RESUMO
BACKGROUND: High efficiency in terms of reaction yield and purity has led to the extensive utilization of copper-catalyzed azide-alkyne cycloaddition (CuAAC) in various fields of chemistry. Its compatibility with low molecular weight alcohols promotes the application in surfactant synthesis to tackle the miscibility constraints of the reactants. OBJECTIVE: For the tuning of surfactant properties, double click coupling of the antipode precursors was attempted. Failure of the CuAAC to provide the targeted product in combination with unexpected reaction outputs led to an investigation of the side reaction. METHODS: The CuAAC-based coupling of sugar azide with propargyl building block in the presence of copper- (I) catalyst exclusively led to the mono-coupling product in a respectable yield of almost 80%. Besides the unexpected incomplete conversion, the loss of the remaining propargyl group, as indicated by both NMR and MS. On the other hand, application of substantial amounts of CuSO4 under reducing conditions in refluxing toluene/water furnished the alkyne dimer in a moderate yield of 43%, while no change of azide compound was noticed. RESULTS: The Cu(I)-catalyst applied for azide-alkyne cycloadditions enables the homo-coupling of certain terminal alkynes at a higher temperature. Moreover, aromatic propargyl ethers may be cleaved to furnish the corresponding phenol. The copper-catalyzed coupling appeared highly sensitive towards the alkyne compound. Only selected derivatives of propargyl alcohol were successfully dimerized. CONCLUSIONS: The observed failure of the Huisgen reaction for the synthesis of sugar-based surfactants may indicate non-recognized constrains of the reaction, which could affect its wide application in bioconjugation. The temperature requirement for the alternative dimerization of terminal alkynes renders this side reaction nonrelevant for typical click couplings, while narrow substrate diversity and moderate yield limit its synthetic application.
RESUMO
A series of aryl azide terminated thiols and phosphonic acids has been synthesized, and used to prepare self-assembled monolayers on (respectively) gold and aluminum oxide surfaces. The rates of photoactivation were determined using contact angle measurement and X-ray photoelectron spectroscopy (XPS). The behavior of a diazirine functionalized aryl thiol was also studied. The rates of activation were found to be similar for all five adsorbates. However, the extent of photochemical coupling of a primary amine was significantly greater for the aryl azides than for the diazirine. A range of primary amines was successfully coupled to all of the azides with high yield. Little difference in reactivity was observed following perfluorination of the aromatic ring. Micrometer-scale patterns were fabricated by carrying out exposures of the aryl azide terminated SAMs through a mask submerged under a film of primary amine. Contrasting amines could be introduced to unreacted regions in a subsequent maskless step. A scanning near-field optical microscope was used to fabricate nanopatterns. Exposure of the azides to irradiation at 325 nm in air enabled selective deactivation of azides. The surrounding surface was functionalized with a primary amine in a maskless process; when a protein-resistant oligo(ethylene glycol) functionalized amine was used it was possible to produce protein nanopatterns, by adsorbing protein to features defined using near-field exposure.
