Solvation Accounts for the Counterintuitive Nucleophilicity Ordering of Peroxide Anions

Abstract

The nucleophilic reactivities (N, s ) of peroxide anions (generated from aromatic and aliphatic peroxy acids or alkyl hydroperoxides) were investigated by following the kinetics of their reactions with a series of benzhydrylium ions (Ar CH ) in alkaline aqueous solutions at 20 °C. The second-order rate constants revealed that deprotonated peroxy acids (RCO ), although they are the considerably weaker Brønsted bases, react much faster than anions of aliphatic hydroperoxides (ROO ). Substitution of the rate constants of their reactions with benzhydrylium ions into the linear free energy relationship lg k=s (N+E) furnished nucleophilicity parameters (N, s ) of peroxide anions, which were successfully applied to predict the rates of Weitz-Scheffer epoxidations. DFT calculations with inclusion of solvent effects by means of the Integral Equation Formalism version of the Polarizable Continuum Model were performed to rationalize the observed reactivities.

Intramolecular Hydrogen‐Bonding Modulates the Nucleophilic Reactivity of Ammonium‐Peroxycarboxylates

“…The reactivities of the ammonium‐peroxycarboxylates 2 vary only within less than one order of magnitude. It is noteworthy that 2 react 5 to 10 times slower with 3b – e than structurally related peroxycarboxylates (such as phenylperoxyacetate or ε‐phthalimido‐peroxyhexanoate). Deprotonation of zwitterion 2c and generation of the anionic 2c – increased the reactivity of the RCO 3 – group by a factor of four.…”

“…To characterize the acid/base behavior of 2 + , we determined the Brønsted acidities (p K a ) of their peroxycarboxylic groups by potentiometric titration in water. The acidities of 2 + were found to be in the common range of p K a 7–8 for aliphatic peroxyacids (see Table below) . By assuming that the known p K a of ammonium groups in protonated amino acids (p K a ≈ 10) also apply for the APOCA cations 2 + , it can be anticipated that amino‐substituted peroxycarboxylic acids exist as cations ( 2 + ), zwitterions ( 2 ), or anions ( 2 – ) in aqueous solution (Scheme ), depending on the pH and in analogy to the established equilibria for α‐amino acids.…”

“…In equation (1) the nucleophiles' reactivity is described by the solvent‐dependent nucleophilicity parameter N and the nucleophile‐specific susceptibility s N . These parameters can be determined from kinetic measurements with a set of benzhydrylium ions as reference electrophiles (with known electrophilicity parameters E ), employing the previously reported procedure , . As shown in Scheme , the kinetics were studied in alkaline, aqueous solution, in which zwitterions 2 were generated by equilibrium deprotonation of the corresponding cations 2 + .…”

“…Initial sequential mixing experiments showed that conditions successfully used to determine the nucleophilicity of various peroxide anions could not be applied without modification to study the kinetics of the reactions of 3 with the ammonium‐peroxycarboxylates 2 : When we generated 2a in a first mixing step by dissolving 2a + MsO – ([ 2a + ] 0 = 4 × 10 –4 m ) in aqueous potassium hydroxide solution ([KOH] 0 = 4 × 10 –4 m ), then allowed the solution to age for a defined time and, in a second mixing step, added solutions of 3 ‐BF 4 – ([ 3 ] 0 = 1.1 × 10 –5 m ), we observed mono‐exponential decays of the benzhydrylium absorbances under these pseudo‐first‐order conditions. However, the observed decay rates systematically decreased when the time between the first and the second mixing step, that is, the aging time of the nucleophile solution, was increased (Figure A).…”

“…Recent investigations into the nucleophilic reactivity of inorganic bleach reagents and peroxide anions showed that peroxycarboxylates possess distinctly higher reactivity than the anions of alkyl hydroperoxides (ROO – ) and hydrogen peroxide (HOO – ) , . In this context, some patents on the use of amino‐acid derived peroxycarboxylic acids intrigued us.…”