MECHANISM OF ARYL CHLORIDE OXIDATIVE ADDITION TO CHELATED PALLADIUM(0) COMPLEXES

The mechanism of oxidative addition of aryl chlorides to Pd(dippP)2 (1) has been studied in depth. P-31 NMR monitoring of the reaction of 1 with chlorobenzene in dioxane reveals that cis-(dippp)Pd(Ph)Cl (2) and trans-(eta1-dippp)2Pd(Ph)Cl (3) are formed in parallel pathways and are also in equilibrium with each other, 2 being favored both kinetically and thermodynamically. The reaction is first order in chlorobenzene and approximately first order in Pd(0), the deviation resulting from a parallel equilibrium between 1 and the binuclear complex (dippp)Pd(mu-dippp)-Pd(dippp). Strong rate retardation by added dippp is observed, the reaction being approximately reciprocal first order in dippp. 1 undergoes reversible dissociation of dippp, followed by rate-determining oxidative addition of chlorobenzene to the 14-electron complex (dippp)Pd (5), whose existence was demonstrated directly by inversion transfer NMR experiments using (dipp)-PdPiPr2nBu. The trans complex 3 is formed directly from the 14-electron complex (eta1-dippP)2-Pd. Rate constants for oxidative addition of substituted aryl chlorides show an excellent fit to the Hammet sigma- constants, p = +5.2, whereas the fit to sigma is poor. Activation parameters of DELTAH(double dagger)obs = 28 +/- 3 kcal/mol and DELTAS(double dagger)obs = -2 +/- 8 eu were measured; since DELTAS(double dagger)obs = DELTAS-degrees(diss) + DELTAS(double dagger)OA and DELTAS-degrees(diss) much greater than 0, then DELTAS(double dagger) OA, much less than 0. These observations taken together provide strong evidence for an SNAr-type charged transition state, probably stabilized by partial chloride coordination.