Organic synthesis "on water".
Organic synthesis "on water".
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DOI：
10.1021/cr800448q
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发表时间：
2009-02
期刊：
影响因子：
62.1
通讯作者：
Fokin, Valery V.
中科院分区：
文献类型：
--
作者：
Chanda, Arani;Fokin, Valery V.
水是地球上生命的通用语言，也是大自然进行合成的首选溶剂。相比之下，我们制造复杂有机分子的方法使我们远离了生物合成的水环境。事实上，可以公平地说，在学术实验室和工业中常用的大多数有机反应在水或氧气存在下都会失败。作为我们试图模仿大自然制造新化学键的方式的直接结果，我们学会了依靠高活性的亲核和亲电试剂来控制化学反应活性，并将化学反应引导到所需的途径。保护所有质子性官能团的要求，如醇和胺，是我们依赖这些高能物种的另一个必然结果。然而，水溶液中的化学转化对有机化学家来说并不是什么新鲜事。相反，它们多年来一直吸引着科学家的注意：第一次使用水进行有机反应可以追溯到沃勒从氰酸铵合成尿素。2从真正的有机合成的角度来看，最早的例子可能是1882年Baeyer和Drewsen合成蓝宝石(方案1)。3在它们的合成中，用氢氧化钠溶液处理邻硝基苯甲醛1在丙酮水溶液中的悬浮液，立即形成蓝2的特征蓝色，产物随后沉淀。水具有许多独特的物理和化学性质：保持液态的大温度窗口、广泛的氢键、高热容、大的介电常数以及维持水生生物的最佳氧溶解度。这些独特的性质是水独特结构的结果。水的结构和性质已被代表几乎所有知识领域的科学家研究过，新的理论模型不断涌现。6，7众所周知，水还可以提高各种有机反应的速率并影响其选择性。8、9尽管有这些潜在的优势，水仍然不是有机合成的唯一溶剂，至少部分是因为大多数有机化合物在很大程度上不溶于水，而溶解性通常被认为是反应性的先决条件：“unagant nisi solta”(物质除非溶解，否则不会反应)。因此，在许多“水相反应”的例子中，有机共溶剂被用来增加有机反应物在水中的溶解度。9、10或者，通过引入极性官能团来增加反应物的亲水性，再次使所得到的化合物至少部分可溶于水。然而，这些操作往往会削弱甚至否定水相对于传统溶剂具有的低成本、反应条件简单、工作容易和产品分离的优势。因此，目前正在蓬勃发展的水介质有机合成领域包含了一大类反应。反应物种和产物的溶解度可以从完全到部分甚至几乎为零，因此反应混合物既可以是均相的，也可以是非均相的。水的量也可以有很大的范围，从亚化学计量比到反应物悬浮或溶解的大体积。文献中用了几个术语来描述水溶液中的反应。
Water is the lingua franca of life on our planet and is the solvent of choice for Nature to carry out her syntheses. 1 In contrast, our methods of making complex organic molecules have taken us far away from the watery milieu of biosynthesis. Indeed, it is fair to say that most organic reactions commonly used both in academic laboratories and in industry fail in the presence of water or oxygen. As a direct consequence of our attempts to mimic Nature’s way of making new chemical bonds, we learned to rely on highly reactive nucleophilic and electrophilic reagents to gain control of the chemical reactivity and to channel chemical reactions down a desired pathway. The requirement for the protection of all protic functional groups, such as alcohols and amines, is another corollary of our reliance on these energetic species. Nevertheless, chemical transformations in aqueous solvents are not new to organic chemists. On the contrary, they have attracted the attention of scientists for many years: the first use of water for an organic reaction could be dated back to Wöhler’s synthesis of urea from ammonium cyanate. 2 From a true organic synthesis perspective, the earliest example could be the synthesis of indigo by Baeyer and Drewsen in 1882 (Scheme 1). 3 In their synthesis, a suspension of o-nitrobenzaldehyde 1 in aqueous acetone was treated with a solution of sodium hydroxide.The immediate formation of the characteristic blue color of indigo 2 ensued, and the product subsequently precipitated. Water possesses many unique physical and chemical properties: a large temperature window in which it remains in the liquid state, extensive hydrogen bonding, high heat capacity, large dielectric constant, and optimum oxygen solubility to maintain aquatic life forms. These distinctive properties are the consequence of the unique structure of water. 4, 5 The structure and properties of water have been studied by scientists representing almost all fields of knowledge, and new theoretical models continue to emerge. 6, 7 Water is also known to enhance the rates and to affect the selectivity of a wide variety of organic reactions. 8, 9 In spite of these potential advantages, water is still not commonly used as a sole solvent for organic synthesis, at least in part because most organic compounds do not dissolve in water to a significant extent, and solubility is generally considered a prerequisite for reactivity:“corpora non agunt nisi soluta”(substances do not react unless dissolved). Consequently, in the many examples of “aqueous reactions”, organic cosolvents are employed in order to increase the solubility of organic reactants in water. 9, 10 Alternatively, hydrophilicity of the reactants is increased by the introduction of polar functional groups, again to make the resulting compound at least partially water soluble. 11 However, these manipulations tend to diminish and even negate the advantages of low cost, simplicity of reaction conditions, ease of workup, and product isolation that water has over traditional solvents. Therefore, the currently burgeoning field of organic synthesis in aqueous media encompasses a large family of reactions. The solubility of reacting species and products can range from complete to partial to practically none, so that reaction mixtures can be both homogeneous and heterogeneous. The amount of water can also range widely, from substoichiometric quantities to a large volume in which the reactants are suspended or dissolved. Several terms have been used in the literature to describe reactions in aqueous millieu.
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