Solution Structure and Chelation Properties of 2-Thienyllithium Reagents

The solution and chelation properties of 2-thienyllithium reagents with potential amine and ether
chelating groups in the 3-position and related model systems have been investigated using low
temperature 6Li, 7Li, 13C, and 31P NMR spectroscopy, 15N-labeling, and the effect of solvent additives.
In THF−ether mixtures at low temperature 3-(N,N-dimethylaminomethyl)-2-thienyllithium (4) is
ca. 99% dimer (which is chelated) and 1% monomer (unchelated), whereas 3-(methoxymethyl)-2-thienyllithium (5) is <10% dimer. Compound 5 crystallizes as a THF-solvated dimer, but there is
no indication that the ether side chain is chelated in solution. Both 4 and 5 form PMDTA-complexed
monomers almost stoichiometrically, similar to the model compound 2, in sharp contrast to phenyl
analogues, which show very different behavior. The barriers to dimer interconversion are ca. 2
kcal/mol lower and chelation is significantly weaker in the 2-thienyllithium reagents than in their
phenyl analogues.