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The True Catalyst revealed: The Intervention of Chiral Ca and Mg Phosphates in Brønsted Acid Promoted Asymmetric Mannich Reactions
Journal of the American Chemical Society ( IF 15.0 ) Pub Date : 2018-03-30 , DOI: 10.1021/jacs.7b13678
Luis Simón 1 , Robert S. Paton 2
Affiliation  

The acetylacetone-benzaldimine Mannich reaction catalyzed by Mg(II) and Ca(II) salts of chiral phosphoric acids (CPA) has been investigated computationally by QM/MM methods. Enantioselectivity in this reaction is both larger than and in the opposite sense to that observed for the same reaction catalyzed by the protic CPA catalyst alone. We present a mechanistic model from which the characteristic differences between these metal and metal-free catalysts, which can coexist in the same reaction mixture, can be understood. Alkaline earth salts with chiral phosphate counterions are found to be more catalytically active than the protic form, and the Ca(II) and Mg(II) CPA salts react via different mechanisms, with a higher coordination number favored by calcium over magnesium. In the well-ordered chiral cavities around these metal centers, asymmetric induction arises from the steric interaction with the imine protecting group in the unfavorable pathway, with both substrates adopting well-defined conformations. These mechanistic models have allowed us to rationalize the stereochemical outcome across a range of bimolecular reactions promoted by divalent metal phosphates formed with different CPAs.

中文翻译:

真正的催化剂揭示:手性磷酸钙和磷酸镁在布朗斯台德酸中的干预促进了不对称曼尼希反应

已经通过 QM/MM 方法计算研究了由手性磷酸 (CPA) 的 Mg(II) 和 Ca(II) 盐催化的乙酰丙酮-苯扎二亚胺曼尼希反应。该反应中的对映选择性大于仅由质子 CPA 催化剂催化的相同反应所观察到的对映选择性。我们提出了一个机理模型,从中可以理解这些金属催化剂和无金属催化剂之间的特征差异,它们可以共存于同一反应混合物中。发现具有手性磷酸盐抗衡离子的碱土盐比质子形式具有更高的催化活性,并且 Ca(II) 和 Mg(II) CPA 盐通过不同的机制反应,钙的配位数高于镁。在这些金属中心周围有序的手性腔中,不对称诱导产生于不利途径中与亚胺保护基团的空间相互作用,两种底物均采用明确定义的构象。这些机械模型使我们能够合理化由不同 CPA 形成的二价金属磷酸盐促进的一系列双分子反应的立体化学结果。
更新日期:2018-03-30
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