The Chemistry of Cyclopropylarene Radical Cations
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.
- Doctoral Dissertations