though PPh3 is liable to Staudinger ligation with the azide substituent. Cu2O in water at room temperature was found also to catalyze the same reaction in 15 minutes with 91% yield. The first reaction mechanism proposed included one catalytic copper atom; but isotope, kinetic, and other studies have suggested a dicopper mechanism may be more relevant. Even though this reaction proceeds effectively at biological conditions, copper in this range of dosage is cytotoxic. Solutions to this problem have been presented, such as using water-soluble ligands on the copper to enhance cell penetration of the catalyst and thereby reduce the dosage needed, or to use chelating ligands to further increase the effective concentration of Cu(I) and thereby decreasing the actual dosage. Although the Cu(I)-catalyzed variant was first reported by Meldal and co-workers for the synthesis of peptidotriazoles on solid support, they needed more time to discover the full scope of the reaction and were overtaken by the publicly more recognized Sharpless. Meldal and co-workers also chose not to label this reaction type "click chemistry" which allegedly caused their discovery to be largely overlooked by the mainstream chemical society. Sharpless and Fokin independently described it as a reliable catalytic process offering "an unprecedented level of selectivity, reliability, and scope for those organic synthesis endeavors which depend on the creation of covalent links between diverse building blocks". An analogous RuAAC reaction catalyzed by ruthenium, instead of copper, was reported by the Jia and Fokin groups in 2005, and allows for the selective production of 1,5-isomers. Strain-promoted azide-alkyne cycloaddition (SPAAC) The Bertozzi group further developed one of Huisgen's copper-free click reactions to overcome the cytotoxicity of the CuAAC reaction. Instead of using Cu(I) to activate the alkyne, the alkyne is instead introduced in a strained difluorooctyne (DIFO), in which the electron-withdrawing, propargylic, gem-fluorines act together with the ring strain to greatly destabilize the alkyne. This destabilization increases the reaction driving force, and the desire of the cycloalkyne to relieve its ring strain. This reaction proceeds as a concerted [3+2] cycloaddition in the same mechanism as the Huisgen 1,3-dipolar cycloaddition. Substituents other than fluorines, such as benzene rings, are also allowed on the cyclooctyne. This reaction has been used successfully to probe for azides in living systems, even though the reaction rate is somewhat slower than that of the CuAAC. Moreover, because the synthesis of cyclooctynes often gives low yield, probe development for this reaction