Dynamic 1h Nmr Studies Along With Theoretical Investigation Around Carbon-Carbon Double Bond In Stable Phosphorus Ylide Containing A 2-Thiazolin-2-Thiol

Introduction

The synthesis of phosphorus ylides is important in organic chemistry ^1-6 because they make stable carbanion. These reactions have gained more use in synthetic organic chemistry ^7-11 because of their application in the synthesis of organic products ^12-14, especially in the synthesis of naturally occurring products with biological and pharmacological activities ^{15, 16}. These ylides are often prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually obtained from the phosphine and an alkyl halide. Some ylides exhibited dynamic ¹HNMR effect that affords good information regarding the interchangeable process of rotational isomers that provide important kinetic data ^17-26. In previous works, dynamic ¹H NMR effects were reported for dimethyl-2-(4, 5 dihydrotiaZolin-N-yl)-3-(triethoxyposphoranylidene)-butanedioate ²⁷. Now, we describe dynamic ¹H NMR effect for reaction between dimethyl acetylendicarboxylate 2 and 2- thiaZolin-2-thiol 3 in the presence of triphenylphosphine 1 for generation of phosphorous ylide 4a involving the two Z – and E-rotational isomers . Synthesis of this reaction has been earlier reported ²⁸ (Figure1-i)

Material and methods

Triphenylphosphine, dialkyl acetylenedicarboxylates and 4, 5-dihydrothiaZole-2-thiol were

purchased from Fluka and without further purification. Quantum mechanical calculation has

been performed using the Gaussian 03 package ²⁹ at the HF ^{30, 31} and B3LYP ^{32, 33} methods with 6-31G(d,p) basis set. The ¹H, ¹³C and ³¹P NMR spectra were recorded on a Bruker DRX–500 AVANCE instrument with CDCl as solvent at 500.1, 125.8 and 202.4 MHZ, respectively.

 

Results and discussions

 The reaction between dimethyl acetylenedicarboxylate 2 and 4, 5-dihydrothiaZole-2-thiol 3 in the presence of triphenylphosphine 1 was completed over a few hours (or after 8 hours) at ambient temperature in ethyl acetate as solvent. The ¹H and ¹³C NMR spectra of the product indicated formation of 4a. The structure of compound 4a was verified from elemental analyses data, mass, IR, ¹H, ¹³C and also ³¹P NMR spectra data. The mass spectra of 4a displayed molecular ion peak at m/Z= 523. The ¹H and ³¹P spectra of stable ylide 4a are consistent with the presence of the two rotational isomer forms (Z-4a and E-4a, Figure 1-j), the C2-C₃ has a partial double bond character, due to the conjugation with adjacent carbonyl group of CO₂Me and C=p⁺(PPh)₃, as shown in Figure 1-j.

Figure 1. i) Reaction between dimethyl acetylendicarboxylate 2 and 4, 5-dihydrothiaZole-2-thiol 3 in the presence of triphenylphosphine 1 for preparation of phosphorus ylide 4a.j) Interchangeable process of rotational isomers in the two 4-Ea and 4-Za isomers  Click here to View figure

The rotation around the carbon-carbon double bond (H₃COO-C=C-PPh₃) in the Z-4a and the E-4a geometrical isomers is slow on the NMR time scale at ambient temperature. Thus, this provides a mixture of the two Z-4a and E-4a geometrical isomers. The ¹H NMR spectra of the phosphorous ylide 4a exhibit two doublets for the methine proton(H-C-C-P, ³J_PH= 17.5HZ) at d= 5.5 and 5.55 ppm (Figure 2) and two singlets arising from methoxy groups at 3.53 and 3.75 ppm and also at 3.13 and 3.78 ppm for the two minor (E-4a) and major (Z-4a) geometrical isomers, respectively (Figure 3). Effects of dynamic ¹HNMR around the partial carbon-carbon double bond will be argued next. On the basis of ¹H NMR spectra at high temperature, two sharp signals was observed for the methoxy groups at 40°C (313 K), due to a fast rotational around the carbon-carbon double bond (Figure 3).Investigation of such behavior in the phosphorus ylide 4a at variable temperatures allowed us to calculate the Gibbs free energy barrier (∆G^≠) for the restricted rotation around the carbon-carbon double bond in 4a (Table 1).

