THE CARBON CARBON BOND-FORMING STEP IN CATALYTIC CROSS-COUPLING - MIGRATION OR ELIMINATION

Two alternative pathways for reductive elimination of adjacent sp3 and sp2 carbon substituents from palladium have been examined by the extended Huckel method, with cis-(PH3)Pd(CH3)CH = CH2 as a model. A brief comparison with the nickel analogue has been made. In the first set of calculations, concomitant lengthening of both Pd-C bonds was permitted to proceed synchronously, leading to an isolated molecule of propene and to (PH3)2Pd. This conventional pathway for reductive elimination, which demonstrated behavior very similar to that calculated previously by the extended Huckel method for cis-(PH3)Pd(CH3)2, was compared with an alternative in which cleavage of the Pd-CH3 bond is more advanced and occurs with synchronous migration to the C-alpha carbon of the vinyl group. The intermediate produced in this way can form an eta-2-propene complex by geometric reorganization. When calculations for the two energy surfaces are compared, the migration pathway is favoured. It is particularly advantageous when the P-Pd-P angle is permitted to relax as reaction proceeds. Comparison is made with experimental evidence, and the calculations are in accord with the observed ease of elimination from square-planar 16e aryl or vinyl complexes.