Chen Hongwen, Ussama M, Iyer Jayendran, Kim Sungmin, Haider M Ali, Khare Rachit, Lercher Johannes A
Department of Chemistry and Catalysis Research Center, Technical University of Munich, 85748 Garching, Germany.
Renewable Energy and Chemicals Laboratory, Department of Chemical Engineering, Indian Institute of Technology Delhi, 110016 New Delhi, India.
ACS Catal. 2025 May 26;15(11):9762-9775. doi: 10.1021/acscatal.4c08004. eCollection 2025 Jun 6.
In this work, we show that Cu effectively catalyzes the electroreductive C-C coupling of conjugated aromatic aldehydes (like benzaldehyde). We demonstrate that C-C coupling is not observed for aliphatic aldehydes (like pentanal), nonconjugated aromatic aldehydes (like hydrocinnamaldehyde), or conjugated nonaromatic aldehydes (like crotonaldehyde). The fact that only conjugated aromatic aldehydes undergo C-C coupling on Cu points to the importance of their planar structure and, consequently, their strong interaction with the metal surface. This direct interaction enables electron transfer that eventually leads to the Eley-Rideal (ER)-type reaction with the electrophilic carbon of the reacting physisorbed aldehyde molecule. Two additional factors are indispensable for the C-C coupling pathway: (i) the preferential first hydrogenation of the carbonyl oxygen resulting in the formation of a hydroxy intermediate (i.e., ArCHOH*), and (ii) a relatively slow second H addition resulting in a stable hydroxy intermediate on the surface. In contrast, when other metals or nonconjugated aldehydes are involved, preferential hydrogenation of the carbonyl carbon, fast second H addition, or a high intrinsic barrier for C-C bond formation inhibits the C-C coupling pathway.
在本工作中，我们表明铜能有效催化共轭芳香醛（如苯甲醛）的电还原碳 - 碳偶联反应。我们证明，脂肪醛（如戊醛）、非共轭芳香醛（如氢化肉桂醛）或共轭非芳香醛（如巴豆醛）均未观察到碳 - 碳偶联反应。只有共轭芳香醛在铜上发生碳 - 碳偶联这一事实表明其平面结构的重要性，进而表明其与金属表面的强相互作用。这种直接相互作用使得电子转移，最终导致与物理吸附的反应醛分子的亲电碳发生埃利 - 里德（Eley - Rideal，ER）型反应。碳 - 碳偶联途径还需要另外两个不可或缺的因素：（i）羰基氧优先进行首次氢化，形成羟基中间体（即ArCHOH*），以及（ii）相对较慢的第二次氢添加，从而在表面形成稳定的羟基中间体。相比之下，当涉及其他金属或非共轭醛时，羰基碳的优先氢化、快速的第二次氢添加或碳 - 碳键形成的高固有势垒会抑制碳 - 碳偶联途径。
