2, 3, AND 4 WATER CHAIN MODELS FOR THE NUCLEOPHILIC-ADDITION STEP IN THE WACKER PROCESS

The first two steps of the Wacker process are studied using a density functional method and a variety of chemical models. These two steps ar e, first, the nucleophilic addition of an hydroxyl group to an ethylen e coordinated to palladium dichloride and, second, the beta-eliminatio n step leading to a pi-coordinated vinyl alcohol complex. The most imp ortant result is found for the nucleophilic addition step. It is shown that the nucleophile should be modeled by a chain of water molecules. At least three water molecules are needed to bridge from the point of attack on the olefin to the negative chloride ligand. The H5O2+-Cl- i on pair is formed simultaneously as the nucleophilic addition occurs i n a conceited way with a low barrier. Since a charge separation occurs in this step, solvent effects are quite important. This is also true for the second step of the process where beta-elimination occurs, sinc e in this step a negative chloride is moved away from the positive pal ladium atom in order to make place for the hydride. When very large ba sis sets are used and solvent effects are accounted for, good agreemen t with what is known experimentally is found.