Synthesis of [(R)-DTBM-SEGPHOS]NiCl2 for the enantioselective acetal formation from N-propanoyl-1,3-thiazinane-2-thione and trimethyl orthoformate

Abstract

A. [(R)-DTBM-SEGPHOS]NiCl2 (1). An oven--dried single-necked 25 mL round-bottomed flask (14/23 joint), equipped with a 1.5-cm Teflon-coated magnetic stir bar, is charged with (R)-DTBM-SEGPHOS (1.00 g, 0.85 mmol, 1 equiv) (Note 2) and NiCl2 (110 mg, 0.85 mmol, 1 equiv) (Note 3), and acetonitrile (15 mL) (Note 4). A reflux condenser (14/23 joint) with a rubber septum is attached to the round-bottomed flask, and the system is purged for 5 min with N2, after which a N2-filled balloon is used to seal the system. The mixture is then heated at reflux in an oil bath for 16 h. A Number 3 Glass filter funnel connected with a vacuum adaptor to a single-necked 500 mL round-bottomed flask (29/32 joint) is charged with Celite® (25 g) (Note 5). The Celite® is wetted with acetonitrile (70 mL) (Note 4) and allowed to settle. Whilst the reaction mixture is still warm, the contents are poured over the Celite®. Once absorbed, the Celite® is carefully washed (Note 6) with acetonitrile (300 mL) (Note 4) until the color completely passes to the round-bottomed flask. The filtrate is concentrated on a rotary evaporator (30 °C, 12 mmHg). The resulting solid is dissolved in dichloromethane (20 mL) (Note 7) and transferred to a vial (20 mL). The solution is concentrated on a rotary evaporator (30 °C, 12 mmHg) and the resulting solid is broken up with a spatula and dried on a high vacuum line (0.1 mmHg) for 4 h giving the title compound 1 (1.10 g, 0.84 mmol, 99%) (Note 8) as a fine dark green-black powder.