Samarium(II) diiodide mediated coupling of bis(bromomethyl)arenes. Facile synthesis of [2n]cyclophanes without using high-dilution techniques
Abstract
In the presence of samarium(II) diiodide (Sml2), coupling of 1,3- and 1,4-bis(bromomethyl)benzenes proceeds smoothly to give their cyclic and acyclic oligomers. Use of 1,2 equiv of Sml2 gives 1,2-bis(3- and 4-bromomethylphenyl)ethanes, respectively, in 95% yield, and the use of 5.0 equiv. of Sml2 affords [2n]metacyclophanes and [2n]paracyclophanes (n= 2, 3 and 4), respectively, in satisfactory yields. By using 5.0 equiv. of Sml2, 2-bromo- and 2-methoxy-1,4-bis(bromomethyl)benzenes and 1,4- and 2,6-bis(bromomethyl) naphthalenes each give only one of several expected isomeric [2.2] cyclophanes as the major cyclic oligomer.