Catalysis of Organic..

Falling et al. 329With these process factors in mind, a continuous, homogeneous reaction processconcept was developed in which the starting epoxide 1 is fed as a liquid to thereactor containing the soluble Lewis acid and iodide salt while continuouslyremoving the 2,5-DHF and crotonaldehyde by distillation. Continuously, or asneeded, the catalysts are recovered from the reaction mixture by extraction with analkane and the undissolved oligomer is discarded.Catalyst RequirementsThe rearrangement reaction of 1 requires a two component catalyst systemconsisting of a Lewis acid and a soluble source of iodide anion. The process concepthad a number of requirements for the catalysts in order to achieve commerciallyuseful operation. These were:1. High catalytic activity.2. High selectivity (especially low oligomerization).3. Chemical and thermal stability (for acceptable catalyst lifetime).4. Soluble in 2,5-DHF.5. Alkane solubility (for recovery/oligomer removal).6. Low vapor pressure (to prevent losses during distillation steps).7. Low melting point (for easy handling as liquids).8. Low cost.9. Low toxicity.Iodide Salt Catalyst ComponentFor adequate reaction rates, a high concentration of iodide anion is necessary. Thecation portion of the salt appears to have little or no effect on catalytic activity orreaction selectivity. Inorganic iodides (such as potassium iodide) are the obviousfirst choice based on availability and cost. Unfortunately these catalysts have verypoor solubility in the reaction mixture without added solubilizers or polar, aproticsolvents. These solubilizers (e.g., crown ethers) and solvents are not compatiblewith the desired catalyst recovery system using an alkane solvent. Quaternary oniumiodides however combine the best properties of solubility and reactivity.Quaternary ammonium iodides are attractive choices because they generallyhave good activity, low cost, and solubility in the reaction and recovery processes.Simple quaternization of the wide variety of available tri(n-alkyl)amines with n-alkyliodides allows optimization of the tetra(n-alkyl)ammonium iodide salt properties:R 3 N + R’I R 3 R’N + I -The long-term operation of a continuous reaction process with catalyst recoveryrequires a catalyst with very good thermal stability. Unfortunately the tetra(nalkyl)ammoniumiodides which have good solubility properties also have poorthermal stability due to breakdown by dealkylation. This was observed duringoperation of the continuous process and in thermal analysis. Thermogravimetric