  Macrocycles have attracted much attention due to their specific "endless"
topology, which results in extraordinary properties compared to related linear
(open-chain) molecules. However, challenges still remain in their controlled
synthesis with well-defined constitution and geometry. Here, we report the
first successful application of the (pseudo-)high dilution method to the
conditions of on-surface synthesis in ultrahigh vacuum (UHV). This approach
leads to high yields (up to 84%) of cyclic hyperbenzene ([18]-honeycombene) via
an Ullmann-type reaction from 4,4"-dibromo-meta-terphenyl (DMTP) as precursor
on a Ag(111) surface. The mechanism of macrocycle formation was explored in
detail using scanning tunneling microscopy (STM) and X-ray photoemission
spectroscopy (XPS). We propose that hyperbenzene (MTP)6 forms majorly by
stepwise desilverization of an organometallic (MTP-Ag)6 macrocycle, which
preforms via cyclisation of (MTP-Ag)6 chains under pseudo-high dilution
condition. The high probability of cyclisation on the stage of the
organometallic phase results from the reversibility of the C-Ag bond. The case
is different from that in solution, in which cyclisation typically occurs on
the stage of covalently bonded open-chain precursor. This difference in the
cyclisation mechanism on a surface compared to that in solution stems mainly
from the 2D confinement exerted by the surface template, which to a large
extent prevents the flipping of chain segments necessary for cyclisation.
