Organic Syntheses, CV 7, 479
Submitted by D. A. White
1
Checked by Carl R. Johnson and Debra L. Monticciolo.
1. Procedure
The cell consists of a commercially available
four-necked, 500-mL, round-bottomed flask equipped with a
34/45 standard-taper joint electrode assembly (Note
1), a
24/40 standard-taper joint purge and vent assembly, a
mercury pool cathode (Note
2), a
cathode contact (Note
3), a
magnetic stirring bar (Note
4), and
thermometer (inserted in a 10/18 standard-taper joint neck). The two platinum anodes of the electrode assembly (Note
1) are positioned in a horizontal plane ca. 1 cm above (Note
4) the
mercury (cathode) surface.
The solution is electrolyzed with continuous magnetic stirring and nitrogen purging at a constant current (Note
7) until the theoretical quantity of electricity (1.0
F 
1e
− per mole of
diethyl fumarate) has been passed. The rate at which the cooling water in the bath flows is adjusted to maintain the electrolyte solution at 35°C during the first 2 hr of the electrolysis. It is then kept constant for the remainder of the electrolysis. After conditions have stabilized (ca. 1–2 hr of electrolysis), the reaction does not need constant attention, and may be allowed to run overnight.
The reaction mixture is transferred to a
2-L, round-bottomed flask with
ethanol washing and the
ethanol is removed by rotatory evaporation.
Diethyl ether (1 L) is added to precipitate the electrolyte salt, which is collected by filtration and washed with ether. The crude electrolyte is obtained as a white solid (
32–32.5 g, theory
34.1 g). The filtrate and washings were combined and evaporated to give a viscous brown oil, which was vacuum-distilled through a short
Vigreux column (15 cm × 2.5 cm). After a forerun of 70 mL of material boiling below
150°C (0.15 mm), the product (
92–96 g,
53–56%), bp
150–155°C (0.1 mm), is collected (Note
8) and (Note
9). The forerun contained
diethyl maleate,
diethyl fumarate,
diethyl succinate, and
diethyl ethoxysuccinate. The product is a mixture of diastereomers; on standing some meso isomer, mp
74–75°C, crystallizes.
2. Notes
1. The electrode assembly has been described (see synthesis of
dimethyl decanedioate,
Note 1 and
Figure 1, p. 182). In this case the electrodes have the same polarity and are electrically connected with a
platinum wire dipping into the
mercury contacts.
2. About
65 mL (860 g) of mercury was used, giving a pool with a surface diameter of ca. 6 cm.
3. A
mercury-filled 6-mm o.d. glass tube with a platinum wire sealed through the lower end was used. The tube was bent to fit the contour of the flask. It was connected to the flask through a
24/40 standard-taper joint Teflon thermometer adapter (Ace Glass, Vineland, NJ). Contact to the
mercury was made with a
platinum wire as shown (
Figure 1, p. 183).
4. A
20-cm × 0.5-cm Teflon coated stirring bar was used. This thickness (0.5 cm) is close to the maximum usable with an electrode gap of 1 cm. The rate of stirring was the maximum possible without breaking the
mercury surface into droplets.
7. The checkers used a Heath Schlumberger Model SP-2711 (30 V, 3 A) power supply at a current of 1.5 A. The cell voltage, initially 25 V, slowly rose to 30 V at the end of the electrolysis and the current dropped to ca. 1 A. The electrolysis required 17–24 hr. The submitters used a current of 1.0 A.
8. The submitters reported a yield of
135 g (
78%). In part the reduced yields found by the checkers were caused by mechanical losses during distillation.
9. The product showed
1H NMR (CDCl
3) δ: 1.25 (t, 12 H, CH
3), ca. 2.6 (m, 4 H,-COC
H2), ca. 3.3 (m, 2 H, C
H), 4.2 (two overlapping q, 8 H, OCH
2). Analysis calculated for C
16H
2O
8: C, 55.5; H, 7.6%. Found: C, 55.5; H, 7.8. Molecular weight calculated: 346. Found (osmometrically in CHCl
3): 340, 338.
3. Discussion
This synthesis is an example of electrohydrodimerization of activated alkenes, the scope and mechanism of which have been recently reviewed.
2,3 The individual reactions combining to give the overall result. are the cathodic reduction of the alkene to a dimer dianion (in the general case there are two major mechanisms by which the dianion may be formed and these are discussed in the references cited
2,3), the protonation of the dianion by
ethanol, and the anodic oxidation of
ethanol.
In addition to providing an anode reaction (a suitable reaction at the "other" electrode is a necessity in any electrochemical reaction), reaction (4) also maintains the pH constant by producing protons to neutralize the
ethoxide ions originating from reaction (3).
The present synthesis is an adaptation of a previously reported synthesis
4 in a divided cell (i.e., separate anode and cathode compartments). The overriding consideration in making this modification has been to simplify the operations involved and render the synthesis more attractive to chemists not well acquainted with electrochemical procedures. The main simplification achieved is that the pH is controlled internally via the anodic generation of protons as noted above (in the reported procedure,
4 this is achieved by periodic addition of
acetic acid to the cathode compartment). A further simplification has been to run the reaction with a constant current rather than at controlled cathode potential. After the electrolysis has been initiated, the reaction requires no special attention. A small price is paid for the simplicity of the present synthesis in that the yield is somewhat lower than that obtained previously.
4 The major by-product formed is
diethyl succinate, which results from a 2e
− reduction of
diethyl fumarate or
diethyl maleate: [cf. (2), which consumes 1e
− per mole of ester]. The occurrence of reaction (5) leads to incomplete consumption of ester after passage of the theoretical quantity of electricity (there may also be contributions from other sources).
The by-product,
diethyl 2-ethoxybutanedioate, may be formed via base-catalyzed reaction in the vicinity of the cathode, where conditions may become quite basic.
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