New amphiphilic poly(quaternary ammonium) dendrimer catalyst for effective reduction of citronellal

Three types of new amphiphilic poly(quaternary ammonium) dendrimer catalysts were prepared from poly(propylene imine) generation-2 (PPI-G2) dendrimer with 3 types of alkyl halides having varied chain length. The CH stretching at 2922 and 2853 cm−1 and CN+ stretching at 1108 cm−1 confirmed the attachment of alkyl chains and generation of poly(quaternary ammonium) ions at the dendrimer surface. Their 1H and 13C NMR spectra also supported such confirmations. The hydrophobicity of these 3 poly(quaternary ammonium) based dendrimer catalysts was determined by the static water contact angle (θ). The catalyst derived from attachment of long alkyl chain viz., PPI-G2-(C16H33)16 (CH3)8N+Cl− showed an increased contact angle (77.5 θ) thus proving high hydrophobicity. Their catalytic activity was evaluated through reduction of citronellal under pseudo-first order condition at ambient temperature. The rate constant (kobs), of the catalyst containing medium length alkyl chain viz., PPI-G2-(C12H25)16 (CH3)8N+Cl− was higher than those containing short and long alkyl chains due to its well-balanced amphiphilic character. It gave 100% selectivity/activity compared to commercial single-site phase transfer catalyst for citronellal reduction. The effect of [substrate], [catalyst], [NaOH], and [NaBH4] on the rate of the reaction was studied and the results were discussed.

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► Three types of new amphiphilic poly(quaternary ammonium) dendrimers was prepared.
► The activity of these 3 catalysts was assessed by selective reduction of citronellal.
► The 2-fold enhancement of kobs noticed in catalyst 4 than catalysts 3 & 5.