Figure 2 1H NMR spectra of compound 4a. Click here to View figure

 

Figure 3. Dynamic 1H NMR effects at variant for methoxy groups in compound 4a   Click here to View figure

Using the expression k_c= π∆υ/√2, the first–order rate constant was calculated for this rotation at 40 ºC. Application of the absolute rate theory with a transmission coefficient (k) of one, gave a Gibbs free energy barrier. These values are reported in Table 1. Effect of temperature on the rate constant was investigated on the basis of measurement of different chemical shifts in a series of other separate experiments. The results were too small so that the changes in first-order rate constant and Gibbs free energy barrier were negligible in comparison with the results obtained at 40°C (313K).

Table 1. Selected ¹H chemical shifts (at 500.1 MHZ, in ppm, Me₄Si) for ylide 4a along with activation parameters in CDCl₃, for restricted rotational process around the carbon-carbon double bond P(Ph)₃ P–CC–OMe) in ylide 4a and speculative ylide 5a at 298 K (room temperature) and experimental temperature (313 K).

	∆S# (J⁄mol)	∆H# (kJ⁄mol)	∆G# (kJ/mol)	∆υ (HZ)	δ (ppm)	Tc (K)
Experimental a	28.66	74.67	68.71	9.41	5.55, 5.5	313
Theoretical b	-11.04	65.88	69.16	–	–	298
Theoretical c	-11.31	65.75	69.29	–	–	313
Theoretical d	-23.32	60.12	67.42	–	–	313
Theoretical e	-15.97	68.39	73.15	–	–	298

^a Experimental Data obtained from the ¹H NMR study ²⁸

^b Theoretic data obtained using ab initio calculation at HF/6-31G(d,p) level of theory for a synthesiZed ylide 4a at 298K

^c Theoretical data obtained using ab initio calculation at HF/6-31G(d,p) level of theory at experimental temperature (313K) for synthesiZed
ylide 4a.

^d Theoretical data obtained using ab initio calculation at B3LYP method with 6-31G(d,p) basis set at experimental temperature  (313K)for a synthesiZed ylide 4a.

^e Theoretical data obtained using ab initio calculation at HF/6-31G(d,p) level of theory for a speculative ylide 5a at 298K.

In order to determine theoretical rotational energy barrier around the CC bond in ylide 4a (Figure 1-j), first their structures were optimiZed at HF/6-31G level ^{30, 30} of theory by Gaussian 03 program package ²⁹ and relative energy between the two isomers calculated at HF/6-31G(d, p) level and B3LYP method with 6-311++G(d, p) basis set. The results are given in Table 2.

Table 2. The relative energy (kj/mol) between the two isomers of ylid 4a, obtained at HF/6-31G(d,p) and B3LYP method with 6-311++G(d,p) basis set.

 

 

Then, relative energy was plotted versus dihedral angle O1C2C3P4 (Figure 4) as shown in Figure 5. The plots in Figure 5 and 6 were obtained by scanning method at HF/6-31G(d, p) level of theory by Gaussian 03 and each point in both figures were fully optimiZed. A repetition of this behavior was carried out for a speculative ylide 5a (Figure 4) in comparison with the synthesiZed ylide 4a. In a speculative ylide 5a, sulfur atom in conjugate base of N-H acid has been joined to C5 in phosphorus ylide, while in ylide 4a nitrogen atom of conjugate base has been joined to C5 atom. The results shown in Figure 6 and Table 1.

Figure 4. The performance of O1C2C3P4 dihedral angle in a synthesiZed phosphorus ylide 4a and in a speculative ylide 5a Click here to View figure

 

Table 1: Click here to View figure

 

Figure 6. Relative energy versus dihedral angel O1C2C3P4 in a speculative phosphorus ylide 5a Click here to View figure

The corresponding structures, with respect to all points (a, b and c) in Figure 5 were drawn in Figure 7. Frequency calculations were employed for all transition and stationary points (TS, E–4a and Z-4a) and the results are reported in Table 3. First negative frequency value could be considered as a strong proof for confirming of TS structure.

