The synthesis of a new series of trimethoxysilyl-tethered N-substituted 3,5-dialkylpyrazolylpyridines is reported. The silyl spacers allow the grafting of ligands onto silica via formation of covalent bonds as well as the synthesis of xerogels.Pyrazolylpyridine derivatives are good bidentate ligands for a wide range of metals, 1-3 and we expect that such ligands coordinating a transition metal could be utilized to synthesize 'hybrid catalysts', i.e. soluble organometallic complexes covalently bonded onto an insoluble support. The literature does not mention the chemical anchoring of the pyrazolylpyridine ligand onto silica, although such hybrid material could help develop new economical and environmentally friendly chemical processes. 4 Among the possible applications are sorbents for the recovery of metal ions from aqueous solutions, as well as heterogenized homogenous catalysis. Furthermore, following the work of Heinrichs et al., 5-7 calcination followed by reduction of the material should permit the preparation of highly dispersed metallic nanoparticles on silica with a finely controlled texture.In this paper, we describe the synthesis of a new series of 3,5-disubstituted pyrazole and pyrazolylpyridine derivatives bearing a tethered (trimethoxysilyl)propyl group in position 4 of the pyrazole ring, namely 3,5-dimethyl-4-[3-(trimethoxysilyl)propyl]pyrazole (3a), 2-{4-[3-(trimethoxysilyl)propyl]-3,5-dimethyl-1H-pyrazol-1-yl}pyridine (3b), 2-{4-[3-(trimethoxysilyl)propyl]-3,5-dimethyl-1H-pyrazol-1-yl}-6-methylpyridine (3c) and 2-{3,5-diethyl-4-[3-(trimethoxysilyl)propyl]-1H-pyrazol-1-yl}pyridine (3d) (Figure 1).Pyrazole derivatives are usually readily synthesized from a dione and (substituted or not) hydrazine, generally in alcoholic medium using an acidic catalyst. 8 Such conditions however are not suitable with labile functionalities such as trialkoxysilyl groups and modifications of the general procedure proved to be essential for a succesful synthesis of the alkoxysilyl-substituted heterocycles. Accordingly, molecular sieves were tested as catalysts along the lines previously reported by Sreekumar et al. for the synthesis of pyrroles and pyrazoles. 9 Indeed, 3 Å molecular sieves, beside acting as a water scavenger, also promoted the addition of diones derived either from 2,4-pentanedione or 3,5-heptanedione (1a-b) to both hydrazine and to hydrazinopyridines (2a-c) in anhyd THF yielding 3a-d, respectively (Scheme 1). However, when 2,2,6,6-tetramethylheptane-3,5-dione or 1,3-diphenylpropane-1,3-dione was used with hydrazinopyridine and 3 Å molecular sieves, no reaction was observed by GC after 3 days in refluxing THF. This can possibly be explained by steric hindrance together with a weaker electrophilicity of the carbon atom of the carbonyl group, and consequently a weaker reactivity toward nucleophilic hydrazine under our reaction conditions. The bidentate ligands prepared in this paper have now been used for the preparation of xerogels with chemically bonded organometallic complexes by sol-gel process. The i...
