The 9H‐9‐Borafluorene Dianion: A Surrogate for Elusive Diarylboryl Anion Nucleophiles

Abstract

Double reduction of the THF adduct of 9H‐9‐borafluorene (1⋅THF) with excess alkali metal affords the dianion salts M2[1] in essentially quantitative yields (M=Li–K). Even though the added charge is stabilized through π delocalization, [1]2− acts as a formal boron nucleophile toward organoboron (1⋅THF) and tetrel halide electrophiles (MeCl, Et3SiCl, Me3SnCl) to form B−B/C/Si/Sn bonds. The substrate dependence of open‐shell versus closed‐shell pathways has been investigated.

Anionic Organoboranes: Delicate Flowers Worth Caring for

“…[36] Along the reaction sequence from 9 via Li 2 [9′] to Li 2 [9′′], the original dibenzo [g,p]chrysene-like scaffold is transformed into two mutually connected borafluorene moieties through a stepwise twofold Wagner-Meerwein-type rearrangement at the B 2 core. Importantly, the removal of one hydride ion from [9′′] 2restores a dibenzo [g,p]chrysene framework as it forms the H-bridged diborane (5) [37] In this way [10] is obtained in only moderate yields due to its pronounced sensitivity toward the Lewis acids used to abstract the hydride substituent from [9′′] 2-(see below). A targeted synthesis of [10] was accomplished through the notable deprotonation of its precursor borane 9 (Scheme 6; cf.…”

“…Following the model of the "Wanzlick equilibrium" between donor-substituted olefins and singlet carbenes, [47] diborane(4) dianions [Ar 2 B=BAr 2 ] 2can be viewed as formal dimers of still elusive free diarylboryl anions [: [37,40,48] Yet, early theoretical calculations on the model system Li 2 [H 2 B=BH 2 ] → 2 Li[:BH 2 ] ( 1 A 1 ) indicated prohibitively high dissociation energies. [49] The fundamental problem concerning singlet-state boryl anions is that they are electron-rich and -poor at the same time, because they have an excess of charge density in one B(sp 2 ) orbital and a shortage of electrons in the B(p z ) orbital.…”

“…[A] 2-). [37,56] It was envisaged that [15] 2would have the following advantages over [14] -: (i) a significantly lower steric shielding of the boron center due to the absence of four dangling Ph substituents and the bulky 1,3-dimesitylimidazolidin-2-ylidene base, (ii) a higher accumulation of negative charge onto the boron atom, because the H substituent, unlike the NHC, does not act as a π acceptor in the delocalization of electrons away from the boron center, and (iii) an easier removal of the hydride compared to the NHC ligand from the products of the aimed-for nucleophilic substitution reactions (see section 2). The dianion salts M 2 [15] (M = Li-K) have been prepared in essentially quantitative yields through the double reduction of the monomeric adduct 15•thf [57] with Scheme 8.…”

“…The key to success was the strict adherence to a carefully elaborated temperature regime during the synthesis process. [37]…”

“…[A] 2-) reacts with the electrophiles MeCl, Et 3 SiCl, Me 3 SnCl as a formal boron nucleophile (Li[16H]-Li[18H]; Scheme 10). [37] When working with excess MeCl, the reaction does not stop at the stage of the methyl(hydrido)borate Li[16H], but a second equivalent of…”