S. Boiadjiev, D. Lightner
Aug 27, 2002
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Journal
Tetrahedron-asymmetry
Abstract
Abstract As observed by NMR, iodopyrroles 1a and 1b (ethyl and methyl 3,5-dimethyl-4-[(1′-iodo-2′,2′-dimethyl)propyl]pyrrole-2-carboxylate) and a variety of related derivatives with iodine replaced by methoxy 2 , thiomethyl 3 , acetic acid esters 4 , propionic acid ester 5 or malonic esters 6 exhibit restricted rotation about the C(4)C(1′) bond due to the bulky tert -butyl group and an ortho effect from the sterically crowded 3,5-dimethylpyrrole. Most of the compounds, which are members of the rare class of atropisomers due to restricted rotation about an sp 3 – sp 2 CC bond, undergo diastereomeric enrichment by preparative TLC and crystallization. From dynamic NMR studies of the enriched diastereomers one can determine kinetic and thermodynamic parameters associated with the atropisomerism, e.g., Δ G ‡ ∼24 kcal/mol for 1 and 5 (313 K), ∼22 kcal/mol for 3 (273 K), and ∼25 kcal/mol for 6 (313 K) in C 2 D 2 Cl 4 solvent.