Computational study for the ylide isomers from the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates in the presence of 6-chloro-2-benzoxazolethiol

2.1 Calculations

Assignment of the stability of the two Z- and E-isomers is possible in phosphorus ylides by experimental methods such as ¹H and ¹³C NMR and IR spectroscopy, mass spectrometry and elemental analysis data. For this reason quantum mechanical calculations have been performed in order to gain a better understanding of the most important geometrical parameters and also relative energies of both the geometrical isomers.

In order to determine more stable form of both the geometrical isomers [Z-4(a,c) or E-4(a,c)] of ylides (4a or 4c), first their structures were optimized at HF/6-31G (d,p) level of theory by Gaussian03 program package. Also relative stabilization energy of the two isomers has been calculated at HF/6-31G (d,p) and B3LYP/6-311++G (d,p) levels (See Figs. 2 and 3). The relative stabilization energies for the two isomers [Z-4 (a,c) and E-4 (a,c)] are reported in Table 1, as can be seen, the Z-4a and the Z-4c isomers are more stable than the E-4a and E-4c forms (0.63 and 1.64 kcal/mol), (0.62 and 0.88 kcal/mol) (0.78 and 1.84) and (0.71 and 1.03) at both HF and B3LYP levels in the gas and solvent phases, respectively. The results of zero point energy (ZPE) have been accumulated in Table 1. They are with the relative stabilization energies in the absence of these corrections in agreement.

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Figure 2

(i) Intramolecular hydrogen bonds (dotted lines) in the two E-4a and Z-4a geometrical isomers of stable ylide 4a. (j) A part of molecular graphs, including intermolecular hydrogen bond at critical points (BCPs) for the two E-4a and Z-4a geometrical isomers of stable ylide 4a.

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Figure 3

(i) Intramolecular hydrogen bonds (dotted lines) in the two E-4c and Z-4c geometrical isomers of stable ylide 4c. (j) A part of molecular graphs, including intermolecular hydrogen bond at critical points (BCPs) for the two E-4c and Z-4c geometrical isomers of stable ylide 4c.

