Magnesium Boryl Reactivity with 9‐BBN and Ph3B: Rational B−B′ Bond Formation and Diborane Isomerization

Abstract Reactions of a magnesium‐based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9‐BBN and Ph3B, provide facile B−B′ single bond formation. Although the Ph3B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9‐BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn‐diboration of the resultant alkenyl carbocycle.


General considerations and starting materials
All manipulations were carried out using standard Schlenk line and glovebox techniques under an inert atmosphere of argon. NMR experiments were conducted in J Young tap NMR tubes made up and sealed in a Glovebox. NMR spectra were collected on a Bruker AV300 spectrometer operating at 300.2 MHz were synthesized by literature procedures. [1,2] Elemental analysis was carried out at Elemental Microanalysis Ltd., Okehampton, Devon. UK.

Compound 9
In a J Young NMR tube, d8-toluene (0.5 mL) was added to a mixture of (5) (200 mg, 0.4 mmol) and bis(pinacolato)diboron (107 mg, 0.4 mmol). After 2 hours, 0.5 equivalents of 9-BBN dimer (48.9 mg, 0.2 mmol) were added. After a further 2 hours, the solvent was removed under reduced pressure and the solid was washed with hexane to yield compound 9 (160 mg, 57.8%). Colorless crystals suitable for X-ray diffraction analysis were obtained from a saturated toluene solution at 35˚C. 1

Compound 10
In a J Young NMR tube, d8-toluene (0.5 mL) was added to a mixture of compound (5)

Computational Details / Methodology
DFT calculations were run with Gaussian 09 (Revision D.01). 3 Mg centers were described with the Stuttgart RECPs and associated basis sets, 4 and 6-31G** basis sets were used for all other atoms (BS1). 5 Initial BP86 6 optimizations were performed using the 'grid = ultrafine' option, with all stationary points being fully characterized via analytical frequency calculations as either minima (all positive eigenvalues) or transition states (one negative eigenvalue). All energies were recomputed with a larger basis set (BS2) featuring 6-311++G** on all atoms. Corrections for the effect of toluene (ε = 2.3741) solvent were run using the polarizable continuum model and BS1. 7 Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping as implemented in Gaussian. 8

Breakdown of Energy Contributions
The following tables detail the evolution of the relative energies as the successive corrections to the initial SCF energy are included. Terms used are:

ΔEBS1
SCF energy computed with the BP86 functional with BS1

ΔHBS1
Enthalpy at 0 K with BS1

ΔGBS1
Free energy at 298.15 K and 1 atm with BS1

ΔGBS1/tol
Free energy corrected for toluene solvent with BS1

ΔGBS1/tol+D3
Free energy corrected for toluene and dispersion effects with BS1

ΔGtol
Free energy corrected for basis set (BS2), dispersion effects and toluene solvent Energy Tables   Table S1 -Computed relative energies (kcal/mol) for the reaction of complex 9. Data in bold are those used in the main text. All energies are quoted relative to 9 at 0.0 kcal/mol.

Computational Structural Discussion
Two different conformers of 9, the unsymmetrical diborane anion complex with a {Bpin} and a 9-BBN unit, have been computationally optimised. The X-ray crystal structure, 9Xray, is slightly raised in free energy by 3.8 kcal mol -1 , compared to the computationally built geometry 9. Comparison of the structures by overlaying the two geometries (mapping the equivalent Mg centres, nacnac ligands and B atoms to be aligned), shows the majority of misalignment is due to the 9-BBN unit position ( Figure S1). Upon closer inspection the conformational difference can be best described by the torsion O-BBpin-B9BBN-H (shown in red in Figure S2), which is ~ -5° for the crystal structure geometry (9Xray, magenta) and ~ 47° for the computationally optimised structure 9 (shown in blue / elemental colours).
Looking at the breakdown in energy corrections for the two structures, the difference in ΔGtol arises from dispersion contributions, with the computed 9 involving more favourable and closer contacts between the 9-BBN unit and the Dipp groups of the nacnac ligand in comparison to the solid state structure 9Xray.

Single Crystal X-ray Diffraction Analysis
Data were collected for compounds 9 -12 on a SuperNova, Dual Cu at zero, EosS2 diffractometer. The crystals were all kept at 150(2) K during data collection. Using Olex2, 9 the structures were solved via the olex2.solve routine and refined with the ShelXL 10 refinement package using Least Squares minimization.
For compound 9, H2 and H40 (attached to B2 and C40, respectively) were located and refined without restraints. From the raw data for compound 11 it was evident that the diffraction pattern resulted from a twinwherein the second component was small. The twin angle was determined to be in the region of 178°and, as such, most reflections were overlapped. Data integration to account for the twin was carried with as small a mask as possible in an effort to maximize the quality of the arising data sets. The R(int) for the data extracted pertaining to the minor component was poor and, after parallel refinements, the results presented here are based solely on the data arising from the major component in the sample.
Overall, we have here an unambiguous characterization of this material, and a credible convergence.
Residual electron density is in the region of C40-C42, for which some soft ADP restraints were included in the model. There is either some minor disorder in this regionor the maxima are artifacts of the crystal quality/twinning. In any event, it was not possible to attain a chemically sensible disorder model for this region with an accompanying enhancement of the residuals. H2a and H2b were located and refined freely, subject to being equidistant from B2. For 12 the hydrogens attached to B2 and C36 were located and refined without restraints. A further crystal resulting from a toluene solution of a mixture of 11 and 12 was found to represent an average of these two structural isomers. In particular, C38-C42 were each found to be disordered in a 78:22 ratio. ADP restraints were applied to the fractional occupancy carbons in the final least squares. H36 was located and refined subject to being a distance of 0.98 Å from C36, while H2A and H2A (attached to B2) were located and refined freely. .