Gas–liquid phase-transfer catalysis: Wittig–Horner reaction in heterogeneous conditions
The Wittig–Horner synthesis of alkenes has been carried out under gas–liquid phase-transfer catalysis (g.l.–p.t.c.) conditions. With this technique the carbonyl compound and the phosphonate flow, under pressure, through solid potassium carbonate contained in a thermostatted column where the conditions (temperature and pressure) are such as to ensure that the reactants are gaseous. The alkene so produced was cooled and collected at the column outlet. Reaction yields were higher when the solid potassium carbonate was coated with poly(ethylene glycol)–Carbowax 6000. The function of the poly(ethylene glycol) is discussed. Aromatic aldehydes react successfully with triethyl phosphonoacetate and diethyl (cyanomethyl)phosphonate; ketones react only with diethyl (cyanomethyl)phosphonate. Reaction conversions (based on the purity of the recovered alkene) are always high, while yields (measured on the basis of the actual amount of pure alkene recovered, with respect to the reacting carbonyl compound) are lower.