Pd-based systems

So far only Pd-based systems have been highlighted in this section however, the use of other metals such as Ni has clear economic advantages. In this regard, Chiu and co-workers have used a bis-carbene tetradentate ligand to catalyse the coupling of aryl bromides and chlorides with both electron rich and electron poor aromatic rings however, the reaction with electron poor aryl bromides lead to superior yields (Scheme 6.30) [113]. 

Kumada-Tamao-Corriu reaction Fe-based systems, 11, 21 Ni-based systems, 11, 20 Pd-based systems, 11, 22 reagent systems, 11, 23... 

Figure 4-24 presents the energy profile for the acrylate insertion, resulting from the calculations performed for the structures based on the model catalyst. The lowest energy reaction intermediates for the 2,1-insertion are present in the figure, as the 2,1-insertion was found to have higher barriers for both, the Ni- and Pd-based systems. 

Hartwig s DPPF/Pd-based system is able to effect similar C-N bond formation reactions between aryl bromides and aniline derivatives, Eq. (80) [29]. 

Hartwig reported that ferrocene-based diphosphine 13 catalyzes the arylation of aniline at room temperature, Eq. (91) [72]. Additionally, the (f-BuljP/Pd-based system effects the room-temperatuxe condensation of anilines and aryl bromides. However, the (f-BuljP/Pd-catalyst is considerably more active [50]. While the reaction with 4-bromotoluene and anihne proceeded in 20 h using 5 mol% 15/Pd(dba)2, the reaction between bromoben-zene and aniline was complete in 1 h using only 1 mol% of the (f-BuljP-derived catalyst. 

The Tosoh group described the first amination of bromothiophenes with a (t-Bu)3P/Pd-based system [131]. The reactions between several bromothiophenes and diphenylamine proceeded with moderate yields, Eq. (164). 

Buchwald and co-workers disclosed that the palladium-catalyzed diaryl ether formation can be performed with a 6/Pd-based system [152]. These reactions are accomplished using a mild base such as K3PO4, and with electron-poor aryl bromides, the reactions proceed in good to excellent yield, Eq. (194). 

The alternating copolymerization of CO and ethylene has been studied in much detail over the past two decades. Although a number of Ni-based systems are known to catalyze this reaction,comparative studies have shown that Pd-based systems are by far the most active catalysts for this reaction. The lower activity of the Ni-based systems is somewhat surprising because the requisite CO and ethylene insertion steps are expected to be energetically more facile for the Ni-based systems. A recent study has shown that the energy barrier for CO insertion into the Ni-Me bond is less than 10 kcal mol" (Scheme 25), significantly lower... 

The DSHP, which is catalyzed by C-supported Pd-based systems, may even occur in aqueous solutions, in the presence of strong mineral acids and halide ions, at temperatures inthe range 0-25 °C and at 100 atm pressure [22]. DuPont have studied extensively the use of Pt and Pd bimetallic catalysts on silica the optimal weight ratio of Ptto metal loading on silica was 0.02-0.2 the selectivity to HP obtained was about 70%, with a concentration of HP exceeding 20%. Key promoters were halogens, such as Cl or Br salts. However, the presence of strong acids may be not compatible with the epoxidation of olefins. 

Examination of catalytic performance of synthesized Pd-based systems. 

In the case of Pd-diimine catalyst, the isomerisation barriers (5.5-8.0 kcal /mol) are substantially lower than the insertion barriers (17.5-18.5 kcal/mol) [ 13c,d, 17,33-35]. The difference between the isomerisation and insertion barriers is substantially larger than for the corresponding Ni-complexes, for which the isomerisation barriers are higher by ca. 5-8 kcal/mol and the insertion barriers are lower by 4-5 kcal/mol [ 1 ld,e, 13a-c]. As we will show later, this is responsible for a difference in the pressme effect on the branching numbers observed for Ni- and Pd-based systems [29]. 

The results (Table 3) clearly indicate that in the case of the Ni-based Brookhart catalyst the polar monomers are bound by the carbonyl oxygen atom, while in the Pd-systems the n-complexes are preferred. Tables 3 and 4. The difference between the Ni- and Pd-systems has mainly a steric origin with larger repulsive interaction between the occupied d-orbitals on palladium and the oxygen lone-pair orbitals. On the other hand, there is practically no difference in the orbital-interaction contribution (between occupied and virtual orbitals ) to the binding energy between Ni- and Pd-based systems, as far as a... 

The Mizoroki-Heck reaction [eqn (9.1)] is a valuable synthetic tool for the catalytic formation of a range of aryl-olefins, and other related building blocks, widely used in the pharmaceutical and fine chemicals industries. Many palladium-containing compounds are known to catalyse the reaction to varying extents. Over the past two decades [(NHC)Pd]-based systems have been increasingly employed, and seem to provide some advantages with respect to other catalysts. 

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