Reactivity of Pd(0)(NHC)₂ (NHC: N-Heterocyclic Carbene) in Oxidative ­Addition with Aryl Halides in Heck Reactions

Abstract

Pd(0)(NHC)₂ complexes (saturated or unsaturated NHC) may react in oxidative addition of aryl halides by two mechanisms: a dissociative mechanism involving Pd(0)(NHC) as reactive complex formed after dissociation of one carbene or an associative mechanism involving Pd(0)(NHC)₂ as the reactive complex. The mechanism of the oxidative addition of aryl halides with Pd(0)(2)₂ (2: 1,3-di-benzyl-4,5-di-tert-butylimidazolidin-2-ylidene) is established in this work (associative mechanism) and compared to that of Pd(0)(1)₂ (1: 1,3-di-tert-butylimidazolin-2-ylidene) which reacts via Pd(0)(1) in a dissociative mechanism, as reported by Cloke et al. Both complexes activate aryl chlorides at room temperature. The more reactive complex with aryl chlorides is Pd(0)(2)₂ which directly reacts in an associative mechanism. Pd(0)(2)₂ is even more reactive than Pd(0)(1). Consequently, the reactivity of Pd(0)(NHC)₂ complexes in oxidative additions is not connected to the structure of the reactive species, i.e., Pd(0)(NHC)₂ vs. Pd(0)(NHC) but is more relevant to the electronic and steric properties of the carbene ligand.

palladium - NHC carbene - Heck reactions - oxidative addition - kinetics - mechanism

References and Notes

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<A NAME="RS08006ST-18A">18a</A>

The slight difference in the oxidation potentials of the Pd(0)(2)₂ complexes generated by the electrochemical reduction of PdCl₂(2)₂ and PdI₂(2)₂ arises from the oxidation process which first generates a Pd(I) complex prompted to react with the halide ions generated in the reduction of the Pd(II) complexes (ECE mechanism). [18b]

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(a) Under stoichiometric conditions {[Pd(0)(2)₂] = [PhBr] = C ₀}, one has 1/x = kC ₀ t + 1. When x = 0.5, one has 2 = kC ₀(ΔE)/v _1/2 + 1. C ₀/v _1/2 was determined as indicated in Figure [7] with ΔE = (|E^pR_inv| - |E^pR₁|)+(|E^pR_inv| - |E^pO₁|) (E^pR_inv is the inversion potential of the scan). (b) PhCl and 4-CF₃C₆H₄Cl were used in large excess. The kinetic law is: lnx = -k[ArCl]t. When x = 0.5, one has ln0.5 = -k[ArCl]t _1/2 = -k[ArCl](ΔE)/v _1/2 with ΔE determined as indicated just above. [ArCl]/v _1/2 was determined as indicated in Figure [8] .

<A NAME="RS08006ST-21A">21a</A>

See for example the detection of Pd(0)(PPh₃)₂ in its non favored equilibrium with Pd(0)(dba)(PPh₃)₂ by cyclic voltammetry even if the low thermodynamic concentration of Pd(0)(PPh₃)₂ in this equilibrium excludes its detection in ³¹P NMR. [19] What is detected in cyclic voltammetry performed at low scan rate at a steady electrode is a dynamic concentration of Pd(0)(PPh₃)₂ as a consequence of the continuous shift if the equilibrium towards Pd(0)(PPh₃)₂ due to its oxidation and consumption in the diffusion layer (CE mechanism). [18b] Consequently, species present at low thermodynamic concentrations in endergonic equilibrium may be detected by cyclic voltammetry performed at low scan rates.

<A NAME="RS08006ST-21B">21b</A>

If Pd(0)(2)₂ dissociated to Pd(0)(2) and the unstable carbene 2: Pd(0)(2)₂ ↔ Pd(0)(2) + 2 during the cyclic voltammetry, this would result in a continuous shift of the equilibrium to its right-hand side by the irreversible decomposition of the unstable carbene 2, making thus the Pd(0)(2)₂ unstable. In this work, the Pd(0)(2)₂ complex was generated transiently by the electrochemical reduction of PdI₂(2)₂ and was found to be quantitatively formed within the longest time scale investigated here (v = 0.2 Vs^-1, δt = 100 s).

<A NAME="RS08006ST-22">22</A>

From DFT calculations it comes out that PMe₃ is a less good ligand than carbenes for Pd(0) complexes. [9a] This effect must be amplified for PPh₃.

<A NAME="RS08006ST-23">23</A>

Procedure for the Determination of the Rate Constant of the Oxidative Addition of Aryl Chlorides with the Electrogenerated Pd(0)(2) ₂ .
Cyclic voltammetry was performed with a generator PAR Model 175 and a home-made potentiostat. The voltammograms were recorded with a digital oscilloscope Nicolet 3091. Experiments were carried out in a three-electrode thermostated cell connected to a Schlenk line. The counter electrode was a platinum wire of ca. 1 cm² apparent surface area; the reference a saturated calomel electrode (Radiometer) separated from the solution by a bridge filled with 2 mL of DMF containing n-Bu₄NBF₄ (0.3 M). The working electrode was a steady gold disk electrode (d = 1 mm). To 8 mL of DMF containing n-Bu₄NBF₄ (0.3 M) was added 26 mg (0.024 mmol, 3 mM) of PdI₂(2) ₂ . The cyclic voltammetry was performed at different scan rates (from 0.2-1 Vs^-1) in the absence of PhCl and then in the presence of increasing amounts of PhCl (from 0.03-1.05 M) at the same scan rates. A similar experiment was performed from 4-MeC₆H₄Cl, at the scan rate of 0.2 Vs^-1 in the presence of 4-MeC₆H₄Cl (from 0.4 M to 1.7 M). Another experiment was performed from 4-CF₃C₆H₄Cl. The scan rate was varied from 0.2-0.5 Vs^-1 in the presence of 4-CF₃C₆H₄Cl (from 0.03-0.45 M).