Density Heck reaction

Although sulfur is unHkely to chelate the metal in this case, it is worth mentioning the axially chiral diphosphine Hgands, based on hz-thienyl systems which increase the electronic density at phosphorus such as 159 (used in Ru-catalyzed reduction of /1-keto esters with 99% ee) [llla],BITIANP 160,andTMBTP 161 (in a Pd-catalyzed Heck reaction, the regio- and enantioselectivity are high with 160 but low with 161) [mb]. 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

When a series of STO characterized Pd/A Og catalysts were used to promote the Heck reaction (Eqn. 1) the amount of the fi aryl enol ethers, 1 and 2, formed after a 60 minute reaction was directly related to the comer site densities on these catalysts. Thus, this reaction and presumably, others such as the diene cyclization shown in Eqn. 2, which require the adsorption of two reactive species on a single surface atom, must take place on the more coordinatively unsaturated comer atoms. 

The first studies that intentionally used colloidal nanocatalysts were reported independently by Beller et al. [50] and Reetz et al. [51] using chemical reduction and electrochemical techniques, respectively, to synthesize colloidal palladium nanoparticles for the Heck reaction. Both Beller and Reetz concluded that the solution-phase catalysis occurred on the surface of the nanoparticle, without confirming that a homogeneous catalytic pathway was nonexistent. Le Bars et al. [52] demonstrated an inverse relationship between the size of Pd nanoparticles and the TOF (normalized to the total number of surface atoms) for the Heck reaction (Fig. 18.4a). After normalizing the rate to the density of defect sites (for each nanoparticle size) (Fig. 18.4b), the TOF for all particle sizes was identical. Colloidal PVP-capped palladium nanoparticles synthesized by ethanol reduction are effective catalysts for Suzuki cross-coupling reactions in aqueous solution [53]. The El-Sayed group reported that the initial rate of reaction increased linearly with the concentration of Pd nanoparticles [53] and the catalytic activity was inversely proportional to the... 

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... 

Albert, K., Gisdakis, P. and Rbsch, N. (1998) On C—C coupling by carbene-stabiUzed palladium catalysts a density functional study of the Heck reaction. Organometallics, 17,1608-16. 

Gron, L.U., LaCroix, J.E., Higgins, C.J. et al. (2001) Heck reactions in hydrothermal, subcritical water water density as an important reaction variable. Tetrahedron Lett., 42, 8555-7. 

In an electrochemical way of making metal nanopartides, a solution of stabilizer e.g., tetraalkylammonium bromide in THF) is electrolyzed using an anode made of the metal of interest and an inert cathode.l Under the conditions of the electrolysis, dissolution of the anode material takes place via oxidation. The metal cations transfer to the cathode, where reduction, nucleation and finally stabilization occur. The size of thus-prepared nanopartides can be easily controlled by the current density. The method is applicable to many transition metals. Moreover, systems with two anodes of different metals result in the formation of bimetallic nanopartides. When oxidation of the anode metal is difficult, the metal of interest can be introduced as inorganic salt.h67] pd nanopartides, e.g., prepared via this method were found to be catalyticaUy active in the Heck reaction and the... 

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