Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

Abstract An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B2Pin2)]− using “soft” BR3 electrophiles (BR3=BPh3 or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR3‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B2Pin2 and BR3 and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B2Pin2, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}− transfer to BPh3, thus enabling simple access to unsymmetrical sp2−sp3 diboranes.


General Remarks
Unless otherwise indicated all manipulations were conducted under nitrogen atmosphere. B 2 Pin 2 was kindly provided by AllylChem. Vinyl Grignard (1 M in THF), isopropenyl Grignard (0.5 M in THF), (E/Z)-1-propenyl Grignard (0.5 M in THF), BPh 3 (0.25 M in THF) were purchased from commercial sources and used as received unless otherwise stated. B(C 6 F 5 ) 3 was dried over Et 3 SiH (in a pentane solution), followed by sublimation. 9-Aryl-BBN compounds were synthesized from commercially available 9-methoxy-BBN and ArylMgBr, following the procedure reported in the literature. 1 THF was dried over elemental potassium. NMR spectra were recorded with a Bruker AV-400 spectrometer (400 MHz 1 H; 100 MHz 13 C; 128 MHz 11 B; 376 MHz 19 F). 1 H NMR chemical shifts are reported in ppm relative to protio impurities in the deuterated solvents and 13 C NMR chemical shifts using the solvent resonances unless otherwise stated. 11 B NMR spectra were referenced to external BF 3 :Et 2 O, 19 F to Cl 3 CF). Coupling constants J are given in Hertz (Hz), while the multiplicity of the signals are indicated as "s", "d", "t", "q", "pent", "sept" or "m" for singlet, doublet, triplet, quartet, pentet, septet or multiplet, respectively. A sealed capillary containing d 6 -DMSO is inserted in the J. Young NMR tube for the locking of the NMR sample. Mass spectra were recorded on a Waters QTOF mass spectrometer.

Formation of [B 2 Pin 2 -vinyl] ([2] -)
In a J. Young NMR tube, B 2 Pin 2 (53 mg, 0.200 mmol, 1.0 eq.) was dissolved in dry THF (0.3 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1 M solution of vinyl magnesium bromide (200 µL, 0.200 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy. The adduct [2]shows two signals in 11 B{ 1 H}-NMR spectrum (broad signal at 37.3 ppm for the sp 2 boron and a sharp peak at 4.8 ppm for the sp 3 boron), while the vinyl signals appeared in the 1 H-NMR spectrum at 6.12 and 5.14 ppm. The singlet at 5.30 ppm is attributed to ethene due to protic traces, in accordance with the literature. 2 Figure S1. In-situ 1 H and 11 B{ 1 H}-NMR spectra of an equimolar mixture of B 2 Pin 2 and vinyl Grignard in dry THF. Blue (reference NMR for vinylMgBr), red (the equimolar mixture after 2 hours at -78 o C and 15 min at RT).

