Methylenecyclopropane Rearrangement as a Probe for Free Radical Substituent Effects. σ ^• Values for Potent Radical-Stabilizing Nitrogen-Containing Substituents

Abstract

A series of nitrogen-containing 2-aryl-3,3-dimethylmethylenecyclopropanes have been prepared and rearrangement rates to the corresponding 2-arylisopropylidenecyclopropanes have been measured. These rates are dependent on the nature of the nitrogen-containing group in the para-position of the aryl group. Rearrangement rates have been used to calculate sigma (*) values, which are a measure of the radical stabilizing ability of the substituent. Groups such as p-N=N-Bu-t, p-CH=N-Bu-t, p-NH(2), p-CH=N-OH, and p-CH=N-OCH(3), are "good" radical stabilizers. We have also classified groups such as p-CH=N-NMe(2), p-N=N-Ph, p-N=N(O)-Bu-t, p-CH=N(O)-Bu-t, and p-CH=N-O(-) M(+), which have an extraordinarily large radical stabilizing effect, as "Super Stabilizers". These substituents stabilize the transition state of the methylenecyclopropane rearrangement by extensive spin delocalization. In the case of the latter three substituents, nitroxyl type stabilization is proposed. Density functional calculations (B3LYP/6-31G) have been carried out on a series of nitrogen-containing substituted benzylic radicals. Rates of the methylenecyclopropane rearrangement correlate with radical stabilization energies (DeltaE) determined from an isodesmic reaction of substituted benzylic radicals with toluene. These calculations confirm substantial spin delocalization onto the nitrogen-containing substituents on the para-position of the benzylic radical.

Singlet stabilization of oxazole- and isoxazolenitrene-n-oxides by radical delocalization

“…Remarkably, the slope of the line is found to be ca. 1.9, indicating that the isomeric stabilization of the singlet states is about twice as large as that of the triplet states, the same as what was found previously in the comparison of singlet-triplet splittings and a-BDEs of substituted systems [29].…”

“…While the effects on the singlet-triplet splittings by common substituents are generally small (<3 kcal/mol) in an absolute sense [5,27,28], they are significant considering the extent to which pradicals themselves can be stabilized by substitution. In fact, the effects of substitution on the singlet-triplet splittings are predicted to be similar to those on bond dissociation energies in substituted toluenes [25,26,29]. Because the effects on the singlet-triplet splittings result from differential stabilization, the fact that they are found at all indicates that the substituents have a spin-state dependent radical substituent effect.…”

Electronic Properties of Triazoles. Experimental and Computational Determination of Carbocation and Radical-Stabilizing Properties

“…Computational studies again were used to gain insight into these radical-stabilizing effects. Our previous studies have shown a very good correlation between the rearrangement rate of methylenecyclopropanes 33 and the B3LYP/6-31G* calculated radical stabilization energy of benzylic radicals. In fact, data for 36 – 38 (shown in red) fit very nicely on this plot as shown in Figure .…”

Correlation analysis in the chemistry of free radicals

“…In addition, it is not quite clear to what extent the centre in the transition state 6 is a radical centre. Nevertheless, this approach has been successfully developed; it was shown in a recent publication 94 that the introduction of nitrogen-containing substituents into the para-position of the substrate 4 provides considerable radical stabilisation. Moreover, some substituents, for example, CH=NNMe 2 , N=NPh, N=N(O)Bu t and CH=N(O)Bu t (s C = 0.92, 1.08, 1.08 and 1.13, respectively), displayed extraordinary radical-stabilisation capacity; they were called super-stabilisers.…”