A redox-active diborane platform performs C(sp3)–H activation and nucleophilic substitution reactions

Targeted C(sp3)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.

(3 x C Ar ). Resonances of the same color belong to the same phenyl ring (as confirmed by 2D NMR experiments). Signals marked in red could be unequivocally assigned, because they possess double intensity; the blue/green resonances were tentatively assigned by comparison of their chemical shift values with those of a related compound featuring a B-B bond instead of the bridging H atom. S5 Reaction of 1H2 with exc. LiH. In an NMR tube, THF-d8 (0.6 mL) was added at room temperature to a solid mixture of 1H2 (7 mg, 13 μmol) and LiH (15 mg, 1.9 mmol). 1 H and 11 B NMR spectra recorded on this mixture after 10 h showed no reaction. However, upon heating (60 °C, 1 d) the lithium dihydrido-2,7-di(tBu)-9-boratafluorene S3 evolved as the major product (ca. 80%); further heating at 100 °C for 1d resulted in the quantitative conversion of 1H2 to the lithium dihydrido-2,7-di(tBu)-9boratafluorene.

Deuterium-labeling experiments (A) Synthesis of 1D2
Compound 1D2 was obtained according to the published synthesis of 1H2, S1 but by using Et3SiD instead of Et3SiH (Scheme S1).

1D2
The 1 H, 11 B{ 1 H}, and 13 C{ 1 H} NMR spectra (C6D6) of 1H2 S1 and 1D2 are identical with the exception of a broad proton resonance at 3.5 ppm, which has been assigned to the bridging H-atoms in 1H2 S1 and which is missing in 1D2. Instead, 1D2 shows a signal at 3.4 ppm in the 2 H NMR spectrum.

(B) Reaction of 1H2 with D3CLi
A solution of D3CLi·LiI in Et2O (0.5 M, 0.05 mL, 25 μmol) was evaporated to dryness in an NMR tube. The resulting colorless residue was dissolved in THF-d8 and the solution was frozen at -196 °C. Colorless 1H2 (14 mg, 25 μmol) was added, the NMR tube was vacuum sealed, and warmed to room temperature. The orange reaction mixture was analyzed by NMR spectroscopy (see section D).

(C) Reaction of 1D2 with H3CLi
A solution of H3CLi in Et2O (0.25 M, 0.14 mL, 35 μmol) was evaporated to dryness in an NMR tube. The resulting colorless residue was dissolved in THF-d8 and the solution was frozen at -196 °C. Colorless 1D2 (20 mg, 36 μmol) was added, the NMR tube was vacuum sealed, and warmed to room temperature. The orange reaction mixture was analyzed by NMR spectroscopy (see section D).

(D) Results of the deuterium-labeling experiments
The reactions D3CLi/1H2 and H3CLi/1D2 in THF-d8 give complex product mixtures in which Li[1H]/ Li [1D] and Li[2]/Li [2-d3] are present as major constituents ( Figure S4). The additional, poorly resolved signals in the range 7.7 ppm to 6.5 ppm are due to hydride-trapping products. The product distributions critically depend on the exact stoichiometries employed. Figure S4. Aromatic regions of 1 H NMR spectra (THF-d8) recorded on the reaction mixtures of D3CLi/1H2 (blue; 250.1 MHz) and H3CLi/1D2 (green; 300.0 MHz). Matching spectra of Li[1H] (orange; 500.2 MHz) and Li[2] (red; 500.2 MHz) are shown for comparison. Note: The spectra have been recorded at different spectrometer frequencies, which leads to slight differences in the line shapes of corresponding signals.
Note: The orange-colored arrows mark the 13 C satellites of the solvent. The spectra have been recorded at different spectrometer frequencies, which leads to slight differences in the line shapes of corresponding signals and in the positions of the satellites. The integral values of the satellite signals are negligible compared to those of the other resonances. Both in the green and in the red spectrum, the integral ratio of the µ-H signal relative to the CH2 signal is 1:2.

Reactions of Li2[1] with α,ω-dihaloalkanes
All reactions were conducted by adding a dark red solution of [Li(thf)3]2[1] in THF-d8 (0.3 mL) at room temperature with stirring to a small excess of an α,ω-dihaloalkane X(CH2)nX in THF-d8 (0.2 mL). The amounts of starting materials used in each individual experiment and the products obtained are listed in Table S1. Table S1: Starting materials and products of the reactions of α,ω-dihaloalkanes X(CH2)nX with [Li(thf)3]2[1]. The percentages of conversion of the boron compound (determined through integration of the 1 H NMR spectra) are listed in parentheses.
The neutral compounds 14 C2 , 14 C3 , and 14 C4 were crystallized by slow evaporation of the solvent. The crystals were rinsed with small volumes of hexane (14 C2 : 0 mL, 14 C3 : 3 x 0.2 mL, 14 C4 : 3 x 0.2 mL) and THF (14 C2 : 0 mL, 14 C3 2 x 0.1 mL, 14 C4 : 0 mL), and dried in a dynamic vacuum.       , the poor resolution of some signals in the aromatic and aliphatic spectral region originates from a dynamic behavior of the system in solution, which arises from conformational changes of the twisted B2C4 ring and/or from an association-dissociation equilibrium between the cations and the anions. S5 In the cases of Li2[4] and Li2[11], the phenomenon is even more pronounced due to further symmetry breaking by the boronbonded organyl substituents.              Figure

