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| Some Examples of Metal Reactions |
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Metalation is a metal reaction which results in a metal atom being attached to a, usually organic molecule. Lithium metal that comes in contact with an organohalide lithiates results in the organic molecule and gives the organolithium reagent and lithium halide. Magnesium similarly metalates organohalides to give Grignard reagents. Protons on a molecule that are acidic, even only slightly, can be exchanged for a metal by a suitable butyllithium reagent mixture. Transmetalation is another type of metal reaction that is the exchange of two metals between organic molecules. An example would be, transmetalations can be accomplished by reacting an organolithium reagent with a metal salt.
A template reaction explains a class of ligand-based reactions that occur which include two or more adjacent coordination sites on a metal center. In the absence of the metal ion, the same organic reactants produce different products. In a more practical sense, template metal reactions describe the pathways for transition of metal-based catalysis. Reactants coordinate to the adjacent site on the metal ion. Due to their adjacency, the two reactants tend to interconnect. These reactions take place within the coordination complex, and when this is done it reduces the activation energy.
In the space of homogeneous catalysis, the cyclo-oligomerization of acetylene to Cyclooctatetraene at a Nickel(II) centre is most often considered to show the templating effect of the nickel. It is supported that four acetylene molecules occupy four sites around the metal, and react simultaneously to produce the product. This simplistic and incorrect picture guided early mechanistic hypotheses. An example of this would if a competing ligand like Triphenylphosphine is present in large enough proportion to occupy one coordination site, then there is room left for only three acetylene molecules, and these come together to form Benzene.
Another form of metal reaction is the coupling reaction in organic chemistry. It is a catch-all for a range of reactions in organometallic chemistry where two hydrocarbon fragments are coupled with the aid of a metal containing catalyst. Coupling reactions are usually divided into two main classes; the first is the cross couplings in which two different molecules react to form one new molecule. An example would be the nickel chloride catalyzed reaction of an aryl magnesium halide with an aryl halide to form a biaryl. An example of the second type of coupling or homocoupling, would be an Ullmann reaction. This is the reaction of copper metal with two molecules of an aryl halide to form a biaryl. The Ullmann reaction often requires very high temperatures, and has partly been replaced in synthetic chemistry due to the palladium-catalyzed coupling reactions.
Many coupling reactions involve phenols. BINOL is the C-C coupling reaction product of 2-naphthol with copper(II) chloride and 2,6-xylenol dimerises as well with it is iodosobenzene diacetate. A common metal in this type of chemistry is palladium which is often added in the form of tetrakis (triphenylphosphine) palladium (0). This is an air-sensitive compound that is very good for coupling unsaturated halogen compounds with organometallics such as tributyltin hydride.
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