Flow Macrocyclization Using Copper Tubing

Key words

flow chemistry - macrocycles - copper tubing - click chemistry

Significance

Flow macrocyclization with a ­copper surface catalyst via the azide-acetylene cycloaddition reaction was described. The flow macrocyclization of an azidealkyne 1 with TTTA and DIPEA was performed in copper tubing (3 m length, 0.75 mm inner diameter) to give the tri­azole-containing macrocycle 2 in 73% isolated yield within five minutes of residence time without addition of extraneous copper(I) salt in the reaction mixture. The structure of the 12-memberd macrocycle 2 was confirmed by X-ray crystallography.

Comment

A series of 12- to 22-membered ­macrocycles was prepared under similar flow ­conditions (14 examples, 28-90% yield). Elemental analysis in the ethanolic reaction mixture indicated less than 5 ppm of copper. In flask reactions, refluxing the reaction mixture of azidealkyne 1, TTTA, and DIPEA in ethanol with copper turnings gave no detectable product 2 after five minutes, and only a trace amount of 2 after 90 min. Under similar conditions, CuI (0.2 or 1.0 equiv) was also not effective.