The mechanism of oxidative addition of aryl chlorides to Pd(dippp)2 (1) has been studied in depth. 31P NMR monitoring of the reaction of 1 with chlorobenzene in dioxane reveals that cis-(dippp)Pd(Ph)Cl (2) and trans-(η1-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(μ-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 (dippp)-PdPiPr2 nBu. The trans complex 3 is formed directly from the 14-electron complex (η1-dippp)2-Pd. Rate constants for oxidative addition of substituted aryl chlorides show an excellent fit to the Hammet σ1 constants, ρ = +5.2, whereas the fit to σ is poor. Activation parameters of ΔH‡ obs = 28 ± 3 kcal/mol and ΔS‡ obs = -2 ± 8 eu were measured; since ΔS‡ obs = ΔS°diss + ΔS‡ OA and ΔS°diSS ≫ 0, then ΔS‡ OA, ≪ 0. These observations taken together provide strong evidence for an SNAr-type charged transition state, probably stabilized by partial chloride coordination.
|Number of pages||9|
|Publication status||Published - 1993|
ASJC Scopus subject areas
- Inorganic Chemistry
- Organic Chemistry