Visit our sponsor at www.chemglass.com

	Alpha-Hydride Elimination and Abstraction

General Information

Alpha-hydride elimination is the transfer of a hydride (hydrogen atom) from the alpha-position on a ligand to the metal center. The process can be thought of as a type of oxidative addition reaction as the metal center is oxidized by two electrons (Eq 1). As the reaction involves a formal oxidation of the metal, alpha-elimination can not occur in a d⁰ or d¹ metal complex. In these cases, a variant called alpha-abstraction can occur. Alpha-abstraction does not result in a change of oxidation state and the alpha-hydrogen is transferred directly to an adjacent ligand instead of the metal center (Eq 2):



Alpha-abstraction is a common method for synthesizing high oxidation state alkylidene and alkylidyne complexes. While alpha-elimination requires an open coordination site on the metal complex, alpha-abstraction does not. As shown below, alpha-abstraction may be induced by a steric crowding of the metal; the abstraction can occur spontaneously or be induced by addition of a donor ligand such as a phosphine:



The mechanism of both alpha-elimination and alpha-abstraction proceeds through the same sort of four-center transition state as beta-hydride elimination or sigma bond metathesis. Alpha elimination/abstraction can occur when the metal contains both alpha and beta hydrogens, but is most common when beta-hydride elimination is blocked.

The microscopic reverse of a alpha-hydride abstraction would be a called a carbon-hydrogen bond activation in which the C-H bond adds across a metal-ligand bond. In practice, such activations are rare, although highly desirable from the context of functionalization of hydrocarbons.

A third type of elimination reaction encountered with alkyl ligands is the delta elimination reaction.

[Index]   [Keyword Search]   [Books & Software]   [ILPI Home Page]

Please visit our sponsor to thank them for supporting this site!

This page was last updated Tuesday, March 31, 2015
This document and associated figures are copyright 1996-2024 by Rob Toreki or the contributing author (if any) noted above. Send comments, kudos and suggestions to us by email. All rights reserved.