Organic Syntheses, CV 6, 187
Submitted by Stanely R. Sandler
1
Checked by D. W. Brooks and S. Masamune.
1. Procedure
A.
1,1-Dibromo-2,2-diphenylcyclopropane. A
500-ml., three-necked, round-bottomed flask equipped with a
mechanical stirrer, a
dropping funnel, and a
condenser fitted with a drying tube is flushed with dry
nitrogen, then charged with
25.0 g. (0.139 mole) of 1,1-diphenylethylene (Note
1),
100 ml. of pentane, and
28 g. (0.25 mole) of potassium tert-butoxide (Note
2). The mixture is stirred and cooled to 0° before
66.0 g. (0.261 mole) of bromoform (Note
3) is added dropwise over 30–45 minutes. Stirring is continued for an additional 2–3 hours at room temperature, and 200 ml. of water is added. The yellowish insoluble product is filtered, dried, and digested with
300 ml. of refluxing 2-propanol for 30 minutes. After cooling, the product is filtered and washed with
100 ml. of 2-propanol, yielding
31–38 g. (
63–78%) of colorless crystals, m.p.
151–152°.
B.
2-Bromo-3,3-diphenyl-2-propen-1-yl acetate. A
250-ml. flask equipped with a condenser is charged with
17.6 g. (0.0500 mole) of 1,1-dibromo-2,2-diphenylcyclopropane,
12.5 g. (0.0748 mole) of silver acetate (Note
4), and
50 ml. of glacial acetic acid, then immersed in an
oil bath at 100–120° for 24 hours (Note
5). After cooling, the mixture is diluted with
200 ml. of diethyl ether and filtered. The ethereal filtrate is washed with two 100-ml. portions of water, two
100-ml. portions of aqueous saturated sodium carbonate, and finally with two 100-ml. portions of water. After drying over anhydrous
sodium sulfate, the
ether is removed on a
rotary evaporator. Distillation of the resulting residue under reduced pressure yields
12.0 g. (
72%) of the product, b.p.
142–145° (0.15 mm.),
n22D 1.6020–1.6023 (Note
6).
2. Notes
2.
Potassium tert-butoxide was supplied by Mine Safety Appliances (MSA) Research Corporation. See end of Discussion section below.
3.
Bromoform was supplied by the Dow Chemical Company and used without further purification.
5. A 24-hour period may not be required but was found to be convenient.
6. UV (CH
3OH) nm. max. (log ε): 260 (3.94);
1H NMR (CDCl
3), δ (multiplicity, number of protons): 2.08 (s, 3H), 4.87 (s, 2H), 7.3 (m, 10H).
3. Discussion
The present procedure is that of the submitter
2 and illustrates a general method for the chain extension of alkenes
via gem-dihalocyclopropanes, earlier described by Skell and Sandler.
3 The reaction of dihalocyclopropanes with electrophilic reagents yields haloallylic derivatives,
2 the thermal reaction yields haloallylic halides or halodienes, and the reaction with magnesium, sodium, or lithium alkyl reagents yields allenes.
2 These reactions are summarized in
f.htmigure 1, and examples are given in Table I.
Figure 1.
TABLE I
CHAIN ELONGATION OF ALKENES via gem-DIBROMOCYCLOPROPANES
|
Alkene |
Conditions for Dibromocyclopropane Opening |
Product |
|
|
AgNO3, H2O |
|
|
Heat |
|
|
CH3CO2Ag, CH3CO2H |
|
|
CH3CO2Ag, CH3CO2H |
|
|
CH3CO2Ag, CH3CO2H |
|
|
Heat or CH3CO2Ag, CH3CO2H |
|
|
CH3CO2Ag, CH3CO2H |
|
|
CH3CO2Ag, CH3CO2H |
|
|
This general method has been used by Parham and co-workers
4 to transform indenes into β-halonaphthalenes. The method is also useful for the conversion of pyrroles to β-substituted pyridines and of indoles to β-haloquinolines.
3 More recently, phase transfer agents have been used to aid the preparation of
gem-dihalocyclopropanes by the reaction of olefins with haloforms, using aqueous
sodium hydroxide.
5,6,7
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