Synthesis, characterization and intramolecular cyclisation study of

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methylene group in the ortho position to the diazo linkage was investigated in our laboratory [1-2]. The reaction involved in the cyclization has been studied since a long time to ... of 0.25 mL of this solution was transferred to several 1 mL vial.

J. Phys. IV France 122 (2004) 171-175  EDP Sciences, Les Ulis DOI: 10.1051/jp4:2004122026

Synthesis, characterization and intramolecular cyclisation study of three new liquid crystals B. Saïdat1, M.H. Guermouche2 and J.-P. Bayle3 1

Centre Universitaire, Laghouat, Algérie Institut de Chimie, BP. 32 El-Alia, Bab-Ezzouar, Alger, Algérie e-mail: [email protected]à.fr 3 Laboratoire de Chimie structurale, ICMO, Bât. 410, Université de Paris-Sud, 91405 Orsay Cedex, France 2

Abstract. Internal cyclization of three new phenyldiazene liquid crystals (R is an alkyl substituent with 4, 6 or 8 carbons) with activated methylene group in the ortho position to the diazo linkage was studied . The initial liquid crystals was synthesised and characterized by 1H NMR, electrospray mass spectrometry and differential scanning calorimetry. The final compound was characterized by 1H NMR and differential scanning calorimetry. The kinetic of cyclization was studied at different temperatures and followed by reversed phase HPLC and a UV detection. For all the temperatures used, it appeared that the cyclisation was a first order reaction for the three compounds. The Arrhenius plot (ln reaction constant k against 1000/T) gave the mean activation energy of the cyclisation.

1 INTRODUCTION In the last few years, the internal cyclization of some new phenyldiazene liquid crystals with activated methylene group in the ortho position to the diazo linkage was investigated in our laboratory [1-2]. The reaction involved in the cyclization has been studied since a long time to obtain the 2H-indazole ring [3-5]. Recently [6], we studied the internal cyclization kinetic of a related phenyldiazene compound with water as leaving group by HPLC. In this paper, we will follow the reaction kinetics by DSC and HPLC in a three new related compounds having a different and havier leaving group, the scheme of cyclization are shown in Fig 1. Initial Liquid Crystal

R

N

Final Liquid Crystal

R

CH2 CH3

CO2

N=N

O

CO2

H3C

Cl

Cyclisation

Figure 1. Cyclisation scheme of the considered liquid crystals.

H O

O

N

N

O

Cl

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2 EXPERIMENTAL 2.1

Reagents

Heptane, acetonitrile and THF (chromatographic grade) used are from Fluka (Switzerland). 2.2

Synthesis of the three liquid crystals

The scheme of the three liquid crystals synthesis is given in Fig 2. R

R O

H 3C

+

O

N2 Cl

R N

+

Dioxane

O

H 3C

N

O

TBuO

OH

R N

OH

N

N

CHCl3 During the cyclisation, one of the hydrogens disappears R

Phase C4 C6 C8

R C4H9 C6H13 C7H17

R N

O H3 C

O

N

N

O O

Cl

Figure 2. Schematic synthesis of the three compounds. 2.3

Kinetic study

Kinetics was studied by preparing 1mg.mL-1 chloroformic solution of the liquid crystal A. A number of 0.25 mL of this solution was transferred to several 1 mL vial. Chloroform was evaporated. The residue was then heated at the appropriate temperature in a regulated temperature oven. At the selected times, the vials were taken , the residues were dissolved in 1 mL of THF and analysed by HPLC. Quantitation was carried out using a standard solutions containing an appropriate amount of the initial A et final B liquid crystals. 2.4

Apparatus

Transition temperatures were measured on 2920 modulated DSC TA instruments with a heating rate of 10 °C per minute. Waters chromatograph with a 600 pump, 7625 Rheodyne injector with 20 µl sample loop, Waters diode array detector 900 was used. Separations were carried out on a 30 x 0,4 cm Novapak C18 column (Waters) with the following mobile phase : Aqueous sodium heptane sulfonate (5 mM) –THFAcetonitrile ; 20-40-40 ; v-v-v. Flow rate was fixed to 1 mL.min-1. Data were collected with Millenium 32 (Waters). Quantitation was made at 250 nm.

