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. 2020 Jan 22;59(14):5621–5625. doi: 10.1002/anie.201914219

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

Jannik Gilmer 1,†, Hendrik Budy 1,†, Thomas Kaese 1, Michael Bolte 1, Hans‐Wolfram Lerner 1, Matthias Wagner 1,✉
PMCID: PMC7155136  PMID: 31834978

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.

Keywords: boron, boryl anions, nucleophilic substitution, radical reactions, umpolung

Boron nucleophile: The 9H‐9‐borafluorene dianion behaves as a boron‐centered nucleophile with minimal steric shielding. The boron‐bonded hydrogen atom can subsequently be abstracted from the primary products, thereby rendering the title compound a masked diarylboryl anion.

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The quest for boron‐centered nucleophiles remains a major challenge, but is rewarded with fundamentally new opportunities for element‐organic synthesis. Three‐coordinate, hypovalent boron compounds are electrophiles. Polarity inversion can, in principle, be achieved through the addition of two electrons into the vacant boron pz orbital to generate an electron lone pair. Yet, this approach is impeded by the low electronegativity of the boron atom.1 Thus far, the arguably most versatile strategy to circumvent this problem relies on the in situ generation of nucleophilic boron species from tetra(organyloxy)diboranes(4), B2(OR)4: Upon addition of suitable Lewis bases, monoadducts are formed in which the B(sp3)−B(sp2) bond is polarized in such a way as to render the sp2 boryl unit a masked nucleophilic [B(OR)2]− equivalent.2, 3, 4 The combined −I and +M effects of the oxygen atoms decisively stabilize the [B(OR)2]− fragment—ultimately to an extent that allowed for the structural characterization of the corresponding magnesium boryl complexes by Hill and co‐workers.5 Until today, the system B2(OR)4/Lewis base has been developed into a broadly applicable tool for the preparation of boronic acid esters,6 which serve as key building blocks in organic synthesis.7

Also the important field of organic optoelectronic materials increasingly demands for novel organoboranes, and specifically for boron‐doped polycyclic aromatic hydrocarbons (B‐PAHs) to be applied as electron acceptors or light emitters.8, 9, 10 Here, nucleophilic boron precursors would be equally desirable, but now they should be devoid of π‐donating heteroatom substituents so that the boron atom can unfold its full capacity as an electronically perturbing element in the final products. The first firmly established, heteroatom‐free boryl anion intermediate was the base‐stabilized species Li[BH2(PCy3)] (Cy=cyclohexyl), an isoelectronic analogue of the well‐known phosphonium ylides.11 A related N‐heterocyclic carbene adduct, Li[BH2(NHC)], was described by Lacôte and co‐workers,12 while the Bertrand group reported on a cyclic‐(alkyl)(amino)carbene‐supported dicyanoboryl anion, [B(CN)2(CAAC)]−.13 Willner and Finze recently isolated alkali metal salts of the nucleophilic tricyanoborate dianion, [B(CN)3]2−.1 Here, the flow of negative charge from the doubly occupied boron pz orbital into the antibonding CN orbitals counterbalances the electropositive character of the boron center.14

Despite recent progress in the field of boryl nucleophiles, examples of diarylboryl anions ([BAr2]−), which would be the building blocks of choice for the synthesis of B‐PAHs, are still scarce: Eisch and co‐workers claimed that UV irradiation of Na[BPh4] generated [BPh2]− via the reductive elimination of biphenyl,15 but the existence of the free anion remains subject to debate.16, 17 In 2019, Yamashita and co‐workers disclosed that the tetra(o‐tolyl)diborane(4) dianion behaves as a dimer of [B(oTol)2]−.18 One year earlier, our group had postulated the intermediate formation of compound [A]− (Figure 1), which, depending on the reaction conditions, either undergoes boron insertion into one of the methyl C−H bonds or nucleophilic substitution of MeI to afford 2 equiv of 9‐methyl‐9‐borafluorene.19

Figure 1.

Figure 1

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The postulated intermediate [A]− containing a Lewis‐acid‐stabilized boryl anion,19 the isolable anion [B]− with a boron‐bonded NHC ligand,20 and the borafluorene dianion [1]2− (this work). Carbon atoms marked with asterisks bear tBu substituents.

