Mechanism of the lithium–iodine exchange in an iodothiophene

Abstract

Solutions of 2-lithio-5-methylthiophene (4) were characterized using DNMR techniques and shown to be a mixture of monomer and dimer in THF–Et2O (3:2). The hypervalent iodine ate complex 5 (Ar2I–Li+), a presumed intermediate in the Li–I exchange with 2-iodo-5-methylthiophene, was observed by 13C and 7Li NMR spectroscopy at low temperatures (–130 °C). At higher temperatures, the ate complex coalesced with 2-lithio-5-methylthiophene. A kinetic scheme was developed, which accounts for the exchange of the monomer 4M, dimer 4D, and 2-iodo-5-methylthiophene (6) with the ate complex 5. The rates of the various exchanges were obtained through a DNMR analysis of the variable temperature 13C and 7Li NMR spectra, and the thermodynamic and activation parameters were calculated. The monomer 4M and the ate complex 5 have similar reactivity as aryl donors in the Li–I exchange reaction, but 4M is at least 1000 times as reactive as the dimer 4D towards the iodide.Key words: halogen–metal exchange, lithium iodinate, iodine ate complex, lithium reagent, aggregate reactivity.