SYNTHESIS OF GAMMA-KETOCYCLOALKANECARBOXYLIC ACID-ESTERS BY REGION-SPECIFIC ALKOXY CARBONYLATION OF ALPHA,BETA-KETOCYCLOOLEFINS CATALYZED BY PALLADIUM

PdCl2(PPh(3))(2), in combination with HCl, is highly active in the alk oxycarbonylation of alpha,beta-ketocycloolefins to 3-oxocycloalkanecar boxylic acid esters. The reaction is region specific. The catalytic sy stem is stabilized by addition of PPh(3), which depresses the activity but prevents decomposition to inactive palladium metal. Typical react ion conditions are: Pd/P/cyclohexenone/HCl = 1/2-4/200-600/50-100; [Pd ] = 0.1-1 . 10(-2) mol l(-1); P-CO = 100 atm; temperature 100 degrees C; solvent dioxane, THF, or benzene. With 2-cyclohexen-1-one the effec t of the following variables on the yield has been examined: temperatu re, pressure of carbon monoxide, concentration of cyclohexenone, ethan ol, catalyst, Pd/P ratio, cyclohexenone/HCl ratio, reaction time, and solvent. As expected the yield increases with increasing reaction time , pressure of carbon monoxide, concentration of catalyst at constant P d/P ratio, and temperature. The yield passes through a maximum when in creasing the concentration of HCl and EtOH, after which the yield decr eases because the catalyst decomposes to palladium metal. The yield is almost insensitive to the polarity of solvents such as dioxane, THF, or benzene, but it rather depends on the bulkyness of the alkanol as t he reaction rate decreases in the order: MeOH > EtOH > n-PrOH > n-BuOH > s-PrOH. The proposed reaction mechanism proceeds through the follow ing steps. (i) The starting Pd-II precursor is reduced to a Pd-0 compl ex which gives oxidative addition of HCl with formation of a Pd-hydrid e species; (ii) cyclohexenone inserts into a Pd-H bond giving a beta-k etocyclohexyl-Pd intermediate; (iii) this inserts CO with formation of the corresponding carbonyl derivative; (iv) alcoholysis of this inter mediate yields the final product with regeneration of the hydride. The reaction is region specific because of the keto group which directs t he anti-Markownikoff addition of Pd-H to the conjugated C=C double bon d.