Influence of chelating phosphines on the insertion of isocyanides into palladium-methyl bonds in (P-P)Pd(Me)Cl complexes and their further reaction with olefins and isothiocyanates

A systematic study of the insertion reactions between (P-P)Pd(Me)Cl (where P-P = dppe, dppp, dppf) and CNR (where R = 2,6-dimethylphenyl, tert-butyl) is presented. The results have demonstrated that the insertion of CNR into the Pd-Me bond is highly dependent on the nature of the chelating phosphine. For the more strained complex formed with dppe, the insertion of isocyanides is slower and multiple insertion is not observed. This is in contrast to (dppp)Pd(Me)Cl, which reacts readily with isocyanides to produce single-, double-, and multiple-inserted products. Results analogous to those of dppp have been observed for the complexes with dppf, although the reactions are more controllable, indicating a less reactive complex. In an attempt to establish some structure-reactivity correlations, structural characterizations of (dppe)Pd{C(Me)=NXyl}Cl (1) and (dppp)Pd{(C=NXyl)(C(Me)=NXyl)}Cl (3) have been carried out. The structural parameters obtained are consistent with the observed reactivity. The reactions between (dppe)Pd{C(Me)=NXyl}Cl and unsaturated (olefins and isothiocyanates) species have also been studied. While norbornadiene (a strained diolefin) inserts into the palladium-iminoacyl bond, an addition is observed when reacting 1 with isothiocyanates.