On-Surface Ullmann Reaction for the Synthesis of Polymers and Macrocycles

Abstract

Compared to organic synthesis in solution, the benefits of on-surface synthesis are apparent especially when it involves insoluble reactants or when in situ characterizations of products with large molecular weight are required. This article covers the on-surface synthesis of hydrocarbon polymers and macrocycles via an Ullmann type reaction of haloarenes on metal single-crystal surfaces. The related formation of stable organometallic reaction intermediates with carbon-metal-carbon bonds is also discussed. Regarding the on-surface synthesis of polymers, deposition onto hot metal surfaces (in contrast to post-annealing) as well as high diffusion rates of the monomers lead to the increase of their chain lengths. To obtain high yields of macrocycles, it is important to enable the ring closure at the stage of the organometallic species with reversible carbon-metal-carbon bonds and to apply high-dilution conditions. In contrast, the formation of macrocycles by cyclisation of polymer chains with only covalent bonds is of low probability due to the particular mechanism of the ring-closure process in two-dimensional confinement.