A.
Di-tert-butyl tricarbonate. A
1-l., three-necked flask, fitted with a
mechanical stirrer, a
200-ml. pressure-equalizing dropping funnel, a
calcium chloride-filled drying tube, and
gas-inlet tube with a minimum internal diameter of 6 mm. (Note
1) extending nearly to the bottom of the flask, is dried either by heating with a free flame while passing anhydrous
nitrogen through the apparatus, or by heating to 120° for several hours in an
oven. Before use, the dropping funnel should be calibrated to indicate levels corresponding to 85 ml. and 105 ml. of liquid. While an atmosphere of anhydrous
nitrogen (Note
2) is maintained inside the apparatus, it is allowed to cool before a mixture of
44.8 g. (0.400 mole) of alcohol-free potassium tert-butoxide (Note
3) and
550 ml. of anhydrous tetrahydrofuran (Note
4) is added to the reaction flask. The mixture is stirred under an atmosphere of anhydrous
nitrogen for 5–10 minutes to obtain a solution. (Note
3), (Note
5). The reaction flask is immersed in an
ice–salt bath maintained at −5° to −20°, and all subsequent steps, including solvent removal, are performed with this cooling bath in place. A stream of anhydrous
carbon dioxide (Note
2) is passed through the cold reaction solution for 30 minutes with vigorous stirring, resulting in the formation of a thick, creamy slurry in the reaction flask. While the reaction mixture is being saturated with
carbon dioxide,
85 ml. of anhydrous benzene is added to the dropping funnel. A stream of
phosgene is bubbled through the
benzene until the total volume of the solution is 105 ml., corresponding to the addition of approximately
24 g. (0.24 mole) of phosgene (Note
6). When the addition of
carbon dioxide is complete, the
phosgene solution is added to the cold reaction slurry, dropwise and with vigorous stirring, over 1 hour, maintaining the temperature of the cooling bath at −5° to −15°. During this addition the reaction mixture becomes less viscous but remains a white slurry. When the addition of the
phosgene solution is complete, the cold reaction mixture is stirred for an additional 45 minutes while a stream of anhydrous
nitrogen is passed through the reaction solution to sweep out most of the excess
phosgene. The fittings are removed from the reaction flask, two of the three necks are stoppered, and the volume of solvents in the reaction flask is reduced from about 650 ml. to 100 ml. with a
rotary evaporator. During evaporation the flask should be continuously cooled in an ice–salt bath maintained at −5° to 0°. This evaporation should be performed with either a very
efficient aspirator or with a mechanical vacuum pump fitted with an
efficient cold trap. Since some
phosgene is still present in the reaction mixture, the exhaust from the aspirator or the vacuum pump should be discharged in the hood and any material collected in a cold trap should be emptied in the hood. The residual slurry of finely divided
potassium chloride is filtered with suction in a
large-diameter, fritted-glass funnel, precooled with
50 ml. of ice-cold pentane. During filtration the filter funnel may be loosely covered with an inverted large-diameter funnel through which a stream of
nitrogen is passed to protect the contents of the funnel from atmospheric moisture. The residue in the reaction flask is washed into the filter funnel with
350 ml. of ice-cold pentane and washed with two additional
100-ml. portions of ice-cold pentane, leaving white
potassium chloride as a residue. The combined filtrate and
pentane washings are concentrated to dryness at 0° with a rotary evaporator, under reduced pressure supplied by a vacuum pump equipped with an efficient cold trap, yielding
33.7–39.6 g. (
64–75%, (Note
7)) of
di-tert-butyl tricarbonate as a colorless solid. This crude product is recrystallized by dissolving it in
1250 ml. of pentane at room temperature, and cooling the solution to −15°. The
pentane mother liquors are concentrated with a rotary evaporator, giving two additional crops of crystalline product. The total yield is
31.2–32.8 g. (
59–62%) of the pure
di-tert-butyl tricarbonate as colorless prisms, m.p.
62–63° (dec.) (Note
8).