Boron‐ versus Nitrogen‐Centered Nucleophilic Reactivity of (Cyano)hydroboryl Anions: Synthesis of Cyano(hydro)organoboranes and 2‐Aza‐1,4‐diborabutatrienes

Abstract Cyclic alkyl(amino)carbene‐stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron‐centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano‐nitrogen‐centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2‐aza‐1,4‐diborabutatrienes.


Introduction
As anionic boron analogues of carbenes, boryl anions ([R 2 B] À ) long remained in the realm of computational curiosities. [1][2][3] While the transient formation of [Ph 2 B] À upon photolysis of [Ph 4 B] À salts had been deduced from trapping reactions as early as 1967, [4] these claims were later disputed. [5][6][7] Although Curran, Fensterbank, Malacria and Lacôte demonstrated in 2010 that Lewis base-stabilized parent boryl anions [L!BH 2 ] À can indeed be generated in situ by the reduction of L!BH 2 I, and trapped with electrophiles, these species are too thermodynamically unstable to be isolated. [8][9] The first isolable boryl anion, compound I (Figure 1), was reported in 2006 by Yamashita and Nozaki, making use of the same features that help stabilize N-heterocyclic carbenes (NHCs). [10] The anionic boron center of I is incorporated within a five-membered ring, electronically stabilized by the two adjacent nitrogen π donors and sterically shielded by the bulky Dip groups (Dip = 2,6-diisopropylphenyl), while complexation to the lithium cation provides further thermodynamic stability. [11,12] Diazaborolyl anions such as I have since been used as powerful Brønsted bases, reducing agents and boron nucleophiles to access a plethora of new boron-element bonds. [13][14][15][16][17][18][19] In the last decade the available repertoire of stable boryl anions has been steadily increasing. In 2010 our group reduced a NHC-stabilized bromoborole to the corresponding aromatic borolyl anion, II, which reacts as a boron nucleophile towards MeI. [20] Taking further inspiration from NHCs, the electronic properties of which can be fine-tuned by changes in the heterocycle, Kinjo and co-workers designed the 1,2,4,3-triazaborol-3-yl lithium complex III, which is capable of coupling two CO molecules to a diboraalkene and cyclizing arylisonitriles to 2-boranylindole derivatives. [21] The reactivity of I was further increased by synthesizing the mixed lithium-potassium salt of the diazaborolyl anion dimer, IV, which is capable of deprotonating benzene, [22] or the dimeric potassium salt V, in which both anionic boron centers are "naked" and promote facile CÀ H activation at a pendant Dip-isopropyl group. [23] While boryl anions I-V are accessed by reduction of haloboron precursors, Bertrand and co-workers showed that the BÀ H bond of a cyclic alkyl(amino)carbene (CAAC)-stabilized (dicyano)hydroborane undergoes an "umpolung" due to the electron-withdrawing nature of the cyano ligands, which enables the synthesis of the corresponding boryl anion VI by deprotonation. [24] The negative charge at boron in VI is further stabilized by π backdonation to the π-acidic CAAC ligand. Using the same umpolung principle, Finze and co-workers deprotonated [BH(CN) 3 ] À to the corresponding dianion [B(CN) 3 ] 2À , [25] while Hörner and Frank synthesized stable NHC analogues of VI, such as the nonameric potassium salt VII. [26] The latter was shown to react as a boron-centered nucleophile with a variety of organic, main group and transition metal electrophiles.
The divergent reactivity of 2 a, both as a soft boron-and as a hard nitrogen-centered nucleophile, prompted us to undertake further investigations into the reactivity of (cyano) hydroboryl anions with electrophiles. In this contribution we present the synthesis of monomeric and dimeric CAACstabilized (cyano)hydroboryl anions and show their divergent reactivity towards a wide range of organic electrophiles and haloboranes, leading to fully unsymmetrically-substituted cyano (hydro)boranes and the first examples of 2-aza-1,4-diborabutatrienes (i. e. LHB=C=N=BR 2 ), respectively.
Unlike in 2 a, the lithium cation in 2 b is terminally bound to the cyano substituent and stabilized by three THF molecules. The boron center is trigonal planar (Σ(ffB) = 359.8(2)°) while the B1À C1 bond length of 1.450(5) Å is similar to that of 2 a (1.447(3) Å) [27] and denotes a B=C double bond, indicating strong π backdonation from the boryl anion lone pair into the empty p orbital at the CAAC ligand. Due to the high Brønsted basicity of the borylene all attempts to recrystallize 2 b in higher yields from hexanes resulted in partial hydrolysis back to 1 b and crystallization of the cubane-type cluster 3 b (Scheme 2b, Figure 2). Four [(CAAC Cy )BH(CN)Li] units generate the central Li 4 N 4 cube of C 2v symmetry, the opposing lithium apices of which are stabilized by two (CAAC Cy )BH 2 (CN) units coordinating as nitrogen donors, and two π interactions with adjacent Dip substituents, respectively. The structure of 3 b is reminiscent of that of [(PhC=N)Li(NC 5 H 5 )] 4 , which also crystallizes as a tetramer with a cubic Li 4 N 4 core and pyridine-stabilized lithium apices. [28] In order to isolate 2 b in high yields and analytical purity its tetramethylethylenediamine (TMEDA) analogue 2 b-TMEDA was synthesized by the addition of TMEDA to crude 2 b (Scheme 2c), or alternatively by deprotonating 1 b in hexanes at À 35°C in the presence of TMEDA (Scheme 1d).

