Mechanistical Study on Substrate-Controlled Highly Selective [2+2] and [2+3] Cycloaddition Reactions

Polycyclic conjugated hydrocarbons have acquired increased interests recently because of their potential applications in electronic devices. On metal surfaces, the selective synthesis of four- and five-membered carbon rings remains challenging due to the presence of diverse reaction pathways. Here, utilizing the same precursor molecule, we successfully achieved substrate-controlled highly selective cycloaddition reactions towards four- and five-membered carbon rings. A 97 % yield for four-membered carbon rings on Au(111), while a 96 % yield towards five-membered carbon rings is achieved on Ag(111). The detailed topological structures of the reaction products are carefully examined by bond-resolving scanning tunneling microscopy (BR-STM) imaging with a CO functionalized tip. The underlying mechanism of the novel surface-directed reaction selectivity is elucidated by extensive density functional theory (DFT) calculations. Our study paves the way for high selective synthesis of polycyclic conjugated hydrocarbons with non-benzenoid rings.