This work represents our initial effort in identifying azide/alkyne pairs for optimal reactivity in copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions. In previous works, we have identified chelating azides, in particular 2-picolyl azide, as "privileged" azide substrates with high CuAAC reactivity. In the current work, two types of alkynes are shown to undergo rapid CuAAC reactions under both copper(II)-(via an induction period) and copper(I)-catalyzed conditions. The first type of the alkynes bears relatively acidic ethynyl C-H bonds, while the second type contains an N-(triazolylmethyl)propargylic moiety that produces a self-accelerating effect. The rankings of reactivity under both copper(II)-and copper(I)-catalyzed conditions are provided. The observations on how other reaction parameters such as accelerating ligand, reducing agent, or identity of azide alter the relative reactivity of alkynes are described and, to the best of our ability, explained.
Copper(II) acetate under aerobic conditions catalyzes the formation of 5,5'-bis(1,2,3-triazole)s (5,5'-bistriazoles) from organic azides and terminal alkynes. This reaction is an oxidative extension of the widely used copper-catalyzed azide-alkyne "click" cycloaddition. The inclusion of potassium carbonate as an additive and methanol or ethanol as the solvent, and in many instances an atmosphere of dioxygen, promote the oxidative reaction to afford 5,5'-bistriazole at the expense of 5-protio-1,2,3-triazole (5-protiotriazole). If needed, tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) as a ligand additive further accelerates the formation of 5,5'-bistriazoles. A convenient procedure to prepare TBTA is also reported to facilitate the adoption of this method for preparation of 5,5'-bistriazoles. Aromatic azide-derived 5,5'-bistriazoles possess rigid axially chiral structures with a broad distribution of dihedral angles, which may be explored as chiral ligands in enantioselective catalysis if decorated with proper functional groups.
In this work, a wideband low sidelobe slot array antenna for wireless communication system at E-band is presented. The slots are excited by a parallel feeding network where the Taylor distribution is applied. Two types of wideband unequal power divider with compact structures are designed for various output ratios to fit the limited feeding network space. One is integrated vertically inside a 2 × 1 elements subarray using a set of pins with asymmetrical offset. The other type is designed inside an E-plane T-junction with unbalance impedance at two output ports. To avoid main lobe tilt and asymmetrical radiation patterns, the feeding network is designed to be biaxially symmetrical, and the feed port is placed in the center of the back. A prototype is fabricated with brass and measured in an anechoic chamber. It achieves a peak gain of 25.8 dBi and a fractional impedance bandwidth of 19.2 % with S 11 below −10 dB. The measured radiation patterns agree with the simulated patterns very well, and the sidelobe level is lower than −21 dB.
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