Magnesium-catalysed nitrile hydroboration

A β-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin).


Experimental Data General Experimental Procedures
All manipulations were carried out using standard Schlenk line and glovebox techniques under an inert atmosphere of argon. NMR experiments were conducted in Youngs tap NMR tubes made up and sealed in a Glovebox. NMR spectra were collected on a Bruker AV300 spectrometer operating at 300.2 MHz ( 1 H), 75.5 MHz ( 13 C), 96.3 MHz ( 11 B). The spectra were referenced relative to residual solvent resonances or an external BF 3 .OEt 2 standard ( 11 B). Solvents (Toluene, THF, hexane) were dried by passage through a commercially available (Innovative Technologies) solvent purification system, under nitrogen and stored in ampoules over molecular sieves. C 6 D 6 and d 8 -toluene were purchased from Fluorochem Ltd. and dried over molten potassium before distilling under nitrogen and storing over molecular sieves. Di-n-butylmagnesium (1.0 M solution in n-heptane) and pinacolborane were purchased from Sigma-Aldrich Ltd. and used without further purification. [HC{(Me)CN(2,6i Pr 2 C 6 H 3 )} 2 MgnBu] was synthesised by a literature procedure. 1

Stoichiometric Reactions
Scheme S1: Stacked 1 H NMR spectra in C 6 D 6 recorded during the stoichiometric reduction of t-BuCN with HBpin. (i) Magnesium aldimide formation after addition of HBpin and t-BuCN to V; (ii) Magnesium aldimidoborate formation on addition of a further equivalent of HBpin; (iii) Intramolecular hydride transfer with formation of magnesium borylamide; (iv) Bis(boryl)amine formation after addition of a further equivalent of HBpin.

X-ray Structural Analyses
Diffraction data for compounds 1 -5 were collected on a Nonius Kappa CCD with a low temperature device at 150 K, utilizing Mo-Kα radiation monochromated with graphite (λ = 0.71070 Å). Processing utilized the Nonius software, 2 with structure solution and refinement using XSeed 3 or WINGX-1.70 4 suite of programs throughout and visualized utilizing ORTEP 3. 5 The asymmetric unit of 2 comprises half of a dimer which straddles a crystallographic inversion center. Compound 5 co-crystallized with one molecule of toluene and a molecule of hexane which was half occupied. All bond lengths in the hexane molecule have been restrained. The phenyl group in the toluene molecule has been refined using constraints.

Kinetic Experiments
All NMR Data were recorded on a Bruker AV400 NMR operating at 400.13 MHz ( 1 H) and were recorded at 323 K unless stated otherwise. All data were processed using ACD/Labs group spectra common integral analysis software.
In a glovebox a stock solution of the precatalyst was made to the relevant concentration, 0.5 mL of the catalyst solution was transferred to a Youngs tap NMR tube followed by addition of the relevant quantity of HBpin, followed by the chosen substrate. The tube was sealed, removed from the glovebox, immediately frozen with liquid nitrogen and thawed just prior to loading into the NMR spectrometer which had been preheated to a chosen temperature (if required). 1

Determination of reaction order with respect to [HBpin]
Varying

Kinetic Isotope Effect -EtCN
Using the standard reaction: LMgBu (10 mg, 0.02 mmol ie. 10 mol%) was dissolved in 0.5 ml of C 6 D 6 , 110.5 μL (0.42 mmol) of deuterated pinacolborane was then added followed by 0.2 mmol of propionitrile. 1 H NMR spectra were collected at consistent intervals until the reaction reached the desired 3 half-lives (80 % product conversion). All reactions were carried out at 323 K. Using the standard reaction: LMgBu (10 mg, 0.02 mmol ie. 10 mol%) was dissolved in 0.5 ml of C 6 D 6 , 110.5 μL (0.42 mmol) of deuterated pinacolborane was then added followed by 0.2 mmol of 4methoxybenzonitrile. 1 H NMR spectra were collected at consistent intervals until the reaction reached the desired 3 half-lives (80 % product conversion). All reactions were carried out at 323 K.

EDG Nitrile
Kinetic Isotope Effect -m-MeOC 6 H 4 CN Using the standard reaction: LMgBu (10 mg, 0.02 mmol ie. 10 mol%) was dissolved in 0.5 ml of C 6 D 6 , 110.5 μL (0.42 mmol) of deuterated pinacolborane was then added followed by 0.2 mmol of 3methoxybenzonitrile. 1 H NMR spectra were collected at consistent intervals until the reaction reached the desired 3 half-lives (80 % product conversion). All reactions were carried out at 323 K.