Mechanism of Formation of Silver N-Heterocyclic Carbenes Using Silver Oxide: A Theoretical Study

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• 作者：Joseph M. Hayes ; M&oacute ; nica Viciano ; Eduardo Peris ; Gregori Ujaque ; Agustí ; Lled&oacute ; s
• 刊名：Organometallics
• 出版年：2007
• 出版时间：December 3, 2007
• 年：2007
• 卷：26
• 期：25
• 页码：6170 - 6183
• 全文大小：983K
• 年卷期：v.26,no.25(December 3, 2007)
• ISSN：1520-6041

文摘

The reasons for the efficiency of the silver oxide route in the synthesis of Ag(I) N-heterocyclic carbenecomplexes have been investigated by means of DFT calculations. A general reaction system thatincorporates two N,N-dimethyl imidazolium cations, two iodides as the counterions, Ag₂O, anddichloromethane solvent was considered. Exploration of several pathways for the formation of the twosilver-NHC cations gives a clear picture for the reaction mechanism. The favored route involves abarrierless and very exergonic deprotonation of the first imidazolium followed by a low barrier and alsoexergonic metalation, affording the first silver-NHC. The second imidazolium assists these two stepsstabilizing intermediates and the transition state by the formation of a strong C_imidazolium-H···OAg hydrogenbond. The formed [R₂NHC]-AgI then diffuses in the solution, while silver hydroxide deprotonates thesecond imidazolium salt in a slightly endergonic process. After metalation, the second silver-NHC isobtained. The overall reaction is thermodynamically driven, the conversion of two imidazolium salts totwo silver carbenes entailing a system stabilization of more than 70 kcal mol^-1. As the acid-base reactionbetween the imidazolium salt and the silver base plays a key role in the process, pK_a calculations wereperformed to compare the basicity of the N-heterocyclic carbene with those of the silver bases Ag₂O,AgOH, and AgOAc. From the pK_a values obtained, the advantage of silver oxide in the generation ofsilver carbenes is attributed to its stronger basicity.