Schiff Base Cobalt(II) Complex-Catalyzed Highly Markovnikov-Selective Hydrosilylation of Alkynes

A bench-stable cobalt(II) complex, with 3N-donor socket-type benzimidazole-imine-2H-imidazole ligand is reported as a precatalyst for regioselective hydrosilylation of terminal alkynes. Both aromatic and aliphatic alkynes could be effectively hydrosilylated with primary, secondary, and tertiary silane to give α-vinylsilanes in high yields with excellent Markovnikov selectivity and extensive functional-group tolerance. Catalyst loading varies within 0.5–0.05 mol %, which is one of the most efficient reported so far in the literature on cobalt-catalyzed alkyne hydrosilylation.

Gas chromatography was performed on a Bruker Scion 436-GC with a 30 m Agilent VF5-ms 0.53 mm Megabore column and a TCD detector. The temperature program was as follows: 60 °C (3 min), 20 °C/min, 280 °C (20 min). Decane was used as a reference. GC-MS analyses were performed on a Bruker Scion 436-GC with a 30 m Varian DB-5 0.25 mm capillary column and a Scion SQ-MS mass spectrometry detector. The temperature program was as follows: 60 °C (3 min), 10 °C/min, 250 °C (15 min). NMR analyses were performed on a Bruker Fourier 300 MHz or 400 MHz spectrometer.
FT-IR spectra were recorded on a Nicolet iS50 (Thermo Scientific) Fourier transform spectrophotometer equipped with a diamond ATR unit. In all cases, 16 scans at a resolution of 2 cm -1 were collected, to record the spectra in the range of 4000-400 cm -1. HRMS Spectra were recorded on a QTOF type mass spectrometer (Impact HD, Bruker) in positive ion mode.

General procedure of alkyne hydrosilylation:
General procedure for hydrosilylation of alkynes A: To a flame-dried Schlenk bomb flask charged with argon, precatalyst (0.1 mol %), THF (0.5 mL), silane (1.0 mmol), and alkyne (1.0 mmol) were placed. Mixture was then heated to 40°C with stirring in oil bath. After 10 minutes LiHBEt3 was added (0.3 mol %), and reaction vessel was then closed. After 20 hours, hexane was added to reaction mixture. Solution was then filtered through silica plug, and concentrated on rotavapor. The crude mixture was purified by evaporating volatiles on vacuum line followed by extraction with hexane. Regioselectivity was monitored by 1 H NMR.
General procedure for hydrosilylation of alkynes B: To a flame-dried Schlenk bomb flask charged with argon, precatalyst (0.5 mol %), THF (0.5 mL), silane (1.0 mmol), and alkyne (1.0 mmol) were placed. Mixture was then heated to 60°C with stirring in oil bath. After 10 minutes LiHBEt3 was added (1.5 mol %), and reaction vessel was then closed. After 20 hours, hexane was added to reaction mixture. Solution was then filtered through silica plug, and concentrated on rotavapor. The crude mixture was purified by evaporating volatiles on vacuum line followed by extraction with hexane. Regioselectivity was monitored by 1 H NMR.
General procedure for hydrosilylation of alkynes C: To a flame-dried Schlenk bomb flask charged with argon, precatalyst (0.05 mol %), THF (0.5 mL), silane (1.0 mmol), and alkyne (1.0 mmol) were placed. Mixture was then heated to 40°C with stirring in oil bath. After 10 minutes LiHBEt3 was added (0.15 mol %), and reaction vessel was then closed. After 20 hours, hexane was added to reaction mixture. Solution was then filtered through silica plug, and concentrated on rotavapor. The crude mixture was purified by evaporating volatiles on vacuum line followed by extraction with hexane. Regioselectivity was monitored by 1 H NMR..

Ligand(L) and complex 1 synthesis:
Ligand was synthesized according to reported procedure [1]. 2-chlorobenzimidazole (8.00 g 0.05 mol) was placed under argon atmosphere in two-necked round-bottomed flask. Methylhydrazine (11.9 g, 0.25 mol), in five-fold excess, was dissolved in anhydrous ethanol and was added to the reaction mixture, which was heated in heating mantle to 80°C and stirred for 20 h. The white, crystalline product was filtered on Büchner funnel and dried under vacuum. Yield 68.2% (5.8 g, 0.036 mol). Then condensation of 2-imidazolecarboxyaldehyde with 2-(1-methylhydrazine)benzimidazole was performed. At two-necked round-bottomed flask 2-(1-methylhydrazine)benzimidazole (1 g, 6.16 mmol) placed under argon atmosphere. The 4-imidazolecarboxyaldehyde (0.591 g, 6.16 mmol) was dissolved in anhydrous ethanol and was added to the reaction mixture. Reaction mixture was stirred for 24 h at 60°C. Yellow clear solution was cooled to room temperature. Precipitate, was then filtered by vacuum filtration, washed with anhydrous ethanol and dried under vacuum to give 1.24 g (5.16 mmol) of L. The supernatant was concentrated to minimal amount of volume and the next part of precipitate was obtained 0.140 g (0.58 mmol). Total yield is 91% (1.38 g, 5.74 mmol).