The Synthesis and Catalytic Application of a New Class of Imprinted Silica
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The Synthesis and Catalytic Application of a New Class of Imprinted Silica John D. Bass, Sandra L. Anderson, Alexander Katz Department of Chemical Engineering University of California at Berkeley Berkeley, CA 94720-1462 (USA)
Abstract The effect of chemical environment surrounding a synthetic heterogeneous catalyst active site is investigated using the hydrophilic imprinting of silica. Two model reaction systems have been used for this study: (i) Knoevenagel condensation of 3-nitrobenzaldehyde and malononitrile and (ii) Suzuki coupling of bromobenzene and phenylboronic acid. Using a catalyst in which isolated imprinted amines are surrounded by an acidic silanol-rich environment led to rate accelerations of over 120-fold relative to catalysts in which the amines are surrounded by a hydrophobic environment consisting of trimethylsilyl functional groups for system (i). This result parallels our previous study on the effect of the outer sphere composition on rate acceleration of Knoevenagel reactions using isophthalaldehyde as the aldehyde reactant. We also extended our method for the hydrophilic imprinting of bulk silica to organometallic systems, by successfully synthesizing a tethered palladium complex within the imprinted pocket. This material was used as an active catalyst for (ii). Our results show that a hydrophobic framework environment results in higher initial turnover frequencies than an acidic silanol-rich framework for the Suzuki coupling reaction of bromobenzene and phenylboronic acid, albeit with a lower overall effect than observed in the Knoevenagel system (i). Altogether, these results demonstrate the control of chemical reactivity via the rational design of the outer sphere using an imprinting approach.
Introduction Chemical reactivity is known to strongly depend on the environment surrounding a reactive center [1]. Biological catalysts, by virtue of achieving intricate organic functional group organization within an active site, commonly show perturbed reactivity. One of the interesting ways that biological systems accomplish this for base catalysis is by enclosing the active site within a hydrophobic environment [2]. For example, the catalytic lysine in a number of aldolase antibodies shows a decrease in pKa from its unperturbed value of 10.5 to about 6.0 within the hydrophobic pocket of the antibody [3,4]. Unlike biological systems, where active site uniformity and isolation permit outer-sphere hydrophobicity to be controlled and optimized via rational design, achieving the same in synthetic heterogeneous systems that consist of tethered organic functional groups has been difficult. This is because such synthetic catalysts typically consist of a distribution of active sites that can range from sub-nanometer clusters to monolayers with macroscopic dimensions [5]. This distribution produces heterogeneous environments surrounding an active site, which are not amenable to rigorous control via synthetic manipulation. It would be highly desirable to use the framework surrounding an active site
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