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Organic Synthess - Johnson S.K.

Johnson S.K., Brossi A., Seebach D. Organic Synthess - Michigan, 1977. - 138 p.
Download (direct link): organicsynthess1977.pdf
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to 33.7-39.6 g. (6475%, Note 7). This crude product is recrystallized from pentane by dissolving the crude product in 1250 ml. of pentane at room temperature and then cooling the solution to 15. The pentane mother liquors are concentrated with a rotary evaporator to separate two additional crops of crystalline product. The total yield is
31.2-32.8 g. (59-62%) of the pure di-?er?-butyl tricarbonate as colorless prisms, m.p. 62-63 dec. (Note 8).
B. Di-tert-butyl dicarbonate. A solution of 20.0 g. (0.076 mole) of di-i-butyl tricarbonate in 75 ml. of carbon tetrachloride is placed in a 600-ml. beaker fitted with a magnetic stirrer, and 0.10 g. (0.0009 mole) of freshly sublimed l,4-diazabicyclo[2.2.2]octane (DABCO) is added (Note 9). Rapid evolution of carbon dioxide begins at once. The reaction mixture is stirred at 25 for 45 minutes to complete the loss of carbon dioxide (Note 10), and then 35 ml. of water, containing sufficient citric acid to make the aqueous layer slightly acidic, is added. The layers are separated and the organic layer is dried over anhydrous magnesium sulfate and then concentrated at 25 with a rotary evaporator. The residual liquid is distilled under reduced pressure to separate 13.3-15.1 g. (80-91%) of di-?er?-butyl dicarbonate as a colorless liquid, b.p. 55-56 (0.15 mm.) or 62-65 (0.4 mm.) w25d 1.4071-1.4072 (Note 11).
2. Notes
1. A gas-inlet tube of smaller diameter or a tube fitted with a fritted-glass outlet tends to become clogged during this preparation and is not recommended.
2. The submitters dried this gas by passing it succesively through an empty trap, through a trap containing concentrated sulfuric acid, and through another empty trap. The checkers used this drying procedure for carbon dioxide but dried the nitrogen by passing it through a column of molecular sieves.
3. The submitters employed alcohol-free potassium ierf-butoxide,
purchased from & Laboratories, without further purification; the
checkers employed comparable material taken from a freshly opened
bottle purchased from MSA Research Corporation. The submitters
report that among approximately ten different bottles of commercial
potassium terf-butoxide used, only material from one bottle failed to
form the tricarbonate in this procedure. The defective material in this
one bottle was an extremely fine powder that failed to dissolve when
stirred with tetrahydrofuran. Solubility in tetrahydrofuran appears to be a good criterion for the purity of alcohol-free potassium teri-butoxide. The checkers have observed that the 1:1 complex of potassium tert-butoxide and iert-butyl alcohol is much less soluble in ethereal solvents than is alcohol-free potassium tert-butoxide.
4. A reagent grade of tetrahydrofuran (b.p. 65-66) was distilled from lithium aluminum hydride before use.
5. The submitters reported that their solution had a faint blue color at this point.
6. If desired, the dropping funnel may be removed from the reaction flask and replaced with a calcium chloride-filled drying tube during the preparation of the phosgene solution. When preparation of the phosgene solution is complete, the drying tube should be removed and quickly replaced with the dropping funnel containing the phosgene solution.
7. Although the submitters report that this crude product is suitable for use in the next step of this preparation, the checkers found that once when using the crude product the subsequent reaction did not go to completion unless an extra quantity of the diamine base was added. This suggests that some potentially acidic impurity such as tferf-butyl chloroformate may be present in the crude product and could interfere with the subsequent reaction. The checkers therefore recommend that the product be purified before use in the next step of this preparation.
8. Although this tricarbonate undergoes thermal decomposition when heated above its melting point (63) to form tertf-butyl alcohol, isobutylene, and carbon dioxide, the product appears to be stable to storage at temperatures of 25 or less. The product exhibits infrared bands (carbon tetrachloride) attributable to ї stretching at 1845, 1810, and 1780 cm.-1; the proton magnetic resonance spectrum (carbon tetrachloride) exhibits a singlet at 8 1.55.
9. The submitters report that both l,4-diazabicyclo[2.2.2]octane and triethylamine have been used to catalyze this decomposition. Tri-ethylamine was less satisfactory as a catalyst because of its relatively rapid reaction with the solvent, carbon tetrachloride, to form triethylamine hydrochloride and because of difficulty encountered in separating triethylamine from the dicarbonate product. The 1,4-diazabicyclo-[2.2.2]octane was efficiently separated from the dicarbonate product by the procedure described in which the crude product was washed with irery dilute aqueous acid.
10. The progress of this reaction may be followed either by observing the disappearance of the band at 1845 cm."1 in the infrared or by following the replacement of the reactant proton magnetic resonance peak (carbon tetrachloride) at S 1.55 by the product peak at S 1.50.
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