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Full Paper doi.org/10.1002/chem.202101025 similar to that of 2 a. The main difference with 2 a is that the borylene plane in 2 b-TMEDA is not coplanar with the central Li 2 N 2 unit but rotated by ca. 70°, presumably due to steric repulsion between the TMEDA ligands and the CAAC-cyclohexyl moiety.

Reactions of 2 a and 2 b as boron-centered nucleophiles
Similarly to I, [10,17,18,29] the reactions of the boryl anions 2 a and 2 b towards organohalides (RX = 1-bromobutane, allyl bromide, dichloromethane, acetyl chloride, benzoyl chloride) at room temperature resulted in the clean formation of the corresponding colorless cyano(hydro)organoboranes 4 a R and 4 b R , respectively, by salt elimination (Scheme 3). The 11 B NMR shifts of each 4 a R /4 b R pair are nearly identical (Δδ 11B � 1.5 ppm) and range from À 23.6 ppm for R = COMe to À 25.1 ppm for R = Br. This boron-centered reactivity reflects that of 2 a with heavier group 14 triorganochlorides (Scheme 1b). [27] It is noteworthy that unlike 2 a and 2 b, the diazaborolyl anion I reacts with 1-bromobutane to provide the corresponding bromoborane rather than the butylborane. [17] Marder and Lin showed that the preference of boryl anion I for halogen abstraction from RX over nucleophilic attack at the halidebound carbon atom increases for the heavier halides, and can be promoted by the stability of the R À fragment in the transition state of the halogen abstraction step. [30] The reaction between 2 a or 2 b with 1,2-dibromopropane in benzene at room temperature was accompanied by rapid gas evolution as the reaction mixture became colorless. In C 6 D 6 the 1 H NMR spectra of the two reaction mixtures showed the formation of propene as a by-product (δ 1H = 5.71, 4.99 and 4.93 ppm for the alkene protons), while the 11 B NMR spectra showed a single boron-containing product with a doublet around À 25 ppm ( 1 J B-H � 100 Hz), identified by X-ray structural analyses as the CAAC-stabilized bromo(cyano)hydroboranes 4 a Br and 4 b Br , respectively (Scheme 4). In this case the entropically favored elimination of propene is likely to be the driving force of the bromine abstraction versus the nucleophilic attack at carbon.
All 4 a/b R boranes crystallized as colorless solids and were characterized by X-ray crystallographic analyses (Figure 3 and Figures S60-S64). In all cases the B1À C1 bond length is ca.

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Full Paper doi.org/10.1002/chem.202101025 arranged so that the N1À C1À B1À H1 torsion angle tends towards 0°(from ca. 0.9°in 4 a nBu to ca. 20.7°in 4 a COPh ), in order to minimize steric repulsion between the Dip substituent and the CN and R substituents. Overall the syntheses presented in Schemes 2 and 3 represent facile routes to fully unsymmetrically substituted boranes, which would otherwise be difficult to access.

Reactions of 2 a and 2 b as cyano-nitrogen-centered nucleophiles
The reactivity of 2 a and 2 b towards haloboranes (R 2 BX) proceeded rather differently. Instead of turning colorless as in the reactions with organohalides the reaction mixtures turned dark red or bright orange for Mes 2 BF (Mes = 2,4,6-Me 3 C 6 H 2 ) or (iPr 2 N) 2 BCl, respectively. The 11 B NMR spectra of the reaction products, compounds 5 a/b R , showed a broad resonance around 36 ppm for 5 a/b Mes and 22 ppm for 5 a/b NiPr2 , upfield-shifted from the precursors Mes 2 BF (δ 11B = 53 ppm) [31] and (iPr 2 N) 2 BCl (δ 11B = 34 ppm), [32] as well as a BH doublet around À 9 ppm for 5 a/b Mes and À 14 ppm 5 a/b NiPr2 , in a similar range to those of precursors 2 a and 2 b (δ 11B = À 10.8 and À 12.9 ppm, respectively). These data suggested nucleophilic attack at the borane by the terminal cyano-nitrogen donor rather than by the boryl anion (Scheme 5), in analogy to the reaction between 2 a and triorganosilyl chlorides (Scheme 1c). [27] This was confirmed by X-ray structural analyses (Figure 4, Figures S65 and S66), which showed that the geometry around B1 remains trigonal planar (Σ(ffB1) � 360°) and the BR 2 moiety coordinates at the cyano-nitrogen N2. The B1À C21À N2À B2 moiety presents a near-linear arrangement with a B1-C21-N2 angle of ca. 174°and a C21À N2À B2 angle of ca. 170°for 5 a/b Mes and ca. 160°for 5 a/b NiPr2 . In the 5 a/b Mes derivatives the B1À C21, C21À N2 and N2À B2 distances (ca. 1.45, 1.17 and 1.41 Å, respectively) are all within the range of double bonds, [33][34] while the C1À B1 and N1À C1 bond lengths (ca. 1.51 and 1.34 Å, respectively) suggest only a small amount of π backbonding from the electron-rich boron to the CAAC ligand. In the 5 a/ b NiPr2 derivatives the electronic structure is slightly different: due

