Palladacycles phosphine-derived

Exposure of the benzo[b]thiophene derivative 44 to a palladium catalyst in the presence of tri(2-furyl)phosphine (TFP) as the ligand led to the product 45, which incorporated two heterocyclic units from the starting material. The mechanistic aspects of this transformation were also discussed, which appears to involve palladacycle intermediates <06JA722>. 

Palladacycles, such as (80), derived from tri(l-naphthyl)phosphine, proved to be very active catalysts for Heck reactions to produce (81) (ArX = Phi, 4-MeCOC6H4Br,... 

A twofold intramolecular Heck reaction of the dibromobenzene derivative 19 has been used to construct the heptacyclic skeleton 20 of cephalostatin analogs. This reaction required a precise control of the reaction time and temperature. The conversion proceeded best with a catalytic amount of the palladacycle from tris(< -tolyl)phosphine and palladium acetate, and gave exclusively (in 80% yield) the heptacycle 20 with an unusual m-annelation of the two newly formed rings (Scheme 10). ... 

Milstein et al. found that Pd complexes with chelating alkylphosphines such as bis(diisopropylphosphino)butane (dippb) efficiently catalyze the olefmation of aryl chlorides with styrenes in the presence of elemental zinc [29]. Unfortunately, these electron-rich phosphines are apparently incompatible with electron-poor olefins such as acrylic acid derivatives. The latter were successfully coupled with activated chloroarenes by Herrmann et al., who used palladacycles or Pd-catalysts with heterocyclic carbenes [30]. 

Many palladacycles, besides those derived from bidenate phosphines, are available for catalyzing organic reactions. These varieties include 0,N-, N,N-, N,C-, P,N-, f C-ligated. species. In the following, a selection of palladacycle catalysts bearing such as well as more exotic bidentate diphosphines are presented. 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers. They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles and tuned catalysts (TunaCat). A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity. ... 

Halogen-bridged dimeric palladacycles can serve as precursors to monomeric derivatives that can readily be obtained by treating the dimers with various reagents, such as phosphines and organometals (Scheme 35). 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

A novel racemic palladacyclic dimeric complex 136 was resolved via recrystallization of its diastereomeric (3 )-prolinate derivatives 137 and 138 (Scheme 20). The chemistry and resolution of chiral PMeBuTh was studied using 139 and 140. Complex 139 was also used in the resolution of stilbenediamine. The P-chiral phosphine PBuTh(4-BrC6H4) was resolved using 141. ... 

Palladacycle catalysts are a class of phosphine-free, highly active, and unstable zero-valent palladium catalysts [16]. Oxime-derived palladacycle was known for its thermal robustness, tolerance to air and moisture, and slow formation of large palladium particles. However, many methods for recycling palladacycle are not successful. In order to recover and reuse the catalyst, a fluorous oxime-derived palladacycle 20 was prepared from 4,4 -dihydroxybenzophenone as a precatalyst for the coupling reactions (Scheme 7.8). 

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