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≈(Ε +Ε )/2 (1) these addents. ΗΟΜΟ LUMO 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. Jasiński, M. Mikulska , A. Barański

transition state bonds in [2+3] cycloadditions. None of them has any feature of an orientation complex (OC), because the reaction centres at this stage do not adopt the orientations found later in TS’s. Moreover, they are not charge transfer (CT) complexes, as is shown by the practically non-existent charge transfer between substructures (t≈0.0e). It is noted that the presence of such LM complexes is also suggested by the B3LYP/6- 31g(d) simulations of [2+3] cycloadditions of nitrone 2 to nitroethene [23], 2-nitroprop-1-ene [11], β-nitrostyrene [24] and maleimide [25]. Further movement of the reaction system along reaction coordinates A-D leads to the respective TS complexes. This in turns leads to an increase in ΔH by 13–16 kcal mol-1 and a decrease in ΔS by approximately 47.0 cal mol-1 K-1 (Table 2). All the located TS’s are biplanar. Two new σ bonds are formed within them, but their lengths (r) and degree of advancement (l) are quite varied. In particular, the C5-O1 bond forms more rapidly

for TSA and TSB structures (r≈1.72 Å, l≈0.8). At the same time the C3-C4 bond is only beginning to be formed

(r>2.5 Å, l<0.4). For TSC and TSD complexes, in turn, the C3-C4 bond is much more advanced (r≈2.02 Å, l≈0.7) compared to the C5-O1 bond (r≈2.40 Å, l<0.3). Therefore the located TS’s are highly asymmetric, as confirmed by

the values of the respective ∆l indices (Table 3). TSA

and TSB complexes are most asymmetric (∆l=0.52 and 0.42). As expected, their asymmetry is higher than that of similar complexes in the [2+3] cycloaddition reactions Figure 1. Enthalpy profiles for the [2+3] cycloaddition of between nitrone 2 and 2-nitroprop-1-ene (∆l=0.35–0.28 a-phenylnitroethene 1 and (Z)-C,N-diphenylnitrone 2 in gas phase according to B3LYP/6-31G(d) calculations [12]). Even though all the located TS’s are polar, as (298 K). evidenced (Table 3) by their dipole moments (µD) and the extent of charge transfer between the substructures A [TS-1A] [I] [TS-2A] 3 (t), and their asymmetry (∆l) is not sufficient to induce a B [TSB] 4 two-step reaction. The hill drop movement from transition 1 + 2 C states TSA-D leads directly to corresponding cycloadducts [TSC] 5 D (3-6). Therefore, despite the large differences in the [TS ] D 6 shielding of the reaction centres and high electrophilicity Scheme 3. Critical structures in reaction between nitroalkene 1 of nitroalkene 1 in the gas phase, its [2+3] cycloaddition and nitrone 2 in nitromethane and water. with nitrone 2 in pathways A-D occurs as a one-step unambiguously confirmed that those were the only process. critical structures (Fig. 2) in the conversion pathways of It should be noted at this point, that in the 1+2 addents 1 and 2 towards cycloadducts 3-6. Attempts to reaction, the driving force of the polar mechanism is the find zwitterionic intermediates were unsuccessful. strong electrophilic character of nitroalkene and as a The formation of LM complexes involves the consequence, the bond formation at the β-conjugated enthalpy (ΔH) of the reaction system being reduced position of 1 in the four reaction paths (the C5 in A and by 1.4–3.9 kcal mol-1 and entropy (ΔS) reduced by B channels, and the C4 in D and E channels). 26.2–31.6 cal mol-1 K-1 (Table 2). Due to the negative When a dielectric medium with a polarity entropy value, LM structures cannot exist at room corresponding to that of toluene (ε=2.38) is added to the temperature as stable intermediates (ΔG>0). reaction, this does not change the reaction mechanism. The distances between the reaction centres within There is only a slight change in the quantitative the LMs are much higher than the range typical [22] for description of the critical structures. Specifically, LM

407 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 †

Figure 2. Critical structures for the [2+3] cycloaddition of a-phenylnitroethene 1 and (Z)-C,N-diphenylnitrone 2 in gas phase according to B3LYP/6-31G(d) calculations.

