Organic Syntheses, CV 7, 200
Submitted by R. T. Taylor and L. A. Paquette
1.
Checked by David A. Cortes and M. F. Semmelhack.
1. Procedure
A.
7,7-Dibromo-1,6-dimethylbicyclo[4.1.0]hept-3-ene. Into a
3-L, three-necked flask equipped with an
overhead stirrer,
1-L addition funnel, and
reflux condenser capped with a
nitrogen-inlet tube are introduced
44.8 g (0.4 mol) of powdered potassium tert-butoxide (Note
1) and
1 L of olefin-free petroleum ether (bp
35–55°C; (Note
2)). To this stirred mixture is added a solution containing
38.0 g (0.35 mol) of 1,2-dimethyl-1,4-cyclohexadiene (Note
3) in 200 mL of the same solvent. With external cooling from an
ice bath and under
nitrogen,
102.4 g (0.4 mol) of bromoform in 400 mL of petroleum ether is added dropwise during 1 hr. The ice bath is removed and the resultant slurry is stirred at room temperature under
nitrogen for 6 hr. Water (500 mL) is added and the mixture is poured into a
3-L separatory funnel containing
300 mL of benzene. The organic layer is washed with four 500-mL portions of water, dried over anhydrous
magnesium sulfate, and concentrated on a
rotary evaporator (Note
4). Further evacuation at 0.5 mm produces a solid that is recrystallized from
ether–
petroleum ether (1 : 3) to afford
55–62 g (
56.5–63.5%) of colorless solid, mp
95–98°C (Note
5).
B.
1,6-Dimethyltricyclo[4,1.0.02,7]hept-3-ene. A solution of
20.95 g (0.075 mol) of 7,7-dibromo-1,6-dimethylbicyclo[4.1.0]hept-3-ene in
500 mL of anhydrous ether is placed in a
1-L, three-necked flask equipped with a
magnetic stirring bar, reflux condenser, addition funnel, and nitrogen-inlet tube. With stirring under
nitrogen and external cooling in an ice bath,
50 mL of 1.6 M ethereal methyllithium (Note
6) in
70 mL of ether (0.08 mol) is introduced by dropwise addition during 30 min. The ice bath is removed and the mixture is stirred at room temperature for 1 hr. After 100 mL of water has been cautiously introduced, the mixture is transferred to a separatory funnel and the organic layer is separated. This solution is washed with water (3 × 100 mL), dried over anhydrous
sodium sulfate (Note
7), and carefully concentrated by slow distillation through a
40-cm Vigreux column at atmospheric pressure, heating at <60°C (Note
8). The residual liquid is distilled through a
short, unpacked column to give
4.2–4.4 g (
46–49%) of colorless oil, bp
48–49°C (23 mm) (Note
9). Under the proper conditions, this hydrocarbon can be stored for 2 weeks at −5°C without deterioration.
2. Notes
1.
Potassium tert-butoxide can be obtained commercially from MSA Research Corporation, Callery, Pennsylvania. The checkers used a sample from Aldrich Chemical Company, Inc.
2. A
liter of technical grade petroleum ether was treated in a separatory funnel with
200 mL of concentrated sulfuric acid, washed with water, and dried over anhydrous
magnesium sulfate.
3. This diene was prepared by the procedure of Paquette and Barrett;
2 satisfactory results can be realized with material of 70–85% purity (15–30% contamination by
o-xylene) since the aromatic impurity does not react subsequently and is easily removed.
4. Any residual
o-xylene should be removed prior to crystallization because the dibromide is exceedingly soluble in aromatic solvents.
5. Further recrystallization is not necessary, but pure crystals, mp
107–108°C, can be obtained in the manner described by Vogel and co-workers.
3
6. The
ethereal methyllithium solutions were purchased from Alfa Inorganics. The concentration of
methyllithium in such solutions may be conveniently determined by a procedure described elsewhere
4,5 in which the
lithium reagent is titrated with
sec-butyl alcohol, utilizing the charge transfer complex formed from
bipyridyl or o-phenanthroline and the
lithium reagent as indicator.
7. Anhydrous
magnesium sulfate is too acidic for this purpose and promotes rearrangement of the hydrocarbon.
8. All glassware that is to contain the cyclized product should be washed in base and dried (where necessary) prior to use.
9. The checkers found bp
55–56°C/30 mm. Attempted distillation at ca. 50 mm (bp 75°C) led to significant rearrangement to a
dimethylcycloheptatriene. The product exhibits the following
1H NMR spectrum (CDCl
3) δ: 1.08 (s, 3 H, CH
3), 1.33 (d, 1 H,
J = 2, methine C-
H), 1.52 (s, 3 H, CH
3), 2.15–1.80 (m, 3 H, allylic methylene and methine), 5.50–5.15 (m, 1 H, olefinic C-
H), 6.10–5.70 (m, 1 H, olefinic C-
H).
3. Discussion
The tricyclo[4.1.0.0
2,7]hept-3-ene ring system, with its conjugated bicyclobutane ring and double bond and its isomeric relationship to
cycloheptatriene, has recently commanded attention as a precursor of yet more highly strained molecules. However, the preparation of the parent hydrocarbon by reaction of
7,7-dibromo-3-norcarene with
methyllithium at 0°C, first reported by Klummp and Vrielink,
6 does not proceed in yields above 1–5%.
6,7 Placement of a single methyl group at a ring juncture position of the transient
norcarenylidene intermediate is, however, adequate to promote efficient ring closure through C-H alpha insertion.
7,8 The procedure described above is exemplary. Although two alternative routes to tricyclo[4.1.0.0
2,7]hept-3-enes are currently available,
6,9 alkyllithium-promoted cyclization of readily available
7,7-dibromobicyclo[4,1.0]hept-3-enes constitutes the most direct and efficient approach. In addition, this procedure illustrates an entirely general method for converting norcarane derivatives to
endo,endo-1,3-bridged bicyclobutanes.
10,11,12
Exposure of tricyclo[4.1.0.0
2,7]hept-3-enes to catalytic amounts of Ag
+ leads instantaneously and quantitatively to cycloheptatriene derivatives.
7 Promise of their usefulness as synthetic intermediates is growing rapidly.
13,14
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
petroleum ether
olefin-free petroleum ether
bipyridyl or o-phenanthroline
7,7-dibromobicyclo[4,1.0]hept-3-enes
endo,endo-1,3-bridged bicyclobutanes
sulfuric acid (7664-93-9)
Benzene (71-43-2)
ether (60-29-7)
sodium sulfate (7757-82-6)
nitrogen (7727-37-9)
bromoform (75-25-2)
lithium (7439-93-2)
magnesium sulfate (7487-88-9)
Methyllithium (917-54-4)
cycloheptatriene (544-25-2)
1,2-Dimethyl-1,4-cyclohexadiene (17351-28-9)
sec-butyl alcohol (78-92-2)
1,6-Dimethyltricyclo[4.1.0.02,7]hept-3-ene (61772-32-5)
7,7-Dibromo-1,6-dimethylbicyclo[4.1.0]hept-3-ene (38749-43-8)
dimethylcycloheptatriene
7,7-dibromo-3-norcarene
norcarenylidene
o-Xylene (95-47-6)
potassium tert-butoxide (865-47-4)
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