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3 RESULTS AND DISCUSSION 3.1

Thermal properties

Starting with the solid sample, the DSC curves of C4 , C6 and C8 during the first cycle (see Fig.3) showed an endothermic transition corresponding to the melting of the three compounds. On a further increase of the temperature, a broad exothermic peak corresponding to the cyclization reaction is observed. After standing at 250°C during 1 hour, the compound was cooled down to room temperature. The DSC curve during the second heat revealed that the exothermal peak is absent as the sample now contained the final product indicating the sample did not have anymore chemical change with temperature. For three compounds, the same final product is formed which exhibits two transitions, the first corresponds to its melting (solid giving nematic phase), the second to its clarification ( nematic giving isotropic phase) as shown in figure 3. 2

C6

2

Heat Flow J/mV

Heat Flow J/mV

0

C4

0

-2

-4

-2

-4

-6

-6

-8

-8

-10 40

60

80

100

120

140

160

180

200

220

240

260

-10

280

40

T°C 2

80

C8 Heat Flow J/mV

-4

-6

120

140

160

180

200

220

240

260

280

Final compound

0

-2

100

T°C

2

0

Heat Flow J/mV

60

-2

-4

-6

-8

-8 -10 40

60

80

100

120

140

160

T°C

180

200

220

240

260

280

-10 40

60

80

100 120 140 160 180 200 220 240 260 280

T°C

Figure 3. DSC curves of C4 , C6 and C8 during the first cycle and DSC of the common final compound. 3.2

Kinetic study

For example, chromatograms appearing in figure 4 (4a, 4b, 4c) showed the evolution of C6 when it was heated at 373K during 25 minutes (4a), 45 minutes (4b) and 200 minutes (4c) of cyclisation.

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4a

4b C6

4c FC

FC

FC

C6

C6 5.00

10.00

5 .0 0 M inute s

Minutes

0,0

1 0 .0 0

5.00

10.00

Minutes

4d

Ln(c)

-0,4

-0,8

C4 C6 C8

-1,2

-1,6

20

40

60

80

100

time (min) Figure 4. Evolution of C6 when it was heated at 373K for different times; 25 min (4a), 45 min (4b), 200 min (4c), FC is he final compound. First order curves for the three compounds at 373 K(4d).

For the temperatures chosen, varying from 343 to 403K, treatment of the HPLC data obtained showed that the cyclization reaction obeyed a first order kinetic for the three compounds as shown, for example, in figure 4 (4d) at 373K. The kinetic of cyclisation is relatively quick compared to those studied in the reference 6. The half life times found are 53.5 minutes for C4, 46.3 minutes for C6 and 35.8 minutes for C8. The Arrhenius plots (ln k against 1000/T) give the activation energies ( 108.6 KJ.moL-1 for C4, 123.7 KJ.moL-1 for C6 and 106.9 KJ.moL-1 for C8.

4 CONCLUSION DSC measurements established that the three new phenyldiazene liquid crystal with activated methylene group in the ortho position to the diazo linkage cyclized exothermically. For the three compounds, the cyclization kinetics studied using a normal HPLC method showed that the cyclization reaction obeys a first order kinetics giving give the activation energies ( 108.6 KJ.moL-1 for C4, 123.7 KJ.moL-1 for C6 and 106.9 KJ.moL-1 for C8).

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References [1] Canlet C., Judeinstein P., Bayle J.P., Roussel F. and Fung B.M., Liq. Crystals 26 (1999) 281 [2] Canlet C., Khan M. A., Judeinstein P., Bayle J.P., Roussel F. and Fung B.M, New J.Chem. 23 (1999) 1223 [3] Cava M. P., Noguchi I. and Buck K.T., J. Org. Chem., 38 (1973) 2394 [4] Fusco R., Marchesini A. and Sannicolo F., J. Heterocyclic Chem., 24 (1987) 773 [5] Saidat B., Guermouche M.H., Canlet C., Berdagué P. and Bayle J.P., Chromatographia , 55 (2002) 55

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