The 9<i>H</i>‐9‐Borafluorene Dianion: A Surrogate for Elusive Diarylboryl Anion Nucleophiles

Gilmer, Jannik; Budy, Hendrik; Kaese, Thomas; Bolte, Michael; Lerner, Hans‐Wolfram

doi:10.1002/ange.201914219

Angewandte Chemie

2020

DOI: 10.1002/ange.201914219

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

Jannik Gilmer

Hendrik Budy

Thomas Kaese

et al.

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.

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Cited by 19 publications

References 48 publications

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Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

Prey

Wagner

2020

Adv Synth Catal

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Certain electron‐rich 1,4‐diborabenzene derivatives efficiently activate single, double, and triple bonds and thereby increasingly compete with transition metals in homogeneous catalysis. This review compares the activation of three model substrates (H2, H2C=CH2, CO2) by (i) 9,10‐dihydro‐9,10‐diboraanthracene dianions, (ii) their neutral carbene‐stabilized congeners, (iii) 1,3,2,5‐diazadiborinines, and (iv) 1,4,2,5‐diazadiborinines. Distinct structure‐properties relationships become apparent, the most influential factors being (i) the steric demands of the B‐bonded substituents, (ii) the charges on the B‐doped (hetero)arenes, (iii) charge polarization as a result of additional N‐doping, and (iv) the energies and nodal structures of the frontier orbitals. The observed reactions are explained by a transition metal‐like activation mechanism. If the two boron atoms are chemically inequivalent, contributions of a B(+I)/B(+III) mixed‐valence state determine the observed regioselectivities when polar substrates are added. The lessons learned from the conversions of the model substrates are subsequently used to rationalize the behavior of the B2 heterocycles also toward more sophisticated substrate molecules. Finally, catalytic cycles based on H2‐ and H−‐transfers, hydroboration reactions, and CO2 reductions will be covered.

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Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

Prey

Wagner

2020

Adv Synth Catal

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B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

2020

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The tetraaryl μ‐hydridodiborane(4) anion [2H]− possesses nucleophilic B−B and B−H bonds. Treatment of K[2H] with the electrophilic 9‐H‐9‐borafluorene (HBFlu) furnishes the B3 cluster K[3], with a triangular boron core linked through two BHB two‐electron, three‐center bonds and one electron‐precise B−B bond, reminiscent of the prominent [B3H8]− anion. Upon heating or prolonged stirring at room temperature, K[3] rearranges to a slightly more stable isomer K[3 a]. The reaction of M[2H] (M+=Li+, K+) with MeI or Me3SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me3Si). Thus, [2H]− behaves as a masked [:BFlu]− nucleophile. The HBFlu by‐product was used in situ to establish a tandem substitution‐hydroboration reaction: a 1:1 mixture of M[2H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[2H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R2BFlu].

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B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO₂, Isocyanates, or Carbodiimides

et al. 2021

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The intriguing (m-hydrido)diboranes(4) with their prominent pristine representative [B 2 H 5 ] À have mainly been studied theoretically.W en ow describe the behavior of the planarizedt etraaryl (m-hydrido)diborane(4) anion [1H] À in cycloaddition reactions with the homologous series of heterocumulenes CO 2 ,i PrNCO,a nd iPrNCNiPr.W es how that a C = Obond of CO 2 selectively activates the B À Bbond of [1H] À , while the m-H ligand is left untouched( [ 2H] À ). The carbodiimide iPrNCNiPr,i nc ontrast, neglects the BÀBb ond and rather adds the B-bonded H À ion to its central Ca tom to generate aformamidinate bridge across the B 2 pair ([3] À ). As ah ybrid, the isocyanate iPrNCO combines the reactivity patterns of both its congeners and gives two products:o ne of them ([4H] À )isrelated to [2H] À ,the other ([5] À )isananalog of [3] À .W ef inally propose am echanistic scenario that rationalizes the individual reaction outcomes and combines them to ac oherent picture of B-B vs.B -H bond activation.

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

Cited by 19 publications

References 48 publications

Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO₂, Isocyanates, or Carbodiimides

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

Cited by 19 publications

References 48 publications

Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO2, Isocyanates, or Carbodiimides

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Product

Resources

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B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO₂, Isocyanates, or Carbodiimides