Reactivity in Chemistry
Oxidative Addition & Reductive Elimination
OA.3. Polar Oxidative Addition
The polar oxidative addition mechanism is very similar to an aliphatic nucleophilic substitution (SN1 or SN2) reaction.
Figure OA3.1. An example of a polar oxidative addition.
In an oxidative addition, the metal can act as a nucleophile in the first step in an SN2 process. In the second step, the liberated halide binds to the metal. That doesn't happen in a normal nucleophilic substitution. In this case, the metal has donated its electrons and is able to accept another pair from the halide.
Figure OA3.2. Mechanistic steps in a polar oxidative addition.
Polar oxidative addition has some requirements similar to a regular SN1 or SN2 reaction:
Requires good leaving group
Requires tetrahedral carbon (or a proton) as electrophile
a) What do you think is the most difficult step (i.e. the rate-determining step) for the reaction in Figure OA3.2? Why?
b) Suggest the probable rate law for this reaction.
The platinum compound shown below is capable of reductively eliminating a molecule of iodobenzene.
a) Show the products of this reaction.
The starting platinum compound is completely stable in benzene; no reaction occurs in that solvent. However, reductive elimination occurs quickly when the compound is dissolved in methanol instead.
b) Explain why the solvents may play a role in how easily this compound reacts.
The reaction in methanol is inhibited by added iodide salts, such as sodium iodide.
c) Provide a mechanism for the reductive elimination of iodobenzene from the platinum complex, taking into account the solvent dependence and the inhibition by iodide ion.
For the following reaction,
a) Identify the oxidation state at platinum in the reactant and the products.
b) Assign stereochemical configuration in the product and the reactant.
c) Explain the steresochemistry of the reaction.
Reaction of the following deuterium-labeled alkyl chloride with tetrakis(triphenylphosphine) palladium produces an enantiomerically pure product (equation a). Draw the expected product.
However, reaction of a very similar alkyl halide produces a compound that is only 90% enantiomerically pure. Draw the major product and explain the reason that there is some racemization.
Frequently, oxidative additions and reductive eliminations are preceded or followed by other reactions. Draw a mechanism for the following transformation.
Because transition metals can actually donate electrons to electrophiles, in some cases they are capable of being protonated. This step can be thought of as an oxidative addition. Just like in the protonation of an oxygen or a nitrogen, the metal complex will end up with a positive charge. The hydrogen that bonds to the metal becomes a ligand. In our approach to figuring out the charge or oxidation state on a metal in a complex, we generally consider all of the ligands to have their full octet. For a hydrogen atom, the octet is two electrons. That would give the hydrogen atom a negative charge. As a result, the charge on the metal will increasde by +2 upon protonation (+1 for the overall charge, and +1 because it now has an additional, anionic ligand to balance).
Thinking about it mechanistically, the metal has just donated a lone pair to a proton (H+) so that it can become a hydride ligand (:H-). The metal has given away two of its electrons to this donor, so its charge or oxidation state increases by 2+.
Protonation of metals can be important in catalytic reactions. For example, it is believed that this step may play an important role in a couple of biological systems. Nitrogenase is an iron-containing enzyme found in certain bacteria that reduces atmospheric nitrogen (N2) to ammonium ion (NH4+). This reaction is very important in providing soluble nitrogen to plants so that they can make amino acids and other nitrogen-containing metabolites. Some researchers have found evidence that protonation of iron atoms in the enzyme plays a key step in this crucial reaction.
Show the products of protonation in the following complexes. In addition, show the oxidation state of the metal, both before and after protonation.
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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
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