Trishomoaromatic (B₃N₃Ph₆) Dianion: Characterization and Two‐Electron Reduction

Abstract

Benzene, a common aromatic compound, can be converted into an unstable antiaromatic 8π‐electron intermediate through two‐electron reduction. However, as an isoelectronic equivalent of benzene, borazine (B3N3Ph6), having weak aromaticity, undergoes a totally different two‐electron reduction to afford (B3N3R6)2− homoaromatic compounds. Reported here is the synthesis of homoaromatic (B3N3Ph6)2− by the reduction of B3N3Ph6 with either potassium or rubidium in the presence of 18‐crown‐6 ether. Theoretical investigations illustrate that two electrons delocalize over the three boron atoms in (B3N3Ph6)2−, which is formed by the geometric and orbital reorganization and exhibits (π,σ)‐mixed homoaromaticity. Moreover, (B3N3Ph6)2− can act as a robust 2e reductant for unsaturated compounds, such as anthracene, chalcone, and tanshinones. This 2e reduction is of high efficiency and selectivity, proceeds under mild reaction conditions, and can regenerate neutral borazine.

From BN‐Naphthalenes to Benzoborole Dianions

“…However, to the best our knowledge, their reduction chemistry has been almost unexplored. Only very recently have Xi and co‐workers shown that reduction of Ph 6 B 3 N 3 six‐membered ring produced the corresponding trishomoaromatic dianion [14] . Recently, we reported the efficient synthesis of a series of BN‐naphthalene derivatives by a base‐promoted C−H borylation strategy [15] .…”

Reversible Single Electron Redox Steps Convert Polycycles with a C3P3 Core to a Planar Triphosphinine

“…Also, a number of heterocyclic derivatives of [C 6 R 6 ] 2− were prepared ( V – IX , Figure 1). [16–19] Due to their quinoid structure, V and VIII might be interpreted as the respective silicon‐ and boron‐substituted analogues of I – III . The other examples show structures which severely deviate from a planar ring as seen in the bicyclic diaza‐dipnictobicyclohexene VI , [20, 21] the tricyclic hexasilabenzene dianion VII [17] and the tetracyclic, trishomoaromatic species [B 3 N 3 Ph 6 ] 2− ( IX ) [19] .…”

“… [16–19] Due to their quinoid structure, V and VIII might be interpreted as the respective silicon‐ and boron‐substituted analogues of I – III . The other examples show structures which severely deviate from a planar ring as seen in the bicyclic diaza‐dipnictobicyclohexene VI , [20, 21] the tricyclic hexasilabenzene dianion VII [17] and the tetracyclic, trishomoaromatic species [B 3 N 3 Ph 6 ] 2− ( IX ) [19] . With the exception of VI and VII , all dianions shown in Figure 1 were prepared by two‐electron reduction of the respective neutral benzene derivative.…”