LEVULINIC ACID SYNTHESIS VIA REGIOSPECIFIC CARBONYLATION OF METHYL VINYL KETONE OR OF ITS REACTION-PRODUCTS WITH HYDROCHLORIC-ACID OR AN ALKANOL OR OF A MIXTURE OF ACETONE WITH A FORMALDEHYDE PRECURSOR CATALYZED BY A HIGHLY-ACTIVE PD-HCL SYSTEM

Levulinic acid and its esters are synthesized by the palladium-catalyzed carbonylation of (i) 3-buten-2-one, (ii) 4-alkoxy-2-butanone, (iii) 4-chlorobutanone, and (iv) the products of reaction of acetone with formaldehyde or a formaldehyde precursor, preferably trioxane, in the presence of water or an alkanol containing dissolved hydrochloric acid. The synthesis is regio-specific. The catalytic system is highly active only in the presence of excess hydrochloric acid at 100-110 °C. The presence of the acid is essential for catalytic activity. The yield increases with increasing carbon monoxide pressure, concentration of palladium, concentration of the acid (up to a HCl/substrate ratio of 2.5-3), concentration of the substrate and of the alkanol dissolved in a solvent such as benzene. The following reactivity order has been established: primary alcohol > secondary > tertiary and n-BuOH = i-BuOH > n-PrOH > EtOH. Methanol almost suppresses the reaction. Palladium(O) or palladium(II) complexes can be used as catalyst precursors. They decompose to palladium metal, which is believed to be the true catalyst. Pd/C is also active. The possible modification of the microstructure of the catalyst is discussed on the basis of the reactivity of the metal with the reaction medium. The precursor PdCl2(PPh3)2, which is usually stable in carbonylation reactions up to ca. 110 °C, decomposes to metallic palladium on dissociating the PPh3 ligand, which reacts with the β-ketochloride to yield a phosphonium salt, [PPh3(CH2CH2COCH3)]+ Cl-. Thus the PPh3 ligand cannot further stabilize the palladium(II) complex. Under milder conditions (80 °C), the precursor is recovered as [PdCl3(PPh3)]-[PPh3(CH2 CH2COCH3)]+. Under the reaction conditions employed, the β-ketochloride is present even when starting with a different substrate. The proposed reaction mechanism begins with the oxidative addition of the β-ketochloride to yield an organopalladium(II) intermediate, which inserts carbon monoxide to give a ketoacylpalladium species. This interacts with water or alkanol to yield the final product. Alternatively, the initial organopalladium(II) intermediate may form after insertion of the keto-olenn into the Pd-H bond that forms upon oxidative addition of hydrochloric acid to palladium metal. It is believed that the reaction is regiospecific because of the presence of a keto group in the β-position, rather than of any peculiar ability of the catalyst itself. © 1990.