Boryl Anions

Besides their well established role in organic synthesis as hydro- and diboration reagents, organoboranes have recently received much attention owing to their interesting chemical and electronic properties. This development raises the need for new alternative synthetic routes to boron-containing molecular systems. Borylation reactions commonly rely on reactants that provide a formal R2B+ moiety and are therefore restricted to substrates that readily accept an intrinsically electrophilic boron center. Hence, a complementary synthon, providing a nucleophilic boron center would be highly desirable. This long-standing goal in boron chemistry however is not easily achieved. Self-evident attempts to obtain so called boryl anions by reduction of (diorganyl)haloboranes with alkali metals are usually hampered by the formation of highly reactive radical species which lead to the formation of unwanted insertion or hydrogen-abstraction products.[1]

In 2006 Nozaki and Yamashita solved that Problem by reduction of a diazaborole, yielding a stable three-coordinate boryl anion (1). The elusive boryl anion was made accessible by using the typical N-heterocyclic carbene (NHC)-style backbone in connection with sterically demanding aryl groups effectively protecting the reactive site. In 2008 the linear boryl anions [{(η5-C5H4R)(CO)2Mn}2B] (R = H, Me) (2) were synthesized in our group by treatment of a bridging chloroborylene with Li.[2] Unlike 1, these boryl anions are linear at the boron center (sp-hybridization is assumed) and fully separated from their counter ions. The two latest additions to this interesting class of compounds are 3, which was obtained - also in our group - by reduction of a NHC-stabilized 1-chloroborole[3] and 4 which is inferred as an intermediate in the reduction of a NHC-iodoborane adduct with lithium di-tert-butylbiphenylide and in situ quenching.

Fig. 1: Boryl Anions.

The reactivity of the boryl anions 2 and 3 is a current focus of our research. Treatment of 2 with methyliodide hinted at its nucleophilicity,[2] and with NHC complexes of group 11 metal chlorides, substitution reactions at the metal centre were observed, leading to trimetalloboranes 6 and 7a,b.[4]

Furthermore, the reaction of 2 with [Pt(PCy3)2] reveals the electrophilic nature of 2 yielding an anionic, base-stabilized metalloborylene (8).[5]

Fig. 2: Reactivity of 2.

For the synthesis of 3, the SIMes (1,3-dimesitylimidazolidin-2-ylidene) adduct of 1-chloro-2,3,4,5-tetraphenyl-borole (10) was reduced with excess KC8 to give the monoanionic derivative 3 in 37% yield. For probing the nucleophilicity of the boron atom, compound 3 was treated with an excess of MeI in Et2O at ambient temperature. After workup, the NHC-coordinated 1-methyl-2,3,4,5-tetraphenylborole (11) was isolated as a colorless solid in 71% yield.[3] Being the first anion of its kind, this remarkable compound received great attention in several highlight articles (Nature, Angew. Chem., Treffpunkt Forschung).

Fig. 3: Synthesis and Reactivity of 3.

In comparison with the neutral boroles, the intraannular CC bond-length alteration within the C4B rings is significantly decreased. The BC1 and BC4 bonds are also shorter than those observed in neutral boroles. In fact, the structural features of 3 within the borole ring resemble those of borole dianions, thus emphasizing the aromatic character of the C4B ring in 3.
To better understand the electronic structure of 3, computations of the model complex 3' were performed with DFT methods at the B3LYP/6-31g* level of theory. Complex 3', in which the mesityl groups are replaced by 2,6-dimethylphenyl groups, was optimized as a monomer in the absence of the potassium ion coordination. The energy-optimized geometry of 3' is in good agreement with that determined experimentally, thus indicating that the critical bonding features are captured in the model complex. Examination of the electron density distribution of the HOMO of 3' indeed reveals a p-like bonding orbital between the boron and carbene carbon center with significant contribution from boron atom (14.6%), suggesting the presence of a π-nucleophilic boron atom (Fig. 4). This finding also implies the existence of a considerable π-back bond contribution from boron to carbon to the overall borole-carbene bonding interaction.

Fig. 4: Depiction of the HOMO of 3'.

Selected references:

  1. H. Braunschweig, Angew. Chem. 2007, 119, 19901992; [pdf] Angew. Chem. Int. Ed. 2007, 46, 19461948. [pdf]
  2. H. Braunschweig, M. Burzler, R. D. Dewhurst, K. Radacki, Angew. Chem. 2008, 120, 5732–5735; [pdf] Angew. Chem. Int. Ed. 2008, 47, 5650–5653. [pdf]
  3. H. Braunschweig, C.-W. Chiu, K. Radacki, T. Kupfer, Angew. Chem. 2010, 122, 2085–2088; [pdf] Angew. Chem. Int. Ed. 2010, 49, 2041–2044 (VIP). [pdf]
  4. H. Braunschweig, P. Brenner, R. D. Dewhurst, M. Kaupp, R. Müller, S. Östreicher, Angew. Chem. 2009, 121, 9916–9919; [pdf] Angew. Chem. Int. Ed. 2009, 48, 9735–9738. [pdf]
  5. H. Braunschweig, K. Kraft, S. Östreicher, K. Radacki, F. Seeler, Chem. Eur. J. 2010, 16, 10635–10637 (VIP). [pdf]

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