X-ray
Data for all structures were collected on a STOE IPDS II two-circle diffractometer with a Genix Microfocus tube with mirror optics using MoK radiation ( = 0.71073 Å). The data were scaled using the frame-scaling procedure in the X-AREA program system. S6 The structures were solved by direct methods using the program SHELXS S7 and refined against F 2 with full-matrix least-squares techniques using the program SHELXL. S7

Structure
Internal code CCDC reference number [Li(thf) The H atom bridging B1 and B2 was isotropically refined; the coordinates of the two H atoms bonded to C1 were also refined. One tBu group is disordered over two positions with a site occupation factor of 0.793(9) for the major occupied site. In two thf ligands, two methylene groups are disordered over two positions with site occupation factors of 0.66(2) and 0.51(2) for the major occupied sites. In one thf ligand, three methylene groups are disordered over two positions, each with a site occupation factor of 0.63(2) for the major occupied site. The lengths of the C-C bonds involving the disordered atoms in the thf ligands were restrained to 1.50(1) Å and the 1-3 distances involving disordered C atoms in the thf ligands were restrained to 2.3(1) Å or 2.3(3) Å. The displacement ellipsoids of all disordered atoms were restrained to an isotropic behavior.

[Li(thf)3][2]
Colorless The asymmetric unit contains two crystallographically independent molecules of [Li(thf)3][2]. The H atom bridging the boron atoms and the H atoms located on the C atoms bonded to both boron atoms were isotropically refined for both independent molecules. In the asymmetric unit, two tBu groups are disordered over two positions with site occupation factors of 0.78(2) and 0.53(4) for the major occupied sites. In two thf ligands, two methylene groups are disordered over two positions with site occupation factors of 0.63(2) and 0.53(1) for the major occupied sites. In one thf ligand, three methylene groups are disordered over two positions, each with a site occupation factor of 0.62(4) for the major occupied site. The displacement ellipsoids of all atoms in the coordinating [Li(thf)3] + ion were refined with a rigid bond restraint. The displacement ellipsoids of the disordered atoms were restrained to an isotropic behavior. Due to the absence of anomalous scatterers, the absolute structure could not be determined (Flack-x-parameter 1.5(10)).

[Li(thf)3(Et2O)][7]
Colorless single crystals of [Li(thf) All the H atoms bonded to B were isotropically refined. Three tBu groups are disordered over two positions with site occupation factors of 0.62(1), 0.59(2), and 0.52(2) for the major occupied sites. In one thf ligand, three methylene groups are disordered over two positions, each with a site occupation factor of 0.62(2) for the major occupied site. The disordered atoms were isotropically refined. Bond lengths and bond angles of one disordered tBu group were restrained to be equal to those of the non-disordered tBu group. Bond lengths and bond angles of the disordered thf ligand were restrained to be equal to those of a non-disordered thf ligand.

)(thf)][Li(thf)2][11] formed.
The H atom bonded to B1 was isotropically refined. Two tBu groups are disordered over two positions with site occupation factors of 0.58(2) and 0.51(2) for the major occupied sites. In one thf ligand, one methylene group is disordered over two positions with a site occupation factor of 0.79(3) for the major occupied site. The displacement ellipsoids of the disordered atoms were restrained to an isotropic behavior.  Boron-bridging H atoms were isotropically refined. In one of the independent molecules three tBu groups are disordered over two positions with site occupation factors of 0.54(1), 0.75(2), and 0.83(2). In the other independent molecule, three tBu groups are disordered over two positions with site occupation factors of 0.52(3), 0.55(1), and 0.57(1). In one thf ligand, the O atom and two methylene groups are disordered over two positions, each with a site occupation factor of 0.69(1) for the major occupied site. In one thf ligand, two methylene groups are disordered over two positions, each with a site occupation factor of 0.51(1) for the major occupied site. The displacement ellipsoids of all disordered atoms and all atoms in the [Li(thf)4] + ions were restrained to an isotropic behavior. Bond lengths and angles in the disordered thf ligands were restrained to be equal to those in a nondisordered thf ligand.
One of the two boron atoms is tetracoordinated by the thf ligand. Two tBu groups are disordered over two positions with site occupation factors of 0.818(6) and 0.61(2) for the major occupied sites. The displacement ellipsoids of the disordered atoms were restrained to an isotropic behavior. Yellow plates of 14 C3 suitable for X-ray crystallography were grown from the reaction mixture 1,3dibromopropane/Li2[1] in THF by slow evaporation of all volatiles at room temperature.
Due to the absence of anomalous scatterers, the absolute structure could not be determined (Flackx-parameter 1.8(10)). Yellow single crystals of 14 C4 were grown from the reaction mixture 1,4-dibromobutane/Li2[1] in THF by slow evaporation of all volatiles at room temperature.
The asymmetric unit contains two half crystallographically independent molecules of 14 C4 both located on a crystallographic center of inversion. The crystal was non-merohedrally twinned with a fractional contribution of 0.341(3) of the minor component.

ESI56
[Li (12-crown-4) The asymmetric unit contains one-half of a molecule of [Li(12-crown-4)2][15 C5,Cl ] located on a crystallographic C2 axis. Three methylene groups and the terminal Cl atom of the pentyl chain are disordered over the C2 axis with two equally occupied positions. One tBu group is disordered over two positions with a site occupation factor of 0.83(3) for the major occupied site. The C-C bond lengths in the pentyl chain were restrained to 1.50(1) Å and 1-3 C-C distances in this chain were restrained to 2.50(1) Å. The C-Cl bond length was restrained to 1.80(1) Å. The displacement ellipsoids of the disordered C atoms were restrained to an isotropic behavior. The Flack-x-parameter refined to -0.2(2). The boron-bridging H atom was isotropically refined.