blue light‐emitting materials (Figure b) .…”