Ligands with 1,10-phenanthroline scaffold for highly regioselective iron-catalyzed alkene hydrosilylation

Transition-metal-catalyzed alkene hydrosilylation is one of the most important homogeneous catalytic reactions, and the development of methods that use base metals, especially iron, as catalysts for this transformation is a growing area of research. However, the limited number of ligand scaffolds applicable for base-metal-catalyzed alkene hydrosilylation has seriously hindered advances in this area. Herein, we report the use of 1,10-phenanthroline ligands in base-metal catalysts for alkene hydrosilylation. In particular, iron catalysts with 2,9-diaryl-1,10-phenanthroline ligands exhibit unexpected reactivity and selectivity for hydrosilylation of alkenes, including unique benzylic selectivity with internal alkenes, Markovnikov selectivity with terminal styrenes and 1,3-dienes, and excellent activity toward aliphatic terminal alkenes. According to the mechanistic studies, the unusual benzylic selectivity of this hydrosilylation initiates from π–π interaction between the phenyl of the alkene and the phenanthroline of the ligand. This ligand scaffold and its unique catalytic model will open possibilities for base-metal-catalyzed hydrosilylation reactions.


Supplementary Methods
All manipulations were carried out using standard Schlenk, high-vacuum and glovebox techniques. THF, Et2O, 1,4-dioxane, and toluene were distilled from sodium benzophenone ketyl prior to use. Iron(II) chloride (99.99%) was purchased from Sigma-Aldrich and used as received. All commercial available olefins were purchased from Sigma-Aldrich, Alfa Aesar, Acros, and TCI. All olefins are dried over LiAlH4 or CaH2 and distilled prior to use. Phenylsilane was purchased from Sigma-Aldrich or J&K Chemical, dried over LiAlH4 and distilled prior to use. Melting points were measured on a RY−I apparatus and uncorrected. Infrared spectra were recorded on a Bruker Fourier transform spectrometric (FT-IR) and reported in wave number. High resolution mass spectrometric (HRMS) analyses spectrum was determined on an IonSpec FT-ICR mass spectrometer and Waters GCT Premier mass spectrometer. Trace metal contamination analysis of iron precatalysts by ICP-OES (spectro-blue) was performed using a X7 (Thermo Electron Corporation) instruments. Magnetic moment was measured on SQUID VSM (Quantum Design). 1 H NMR, 13 C NMR, 29 Si NMR, 19 F NMR spectra were recorded with a Bruker AV 400 spectrometer at 400 MHz ( 1 H NMR), 101 MHz ( 13 C NMR), 79 MHz ( 29 Si NMR), and 376 MHz ( 19 F NMR) in CDCl3.
Chemical shifts were reported in ppm down field from internal Me4Si ( 1 H NMR) and CDCl3 ( 13 C NMR). Gas Chromatography (GC) analysis were performed using a Hewlett Packard Model HP 7890 Series instruments equipped with an FID detector and a capillary column, HP-5 (Agilent Technologies, 30 m × 0.032 mm × 0.25 m film thickness).
All stationary points were fully optimized at the density functional theory level in Gaussian 09 1 , using the unrestricted ωB97XD 2 functional without symmetry constraints.
The triple-ζ valence basis set TZVP 3,4 were used for Fe, and the 6-31G(d) basis set was used for H, C, N and Si. The energies were further evaluated using a larger basis set def2-TZVPP 5,6 for all atoms involving the solvation effect with an implicit description of toluene using the CPCM treatment 7,8 , where the United Atom Topological Model (UAHF) was used to define the solute cavity. For selected key steps of energy profile were checked by M06/def2-TZVPP 9 and SMD solvent correction 10 at single point energy calculations. All optimized species were verified as either minima or transition structures by the presence of zero or a single imaginary vibrational frequency. Saddle points were connected to minima in the usual way with intrinsic reaction coordinate (IRC) calculations 11,12 . Computed structures are displayed with CYLview 13 .

