Heck palladium black formation

Herrmann emphasized that aryl chlorides are unsuitable as arylating agents in Heck reactions primarily because P—C cleavage and loss of Pd(0)-stabilizing phosphines ultimately occur, leading to catalyst deactivation and palladium black formation, rather than resistance of aryl chlorides to oxidative addition.f " ... 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

In a similar approach, naproxen is prepared from an olefin, the product of a Heck reaction (see Section 5.3.2.1). As described above, the reaction proceeds in the presence of water and HC1, additionally copper(n)chloride is added, possibly to prevent the formation of palladium black (Scheme 5.39). Addition of HC1 to the double bond and subsequent oxidative addition reaction of the benzylic chloride with the active Pd° species initiates the catalytic cycle, which proceeds similarly to the ibuprofen synthesis [70-73]. 

The development of Pd colloids as catalyst for C—C coupling reactions is rather recent [5]. The first example was reported by BeUer et al. in 1996 they used preformed Pd coUoids stabihsed by tetra-octylammonium bromide prepared following the Bonneman procedure in the Heck arylation [6]. The colloidal system was effective for the Heck arylation of styrene or butyl acrylate by activated aryl bromides, but showed only moderate to little activity for deactivated aryl bromides and aryl chlorides. To obtain these results, the authors found that the colloidal pre-catalysts must be added slowly to the reaction mixture to avoid the formation of inactive palladium black at the beginning of the reaction. 

As stated earlier, it is very important to keep the size of the clusters down to prevent formation of palladium black. Apart from the kinetic approach described above a mechanical approach is also possible [76]. Two groups have described an ultrasound promoted Ugand-free Heck reaction of aryl iodides [77, 78]. In one case, the yield of product was found to be 8-fold higher than under comparable thermal conditions. 

The same catalyst precursor PPI(G2)-[(diphosphine)PdMe2)i5 was employed by Reetz and co-workers for Heck reactions [9]. By addition of diethyl ether the polymer-bound catalyst could be precipitated and isolated by filtration. Upon its repeated use for catalysis, a slight decrease in activity was observed. By contrast to analogous low molecular-weight catalysts that were not polymer-bound, no formation of palladium black was observed with the dendrimer-bound catalyst. 

Extended X-ray absorption fine stmcture (EXAFS) analysis has been employed recently for the investigation of the role of colloids in catalysis. For Heck reactions with Herrmarm s paUadacycle or with [Pd(OAc)2]/P(tert-Bu)3, no indication of formation of coUoidal palladium was found [44]. By contrast, Koningsberger and co-workers found that in aUyHc amination with allylpalladium complexes with bidentate phosphine Hgands, from the early stages of the reaction onward, deactivation occurs by formation of di- and trinuclear clusters, which aggregate to larger clusters and finally to palladium black [45]. 

Thus, low loadings of palladium without added ligand can be used in cross-coupling reactions such as the Mizoroki-Heck reaction of atyl bromides, but only when the palladium-substrate ratio is kept particularly low, typically from 0.01-0.1 mol%. Quenching of ionic species (into metal colloids) is less likely at lower concentrations due to a lower collision frequency. Pd black formation is a catalytic dead end. At too low a concentration, the reaction will proceed slowly, if at all. - ... 

De Vries has developed ligand-free Heck reactions involving ArBr in the presence of low loading of Pd(OAc)2 (0.01-0.lmol%) and NaOAc as a base [6c]. Soluble Pd" clusters stabilized by Na X" (X"=AcO", Br") are formed that inhibit the reversible formation of inactive palladium black and slowly deliver the active ligand-free Pd (Scheme 19.3b) [6d]. Lower the Pd(OAc)2 loading, higher the TOR Aryl chlorides are unreactive. 

Catalyst deactivation in Heck reaction is considered as a result of palladium cluster aggregation with subsequent precipitation as palladium black. Besides formation of Pd (0), other palladium species can be present in the reaction media, all contributing to catalyst deactivation. An approach proposed by Rothenberg et al. [7] suggests that all palladium species deactivate via a first-order process with a single deactivation constant. 

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