Checked by Ed Fewkes and Martin F. Semmelhack.
1. Procedure
A.
Dicarbonyl(cyclopentadienyl)(ethyl vinyl ether)iron tetrafluoroborate (1). A
500-mL, three-necked flask, with a
stopcock at the bottom, is fitted with a
nitrogen inlet and a
mechanical stirrer with a Teflon paddle.
Nitrogen is passed through the flask while it is flame-dried (Note
1) and then
70 mL of mercury is introduced. The
mercury is stirred vigorously as
7.2 g (0.31 mol) of sodium metal, cut into small pieces, and added slowly under a strong flow of
nitrogen; after which the remaining neck is capped with a
rubber septum. The amalgam is allowed to cool to room temperature and
100 mL of tetrahydrofuran is added (Note
2) and (Note
3). While the system is flushed with
nitrogen, one septum is removed and
35.4 g (0.1 mol) of dicarbonyl(cyclopentadienyl)diiron [(CO)2CpFe]2 (Note
4) is added at once. Vigorous stirring is continued for 40 min. The
mercury is drained through the stopcock, and the deep-yellow-red solution of
sodium dicarbonyl(cyclopentadienyl)ferrate, which is ready for use without further purification, is transferred (Note
2) to a
500-mL, round-bottomed flask containing a
magnetic stirrer. An additional
50 mL of tetrahydrofuran is used to rinse the amalgam flask.
Chloroacetaldehyde diethyl acetal (31.11 g, 0.20 mol) (Note
5) is added by syringe slowly, since the initial reaction is exothermic. The resulting solution is heated at 50°C with stirring for 2 hr. After the solution is cooled to room temperature, solvent is removed, first with a
rotary evaporator and then with an
oil pump overnight (Note
6). The residue is taken up in
ethyl ether (Note
7), and filtered by suction through a
1
-in. plug of Celite packed in a 250-mL, coarse-porosity, fritted Schlenk tube (Note
8) and (Note
9). The filtrate is collected in a 1-L, round-bottomed flask containing a
magnetic stirring bar. The
sodium chloride residue is washed several times with fresh
ether until the washings are nearly colorless. The filter tube is removed and the flask is capped with a rubber septum and a nitrogen inlet; air is displaced by flushing the flask is capped with
nitrogen.
The solution is cooled to −78°C in a
dry ice–acetone bath, and
38.19 g (0.23 mol) of tetrafluoroboric acid–diethyl ether complex (Note
10) is added dropwise by syringe over a 30-min period. The solution is allowed to warm to room temperature. The yellow precipitate is filtered off and collected in a 250-mL Schlenk tube, washed with
ether, and dried under a stream of
nitrogen and finally under reduced pressure (oil pump). The bright-yellow salt
1 weighs
43.2–57.6 g (
60–80%) and may be used without further purification (Note
11). It may be stored indefinitely under
nitrogen at 0°C without decomposition.
B.
Dicarbonyl(cyclopentadientyl)(trans-3-methyl-2-vinylcyclohexanone)iron tetrafluoroborate (3). Under a
nitrogen atmosphere,
cuprous iodide (Note 12) (24.76 g, 0.13 mol) and
150 mL of ether are placed in a
1-L, three-necked, round-bottomed flask containing a magnetic stirring bar, and cooled to 0°C in an
ice–salt bath. First
172 mL (0.26 mol) of methyllithium in ether (Note
13) is added by syringe, then
12.60 g (0.13 mol) of 2-cyclohexen-1-one (Note
14) is added dropwise by syringe while the mixture is stirred at 0°C. A bright yellow precipitate forms immediately. After 15 min,
200 mL of tetrahydrofuran is added and the mixture is cooled to −78°C in a dry ice–acetone bath. While vigorous stirring and a strong flow of
nitrogen are maintained, one septum is removed and
43.70 g (0.13 mol) of complex salt 1 is added at once. The septum is replaced and
50 mL of fresh tetrahydrofuran is used to wash solid
1 from the neck and sides of the reaction vessel. After 1 hr at −78°C, the mixture is allowed to warm to room temperature while stirring is continued. Stirring is halted to allow insoluble copper salts to settle, leaving a red supernatant liquid. A
250-mL, coarse-frit, Schlenk filter is prepared with a Celite mat, topped with 2 in. of activity-IV neutral alumina (Note
15), which is further deactivated by washing in the Schlenk tube with
100 mL of diethyl ether. The supernatant solution is transferred to the Schlenk tube by cannula and filtered by suction into a 1-L, round-bottomed flask containing a magnetic stirring bar. The copper salts remaining in the reaction vessel are repeatedly washed with fresh
ether until the filtered washings are nearly colorless. Removal of solvent from the filtrate leaves product
2 as a deep red oil (Note
16).
