Organic Syntheses, CV 9, 203
Submitted by Thomas K. Hansen
1, Jan Becher
1, Tine Jørgensen
1, K. Sukumar Varma
2, Rajesh Khedekar
3, and Michael P. Cava
3.
Checked by John Hynes and Amos B. Smith, III.
1. Procedure
CAUTION! The preparation should be carried out in an
efficient hood, since
carbon disulfide is both toxic and easily ignited.
A.
Tetraethylammonium bis(1,3-dithiole-2-thione-4,5-dithiol) zincate (2). An oven-dried (105°C for 6 hr) apparatus consisting of a
3-L, round-bottomed flask, equipped with a
mechanical stirrer (Note
1), a
250-mL pressure-equalizing dropping funnel, and a
gas inlet tube, is connected to
nitrogen (N
2) via a
Firestone valve (Note
2). To avoid water condensing on the surfaces, N
2 should be allowed to flow through the flask and out of the dropping funnel while the glassware is still hot. While the dropping funnel is temporarily removed, the flask is charged with
23.0 g of sodium shavings (1.0 mol) (Note
3) via a solids addition funnel (Note
4) and (Note
5) and, after reassembly, the apparatus is flushed with N
2 for 5 min and placed in an
ice-water bath.
Carbon disulfide (CS2, 180 mL, 3.0 mol) (Note
6) is introduced into the flask through the dropping funnel, after which the dropping funnel is immediately loaded with
200 mL of dimethylformamide (DMF) (Note
7), the system is closed, and the N
2 atmosphere is maintained throughout the reaction by adjusting the Firestone valve to a slightly positive pressure (Note
8). The
DMF is added dropwise with stirring over 4 hr during which a red color appears (after addition of several mL of
DMF). During the addition, the temperature of the reaction mixture is maintained with ice-water cooling. Upon completion of the addition, the reaction mixture is allowed to warm to room temperature and stir overnight, during which time it becomes increasingly deep red/violet. The reaction mixture is visually inspected for residual
sodium (Note
9). As a precaution, ice is added to the cooling bath and
50 mL of methanol is added slowly through the dropping funnel (Note
9). A mixture of
400 mL methanol and 500 mL of deionized water, which is degassed by applying vacuum, is then added rapidly through the dropping funnel. A solution of
20 g (0.15 mol) of zinc chloride (ZnCl2) in a mixture of
500 mL of concentrated aqueous ammonium hydroxide and
500 mL of methanol is then added through the dropping funnel (Note
10). A solution of 53 g
(0.25 mol) of tetraethylammonium bromide in 250 mL of deionized water is added dropwise via the dropping funnel with vigorous stirring over at least 4 hr, and the solution is stirred overnight. A large amount of red precipitate develops in the flask overnight. This salt is collected by suction on a
Büchner funnel and washed immediately with 500 mL of deionized water, then with
400-mL portions of isopropyl alcohol until the filtrate is colorless, and finally once with
200 mL of diethyl ether. The product is then dried in a
desiccator under vacuum affording
74–76 g (
83–84%) of zincate
2 as a red powder. The product is sufficiently pure for further reactions including alkylation and acylation (Note
11) and (Note
12).
B.
4,5-Dibenzoylthio-1,3-dithiole-1-thione (3). A
1-L, round-bottomed flask, equipped for magnetic stirring, is charged with
400 mL of acetone (Note
13) and
16 g (0.0223 mol) of tetraethylammonium bis(1,3-dithiole-2-thione-4,5-dithio) zincate (2). Magnetic stirring is initiated,
40 mL (48.4 g, 0.345 mol) of benzoyl chloride (Note
14) is added via a dropping funnel over 4 hr, and the mixture is left overnight. The resulting yellow-light brown precipitate is collected by suction and washed on the filter with 500 mL of deionized water and
300 mL of acetone. This crude material is dissolved in
350 mL of chloroform,
0.5 g of Norit is added and the mixture heated under reflux for 10 min (Note
15). The mixture is filtered while still hot, and the filter cake washed with an additional
50 mL of hot chloroform to dissolve any residual product on the filter. The combined
chloroform solutions are concentrated to 150 mL by use of a
rotary evaporator. The resulting mixture is warmed to effect complete solution of diester
3 and
50 mL of methanol is added portionwise with stirring. The solution is then left overnight in the refrigerator. The resulting crystalline precipitate is collected by suction and air-dried, affording
11.8–12.0 g (
65–66%) of dibenzoyl ester
3 (mp
143–144°C) (Note
16)
2. Notes
1. The mechanical stirrer must be made of inert material (glass or Teflon).
2. If a Firestone valve (Ace Glass Inc., Vineland NJ) is not available, a bubbler connected to the N
2 inlet tube via a
T-adapter is sufficient.
3.
Sodium shavings (23.0 g, 1 mol) are conveniently prepared in the following way: A clean piece of
sodium, 40–60 g, is weighed. Shavings are prepared from this using a commercial household grater directly into a
beaker containing dry
ether. The weight of shavings is determined by differential weighing of the remaining piece of
sodium.
CAUTION: Sodium is a corrosive and potentially flammable solid. Handle with care using appropriate gloves for protection in a hood avoiding contact with moisture.
4.
Extreme caution is necessary during the preparation and transfer of the sodium shavings because of the large amount of sodium placed in the flask. The flask must be free of flaws and
sodium waste must be carefully destroyed with
ethanol.