Synthesis and analytical data of Darphen-Fe
In an argon-filled glovebox, a Schlenk flask (50 mL) was charged with 2,9-diaryl-1,10 phenanthrolines 1 (2 mmol), FeCl2 (253.5 mg, 2 mmol) and dry THF (20 mL). The reaction mixture was stirred at room temperature for 24 h. The solvent was partially removed under vacuum (about 5 mL left), then dry n-hexane (15 mL) was added, and solids precipitated. The product was collected by filtration, washed with 20 mL nhexane, and dried under vacuum.  Typical procedure for synthesis of 13a-f To a mixture of methyltriphenylphosphonium bromide (7.14 g, 20 mmol) and potassium tert-butanolate (2.47 g, 22 mmol), THF (100 mL) was added at 0 o C. The mixture was warmed to room temperature and stirred for 1 h. Then a solution of cinnamaldehyde (2.0 mL, 16 mmol) in THF (10 mL) was added and the resulting mixture was stirred for additional 12 h. A saturated solution of NH4Cl (aq., 50 mL) was added and the mixture was extracted with Et2O (3 × 100 mL). The combined organic phases were washed with saturated brine (100 mL), dried over Na2SO4, and the solvents were removed under reduced pressure. The residue was applied to a plug of silica, eluted with hexane, and the solvent was removed carefully under reduced pressure to obtain the desired product 13a (1.89 g, 91%) as a colorless liquid.

Typical procedure for hydrosilylation of β-alkyl styrenes
In an argon-filled glovebox, a vial (4 mL) was charged with alkene (2 mmol

Background experiments
In an argon-filled glovebox, a vial (4 mL) was charged with metal salt (0.025 mmol), 1e (14.6 mg, 0.025 mmol) and THF (0.5 mL). The reaction mixture was stirred at room temperature for 12 h, then 7a (59.1 mg, 0.5 mmol), PhSiH3 (59.5 mg, 0.55 mmol) and EtMgBr (1 M in THF, 55 μL, 0.055 mmol, 11 mol %) was added. After stirring for 24 hours at 30 o C, the vial was removed from the glovebox and the reaction mixture was concentrated by rotating evaporation. Iron was removed by flash column chromatography with hexane and 8a detected by GC with n-dodecane as internal standard. RuCl3 0

The robustness screen experiments
In an argon-filled glovebox, a vial (10 mL) was charged with 7a (59.

Hydrosilylation of other internal alkenes
In an argon-filled glovebox, a vial (10 mL) was charged with alkene (0.5 mmol), silane (0.6 mmol), anhydrous THF (1 mL) and complex 2e (0.025 mmol). The reaction mixture was stirred at 30 o C for 1 minutes, then EtMgBr (1 M in THF, 55 μL, 0.055 mmol, 11 mol %) was added. After stirring for 24 hours at 30 o C, the vial was removed from the glovebox and the reaction mixture was concentrated by rotating evaporation.
The residue was purified by flash chromatography to afford the desired product.

Hydrosilylation of 1-substituted buta-1,3-dienes
In an argon-filled glovebox, a vial (4 mL) was charged with alkene (0.5 mmol), silane μL, 0.011 mmol, 2.2 mol %) was added. After 10 hours, the vial was removed from the glovebox and the reaction mixture was concentrated by rotating evaporation. The residue was purified by flash chromatography to afford the desired product.

Deuteration experiments
In an argon-filled glovebox, a vial (10 ml

Reduction experiments
In an argon-filled glovebox, a bottle (25 mL Table 1, entry 9. Extend the reduction time to 3 hours, the same results were obtained.

Electron paramagnetic resonance analysis
In an argon-filled glovebox, a bottle (25 mL) was charged with complex 2e (284.   Table 5  and Int 1'''. Although the predicted π-π and Fe-π allyl interaction can be found in the calculated low spin Fe(I) intermediates, the splitting energy of d orbital still dominate the total energy of Fe(I) intermediates and therefore the electrons tend to occupy all the d orbital rather than maximizing paired electrons.