Rearrangements of 2-Nitrobenzyl Compounds. 1. Potential Energy Surface of 2-Nitrotoluene and Its Isomers Explored with ab Initio and Density Functional Theory Methods

Reaction paths leading from 2-nitrotoluene ( 1 ) to 2-nitrosobenzyl alcohol ( 4 ) were investigated by using ab initio and density functional theory. Overall, 25 minima and 12 transition states were located for molecules of composition C 7 H 7 NO 2 . Nine conformational minima were found for 4, all being slightly lower in energy than 1 . The lowest-energy path from 1 to 4 was predicted to have an overall activation energy of 51.7 kcal mol -1 at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d,p) level of theory. The rate-determining step is a 1,3-hydrogen shift between two oxygen atoms in the aci- form 2 . The activation barrier of 20.3 kcal mol -1 predicted for this shift is substantially higher than that of 9.7 kcal mol -1 for the retautomerization 2 → 1 . To account for reaction paths involving ionization of the moderately strong acid 2 in aqueous solution, anions obtained by deprotonation of 1 − 4 were studied. A common anion 2 - was found by CH-deprotonation of 1 and by OH-deprotonation of two aci -isomers 2 . An anion 3 - was obtained by deprotonation of benzisoxazoline 3 and nitrone 6 . A single conformer 4 - was located by deprotonation of 4 . At the B3LYP/6-311+G(2d,p) level the activation barrier of 39.2 kcal mol -1 for the reaction 2 - → 3 - is much higher than that of 18.4 kcal mol -1 for the cyclization of the neutral aci -form, trans - 2 → 3a . The barrier height for the latter reaction was reproduced within 1.0 kcal mol -1 by calculating QCISD(T)/6-31G(d,p) energies at the B3LYP and MP2/6-31G(d) geometries. On the other hand, the reaction 3 - → 4 - is expected to be very facile in contrast to ring opening in the neutral species, 3 → 4 . These results indicate that the cyclization 2 → 3 is acid-catalyzed around neutral pH and that the subsequent ring opening 3 → 4 is base-catalyzed.