Synthesis of a New Trialdehyde Template for Molecular Imprinting

Abstract

A recent approach in the area of molecular recognition involves template-assisted imprinting' of the surface of rigid polymers such as silica gel. Siloxy-Schiff bases of structurally rigid dialdehydes have been used as templates to introduce two appropriately spaced amino groups for substrate binding on the solid surface. The distance between the two amino groups is defined by the structure of the dialdehydes. Moderate selectivity has been reported in rebinding of the dialdehydes to the modified surface.2 Current objectives in this quest involve the development of multidentate template molecules to expand the repertoire of functional groups available a t the binding sites3 We report herein the synthesis of a trialdehyde and its Schiff bases as a new template for molecular imprinting of solid surfaces. The reaction of cyanuric chloride with 3 equiv of p hydroxybenzaldehyde in benzene gave the desired trialdehyde, TRIPOD (1) (Scheme 11, in a single step. Trialdehyde 1 was then reacted at room temperature with (aminopropy1)triethoxysilane (APTES) or p-(methoxydimethylsily1)aniline (MODSA)4 to afford the corresponding silane-TRIPODS, APTES TRIPOD (2), and MODSA TRIPOD 4 (Scheme 2), respectively. Model compounds PROPYL TRIPOD (3) and ANILINE TRIPOD (5) were also synthesized for future mechanistic studies of Schiff base formation. Acid hydrolysis of all the Schiff bases regenerated the intact TRIPOD. Molecular imprinting was carried out with silaneTRIPOD molecules 2 and 4 on silica gel to yield modified silicas 7 and 8, respectively. Porous silica gel, Fractosil 500, was refluxed in dry toluene for 2 days28 with an appropriate amount of the silane-TRIPODS to get a low substitution level on the silica and ensure site isolation (Scheme 3). Initial experiments with APTES TRIPOD (2) indicated that a partial degradation of 2 took place during the imprinting reaction. Refluxing of the model compound 3 in toluene for 2 days produced p-hydroxybenzaldehyde, suggesting that the degradation was caused by the partial hydrolysis of the Schiff base and subsequent attack on the ether linkage by the amine. On the other hand, MODSA TRIPOD (4) did not decompose, presumably due to the greater strength of the aromatic Schiff