Palladium species

Transmetallation—Transfer of substituent R from tin to the palladium center thus generating a palladium species 7 that contains both the fragments R and R that are to be coupled. 

Note. Enol acetates undergo a similar oxidation, using the palladium species and tributyltin methoxide as dual catalysts. 

Ph3P)4Pd and certain Pd(II) complexes in the presence of an excess of a tertiary phosphine also function as active catalysts (128). This indicates that palladium species may have potential provided they are protected from destructive reduction by the choice of suitable ligands. A complex species [(Ph3P)2Pd]jf gradually forms in the PhjP—Pd metal mixture ( 28). 

Yamamoto Y,Nakamura I (2005) Nucleophilic Attack by Palladium Species. 14 211-240 Yasuda H (1999) Organo Rare Earth Metal Catalysis for the Living Polymerizations of Polar and Nonpolar Monomers. 2 255-283 Yasuda N, see King AO (2004) 6 205-246... 

Figure 22.1. Schematic representation of use of insoluble poisons to selectively terminate catalysis from leached palladium species in couphng reactions.

Under all the conditions studied, addition of bare Si02-SH to Heck or Suzuki coupling reactions using a variety of bases, aryl halides and solvents resulted in complete cessation of the catalytic activity (35). These results suggest that catalysis with this precatalyst is also associated with labile palladium species that... 

Most commercial palladium scavengers are designed for use after a reaction - thus at low temperatures. Is the solid poison being considered stable at the temperatures of the reaction Many Heck couplings are used at well over 100°C and some polymeric poisons may bre down under these conditions. This could lead to ineffective poisoning of solution species as well as unwanted interaction of small molecule degradation products with supported palladium species. 

Allyl carbamates also can serve as amino-protecting groups. The allyloxy group is removed by Pd-catalyzed reduction or nucleophilic substitution. These reactions involve formation of the carbamic acid by oxidative addition to the palladium. The allyl-palladium species is reductively cleaved by stannanes,221 phenylsilane,222 formic acid,223 and NaBH4,224 which convert the allyl group to propene. Reagents... 

Nucleophilic Substitution of xi-Allyl Palladium Complexes. TT-Allyl palladium species are subject to a number of useful reactions that result in allylation of nucleophiles.114 The reaction can be applied to carbon-carbon bond formation using relatively stable carbanions, such as those derived from malonate esters and (3-sulfonyl esters.115 The TT-allyl complexes are usually generated in situ by reaction of an allylic acetate with a catalytic amount of fefrafcz s-(triphenylphosphine)palladium... 

The various palladium species can be subject to decomposition and deposition of palladium metal, which generally leads to catalyst inactivation. Apart from their effect on the catalyst activity, the ligands and bases also affect catalyst longevity. 

LaCo03. In such a situation, tentative explanations on possible changes in the chemical environment of oxidic palladium species could be proposed with a subsequent stabilisation of oxidic Pd species in the perovskite matrix. The formation of well-dispersed Pd"1 entities could further inhibit the formation of sulphate generally observed on PdO at high temperature [113,115],... 

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

Lamaty and coworkers described a straightforward combination of three Pd-cata-lyzed transformations first, an intermolecular nucleophilic substitution of an al-lylic bromide to form an aryl ether second, an intramolecular Heck-type transformation in which as the third reaction the intermediate palladium species is intercepted by a phenylboronic acid [124]. Thus, the reaction of a mixture of 2-iodophenol (6/1-253), methyl 2-bromomethylacrylate 6/1-254 and phenylboronic acid in the presence of catalytic amounts of Pd(OAc)2 led to 3,3-disubstituted 2,3-di-hydrobenzofuran 6/1-255 (Scheme 6/1.66). In addition to phenylboronic acid, several substituted boronic acids have also been used in this process. 

As described in the preceding sections, many domino reactions start with the formation of vinyl palladium species, these being formed by an oxidative addition of vinylic halides or triflates to Pd°. On the other hand, such an intermediate can also be obtained from the addition of a nucleophile to a divalent palladium-coordinated allene. Usually, some oxidant must be added to regenerate Pd11 from Pd° in order to achieve a catalytic cycle. Lu and coworkers [182] have used a protonolysis reaction of the formed carbon-palladium bond in the presence of excess halide ions to regenerate Pd2+ species. Thus, reaction of 6/1-386 and acrolein in the presence of Pd2+ and LiBr gave mainly 6/1-388. In some reactions 6/1-389 was formed as a side product (Scheme 6/1.98). 

In what Co/Pd-HFER reduction profile is concerned, palladium species are reduced from RT until 130 °C. This reduction process can be decomposed into 2 peaks, meaning that, besides the reduction of cationic Pd2+, the reduction of PdO also occurs at relatively low temperatures [14], At 200-250 °C a reduction process is detected in the bimetallic catalyst, currently assigned to the reduction of cobalt oxo-ions [9,15] reduction of Co304 takes place in the temperature range of 350-500 °C [11]. At higher... 

---