Kinetics and Mechanism of Allene Racemization Catalyzed by a Gold N-Heterocyclic Carbene Complex

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• 作者：Hao Li ; Robert J. Harris ; Kohki Nakafuku ; Ross A. Widenhoefer
• 刊名：Organometallics
• 出版年：2016
• 出版时间：July 11, 2016
• 年：2016
• 卷：35
• 期：13
• 页码：2242-2248
• 全文大小：441K
• 年卷期：0
• ISSN：1520-6041

文摘

The kinetics of the racemization of 1,3-disubstituted allenes catalyzed by (IPr)AuOTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) has been investigated. The rate of gold-catalyzed allene racemization obeyed the following second-order rate law: rate = k_rac[allene][Au]. An analysis of the rate of the gold-catalyzed racemization of 1-aryl-1,2-butadienes as a function of allene electron donor ability established moderate depletion of electron density on the C1 allenyl carbon atom in the rate-limiting transition state for racemization. Analysis of the temperature dependence of the rate of racemization of 1-(p-tolyl)-1,2-butadiene established the activation parameters ΔH^⧧ = 8.3 ± 1 kcal mol^–1 and ΔS^⧧ = −28 ± 4 eu. These observations were in accord with a mechanism for allene racemization involving turnover-limiting, intermolecular allene exchange followed by rapid allene stereomutation.