Cross coupling reactions steric effect

Phosphinocarbene or 2 -phosphaacetylene 4, which is in resonance with an ylide form and with a form containing phosphoms carbon triple bond, is a distillable red oil. Electronic and more importantly steric effects make these two compounds so stable. Carbene 4 adds to various electron-deficient olefins such as styrene and substituted styrenes. Bertrand et al. have made excellent use of the push-pull motif to produce the isolable carbenes 5 and 6, which are stable at low temperature in solutions of electron-donor solvents (THF (tetrahydrofuran), diethyl ether, toluene) but dimerizes in pentane solution. Some persistent carbenes are used as ancillary ligands in organometallic chemistry and in catalysis, for example, the ruthenium-based Grubbs catalyst and palladium-based catalysts for cross-coupling reactions. 

In comparison to the nickel-catalysed aryl halide coupling reactions, the palladium-catalysed analogues are not sensitive to the steric hindrances and to the presence of nitro group. The nickel(O) complexes are more reactive than the appropriate palladiums, and thus more effective in the reactions with the least reactive aryl chlorides. However, the palladium complexes are more convenient and selective, even in the cross-coupling reaction of two electronically different aryl halides. 

Polymers can be effective stabilisers of NPs through electronic and steric effects. Dendrimers are highly branched polymers only a few nanometers in diameter. Dendrimers are defined by a central core, interior polymeric branches and an exterior functionalized surface. Their uniform size, dispersity and water solubility (due to outer hydrophilic and metal encapsulating functional groups) make them ideal candidates as polymeric stabilizers for NP catalysed cross-coupling reactions. The encapsulated NPs are primarily eonfined sterically... 

Palladium NPs confined in hoUow graphite nanofibers exhibit attractive catalytic properties in Suzuki-Miyaura cross-coupling reactions [209]. There also, confinement of NPs at the step-edges facilitates retention of catalytic centers and recycling without any significant loss of activity or selectivity over multiple catalytic cycles. Furthermore, careful comparison of the catalytic properties of Pd NPs either on or in graphite nanofibers reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the Suzuki-Miyaura reaction. The yield and selectivity for the cross-coupled product are altically dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are observed for aryl boronic add reactants possessing substituents in different positions. 

Preparation and Cross-Coupling of2-Pyhdyl and 3-Pyridylzinc Bromides 83 Table 3.33 Steric effect on cross-coupling reaction. 

Table 2. The base and cation effect on the yield of 236 in the cross-coupling reaction of sterically hindered boronic acid 234 with 3-methyl-2-bromopyridine (235) [15]...

As the other steps in cross-coupling reactions, the rate of the reductive elimination step is also influenced by the steric and electronic effects induced by the ancillary ligands bound to the metal center. Consequently, the influence of the steric and electronic properties of these ligands on this step has been theoretically investigated by several groups. Ananikov, Musaev and Morokuma analyzed by means of the ONIOM approach the C-C coupling from the complexes c -[Pd(R)2(L) ] (R = Me, Ph, vinyl, ethynyl L = PPhs, PCys, PMes and PH3 n = 1,2) [116]. According... 

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