Checked by Stephen T. Wrobleski, Alan T. Johnson, and Robert K. Boeckman, Jr..
1. Procedure
A.
1,1-Dibromopentane(Note
1). A dry,
3-L, three-necked, round-bottomed flask is equipped with a
mechanical stirrer,
nitrogen inlet,
rubber septum, and a
200-mL, graduated, pressure-equalizing addition funnel that is sealed with a
rubber septum. After placing the system under a
nitrogen atmosphere, the flask is charged with
280 mL of dry tetrahydrofuran (Note
2),
400 mL of dry diethyl ether (Note
2), and
20 g (0.20 mol) of dry diisopropylamine (Note
3). Stirring is initiated, and the contents of the flask are cooled to −10°C (
dry ice-acetone bath). After transfer of
120 mL (0.20 mol) of 1.7 M butyllithium in
hexane (Note
4) to the
addition funnel by syringe, the solution of alkyllithium is slowly added to the stirred solution at such a rate as to maintain a temperature of −10°C. After the addition is complete,
10 mL of dry THF is introduced via syringe to rinse the walls of the addition funnel and then added to the reaction mixture. The mixture is stirred at −10°C for 15 min and cooled to −90°C (
toluene-liq. nitrogen bath). The addition funnel is charged by syringe with a solution of
15 mL (38 g, 0.22 mol) of dibromomethane (Note
5) in
100 mL of dry THF, which is then added at such a rate that the temperature does not exceed −85°C (Note
6). The mixture is stirred for 15 min. A solution of
22 mL (37 g, 0.20 mol) of 1-iodobutane (Note
7) in
100 mL of THF, and
120 mL of hexamethylphosphoric triamide (Note
8) are successively added by syringe at such a rate (over an

25 min period) that the temperature of the reaction mixture does not exceed −85°C (Note
6). After the addition is complete, the reaction mixture is stirred at −90°C (toluene-liq. nitrogen bath) for 2 hr, −78°C (
dry ice-acetone bath) for 1 hr, −48°C (
dry ice-acetonitrile bath) for 2 hr, and −23°C (
dry ice-carbon tetrachloride bath) for 3 hr (Note
6) and (Note
9). The solution is poured into
200 mL of 1 M hydrochloric acid solution and transferred to a
2-L separatory funnel. The resulting mixture is extracted with three
100-mL portions of hexane. The organic extracts are combined and washed with three 300-mL portions of water,
100 mL of saturated aqueous sodium sulfite solution, and
100 mL of brine. The organic layer is dried over anhydrous
magnesium sulfate and then concentrated with a
rotary evaporator (0°C
water bath, trapping at −78°C with a
dry ice-acetone condenser) at aspirator pressure. The brown liquid residue is distilled to afford
27–32 g (
59–70%) of
1,1-dibromopentane as a colorless liquid, bp
70–72°C (15 mm) (Note
10).
B.
(Z)-1-Ethoxy-1-phenyl-1-hexene. A dry,
3-L, three-necked, round-bottomed flask is equipped with a mechanical stirrer, nitrogen inlet, rubber septum, and a 200-mL, pressure-equalizing addition funnel that is sealed with rubber septum. After placing the system under a
nitrogen atmosphere, the flask is charged with
350 mL of dry tetrahydrofuran (Note
2). The contents of the flask are cooled to 0°C with an ice bath, and
140 mL (0.28 mol) of a 2.0 M solution of titanium tetrachloride in
dichloromethane (Note
11) is added slowly by syringe to the stirred
THF over a period of 10 min. To the yellow solution at 0°C is added slowly
84 mL (0.56 mol) of tetramethylethylenediamine> (Note
12) by syringe. After being stirred at 0°C for 20 min,
41 g (0.63 mol) of zinc dust (Note
13) is added to the reaction mixture at 0°C in five portions in such a manner that the temperature remains at 0°C (Note
14), followed by addition of
0.88 g (3.2 mmol) of lead (II) chloride (Note
13) and (Note
15), and then the resulting suspension is warmed to 25°C. The color of the suspension turns from brownish yellow to dark greenish blue while being stirred at 25°C for 30 min. The addition funnel is then charged by syringe with a solution of
11 g (70 mmol) of ethyl benzoate (Note
12) and
