Organic Syntheses, CV 5, 852
Submitted by Donald C. Kleinfelter and Paul von R. Schleyer
1.
Checked by William E. Parham, Wayland E. Noland, and Lynette E. Christensen.
1. Procedure
A.
2-exo-Norbornyl formate. Approximately
800 g. (17.4 4 moles) of 98–100% formic acid (Note
1) is added to
400 g. (4.25 moles) of nobornene (Note
2) in a
2-l. round-bottomed flask equipped with a
condenser, and the mixture is boiled under reflux for 4 hours (Note
3). The dark solution is cooled and the flask arranged for distillation using a
30-cm. Vigreux column. The excess
formic acid is removed under reduced pressure (b.p.
26–30°/21–30 mm.). Distillation of the residue then gives a forerun of about 100 ml. of a mixture of
formic acid and ester followed by about
485 g. of
2-exo-norbornyl formate, a colorless oil, b.p.
65–67°/14–16 mm. nD25 1.4594–1.4597. Another
55–65 g. of ester is obtained by adding water to the forerun, extracting with 30–60°
petroleum ether, washing the extracts with dilute
sodium carbonate solution, drying over
sodium sulfate, and distilling. The total yield is
540–550 g. (
90.5–92.5%) (Note
4).
B.
2-Norbornanone. A solution of
510 g. (3.64 moles) of 2-exo-norbornyl formate in
1.5 l. of reagent grade acetone is contained in a
5-l. three-necked flask equipped with a
thermometer,
stirrer, and
dropping funnel containing
8N chromic acid solution (Note
5). The flask is cooled with an
ice bath and the oxidant is added at a rate such that the reaction temperature is maintained at 20–30°. Approximately 1870 ml. of oxidant solution is required, completion of the reaction being shown by the persistence of the brownish orange color. A slight excess of oxidant is added, and the solution is stirred overnight at room temperature. Solid
sodium bisulfite is added in portions to reduce the excess oxidant.
The reaction mixture is poured into a
large separatory funnel. The dark green
chromic sulfate sludge, which has formed during the course of the reaction, is separated either by decantation and washing or by drawing it off from the bottom of the funnel. The
acetone solution is washed three times with
200–250 ml. portions of an aqueous saturated potassium carbonate solution and finally is dried over anhydrous
potassium carbonate. The
acetone is removed by distillation through a 30-cm. Vigreux column at atmospheric pressure;
benzene may be added near the end to assist in the removal of water by azeotropic distillation. When it is observed that the distillation of solvent is complete and the considerably hotter vapors of product begin to ascend the column, the condenser is removed from the top and replaced by an
adapter and collection flask immersed in ice water. The adapter is heated and maintained above 100° by a free flame until the product begins to distil (Note
6).
2-Norbornanone,
335–350 g. (
83–87%), distils at
170–173° and crystallizes immediately in the collection flask. The crystals melt at
90–91° (Note
7) and are sufficiently pure for most preparative purpose (Note
8).
2. Notes
3.
Norbornene is not soluble in cold
formic acid; initially there are two layers. As heat is applied to the flask, solution occurs and the reaction becomes quite exothermic. It is recommended that a splash
trap be mounted at the top of the condenser and that an ice bath be nearby in case the refluxing becomes too rapid.
Caution! Formic acid causes severe burns!
6. The product solidifies readily. Care should be taken to prevent clogging of the adapter. Once begun, there is no difficulty if the distillation proceeds smoothly to completion.
7. The melting points given in the literature vary from
90–91° (Ref.
3) to
95.5° (Ref.
4). Pure
2-norbornanone, m.p.
97.2–98.0°, may be made by regeneration from its semicarbazone derivative, m.p.
196.5–197.6°.
3. Discussion
2-Norbornanone is generally prepared from the Diels-Alder adduct of
cyclopentadiene and
vinyl acetate by hydrogenation, saponification, and oxidation with
chromic acid in
acetic acid solution.
5 The present procedure, which gives higher over-all yields in fewer steps, makes use of the superior solvent,
acetone, for mild
chromic acid oxidations
2 and of the observation that formate esters of secondary alcohols can be oxidized directly to ketones.
6
4. Merits of Preparation
2-Norbornanone is a useful starting material for various bicyclic derivatives of theoretical interest. The present procedure provides a convenient method for its preparation and illustrates a general method for the oxidation of formate esters to ketones.
This preparation is referenced from:
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