agent makes up for any oxygen which may have gotten into the system. Oxygen oxidises the Cu(I) to Cu(II) which impedes the reaction and results in low yields. One of the more commonly used Cu compounds is CuSO4. Oxidation of Cu(0) metal Halides of copper may be used where solubility is an issue. However, the iodide and bromide Cu salts require either the presence of amines or higher temperatures. Commonly used solvents are polar aprotic solvents such as THF, DMSO, acetonitrile, DMF as well as in non-polar aprotic solvents such as toluene. Neat solvents or a mixture of solvents may be used. DIPEA (N,N-Diisopropylethylamine) and Et3N (triethylamine) are commonly used bases. Mechanism A mechanism for the reaction has been suggested based on density functional theory calculations. Copper is a 1st row transition metal. It has the electronic configuration [Ar] 3d10 4s1. The copper (I) species generated in situ forms a pi complex with the triple bond of a terminal alkyne. In the presence of a base, the terminal hydrogen, being the most acidic, is deprotonated first to give a Cu acetylide intermediate. Studies have shown that the reaction is second order with respect to Cu. It has been suggested that the transition state involves two copper atoms. One copper atom is bonded to the acetylide while the other Cu atom serves to activate the azide. The metal center coordinates with the electrons on the nitrogen atom. The azide and the acetylide are not coordinated to the same Cu atom in this case. The ligands employed are labile and are weakly coordinating. The azide displaces one ligand to generate a copper-azide-acetylide complex. At this point cyclization takes place. This is followed by protonation; the source of proton being the hydrogen which was pulled off from the terminal acetylene by the base. The product is formed by dissociation and the catalyst ligand complex is regenerated for further reaction cycles. The reaction is assisted by the copper, which, when coordinated with the acetylide lowers the pKa of the alkyne C-H by up to 9.8 units. Thus under certain conditions, the reaction may be carried out even in the absence of a base. In the uncatalysed reaction the alkyne remains a poor electrophile. Thus high energy barriers lead to slow reaction rates. Ligand assistance The ligands employed are usually labile i.e. they can be displaced easily. Though the ligand plays no direct role in