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.