[Molecules: 32] [Related articles/posters: 079 025 024 104 066 ]

Selective halogen migration reactions at mixed halothiophenes

J. Fröhlich and B. Winter

Institute of Organic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria

Introduction

Amide induced halogen dance (HD) reactions at aromatic and heteroaromatic compounds are well known in the literature although significant disadvantages have not allowed their preparative application:

Reactions are equilibrium driven and therefore mainly produce mixtures
In some cases the desired reaction products are accompanied by amino-derivatives via competitive substitution

Within our research group what we see as a major breakthrough towards preparative applicability of this reaction type has been achieved: the final intermediates are organolithium compounds, which allow introduction of further substituents by reaction with various electrophiles.

Moreover, extension of the synthetic scope by selectively controlling halogen dance vs. its prevention now allows access to a variety of specifically substituted heteroaromatic compounds.

As a result of these studies a series of new tri-substituted thiophenes and furans was obtainable from 2-bromo-(3 or 5)-R-thiophenes or furans (R=Br, CH₃, SiMe₃, SnBu₃). see references

Until now no investigations utilizing "mixed" di-halogenated thiophenes as starting materials for HD chemistry have been made.

We now report the first sucessful HD reactions of "mixed" halo-substituted thiophenes. As a result we have discovered that the iodo-substituent migrates preferentially over the bromo-substituent, and the bromo-substituent over the chloro-substituent.

The present work aims at selective migration of one of the two halogen atoms followed by subsequent reaction with various electrophiles in order to enable access to new thiophene derivatives with a 2-E-3,5-di-halo substitution pattern.