The oil is dissolved in
500 mL of diethyl ether under a
nitrogen atmosphere, cooled to −78°C in a dry ice–acetone bath and
18 g (0.11 mol) of tetrafluoroboric acid–diethyl ether complex is added dropwise by syringe over a 30-min period, while the bath temperature is maintained at −78°C. The solution is allowed to warm to room temperature, and the powdery yellow solid is isolated by filtration through a 250-mL, coarse-frit, Schlenk filter tube. The product is washed several times with fresh
ether and dried under a stream of
nitrogen and then under reduced pressure (oil pump). The yield of salt
3 is
31–39.1 g (
60–75%). The product may be used without further purification (Note
17) and may be stored under
nitrogen of 0°C for several weeks with no observable decomposition (Note
18).
C.
trans-3-Methyl-2-vinylcyclohexanone (4). Compound 3 (31.5 g) and
25 mL of acetonitrile (0.47 mol, 6-fold excess) (Note
19) are placed in a
100-mL round-bottomed flask (Note
20) fitted with a magnetic stirring bar and a
reflux condenser. The mixture is heated to reflux for 2 hr under
nitrogen, cooled to room temperature, and slowly added to
300 mL of diethyl ether. The
acetonitrile complex 5 precipitates as a bright yellow solid, and may be removed by suction filtration (Note
21).
The filtrate is washed 3 times with distilled water to remove excess
acetonitrile and then dried over anhydrous
magnesium sulfate. Filtration followed by removal of
ether leaves the product
4 as a yellow oil (
7.1–7.6 g,
64–70%), which may be further purified by short path or bulb-to-bulb distillation (bp
30°C at 0.1 mm) to a colorless liquid (Note
22) and (Note
23).
2. Notes
1. All glassware, syringes, and cannulae were routinely flame- or oven-dried and cooled under dry
nitrogen.
2. In general, transfers of dry solvent or of solutions are made by 2-mm cannulae inserted through rubber septa capping delivery and
receiver vessels. Transfer is made by positive
nitrogen pressure applied through a hypodermic needle, while a second needle in the receiver vessel is employed as a vent. Cannulae are available from Hamilton Company, P.O. Box 10030, Reno, NV 89510.
4.
Dicarbonyl(cyclopentadienyl)diiron can be readily prepared on a large scale from
iron pentacarbonyl and
dicyclopentadiene.
4 Alternatively it can be purchased from Alfa Products, Johnson Mathey Co. or from Strem Chemical Company.
5.
Chloroacetaldehyde diethyl acetal was purchased from Aldrich Chemical Company, Inc. This substance is listed as an irritant. Proper care should be exercised when handling it.
6. It is important to stir the product continually to ensure effective removal of
tetrahydrofuran. If appreciable solvent remains, the vinyl ether complex
1 may not crystallize readily.
7.
Ethyl ether was freshly distilled from
sodium benzophenone ketyl.
8. Schlenk tubes were purchased from Ace Glass Company, Catalog No. 7761-36.
9. Filtration is smoothly accomplished by allowing the
sodium chloride to settle and filtering the clear supernatant liquor in small portions, transferring the solution to the filtering tube by cannula. If the Celite should become clogged, the surface may be scraped clean, under a strong stream of
nitrogen, using a
long spatula, in order to increase the filtration rate.
