Reductive Elimination

Reductive Elimination. An Inorganic Chemistry Topic

1. Submitted by Shahdan Borbora M.Sc. 4 th Semester Roll No. 11 REDUCTIVE ELIMINATION

2. An elementary step in which both the coordination number as well as the oxidation state of the metal decreases while forming a new covalent bond, therefore most often seen in higher oxidation state It is the microscopic reverse of oxidative addition Introduction

3. General Information Reductive elimination is more common in higher oxidation states Can involve a two-electron change at a single metal center (mononuclear) or a one electron change at each of two metal centers (binuclear, dinuclear or bimetallic)

4. Factors Affecting Reductive Elimination F or Reductive Elimination to occur The eliminating groups must be cis -oriented to each other A high formal positive charge on the metal The presence of bulky groups on the metal An electronically stable organic product

5. Mononuclear Systems Cis Trans This pathway is common for d 8 metals Ni(II), Pd(II) and Au(III) and d 6 metals Pt(IV), Ir (III) and Rh (III). Mononuclear reductive elimination requires that the groups being eliminated must be cis to one another on the metal center

6. Reductive elimination reactions are intramolecular and this can be seen from the following example where no cross product is formed.

7. For binuclear reductive elimination, the oxidation state of each metal decreases by one, while the d-electron count of each metal increases by one. This type of reactivity is seen with first row metals, which prefer one unit change in oxidation state, but has been observed in both second and third row metals. Binuclear Systems

8. Reductive Elimination – O h complexes In octahedral complexes, reductive elimination can be very slow from the coordinatively saturated center, and often, reductive elimination only proceeds via a dissociative mechanism, where a ligand must initially dissociate to make a five-coordinate complex. This complex adopts a distorted trigonal bipyramidal structure and the two groups to be eliminated are brought very close together. After elimination, a T-shaped three-coordinate complex is formed, which will associate with a ligand to form the square planar four-coordinate complex. The rate of reductive elimination is greatly influenced by the geometry of the metal complex

9. Reductive Elimination – Square Planar Complexes Reductive elimination of square planar complexes can progress through a variety of mechanisms: dissociative, nondissociative, and associative dissociative mechanism for square planar complexes initiates with loss of a ligand, generating a three-coordinate intermediate that undergoes reductive elimination to produce a one-coordinate metal complex

10. For a nondissociative pathway, reductive elimination occurs from the four-coordinate system to afford a two-coordinate complex. If the eliminating ligands are trans to each other, the complex must first undergo a trans to cis isomerization before eliminating. In an associative mechanism, a ligand must initially associate with the four-coordinate metal complex to generate a five-coordinate complex that undergoes reductive elimination synonymous to the dissociation mechanism for octahedral complexes.

11. Effect of Added Ligands Added ligands can inhibit, increase, or have no effect on the rate of reductive elimination Addition of a ligand induces the elimination reaction. Here, the incoming phosphine creates a fluxional 5-coordinated intermediate

12. Effect of solvent Solvent leads to the formation of different elimination products because prior ligand dissociation occurs in polar solvents and forms coordinatively unsaturated intermediate that leads to final product

13. Oxidatively Induced Reductive Elimination Oxidizing a stable complex to an unstable oxidation state can induce a reductive elimination, a process called oxidatively induced reductive elimination. Rate is inhibited by excess bipy Homolytic cleavage rather than a straight reductive elimination Occurs via concerted reductive elimination Fe II Fe III Fe IV

14. Rates of Reductive Elimination The rates of reductive elimination depend mainly on the thermodynamic factor. Usually fast, Reversible Very fast, rarely reversible Slow, Most likely non-reversible If we consider that the D H-H = 104 kcal/mol and that the D M-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 D R-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. D R-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.

15. Application Reductive elimination has found widespread application in academia and industry, most notable being hydrogenation, the Monsanto acetic acid process, hydroformylation and cross coupling reactions. In many of these catalytic cycles, reductive elimination is the product forming step and regenerates the catalyst; however, in the Heck reaction and Wacker process, reductive elimination is involved only in catalyst regeneration

16. References Basic Organometallic Chemistry , 2 nd Edition, B D Gupta, A J Elias, 134-146, 2013, Universities Press(India) Pvt. Ltd. Inorganic Chemistry , 3 rd Edition, Gary L. Miessler , Donald A. Tarr , 525-526, 2015, Pearson India Education Services Pvt. Ltd. Inorganic Chemistry , 4 th Edition, James E. Huheey , Ellen A. Keiter , Richard L. Keiter , Okhil K. Medhi , 637-642, 2013, Dorling Kindersley(India) Pvt. Ltd. http://www.ilpi.com/organomet/reductive.html https://en.wikipedia.org/wiki/Reductive_elimination Gillie, A.; Stille , J. K. (1980). "Mechanisms of 1,1-Reductive Elimination from Palladium". J. Am. Chem. Soc. 102 : 4933. doi : 10.1021/ja00535a018 . Gillie, Stille J. Am. Chem. Soc. 1980 , 102 , 4933. Kochi et. al. Organometallics 1982 , 1 , 155 Komiya, Albright, Kochi, Hoffmann J. Am. Chem. Soc. 1976 , 98 , 7255

17. THANK YOU !! 17