Activation/Driving Force Relationships for Cyclopropylcarbinyl → Homoallyl-Type Rearrangements of Radical Anions

Abstract

By using direct and indirect electrochemical methods, rate constants (ko) for cyclopropane ring opening of radical anions derived from the one-electron reduction of trans-1-benzoyl-2-phenylcyclopropane, trans-1-benzoyl-2-vinylcyclopropane, 2-methylenecyclopropyl phenyl ketone, spiro[anthracene-9,1'-cyclopropan-10-one], 3-cyclopropylcyclohex-2-en-1-one, and 3-(1-methylcyclopropyl)cyclohex-2-en-1-one were determined. Qualitatively, rate constants for ring opening of these (and other cyclopropyl- and cyclobutyl-containing radical anions) can be rationalized on the basis of the thermodynamic stability of the radical anion, the ability of substituents on the cyclopropyl group to stabilize the radical portion of the distonic radical anion, and the stability of the enolate portion of the distonic radical anion. On the basis of this notion, a thermochemical cycle for estimating deltaG(o) for ring opening was presented. For simple cyclopropyl-containing ketyl anions, a reasonable correlation between log(ko) and deltaG(o) was found, and stepwise dissociative electron transfer theory was applied to rationalize the results. Activation energies calculated with density functional theory (UB3LYP/6-31+G*) correlate reasonably well with measured log(ko). The derived log(ko) and deltaG(o) and log(ko) vs E(a) plots provide the basis for a "calibration curve" to predict rate constants for ring opening of radical anions derived from carbonyl compounds, in general.

A Radical‐Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical Anions

“…While our understanding of distonic radical-anion fragmentations is improving, there is still a lack of fundamental information for predicting the reactivity pathway and for establishing activation/driving force relationships. [4] Among the many findings in this study, we highlight that the incorporation of phenyl substituents is not the only criteria for observing a competing fragmentation reaction from the distonic radical anion of a bicyclic endoperoxide.…”

“…In the studies of the rigid bicyclic endoperoxides ASC, DASC, and ART with alkyl bridgehead substituents, only reduction of the distonic radical anion was observed to yield the corresponding cis-diol [Eq. (4)]. However, with endoperoxides with phenyl substituents, as with DPA-O 2 and TPD, a fragmentation pathway is observed in competition with reduction from the electrode.…”

“…The thermochemical cycle identifies three contributing factors to the driving force of the distonic radical-anion fragmentation reactivity: [4] 1) the thermodynamic stability of the delocalized distonic radical anion as reflected by the oxidation potential, 2) the fragmentation of the biradical, and 3) the stability of the localized radical anion as measured by the reduction potential.…”

The competing radical eliminations in the tandem mass spectrometry of the OH‐deprotonated benzyl vanillate

“…[5][6][7] However, several exceptional cases have been documented in literatures over the years that dissociation of an EE ion affords an odd-electron (OE) ion in the CID-MS process, which has attracted the interest of many mass spectrometrists. [7][8][9][10][11][12][13][14][15][16][17] Generation of radical ions can be attributed to resonance stabilization of the product ions, [13] or the dynamically favorable process via an intramolecular electronic excitation of a transient structure, rather than a simple homolytic cleavage. [12,14,15] Katritzky et al proposed a concept of the radical merostabilization, in which an electron-accepting group and an electron-donating group can significantly stabilize a radical.…”

A utilização da química computacional em processos químicos relacionados à ionização por electrospray

“…Por muitos anos, assumiu-se que a reatividade dos ânions radicais, em particular seu rearranjo e reações de dissociação eram similares às dos radicais neutros. [284][285][286][287] Contudo, Tanko e colaboradores demonstraram que existem exceções, e que ambos os sítios de carga e spin governam a reatividade dos ânions radicais, principalmente em seus rearranjos. 287 Existe um grande número de artigos científicos envolvendo ânions radicais, cuja clivagem resulta em espécies distônicas.…”