A.
[(Methyl)(methoxy)carbene]pentacarbonyl chromium(0).2 A
1-L, two-necked, round-bottomed flask equipped with
magnetic stirring bar,
100-mL addition funnel,
reflux condenser, and
gas inlet is charged with
17.8 g (0.085 mol) of chromium hexacarbonyl (Note
1). The apparatus is evacuated (
oil pump) and filled with
argon (four cycles), and a positive pressure is maintained with an argon-filled balloon on a T-tube. Dry
diethyl ether (500 mL) is transferred via cannula into the flask and stirring is commenced. The addition funnel is charged with
60 mL (1.42 M in ether, 0.085 mol) of methyllithium via cannula and rapid dropwise addition is begun. The
methyllithium is added over a 15-min period, during which time the solution turns from bright yellow to dark brown. The solution is heated at reflux for approximately 1.5 hr. After the solution is cooled, the solvent is removed by rotary evaporation. The dark-brown residual solid is taken up in 80 mL of water (in air), and
13.0 g (0.088 mol) of trimethyloxonium tetrafluoroborate (Note
2) is added over a 30-min period with stirring (Note
3). The mixture is extracted several times with
200-mL portions of cold pentane (Note
4). The combined
pentane layers are dried over anhydrous
magnesium sulfate and filtered through a bed of Celite. The solution is concentrated by rotary evaporation to approximately 60 mL and is cooled to −20°C under
argon. After 1 hr the resulting bright-yellow crystals (
17.6 g,
83%) are collected and dried at 25°C under reduced pressure for 10 min (Note
5).
B.
1,3-Dimethyl-3-methoxy-4-phenylazetidinone. A
250-mL Pyrex Erlenmeyer flask is charged with
1.25 g (5.0 mmol) of [(methyl)(methoxy)carbene]pentacarbonyl chromium(0)]. The flask is fitted with a
rubber septum, evacuated, and filled with
argon (four cycles). Dry
petroleum ether (175 mL) is transferred via cannula into the flask to produce a dark-yellow solution. The solution is charged with
0.59 g (5.0 mmol) of N-methylbenzylidenimine (Note
6). The flask is irradiated with six 20-W Vitalites (Note
7). The solution turns brown and heterogeneous within an hour. After 3 days (Note
8) the solution is filtered through a bed of Celite, the precipitate is washed with dry
petroleum ether, and the now lighter-yellow solution is sealed in a flask, degassed, and irradiated as before. After 5 days of further irradiation, the mixture is filtered and the filtrate is exposed to air and irradiated again until a colorless solution is obtained (ca. 1 day). Filtration through a bed of Celite and removal of solvent by rotary evaporation affords colorless crystals of essentially pure β-lactam
2. Recrystallization from
hexane gives
0.67–0.76 g (
65–74% yield), mp
76–77°C (Note
9).