Heck catalysts

The much more stable MIL-lOO(Cr) lattice can also be impregnated with Pd(acac)2 via incipient wetness impregnation the loaded catalyst is active for the hydrogenation of styrene and the hydrogenation of acetylene and acetylene-ethene mixtures to ethane [58]. MIL-lOl(Cr) has been loaded with Pd using a complex multistep procedure involving an addition of ethylene diamine on the open Cr sites of the framework. The Pd-loaded MIL-lOl(Cr) is an active heterogeneous Heck catalyst for the reaction of acrylic acid with iodobenzene [73]. 

The retentions of the catalysts were also measured on a synthetic reaction mixture. The Heck-catalyst showed a retention of 96% while under experimental conditions retentions lower than 90% were obtained. For the PTC the values are both higher than 99%. The authors assume that this big difference for the Heck-catalyst is caused by the formation of smaller Pd species in the catalytic cycle. However, no precipitation or Pd-black formation was observed. 

Scheme 20 Immobilization of a Heck catalyst on a ROMP-derived monolithic support...

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

E Peris, JA Loch, J Mata, RH Crabtree. A Pd complex of a tridentate pincer CNC bis-carbene ligand as a robust homogeneous Heck catalyst. Chem Common 201-203, 2001. 

Terminally functionalized PIB oligomers have been used in a thermomor-phic heptane/JV,N-dimethylacetamide mixture as supports for thermally stable SCS-Pd(II) Heck catalyst precursors. A PIB-supported SCS-Pd(II) Heck catalyst was prepared from the carboxyl-terminated PIB oligomer by the sequence of reactions shown in Eq. 61. The PIB-bound SCS-Pd(II) species 125 so ormed was then used to carry out Heck chemistry (Eq. 62). As was true for SCS-Pd(II) species on other polymers [154,155,162], this catalyst was only effective for aryl iodides or activated aryl bromides. However, the catalyst could be used through... 

A non-covalent immobilization of Heck catalyst on silica (SILP concept) has been realized by Hagiwara et al. [217]. They used a silica surface, supported with Pd(OAc)2 dissolved in [BMIM][PFe]. This catalyst was appUed to the Mizoroki-Heck reaction of aryl halides with acrylate without a ligand in n-dodecane as solvent. It was six times reused and the overall TON reached 68 400 (for more details see Section 5.6). 

Table 1 Efficacy of two room temperature Heck catalysts discovered in Hartwig s high-throughput screen... 

Sealed glass 96-well microtiter plate, 50 pi, 70 °C FRET technique, fluorescence plate reader Room temperature Heck catalyst discovered 2003 56... 

Even though aminophosphine- and phosphite-based pincer complexes, as well as other systems such as SCS-based pincer-type Heck catalysts, were shown to serve as stable and clean sources of palladium nanoparticles in Heck reactions [24a, 25], and also xylene-derived phosphine-based systems were found to decompose under Heck reaction conditions in the presence of organic bases and hence palladium nanoparticles generally are considered to be the catalytically active form of palladium pincer Heck catalysts [27], catalytic cycles with the involvement of Pd intermediates could not have been excluded to be operative in pincer-catalyzed versions of the Heck reaction. In contrast, experimental observations, such as halide exchange reactions... 

Figure 10.4 Experimentally applied pincer-type Heck catalysts [2,6-CjH3(XPRj)2Pd(CI)] (X=NH, R = piperidinyl, 10 X = 0, R = piperidinyl, 20 X = 0, R = isopropyl, 3 X = CH2, R = isopropyl, 21) selected for the computational study published by Freeh in 2010.

Figure 10.9 DFT reaction profiles of the initial reaction steps of the Heck cycle catalyzed by pincer-type Heck catalysts (according to Scheme 10.10), exemplarily depicted for 3 for electron-poor palladium pincercomplexes (left)...

These computational investigations showed for the first time that catalytic cycles with the involvement of Pd intermediates are indeed thermally accessible for palladium pincer complexes under Heck reaction conditions and hence are a true alternative to palladium nanoparticle-catalyzed versions of the Heck reaction. This, however, does not imply that Pd /Pd mechanisms are operative in any case for palladium pincer complexes in the Heck reaction. In contrast, palladium nanoparticles have been often shown to be the catalyticaUy active form of pincer-type Heck catalysts, as it is, for example, the case for the aminophosphine-based palladium pincer Heck catalyst [2,6-CgH3(NHP(piperidinyl)2)2Pd(Cl)] (10) (under the reaction conditions applied) - the pincer complex with the highest electron density on the metal center and thus where the lowest energy path was calculated. Therefore, it is reasonable to anticipate that palladium pincer Heck catalysts exist that operate via Pd /Pd mechanisms whereas others serve as sources of palladium nanoparticles. This hypothesis got strong experimental support from... 

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