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The Reaction Mechanism of the [2+3] Cycloaddition Between A‑Phenylnitroethene and (Z)‑C,N‑Diphenylnitrone in the Light of a B3LYP/6‑31G(D) Computational Study†

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The Reaction Mechanism of the [2+3] Cycloaddition Between A‑Phenylnitroethene and (Z)‑C,N‑Diphenylnitrone in the Light of a B3LYP/6‑31G(D) Computational Study† Cent. Eur. J. Chem. • 11(3) • 2013 • 404-412 DOI: 10.2478/s11532-012-0169-0 Central European Journal of Chemistry The reaction mechanism of the [2+3] cycloaddition between a‑phenylnitroethene and (Z)‑C,N‑diphenylnitrone in the light of a B3LYP/6‑31G(d) computational study† Research Article Radomir Jasiński*, Maria Mikulska , Andrzej Barański Institute of Organic Chemistry and Technology Cracow University of Technology, 31155 Cracow, Poland Received 10 September 2012; Accepted 5 November 2012 Abstract: The analysis of reactivity indices suggests the polar nature of the [2+3] cycloaddition of a-phenylnitroethene to (Z)-C,N-diphenylnitrone. Similar conclusions can be drawn from the investigation of the reaction pathways using the B3LYP/6-31g(d) algorithm. This shows that the cycloaddition mechanism depends on the polarity of the reaction medium. A one-step mechanism is followed in the gas phase and toluene in all the theoretically possible pathways. In more polar media (nitromethane, water), a zwitterionic, two-step rather than a one-step mechanism occurs in the pathway leading to 3,4-trans-2,3,5-triphenyl-4-nitroisoxazolidine. Keywords: [2+3] cycloaddition • Nitrone • Nitroalkene • Mechanism • DFT calculations © Versita Sp. z o.o. 1. Introduction of theoretically possible [2+3] cycloaddition pathways of α-phenylnitroethene (1) to (Z)-C,N-diphenylnitrone (2) An increasing number of reports question common views in the gas phase with three dielectric media of different of the one-step mechanism of [2+3] cycloaddition [2] as polarities (Scheme 1). the only possible one, independent of addend structures. The electrophilicity of nitroethene 1 (ω=2.44 eV) is Currently it is known that the [2+3] cycloaddition reactions comparable to that of 2-nitroprop-1-ene (ω=2.46 eV) involving strongly electrophilic dipolarophiles [3] may [14], but the shielding of the nitrovinyl moiety’s α carbon occur as two-step processes through a zwitterionic atom is much higher. Therefore we expected an ionic intermediate [4-9]. This mechanism is particularly reaction mechanism, particularly in the presence of polar likely when one of the dipolarophile reaction centres is solvents. intensely sterically shielded, and the other is deshielded. Such dipolarophiles include for example α-substituted nitroethenes; we have studied their [2+3] cycloaddition 2. Computational procedure reactions with nitrile N-oxides [10] and nitrones [11-13] over a number of years. In particular [12], a study The quantum-chemical calculations were performed on with B3LYP/6-31g(d) simulations of competing [2+3] a SGI-Altix-3700 computer in the Cracow Computing cycloaddition pathways of 2-nitroprop-1-ene to (Z)-C,N- Center “CYFRONET”. Hybrid B3LYP functional and diarylnitrones demonstrated that reaction transition 6-31G(d) basis set included within Gaussian 03 software complexes are kinetically preferred pathways and are [15] were applied. significantly asymmetric. However, the asymmetry is not Global reactivity indices were estimated according great enough to induce a zwitterionic, two-step reaction. to equations recommended by Parr and Yang [16] and In this paper we discuss the quantumchemical simulations Domingo [17]. In particular, electronic chemical potentials * E-mail: [email protected] 404 † Part 15 of the series ‘Conjugated Nitroalkenes in Cycloaddition Reactions’; Part 14 see [1] R. Jasiński, M. Mikulska , A. Barański Ph Ph The critical structures on the potential energy surface A were localised in an analogous manner as in the case NO2 Ph N of the previously analysed reaction of 2-nitroprop-1-ene O 3 with (Z)-C,N-diphenylnitrone [12]. The charge transfer (t) [19] was calculated according to the formula: Ph NO2 B Ph Ph N t=-ΣqA Ph NO2 Ph O + + 4 N Ph where qA is the net charge and the sum is taken over all Ph O 1 2 the atoms of dipolarophile. For the calculations of the C NO2 N solvent effect, the PCM algorithm of Tomasi et al. [20] Ph Ph O was