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

Abstract

Electron‐deficient borole compounds exhibit a pronounced Lewis acidity that is enhanced due to their antiaromatic character so that even weak donors datively coordinate to form Lewis acid–base adducts. This contribution presents the synthesis and structural characterization of Lewis acid–base adducts formed by the reaction of 1‐mesityl‐2,3,4,5‐tetraphenylborole and 4‐picoline as well as 1‐chloro‐2,3,4,5‐tetraphenylborole with various donors. The new compounds are characterized by means of multinuclear NMR spectroscopy and single‐crystal X‐ray diffraction techniques and compared to related systems.

Synthesis of Zwitterionic Cobaltocenium Borate and Borata‐alkene Derivatives from a Borole‐Radical Anion

“…Inclusion of a boron atom into an antiaromatic conjugated ring system, as was realized in 1‐ H ‐borole derivatives, further reduces the electron density at boron to generate potent Lewis acids 1–2. The electrophilic nature of boroles is illustrated by formation of Lewis adducts with a wide variety of donor molecules 3–10. Furthermore, the low‐energy LUMO of boroles allows reduction processes, which are associated with the vacant p z orbital at boron 11…”

Preparation and Characterization of a π‐Conjugated Donor–Acceptor System Containing the Strongly Electron‐Accepting Tetraphenylborolyl Unit

“…The presence of an intact Lewis acidic center was probed by addition of the Lewis base IMes (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene) . Stoichiometric addition of the carbene to 1 in toluene was accompanied by an immediate decoloration of the solution and gave rise to a new 11 B NMR signal at δ ( 11 B)=−8.1 ppm, indicative of a tetracoordinate boron species and in line with the chemical shift values of other borole Lewis acid–base adducts . Adduct 2 was isolated as a pale‐yellow solid in 78 % yield.…”

“…[8] Results and Discussion Synthesis Accesst ot he thiophene-bridged donor-acceptor system 1 was achieved via tin-boron exchange of 1,1-dimethyl-2,3,4,5tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole, [9] which afforded the product in 70 %y ield after filtration andw ashing with n-hexane (Scheme 1). The successful formation of the borole ring was readily confirmed by its broad 11 BNMR resonance at d( 11 B) = 58.5 ppm. [6d] Heteronuclear and two-dimensionalN MR experiments (HSQC and HMBC) as well as high-resolution mass spectrometry further supported the molecularc omposition of 1.T he presence of an intact Lewis acidic centerw as probedb ya ddition of the Lewis base IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).…”

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

“…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.…”