Org. Synth. 1987, 65, 68
DOI: 10.15227/orgsyn.065.0068
DIALKOXYACETYLENES: DI-tert-BUTOXYETHYNE, A VALUABLE SYNTHETIC INTERMEDIATE
[Propane, 2,2'-j[1,2-ethynediylbis(oxy)]bis[2-methyl-]
Submitted by Anna Bou, Miquel A. Pericàs, Antoni Riera, and Fèlix Serratosa.
Checked by Terry Singleton, Jae Chan Park, Martin F. Semmelhack, Sean M. Kerwin, and Clayton H. Heathcock.
1. Procedure
A. (1)
Preparation of trans-2,3-dichloro-1,4-dioxane. (See
(Note 1).) To a
2-L, three-necked, round-bottomed flask, equipped with two inlet tubes (with sintered-glass diffusers at the end) connected to a
chlorine cylinder, and a
reflux condenser connected to an outlet tube immersed in a
potassium hydroxide solution, are added
1200 g (13.64 mol) of anhydrous dioxane (
free of peroxides!) and
8 g (0.03 mol) of iodine. A stream of
chlorine is passed through the
dioxane/
iodine solution heated at 90°C, and the reaction is monitored by NMR spectroscopy. After 9 hr, the conversion is 50% complete
(Note 2); after 33 hr, about 90% complete. At this point, the stream of
chlorine is interrupted.
Reinitiation of the chlorine stream after some hours (next morning, for example) may be dangerous because it was observed in one case the mixture inflamed spontaneously! The reaction mixture is allowed to cool to room temperature,
500 mL of ether is added, and the solution is washed with aqueous
sodium thiosulfate solution. The organic layer is separated, dried over
sodium sulfate, the
ether is evaporated under reduced pressure, and the residue is distilled through a
20-cm Vigreux column, to yield
1200–1300 g of
trans-2,3-dichloro-1,4-dioxane, bp
89°C/16 mm (lit.
1 bp
82.5°C/14 mm; mp
31°C)
(Note 3).
(2) 2,3-Di-tert-butoxy-1,4-dioxane. To a 2-L, three-necked, round-bottomed flask, equipped with a mechanical stirrer, a reflux condenser protected from moisture by a drying tube, and an inlet tube for dry nitrogen, are added 103.6 g (0.66 mol) of trans-2,3-dichloro-1,4-dioxane, 979.1 g (13.23 mol) of anhydrous tert-butyl alcohol (distilled from CaH2), and 365.1 g (2.64 mol) of potassium carbonate (ground with a mortar and pestle and activated at 250°C for 3 hr) (Note 4). The mixture is stirred vigorously and heated under reflux for 24–30 hr, the progress of the reaction being monitored by 1H NMR spectroscopy. Once the singlet at δ 5.95, corresponding to the methine protons of the starting material, has completely disappeared, the reaction mixture is allowed to cool to room temperature, and poured into 500 mL of ether, and enough water (750–850 mL) is added to dissolve all of the inorganic salts. The organic layer is separated, and the aqueous layer is extracted with two 200-mL portions of ether. The combined ether extracts are dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to give 128–147 g of an oily residue (Note 5). Hexane (250 mL) is added and the solution is allowed to stand in a refrigerator. The resulting crystals are separated by suction filtration and washed thoroughly with 150–250 mL of hexane. The residue of 3–6 g of crystalline product, mp 106–107°C, which remains insoluble, is identified as trans-2-tert-butoxy-3-hydroxy-1,4-dioxane (Note 6). The filtrate (which contains approximately a 25 : 75 mixture of cis and trans isomers) is evaporated under reduced pressure to one-half its volume and is cooled to 0°C. Massive crystals appear, which are collected by suction filtration. The crystallization process is repeated once more to give 40–60 g of trans-2,3-di-tert-butoxy-1,4-dioxane, mp 64–65°C. The hexane solution is evaporated under reduced pressure and the oily residue distilled at 50–57°C/0.25 mm to give 54–84 g of a mixture of cis- and trans-2,3-di-tert-butoxy-1,4-dioxane (115–124 g combined; 75–81% yield) (Note 7).