11. The salt may be reprecipitated by dissolution in
methylene chloride containing a small amount of
ethanol, followed by the addition of
ether. The product shows the following spectra: IR (CH
2Cl
2) cm
−1: 2095, 2045, 1545; NMR (CD
3NO
2) δ: 1.40 (t, 3 H, Me), 2.73 (dd, 1 H,
trans-CH=), 3.00 (dd, 1 H,
cis-CH=), 4.37 (q, 2 H, OCH
2), 5.50 (s, 5 H, Cp), 7.92 (dd, 1 H, CHOEt). The checkers realized the higher yield only when the preparation was carried out on one-tenth the scale specified here.
14.
2-Cyclohexen-1-one was purchased from Aldrich Chemical Company, Inc., and purified by vacuum distillation.
15. Alumina was purchased from Woelm and brought to activity IV as directed.
16. As in the preparation of the vinyl ether complex,
1, it is very important to remove
tetrahydrofuran effectively to promote facile crystallization of product. This is most easily done by removing most of the solvent with a rotary evaporator and then stirring the resulting oil under reduced pressure (oil pump) overnight. Mixing the oil with about
50 mL of ether followed by solvent removal under reduced pressure helped to facilitate removal of traces of
tetrahydrofuran. Pure
2 may be obtained as a yellow crystalline solid, which decomposes on heating, by chromatographing the oil on basic alumina (activity IV), eluting with
5% ether in hexane. Compound
2 is characterized by the following spectra: IR (CH
2Cl
2) cm
−1: 2000, 1940, 1700; NMR (CDCl
3) δ: 1.17 (t, 6 H, CH
3), 1.43 (d, 2 H, FeCH
2), 1.65 (dd, 1 H, CHCO), 1.70–2.55 (m, 7 H, CH, CH
2), 3.41 (dq, 2 H, OCH
2), 3.76 (dt, 1 H, CHOEt), 4.87 (s, 5 H, Cp).
17. Compound
3 may be recrystallized by dissolution in a minimum volume of
nitromethane at 0°C followed by the addition of
diethyl ether. The crystalline material decomposes on heating. It is characterized by the following spectra: IR (CH
2CH
2) cm
−1: 2090, 2050, 1708; NMR (CD
3NO
2) δ: 1.1 (m, 3 H, CH
3), 1.4–2.6 (m, 8 H, CH, CH
2), 3.28 (d, 1 H,
trans-CH
2=), 4.00 (d, 1 H,
cis-CH
2=), 5.0 (m, 1 H, CH=), 5.65 (s, 5 H, Cp).
18. However, salt
3 is unstable at room temperature as a solid and in solution and rearranges to the isomeric complex in which the iron center is bound to the carbonyl group of the substituted cyclohexanone.
20. Because of the ease with which product
4 isomerizes to the conjugated enone in the presence of base, it is imperative that the demetallation and subsequent purification steps be carried out in glassware that is free of basic residues.
21. An inert atmosphere is not necessary.
22. The considerable vapor pressure of
4 causes some loss during vacuum distillation. Although the color of the product is improved by distillation, both IR and NMR spectra of the product before and after distillation show little change.
23. Compound
4 is observed to darken on standing in air at room temperature for prolonged periods. Compound
4 is characterized by the following spectra: IR (neat) cm
−1: 1708; NMR (CDCl
3) δ: 0.99 (d,d, 1 H, CH
3), 1.3–2.8 (m, 8 H, CH, CH
2), 4.98 (dd, 1 H,
trans-CH
2=), 5.2 (dd, 1 H,
cis-CH
2=), 5.76 (m, 1 H, CH=).
3. Discussion
Because of their high reactivity, these complex salts react rapidly and regiospecifically, at low temperature, with a number of carbon and heteroatomic nucleophiles, including thiols, amines, and alcohols.
21 Finally, exposure of the double bond takes place under particularly mild conditions so that isomerization of the β,γ-unsaturated carbonyl system may be avoided. The present scope of reactions with these vinyl cation synthons is summarized in Table I.
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved