DFT calculations were performed to shed light on the molecular mechanism of [3 + 2] cycloadditions of simple conjugated nitroalkenes to benzonitrile N-oxides. In particular, it was found that these processes proceed by a one-step mechanism through asynchronous transition states. According to the latest terminology, they should be considered polar but not stepwise processes.

C H R O N I C L E A B S T R A C TAnalysis of addent interactions with respect to the theory of electrophilicity and nucleophilicity indexes and kinetic studies shows the polar nature of,3-trichloro-1-nitroprop-1-ene. However, PES exploration results do not confirm any zwitterion on the reaction paths. On the other hand, the nature of the solvent effect and the experimentally determined activation parameter values do not exclude a scenario in which one-step and stepwise processes occur simultaneously.

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Quantum-chemical calculations using the semiempirical AM1/COSMO method have shown that the regioselectivity of [2+3] cycloaddition of 3-nitro-1-propene to triphenylnitrone is a function of thermodynamic factors.In previous work [2], we reported that the cycloaddition of 3-nitro-1-propene (1) to triphenylnitrone (2) proceeds regioselectivity. Of the two theoretically possible nitromethylisoxazolidines 3 and 4, only 5-nitromethyl-2,3,3-triphenylisoxazolidine (4) is formed in 15% yield. However, this pathway is not the only direction for the conversion of nitroalkene 1, which is partially converted under the reaction conditions to 1-nitro-1-propene (5). The reaction of propene 5 with nitrone 2 gives 4,5-trans-5-methyl-4-nitro-2,3,3-triphenylisoxazolidine (6) in 75% yield. Thus, the overall reaction is complex in nature (Scheme 1).In order to understand the specifics of these reactions, we carried out a quantum-chemical analysis of the cycloaddition of nitropropene 1 to nitrone 2 through regioisomeric pathways A and B using the AM1/COSMO program from the MOPAC93 package [3]. In previous work [4, 5], we carried out similar calculations for the reaction of this nitrone with nitropropene 5 (pathways C and D).The reaction analyzed was carried out without solvent with a ten-fold excess of nitroalkene 1. Since, the dielectric constant for nitroalkene 1 is unknown, the EPS key word with value 38 was used for taking account of the solvent effect. This permits us to simulate a dielectric continuum for a reaction carried out in nitromethane [3]. (According to AM1 data, the dipole moments of nitropropene 1 and nitromethane differ only slightly and, thus, we assumed that the two compounds have similar EPS dielectric constants). 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. _______ * For Communication 62 see [1].