The Mechanism of Formation of Pi-Allyl Palladium-Chloride Complexes From Olefins
Chrisope, Douglas Ray
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https://hdl.handle.net/2142/70367
Description
Title
The Mechanism of Formation of Pi-Allyl Palladium-Chloride Complexes From Olefins
Author(s)
Chrisope, Douglas Ray
Issue Date
1987
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
The reaction of ${\rm PdCl}\sb2$ with olefins bearing allylic hydrogens to give $\pi$-allyl palladium complexes is accepted to be a two-step process. Previous workers reported isolation of the presumed $\pi$-olefin complex intermediates and their conversion to $\pi$-allyl complexes under typical reaction conditions. Mechanistic proposals for the H removal step include palladium insertion into the allylic C-H bond, allylic proton abstraction by chloride ligand, and proton abstraction by an external base, none of which are supported by substantial evidence.
Experimental and theoretical results lead to the expectation of a small isotope effect for the palladium insertion mechanism since the H transfer is very non-linear. Kinetic considerations suggest a second-order rate expression involving the $\pi$-olefin complex and the added base would be expected for the external base mechanism.
Product isotope effects were determined for the reaction of 2,2-dideuteriomethylenecyclohexane with ${\rm PdCl}\sb2$ under various conditions. The reaction in DMF solvent at $60\sp\circ$ displays an isotope effect of 4.55 $\pm$ 0.1. In AcOH, the reaction with ${\rm Na\sb2PdCl}\sb4$ has an isotope effect of 3.5 $\pm$ 0.1 at $60\sp\circ.$ The yield and isotope effect in AcOH solvent are unaffected by the presence of sodium acetate, disfavoring any mechanism requiring such an added base. The moderate to large isotope effects found in the reaction are inconsistent with the palladium insertion into the allylic C-H bond.
Kinetic studies of the reaction of methylenecyclohexane, 2,3-dimethyl-2-butene, and 2,4,4-trimethyl-2-pentene in DMF and AcOH solutions were consistent with first-order behavior in both olefin and ${\rm PdCl}\sb2.$ No rate equation with a first-order dependence in acetate was applicable. On the basis of the kinetics and the isotope effect results, the intramolecular proton abstraction by chloride ligand is the best mechanism, but abstraction by external chloride cannot be ruled out.
Additional support for this intramolecular mechanism by chloride ligand is provided by examination of X-ray structures of 21 Pt-olefin complexes. The unusually short distances between allylic hydrogens and chloride ligands indicate an attractive interaction is present in these species.
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