B. 1,2-Di-tert-butoxy-1,2-dichloroethane. To a 250-mL, round-bottomed flask, equipped with a pressure-equalizing dropping funnel protected from moisture by a drying tube, and a magnetic stirring bar, are added 43.2 g (0.21 mol) of phosphorus pentachloride and 50 mL of hexane, and the flask is cooled with an ice–salt bath. While the solution is stirred, a solution of 30 g (0.13 mol) of 2,3-di-tert-butoxy-1,4-dioxane in 100 mL of hexane is added dropwise. After the addition is complete, the cooling bath is removed and the mixture is stirred at room temperature for 90–180 min until no starting material is observed in the NMR spectrum of a sample (Note 8); the reaction mixture is then filtered through a sintered-glass filter to remove excess phosphorus pentachloride. The resulting hexane solution (~ 200 mL), which contains 1,2-di-tert-butoxy-1,2-dichloroethane, 2-chloroethyl dichlorophosphate, 1,2-dichloroethane, phosphorus oxychloride, and traces of phosphorus pentachloride, is transferred into a 1-L, three-necked, round-bottomed flask, equipped with a magnetic stirrer, a dry-ice condenser protected from moisture by a potassium hydroxide tube, and a short inlet tube connected to an ammonia cylinder. The reaction flask is cooled with a dry ice–acetone bath and, while the solution is stirred (Note 9), a fast stream of gaseous ammonia is introduced. A vigorous reaction takes place and a copious white precipitate forms. The stream of gaseous ammonia is continued for 15–60 min (Note 10). The cooling bath and condenser are removed, the reaction flask is connected to an ordinary aspirator line through a potassium hydroxide drying trap, and the ammonia is evaporated under aspirator vacuum with efficient stirring. The ammonia-free solution is filtered through a sintered-glass filter and the precipitate is washed with hexane. The resulting hexane solution (400–500 mL), which contains 25–26 g (0.12 mol) of pure 1,2-di-tert-butoxy-1,2-dichloroethane, is suitable for the next operation (yields, calculated from an aliquot, are 95–97%) (Note 11).
C. (E)-1,2-Di-tert-butoxy-1-chloroethene. The solution prepared above is placed in a 1-L, round-bottomed flask, equipped with a magnetic stirring bar and a Liebig condenser (protected from moisture by a drying tube) and cooled with an ice bath; 28.9 g (0.258 mol) of solid potassium tert-butoxide is added in small portions through the condenser over a 30-min period. After addition, the cooling bath is removed and stirring is continued for 90–120 min until no more starting material is detected in the 1H NMR spectrum of a sample; enough ice water is then added to just dissolve all the inorganic salts. The organic layer is separated and the aqueous layer is extracted with hexane (2 × 100 mL). The combined hexane extracts are dried over anhydrous sodium sulfate, filtered, and concentrated at aspirator vacuum. The residue is distilled at 40°C/0.2 mm, with the collection flask at −78°C, to give a center cut of (E)-1,2-di-tert-butoxy-1-chloroethene (15.4–17.0 g, 58–63% yield from 2,3-di-tert-butoxy-1,4-dioxane) (Note 12) and (Note 13).
D. Di-tert-butoxyethyne. In a 2-L, three-necked, round-bottomed flask, equipped with a magnetic stirring bar (Note 14), a dry-ice condenser protected from moisture by a potassium hydroxide tube, and a pressure-equalizing dropping funnel, 0.5 mol of sodium amide is prepared in 500 mL of liquid ammonia (Note 15), and 20 g (0.0968 mol) of (E)-1,2-di-tert-butoxy-1-chloroethene dissolved in 150 mL of anhydrous ether is added in a 5-min period with efficient stirring. After the addition, stirring is continued for 80 min. The reaction mixture is diluted with 200 mL of cold pentane (−20 to −30°C), and 400 mL of cold water is added very cautiously. The organic layer is washed with 50 mL of a 0.1 M buffered phosphate solution (NaH2PO4/Na2HPO4), dried over anhydrous sodium sulfate (Note 16), filtered, and concentrated at aspirator vacuum without heating to give 10–12 g (78–86% yield) of di-tert-butoxyethyne as a pale-yellow oil. The product is sufficiently pure for further reactions (Note 17), but it may be distilled at 30°C/0.05 mm; freezing point 8.5°C.
2. Notes
1.
This procedure was reported by J. J. Kucera and D. C. Carpenter.
1,22.
