Surface-Dependent Chemoselectivity in C-C Coupling Reactions

Surface-confined covalent coupling reactions of the linear compound 4-(but-3-en-1-ynyl)-4'-ethynyl-1,1'-biphenyl (1), which contains one alkyne and one enyne group on opposing ends, have been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The reactions show a surface-dependent chemoselectivity: on Au(111), compound 1 preferentially yields cyclotrimerization products, while on Cu(111), a selective coupling between the enyne and alkyne groups is observed. Linear, V-shaped string formations combined with Y-shaped bifurcation motifs result in a random reticulation on the entire surface. DFT calculations show that the C-H⋅⋅⋅π^δ- transition state of the reaction between the deprotonated alkyne group and a nearby H-donor of the alkene group plays a key role in the mechanism and high chemoselectivity. This study highlights a concept that opens new avenues to the surface-confined synthesis of covalent carbon-based sp-sp² polymers.