January 24, 2019 Critical Thinking

? Introduction :
Higher olefins are important industrial intermediates , which forms by dimerization of akenes .moreover in order to study the reactivity and metal carbon bond characterization , they serve as good model .
? History :
Dimerization of alkenes was firstly done by the zeigler in the early 1950s.the compound that is obtained by the selective dimerization of alkenes that is organo aluminium compound .their use is also determined by the zeigler.
? Mechanism :
Three types of mechanism are proposed for dimerization of olefins.
• Degenerated polymerization :
? The three important steps are included in this mechanism are
(1) initiation reaction
(2) monomer insertion into the activated complex.
(3) transfer reaction
? Initiation :
At the metal hydride center coordination of olefins are done and in classical polymerization initial step is, in the metal hydride succeeding insertion of the coordinated oleifins (by carbon carbon double bonds ).
? Monomer insertion in activated complex :
• The metal-carbon bond, that is established in this way , inserts a second monomer molecule coordinated formerly in same metal center (which is the propagation step).
• After the ? hydride extraction dimer is formed , a usual cleavage reaction of the transition metal-carbon bonds.”along with the formation of metal hydride The ?-hydrogen of alkyl group which is attached to the metal is transfer to the last , and leaves the metal center as a vinylic olefin.
• The metal, its valency state, along with the ligand environment all these are affected on the ?-hydrogen abstraction . The metals present at extreme end of the transition series are able to ?-hydrogen abstraction easily by an attached alkyl group.and for dimerization of olefins these are good catalyst.
• Selectivity to dimerization is actually the ratio of the chain transfer rate to the propagation rate . The temperature does not appear to control the ratio. Steric course of insertion ambiguous sometimes. The vital factors which decide the products are not obviously understood. metal-carbon and metal hydrogen bonds react with olefins either like protons and carbonium ions or like hydride ions and carbanions . The factors that affect the a/b ratio are however to be ascertained. That depend on the nature of olefins, the temperature and the complex involved .
? Ligand effect
Electron-withdrawing ligands plays their rule by withdrawing electron density from the metal as a result increasing the positive charge. That polarizes the adjacent bonds involving the ?-hydrogen, allow the addition of the ?-hydrogen into the polar and six-centered transition state . Donor ligands, while , reduce the charge on the metal (positive ). Adjacent bond polarization is less rigorous and thus rea