Enhanced imine synthesis in water: from surfactant-mediated catalysis to host–guest mechanisms

Kamel Meguellati (1) , Ali Fallah-Araghi (1) , Jean-Christophe Baret (2) , Abdeslam El Harrak (3) , Thomas Mangeat (4) , Carlos Marques (5) , Andrew D. Griffiths (1) , Sylvain Ladame (1)

1 ISIS - Institut de Science et d'ingénierie supramoléculaires
2 CRPP - Centre de Recherche Paul Pascal
3 LLB - UMR 12 - Laboratoire Léon Brillouin
4 FEMTO-ST - Franche-Comté Électronique Mécanique, Thermique et Optique - Sciences et Technologies (UMR 6174)
5 ICS - Institut Charles Sadron

An environment-responsive and fluorogenic reaction is reported and used as a model system to demonstrate experimentally three mechanisms of enhanced imine synthesis in water using either surfactants (below and above their CMC) or double-stranded DNA (acting as a reaction host). In solution, entropic factors thermodynamically disfavour the formation of larger molecules from smaller ones. The enthalpy of the reaction (DH) depends mainly on the change in bond energy and solvation energy. The entropy of the reaction (DS) is a measure of the change in possibility of movement of the constituents of the reaction: the change in the degrees of freedom of overall transla-tional movement and rotational movement is most significant whereas the contribution by vibrations and internal rotations is small. 1 The formation of one product molecule from two reactant molecules (2-1 reaction) in solution will result in a high loss of mobility (and hence a loss of entropy) equivalent to 6 degrees of freedom (3 translational and 3 rotational). Compartmentalisation (e.g. in receptors or capsules) is one of the mechanisms that have been invoked to overcome this entropic problem. 2 In confined geometries, chemical reactions take place under crowded conditions that can affect the availability of reaction partners (on the reaction time scale) and by this means will influence the outcome of the reaction. Herein, we demonstrate experimentally that synthetic reactions of imine formation, otherwise highly unfavourable in water, can be enhanced through (1) surfactant-mediated catalysis, (2) confinement within micelles or (3) a host-guest mechanism, using double-stranded DNA as a host. As a model system to study these three mechanisms we used the reversible reaction of a non-fluorescent amine 1 with a very weakly fluorescent aldehyde 2 to form a fluorescent imine 3 in water (Fig. 1). Fluorescent imine 3 differs from the well-known trimethine cyanine dyes by one C-N substitution in the polymethine chain, thus making its formation reversible and thermodynamically controlled. 3 Because of the intrinsic fluorogenic properties of this reaction, we used fluorescence spectroscopy to monitor changes in reaction efficiency (apparent equilibrium constant K eq , first and second order rate constants k À1 and k 1) in response to various environmental stimuli. Water soluble Fischer's base aldehyde 2 was synthesised in 4 steps from commercially available 5-hydrazinobenzoic acid (see ESI †). N-Methyl-2-amino-benzothiazolium salt 1 was synthe-sised in one step from 2-aminobenzothiazole. 3 The formation of imine 3 from amine 1 and aldehyde 2 was first investigated in bulk solvent and its photophysical properties determined by fluorescence spectroscopy. In aqueous solution, Fig. 1 Reversible synthesis of fluorescent imine 3 from non-fluorescent amine 1 and weakly fluorescent aldehyde 2.

Domaines: Matière Condensée [cond-mat]; Matière Molle [cond-mat.soft]; Chimie

Kamel Meguellati, Ali Fallah-Araghi, Jean-Christophe Baret, Abdeslam El Harrak, Thomas Mangeat, et al.. Enhanced imine synthesis in water: from surfactant-mediated catalysis to host–guest mechanisms. Chemical Communications, 2013, 49 (96), pp.11332. ⟨10.1039/c3cc46461j⟩. ⟨hal-02129804⟩