35 g (0.15 mol) of 1,1-dibromopentane (part A) in
100 mL of dry THF. The resulting solution is then added to the stirred reaction mixture over a period of 10 min at 25°C and stirring is continued for 3.5–4.5 hr. The color of the resulting mixture gradually turns dark brown as the reaction proceeds (Note
16),(Note
17),(Note
18). The reaction mixture is then cooled to 0°C and
70 mL of triethylamine (Note
12) and
91 mL of saturated aqueous potassium carbonate solution are successively added slowly at 0°C by syringe. After stirring at 0°C for an additional 15 min,
200 mL of ether-triethylamine (200/1, v/v) is added to the reaction mixture. The entire reaction mixture is then passed rapidly through a thin pad of activity III basic alumina (Note
19) on a
1-L glass filter using
500 mL of ether-triethylamine (200/1, v/v) as eluent. The filtrate is concentrated with a rotary evaporator (25°C, water bath). If a white solid appears at this point, the mixture is diluted with
100 mL of hexane-triethylamine (200/1, v/v) and the mixture is again filtered through a thin pad of basic alumina (Akt. III). The pad is washed with
100 mL of hexane-triethylamine (200/1, v/v) and the total eluent is concentrated again with a rotary evaporator. The resulting crude material is then vacuum distilled to give
11.0–11.4 g (
77–80%) of a
93:7 mixture of (Z)- and (E)-1-ethoxy-1-phenyl-1-hexene, bp
73–75°C (0.20 mm) (Note
20) and (Note
21).
2. Notes
1. This procedure was reported by J. Villieras, C. Bacquet, and J. F. Normant.
3
4. A
1.7 M hexane solution of butyllithium was obtained from Kanto Chemical Co. It may be standardized; however, the submitters chose to use a fresh reagent and forego the titration. The checkers employed a
1.6 M solution of n-butyllithium in hexane obtained from Lithco Inc., which was standardized before use.
5.
Dibromomethane was freshly distilled, bp
96–97°C. The checkers noted some variability in the yield which in part appeared to be associated with the source of the
dibromomethane.
6. The checkers monitored the internal temperature of the reaction mixture via
thermocouple using an immersion well. The checkers observed that accurate temperature control is essential to obtain the reported yields reproducibly.
9. The color of the mixture changed from brown to white after 30-min stirring at −90°C. Then the color of the mixture changed gradually from white to light brown at −23°C.
10. The infrared spectrum (neat) has absorptions at 2956, 2930, 2860, 1465, 1431, 1238, 1158, 927, 732, 667, 596 cm
−1;
1H NMR (CDCl
3) δ: 0.95 (t, 3 H, J = 7.2), 1.28–1.61 (m, 4 H), 2.41 (dt, 2 H, J = 8.4, 6.2), 5.72 (t, 1 H, J = 6.2);
13C NMR (CDCl
3) δ: 13.8, 21.4, 30.1, 45.1, 46.2.
13.
Zinc dust purchased from Wako Pure Chemical Industries, Ltd. (GR grade) was activated by washing several times with
5% hydrochloric acid washing in turn with water,
methanol, and
ether, and drying in vacuo according to Fieser and Fieser.
4 The lots employed by the submitters were found to contain

0.05 mol% Pb based on the Zn content by X-ray fluorescence analysis (Note
15). The checkers employed
Zn dust (−325 mesh, 99.998% purity) obtained from Aldrich Chemical Company, Inc.
14. The reduction is a slightly exothermic process.
15. Addition of a catalytic amount of
PbCl2 (Rare Metallic Co., 99.999% purity) to a commercial lot of
Zn dust (Aldrich Chemical Company, Inc. (99.998% purity) or Rare Metallic Co. (99.999% purity)) has shown reproducible results as were previously reported.
5,6 The yield of
(Z) and (E)-1-ethoxy-1-phenyl-1-hexene fell to
10–15% ((Z)/(E) = 92/8–95/5) without the addition of
PbCl2.
7
17. The following ratios of reactants, ester/1,1-dibromoalkane/zinc/TiCl
4/PbCl
2/TMEDA = 1/2.2/9/4/0.045/8, gave the best results. When the amount of the reagent was reduced to 1/1.1/4.5/2/0.023/4, only 44% of the
1-ethoxy-1-phenyl-1-hexene was isolated under the same reaction conditions and 44% of
ethyl benzoate remained.
