Cyclopropyl Conjugation and Ketyl Anions:  When Do Things Begin to Fall Apart?

Abstract

Results pertaining to the electrochemical reduction of 1,2-diacetylcyclopropane (5), 1-acetyl-2-phenylcyclopropane (6), 1-acetyl-2-benzoylcyclopropane (7), and 1,2-dibenzoylcyclopropane (8) are reported. While 6*- exists as a discrete species, the barrier to ring opening is very small (<1 kcal/mol) and the rate constant for ring opening is >10(7) s(-1). For 7 and 8, the additional resonance stabilization afforded by the benzoyl moieties results in significantly lower rate constants for ring opening, on the order of 10(5)-10(6) s(-1). Electron transfer to 8 serves to initiate an unexpected vinylcyclopropane --> cyclopentene type rearrangement, which occurs via a radical ion chain mechanism. The results for reduction of 5 are less clear-cut: The experimental results suggest that the reduction is unexceptional, with a symmetry coefficient alpha </= 0.5, and reorganization energy consistent with a simple electron-transfer process (one electron reduction, followed by ring opening). In contrast, molecular orbital calculations suggest that 5*- has no apparent lifetime and that reduction of 5 may occur by a concerted dissociative electron transfer (DET) mechanism (i.e., electron transfer and ring opening occur simultaneously). These seemingly contradictory results can be reconciled if the increase in the internal reorganization energy associated with the onset of concerted DET is offset by a lowering of the solvent reorganization energy associated with electron transfer to a more highly delocalized LUMO.

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

“…(1)]. [2][3][4][5] However, examples involving the fragmentation of a spatially separated radical anion yielding a localized radical anion and a neutral molecule are rather unknown [Eq. (2)].…”

“…Table 3 summarizes the various thermochemical values for the diphenyl-and di-A C H T U N G T R E N N U N G alkyl-substituted endoperoxides and corresponding reactive intermediates. The overall driving force for the fragmentation of the distonic radical anion DG fragðCORRO À Þ is given by Equation (5).…”

Efficient Homogeneous Radical‐Anion Chain Reactions Initiated by Dissociative Electron Transfer to 3,3,6,6‐Tetraaryl‐1,2‐dioxanes

“…However, a body of work by Tanko and coworkers has provided evidence that this assumption is not necessarily valid. [3][4][5][6] They demonstrated by electrochemically generating a series of cycloalkylketone radical anions that both charge and spin are governing factors in the reactivity of radical-anion rearrangements. The radical anions in these studies were observed to undergo a carbon-carbon bond fragmentation yielding a single species containing a spatially separated radical and anion, which is otherwise known as a distonic radical anion [Eq.…”

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.…”

“…[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. 200,284,287 Por outro lado, exemplos envolvendo a dissociação de ânions distônicos, cujos produtos da reação sejam um ânion radical e uma molécula neutra ainda são desconhecidos.…”