A computational study of the electron affinities of substituted Cope rearrangement transition states †

Abstract

The structures and properties of the negative ions of the transition states of the Cope rearrangement of hexa-1,5diene and 2,5-dicyanohexa-1,5-diene and the negative ion of cyclohexanedione are examined at the Becke3LYP/6-31ϩG* level of theory. The negative ion of the hydrocarbon cyclohexane-1,4-diyl is calculated to be unstable with respect to electron detachment and with respect to ring opening. Addition of two cyano substituents results in an ion that is stable with respect to both electron detachment and with respect to ring opening, such that the Cope rearrangement has an inverted potential energy surface for the negative ion. The negative ion of cyclohexanedione is calculated to be stable with respect to ring-opening, but unstable with respect to electron detachment in the chair conformation. The ion in the boat geometry is predicted to be stable with respect to vertical electron detachment, suggesting that bicyclo[4.2.0]octa-2,5-dione will form a stable negative ion in the gas phase.

An Electron-Catalyzed Cope Cyclization. The Structure of the 2,5-Dicyano-1,5-hexadiene Radical Anion in the Gas Phase

“…28 For example, cyclization of the radical anion of 2,5-dicyano-1,5-hexadiene (2a, eq 1) to form the corresponding cyclohexane-1,4-diyl radical anion 2b is calculated to be exothermic by 16.2 kcal/mol (B3LYP/6-31+G*). 28 In this work, we provide experimental evidence that 1 does indeed cyclize upon reduction in the gas phase, confirming the theoretical predictions.Addition of neutral 2,5-dicyano-1,5-hexadiene 10 cm downstream of the electron emission filament in a flowing afterglow-triple quadrupole apparatus 29,30 (operating at room temperature with 0.4 Torr of helium buffer gas flowing at a rate of 200 STP cm 3 /s) results in the formation of molecular anion 2 (m/z 132) by thermal electron capture.The structure of 2 was established by using chemical reactivity. Ion 2 is found to undergo reactions common to organic anions, such as condensation with CO 2 and CS 2 .…”

“…Recently reported electronic structure calculations predict that, in select cases, single electron reduction can also catalyze the Cope rearrangement because the electron affinity of the openshell transition state is much greater than that of the closedshell hexadiene (Figure 1). 28 For example, cyclization of the radical anion of 2,5-dicyano-1,5-hexadiene (2a, eq 1) to form the corresponding cyclohexane-1,4-diyl radical anion 2b is calculated to be exothermic by 16.2 kcal/mol (B3LYP/6-31+G*). 28 In this work, we provide experimental evidence that 1 does indeed cyclize upon reduction in the gas phase, confirming the theoretical predictions.…”

Antiaromaticity-promoted radical anion stability in α-vinyl heterocyclics

“…44 Through DFT calculations, Alabugin and coworkers demonstrated in 2003 that remote substituent effects and benzannelation of enediyne lead to the dynamic interplay between antiaromaticity (reactant radical anions) and aromaticity (transition and product), promoting radical anionic cyclorearomatization reactions with much higher efficiency than the thermal mode. 45 Similarly, both theoreticians 46 and experimentalists 47 confirmed the formation of cyclohexane-1,4-diyl radical anions during the electron-catalyzed Cope cyclization reaction.…”

Reaction Mechanisms. Part 2. Pericyclic Reactions

“…60 Wenthold describes the effects of substituents on the electron af¢nity of the transition state for the Cope rearrangement reaction. 62 Negative ion photoelectron spectroscopy has been used to study the electronic structure of transition states, and this study aims to ¢nd good candidates to study Cope rearrangement. Calculations at the B3LYP/6^31G* level of theory show that the addition of electron-withdrawing groups at C1 and C4 stabilizes the negative ions of Cope transition states such that electron detachment or ring opening is less likely to occur.…”