5. Decanting the
ether off immediately before addition is recommended to avoid oxidation of the shavings. If the shavings are a little wet from
ether this does not harm the reaction.
6. The
CS2 was HPLC grade obtained from Aldrich Chemical Company, Inc. and used without purification. Lower grades of CS
2 can be used; however, a minor decrease in yield will occur.
7. The
DMF used was obtained from Fisher Scientific Company (certified A.C.S.) and used without further purification.
8. It is important that the stirrer assembly be nearly gas tight, so that only small amounts of N
2 and CS
2 are lost.
9. Normally there will not be any
sodium metal left, but small pieces of
sodium may have adhered to the walls of the flask above the liquid level. These pieces of residual
sodium metal must be destroyed with
methanol. If any
sodium is left, gas evolution is seen and the mixture is left for 1 hr with cooling and stirring.
10. This is most easily accomplished by adding the ZnCl
2 in portions to the rapidly stirred
ammonium hydroxide followed by addition of the
methanol.
11. Recrystallization of the
zinc complex can be carried out as follows: Dissolve 20 g of the salt in
300 mL of warm acetone and add
0.5 g of Norit. Heat at reflux for 10 min, filter the hot mixture to remove the
Norit, concentrate the filtrate under reduced pressure to one half the original volume, and add
100 mL of isopropyl alcohol. Collect the resulting precipitate by suction and wash the precipitate with
ether. Provides
15.8 g (
80% recovery) of pure salt (mp
206–208°C).
12. The product exhibits the following properties: IR (KBr) cm
−1: 1460, 1410, 1165, 1050, 986, 878, 775, 450;
13C NMR (125 MHz, CDCl
3) δ: 7.9, 53.2, 136.3, 209.5.
13. Commercial
acetone was first dried with
CaCl2, then distilled.
14.
Benzoyl chloride was obtained from Merck & Company, Inc., and used without purification.
15. Any brand of decolorizing charcoal may be used.
16. The product exhibits the following properties: IR (CHCl
3) cm
−1: 3001, 1695, 1601, 1586, 1450, 1200, 1179, 1068, 880, 660, 637;
13C NMR (62.9 MHz, CDCl
3) δ: 127.8, 129.0, 133.5, 134.6, 134.8, 185.2, 212.1.
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
This synthesis provides easy and inexpensive access to the 4,5-dithio-1,3-dithiole-2-thione system. By alkylation with
1,2-dibromoethane,
4,5-ethylenedithio-1,3-dithiole-2-thione, a key intermediate in the synthesis of
bis(ethylenedithio)tetrathiofulvalene (BEDT-TTF) is obtained.
4,5 BEDT-TTF forms charge-transfer salts, many of which are conducting and superconducting, and known as "organic metals". Extensive reviews on the synthesis and properties of
BEDT-TTF6 and other TTF derivatives have been published.
7
Originally, the reduction of
carbon disulfide was carried out electrochemically, but this procedure is not practical for preparative amounts of material.
8 In 1979, a chemical reduction method was discovered by Steimecke.
9 The present procedure is based on that 1979 report, but
sodium is substituted for
potassium (which presents an explosive hazard).
10 Practical improvements have allowed the development of a one-pot procedure that is easily scaled up to provide 70–80 g of material.
Complexation with
zinc effectively separates the C
3S
5 dianion from the trithiocarbonate ion in high yield, and the material obtained is sufficiently pure for further reactions. The quaternary ammonium zincate salt is alkylated slowly but smoothly by many halides at room temperature in solvents such as
acetone or
tetrahydrofuran. Reaction of the zincate with
benzoyl chloride followed by cleavage of the resulting benzoate by
sodium ethoxide in
ethanol provides the much more reactive species Na
2C
3S
5. The sodium salt is, however, very air sensitive,
5 whereas the zinc salt is completely stable.
An alternative to this synthesis is provided by Shumaker via tetrathiapentalene.
11 For most purposes, this procedure offers no advantages and requires more expensive starting materials. The procedure described here is applicable not only for the preparation of BEDT-TTF, but intermediates
2 and
3 may also be used for the synthesis of a variety of BEDT-TTF analogs.
12 13
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
Tetraethylammonium bis(1,3-dithiole-2-thione-4,5-dithiol) zincate
BEDT-TTF
tetraethylammonium bis(1,3-dithiole-2-thione-4,5-dithio) zincate
ethanol (64-17-5)
methanol (67-56-1)
ether,
diethyl ether (60-29-7)
chloroform (67-66-3)
nitrogen (7727-37-9)
acetone (67-64-1)
Norit (7782-42-5)
benzoyl chloride (98-88-4)
zinc (7440-66-6)
sodium (13966-32-0)
isopropyl alcohol (67-63-0)
sodium ethoxide (141-52-6)
carbon disulfide (75-15-0)
1,2-dibromoethane (106-93-4)
zinc chloride (7646-85-7)
ammonium hydroxide (1336-21-6)
potassium (7440-09-7)
Tetrahydrofuran (109-99-9)
dimethylformamide,
DMF (68-12-2)
tetraethylammonium bromide (71-91-0)
4,5-Dibenzoyl-1,3-dithiole-1-thione
4,5-ethylenedithio-1,3-dithiole-2-thione
bis(ethylenedithio)tetrathiofulvalene
4,5-Dibenzoylthio-1,3-dithiole-1-thione
Benzenecarbothioic acid, S,S'-(2-thioxo-1,3-dithiole-4,5-diyl) ester (68494-08-6)
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