Borane activation of [2] -3.1 Addition of B(C 6 F 5 ) 3
In a J. Young NMR tube, B 2 Pin 2 (15 mg, 0.057 mmol, 1.0 eq.) was dissolved in dry THF (0.3 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1 M solution of vinyl magnesium bromide (57 µL, 0.057 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min . While still at -78 o C, a solution of B(C 6 F 5 ) 3 (30 mg, 0.057 mmol, 1.0 eq.) in THF (0.2 mL) was added to the solution. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy, revealing significant decrease in the resonances attributable to [2]and the formation of an alkoxy-B(C 6 F 5 ) 3 species, as confirmed by the resonance at -3.2 ppm in the 11 B{ 1 H}-NMR spectrum ([alkyl-B(C 6 F 5 ) 3 ] have a resonance around -15 ppm). The nature of the anionic species was further confirmed by the Δδ 19 F < 4ppm between the orthoand para-fluorine of the phenyl groups in the 19 F{ 1 H}-NMR spectrum. In the 11 B{ 1 H}-NMR spectrum a new resonances at 48.0 appeared as well, typically of borinic acid / R 2 B(OR) species. Moreover, the vinyl signals still persisted, although with a different chemical shift compared to those of [2] -. The J. Young NMR tube was then left at room temperature for 18 hours and periodically inverted. After this time, the alkoxy-B(C 6 F 5 ) 3 species was found as major product (by multinuclear NMR spectroscopy). An aliquot of the sample was analysed by ESI-MS (negative mode), observing the following alkoxy species as major products: To prove that vinyl transfer from [2]to B(C 6 F 5 ) 3 did not happen, [vinyl-B(C 6 F 5 ) 3 ]was independently synthesized by mixing equimolar amounts of B(C 6 F 5 ) 3 (15 mg, 0.028 mmol, 1.0 eq.) and vinyl magnesium bromide (28 µL, 0.028 mmol, 1.0 eq.) in dry THF (observed 11 B{ 1 H}-NMR = -14.60 ppm).  Figure S3. In-situ 1 H and 11 B{ 1 H}-NMR spectra of an equimolar mixture of B 2 Pin 2 , vinylMgBr and BPh 3 in dry THF. Blue (after 2 hours at -78 o C and 10 minutes at RT), red (after a further 18 hours at RT).

Addition of 9-Ph-BBN
In a J. Young NMR tube, B 2 Pin 2 (30 mg, 0.113 mmol, 1.0 eq.) was dissolved in dry THF (0.5 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1M solution of vinyl magnesium bromide (114 µL, 0.113 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. While still at -78 o C, 9-Ph-BBN (24 µL, 0.113 mmol, 1.0 eq.) was added to the solution. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy, revealing the formation of the desired product   Figure S9. In-situ 1 H and 11 B{ 1 H}-NMR spectra of an equimolar mixture of B 2 Pin 2 , vinylMgBr and 9-Ph-BBN in dry THF. Blue (after 2 hours at -78 o C and 10 mins at RT), red (after 18 hours at RT). S15 Figure S10. 1

Addition of 9-Mesityl-BBN to [2] -
In a J. Young NMR tube, B 2 Pin 2 (30 mg, 0.113 mmol, 1.0 eq.) was dissolved in dry THF (0.5 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1M solution of vinyl magnesium bromide (114 µL, 0.113 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. While still at -78 o C, 9-Mesityl-BBN (33 µL, 0.113 mmol, 1.0 eq.) was added to the solution. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy, revealing no reaction. The steric hindrance around the boron centre prevents THF binding as well (in contrast to PhBBN). S19 Figure S14. In-situ 1 H and 11 B{ 1 H}-NMR spectra of an equimolar mixture of B 2 Pin 2 , vinylMgBr and 9-Mesityl-BBN in dry THF. Blue (after 2 hours at -78 o C and 10 mins at RT), red (after 18 hours at RT).

Addition of 9-o-tolyl-BBN
In a J. Young NMR tube, B 2 Pin 2 (30 mg, 0.113 mmol, 1.0 eq.) was dissolved in dry THF (0.5 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1M solution of vinyl magnesium bromide (114 µL, 0.113 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube was inverted each 30 min. While still at -78 o C, 9-o-tolyl-BBN (25 µL, 0.113 mmol, 1.0 eq.) was added to the solution. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy, revealing the partial formation of the desired product

