Ligand-free catalysts basic mechanisms

A-[Co(en)3l is not racemized in the presence of OH at high temperatures the observed loss of optical activity is associated with the formation of cis and trans [Co(en)2(OH)2] (164). This is in keeping with the observation that cis-[Co(en)2en(OH) (i.e., with one monodentate en ligand) in basic aqueous solution does not cyclize to form [Colenla] (165). In the presence of a large excess of free en, however, racemization is observed the rate is dependent on en concentration, and the complex exchanges ligands at about the same rate as racemization (164, 166). Carbon black catalyzed racemization follows the rate law V = Ai[Co(en)3 ]ads[OH ]ad8. where the concentrations refer to the amount of reactant adsorbed onto the surface of the catalyst, ki has a value of (9 1) X 10 M s at 25°C (167). An effect of [en] was observed but was attributed solely to the pH of the basic amine. The reaction on the surface of the catalyst was proposed to occur via the SnI(CB) mechanism. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

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