In both instances, the actual reactive species would be the 9‐borafluorenyl anion, [BFlu]−, a cyclized derivative of [BPh2]− (consider the conceptual relationship between [A]− and the above‐mentioned B2(OR)4/Lewis base system). In compound [A]−, the [BFlu]− moiety is stabilized through adduct formation of its electron lone pair with a Lewis acidic 9‐methyl‐9‐borafluorene molecule. As an alternative option, [BFlu]− could be tamed through adduct formation of its boron pz orbital with a suitable Lewis base. Precedence exists in the form of Braunschweig's tetraphenylborolyl–NHC complex [B]− (37 % yield),20, 21 and we thus wondered whether the 9‐borafluorenyl dianion [1]2− (Figure 1), a formal hydride adduct of [BFlu]−, can also be made accessible on a preparative scale. We envisage two advantages of [1]2− over [B]− as a nucleophilic building block: 1) Its molecular framework is fully conjugated, and the boron center is not sterically shielded, given that an H− ion is the smallest possible ligand. 2) After reaction with an electrophile El−X, the hydride substituent should be easily removable from the primary intermediate [El(H)BFlu]− to generate three‐coordinate ElBFlu, thereby rendering [1]2− a true synthetic equivalent of the thus far elusive [BFlu]−.

Herein, we report high‐yielding syntheses of M2[1] salts (M=Li, Na, K) and their use as boron nucleophiles for establishing B−B and B−C/Si/Sn bonds. We also investigated key mechanistic issues regarding the formation and closed‐shell versus open‐shell reactivity of M2[1].

The synthesis of M2[1] is based on the two‐electron reduction of parent 9H‐9‐borafluorene (1), which is stable in monomeric form as its THF adduct 1⋅THF (Figure 2 a).22, 23, 24 Treatment of 1⋅THF in THF with excess alkali metal M for one to two days led to a color change from pale yellow to dark green (M=Li, Na) or dark red (M=K). The isolation of the corresponding salts M2[1] was achieved through simple removal of unconsumed alkali metal and evaporation of the liquid phase under vacuum. The purity of the products, which were formed in essentially quantitative yields, was confirmed by 1H, 11B, and 13C{1H} NMR spectroscopy; Li2[1] (Supporting Information) and Na2[1] (Figure 3 a) were structurally characterized by X‐ray crystallography.

Figure 2.

Figure 2

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a) The neutral THF adduct 1⋅THF and the anions [1H]−, [2]2−, and [3]2−. b) 1H NMR spectra ([D8]THF) of the reaction products obtained from mixtures of 1⋅THF and excess sodium metal at room temperature (top), at −78 °C (1 h)→room temperature (1 d; middle), and at −78 °C (1 h; bottom). Carbon atoms marked with asterisks bear tBu substituents.

Figure 3.

Figure 3

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a) Molecular structure of [Na(thf)3][Na(thf)][1] in the solid state. Hydrogen atoms (except BH) and tBu groups are omitted for clarity. b) Differences between selected (averaged) bond lengths [Å] of [Na(thf)3][Na(thf)][1] and 1C. c) LUMO of 1C in two different orientations. d) 11B NMR shifts of Na2[1] and BrBFlu (green); differences between selected 13C NMR shifts [ppm] of Na2[1] ([D8]THF) and BrBFlu (C6D6). Carbon atoms marked with asterisks bear tBu substituents.

The key to success is to maintain a temperature of −78 °C during the first hour of the reaction in order to avoid the formation of unwanted side products, primarily [1H]− and [3]2− (Figure 2 a and b, top vs. middle).25 Why is an initial low temperature of such critical importance, given that further stirring of the respective mixture at room temperature for a much longer time span is still required to drive the reaction to completion?

To answer this question, we will exemplarily refer to the sodium species. We first note that immediately after reaching room temperature, the reduction mixture already contains appreciable amounts of Na2[1], together with the B−B‐bonded dimer Na2[2]26 as the major product (Figure 2 b, bottom). Precedence exists for an analogous intramolecular reductive B−B coupling reaction between two methylene‐bridged borafluorene moieties.27 In a first control experiment, we confirmed that also an intermolecular variant is possible by mixing equimolar amounts of Na2[1] and 1⋅THF at −78 °C (Scheme 1).

Scheme 1.