used. All calculations were performed at a T=298 K 5 and p=1 atm. The results are shown in Tables 2 and 3. Ph In this text the letters LM, I and TS denote the D Ph pre-reaction complex, zwitterionic intermediate and Ph N O NO2 transition state, respectively, and a subscript is added to 6 denote the reaction path. Scheme 1. Theoretically possible reaction paths of nitroalkene 1 with nitrone 2. A 3. Results and discussion [LMA] [TSA] 3 B [ ] [ ] LMB TSB 4 To investigate the tested reaction more thoroughly 1 + 2 C [LMC] [TSC] 5 before beginning exploration of its PES, we analysed D the electronic interactions of addents 1 and 2 using [LMD] [TSD] 6 Domingo’s theory of reactivity indices [3,17]. We Scheme 2. Critical structures in reaction between nitroalkene 1 and nitrone 2 in gas phase and toluene. hoped that they could provide an insight into the [2+3] mechanism, which had not previously been kinetically Table 1. Global reactivity indices for nitroalkene 1 and nitrone 2. studied. µ ω N The calculated electronic chemical potential values [a.u.] [eV] [eV] (Table 1) show that during the [2+3] cycloaddition 1 -0.1711 2.44 2.24 1+2 charge transfer should occur from nitrone 2 2 -0.1312 1.67 3.64 (µ=–0.1312 a.u.) to nitroalkene 1 (µ=–0.1711 a.u.). The global electrophilicity of the nitroalkene (ω=2.44 eV) (µ) and chemical hardness (η) of reactants 1 and 2 were confirms its strongly electrophilic nature [3]. Nitrone’s evaluated in terms of one-electron FMO energies using electrophilicity is much lower (ω=1.67 eV); therefore it Eqs. 1 and 2. The µ and η values were then used for functions as a nucleophile in the reactions tested. Its the calculation of global electrophilicity (ω) according to global nucleophilicity [21] is 3.64eV. The electrophilicity Eq. 3. difference of addents 1 and 2 (∆ω=0.77 eV) suggests the polar nature of [2+3] cycloaddition reactions involving µ≈(ΕΗΟΜΟ+ΕLUMO)/2 (1) these addents. The analysis of reactivity indices enables us to η≈Ε −Ε (2) forecast reaction polarity [17]. However, it provides LUΜΟ HOMO no information on the critical structures which occur ω=µ2/2 η (3) on the reaction pathways. We derived such data from the B3LYP/6-31(d) simulations of reaction pathways The global nucleophilicity (N) of diphenylnitrone 2 A-D (Scheme 1) in the gas phase and for a dielectric was calculated as the difference [18]: medium. The enthalpy profiles of reactions A-D turned out Ν=ΕΗΟΜΟ (nitrone) − ΕHOMO (tetracyanoethene) (4) to be quite similar in the gas phase (ε=1.00) (Fig. 1, Scheme 2). In each case, the LM and the TS complex The values of the resulting global and local reactivity structures were located between the addent (1+2) indices are presented in Table 1. and the cycloadduct (3-6) trough. IRC calculations 405 The reaction mechanism of the [2+3] cycloaddition between a‑phenylnitroethene and (Z)‑C,N‑diphenylnitrone in the light of a B3LYP/6‑31G(d) computational study † Table 2. Kinetic and thermodynamic parameters of [2+3] cycloaddition between a-phenylnitroethene 1 and (Z)-C,N-diphenylnitrone 2 according to B3LYP/6-31G(d) calculations (T=298 K; ∆H and ∆G values are in kcal mol-1; ∆S values are in cal mol-1 K-1). Solvent (ε) Path Transition ΔH ΔG ΔS 1+2→LMA -3.0 4.8 -26.3 A 1+2→TSA 14.1 28.3 -47.8 1+2→3 -10.6 4.2 -49.5 1+2→LMB -1.4 7.1 -28.4 B 1+2→TSB 15.1 29.3 -47.7 Gas 1+2→4 -8.4 6.5 -49.9 phase (1.00) 1+2→LMC -3.9 5.5 -31.6 C 1+2→TSC 13.1 27.3 -47.8 1+2→5 -17.2 -2.7 -48.8 1+2→LMD -3.0 4.8 -26.2 D 1+2→TSD 16.0 30.2 -47.5 1+2→6 -17.3 -3.6 -46.0 1+2→LMA -1.3 12.0 -44.4 A 1+2→TSA 14.6 28.5 -46.9 1+2→3 -7.6 7.0 -48.9 1+2→LMB -0.1 7.7 -26.2 B 1+2→TSB 15.7 30.0 -47.9 Toluene 1+2→4 -6.3 8.6 -50.0 (2.38) 1+2→LMC -2.4 9.4 -39.7 C 1+2→TSC 14.3 28.4 -47.1 1+2→5 -14.9 -0.6 -47.8 1+2→LMD -1.2 6.6 -26.0 D 1+2→TSD 17.2 31.2 -47.0 1+2→6 -15.5 -1.9 -45.6 1+2→TS-1A 11.4 23.6 -40.9 1+2→I 11.0 25.8 -49.8 A 1+2→TS-2A 15.1 30.4 -51.4 1+2→3 -3.7 10.8 -48.8 Nitromethane 1+2→TSB 15.2 30.1 -49.9 B (38.20) 1+2→4 -4.1 11.0 -50.7 1+2→TSC 15.3 29.4 -47.3 C 1+2→5 -12.1 2.1 -47.9 1+2→TSD 17.7 31.8 -47.2 D 1+2→6 -13.4 0.5 -46.9 1+2→TS-1A 14.8 30.0 -50.9 1+2→I 11.1 23.7 -42.5 A 1+2→TS-2A 15.1 30.5 -51.8 1+2→3 -3.8 10.6 -48.6 Water 1+2→TSB 16.1 33.0 -56.8 B (78.39) 1+2→4 -4.1 11.1 -51.0 1+2→TSC 15.0 29.2 -47.6 C 1+2→5 -12.2 2.2 -48.4 1+2→TSD 17.4 31.3 -46.8 D 1+2→6 -13.3 0.4 -46.1 406 R.
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