408 R. Jasiński, M. Mikulska , A. Barański

Table 3. Physical properties of critical structures 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 Structure C3-C4 C5-O1 ∆l µ t (ε) r [Å] l *) r [Å] l *) [D] [e]

1 4.01

2 3.10

LMA 5.503 3.282 4.20 0.04

TSA 2.747 0.270 1.723 0.791 0.52 4.95 0.17 3 1.588 1.425 2.08 0.21

Gas LMB 4.403 3.134 1.46 0.01

phase TSB 2.595 0.378 1.722 0.794 0.42 5.86 0.15 (ε=1.00) 4 1.600 1.427 3.87 0.22

LMC 4.924 3.372 2.27 0.00

TSC 2.018 0.695 2.404 0.294 0.40 5.28 0.12 5 1.546 1.409 3.90 0.26

LMD 7.004 4.050 3.71 0.02

TSD 2.028 0.688 2.376 0.296 0.39 6.62 0.09 6 1.546 1.394 3.99 0.25

1 4.55

2 3.71

LMA 4.376 3.337 1.39 0.02

TSA 2.883 0.185 1.697 0.809 0.62 6.80 0.25 3 1.588 1.425 2.41 0.21

Toluene LMB 4.004 3.776 1.59 0.02

(ε=2.38) TSB 2.639 0.351 1.669 0.831 0.48 7.42 0.19 4 1.600 1.428 4.54 0.22

LMC 4.828 3.426 2.85 0.00

TSC 2.008 0.701 2.443 0.267 0.43 6.52 0.13 5 1.546 1.410 4.50 0.19

LMD 6.998 4.057 4.36 0.03

TSD 2.012 0.698 2.418 0.266 0.43 8.11 0.12 6 1.545 1.394 4.67 0.25

1 5.16

2 4.70

TS-1A 3.928 1.767 0.762 11.22 0.37

IA 3.744 1.559 13.44 0.49

TS-2A 2.447 0.460 1.531 7.16 0.18 Nitromethane 3 1.589 1.427 2.71 0.22

(ε=38.20) TSB 2.530 0.419 1.567 0.904 0.49 9.01 0.22 4 1.600 1.430 5.55 0.23

TSC 2.000 0.706 2.511 0.220 0.49 8.60 0.17 5 1.545 1.410 5.36 0.25

TSD 1.993 0.709 2.495 0.209 0.50 10.69 0.16 6 1.544 1.393 5.83 0.25

409 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 †

a ContinuedTable 3. Physical properties of critical structures of [2+3] cycloaddition between -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 Structure C3-C4 C5-O1 ∆l µ t (ε) r [Å] l *) r [Å] l *) [D] [e]

1 5.19

2 4.78

TS-1A 3.955 1.777 0.754 11.31 0.37

IA 3.759 1.554 13.64 0.50

TS-2A 2.447 0.460 1.530 7.26 0.19 Water 3 1.589 1.427 2.63 0.21

(ε=78.39) TSB 2.536 0.415 1.565 0.906 0.49 9.21 0.23 4 1.600 1.430 5.65 0.23

TSC 2.001 0.705 2.521 0.212 0.49 8.79 0.18 5 1.545 1.410 5.40 0.26

TSD 1.993 0.709 2.508 0.199 0.51 10.97 0.17 6 1.544 1.393 5.92 0.24

*)

TS P where r X-Y is the distance between the reaction centers X and Y at the transition structure and r X-Y is the same distance at the corresponding product [11]. When nitromethane (ε=38.20) is used as a dielectric medium, the LM complex valleys in the A-D energy profiles disappear, and the addent conversion to cycloadducts 3-6 follows a different mechanisms. For example, for reaction A, two TS’s separated by a shallow intermediate trough I are located in the energy profile (Figs. 3 and 4, Scheme 3).

