Org. Synth. 1998, 75, 210
DOI: 10.15227/orgsyn.075.0210
SYNTHESIS OF 8,8-DICYANOHEPTAFULVENE FROM CYCLOHEPTATRIENYLIUM TETRAFLUOROBORATE AND BROMOMALONONITRILE
[
Propanedinitrile, 2,4,6-cycloheptatrien-1-ylidene-
]
Submitted by Hitoshi Takeshita, Akira Mori, and Kanji Kubo
1
.
Checked by Yuping Qiu and Amos B. Smith, III.
1. Procedure
Caution! All operations should be conducted in a well-ventilated hood with breathing protection.
An
oven-dried, 1-L, three-necked, round-bottomed flask, equipped with a
nitrogen gas inlet,
magnetic stirring bar,
dropping funnel,
thermometer, and
condenser, under an inert
nitrogen atmosphere, is placed in an
ice bath and charged, while stirring, with
500 mL of pyridine
and
17.79 g of cycloheptatrienylium tetrafluoroborate 1
2 (0.1 mol) at 0°C
(Note 1).
Bromomalononitrile (14.49 g, 0.1 mol)
(Note 2) is added drop by drop within a 10-min period, and the mixture is vigorously stirred at that temperature for 1 hr. The ice bath is replaced by a
water bath, the temperature is gradually raised to 40°C, and the stirring mixture is kept at that temperature for 5 hr. The solvent is distilled off under reduced pressure below 40°C. To the resultant residue,
1 L of chloroform
(CHCl
3) is added and the solution is filtered to remove insoluble
pyridinium tetrafluoroborate
. The
chloroform solution is evaporated to remove a trace amount of
pyridine
(Note 3), and the residue is chromatographed with
Wakogel C-300 (400 g) and
chloroform
(Note 4). The red solution that is eluted is evaporated and the mass obtained recrystallized from
ethanol to yield
12.34 g (
80.0% of
8,8-dicyanoheptafulvene
,
2, red needles, mp
199-200°C (lit.
3 198-199°C, lit.
4 201-202°C)
(Note 5).
2. Notes
1.
Pyridine, purified by distillation immediately before use must be anhydrous.
Cycloheptatrienylium (tropylium) tetrafluoroborate, 1, was prepared by the method of Conrow2 and its purity was at least 99%. Aldrich Chemical Company, Inc.
, also provides
1.
2.
Bromomalononitrile was prepared by bromination of
malononitrile by the method of Ferris, et al.
5 To this end,
bromine (48.5 g, 0.30 mol) was added over a period of 5 hr to
malononitrile (20 g, 0.30 mol) dissolved in water (300 mL) in an ice bath, and kept overnight under those conditions. A slight brown-white precipitate separated from the solution. It was washed with water and taken up in CHCl
3 (100 mL). After drying (Na
2SO
4) and concentrating under reduced pressure to about one half the original volume, the solution was cooled in a refrigerator to deposit colorless crystals, which were then purified by recrystallization from CHCl
3 to give
21 g (
48%) of the desired
bromomalononitrile
, mp
63-64°C (Ferris, et al.,
5 reported
20% yield).
3.
Prior to chromatography,
pyridine must be thoroughly eliminated by distillation. Use of dilute acid fractionation gave an inferior result.
4.
From the less polar yellowish fractions,
β,β-dicyanostyrene (0.23 g, 1.5%) was obtained before elution of the desired compound.
5.
8,8-Dicyanoheptafulvene
2 had the following spectroscopic properties:
1H NMR (500 MHz, CDCl
3) δ: 6.88-7.00 (m, 4 H) and 7.34 (dm, 2 H, J = 12.1)
;
13C NMR (125 MHz, CDCl
3) δ: 70.1, 114.6 (2C), 135.4 (2C), 137.4 (2C), 138.9 (2C), and 163.7
; IR (KBr) cm
−1: 2196, 1634, 1584, 1520, 1488, 1404, 1372, 1267, 886, 829, 763, 603, and 538
.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
Previously,
8,8-dicyanoheptafulvene,
2, was prepared by (a) thermolysis of
2,2-bis(cycloheptatrienyl)malononitrile
3 and (b) acetic anhydride-mediated condensation of
tropone
with
malononitrile.
4 Since
8,8-dicyanoheptafulvene,
2, is a representative stable heptafulvene, its synthetic utility and potential for functional materials warrant an efficient synthesis. As method (a) involves disproportionation of two cycloheptatriene units, and method (b) uses tropone derivatives as the starting material, it was desirable to develop a practical method of synthesizing
8,8-dicyanoheptafulvene,
2, from the cycloheptatrienylium salt. The late Kitahara and co-workers attempted the preparation of
8,8-dicyanoheptafulvene,
2, based on this concept. Although they were partially successful, no experimental information is available; the literature
6 simply states that (an unspecified) base-treatment of
cycloheptatrienylium salt with
bromomalononitrile, according to Kitahara, formed
β,β-dicyanostyrene.
The present procedure emphasizes pyridine as the solvent with mild heating. When pyridine is partially replaced with acetonitrile, the yield of 8,8-dicyanoheptafulvene, 2, drops to less than 40%.
The present procedure is applicable only to
bromomalononitrile; various α-halogen active methylene derivatives, (e.g.,
diethyl chloromalonate,
methyl bromoacetoacetate) led predominantly to the formation of ring-contracted styrene derivatives. On the other hand, substituted cycloheptatrienylium salts with
bromomalononitrile gave the desired dicyanoheptafulvene derivatives in excellent yields. One notable example is the synthesis of
5- and 7-(dicyanomethylene)-2,3-dihydrocyclohepta-1,4-dithiins.
7
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
8,8-Dicyanoheptafulvene:
2,4,6-Cycloheptatriene-Δ1,α-malononitrile (8);
Propanedinitrile, 2,4,6-cycloheptatrien-1-ylidene- (9);
(2860-54-0)
Cycloheptatrienylium tetrafluoroborate: Aldrich:
Tropylium tetrafluoroborate:
Cycloheptatrienylium, tetrafluoroborate (1−) (8,9);
(27081-10-3)
Bromomalononitrile, Malononitrile, bromo- (8);
Propanedinitrile, bromo- (8);
(1885-22-9)
Pyridinium tetrafluoroborate:
Pyridine, tetrafluoroborate (1−) (8,9);
(505-07-7)
Malononitrile: HIGHLY TOXIC (8);
Propanedinitrile (9);
(109-77-3)
Bromine (8,9);
(7726-95-6)
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