The on-surface dimerization reaction of an organic nitrile on Au(111) is reported. The reaction cascade yielding a diazapyrene core was investigated by scanning tunneling microscopy, computational studies and reference compounds.

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to desorb from the surface [1a,3a] (a rather general problem of on-surface synthesis) and in addition side reactions, namely hydroalkynylation leading to enynes and trimerization, [1b,3b,4,5a] can occur. To overcome these critical issues, different strategies have been followed in the coupling of two alkynes. Along with the variation of the surface [3] thereby also selecting different surface orientations, [5] monomer design was found as a key tool for reaction optimization. Especially the installation of sterically demanding groups next to the alkyne moiety has been found to enhance the selectivity of the Glaser coupling (Scheme 1a). [1b,5b] If o-coordinating groups are introduced, the monomers can be located at the step edges of the surface to enable the selective formation of the Glaser-coupling product by preorganization (Scheme 1a). [6] Moreover, the effect of surface coverage was investigated and it was found that at high surface loadings in the reaction of alkynyl aryl carboxylic acids, trimerization could be suppressed (Scheme 1b). [7] Further, a photochemical version [8] and a protecting group strategy [9] were developed to increase efficiency and selectivity in Glaser-couplings. Notably, along with the parent CH alkynes, silylated [10] and brominated [11] congeners have been selected as coupling partners that engage in "cleaner" Glaser coupling reactions. However, in these cases, high annealing temperatures were required either for the coupling [10] or to remove the liberated bromine atoms [11a] from the surface. Of note, cleavage of bromine atoms or carbon monoxide from disubstituted alkynes in cyclic enynes was achieved for the preparation of cyclic polyynes. [12] Besides the well-known Glaser coupling, a variety of other on-surface reactions have been successfully established, such as the Ullmann coupling, [13] imine bond formation, [14] decarboxylative coupling, [15] dehydrofluorination reaction, [16] dehydrogenative SiSi coupling [17] and disilabenzene framework formation. [18] Despite these achievements, it is still of high interest to evaluate new chemical functionalities as reactive entities in on-surface chemistry. As compared to homocoupling reactions, the intermolecular coupling of two differently functionalized monomers is far more challenging. The underlying reason is that both monomers are required to constitute a perfectly mixed self-assembly on the surface in order to enable the targeted cross-reaction. Consequently, there Aryl propiolic acids are introduced as a new class of monomers in the field of on-surface chemistry to build up poly(arylenebutadiynylenes) through decarboxylative Glaser coupling. As compared to aryl alkynes that are routinely used in the on-surface Glaser coupling, it is found that the decarboxylative coupling occurs at slightly lower temperature and with excellent selectivity. Activation occurs through decarboxylation for the propiolic acids, whereas the classical Glaser coupling is achieved through alkyne CH activation, and this process shows poor se...

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