These data were obtained by the checkers. The submitters report conversion of 76–80% after only 9 hr. It seems likely that the rate of the reaction may be sensitive to the dimensions and mechanical features of the
chlorine introduction system, and/or an induction period. It is easy and important to monitor the process.
3.
trans-2,3-Dichloro-1,4-dioxane has the following spectra: IR (CCl
4) cm
−1: 2990, 2940, 2885, 1455, 1385, 1375, 1337, 1160, 1115, 1032, 900, 875, 670;
1H NMR (CCl
4)
2b δ: 3.40–4.57 (AA'BB', 4 H, CH
2), 5.95 (s, 2 H, ClCHO).
4.
The yields of this reaction are very sensitive to the presence of traces of moisture and to the ratio of reagents. If one works without a
nitrogen atmosphere and with
tert-butyl alcohol that has not been previously dried over
calcium hydride, with a 1 : 10 ratio of dichloro derivative/alcohol, the yields drop to
65%.
5.
Caution should be exercised in evaporation of the ether, as the di-tert-butoxy compounds are appreciably volatile at reduced pressure. If a rotary evaporator is used for the concentration, the water bath should be kept at or below room temperature, and the residue should not be pumped after it is clear that the bulk of the ether has been evaporated.The submitters report ca.
130 g of oily residue at this stage. Treatment of the oily residue with
105 mL of ether leads to partial crystallization. During washing of the crystals in a Büchner filter with more
ether, an almost complete solubilization takes place, but eventually
0.1–0.5 g of cis-2,3,7,10-tetraoxabicyclo[4.4.0]decane remains as an insoluble residue. This compound was prepared for the first time in 1931;
3 the
cis configuration was established only in 1966.
4 It has the following properties: mp
136°C; IR (CCl
4) cm
−1: 2980, 2955, 2930, 2910, 2875, 1460, 1350, 1285, 1260, 1250, 1150, 1140, 1095, 1080, 1025, 910, 870, 780;
1H NMR (CCl
4) δ: 3.30–4.20 (AA'BB', 8 H CH
2), 4.60 (s, 2 H, OCHO).
6.
The crystalline trans isomer epimerizes in
chloroform solution to give a nearly 70 : 30 mixture of
cis and
trans isomers. The
trans isomer, mp
106–107°C, shows the following spectroscopic properties: IR (KBr) cm
−1: 3450, 2980, 2935, 2890, 1445, 1390, 1370, 1335, 1280, 1260, 1200, 1135, 1105, 1060, 1045, 1035, 1020, 910, 855, 780;
1H NMR (CCl
4) δ: 1.27 (s, 9 H, CH
3), 3.33–4.13 (ABCD + OH, 4 H + 1 H) 4.57 (br, 2 H, OCHO). Anal. calcd. for C
8H
16O
4: C, 54.53; H, 9.15. Found: C, 54.53; H, 9.29.
7.
trans-2,3-di-tert-butoxy-1,4-dioxane has the following spectra: IR (CCl
4) cm
−1: 2975, 2930, 1390, 1367, 6523, 1145, 1100, 1060, 1040, 857;
1H NMR (CCl
4) δ: 1.19 (s, 18 H, CH
3), 3.05–4.20 (m, AA'BB', 4 H, CH
2), 4.30 (s, 2 H, OCHO).
cis- +
trans-2,3-Di-tert-butoxy-1,4-dioxane have the following additional signals: IR (CCl
4) cm
−1: 1170, 1130, 1120, 1080, 1020, 1000, 960, 879;
1H NMR (CCl
4) δ: 4.43 (s, 2 H, OCHO,
cis).
8.
The peaks corresponding to
2,3-di-tert-butoxy-1,4-dioxane overlap with those of the
methylene protons of
2-chloroethyl dichlorophosphate, a by-product from the reaction, but the absence of the
acetal protons of the starting material is clear from the symmetry of the multiplet.
9.
The solution is thick at −78°C. Dilution with additional
hexane may be necessary.
10.
The end of the reaction can be easily detected because, when all of the
2-chloroethyl dichlorophosphate has been destroyed, the reaction mixture
does not crackle any more when condensed
ammonia drops on the stirred mixture, or, much more easily, when the reaction mixture becomes basic to pH paper.
11.
