CAPÍTULO III MOTIVACIONES DEL SECUESTRO
4.1 Teorías que explican la violencia humana
4.1.4 Otras teorías
hydrophobicity is a key parameter to achieve high yields in this reaction. 7.4.2.3 Effect of Photoisomerisation
The effect of photoisomerisation on the performance of the reaction was investigated. Three approaches were considered. Initially, the in situ photoisomerisation was performed at the end of the 2.5 hours of reaction, for trans-C8AzoOC2TAB at 0.5 mM. At this
concentration, cis-form micelles return to unimers. It is assumed that the dissolution of micelles induced by photoisomerisation facilitates the recuperation of the product, as observed for the preliminary experiment with NR. However, the final product was obtained with a 31% yield (Table 7.4, Entry 2), which is significantly lower than the one obtained without light irradiation (Table 7.4, Entry 1). This could be explained by the light sensitivity of the
α,β
-unsaturated double bond, which leads to the degradation of the product (Table 7.4).41169 In the second approach, micellar solutions at 10 mM were prepared and irradiated to the cis-isomer before addition to the reagents. The use of cis-form micelles is particularly interesting in the nanoscale perceptive. According to the results obtained in Chapter 4 and 5, micelles are expected to be spherical at 10 mM, with a short interparticle distance and a small aggregation number (except for cis-C6AzoOC4TAB). It is worth mentioning that
cationic spherical micelles are not commonly reported in the literature, as micelles are often cylindrical or ellipsoidal.42 Therefore, investigating the effect of the nanoscale organisation of cis-AzoTAB on the yield of the reaction is particularly interesting. Table 7.4 shows that the obtained yields vary from 48% to 78%, calculated by 1H NMR. Interestingly, the yield increases compared to those obtained for trans-AzoTABs, except for C6AzoOC4TAB, which decreases from 66% to 48%. Specifically, the yield of the
reaction increases upon photoisomerisation from 46% to 70% using C4AzoOC4TAB, from
48% to 60% using C4AzoOC6TAB, from 53% to 78% using C8AzoOC2TAB and from
55% to 62% using C8AzoOC6TAB. The 78% yield of the reaction using a micellar
solution of cis-C8AzoOC2TAB is higher than the yield obtained with the control reaction
using CTAB, which was 66%, and the highest among all the reactions carried performed in this study. From these results, the highest yields are obtained for short spacers after photoisomerisation and with the largest differences between the yields of trans- and cis- isomers are obtained for the most hydrophilic AzoTABs. It is believed that the large thickness of the shell formed by cis-AzoTABs enhances the reaction. The thickness of the positively-charged shell, induced by the trimethylammonium cation, may stabilise the enolate formed during the second step of the reaction mechanism (Scheme 7.2), which is crucial for micellar catalysis.43
170 Table 7.4. Effect of photoisomerisation on the obtained product yield as obtained by 1H NMR for
the Claisen-Schmidt condensation reaction under micellar conditions. Difference between relative concentration (mol %) and real concentration (mM) is expressed as described in the text. K2CO3 is
used as base to help the reaction. The yield obtained after filtration corresponds to the quantity of powder recuperated at the end of the filtration. The yield calculated by 1H NMR was obtained
based on the ratio of recognisable peaks. T = 35 °C.
Entry Surfactant Relative Conc.
(mol %) Real Conc. (mM) Base Yield after filtration (%) Yield calculated by 1H NMR (%) 1 C8AzoOC2TAB 2 0.5 K2CO3 63 52 2a C 8AzoOC2TABa 2 0.5 K2CO3 36 31 3b C 8AzoOC2TAB 15 10.5 K2CO3 89 78 4b C 4AzoOC6TAB 15 10.5 K2CO3 68 60 5b C 6AzoOC4TAB 13 10.5 K2CO3 58 48 6b C 4AzoOC4TAB 13 10.5 K2CO3 77 70 7b C 8AzoOC6TAB 13 10.5 K2CO3 69 62 8c C 8AzoOC2TAB 13 10.5 K2CO3 38 38
a The real concentration is below the CMC of the
cis-isomer. The solution was irradiated at λex =
365nm for 5 minutes at the end of the reaction, to facilitate the recuperation of the product.
b The solution was irradiated at λ
ex = 365nm for 5 minutes prior the addition of reagents and the
start of the reaction.
c The product was extracted from water after irradiation of the organic layer, to separate the
AzoTAB from the product.
The enolate, located at the micelle surface, and the hydrophobic aldehyde, solubilised in the micelles, are in close proximity and the reaction efficiency is enhanced. Conversely, the obtained yield using cis-C6AzoOC4TAB decreases compared to the trans-form (Table
7.4, Entry 5). This AzoTAB was shown to form ellipsoid micelles, which explains the difference to the other AzoTABs studied here.
A third approach was taken, where dichloromethane (50 mL) was added to the mixture at the end of the reaction. The organic layer was irradiated for 5 minutes at
λ
ex = 365 nmand washed twice with water (50 mL). It is expected that the hydrophilic cis-isomer remains in the aqueous layer, which can help to separate the surfactant from the products. However, the photosurfactants could not be recuperated from the water phase and could not be recycled to catalyse a new reaction, as photosurfactants were diluted in water to a concentration below the CMC. The final product was collected after rotary evaporation of the solvent and gave a moderate yield (38%, Table 7.4, Entry 8), with a clean 1H NMR spectrum. Therefore, this method was considered as less efficient than simple filtration to collect the product, due to loss of yield induced by post-treatment and light degradation.
171 7.4.2.4 Effect of Temperature
The effect of the temperature on the reaction yield was investigated for trans- C8AzoOC2TAB at 20 °C, 35 °C and 70 °C and compared to that obtained using CTAB in
the same conditions. At 20 °C, the reaction affords a 14% yield for CTAB, which is not surprising as the Krafft point is not reached (TKrafft = 25 °C)44 and a 19% yield for trans-