Structure and Reactivity of Radical Ions: New Twists on Old Concepts

Abstract

Electron transfer is the simplest reaction possible, yet it has a profound impact on the structure and reactivity of organic compounds. These changes allow a new look at some of the fundamental concepts that are used to explain organic chemistry, such as symmetry, aromaticity, and bonding. The results from high-level electronic structure calculations are used to analyze the mechanistic differences in the pericyclic reactions of simple hydrocarbons and their radical cation counterparts. The importance of state symmetry correlation, Jahn-Teller distortions, delocalization, and fractional bonding for the reaction pathways of hydrocarbon radical cations is discussed.

A computational study of the Diels-Alder reactions between 2,3-dibromo-1,3-butadiene and maleic anhydride

“…In summary, these results corroborate the picture that neutral reactions usually occur in a concerted fashion while in cationic systems a non-concerted mechanism is favored [3,14]. However, the border between asynchronous, concerted and stepwise mechanisms is not yet clear [9,10,13].…”

“…Two σ bonds and one π bond are formed from three π bonds as depicted in Scheme 1. This reaction has been the subject of many experimental and computational studies aiming to decide if it takes place in a concerted fashion and if so, whether or not it is a synchronous process and how the mechanism depends on the geometric and electronic properties of the reactants [3,4,5,6,7,8,9,10,11,12,13,14].…”

“…There have been some studies on the conservation of orbital symmetry to try to construct rules analogous to the Woodward-Hoffmann rules widely used for the neutral reactions [28,29,30]. Wiest and Donoghe proposed a model in which the electronic state symmetry must be conserved throughout the reaction [14]. Gas-phase experiments on the cationic Diels-Alder reaction between butadiene and ethene have been unable to isolate the Diels-Alder product.…”

Selectivity in Radical Cation Cycloadditions

“…One possible reason is the lack of predictability of selectivity due to various competing mechanisms as well as the generally low familiarity of many organic chemists with the selectivity determining factors. This review complements the previous, mechanistically oriented reviews [9][10][11]17,18,[58][59][60] in addressing some of these questions by providing an overview of patterns governing the selectivity that has been observed in ET catalyzed cycloadditions. Using selected literature examples rather than a comprehensive overview, we will discuss the experimental findings and provide an overview of the factors determining them.…”

“…Mechanistically, radical cation Diels-Alder reactions are generally considered to be stepwise reactions, [16][17][18] as demonstrated by experimental 19,20 and computational [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] studies. The stepwise mechanism is the result of removing an electron from the HOMO (or, less commonly, the addition of an electron to the antibonding orbital in the case of radical anions), weakening the bond and making the bond easier to break.…”