Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole

Abstract

Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C 4 B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acidÀbase adducts of the title compound with 3,5-lutidine, PCy 3 , N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared.

A theoretical study of the aromaticity in neutral and anionic borole compounds

“…In fact, it was found that [14] 2− is better described as a closed-shell singlet species with a bipolaron structure, in which the unpaired electrons on the borolyl units are paired across the thiophene bridge. [60] Again, when [14] 2− and [15] 2− are further reduced by two electrons resulting in tetraanionic borole derivatives, the aromatic character of each borole is further increased, as was previously found for dianionic monoborole species (vide supra). [9b,61] The same happens for the hexaanionic [12] 6− species.…”

“…In addition, we have investigated corresponding NICS values for the negatively charged derivatives. As previously reported by the groups of Herberich, [52] Yamaguchi [18] and Braunschweig, [15,17,[53][54][55] the synthesis and isolation of various dianionic and a monoanionic species has been successfully achieved ( Figure 4). The addition of one electron noticeably decreases the antiaromatic character of boroles, as judged by both the NICS and multicenter indices.…”

“…[51] Other significant differences are found by comparison of transition-metal-containing mono-and bis(borole)s. For the borolyl-functionalized ferrocenes, [58] The (anti)aromatic character of anionic derivatives of bisand tris(borole)s seems to follow the same trend as for the above-discussed mono(borole)s (Figure 10). In other words, the anionic [5] 2− , [14] 2− , and [15] 2− (calculated as open-shell, triplet state molecules), which have one extra electron in each borolyl unit, are less antiaromatic than their neutral counterparts. In fact, it was found that [14] 2− is better described as a closed-shell singlet species with a bipolaron structure, in which the unpaired electrons on the borolyl units are paired across the thiophene bridge.…”

“…Since the first report of a stable borole by the group of Eisch in 1969, a pentaphenylsubstituted borole (1), [4] several other aryl [5][6][7] and heteroaryl ring substituents [8] have been found to be effective in the stabilization of borole compounds, thereby enabling detailed investigations of their chemical and physical properties. [9] Nevertheless, isolable boroles are typically highly reactive species and follow various pathways to reduce their antiaromaticity, namely by activation of H−H and Si−H bonds, [10][11][12] [4+2] cycloaddition reactions, [13,14] adduct formation with Lewis bases [15,16] or one-and two-electron reductions to form the corresponding radical anions and dianions, respectively. [17,18] Besides their rich reactivity profile, boroles display chromophoric properties, i.e.…”

Lewis Acid–Base Adducts of 1‐Mesityl‐ and 1‐Chloro‐2,3,4,5‐tetraphenylborole

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Challenges to synthesis, structural and photophysical characterization of boroles, as well as their rich and often unexpected reactivity can be ascribed to the inherent antiaromatic destabilization of boroles. Typical reaction types that cause disruption to the antiaromatic π system include cycloaddition reactions [8,12,21,25] and the activation of small molecules such as H 2 . [22] The essential strategy to isolate a borole molecule is steric and electronic shielding of the backbone in the C 4 B annulus.…”