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锆催化直接酰胺化的机理阐明。

Mechanistic Elucidation of Zirconium-Catalyzed Direct Amidation.

机构信息

Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University , SE-106 91 Stockholm, Sweden.

Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz , Campus Universitario de Puerto Real, 11510 Puerto Real, Spain.

出版信息

J Am Chem Soc. 2017 Feb 15;139(6):2286-2295. doi: 10.1021/jacs.6b10973. Epub 2017 Feb 7.

Abstract

The mechanism of the zirconium-catalyzed condensation of carboxylic acids and amines for direct formation of amides was studied using kinetics, NMR spectroscopy, and DFT calculations. The reaction is found to be first order with respect to the catalyst and has a positive rate dependence on amine concentration. A negative rate dependence on carboxylic acid concentration is observed along with S-shaped kinetic profiles under certain conditions, which is consistent with the formation of reversible off-cycle species. Kinetic experiments using reaction progress kinetic analysis protocols demonstrate that inhibition of the catalyst by the amide product can be avoided using a high amine concentration. These insights led to the design of a reaction protocol with improved yields and a decrease in catalyst loading. NMR spectroscopy provides important details of the nature of the zirconium catalyst and serves as the starting point for a theoretical study of the catalytic cycle using DFT calculations. These studies indicate that a dinuclear zirconium species can catalyze the reaction with feasible energy barriers. The amine is proposed to perform a nucleophilic attack at a terminal η-carboxylate ligand of the zirconium catalyst, followed by a C-O bond cleavage step, with an intermediate proton transfer from nitrogen to oxygen facilitated by an additional equivalent of amine. In addition, the DFT calculations reproduce experimentally observed effects on reaction rate, induced by electronically different substituents on the carboxylic acid.

摘要

使用动力学、NMR 光谱和 DFT 计算研究了锆催化羧酸和胺的缩合反应直接形成酰胺的机理。研究发现,该反应对催化剂呈一级反应,且对胺浓度具有正的速率依赖性。在某些条件下,观察到羧酸浓度呈负的速率依赖性,以及 S 型动力学曲线,这与形成可逆的非循环物种一致。使用反应进度动力学分析方案进行的动力学实验表明,通过使用高浓度的胺可以避免酰胺产物对催化剂的抑制。这些见解导致了反应方案的设计,提高了产率并降低了催化剂的负载量。NMR 光谱提供了锆催化剂性质的重要细节,并作为使用 DFT 计算研究催化循环的理论研究的起点。这些研究表明,双核锆物种可以催化反应,具有可行的能量垒。据推测,胺在锆催化剂的末端η-羧酸盐配体上进行亲核攻击,然后进行 C-O 键断裂步骤,同时通过额外当量的胺促进氮到氧的中间质子转移。此外,DFT 计算再现了实验观察到的反应速率的影响,这是由羧酸上电子不同的取代基引起的。

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