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Synlett 2006(18): 3088-3094  
DOI: 10.1055/s-2006-951519
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Sylvain Rolanda, Pierre Mangeneya, Anny Jutand*b
a Laboratoire de Chimie Organique, UMR CNRS 7611, Université Pierre et Marie Curie, Tour 44-45, 2ème étage. 4 Place Jussieu, 75252, Paris Cedex 5, France
b Ecole Normale Supérieure, Département de Chimie, UMR CNRS-ENS-UPMC 8640, 24 Rue Lhomond, 75231, Paris Cedex 5, France
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Publication History

Received 17 April 2006
Publication Date:
25 October 2006 (online)

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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.

Key words

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

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    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).

20

(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 = (|EpRinv| - |EpR1|)+(|EpRinv| - |EpO1|) (EpRinv 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] .

22

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.

23

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).