Phosphine-based palladium catalysts

The novel catalyst system based on palladium(O) /V-heterocyclic carbene complexes was developed by the group of Beller, in part prompted by the strong patent position of Dow on phosphine-based palladium catalysts [8]. The catalyst [37], either generated in situ from the corresponding imidazolium salt or the molecularly defined divinyldisiloxane complex [Pd(Imes)(dvds)] (Fig. 3), was used in the telomerization of 1,3-butadiene with methanol [38—40]. The /V-heterocyclic carbenes are in general better a-donor ligands and come with considerably different steric requirements than the phosphines. The [Pd(Imes)(dvds)] complex resembles the final telomer-palladium product complex and thus offers a facile and clean entry into the catalytic cycle. The metal carbene complex was shown to be... 

Monarch butterfly 366, 367 ( )-phorbol 248, 250 phosphine-based palladium catalysts 71... 

Phosphine-based palladium catalysts are generally used since they are stable on prolonged heating however, extremely high coupling reaction rates can sometimes be achieved by using palladium catalysts without a phosphine ligand. ... 

In a modified version of the Suzuki reaction arylboronates or boranes are utilized instead of arylboronic acid. Under the action of phosphine-free palladium catalysts NaBPh4 and tra(l-naphtyl)borane were found suitable phenyl-sources for arylation of haloaromatics in fully or partially aqueous solutions at 20-80 °C with good to excellent yields (Scheme 6.12) [32-34]. Aryl halides can be replaced by water-soluble diaryliodonium salts, At2IX (X = HSO4, BF4, CF3COO) in the presence of a base both Ar groups take part in the coupling [35]. 

Also a pyridazin-3(2//)-one, namely 4-bromo-5-methoxy-2-(4-trifluoromethylphenyl)pyridazin-3(2//)-one 169, has been successfully tackled (Equation 27) <2005JHC427>. In this case, the use of a tri(2-furyl)phosphine rather than a triphenylphosphine-based palladium catalyst was essential to achieve high yields. 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

The cross-coupling reactions of various aryl halides and triflates with vinyl- or arylboronic acids and esters (Suzuki cross-coupling reaction) was also carried out in water in the presence of tetrabutylammonium bromide and a base such as Na2C03, using a phosphine-free palladium catalyst to give biaryl derivatives [Eq. 18)1 [108,109]. More recently, Casalnuovo [101] and Gen t [102,110] have performed this reaction using water-soluble palladium catalysts PdCla (tppms)2 and Pd(OAc)2/tppts in water/acetonitrile. 

While the visual appearance of the products was sufficient to identify the active catalysts, the authors employed a parallel UV plate reader to quantify the results. The rhodium catalysts identified provided encouragement that metals other than palladium could be developed for this transformation, and in a subsequent experiment, the colorimetric technique was used to discover the first non-phosphine-based iridium catalyst for allylic alkylations. A high-throughput approach to the discovery of catalysts for asymmetric allylic alkylations is also described in Section 1.13.3.6. 

By comparison of the a-diimine catalysts with the phosphine sulfonate-based palladium catalyst system, the change from a symmetric to an asymmetric ligand stmcture is apparent on first sight. Here (as well as in other ligand systems Section 3.24.4.1.4), it could be shown that this leads to major differences in reactivity. Despite the partially open catalyst stmcture, these complexes are of remarkable stability and activity in... 

A palladium catalyst with a less electron-rich ligand, 2,2-dipyridyl-methylamine-based palladium complexes (4.2), is effective for coupling of aryl iodides or bromides with terminal alkynes in the presence of pyrrolidine and tetrabutylammonium acetate (TBAB) at 100°C in water.37 However, the reactions were shown to be faster in NMP solvent than in water under the reaction conditions. Palladium-phosphinous acid (POPd) was also reported as an effective catalyst for the Sonogashira cross-coupling reaction of aryl alkynes with aryl iodides, bromides, or chlorides in water (Eq. 4.18).38... 

Table 2 Catalysts performance for different palladium phosphine complexes with lead based co-catalyst (0.25 mM Pd, 50 eg. PbO, 400 eg. TEAB). ...

---