Scheme 1

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Synthesis of Na2[2] from Na2[1] and 1⋅THF; reductive cleavage of Na2[2] to give Na2[1]; hydride abstraction from Na2[2] with 1⋅THF and skeletal rearrangement to furnish Na[3H] and Na[1H]. Carbon atoms marked with asterisks bear tBu substituents. Conditions: i) THF, −78 °C, 1 h; ii) [D8]THF, room temperature, 21 h; iii) [D8]THF, room temperature, 1 h.

NMR spectroscopy subsequently revealed the quantitative conversion into Na2[2],28 which was isolated in 98 % yield and structurally characterized by X‐ray crystallography (see the Supporting Information). In a second control experiment, Na2[2] was stirred in a glovebox with sodium metal in [D8]THF. NMR monitoring of the reaction progress over 21 h showed the gradual reductive cleavage of the B−B bond to ultimately furnish 2 equiv of Na2[1], which unveils the role of Na2[2] as the key intermediate on the way from 1⋅THF to Na2[1] (see the work of Kinjo29 for a comparable B−B bond cleavage reaction). In a third control experiment, Na2[2] was treated at room temperature with a substoichiometric amount of the masked Lewis acid 1⋅THF. The mixture subsequently contained two new components: Na[1H] and Na[3H] (Scheme 1).26 It has been shown previously that M[3H] reacts with alkali metals M to afford the diborene M2[3] (see Figure 2 a).24, 30 We therefore propose that the hydride transfer between Na2[2] and 1⋅THF paves the way to the formation of the side products generated in the room‐temperature reduction of 1⋅THF. The sole purpose of the low‐temperature step is therefore to reductively quench the Lewis acidity of 1⋅THF under conditions where its reaction with Na2[2] is kinetically prohibited.

With the doubly reduced 9H‐9‐borafluorene [1]2− in hand, we were then able to tackle long‐standing questions regarding the electronic structure of the central C4B ring in its dianionic (formally aromatic) and neutral (formally antiaromatic) states.31 The pristine system is particularly valuable because here perturbations originating from steric or electronic influences of the boron‐bonded substituent are not an issue (consider the π back‐bonding contribution from B to NHC in K[B]20). In lieu of the experimentally inaccessible monomeric 1,22, 23 we will rely on the computed structure 1C (B3LYP/TZVP) for comparison with the (averaged) bond lengths of Na2[1] (Figure 3 a). Two‐electron injection into neutral 9H‐9‐borafluorene significantly changes its heavy‐atom scaffold (Figure 3 b). The pattern of bond‐length elongations and contractions matches perfectly with the nodal structure of the LUMO of 1C, which becomes doubly occupied upon reduction (Figure 3 c): 1) Na2[1] shows enhanced bond‐length alternations within its benzene rings; specifically, the peripheral rims adopt a butadiene‐like character. 2) The B−C bonds in Na2[1] (avg. 1.537 Å) are shorter by 0.022 Å than those of 1C, which is consistent with an increased double‐bond character in the dianion. Moreover, the difference between the length of the C2−C12 bond minus the averaged C1−C2/C11−C12 bonds amounts to Δ=−0.059 Å in Na2[1] and +0.066 Å in 1C. The absolute values are similar, but the relative bond orders are inverted. Taken together, these structural features indicate that the two added electrons are not constrained to the boron pz orbital, but to some extent distributed over the entire π‐electron system of Na2[1]. Somewhat surprisingly in view of the aromatic character of [B]− (Figure 1),20 the bond‐length alternation in the central C4B ring of [1]2− does not support the view of a cyclic delocalized “Clar sextet” within this substructure, but rather points toward an allylic CBC fragment in combination with a C2−C12 bond of increased double‐bond character. Accumulation of negative charge on the benzene rings of Na2[1] also becomes evident from the 13C NMR spectrum, which shows that all except one of the C(sp2) atoms are significantly shielded (Figure 3 d). 2,7‐Di‐tert‐butyl‐9‐bromo‐9‐borafluorene (BrBFlu) was used as a substitute for the uncharged reference system 1 because BrBFlu is the only derivative of trigonal‐planar 2,7‐di‐tert‐butyl‐9‐borafluorene, for which 13C{1H} NMR data have been recorded in a non‐donor solvent (C6D6) and fully assigned.23

