4-Acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium Tetrafluoroborate

Typical procedure (R = PhCH₂CH₂-): To a round bottom flask is added 3-phenylpropionaldehyde (0.671 g, 5 mmol), pyridine (5.04 g, 63.75 mmol) and HFIP (2.52 g, 15 mmol). The mixture is allowed to stir at room temperature for five minutes. 4-Acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (3.75 g, 12.5 mmol) is added all at once (mildly exothermic) and the flask is sealed. The reaction mixture is stirred at room temperature and it gradually turned red. The reaction is monitored by TLC or GC/MS. Once the reaction is judged complete (usually <1 hr), the HFIP alcohol is removed in vacuo (e.g. 15 mmHg, 37 °C water bath). To this residue is added pentane (30 mL) causing immediate precipitation of the oxyl form. The heterogeneous solution is stirred for five minutes, and the solids are filtered off and washed with pentane (250 mL). The solids are saved and the filtrate is washed with 0.5 M HCl (150 mL x 2). The organic layer is washed with deionized water (150 mL) and brine (150 mL). The organic layer is dried over Na₂SO₄, and the solvent is removed in vacuo (e.g. 100 mmHg, 37 °C water bath) to give 1,1,1,3,3,3-hexafluoropropan-2-yl 3-phenylpropanoate (1.31 g, 87%) as a clear yellow liquid.