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The iron hydrocarbonyl formed by the interaction of iron pentacarbonyl and alkali is reported to be a potent catalyst for the isomerisation of olefins1-4. It seemed interesting therefore to investigate the activity of this catalyst for the isomerisation of safrole to isosafrole which is an important step in the synthesis of piperonal.
Experiments were carried out using iron pentacarbonyl catalyst at w/w with varying amounts of aqueous sodium hydroxide at 95°C. The conversion of safrole to isosafrole under these conditions is most effective when a ratio of 3 moles of sodium hydroxide to 1 mole of iron pentacarbonyl is used5. A modification of the isomerisation reaction could be effected by use of solid alkali flake, which enables the reaction to be carried out at higher temperatures. Under these conditions the exothermic isomerisation, once initiated, will accelerate, resulting in rapid and almost complete conversion of safrole to isosafrole. This isomerisation, as is well known, can also be brought about by alkali alone6, but to obtain the same conversion, much higher catalyst concentrations and/or more vigorous conditions as regards to temperature or duration of reaction are required. The iron carbonyl isomerisation therefore has considerable merit and various techniques for moderating or controlling the reaction rate were examined so that conventional cooling techniques could be employed during larger scale preparations. Typical procedures are given in the Experimental part.
Isomerisation Procedures
(a) Without moderation
Safrole (500 g), iron pentacarbonyl (2.5 g). sodium hydroxide (1.6 g commercial flake) were mixed in a 1-litre flask equipped with stirrer. thermometer and condenser. The well stirred reaction mixture was heated to 110°C at which temperature a vigorous reaction commenced. causing the temperature to rise to 180°C in 6 minutes. After cooling the mixture, 250 ml of 2 N acetic acid were added. The organic layer was separated from the aqueous phase and washed with brine to neutrality. After drying and evaporation of the solvent the mixture was distilled from a Claisen flask and gave 485 g. (97%) of a product which according to analysis by gas-liquid chromatography. contained more than 99.5 % of isosafrole.
(b) With Moderation
Isosafrole (50 g) was charged to a 1 litre flask equipped with stirrer, thermometer, condenser and dropping funnel. Iron pentacarbonyl (2.25 g) and sodium hydroxide (1.42 g of commercial flake) were added to the reaction mixture. The temperature was raised to 130°C and safrole (450 g) was added in the course of 30 minutes. After completion of the addition a conversion of 60% had been obtained7. Maintaining the reaction mixture at 130°C enabled a complete conversion of safrole to isosafrole to be achieved in 1 hours total time. The product was isolated in the manner described before.