  The additive-free tetrazine/enol ether click reaction was performed in
ultra-high vacuum (UHV) with an enol ether group covalently linked to a silicon
surface: Dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate molecules were coupled to
the enol ether group of a functionalized cyclooctyne which was adsorbed on the
silicon (001) surface via the strained triple bond of cyclooctyne. The reaction
was observed at a surface temperature of 380 K by means of X-ray photoelectron
spectroscopy (XPS). No indications for tetrazine molecules which bind directly
to the Si(001) surface via the nitrogen atoms were detected. A moderate energy
barrier was deduced for this click reaction in vacuum by means of density
functional theory based calculations, in good agreement with the experimental
results. This UHV-compatible click reaction thus opens a new, flexible route
for synthesizing covalently bound organic architectures.
