School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States.
J Am Chem Soc. 2012 Aug 1;134(30):12604-10. doi: 10.1021/ja303178z. Epub 2012 Jul 19.
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.
已经从乙醛和二甲胺的研究中推导出了一种在金属金和银表面上通过氧气辅助合成酰胺的通用机制，这允许对两种表面的反应性进行详细比较。通过与以前的工作相结合，乙醛和二甲胺在氧气覆盖的银和金（111）晶体上通过共同的总机制发生高选择性的易乙酰化，其中两种金属的速率限制步骤不同。吸附的原子氧激活胺的 N-H 键，导致吸附的酰胺的形成，其攻击醛的羰基碳，形成吸附的半亚胺。因为醛已知容易从醇的部分氧化形成，我们的机制还提供了对相关醇和胺的催化偶联的洞察。半亚胺的β-H 消除形成偶联酰胺产物。在银上，β-H 消除从半亚胺是速率限制步骤，而在金上酰胺的脱附是慢步骤。银在吸附氧浓度为 0.01 至 0.1 单层之间表现出高的偶联反应选择性，而金的选择性更强烈地依赖于氧覆盖度，在 0.03 单层时接近 100%。选择性趋势和速率限制步骤的差异可能归因于两个表面上吸附的羟基相对稳定性的影响。低表面氧覆盖率导致最高的选择性。这项研究为在气相或液相中金和银基催化剂上醇、醛与胺的氧气辅助偶联提供了一个通用框架。
