Activation in Heck-type processes

The combination of a Mizoroki-Heck reaction with a C—H activation is a domino process that has been encountered with increasing frequency in the past few years. This type of transformation takes place if the organo-palladium intermediate does not react in the usual way but activates an aryl- or vinyl-H bond in close vicinity, forming a new palladium(II) species that can react further with electrophiles or nucleophiles and giving rise to the formation of another C—C or C-Tieteroatom bond. 

The mechanism of direct arylation has been studied experimentally and computationally and possible pathways include electrophilic aromatic substitution, Heck-type coupling and concerted metalation-deprotonation (CMD). The reaction pathway is dependent on the substrate and the catalytic system employed,however, most electron-rich (hetero)arenes seem to follow a base-assisted CMD pathway. Two catalytic cycles for the coupling of bromo-benzene and thiophene are shown in Schemes 19.5 and 19.6. Scheme 19.5 depicts a carboxylate-mediated process where C-H activation occurs... 

Transition metal catalysts encapsulated within the ligand-template nanoreactor G, P(Py)s [ZnJs, have been applied to catalyze industrially relevant processes such as hydroformylation and Heck reaction. Nanoreactor [G 3 Rh(CO)(acac)] encapsulates a Rh-species that contains only one tris(meta-pyridyl)phosphine ligand, P(m-Py)3, surrounded by three Zn-porphyrins or Zn-salphens. Under syngas pressure (H2/CO), rhodium species like Rh(CO)(acac)P(Py)3 transform into a complex of type HRh(CO)3P(Py)3, which is the active species for the hydroformylation reaction, hi this reaction terminal alkenes are converted into linear and/or branched aldehydes, and the ratio of these products strongly depends on the specific catalyst applied. Hydroformylation of 1-octene by encapsulated rhodium, [G 3 HRh(CO)3], resulted in a 10-fold rate enhancanent compared... 

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