A Highly Efficient Preparation of 3,3′-Dipyrrolyl Sulfides

Abstract

Dipyrrolyl sulfides, useful building blocks for linear or cyclic polypyrrolic ligands, were prepared, in high yield, by the reaction of sulfur dichloride with 3-unsubstituted pyrroles at low temperature. Recently, our laboratory has developed several novel dipyrromethene ligands to build supramolecular assem- blies, such as double helical dimers 1 and macrocyclic tri- mers. 2 Each dipyrromethene unit was linked by one or more methylene groups. To extend our work, we were in- terested in preparing dipyrromethene ligands linked by a sulfur bridge. 3,3¢-Dipyrrolyl sulfides are the key precur- sors for the syntheses of such ligands Symmetric 2,2¢-dipyrrolyl sulfides have been known for many years and have been used for the preparation of meso-thiaporphyrins. 3 Unsymmetrical 2,2¢-dipyrrolyl sul- fides were reported in 1994 by Scott's group. 4 A literature survey 5 showed only one example of a 3,3¢-dipyrrolyl sul- fide, which was prepared from the reaction of a 3-unsub- stituted pyrrole with sulfur dioxide using AlCl 3. 6 In this case 2,4-dimethyl-5-ethoxycarbonylpyrrole gave a yield of bis(2,4-dimethyl-5-ethoxycarbonyl-pyrrole-3-yl)sul- fide of about 43%. Gaseous SO2 is less convenient to han- dle than SCl2, which we report here, and which provides an efficient means to 3,3¢-dipyrrolyl sulfides.