The Chemistry of Cyclopropylarene Radical Cations
Abstract
Cyclopropane derivatives are frequently utilized as
"probes" for radical cation intermediates in a number
of important chemical and biochemical oxidation.
The implicit assumption in such studies is that if a
radical cation is produced, it will undergo ring
opening. Through a detailed examination of
follow-up chemistry of electrochemically and
chemically generated cyclopropylarene radical
cations, we have shown that the assumption made in
the use of these substrates as SET probes is not
necessarily valid. While cyclopropylbenzene radical
cation undergoes rapid methanol-induced ring
opening (e.g., k = 8.9 E7 s-1M-1), the radical
cations generated from 9-cyclopropylanthracenes
do not undergo cyclopropane ring opening at all.
The radical cations generated from
cyclopropylnaphthalenes disproportionate or
dimerize before undergoing ring opening. Utilizing
cyclic, derivative cyclic, and linear sweep
voltammetry, it was discovered that decay of radical
cations generated from cyclopropylnaphthalenes in
CH3CN/CH3OH is second order in radical cation
and zero order in methanol. Anodic and Ce(IV)
oxidation of all these naphthyl substrates in
CH3CN/CH3OH led to cyclopropane ring-opened
products. However, the rate constant for
methanol-induced ring opening (Ar-c-C3H5+. +
CH3OH -> ArCH(.)CH2CH2O(H+)CH3) is
extremely small for 1-cyclopropylnaphthalenes)
despite the fact that ring opening is exothermic by
nearly 30 kcal/mol. These results are explained on
the basis of a product-like transition state for ring
opening wherein the positive charge is localized on
the cyclopropyl group, and thus unable to benefit
from potential stabilization offered by the aromatic
ring. Reactions of radical cations generated from
9-cyclopropylanthracenes in CH3CN/CH3CN have
also been investigated electrochemically. The major
products arising from oxidation of these anthryl
substrates are attributable to CH3OH attack at the
aromatic ring rather than CH3OH-induced
cyclopropane ring opening. Ce(IV) oxidation of
9-cyclopropyl-10-methylanthracene and
9,10-dimethylanthracene further showed that radical
cations generated from these anthryl substrates
undergo neither cyclopropane ring opening nor
deprotonation but nucleophilic addition. Side-chain
oxidation products from Ce(IV) oxidation of
methylated anthracenes arose from further reaction
of nuclear oxidation products under acidic and
higher temperature conditions. An analogous (more
product-like) transition state picture can be applied
for cyclopropane ring opening and deprotonation of
these anthryl radical cations. Because of much higher
intrinsic barrier to either nucleophile-induced
cyclopropane ring opening or deprotonation of these
anthryl radical cations, nucleophilic addition
predominates. Stereoelectronic effects may be
another additional factor contributing to this intrinsic
barrier because the cyclopropyl group in these
anthryl systems adopts a perpendicular conformation
which may not meet the stereoelectronic
requirements for cyclopropyl ring opening at either
the radical cation or dication stage.
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