Notwithstanding the evidence of charge delocalization in Na2[1], an upfield‐shifted 11B NMR signal at 3.9 ppm suggests that considerable π‐electron density remains accumulated at the boron center. Thus, [1]2− has the potential of acting as a π nucleophile, and a first experiment in this direction has already proven that a mixture of Na2[1] and 1⋅THF furnishes the B(sp3)−B(sp3) species Na2[2] in quantitative yield (see above). Conceptually related examples of targeted B−B bond‐forming reactions are scarce and restricted to heteroatom‐stabilized boryl nucleophiles, such as Nozaki's and Yamashita's boryllithium32, 33 or (in situ generated) pinacolatoboryl anions,34 and [B(CN)3]2−.35 B−B coupling between Na2[1] and 1⋅THF can take place either in a closed‐shell fashion (as for Finze's K2[B(CN)3]/K[FB(CN)3] system)35 or by comproportionation within an encounter complex constituted by the electron‐rich [1]2− and the electron‐poor 1 (see Power's claim in a related case).36 In fact, the Na2[2] species transiently generated during the synthesis of Na2[1] may also result from the dimerization of two boranyl radicals Na[1].

In order to further assess the scope of [1]2− as a surrogate of the nucleophilic boryl anion [BFlu]−, reactions of Li2[1] with MeCl, Et3SiCl, and Me3SnCl were performed (THF, room temperature). According to NMR analysis, quantitative conversion occurred to afford the B−Me‐ and B−SiEt3‐bonded species Li[1Me] and Li[1SiEt3],25 respectively (Scheme 2). It was next confirmed that the hydride ligand of Li[1Me] can be easily abstracted with excess MeCl or Me3SiCl in a one‐pot reaction to furnish the free borane 4. We take this as a proof of principle that [1]2− can indeed be used as a [BFlu]− equivalent in the synthesis of triorganoboranes. The stannylated hydroborate Li[1SnMe3] was formed to approximately 80 %, together with Li[1H] and Sn2Me6 (Scheme 2). These side products raise the question as to whether the apparent nucleophilicity of [1]2− is, at least in part, a consequence of radical reactivity (as has been observed for the system [B]−/Me3SnCl21): Single‐electron transfer from [1]2− to Me3SnCl yields the [1].− and [Me3Sn]. radicals, which can either recombine in the solvent cage to generate Li[1SnMe3] or undergo radical‐pair separation with subsequent homocoupling (Sn2Me6) and H‐atom abstraction (Li[1H]).

Scheme 2.

Scheme 2

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Synthesis of Li[1Me], Li[1SiEt3], Li[1SnMe3], and Li[1C2H4CH=CH2] from Li2[1] and MeCl, Et3SiCl, Me3SnCl, and 4‐chloro‐1‐butene, respectively; hydride abstraction from Li[1Me] to give the free borane 4. Reaction of Li2[1] with (bromomethyl)cyclopropane to form Li[1C2H4CH=CH2]. Carbon atoms marked with asterisks bear tBu substituents.

The assumption of a possible radical pathway gains further support from the observation that the reaction of Li2[1] with MeI (30 % conversion into Li[1Me]) is far less selective than that with MeCl; for MeI, radical reactivity has been reported.37 Moreover, treatment of Li2[1] with 1 equiv of the fast radical clock (bromomethyl)cyclopropane gave mainly the ring‐opened olefin Li[1C2H4CH=CH2] together with Li2[2] and small amounts of Li[1H], with all three products indicating radical intermediates.38 An authentic sample of Li[1C2H4CH=CH2] was synthesized from Li2[1] and 4‐X‐1‐butene (X=Cl, Br); similar to the case of MeX, only the organochloride gave a high conversion into Li[1C2H4CH=CH2] (75 % by NMR analysis).

In summary, we have disclosed the first example of a fully conjugated diarylboryl ([BFlu]−) anion, stabilized by a hydride ligand ([1]2−). The steric hindrance of the boron center is minimal, and [1]2− straightforwardly reacts with organoboranes or tetrel halides to generate B−B/C/Si/Sn bonds. The hydride ligand of the thereby obtained methyl(hydro)borate [1Me]− can be easily abstracted, thus rendering [1]2− a true surrogate of the elusive [BFlu]− nucleophile. The extent of closed‐shell versus open‐shell reactivity of [1]2− depends on the substrate: Evidence for radical intermediates has been found for the heavier halogen leaving groups (Br, I) and electrophilic tetrel centers (Sn).

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

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J. Gilmer, H. Budy, T. Kaese, M. Bolte, H.-W. Lerner, M. Wagner, Angew. Chem. Int. Ed. 2020, 59, 5621.

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Supplementary

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