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Further investigation was undertaken in order to determine more effective factors on stability of the two Z- and E-isomers, on the basis of AIM calculations (Bader, 1990) at HF/6-31G(d,p) level of theory by the AIM2000 program package (Biegler König et al., 2001). In recent years, AIM theory has often applied in the analysis of H-bonds. In this theory, the topological properties of the electron density distribution are derived from the gradient vector field of the electron density ρ(r) and on the Laplacian of the electron density $\nabla$ ²ρ(r). The Laplacian of the electron density, $\nabla$ ²ρ(r), identifies the regions of space wherein the electronic charge is locally depleted [ $\nabla$ ²ρ(r) > 0] or built up [ $\nabla$ ²ρ(r) < 0] Rozas et al. (2000). Two interacting atoms in a molecule form a critical point in the electron density, where $\nabla$ ρ(r) = 0, called the bond critical point (BCP). The values of charge density and its Laplacian at these critical points give useful information regarding the strength of the H-bonds (Biegler König et al., 2001). The ranges of ρ(r) and $\nabla$ ²ρ(r) are (0.002–0.035) e/ $a_0^3$ and (0.024–0.139) e/ $a_0^5$ , respectively, if H-bonds exist (Grabowski, 2001). The AIM calculation indicates intramolecular hydrogen bonds and critical points (H-BCP) for the two Z-4 (a,c) and E-4 (a,c) isomers. Intramolecular H-BCPs are shown in Figs. 2 and 3 (dotted line) and also Table 4. For each H- bond E2 stabilization energy obtained via NBO analysis. The results (see Tables 2 and 3) confirmed that H-bonds via NBO analysis are in agreement with AIM analysis. The electron densities (ρ), Laplacian of electron density $\nabla$ ²ρ(r), and energy density −H(r) are also reported in Tables 2 and 3. A negative total energy density at the BCP reflects a dominance of potential energy density, which is the consequence of accumulated stabilizing electronic charge (Arnold and Oldfield, 2000). Herein, the number of hydrogen bonds in both categories (E-4a and Z-4a) and (E-4c and Z-4c) are (3 and 3) and (5 and 6), respectively. The values of ρ and $\nabla$ ²ρ(r) are in the ranges (0.007–0.012 and 0.007–0.013) e/ $a_0^3$ , (0.011–0.013 and 0.007–0.012) e/ $a_0^3$ , (0.024–0.044 and 0.024–0.045) e/ $a_0^5$ and (0.044–0.051 and 0.025–0.049) e/ $a_0^5$ , respectively. In addition the Hamiltonian [−H(r)] is in the range (3.09–11.37 and 3.03–12.44 au) and (2.22–13.48 and 11.52–14.04 au) (see Tables 2 and 3). These HBs show $\nabla$ ²ρ(r) > 0 and H(r) < 0, which according to the classification of Rozas et al. (2000) are medium-strength hydrogen bonds. In both ylides the dipole moment for the two E-4a and E-4c isomers are smaller than the two Z-4a and Z-4c isomers in the gas and solvent phases [(8.01, 7.96 D), (10.16, 10.09) and [(11.61, 11.42 D), (14.78, 14.82)]. The value of −H_tot (=ΣH(r)) for the two Z-4a and Z-4c isomers in the gas phase (26.70 and 75.79 au, respectively) is larger than the two E-4a and E-4c isomers (25.47 and 53.06 au, respectively). These differences in the most important geometrical parameters of Z-4 (a,c) with respect to E-4 (a,c), involving a large difference in dipole moment and a considerable difference in −H(r), altogether, make only the slight stability on these isomers in comparison with the E-4 (a,c) forms. On the basis of theoretical calculations (Table 1), the difference between the relative stability of the E-4c and Z-4c isomers is (1.64 k/mol) while it is smaller in the E-4a and Z-4a (0.63 kcal/mol) in gas phase. This result (a slightly difference on stability) is completely consistent with the obtained results on the basis of AIM calculations for determination of the most geometrical parameters (see the value of −H_tot (=ΣH(r)) for the two Z-4a and Z-4c isomers). Moreover, the total number of hydrogen bonds in the two Z-4c and E-4c (6, 5) is more than the two Z-4a and E-4a (3, 3); this leads to rigidity in these geometrical isomers in comparison with the two Z-4a and E-4a forms. The rigidity of the two molecular structures by the more intramolecular hydrogen bonds accompanied by more steric factor of the bulky tert-bulky groups (compare with dimethyl groups in both the E-4a and Z-4a, Figs. 2 and 3) within the structures of E-4c and Z-4c forms makes a good opportunity for the enhancement of energy barrier. Hence, interconversion process between the two isomers needs to pass through very high restricted barrier energy, particular in solution media, for this reason it is possible to see only a single isomer as a lone isomers of 4c. On the contrary, interconversion process for both the E-4a and Z-4a geometrical isomers passes through a considerably low energy barrier, which leads to a plausible observation of the two E-4a and Z-4a [see Fig. 1(j)].

In the synthesis of ylides 4a and 4c (Maghsoodlou et al., 2009), the ¹H, ¹³C and ³¹P NMR data showed a lone isomer of 4c with the experimental abundance percentage of 92% as a major form and also a mixture of isomers E-4a and Z-4a with the abundant percentage of 72% for Z-4a as a major form.

Also, the charge on different atoms which are calculated by NPA and AIM methods and also CHelpG keyword at HF/6-31G (d,p) level are reported in Table 5 for the two Z- and E-isomers of ylides 4a and 4c. There is a good agreement between the results in three methods.

Furthermore, the individual chemical shifts have been characterized by NMR calculations at the mentioned level for the two major E-4 (a,c) and minor Z-4 (a,c) geometrical isomers. The total spin–spin coupling constant is the sum of four components: the paramagnetic spin–orbit (PSO), diamagnetic spin–orbit (DSO), Fermi-contact (FC), and spin-dipole (SD) terms. The values of chemical shifts (δ) and coupling constants (J_x−y) are reported in Tables 6–9. As can be seen there is good agreement between both the experimental (Maghsoodlou et al., 2009) and theoretical chemical shifts (δ) and coupling constants (J_x−y). In the present work, molecular structures of ylides 4 (a,c) involving four large atoms such as sulfur, phosphorus, nitrogen and chlorine are huge with the large numbers of other atoms, for this reason, employment of basis set higher than B3LYP/6-311++G (d,p) is impossible for the performance of more accurate calculations. This limitation causes a small difference between both the experimental and theoretical coupling constants in some functional groups.