Kinetic and Electrochemical Studies of the Oxidative Addition of Demanding Organic Halides to Pd(0): the Efficiency of Polyphosphane Ligands in Low Palladium Loading Cross-Couplings Decrypted

Oxidative
addition (OA) of organic halides to palladium(0) species is a fundamental
reaction step which initiates the C–C bond formation catalytic
processes typical of Pd(0)/Pd­(II) chemistry. The use of structurally
congested polyphosphane ligands in palladium-catalyzed
C–C bond formation has generated very high turnover numbers
(TONs) in topical reactions such as Heck, Suzuki, Sonogashira couplings,
and direct sp2C–H functionalization. Herein, the
OA of aryl bromides to Pd(0) complexes stabilized by ferrocenylpolyphosphane
ligands L1 (tetraphosphane), L2 (triphosphane),
and L3 (diphosphane) is considered. The investigation
of kinetic constants for the addition of Ph–Br to Pd(0) intermediates
(generated by electrochemical reduction of Pd­(II) complexes coordinated
by L1–L3) is reported. Thus, in the
OA of halides to the Pd(0) complex coordinated by L1 the
series of rate constants kapp is found
(mol–1 L s–1): kapp(Ph–Br) = 0.48 > kapp(ClCH2–Cl) = 0.25 ≫ kapp(p-MeC6H4–Br)
= 0.08 ≈ kapp(o-MeC6H4–Br) = 0.07 ≫ kapp(Ph–Cl). Kinetic measurements clarify
the influence that the presence of four, three, or two phosphorus
atoms in the coordination sphere of Pd has on OA. The presence of
supplementary phosphorus atoms in L1 and L2 unambiguously stabilizes Pd(0) species and thus slows down the OA
of Ph–Br to Pd(0) of about 2 orders of magnitude compared to
the diphosphane L3. The electrosynthesis of the complexes
resulting from the OA of organic halides to [Pd(0)/L]
is easily performed and show the concurrent OA to Pd(0) of the sp3C–Cl bond of dichloromethane solvent. The resulting
unstable Pd/alkyl complex is characterized by NMR and single crystal
X-ray structure. We additionally observed the perfect stereoselectivity
of the OA reactions which is induced by the tetraphosphane ligand L1. Altogether, a clearer picture of the general effects of
congested polydentate ligands on the OA of organic halides to Pd(0)
is given.