J. Am. Chem. Soc. 1999, 121, 9429-9434
9429
Cleavage Reactions of Radical Anions that Range from Homolytic to
Heterolytic within the Same Family of Compounds
Zi-Rong Zheng,1a Dennis H. Evans,*,1a Elisa Soazara Chan-Shing,1b and Jean Lessard1b
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19706, and Centre de Recherche en Electrochimie et Electrocatalyse,
De´partement de Chimie, UniVersite´ de Sherbrooke, Sherbrooke, Quebec J1K 2R1, Canada
ReceiVed July 1, 1999
Abstract: Properties of the radical anions of three R-nitrocumenes (R-nitrocumene, p-cyano-R-nitrocumene,
and p-nitro-R-nitrocumene, 1a-c) have been determined by electrochemical methods. In particular, the standard
potentials of the neutral/radical anion couples were found to be -2.20, -2.04, and -1.43 V with respect to
the ferrocenium ion/ferrocene potential and the rate constants for expulsion of nitrite from the radical anions
were 3 × 106, 5 × 106, and 240 s-1 for 1a-c, respectively. Comparison of these potentials with those of
related compounds demonstrates that reduction of the nitroalkyl portion of the molecule occurs in 1a and 1b
while in 1c the electron is added to the nitrophenyl group. Thus, using previously defined terminology, the
cleavage of the radical anions of 1a and 1b to give nitrite and the corresponding cumyl radicals are examples
of homolytic cleavage reactions but the cleavage of the radical anion of 1c is heterolytic. The driving force for
the three cleavage reactions has been estimated and it is concluded that the large decrease in magnitude of the
cleavage rate constants on going from 1a and 1b to 1c is mainly due to the much larger driving force for the
first two.
Introduction
of benzoate esters,4 and additional examples in the chloroaro-
matic family.5
The cleavage reaction of radical ions can be classified as
homolytic (eq 1a) if the cleavage leaves the charge mainly in
the same region as it was in the radical ion or heterolytic (eq
1b) in which there is “regioconservation” of spin density.2a-e
Examples of both homolytic and heterolytic bond cleavages
in radical ions have been investigated including the effect of
variation of substitutents so as, for example, to vary the driving
force for the cleavage reaction. However, in all cases all
members of the family of compounds investigated followed
either the homolytic or heterolytic pathway. In the present paper
we report the first example of a change in mechanism that occurs
when substituents are varied.
The compounds that were studied are the R-nitrocumenes 1.
The radical anions of the parent compound 1a and the cyano
derivative 1b undergo homolytic bond cleavage giving nitrite
and the corresponding cumyl radical. By contrast, nitro deriva-
tive 1c follows the heterolytic pathway giving the same products.
Radical anions are shown in eq 1 as they are the subject of the
present paper. Save´ant and co-workers have described the
heterolytic bond cleavage as an intramolecular dissociative
electron transfer in which the electron in an orbital mainly
centered on A is transferred to B with concerted bond-
breaking.2f-m In addition to the systems investigated in the
papers cited, other reactions that have been studied include
cleavage reactions in the radical anions of R-aryloxyacetophe-
nones,3 cleavage of remote C-Br bonds in the radical anions
The electrochemical reduction of 1a has previously been
investigated6 and it was found that cleavage of nitrite from its
radical anion was very rapid, its half-life being <1 ms.
Nitrocumene 1a has also been used as a test reagent to
distinguish between electronation-protonation or electrocata-
lytic hydrogenation mechanisms in the reduction of nitro
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10.1021/ja9922779 CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/25/1999