The crude reaction mixture is, in fact, an approximately 30 : 70 mixture of
dl- and meso-1,2-di-tert-butoxy-1,2-dichloroethane. Although the
1H NMR spectrum at 60 MHz (CCl
4) shows only one singlet at 5.6 ppm, the 200-MHz spectrum (CDCl
3) shows two sharp singlets separated by 1.8 Hz. The pure meso compound could be isolated by crystallization and purified by sublimation at 40°C/0.05 mm; mp
77–78° (dec). The spectra are as follows: IR (CCl
4) cm
−1: 2975, 2925, 1470, 1458, 1390, 1368, 1310, 1250, 1180, 1130, 1025, 850, 650;
1H NMR at 200 MHz (CDCl
3) δ: 1.36 (s, 9 H, CH
3), 5.73 (s, 1 H, OCHCl).
12.
In later fractions, small amounts (
0.2–0.5%) of
(Z)-1,2-di-tert-butoxy-1-chloroethane have been detected:
1H NMR (CCl
4) δ: 1.25 (s, 9 H, CH
3), 1.33 (s, 9, H, CH
3), 6.03 (s, 1 H, = CH).
13.
(E)-1,2-Di-tert-butoxy-1-chloroethene has the following properties:
nD25 1.4410–1.4415; UV (cylohexane): 217.7 nm (log ε = 3.7); IR (CCl
4) cm
−1: 2972, 1670, 1470, 1390, 1366, 1290, 1260, 1240, 1180, 1140, 1070, 1025, 935;
1H NMR (CCl
4) δ: 1.26 (s, 9 H, CH
3), 1.33 (s, 9 H, CH
3), 5.91 (s, 1 H, = CH).
14.
The
stirring bar must be glass-covered, since
sodium in
ammonia solution attacks Teflon.
15.
The method used for the preparation of
sodium amide is a modification of the procedure described by Nieuwland et al.
5 In a
3-L, three-necked, round-bottomed flask, equipped with a magnetic stirring bar
(Note 14), a dry-ice condenser protected from moisture by a
potassium hydroxide tube, and an inlet tube connected to the
ammonia cylinder, is condensed 500 mL of liquid
ammonia. A slow stream of dry
oxygen is initiated through the inlet tube and 11.5 g (0.5 mol) of
sodium in small pieces is slowly introduced. The addition of
sodium requires 4–5 hr, since the blue color must be discharged before each new addition of
sodium. In this way, a completely white suspension of
sodium amide is obtained, which allows the formation of crude
di-tert-butoxyethyne, free from any
iron impurities.
16.
It is best to minimize exposure of
di-tert-butoxyethyne to light.
17.
Eventually, if a more concentrated solution of
(E)-1,2-di-tert-butoxy-1-chloroethene in
ether is used, the formation of
1,2,3-tri-tert-butoxy-3-cyano-1-propene [
1H NMR (CCl
4) δ: 6.05 (s, 1 H), 4.98 (s, 1 H), 1.28 (br, 27 H)] and
1,2,3-tri-tert-butoxy-1-cyano-1-propene [
1H NMR (CCl
4) δ: 4.03 (s, 2 H), 1.28 (b, 27 H)] is observed. The by-products may be eliminated by column chromatography on neutral
alumina (40 g, 100–125 mesh, activity 1), using a column refrigerated at 0°C and protected from the light, and eluting with
pentane under
nitrogen pressure. From the first 750 mL of eluant,
9–12 g of pure
di-tert-butoxyethyne is obtained.
18.
Di-tert-butoxyethyne has the following properties:
nD25 1.4365; IR (CCl
4) cm
−1: 2972, 2922, 1470, 1450, 1390, 1367, 1301, 1263, 1245, 1150, 825;
1H NMR (CCl
4) δ: 1.31 (s, CH
3).
3. Discussion
The present procedure for the preparation of
di-tert-butoxyethyne is an improvement of a method previously reported by the submitters,
6 who have also reported the preparation from
glyoxal via
1,2-dichloro-1,2-dimethoxyethane.
6Although acetylenic diethers are thermodynamically stable compounds, they show a high kinetic instability that induces polymerization even at low temperatures.
7Different acetylenic diethers have been prepared, either from
glyoxal (method B) or from
dioxane (method A); their stability correlates well, in a qualitative way, with Charton's ν steric parameter,
8 based on effective Van der Waals radii for the corresponding alkoxy groups (Table I).
