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 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.
Important principles to remember about reductive elimination are:
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:
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:
This latter reaction is also an example of an oxidatively induced reductive elimination (see below).
Added ligands such as phosphine can inhibit, increase or have no effect on the rate of reductive elimination!
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.
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.
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.
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. Organometallics1982, 1, 155 for the following work:
Notice that three different decomposition mechanisms are operative for the three different oxidation states of iron!
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.
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.
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
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This page was last updated Thursday, November 20, 2003
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