Additions at Monoenes

Additions at Monoenes.—Results obtained recently have led to a reappraisal of the mechanism of the Wacker process (the [PdCl4] -CuCl2-cataIysed oxidation of ethylene to acetaldehyde). A key feature of the originally proposed mechanism, largely deduced from the kinetic form of the reaction (below) is the formation of 

The formation of a three-co-ordinate species before a fast / -elimination fits in well with other recent observations. 

Nucleophiles other than OH also give stereospecific additions at alkenes. The acetylacetonate ion adds to co-ordinated cis- or trans-C J z in [Pd(CsH5)-(PPh3)(CHD=-CHD)]+ to give [Pd(C5H5)(PPh3) CHDCHDCH(COMe)2 ] by fra 5-addition, and the overall c/j-diamination of alkenes has been achieved by rraAZ5-aminopalladation of alkenes followed by oxidative amination in which the amine displaces the oxidized palladium by an 5 n2 displacement so that there is inversion at the Pd-bound carbon atom. Other carbon-centred nucleophiles (aryl, alkyl, or methoxycarbonyl) had previously been shown to add cis to alkenes, presumably by prior co-ordination at the metal. 

The kinetics of the exo-PPha addition at the j -allyl function of [Co(C5H5)-(l,2,3,5,6-j -cyclo-octadienyl)]+ have been measured. With an excess of PPhj present the observed first-order rate constant for the approach to equilibrium is given by kobs= i[PPh3]+A -i so that rate constants and activation parameters for forward and reverse reactions are obtained. The mechanism is a simple addition, the rate of which is at least 65 times less than that at the l,2,3,4,5- -cycloheptadienyl analogue.  

---