[2 + 3] Cycloaddition of C,C,N‐triphenylnitrone to trans‐substituted nitroethylenes in the light of an AM1 and AM1/COSMO computational study

Abstract

AM1 calculations suggest that the [2 + 3] cycloaddition of triphenylnitrone 1 to nitroalkenes 2a–c in the gas phase takes place in a concerted manner. However, the azolidine ring bonds C3—C4 and O1—C5 are not formed fully synchronously. Kinetic factors favor the formation of cycloadducts with nitro group in position C‐4 of the azolidine ring (path A). Introduction of toluene, acetone or acetonitrile as a reaction medium increases the activation barriers for both regioisomeric paths. Simultaneously, the energy profiles for path A undergo qualitative change. In this case, two transition states and an intermediate with zwitterionic character were localized by means of the AM1/COSMO method. Copyright © 2003 John Wiley & Sons, Ltd.

Synthesis and properties of azoles and azole derivatives 63. Quantum-chemical analysis of [2+3] cycloaddition of 3-nitro-1-propene with triphenylnitrone

“…The difference in the distances between the reaction sites C(3)-C(4) and C(5)-O(1) in the transition state localized on pathway B is about 0.21 Å, while the charge corresponding to transfer does not exceed 0.05 e. The free enthalpies of activation (∆G ≠ ) for both pathways examined are relatively high (>51 kcal/mol) ( Table 1). We should note that the activation barrier thereby calculated for the reaction of nitrone 2 with nitropropene 5 in nitromethane is 45.8 kcal/mol on pathway C and 60.0 kcal/mol on pathway D, while these values in toluene are 40.8 and 51.3 kcal/mol, respectively [4,5]. Thus, it is not remarkable that the [2+3] ycloaddition of nitrone 2 to nitropropene 5 proceeds much more readily through pathway C. The extent of conversion of nitrone 2 to nitroisoxazolidine 6 is apparently limited by the isomerization of nitropropene 1 to nitropropene 5.…”

“…In previous work [4,5], we carried out similar calculations for the reaction of this nitrone with nitropropene 5 (pathways C and D).…”

“…Taking account of the opinion of the authors of the COSMO program [6], the NSPA parameter characterizing the solvation density, was taken equal to 42. The calculations were carried out analogously to those described in our previous work [4]. The critical structure parameters obtained as well as the activation parameters of the reactions examined are given in Table 1.…”

Synthesis and properties of azoles and their derivatives. 68.* [2+3] cycloaddition of 2-nitro-1-propene to (Z)-C,N-diphenylnitrone relative to AM1 and AM1/COSMO quantum-chemical calculations

“…However, in the case of strong electrophilic alkenes or considerable shielding of one of the reaction sites, a stepwise mechanism involving formation of a dipolar intermediate may compete with the concerted mechanism [3,4]. In this regard, in a continuation of a study of the mechanism of (4+2) -electron cycloadditions [5][6][7][8][9][10][11], we investigated the reaction pathways of the [2+3] cycloaddition of 2-nitro-1-propene (1) to (Z)-C,N-diphenylnitrone (2). The formation of four regioisomeric and stereoisomeric diarylnitroisoxazolidines 3-6 is possible in the reaction.…”

“…The global electrophilicity of this compound ( = 2.47 eV) is characteristic for strong -electron-deficient dipolarophiles [12], while the reaction sites are considerably shielded due to the presence of the nitro and methyl groups at the same vinyl carbon atom. Calculations of the possible reaction pathways were carried out with complete optimization of the geometric parameters using the MOPAC 93 program [13] by the standard AM1 method, as in the case of cycloaddition reactions studied in our previous work [5][6][7][8]. The effect of the dielectric medium (toluene and nitromethane) was calculated using the COSMO procedure [14] with NSPA = 42 and RSOLV = 1.…”