Catalysis with Molecular Nanoreactors: Dendrimers and other Highly Branched Macromolecules

在过去的几十年里，催化领域取得了巨大的进步，部分原因是它在经济和生态无害的精细化学品和药物生产中发挥了重要作用。催化领域的研究通常分为两个领域，即均相和多相。虽然前者提供了独特的优势，明确的催化中心，可以系统地进行优化，而后者具有重要的工业意义，因为催化剂的固有保留，这有助于产品分离。近年来，具有催化活性中心的功能聚合物应运而生，它结合了均相催化和多相催化的优点。特别是具有高度支化、球状结构的聚合物大分子，如树枝状大分子，已显示出作为真正的纳米级分子反应器的应用前景，可以在良好控制的环境下影响特定的化学转化。提出了一种利用树枝状和其他高度支化的聚合物结构来设计新型催化剂体系的研究计划。在每种情况下，具有催化活性的反应官能团被放置在球状树枝状结构的核心或内部，从而可以充分实现树枝状包裹所提供的独特性质。树枝状包埋可以提供几个优点，例如：(I)催化所必需的活性物种彼此隔离，防止相互失活；(Ii)催化中心的反应活性可以直接在催化中心，或通过其周围的微环境进行调节；(Iii)催化剂的溶解性可以通过外围修饰来改变；(Iv)催化中心的微环境不仅可以提供增强的活性，还可以提供额外的功能，如底物和产物的运输；(V)多个催化活性亚基之间正向合作作用的可能性。这种由树枝状包裹产生的性质在天然酶体系中经常被发现，并被用于高效率，但它们通常不能同时并入经典的人工均相或多相催化剂中。从拟议的研究中获得的知识将具有双重重要性，既是为了进一步加深我们对一类特殊的、有点不寻常的合成聚合物催化的基本原理的理解，也是为了发现开发可与自然产生的酶相媲美的优质、完全合成的催化剂的线索。随着更好、更有效、更环保和可回收的催化剂的发现，这项研究的影响不仅将在化学和医药产品的生产中感受到，而且将在我们的环境中感受到。在一种可能的实施方式中，可以使用一系列树枝状大分子催化剂在单一介质中以与几种酶在同一整体环境中接连进行复杂转化大致相同的方式进行多步骤转化。

During the last few decades, tremendous progress has been made in the field of catalysis in part due to its important role in the economically and ecologically sound production of fine chemicals and pharmaceuticals. Research in the field of catalysis is typically divided into two areas, namely homogenous and heterogeneous. While the former offers the unique advantage of well-defined catalytic sites that can systematically be optimized, the latter is of great industrial importance due to the inherent retention of the catalysts, which facilitates product separation. In recent years, functional polymers carrying catalytically active sites have emerged that combine the advantages of both homogeneous and heterogeneous catalysis. In particular, polymeric macromolecules with a highly branched, globular architectures, such as dendrimers, have shown promise for application as true nanoscale "molecular reactors" to affect specific chemical tranformations withing a well-controlled environment. A research program utilizing dendritic and other highly branched polymer architectures for the design of new catalyst systems is proposed. In each instance, a reactive functionality with catalytic activity is placed at the core or the interior of a globular dendritic structure so that the unique properties offered by dendritic encapsulation may be fully realized. Dendritic encapsulation, can afford several advantages such as: (i) reactive species essential for catalysis are isolated from each other preventing mutal deactivation; (ii) the reactivity of the catalytic site can be tuned, either directly at the catalytic site, or via its surrounding microenvironment; (iii) the catalyst solubility can be altered by peripheral modification; (iv) the microenviroment of the catalytic site my provide not only enhanced activity but also additional functions such as transport of substrate and product; (v) the potential for positive cooperative interactions between multiple catalytically active subunits. Such properties, resulting from dendritic encapsulation, are frequently found in natural enzymatic systems and are employed to high efficacy, but they generally cannot be incorporated simultaneouly in classical artificial homogeneous or heterogeneous catalysts. The knowldege to be gelaned from the proposed investigations will have two-fold importance, both to further increase our understanding of the fundamental principles regarding catalysis with a specialized and somewhat unusual class of synthetic polymers and to uncover leads for development of superior, fully synthetic, catalysts that rival naturally occurring enzymes. This impact of this research will be felt not only on the production of chemical and pharmaceutical products but also in our environment as better, more effective, ecologically friendly, and recyclable catalysts will be discovered. In one possible implementation, a series of dendrimer catalysts may be used to carry out multistep tranformations in a single medium in much the same way as several enzymes carry out complex tranformations in succession within the same overall environment.