Gas Phase Reactions of Radical Cations of Halogenated Arenes and Alkenes: The Radical Cation Mediated Nucleophilic Substitution at C(sp2) Atom

Hans-Friedrich      Grutzmacher

Abstract

Radial cations of unsaturated organic hydrocarbons are reactive intermediates of many important chemical reactions. In particular, the rate constants of the reactions of these electron deficient species with electron rich reactants are increased by several orders of magnitude compared to the reaction between neutral partners. This effects has been termed “electron hole catalysis”. To get insight into the mechanism(s) of these reactions and the nature of the “electron hole catalysis” the reactions have been investigated in the gas phase using Fourier- transform ion cyclotron resonance (FT-ICR) mass spectrometry, and by molecular orbital calculations. The studies included the reactions of mono- and dihalogenated benzenes and naphthalenes as well as mono- and dihalogenated alkenes. Halogen substituents were chosen for practical reasons since the loss of a halogen substituent during the process can be conveniently used as a monitor reaction. The first part of this account discusses the methods used to determine the kinetics of ion / molecule reactions, in particular FT-ICR-mass spectrometry, and the special features of ion / molecule-reactions in the diluted gas phase. The second part deals with the reactions of halogenated arene radical cations with ammonia and amines as nucleophiles. The reactions correspond to a substitution of one halogen substituent by a radical cation mediated nucleophilic aromatic substitution and are only moderately efficient. Contrary to chemical intuition, the reaction efficiency decreases in the series Cl, Br, and I as substituent. Further, a peculiar dependance on the position of a second substituent is observed. Both effects can be explained by a mechanism involving at least the two steps of an addition of the nucleophile and of an elimination of the substituent and involving a reaction intermediate corresponding to a distonic onium ion. According to the Shaik / Pross model the first step is rate determining and depends strongly on the difference of the ionization energies of the reactants. A second effect comes from the dipole moment of the (neutral) dihalogenated arene, showing a decrease in the activation energy of the addition with an increase in dipole moment. The third part presents results of reactions of the radical cations of mono- and dihalogenated alkenes using ammonia, amines, and aliphatic alcohols as nucleophiles. These reactions are often efficient and may be collision controlled. Besides substitution of a halogen a variety of other reaction products is observed. The efficient reaction of alkene radical cations is in agreement with “electron hole catalysis”. An analysis by the Shaik / Pross model reveals that the initial addition step is now a very fast inner sphere electron transfer which occurs probably without any activation barrier. This is substantiated by molecular orbital calculations for selected reaction systems. Interestingly, experimental results show that addition of the nucleophile to the ionized C-C double bond competes even with intermolecular exothermic electron transfer and exothermic proton transfer. As a consequence, the ß-distonic onium ion, which is formed during the initial addition step, arises as a highly excited intermediate. The further course of the total reaction depends on the properties of this intermediate ß-distonic ion. The consequences of this reaction model for the individual reaction systems are discussed.

Keywords: Nucleophilic, dihalogenated arene, ammonia

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