TABLE I
ACETYLENIC DIETHERS
|
|
RO-C≡C-OR
|
|
-OR
|
Method
|
ν
|
t½ Order of magnitude (by NMR)
|
|
|
-OCH3
|
B9
|
0.38
|
Seconds (0°C, soln.)
|
|
-OC2H5
|
B7
|
0.48
|
Seconds (0°C, soln.)
|
|
-OCH2C(CH3)3
|
A10
|
0.70
|
Minutes (RT, asoln.)
|
|
-OCH(CH3)2
|
A,B6
|
0.75
|
Minutes (RT, soln.)
|
|
-OC(CH3)3
|
A,B6
|
1.22
|
Days (RT, neat)
|
|
|
|
As shown above,
di-tert-butoxyethyne is the only acetylenic diether prepared so far whose stability allows its use as a synthetic intermediate. It has been used in the synthesis of all the members of the series of monocyclic oxocarbons (deltic, squaric, croconic, and rhodizonic acids),
9 10 11 12 as well as in the synthesis of semisquaric acid, the parent compound of the natural mycotoxins,
moniliformins.10Di-tert-butoxyethyne, like other acetylenic ethers having
hydrogen atoms β to
oxygen, is thermally unstable, and when a
benzene solution is heated under reflux, elimination occurs to give
isobutene and
tert-butoxyketene.
tert-Butoxyketene then reacts with the parent
acetylene to afford
2,3,4-tri-tert-butoxycyclobutenone, which is the precursor of squaric and semisquaric acid.
9,10 This thermal instability prevents the use of
di-tert-butoxyethyne in those reactions that proceed at temperatures higher than 40–50°C.
On the other hand,
di-tert-butoxyethyne is prone to undergo a variety of reactions with transition-metal complexes [PdCl
2CH
3CN, Co
2(CO)
8, CpCo(CO)
2, Ni(CO)
4], to afford new, transient, intermediate complexes which are the actual precursors of oxocarbons.
13
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
dl- and meso-1,2-di-tert-butoxy-1,2-dichloroethane
Propane, 2,2'-j[1,2-ethynediylbis(oxy)]bis[2-methyl-
ACETAL (105-57-7)
potassium carbonate (584-08-7)
acetylene (74-86-2)
ammonia (7664-41-7)
Benzene (71-43-2)
ether (60-29-7)
hydrogen (1333-74-0)
glyoxal (107-22-2)
phosphorus pentachloride (10026-13-8)
chloroform (67-66-3)
iron (7439-89-6)
sodium sulfate (7757-82-6)
oxygen (7782-44-7)
sodium thiosulfate (7772-98-7)
1,2-dichloroethane (107-06-2)
nitrogen (7727-37-9)
iodine (7553-56-2)
methine (7782-42-5)
Phosphorus Oxychloride (21295-50-1)
chlorine (7782-50-5)
potassium hydroxide (1310-58-3)
sodium (13966-32-0)
methylene (2465-56-7)
Pentane (109-66-0)
dioxane (123-91-1)
isobutene (9003-27-4)
sodium amide (7782-92-5)
hexane (110-54-3)
tert-butyl alcohol (75-65-0)
calcium hydride (7789-78-8)
phosphate
trans-2,3-Dichloro-1,4-dioxane (3883-43-0)
2-chloroethyl dichlorophosphate (1455-05-6)
1,2-dichloro-1,2-dimethoxyethane
potassium tert-butoxide (865-47-4)
2,3-Di-tert-butoxy-1,4-dioxane,
cis- and trans-2,3-di-tert-butoxy-1,4-dioxane (68470-79-1)
1,2-Di-tert-butoxy-1,2-dichloroethane (68470-81-5)
(E)-1,2-Di-tert-butoxy-1-chloroethene (70525-93-8)
cis-2,3,7,10-tetraoxabicyclo[4.4.0]decane
(Z)-1,2-di-tert-butoxy-1-chloroethane
1,2,3-tri-tert-butoxy-3-cyano-1-propene
1,2,3-tri-tert-butoxy-1-cyano-1-propene
tert-butoxyketene
2,3,4-tri-tert-butoxycyclobutenone
Di-tert-butoxyethyne (66478-63-5)
trans-2-tert-butoxy-3-hydroxy-1,4-dioxane
trans-2,3-di-tert-butoxy-1,4-dioxane
Copyright © 1921-, Organic Syntheses, Inc. All Rights Reserved