Figure 7. Structures corresponding to a, b and c points with respect to Figure 1-j   Click here to View figure

Table 3.

Frequency data obtained for all TS, E-4a and Z-4a structures in the ylide 4a.

	Isomer	First Frequency
	E-4a TS Z-4a	20.68 – 66.16 13.74

 

On the basis of theoretical calculations, for synthesiZed ylide 4a, and data obtained from the Figures 5 and 6, the Gibbs free energy barrier (ΔG^≠), ΔH^≠ and ΔS^≠ calculated around the carbon-carbon double bond for both the speculative and synthesiZed ylide. As can be seen from Table 3; A) there is, relatively, a good agreement between the experimental data (a) and theoretical data (b, c) at 298K and experimental temperature (313). B) there is also a good consistent at both HF level and B3LYP method with 6-31G(d, p) basis set (c, d) with experimental data (a). In addition, theoretical Gibbs free energy barrier for a speculative ylide 5a (Table 1, e) is higher than both Gibbs free energy obtained from experimental data (Table 1, a) and theoretical data (Table 1, b, c and d) in a synthesiZed ylide 4a. These results indicate that why the ylide 4a is practically generated easier than the speculative ylide 5a.

Conclusion

The theoretical results on the basis of mechanical calculations, obtained from ab initio calculation at HF level and B3LYP method with 6-31G (d, p) basis set indicated that activation parameters (ΔG^≠ = 69.29 and 67.42 kj.mol^-1, respectively) for interchangeable process between the two rotational isomers (4-E and 4-Z) were relatively consistent with the experimental dynamic ¹H NMR data (ΔG^≠ = 68.71 kj.mol^-1). In addition, the results show that generation of the synthesiZed ylide, 4a is more possible than the speculative ylide, 5a because, of the less free Gibbs energy barrier.

 

Acknowledgements

We gratefully acknowledge the financial support from the Research Council of the University of Sistan & Baluchestan.

