Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica , Universidad de Santiago de Compostela , 15782 Santiago de Compostela , Spain.
Instituto de Química Orgánica General, CSIC , Juan de la Cierva 3 , 28006 Madrid , Spain.
Acc Chem Res. 2019 Feb 19;52(2):465-479. doi: 10.1021/acs.accounts.8b00567. Epub 2019 Jan 14.
Cycloaddition reactions, by involving the formation of at least two bonds and one cycle in a single operation, represent one of the more practical ways to assemble carbo- and heterocyclic structures from simple acyclic precursors. Especially appealing are formal cycloadditions promoted by transition metals, owing to the ability of these reagents to open mechanisms that are not accessible using classical chemistry. Therefore, along the years, a great variety of annulations based on first-, and particularly second-row transition metals have been discovered. Most of these reactions involve inner sphere mechanisms, with the metal participating via standard oxidative addition or reductive elimination processes. Curiously, metals of the third row like platinum and, especially, gold remained largely unexplored, likely because of the belief that they were inert and expensive. However, from the beginning of this century, many groups realized that these metals can open very interesting mechanistic scenarios and promote novel types of transformations. In particular, the π-acidic, carbophilic behavior of gold(I) complexes, together with the possibility of tuning their reactivity using designed ligands, has triggered important activity in the field. Many gold-catalyzed transformations involved addition or cycloisomerization processes, but during recent years, there have been also important advances in the development of formal cycloaddition reactions. While many of these reactions rely on the activation of alkynes, there has been an increasing number of reports that exploit the peculiar reactivities of allenes and derivatives. In this Account, we present recent efforts on the development of platinum- and gold-catalyzed formal cycloadditions of allenes. For the sake of simplicity, we only include annulations initiated by a direct metal-promoted activation of the allene moiety. Thus, alternative Pt- or Au-catalyzed reactions wherein the allene does not interact with the metal catalyst are not covered. Upon activation by the metals, allenes generate allyl-cation alkenylmetal species that can behave as 1,2- or 1,3-carbon dipoles in cycloaddition processes. Especially relevant is the reactivity of allenamides. The presence of the amide substituent provides for the generation of gold intermediates with a good balance of reactivity and stability, which can therefore react with the corresponding partners in a controlled manner. Moreover, despite the difficulties associated with the transfer of stereochemical information from chiral linear gold(I) complexes, a variety of enantioselective gold-catalyzed annulations have been discovered. This Account is organized considering the number of atoms engaged in the annulation process, and when possible, we present the results in a chronological order.
环加成反应通过在单个操作中形成至少两个键和一个环，代表了从简单的无环前体组装碳环和杂环结构的更实用方法之一。特别吸引人的是由过渡金属促进的形式环加成，因为这些试剂能够打开使用经典化学方法无法获得的机制。因此，多年来，已经发现了基于第一过渡金属，特别是第二过渡金属的大量环化反应。这些反应大多数涉及内球机制，金属通过标准的氧化加成或还原消除过程参与。奇怪的是，第三过渡金属如铂，尤其是金，仍然在很大程度上未被探索，可能是因为它们被认为是惰性的和昂贵的。然而，从本世纪初开始，许多小组意识到这些金属可以开辟非常有趣的机制情景，并促进新型转化。特别是，金（I）配合物的π酸性、碳亲电性以及使用设计的配体来调节其反应性的可能性，在该领域引发了重要的活性。许多金催化的转化涉及加成或环异构化过程，但近年来，在形式环加成反应的发展方面也取得了重要进展。虽然许多这些反应依赖于炔烃的活化，但越来越多的报道利用了丙二烯和衍生物的特殊反应性。在本综述中，我们介绍了最近在开发铂和金催化的丙二烯形式环加成方面的努力。为了简单起见，我们仅包括通过直接金属促进的丙二烯部分的活化引发的环化反应。因此，不涉及丙二烯与金属催化剂相互作用的替代 Pt 或 Au 催化反应不在讨论范围之内。丙二烯通过金属的活化生成烯丙基阳离子烯基金属物种，该物种在环加成过程中可以作为 1,2-或 1,3-碳二极体起作用。特别相关的是丙二酰胺的反应性。酰胺取代基的存在为生成具有良好反应性和稳定性平衡的金中间体提供了条件，因此可以以可控的方式与相应的配体反应。此外，尽管从手性线性金（I）配合物转移立体化学信息存在困难，但已经发现了多种对映选择性金催化的环化反应。本综述按照参与环化过程的原子数进行组织，并在可能的情况下，按时间顺序呈现结果。