18. Under the reaction conditions for alkylidenation, compounds containing the following functional groups were found to be stable: trimethylsilyl ethers of alcohols, olefins, primary alkyl iodides, and ethylene acetals of aldehydes.
19. Basic
alumina (ICN alumina B-Act. I) was purchased from ICN Biochemical GmbH and pretreated by shaking with 6% of water to change its activity (Act I

III).
20. The infrared spectrum (neat) has absorptions at 2924, 2870, 1649, 1492, 1446, 1266, 1072, 768, 696 cm
−1;
1H NMR ((Z) isomer) (CDCl
3) δ: 1.00 (t, 3 H, J = 6.9), 1.23–1.45 (m, 7 H), 2.32–2.39 (m, 2 H), 3.76 (q, 2 H, J = 7.0), 5.38 (t, 1 H, J = 7.3), 7.23–7.50 (m, 5 H);
1H NMR ((E) isomer) (CDCl
3) δ: 0.87 (t, 3 H, J = 7.0), 1.23–1.45 (m, 7 H), 2.00–2.13 (m, 2 H), 3.80 (q, 2 H, J = 7.0), 4.74 (t, 1 H, J = 7.4), 7.23–7.50 (m, 5 H);
13C NMR ((Z) isomer) (CDCl
3) δ: 14.0, 15.3, 22.5, 25.4, 32.0, 65.8, 115.4, 125.8, 127.5, 128.2, 136.7, 153.2. MS m/z (%): 204 (M
+, 48), 161 (100), 133 (55), 55 (49). Anal. Calcd for C
14H
20O: C, 82.30; H, 9.87. Found: C, 82.47; H, 10.04.
21. The ratio of the geometric isomers of the product ((Z)/(E)) was determined by
1H NMR since isomerization has been shown to occur under GLC conditions.
8
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The procedure described here provides a convenient method for the conversion of esters to Z-alkenyl ethers.
5 The results in the Table show the wide applicability and high Z selectivity of the process. As the substituents R
1 or R
3 become bigger, or R
2 becomes smaller, higher Z selectivity is observed. The stereochemistry of the isomers (Table, cases 1–10) was determined by
13C NMR.
8 Since isomerization of alkenyl ethers has been reported to take place under GLC conditions, the remaining Z/E ratios were measured by
1H NMR (200 MHz) analysis. Esters having terminal double bonds reacted to afford the corresponding alkenyl ethers in about 50% yield (cases 7 and 9). Esters with internal double bonds gave better yields and the stereochemistry of double bonds of the reactants was retained except in the instance of case 8 where partial isomerization of the isolated cis double bond occurred. Thus, the reaction provides a convenient and stereoselective access to allyl vinyl ethers (cases 9 and 10) and oxygen-substituted dienes (case 6). Z-Isomers of silyl enol ethers (cases 11–13) and an alkenyl sulfide (case 14) are also produced under good stereocontrol from the corresponding carboxylic acid derivatives.
The preparation of alkenyl ethers is limited to methods which use as starting materials either acetals
10 or acetylenes.
11,12 It is usually difficult to prepare the alkenyl ethers, especially trisubstituted ones, in a regio- and stereoselective manner by these methods. Alkylidenation of carboxylic acid derivatives does not proceed with the Wittig reagent.
13 Methylene transfer (C=O

C=CH
2) of such electron-rich carbonyl compounds has been achieved with the Tebbe reagent
14 or dimethyltitanocene.
15,16 Alkylidenation by using the Schrock-type metal carbene complex of Ta, Ti, Zr, or W has been reported.
17 18 19 However, the method using the Schrock complexes has several drawbacks. For example, i) the preparation of the complex usually requires special techniques and some restrictions exist on the nature of the substitutents R
1 and/or R
2. ii) The alkylidenation reaction using the Schrock complexes does not provide alkenyl ethers with good control over olefin geometry. The present procedure offers an experimentally simple and stereoselective preparation of alkenyl ethers. The reactants, 1,1-dibromoalkanes, are readily prepared from either iodoalkanes (vide supra) or aldehydes
20 9,21 22 and it is not necessary to isolate the reactive organometallic compound.
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