References

1. Hudson H. R., Vol. 1 Wiley: New York,. 386, (1990).
2. Habibi-Khorassani S.M., Maghsoodlou M. T., Ebrahimi A., Zakarianejad M., Fattahi M. J. Solution Chem., 36, 1117, (2007).
3. Cadogan J. I. G. Academic Press: New York, (1979).
4. Maghsoodlou M. T., HaZeri N., Habibi-Khorassani S. M., Kakaei R., Nassiri M., Phosphorus, Sulfur Silicon Relat. Elem., 181, 25, (2006).
5. Habibi-khorassani S.M., Ebrahimi A., Maghsoodlou M.T., Saravani H., ZakarianeZhad M., GhahramanineZhat M., KaZemian M.A., Nassiri M., khajehali Z. Reaction Kinetics Mech., 34,. 261, (2009).
6. Corbridge D. E. C. Phosphorus:, 5th ed.; Elsevier: Amsterdam, (1995).
7. . Maghsoodlou M.T., HaZeri N., Habibi Khorassani S. M., Nassiri, M., Marandi,G., ShahZadeh, A. G., BijanZadeh, H. R.,  Phosphorus, Sulfur, Silicon Relat Elem, 181,. 1117, (2006).
8. . Nair V. C. , Rajesh A. U. Vinod S. Bindu, A. R. Sreekanth J. S. Acc Chem Res., 36,. 899, (2003).
9. . Maghsoodlou M.T., HaZeri. N., Habibi Khorassani S. M., Heydari R., Nassiri M., Moeeni Z., Nirumand u., Eskandari Torbaghan. Z., Phosphorus, Sulfur, Silicon Relat Elem., 181, 865, (2006).
10. . Yavari I., Maghsoodlou M. T., Tetrahedron Letters, 39, 4579, (1998).
11. . Maghsoodlou M. T., HaZeri. N., Habibi-Khorassani S. M., Afshari G., Nassiri. M. J. Chem. Res, 727, (2005).
12. . Maghsoodlou M. T., Heydari. R., Habibi-Khorassani. S. M., Rofouei M. K., Nassiri M., Mosaddegh  E., Hassankhani A., Journal of Sulfur Chemistry, 27,. 341, (2006).
13. . Habibi-KhorassaniS. M., Maghsoodlou M. T., Nassiri M., ZakarianeZhad M., Fattahi M. Arkivoc, xvi,. 168, (2006).
14. Habibi-Khorassani. S. M., Ebrahimi A., Maghsoodlou M. T., Rostami Charati F., KaZemian M, A., Karimi P, Phosphorus, Sulfur Silicon Relat. Elem.,. 559, (2010).
15. Maghsoodlou M. T., Heydari R., HaZeri N., Habibi-Khorasani S. M.; Nassiri M., KaZemian M.A., SalehZadeh J., HajiZadeh M., GhasemZadeh M., Mosaddegh E, Biomedical and Pharmacological Journal, 1,. 51,(2008).
16. .. Habibi Khorassani S.M., Maghsoodlou M.T., Ebrahimi A., ZakarianeZhad M.,. MohamadZadeh p, Shahraki M., Oriental Journal of chemistry, 24, 73. (2008)
17. . Habibi-Khorasani S. M.; Ebrahimi A.; Maghsoodlou M. T.; Same-Salari S.; Nasiri S., Ghasempour H.,  Magnetic Resonance in Chemistry, 49(5), 213, (2011).
18. . Kabiri R.; HaZeri N.; Habibi-Khorasani S. M.; Maghsoodlou M. T.; Ebrahimi A., Saghatforoush L.; Marandi G., RaZmjoob Z., ARKIVOC, 17. 12, (2008).
19. . Habibi-Khorasani S. M., Maghsoodlou M. T., Ebrahimi A., Farahani Vasheghani F., Mosaddeg E.; KaZemain M.A., TETRAHEDRON LETTERS, 50,. 3621, (2009).
20. Maghsoodlou M. T.; Marandi G., Habibi-Khorasani S. M., Saghatforoush L., Aminkhani A.; Kabiri R, Indian Journal of Chemistry, 47B,. 1151, (2008).
21. . HaZeri N., Habibi-Khorasani S. M., Maghsoodlou M. T.,Marandi G., Nassiri M., ShahZadeh, A, journal of Chem.Res., 4,. 215 (3), (2006).
22. . CervinkaO. In Roppoport, Z. Eds., Wiley: New York, Part 1.PP. 219, (1994).
23. . Anet F. A. L.; Anet R.; Cotton F. A.; Jackman, L. M. Eds., Academic Press: New York, Chapter 8,(1979).
24. . Oki M., Eds., VCH: Weinheim, (1985).
25. . Ziyaadini M.; Maghsoodlou M. T., HaZeri. N., Habibi-Khorassani S. M., Hetroatom Chem, 23, 131, (2011).
26. 26.  Maghsoodlou M. T., Shaterian H. R., Marandi. G., Shahraki Poor F., SalehZadeh. J. , Arkivoc (xiii),. 218, (2008).
27. 27.  . HabibiKhorassani S. M., Ansari Fard M., Mohammadi M., Aghdaei E., Shahraki M., HadiZadeh M., HaZeri N. Jordan of chemistry, 7, 23, (2012).
28. . Maghsoodlou M. T.;HaZeriN.; Habibi-Khorasani S. M.; Nassiri M.; Marandi G.; Afshari G.; Niroumand U. Sulfur chemistry, 26,. 261, (2005).
29. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; YaZyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; ZakrZewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; OrtiZ, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; PiskorZ, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; GonZaleZ, C.; and Pople, J. A.; Gaussian, Inc., Wallingford CT, (2004).
30. Petersson G. A., Al-Laham M. A. J Chem Phys, 94,6081, (1991)
31. . Petersson G. A., Bennett A., Tensfeldt T. G., Al-Laham M.A., Shirley WA, MantZaris J J Chem Phys, 89, 2193.(1988), Becke A. D. Phys Rev A, 38, 3098, (1988)
32. Lee C., Yang W., Parr R. G. Phys Rev B, 37,785, (1988)
33. Becke, A. D. Phys Rev A, 38, 3098, (1988)

 

 

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