trans-3-Methyl-2-vinyl (CAS NO.: ), which is also known as Cyclohexanone, 2-ethenyl-3-methyl-, trans-, could be produced through the following synthetic routes.

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 inlet and a mechanical stirrer with a paddle. Nitrogen is passed through the flask while it is flame-dried 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. 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 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 to a 500-mL, round-bottomed flask containing a magnetic stirrer. An additional 50 mL of tetrahydrofuran is used to rinse the amalgam flask.

diethyl acetal (31.11 g, 0.20 mol) 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. The residue is taken up in ethyl ether, and filtered by suction through a 1-in. plug of Celite packed in a 250-mL, coarse-porosity, fritted Schlenk tube. 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 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. 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 (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 is added by syringe, then 12.60 g (0.13 mol) of 2-cyclohexen-1-one 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, 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.

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 and may be stored under nitrogen of 0°C for several weeks with no observable decomposition.

C. trans-3-Methyl-2-vinylcyclohexanone (4). Compound 3 (31.5 g) and 25 mL of acetonitrile (0.47 mol, 6-fold excess) are placed in a 100-mL round-bottomed flask 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.

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.

Notice: Care should be exercised in the preparation of the sodium amalgam since the initial reaction is highly exothermic. This and all subsequent operations should be carried out in a well-ventilated hood. Chloroacetaldehyde diethyl acetal is an irritant and a mutagen. Care should be exercised in its handling.