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OA3. Polar Oxidative Addition

OA3. 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

 

Problem OA3.1.
  1. What do you think is the most difficult step (i.e. the rate-determining step) for the reaction in Figure OA3.2?  Why?
  2. Suggest the probable rate law for this reaction.
Problem OA3.2.

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.

Problem OA3.3.

For the following reaction,

  1. Identify the oxidation state at platinum in the reactant and the products.
  2. Assign stereochemical configuration in the product and the reactant.
  3. Explain the steresochemistry of the reaction.
Problem OA3.4.

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.

 

Problem OA3.5.

Frequently, oxidative additions and reductive eliminations are preceded or followed other reactions.  Draw a mechanism for the following transformation.