The Low Energy of Concert in Many Symmetry‐Allowed Cycloadditions Supports a Stepwise‐Diradical Mechanism

Abstract

The thesis of this paper is that orbital symmetry allowedness does not guarantee concert. The activation energy (E a ) for concerted Diels-Alder and 1,3-dipolar cycloadditions must be substantially lower than that for a stepwise-diradical pathway because the diradical has two fewer bonding electrons than anything on the concerted reaction coordinate, including the transition state. Two bonding electrons provide tens of kcal/mol of stabilization. The difference between the experimental E a and that for diradicals is called the energy of concert (E con ). If experiment represents concert, E con must be large, and if it represents diradicals E con will be very small. In this paper, 42 examples are adduced in which firm experimental data show that E con = 0. These cycloadditions cannot be concerted. While concert remains possible for all cycioadditions not proven stepwise, there is none with compelling evidence for concert. C 2013

Unravelling the Mysteries of the [3+2] Cycloaddition Reactions

“…[69] This MEDT rationalisation of 32CA reactions unravels the long-ranging controversy between Huisgen's [9] and Firestone's [10] mechanistic proposals. [10] However, it is worth emphasizing that while NPA of the charge distribution of zwitterionic TACs suggests that they are neither 1,3nor 1,2-charged structures with full charge separation, [3] pseudodiradical TACs, which are species topologically characterized by the presence of non-bonding electron density integrating less than 1 e at the terminal carbons, [53] do not correspond to pure singlet diradical structures, [10,14] in agreement with Braida's suggestion. [10] However, it is worth emphasizing that while NPA of the charge distribution of zwitterionic TACs suggests that they are neither 1,3nor 1,2-charged structures with full charge separation, [3] pseudodiradical TACs, which are species topologically characterized by the presence of non-bonding electron density integrating less than 1 e at the terminal carbons, [53] do not correspond to pure singlet diradical structures, [10,14] in agreement with Braida's suggestion.…”

“…[11] Note, however, that the experimental observations on which Huisgen based his proposal suggested a one-step mechanism but they did not constitute proof of a concerted mechanism according to the original definition given by Lewis in 1923. [10] Today, Firestone still supports the stepwise diradical mechanism [14] after having gathered a great deal of experimental evidence of the presence of diradical species along the reaction path of several cycloaddition reactions. On the other hand, based on several experimental inconsistencies with respect Huisgen's suggestions, Firestone proposed, in 1968, an alternative two-step mechanism via formation of a ), yet recognising that a duality of mechanisms may exist.…”

Detailed investigations into the Akabori–Momotani reaction for the synthesis of amphetamine type stimulants: Part 2

“…The activation energy for the 1,3-dipolar cycloaddition calculated in this study is approximately 80 kJmol -1 , which is comparable to literature barriers for 1,3-dipolar cycloadditions [17][18][19]. The possibility of the reaction occurring via stepwise mechanisms [20] was also examined theoretically but the barriers calculated were unreasonable 3 .…”

Diradical reaction mechanisms in [3 + 2]-cycloadditions of hetaryl thioketones with alkyl- or trimethylsilyl-substituted diazomethanes

“…In addition to the experimental findings, new reports dealing with computational studies aimed at the demonstration of new zwitterionic [3 + 2]-cycloadditions were published [ 10 – 12 ]. Finally, a third concept for the interpretation of the mechanism of [3 + 2]-cycloadditions, formulated by Firestone, is based on the assumption that they occur via diradical intermediates [ 13 – 15 ].…”