Methylenecyclopropane rearrangement as a probe for free radical substituent effects. sigma(center dot) values for potent radical-stabilizing nitrogen-containing substituents

A series of nitrogen-containing 2-aryl-3,3-dimethylmethylenecyclopropanes h ave been prepared and rearrangement rates to the corresponding 2-arylisopro pylidenecyclopropanes have been measured. These rates are dependent on the nature of the nitrogen-containing group in the para-position of the aryl gr oup. Rearrangement rates have been used to calculate sigma(.) values, which are a measure of the radical stabilizing ability of the substituent. Group s such as p-N=N-Bu-t, p-CH=N-Bu-t, p-NH2, p-CH=N-OH, and p-CH=N-OCH3, are " good" radical stabilizers. We have also classified groups such as p-CH=N-NM e2, 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 methylenecycloprop ane rearrangement by extensive spin delocalization. In the case of the latt er three substituents, nitroxyl type stabilization is proposed. Density fun ctional calculations (B3LYP/6-31G*) have been carried out on a series of ni trogen-containing substituted benzylic radicals. Rates of the methylenecycl opropane rearrangement correlate with radical stabilization energies (Delta E) determined from an isodesmic reaction of substituted benzylic radicals with toluene. These calculations confirm substantial spin delocalization on to the nitrogen-containing substituents on the para-position of the benzyli c radical.