Palladacycles oxime-derived

In a recent article by Botella and Najera, controlled mono- and double-Heck arylations in water catalyzed by an oxime-derived palladacycle were described [22], When the reaction was carried out under microwave irradiation at 120 °C in the presence of dicyclohexylmethylamine with only 0.01 mol% of palladium catalyst (palladium acetate or palladacycle), monoarylation took place in only 10 min with a very high turnover frequency (TOF) of > 40000 (Scheme 6.3). As regards diarylation, 1 mol% of the palladacycle catalyst and 2 equivalents of iodobenzene had to be utilized to obtain moderate to good yields of diarylated product. Whereas microwave heating at 120 °C provided a 31% yield after 10 min, a 66% isolated yield of product was obtained by heating the reaction mixture under reflux for 13 h at 100 °C. Here, the... 

Scheme 6.3 Mono- and double-Heck arylations in water using oxime-derived palladacycles.

This protocol could be extended to a range of different ,/i-unsaturated carbonyl compounds and either activated or deactivated aryl iodides [22], An application of related Heck chemistry to the synthesis of methylated resveratrol (3,4, 5-trihydroxy-( )-stilbene) is shown in Scheme 6.4 [23]. The phytoalexin resveratrol exhibits a variety of interesting biological and therapeutic properties, among them activity against several human cancer cell lines. Botella and Najera have shown that the trimethyl ether of resveratrol (Scheme 6.4) can be rapidly prepared by microwave-assisted Heck reaction of the appropriate aryl iodide and styrene derivatives, using the same oxime-derived palladacycle as indicated in Scheme 6.3. 

A dimeric 4-hydroxyacetophenone oxime-derived palladacycle was used by Najera et al. as a very efficient precatalyst for selective and microwave-assisted arylations of acrolein diethyl acetal, furnishing 3-arylpropanoate products [45]. In the presence of Cy2NMe and tetrabutylammonium bromide (TBAB), higher reaction rates and slightly improved yields were realized under microwave conditions (Scheme 7). 

More recently, Najera et al. performed a number of Heck reactions of deactivated aryl halides and styrenes under phosphane-free conditions using oxime-derived palladacycles or palladium acetate as catalyst [113], Coupling can be performed either vith chcyclohexylmethylamine as base and TBAB as PTC catalyst or in neat vater vith triethylamine in N,N-dimethylacetamide (DMA) solutions under the action of MW irradiation. 

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. ... 

Coelho et al. have developed an improved and highly efficient synthesis of several a-benzyl-p-ketoesters 128 by using MBH adducts as substrates for an intermolecular Heck reaction catalyzed by a Najera oxime-derived palladacycle (129). These efficient catalytic conditions proved to be high selectivity and only provided the corresponding functionalized p-ketoesters 128 in high yields with no decarboxylation products (Scheme 3.50). ... 

A separate hne of oxime-derived palladacycles 182 and 183 is formed using the ferrocene platform, which showed a typical level of activity in type 1 reactions (0.01 mol% 182 or 183, NaOAc, NMP, 140-150 °C) (Figure 2.23) [109]. 

Figure 3.41 Diarylation of ten-butyl acrylate in a sealed vessel using an oxIme-derIved palladacycle.

The coupling of iodo aryls with various olefins was conducted in the presence of palladium chloride (2.0 mol%) at ambient temperature under ultrasonic irradiation in water with yields between 43 and 90% [154], In situ formation of palladium nanoparticles was confirmed by transmission electron microscope and X-ray diffraction analyses, and the palladium nanoparticles could be reused for multiple reactions without significant loss of activity. Biologically relevant stilbene derivates, such as resveratrol, piceatannol and pinosilvine, were efficiently prepared with high regioselectivity and complete stereocontrol (TONS up to 10 , 0.01 to 0.50 mol% catalyst) [148], The activity of the oxime-derived palladacycle catalysts was compared in neat water with a DMF-water mixture, with similar... 

Figure 15.4 Kaiser oxime-derived palladacycle 54 applied in Mizoroki-Heck catalysis in neat water.

Botella, L. and Najera, C. (2004) Controlled mono and double Heck reactions in water catalyzed by an oxime-derived palladacycle. Tetrahedron Lett., 45, 1833-6. 

In 2010, Oxime-Derived Palladacycles as Sources of Palladium Nanoparticles, by Alonso and Nauera [103]... 

Alacid E, Najera C (2008) The Mizoroki-Heck reaction in organic and aqueous solvents promoted by a polymer-supported Kaiser oxime-derived palladacycle. ARKIVOC viii... 

Alternative solvents have been developed for the Heck-Matsuda reaction. This reaction has recently been performed in neat water by Najera s group [95]. The reaction was carried out using an oxime-derived palladacycle catalyst and palladium acetate using a variety of different diazonium tetrafluoroborates giving styrenes, stllbenes, arylideneketones, and cinnamate esters (Figure 1.26). 

Most recently, Botella and Najera have reported that oxime-derived palladacycles such as 51 are air- and water-stable catalysts that are suitable for cross-coupling reactions of different aryl and heteroaryl chlorides with boronic acids in neat water (Equation 84) [123],... 

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). 

A similarly high performance has been reported for oxime-derived (125) and benzylsulfide-derived (126) palladacycles.438 These precatalysts are effective in the cross-coupling of arylboronic acids,438,439 organotin compounds,440 and terminal acetylenes441 with aryl iodides and bromides, and of activated aryl chlorides. SC-palladacycles can effect the Suzuki-Miyaura reaction even at room temperature. 

Formation of dihydroquinol-4-ones from 3-(o-iodoarylamino)propanoate esters involving arylpalladium intermediates which may exist in the palladacycle form and thereby derive special activity for intramolecular attack on the ester group.Under similar conditions, five-membered ring oxime derivatives are prepared. ... 

As conclusion, the high catalytic efficiency of the polymeric palladacycle derived from Kaiser oxime resin similar to that of related unsupported dimeric pal-ladacycles was observed in the Mizoroki-Heck reaction. This polymer showed a good catalytic activity, not only in organic but also in aqueous media, for aryl iodides, bromides, and activated chlorides with high turnover numbers (TONs), rmder aerobic conditions at relatively lower temperature. 

A-Heterocycles were synthesized via insertion of 1,1-dimethylallene into Pd-G bonds of cyclopalladated a-tetralone ketimines. Insertion reactions of alkynes into the Pd-G bond of cyclopalladated ferrocenylimines have also been described.Biomimetic hydrolysis of benzoates has been carried out with a water-soluble cyclopalladated aryl oxime, which was proposed as a potential green catalyst. Ghiral cyclopalladated liquid crystals 88 were obtained from amino acids and could possibly serve as enantioselective catalysts. " Palladacycle 89 has been derived from an anthracene-based Schiff base. 

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