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Reductive Elimination

General Information

As the name implies reductive elimination involves the elimination or expulsion of a molecule from a transition metal complex. In the process of this elimination, the metal center is reduced by two electrons. In the simplest example below the metal goes from the x+2 to the x oxidation state and a coordinatively unsaturated metal center is obtained. In Equation 2 we see a case of a binuclear reductive elimination reaction:

Reductive Elimination Illustrated

Reductive elimination is formally the microscopic reverse of oxidative addition, and it is not surprising that a series of reactions involving an oxidative addition, a rearrangement and then a reductive elimination form the basis for a variety of industrially important catalytic cycles.

Key Facts

Important principles to remember about reductive elimination are:

  1. The groups being eliminated must be in a mutually cis orientation. See Gillie, Stille J. Am. Chem. Soc. 1980, 102, 4933. In this experiment, the authors took a system that was known to reductively eliminate and synthesized an analog where the alkyl groups had to be mutually trans:

    A neat experiment

    To rule out the possibility that the transphos ligand was simply changing the chemistry, the authors performed a crossover experiment which further supported the hypothesis of a mutually cis requirement:

    A neat experiment

    This latter reaction is also an example of an oxidatively induced reductive elimination (see below).

  2. Added ligands such as phosphine can inhibit, increase or have no effect on the rate of reductive elimination!
    1. In some cases, reductive elimination requires the prior dissociation of a ligand, and adding more of that ligand inhibits the reaction. This suggests that the molecule needs to undergo a rearrangement to get the leaving groups into a favorable (cis) position. See Komiya, Albright, Kochi, Hoffmann J. Am. Chem. Soc. 1976, 98, 7255. This reference includes additional data concerning the cis requirement.
    2. In other cases, addition of a ligand induces the elimination reaction! Here, the incoming phosphine creates a fluxional 5-coordinate intermediate that places the H and R groups in a mutually cis orientation.

      A neat experiment

    3. And in still other cases, added L has no effect whatsoever on the rate of the reaction! See Halpern, Acct. Chem. Res. 1982, 25, 332 for a great kinetic isotope study demonstrating the concerted, intramolecular mechanism for reductive elimination in a L2Pt(R)(H) system.
  3. Reductive elimination is more likely for compounds in high and/or unstable oxidation states. In fact, oxidizing a stable complex to an unstable oxidation state can induce a reductive elimination, a process called oxidatively induced reductive elimination. See Kochi et. al. Organometallics 1982, 1, 155 for the following work:

    A neat experiment

    Notice that three different decomposition mechanisms are operative for the three different oxidation states of iron!

  4. As with oxidative addition, there are many different known mechanisms for reductive elimination, including radical pathways. A detailed discussion of these is currently beyond the scope of this text.

Rates of Reductive Elimination

The rates of reductive elimination (as well as oxidative addition) depend on a variety of factors, but one of the most important is thermodynamics. Consider the following examples:

Reductive Elimination Rates

If we consider that the DH-H = 104 kcal/mol and that the DM-H is 50-60 kcal/mol we see that these are essentially balanced and there should be no thermodynamic preference for a dihydride versus a reduced metal center.

But DR-H is typically 100 kcal/mol versus a metal alkyl bond strength of 30 to 40 kcal/mol. We see that the thermodynamic situation is again approximately balanced with a slight preference for the forward reaction.

DR-R is typically around 90 kcal/mol, so for two alkyl substituents, there is a strong thermodynamic driving force for the reaction to go to the right. C-C bond activation is unusually rare, but more examples continue to be found.

Self-Test

If you like the self-test exercises presented here, give me some feedback via email. If there is enough support, I'll add more throughout the OMHTB. - RT

THESE QUIZZES ARE NOT CURRENTLY WORKING - We moved to a new server platform in March 2023 and I have to go back and redo the coding that drives the grading. Stay tuned...

1. Which of the following mechanisms is most plausible for the following reaction?

a reaction

a) loss of phosphine, b) addition of CO, c) insertion of CO, d) addition of excess CO with reductive elimination of acetone.
a) addition of CO, b) loss of phosphine, c) insertion of CO, d) addition of CO with reductive elimination of acetone.
a) addition of CO, b) insertion of CO, c) reductive elimination of acetone, d) addition of CO, e) loss of phosphine.
a) loss of phosphine, b) addition of CO, d) addition of CO, d) reductive elimination of acetone, d) addition of CO.

2. Which of the following is least likely to undergo a reductive elimination reaction?
Cp2Ti(n-Bu)(H) [(CO)2I3Rh(COMe)]-
(P-iPr3)2PtH2 Cp*2Nb(H)(C2H4)

3. If a complex LnM(A)(B)undergoes facile reductive elimination to give AB and LnM, then what can we say about the reverse reaction, oxidative addition of AB to LnM?
LnM must oxidatively add AB with the same ease.
LnM can not undergo oxidative addition of AB.
Whether LnM can undergo oxidative addition depends on both LnM and AB.
The oxidative addition will be slow compared to the reductive elimination reaction.