Role of surface-bound intermediates in the oxygen-assisted synthesis of amides by metallic silver and gold.
Role of surface-bound intermediates in the oxygen-assisted synthesis of amides by metallic silver and gold.
复制标题
表面结合中间体在金属银和金氧辅助合成酰胺中的作用。
DOI：
10.1021/ja303178z
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发表时间：
2012
影响因子：
15
通讯作者：
Friend,CynthiaM
中科院分区：
文献类型：
--
作者：
Siler,CassandraGF;Xu,Bingjun;Madix,RobertJ;Friend,CynthiaM
通过对乙醛和二甲胺的研究，结合前人的工作，得出了在金属金和银表面上氧辅助合成酰胺的一般机理，从而可以详细地比较这两种表面的反应活性。二甲胺与乙醛的乙酰化反应在含氧的银和金(111)晶体上具有高选择性，反应的整个机理相同，但在这两种金属上的反应步骤不同。被吸附的原子氧激活胺的N-H键，形成被吸附的酰胺，后者攻击醛的羰基碳，形成被吸附的半胺。由于醇的部分氧化很容易形成醛，我们的机制也为醇和胺的相关催化偶联提供了洞察力。半氨基β-H消除形成偶联的酰胺产物。在银上，β-H从半胺上的消除是限速的，而在金上，酰胺的解吸是缓慢的步骤。在吸附氧浓度为0.01~0.1个单层时，银表现出较高的选择性，而金表现出更强的选择性依赖于氧覆盖度，在0.03个单分子层时接近100%。选择性趋势和限速步骤的差异可能是由于吸附在两个表面上的羟基的相对稳定性的影响。氧的低表面覆盖率导致了最高的选择性。本研究为醇、醛和胺在金和银基催化剂上的气相或液相氧助偶联反应提供了一般框架。
A general mechanism for the oxygen-assisted synthesis of amides over metallic gold and silver surfaces has been derived from the study of acetaldehyde and dimethylamine in combination with previous work, allowing detailed comparison of the two surfaces’ reactivities. Facile acetylation of dimethylamine by acetaldehyde occurs with high selectivity on oxygen-covered silver and gold (111) crystals via a common overall mechanism with different rate-limiting steps on the two metals. Adsorbed atomic oxygen activates the N–H bond of the amine leading to the formation of an adsorbed amide, which attacks the carbonyl carbon of the aldehyde, forming an adsorbed hemiaminal. Because aldehydes are known to form readily from partial oxidation of alcohols, our mechanism also provides insight into the related catalytic coupling of alcohols and amines. The hemiaminal β-H eliminates to form the coupled amide product. On silver, β-H elimination from the hemiaminal is rate-limiting, whereas on gold desorption of the amide is the slow step. Silver exhibits high selectivity for the coupling reaction for adsorbed oxygen concentrations between 0.01 and 0.1 monolayer, whereas gold exhibits selectivity more strongly dependent on oxygen coverage, approaching 100% at 0.03 monolayer. The selectivity trends and difference in rate-limiting steps are likely due to the influence of the relative stability of the adsorbed hydroxyl groups on the two surfaces. Low surface coverages of oxygen lead to the highest selectivity. This study provides a general framework for the oxygen-assisted coupling of alcohols and aldehydes with amines over gold- and silver-based catalysts in either the vapor or the liquid phase.