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Full Paper doi.org/10.1002/chem.202101025 to partial π backbonding from the amino substituents to B2 (B2-N3/4 ca. 1.43 Å) the B1À C21, C21À N2 and N2À B2 distances (ca. 1.48, 1.18 and 1.45 Å, respectively) are slightly elongated compared to 5 a/b Mes . Moreover, the C1À B1 and N1À C1 bond lengths (ca. 1.47 and 1.36 Å, respectively) also indicate partial double bonds, suggesting that π electron density in 5 a/b NiPr2 is delocalized over the entire [N1À C1À B1À C21À N2À B2À N3/4] framework. The preference for the addition of the boron electrophiles at the cyano-nitrogen rather than at the borylene center is likely owed to the formation of a strong BÀ N multiple bond rather than a comparatively weak BÀ B single bond.
The UV-vis spectra of 5 a Mes and 5 b Mes in hexane showed absorption maxima at 516 and 520 nm, respectively, accounting for their red color, as well as a secondary absorption band around 380 nm. In contrast, the major absorption bands of 5 a NiPr2 (423 nm) and 5 b NiPr2 (429 nm) are blueshifted by ca. 100 nm compared to those of 5 a Mes and 5 b Mes , accounting for their orange color, and overlap with strong secondary absorptions around 400 nm.
Density functional theory (DFT) calculations on 5 a Mes and 5 a NiPr2 performed at the B3LYP/def2-SVP [35] and PBE0-D3/def2-SVP [36][37] levels of theory (see Supporting Information for details) showed that in both compounds the canonical Kohn-Sham HOMO extends over the [N1À B1À C21À N2À B2(-N3/4)] π systems, with a strong π-bonding character between B1 and C21, and to a lesser degree B1 and C1 (Figure 4), as suggested by the X-ray crystallographic data. Nodal planes are found in the N1-C1 and C21-N2 bond regions. Additionally, the 5 a NiPr2 derivative shows small contributions from the lone pairs at N3 and N4, with nodal planes at B2À N3 and B2À N4. Furthermore, bond orders calculated using natural resonance theory (NRT) [38] analysis (see Figure S67 in the Supporting Information) suggest a better delocalization of the [N1À C1À B1À C21À N2] π system in 5 a NiPr2 compared to 5 a Mes , as deduced from the crystallographic data. 5 a/b R may thus be regarded as the first examples of 2-aza-1,4diborabutatrienes. Other reported azaborabutatrienes include a 1-aza-2-borabutatriene rhodium complex generated by borylene transfer from a molybdenum aminoborylene complex to a rhodium vinylidene, [39] as well as a couple of aminoborylacetylenes Et 2 NÀ C�CÀ BY 2 (Y = Mes, NMe 2 ), [40,41] which show significant contribution from their 1-aza-4-borabutatriene resonance forms, Et 2 N=C=C=BY 2 .

Conclusion
We have shown that CAAC-stabilized (cyano)hydroboryl anions may be isolated as either dimeric or monomeric lithium THF adducts, depending on the sterics of the CAAC ligand. The monomeric species, however, is extremely moisture-sensitive and is best generated in situ for further reactivity. The prepared [(CAAC)BH(CN)] À anions acts as boron-centered nucleophiles towards the majority of organohalides, including alkyl, haloalkyl, and allyl halides as well as acid chlorides, resulting in the formation of fully unsymmetrically substituted cyano(hydro) organoboranes. This reaction constitutes a very promising latestage method to elaborate the family of boranes with a wide range of organic electrophiles, compounds that would otherwise be difficult to prepare. In contrast, the reaction with 1,2dibromopropane leads to bromine abstraction and elimination of propene gas, which is likely the driving force of this reaction. The reactions of [(CAAC)BH(CN)] À with haloboranes result in nucleophilic attack by the terminal cyano nitrogen atom and the formation of a linear B=C=N=B chain. X-ray crystallographic and DFT analyses of these orange/red compounds show a significant contribution of the cumulenic B=C=N=B resonance form, making these the first examples of azadiborabutatrienes.