In the first phase, a TS-1A complex is produced in which only the C5-O1 bond is formed (r=1.767 Å, l=0.762). At the same time the distance between the C3 and C4 reaction centres (r=3.928 Å) is well beyond the range typical [22] of a C-C bond in transition complexes of one-step [2+3] cycloadditions. Furthermore,

the TS-1A complex is more polar than the TSA complex in a reaction occurring in the simulated presence of toluene (Table 3). Further movement of the reaction system along the reaction coordinate leads to the trough of zwitterionic

intermediate I (µD=11.22D, t=0.37e). Similarly to TS-1A , the distance between the C3 and C4 reaction centres in I is well beyond the range typical of C-C bonds (r=3.744 Å). At the same time the C5-O1 bond is

Figure 3. Enthalpy profiles for the [2+3] cycloaddition of already fully formed (r=1.559 Å). The I→3 conversion a -phenylnitroethene 1 and (Z)-C,N-diphenylnitrone 2 in occurs through the TS-2A complex, which the C3-C4 nitromethane according to B3LYP/6-31G(d) calculations bond necessary to close the heterocyclic ring forms (298 K). (r=2.447 Å, l=0.46). Similarly to TS-1A, this structure is complex troughs are shallower and activation barriers polar in nature, but not as distinctly (Table 3). slightly increase (Table 2). Furthermore, the asymmetry Competing reactions B-D in nitromethane of TS complexes increases and their polarity is more occur as one-step process. Their TS’s are more distinct (Table 3). asymmetric (∆l≈0.5) than similar structures in toluene

410 R. Jasiński, M. Mikulska , A. Barański

Figure 4. Critical structures from the [2+3] cycloaddition 1+2→3 in nitromethane according to B3LYP/6-31G(d)/PCM calculations.

(∆l=0.43–0.48). However the asymmetry is not great complex formation. Furthermore, a zwitterionic rather enough to induce a zwitterionic reaction. This is than a one-step mechanism occurs on pathway A. confirmed by the IRC calculations. A further increase in the polarity of the reaction medium When nitromethane is replaced by a more polar (e.g. water) does not change the reaction mechanism. solvent like water (ε=78.39), this does not lead to a Finally, it should be noted, that in more nitroalkene- changed mechanism of reactions A-D. The geometries nitrone [2+3] cycloadditions, 4-nitroadducts are favoured. of the respective critical structures and their polarities This is a consequence of a more favourable polar are very similar to those calculated for reactions in interaction that takes place between nucleophilic center nitromethane (Table 3). of the O atom of >C=N(O)-moiety, and the electrophilic center in β-position of nitroalkene [1,4,23]. In the case of the reaction between α-phenylnitroethene and 4. Conclusion (Z)-C,N-diphenylnitrone this is true only in gas phase. With a strong polar environment (e.g. nitromethane), the An analysis of the electronic characteristics of the most favourable channel appear to be path leading to investigated cycloaddition suggests that it should be 3,5-trans-5-nitroizoxazolidine. classified as a polar reaction according to Domingo’s terminology [17]. However, PES investigation results show that the cycloaddition may follow different Acknowledgements mechanisms. In the gas phase and with weakly polar solvents like toluene, a corresponding pre-reaction The generous allocation of computing time by the complex forms on all the theoretically possible pathways regional computer center “Cyfronet” in Cracow (Grant (A-D), being directly converted into the cycloadduct. No. MNiI/SGI2800/PK/053/2003) and financial support Cycloadditions A-D with more polar solvents like from the Polish State Committee (Grant No. C-2/422/ nitromethane does not include the stage of pre-reaction DS/2011) are gratefully acknowledged.

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