Addition of 9-p-anisyl-BBN
In a J. Young NMR tube, B 2 Pin 2 (30 mg, 0.113 mmol, 1.0 eq.) was dissolved in dry THF (0.5 mL) and the solution was then cooled down to -78 o C. After 5 min, a 1 M solution of vinyl magnesium bromide (114 µL, 0.113 mmol, 1.0 eq.) was added. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. While still at -78 o C, 9-p-anisyl-BBN (26 mg, 0.113 mmol, 1.0 eq.) was added to the solution. The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature and then was monitored by multi-nuclear NMR spectroscopy, revealing limited formation of the desired product [6] -( 11 B{ 1 H}-NMR signals at 33.9 ppm [-BPin moieties] and -16.0 ppm [R(Ph)BBN -], while the -CH 2 CH-appeared as broad signals at 0.42 ppm and 0.24 ppm). The J. Young NMR tube was then left at room temperature for 18 hours and periodically inverted. After this time, more product had formed but the [2]adduct and 9-Anisyl-BBN were still the major species. Mesitylene addition (10 µL, 0.070 mmol, 0.62 eq.) allowed the determination of the in-situ yield by the relative integration of the aromatic signal of mesitylene and the aromatic ones of

Addition of BPh 3 to [7] -
In a J. Young NMR tube, B 2 Pin 2 (20 mg, 0.076 mmol, 1.0 eq.) was dissolved in dry THF (0.3 mL) and the solution was then cooled down to -78 o C. After 5 min, a 0.5 M THF solution of (E/Z)-1-propenyl magnesium bromide (151 µL, 0.076 mmol, 1.0 eq.) was added, leaving the solution at -78 o C for 2 hours. The sample was left warming to room temperature and monitored by multi-nuclear NMR spectroscopy. As for the adduct [2] -, signals for tri-and tetra-coordinated the sp 2 -sp 3 boron centres in diborane [7]were observed at 37 ppm and 5-8 ppm, respectively. In this case, as the Grignard is a mixture of E-Z isomers, two different adducts of [7]were generated which presumably leads to the two observed resonances for the four coordinate borons.  Then, the solution was cooled to -78 o C, followed by the addition of a 0.25 M THF solution of BPh 3 (302 µL, 0.076 mmol, 1.0 eq). The solution was kept at -78 o C for 2 hours, with the J. Young NMR tube inverted each 30 min. The sample was left warming to room temperature overnight and then was monitored by multi-nuclear NMR spectroscopy. In the 11 B NMR spectrum, there was a single major signal for tetra-coordinate boron-species at -13.8 ppm which was consistent with [PinB-BPh 3 ] -; minor four coordinate boron containing products were observed at -9.6/-10.0 ppm due to [(E/Z)-1-propenyl-BPh 3 ] -, respectively, in accordance with the literature and confirmed by independent synthesis. PhSiMe 3 addition (10 µL, 0.058 mmol, 0.76 eq.) allowed the determination of the in-situ yield of [PinB-BPh 3 ]by the relative integration of the methyl signals of PhSiMe 3

Computational data
Calculations were performed using the Gaussian09 5 suite of programmes. Geometries were optimized with the DFT method using M06-2X functional and 6-311G(d,p) as a basis set, with PCM (Tetrahydrofuran) solvation. 6 All geometry optimizations were full, with no restrictions. All stationary points located in the potential energy hypersurface were characterized as minima (no imaginary frequencies) or as transition states (one and only one imaginary frequency) by vibrational analysis. The analysis also provided zero-point vibrational energy corrections and thermal corrections to various thermodynamic properties. The transition state was further confirmed by IRC calculations (calcALL, forward, maxpoints=40, stepsize=20 / (calcALL, reverse, maxpoints=40, stepsize=20). Full Cartesian coordinates for the optimised geometries are reported below.

Crystallographic details of [3][MgBr(THF) 2 ]
Crystallographic data for [3][MgBr(THF) 2 ] was recorded on a Rigaku SuperNova Xray diffractometer, at 150 K with Mo Kα radiation (mirror monochromator, λ = 0.71073). The CrysAlisPro 7 software package was used for data collection, cell refinement and data reduction. The CrysAlisPro software package was used for empirical absorption corrections, which were applied using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. All further data processing was undertaking within the Olex2 software. 8 The structure was solved using the ShelXT 9 structure solution program using Intrinsic Phasing. The structure was refined with the SHELXL 10 refinement package using Least Squares minimisation against F2. Non-hydrogen atoms were refined anisotropically. CCDC 1856184.