5-(1,1-Dimethylethyl)-5,6-dihydro-2-phenyl-(5S)-oxazolo[2,3-c]-1,2,4-triazolium tetrafluoroborate

Abstract

[464171-86-6] C16H24BF4N3O (MW 361.1) InChI = 1S/C14H18N3O.BF4/c1-14(2,3)12-9-18-13-15-17(10-16(12)13)11-7-5-4-6-8-11;2-1(3,4)5/h4-8,10,12H,9H2,1-3H3;/q+1;-1/t12-;/m1./s1 InChIKey = VVFLRDDTMDDDGK-UTONKHPSSA-N (catalyst for enantioselective benzoin reactions) Physical Data: ochre solid. Solubility: organic solvents such as THF, methanol. Form Supplied in: not commercially available. Purification: recrystallization from methanol. Analysis of Reagent Purity: not reported. Preparative Methods: synthesized in three steps from the oxazolidin-2-one derived from l-tert-leucine.1-3 Step 1: A dry, argon-flushed Schlenk tube was charged with trimethyloxonium tetrafluoroborate (1.2 equiv) suspended in absolute dichloromethane (3 mL mmol −1).A solution of the oxazolidin-2-one 2 (1 equiv) in absolute dichloromethane (3 mL mmol−1) was added and the mixture was stirred for 15 h at 23°C.The mixture was then diluted with dichloromethane and washed with ice-cold saturated aqueous sodium hydrogen carbonate (3 × 3 mL per mmol−1 of oxazolidinone).The solution was dried over anhydrous magnesium sulfate and concentrated in vacuo to afford iminoether 3 as a colorless liquid in quantitative yield (eq 1). The product was used in the next step without further purification. (1) Step 2: A dry, argon-flushed Schlenk tube equipped with a reflux condenser was charged with the iminoether 3 in absolute THF (1.5 mL mmol−1) and phenylhydrazine (1 equiv). Triethylamine (1 equiv) was then added and the reaction mixture was heated to 80 °C for 1 day. The solution was cooled to 23 °C and concentrated in vacuo. The crude product was purified by washing it several times with petroleum ether/diethyl ether (4:1 mL mmol−1). After drying under high vacuum, the phenylhydrazone 4 was isolated as a pink solid in 77% yield (eq 2). (2) Step 3: A dry, argon-flushed Schlenk tube was charged with the phenylhydrazone 4 (1 equiv) in absolute dichloromethane (10 mL mmol−1) and tetrafluoroboronic acid (1 equiv) in absolute diethyl ether (54 wt %). The solution was stirred for 30 min at 23 °C and then evaporated under reduced pressure. The highly hygroscopic salt was dissolved in orthoformate:methanol (2:1, 20 equiv) and transferred under argon into a pyrex tube; the sealed tube was then heated in a sand bath at 80 °C for 12 h. The mixture was cooled to 23 °C and concentrated in vacuo. The crude product was dried under high vacuum for 2 h. The residue was recrystallized from methanol to give the triazolium salt 1 as an ochre crystalline solid in 65% yield (eq 3). (3) Handling, Storage and Precaution: not reported.