Tricarbonyl[(2,3,4,5-η)-2,4-cyclohexadien-1-one]iron and Tricarbonyl[(1,2,3,4,5-η)-2-methoxy-2,4-cyclohexadien-1-yl]iron(1+) hexafluorophosphate(1+) could be produced through the following synthetic routes.

A. 1-Methoxy-1,4-cyclohexadiene (CAS NO.: ). A 3-l., three-necked, round-bottomed flask equipped with an inlet tube, mechanical stirrer, and an acetone–dry ice condenser fitted with a drying tube is charged with 150 ml. of tetrahydrofuran, 250 ml. of tert-butyl alcohol, and 50 g. (0.46 mole) of anisole. About 1.5 l. of dried liquid ammonia is distilled into the reaction vessel from a steam bath. (11.5 g., 1.66 g.-atoms) is added cautiously with stirring and, when the addition is complete, the stirring is continued for 1 hour with refluxing. The blue color is discharged by cautiously adding methanol dropwise (about 100 ml. is required); 750 ml. of water is then added carefully. The excess ammonia is allowed to evaporate overnight, more water is added, dissolving the lithium salts, and the mixture is extracted three times with 100-ml. portions of petroleum ether (b.p. 30–40°). The combined extracts are washed four times with 75-ml. portions of water, removing tert-butyl alcohol and methanol, and dried over anhydrous magnesium sulfate, and the solvent is removed through a 30-cm. Vigreux column under reduced pressure (20 mm.). Distillation of the residue yields 1-methoxy-1,4-cyclohexadiene (38–40 g., about 75%), b.p. 40° (20 mm.).

B. Tricarbonyl[(1,2,3,4-η)-1- and 2-methoxy-1,3-cyclohexadiene]iron. A 500-ml., three-necked, round-bottomed flask equipped with a nitrogen-inlet tube, a condenser provided with a gas bubbler, and a stopper is flushed with nitrogen and charged with 39 g. (0.35 mole) of 1-methoxy-1,4-cyclohexadiene, 320 ml. of dibutyl ether, and 95 g. (65 ml., 0.49 mole) of filtered iron pentacarbonyl. Using a heating mantle, the mixture is refluxed for 18 hours under a slow nitrogen stream. After cooling, the reaction mixture is filtered by suction through Celite, removing iron particles, the Celite is washed twice with 15-ml. portions of dibutyl ether, and the washings and filtrate are combined. The crude product is obtained by evaporating excess iron pentacarbonyl, unreacted diene, and the dibutyl ether using a rotary evaporator (in a fume hood), with a hot-water bath and ice cooling of the receiver. The distillate is again refluxed for 18 hours under nitrogen as before and worked up in the same manner. This procedure is then repeated again. Distillation of the combined residues using a nitrogen leak yields 54 g. of the product as a yellow oil, b.p. 66–68° (0.1 mm.). The distillation residue, after elution through a short acidic alumina column with light petroleum ether and solvent evaporation, yields an additional 5 g. of product, giving a total yield of 59–68 g. (67–78%).

C. Tricarbonyl[(1,2,3,4,5-η)-1- and 2-methoxy-2,4-cyclohexadien-1-yl]iron(1+) tetrafluoroborate(1-). Triphenylmethyl tetrafluoroborate (34 g., 0.10 mole) is dissolved in a minimum volume of dichloromethane, and 18 g. (0.072 mole) of tricarbonyl (1- and 2-methoxy-1,3-cyclohexadiene)iron dissolved in a like volume of dichloromethane is added. The resulting dark solution is left for 20–30 minutes and added with stirring to three times its volume of diethyl ether. The precipitate is collected and washed with ether, yielding 21–22 g. (87–91%) of product as a yellow solid.

D. Tricarbonyl[(2,3,4,5-η)-2,4-cyclohexadian-1-one]iron. The tetrafluoroborate mixture from Part C (21 g., 0.062 mole) is heated on a steam bath for 1 hour in 450 ml. of water, during which time orange crystals separate. After cooling, the mixture is extracted three times with 100-ml. portions of ether, into which most of the solid dissolves. (The aqueous layer is used in Part E.) The extracts are dried over anhydrous magnesium sulfate, and the ether is evaporated, yielding 7–7.5 g. (47–51%) of the yellow crystalline dienone complex.

E. Tricarbonyl[(1,2,3,4,5-η)-2-methoxy-2,4-cyclohexadien-1-yl]iron(1+) hexafluorophosphate(1+). To the aqueous layer from Part D is added with swirling 7.1 g. (0.044 mole) of ammonium hexafluorophosphate in 30 ml. water. After 30 minutes, the light-yellow product is filtered, washed with water, and air dried, yielding 9–10 g. (35–44%).

Notice: Parts A and B must be conducted in an efficient hood to prevent exposure to ammonia, iron pentacarbonyl, and carbon monoxide.