Oxidation of hydrocarbons. 17. Solvent and substituent effects on the oxidation of styrene derivatives by quaternary ammonium permanganates

Abstract

When alkenes are oxidized by quaternary ammonium or phosphoniurn permanganates in polar organic solvents, such as acetone, the structures of the cations have little or no effect on the rates of reaction. In less polar solvents, such as methylene chloride or toluene, the rates of reaction are, however, dependent on' the identity of the quaternary ammonium or phosphonium ions. It thus appears as if the reacting species may exist as solvent-separated ion pairs in polar solvents and as intimate ion pairs in nonpolar solvents. The rates of reaction are also very sensitive to substituent effects, a nonlinear (concave upward) Hammett plot being obtained for the oxidation of substituted P-bromo and P-methoxystyrenes. The mechanism is best visualized as proceeding by way of a continuum of transition states that can vary from electron rich to electron poor depending on the capacity of the substrate structures to accommodate either negative or positive charges. Lorsque des alctnes sont oxydCs par des permanganates d'ammonium ou de phosphonium quaternaires, dans des solvants organiques polaires c o m e l'acttone, les structures des cations n'ont pas ou peu d'effet sur les vitesses des rkactions. Dans des solvants moins polaires, c o m e le chlomre de mkthylbne ou le tolubne, les vitesses dkpendent toutefois de I'identitC des ions quaternaires. I1 semble donc que les entitCs qui rCagissent peuvent exister sous la forme de paires d'ions solvatCsdans les solvants polaires et de paires d'ions intimes dans les solvants non-polaires. Les vitesses de rkaction sont aussi trbs sensibles aux effets de substituants: on obtient une courbe de Hammett qui n'est pas 1inCaire (concave vers le haut) lors de l'oxydation de P-bromo et de P-mCthoxystyri?nes substituks. La meilleure reprksentation du mkcanisme de la rCaction implique des Ctats de transition continus qui varient suivant leur nature riche ou pauvre en Clectrons, qui dCpend elle-mkme de la capacitk des structures des substrats d'accommoder soit des charges positives ou nCgatives.[Traduit par la revue]

Permanganate oxidation of unsaturated alcohols in alkaline media

“…In this article we do not intend to deal with the mechanism of reactions (2) and (3) as it has been discussed in detail in the literature 7–9, 37–40. We wish to make some remarks, however, on the oxidation mechanism of alkoxide anion (Eq.…”

Iodimetric determinations in organic solvents: Determination of manganese oxidation states in methylene chloride solutions

“…Quaternary ammonium additives are appealing because of the possibility of preparing economical, relatively stable quaternary ammonium permanganates. Many studies of permanganate reactions in non-aqueous media concerning the identification of products (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)) and a few concerning the kinetic and mechanistic aspects of those reactions (17)(18)(19) have been undertaken. Although the application of standard techniques allowed the identification of organic reaction products, a method to determine the oxidation state of product manganese has so far been lacking.…”

Kinetics and mechanism of the oxidation of acetylacetone by permanganate ion

“…The step which plays the decisive role in the kinetics is dependent on the joint effect of all substituents. This assumption can give an explanation for the ambiphilic nature of permanganate ion, which can often be observed 4–7.…”

“…Investigations on unsaturated carboxylic acids, esters, and ethers show that neither steric hindrance nor electronic or mesomeric effects can give solely unambiguous explanation of the reactivity 3–5. These contradictions may be solved by the assumption that in pre‐equilibrium a π‐complex is formed between the Mn atom and the double bond, which can rearrange into a five‐membered cyclic hypomanganate‐diester (probably via a four‐membered metallocyclooxethane) 2–4. Hence, the rate constant is actually k = Kk *.…”