Peripheral mechanism of a carbonyl hydrosilylation catalysed by an SiNSi iron pincer complex

The metal centre remains a spectator through the peripheral mechanism investigated by a combined spectroscopic, crystallographic, and computational analysis.

Removal of diethyl ether and dissolution in CDCl 3 permitted the control also by NMR spectroscopy. The NMR spectra of the corresponding products were in accordance with the reported alcohols in the corresponding references (Table S2). Table S2.
Substrate scope for the hydrosilylation of ketones using iron(0) complex 2 as precatalyst.

Racemisation Experiment with Enantiopure Silyl Ether (S)-8eb
The enantiomerically enriched silyl ether product (S)-8eb (49 mg, 0.19 mmol, 1.0 equiv, e.r. > 99:1) was subjected to the catalytic conditions in the presence of the dimethylphenylsilane (4b, 13 mg, 90 µmol, 0.47 equiv) and the iron hydride complex 7b (4.8 mg, 5.0 µmol, 2.5 mol %). The reaction mixture was heated up for 6 days at 70 °C, and aliquots passed through a short plug of silica gel and analysed by chiral HPLC analysis showed no racemization during the course of the reaction.

Synthesis of Trimethylphosphine-d 9 (6-d 9 )
According to a literature procedure, [S23] in a 100 mL Schlenk flask magnesium turnings (2.3 g, 90 mmol, 3.8 equiv) were thermally and mechanically activated under vacuum. Freshly degassed di-n-butyl ether (15 mL) was added followed by rapid addition of methyl iodide-d 3 (0.36 mL, 5.6 mmol, 0.30 equiv) under nitrogen atmosphere at room temperature. After the initiation of the reaction (colour change to dark brown and heat formation), the solution was cooled to 0 °C and the rest of methyl iodide-d 3 (3.3 mL, 50 mmol, 3.0 equiv) was added dropwise. The solution was allowed to warm to rt and stirred for additional 3 h. The solution was cooled to 0 °C and a solution of triphenylphosphite (5.0 g, 15 mmol, 1.0 equiv) in di-nbutyl ether (40 mL) was added slowly over 2 h. The solution was warmed to rt and stirred for 30 min. The dropping funnel was replaced by a distillation apparatus equipped with a Vigreux column (10 cm) and the desired deuterated phosphine 6-d 9 (560 mg, 6.6 mmol, 44%) was distilled at 160 °C (oil bath).

Phosphine Dissociation
Complex 7b (5 mg, 0.005 mmol) was dissolved in THF (2.0 mL). The closed system was heated up to 70 °C for a period of 2 h. The sample was frozen and the gas phase was changed by 3 purge-cycles with N 2 while thawing the sample. This procedure was repeated 3 times with the same period of time between each other. After 8 h, all volatiles were removed in vacuo and the sample was dissolved in 0.5 mL of C 6 D 6 for 1 H and 31 P{ 1 H} NMR analyses.

Phospine Dissociation in the Presence of Acetophenone (3e)
Complex 7b (5 mg, 0.005 mmol, 1 equiv) was dissolved in THF (2.0 mL). Acetophenone (3e, 2.0 mg, 0.02 mmol, 4 equiv) was added and the closed system was heated up to 70 °C for a period of 2 h. The sample was frozen and the gas phase was changed by 3 purge-cycles with N 2 while thawing the sample. This procedure was repeated 3 times with the same period of time between each other. After 8 h, all volatiles were removed in vacuo and the sample was dissolved in 0.5 mL of C 6 D 6 for 1 H and 31 P{ 1 H} NMR analyses.

Reductive Si-O Bond Cleavage of Silyl Ether 8ed
A Schlenk tube equipped with a magnetic stirrer and a reflux condenser was charged with a solution of the silyl ether 8ed (25 mg, 0.089 mmol, 1.0 equiv) in n-heptane (1.0 mL).
DIBAL−H (0.5 mL, 0.5 mmol, 6 equiv, 1.0M in n-hexane) was added in one portion at room temperature, and the resulting reaction mixture was heated to reflux and maintained at this temperature for 20 h. The reaction mixture was allowed to cool to room temperature and quenched by careful addition of aqueous HCl (1M, 5 mL). The organic layer was separated, and the aqueous phase was extracted with tert-butyl methyl ether (3 × 5 mL). The combined organic layers were washed with brine (5 mL), dried over Na 2 SO 4 , filtered, and the volatiles were evaporated under reduced pressure. The crude product was purified by flash column chromatography on silica gel using n-pentane/diethyl ether mixtures (100:0→10:1) as eluent affording the analytically pure hydrosilane ( Si S)-4d (9.3 mg, 0.057 mmol, 64%, e.r. > 95:5) as colourless oil. [S24]