Proton Transfer from Alkane Radical Cations to Alkanes

Proton Transfer from Alkane Radical Cations to Alkanes 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L. Schowen Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

Three major types of cationic species that can be derived from saturated hydrocarbons are alkyl carbenium ions (R+), alkane radical cations (RH +) and alkyl carbo-nium ions (RH2+). The term carbocations is usually reserved to denote alkyl carbenium and carbonium ions only. Pentacoordinated alkyl carbonium ions (proton-ated alkanes) are the species that result from protonation of alkane molecules they are of paramount importance as reactive intermediates/transition states in the initiation of (Br0nsted) acid-catalyzed conversions of saturated hydrocarbons. Upon dissociation of alkyl carbonium ions, trivalent alkyl carbenium ions are formed and these are responsible for the further progression of acid-catalyzed conversions of alkanes. Alkyl carbenium ions may also be formed by ionization of neutral alkyl radicals and by proton addition to olefins. In both carbenium and carbonium ions, the positive charge is very much located on a particular part of the cation. 

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I 5 Proton Transfer from Alkane Radical Cations to Alkanes... 

Proton transfer from alkane radical cations to alkane molecules results in the transformation of these cations into neutral alkyl radicals (the conjugate bases). The nature of these radicals is determined by the site of proton donation in the alkane radical cation. Information on the site of proton donation in the proton transfer from alkane radical cations to alkane molecules can thus be derived from EPR spectral analysis of the neutral alkyl radicals formed. To aid the reader in appreciating the results that are presented on this matter below and in understanding related spectra from the literature, a section on the characterization of neutral alkyl radicals by EPR spectroscopy in solid systems is included at this point. 

Symmetric Proton Transfer from Alkane Radical Cations to Alkanes An Experimental Study in y-Irradiated n-Alkane Nanoparticles Embedded in a Cryogenic CCI3F Matrix... 

Radiolysis of cryogenic trichlorofluoromethane containing a suitable n-alkane as solute has proven very suitable for the study of symmetric proton transfer from alkane radical cations to alkane molecules. At low concentration of the alkane solute (RH) in the binary CCljF/alkane system, absorption of ionizing radiation mainly occurs by trichlorofluoromethane resulting in its excitation and ionization. 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

Evidence for the occurrence of asymmetric proton transfer in mixed n-alkane crystals has been obtained in close cormection with the demonstration of site selectivity in the donor and acceptor processes. Such evidence can only be gathered properly from species that are related to the solute (higher alkane) molecule, because any site selectivity from matrix spedes is completely wiped out by an overwhelming amount of nonselective processes (see above). With respect to the site of proton donation, proton transfer from solute radical cations to matrix molecules was therefore studied, whereas, with respect to the site of proton acceptance, proton transfer from matrix radical cations to solute molecules was investigated. 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

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