The concept of chelation-assisted copper catalysis was employed for the development of new azides that display unprecedented reactivity in the copper(I)-catalyzed azide-alkyne [3+2] cycloaddition (CuAAC) reaction. Azides that bear strong copper-chelating moieties were synthesized; these functional groups allow the formation of azide copper complexes that react almost instantaneously with alkynes under diluted conditions. Efficient ligation occurred at low concentration and in complex media with only one equivalent of copper, which improves the biocompatibility of the CuAAC reaction. Furthermore, such a click reaction allowed the localization of a bioactive compound inside living cells by fluorescence measurements.

In response to the ever increasing need of chemical biology for new tools, a wide variety of new, highly selective reactions have been described. Herein we report a summary of recent developments and the historical background on bioorthogonal ligation reactions.

A new derivative of the strained 3,3,6,6-tetramethylthiacycloheptyne (TMTH) bearing a functional handle is reported. Following an optimized synthesis, the handle was introduced by mild alkylation of the sulphur atom. The resulting functionalized strained 4,5-didehydro-3,3,6,6-tetramethyl-2,3,6,7-tetrahydrothiepinium (TMTI) proved to be stable and underwent extremely fast [3+2] cycloaddition reaction with benzyl azide in both organic and aqueous solvents. The reaction was equally efficient in cell lysate and serum and therefore opens interesting prospects for chemical-biology applications.

The concept of chelation‐assisted copper catalysis was employed for the development of new azides that display unprecedented reactivity in the copper(I)‐catalyzed azide–alkyne [3+2] cycloaddition (CuAAC) reaction. Azides that bear strong copper‐chelating moieties were synthesized; these functional groups allow the formation of azide copper complexes that react almost instantaneously with alkynes under diluted conditions. Efficient ligation occurred at low concentration and in complex media with only one equivalent of copper, which improves the biocompatibility of the CuAAC reaction. Furthermore, such a click reaction allowed the localization of a bioactive compound inside living cells by fluorescence measurements.

The room-temperature structure of n-propylammonium chloride, which shows rotation of the cations, goes over in a second-order transition into a low-temperature form with fixed positions of the cations. In this process, the crystal of the room-temperature form breaks up reversibly into many domains of the low-temperature form, the domains having differing orientations. The space group of the lowtemperature form is C~-C2/m. The transition is produced by the fixing of the positions of the cations in the (li0) planes of the room-temperature structure in a centrosymmetric arrangement, with the result that the angle between the a axes is contracted, and the c axis is tipped in the direction of the axis [110]. The amount of these distortions varies with the temperature. Approximate positions of the a~.oms in the low-temperature form are given.

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The title compound (I) is stable towards amines, alcohols and carboxylic acids.