Sugar Acyl Iron Complexes: Synthesis and Conformational Studies in Solution and in the Solid State

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Abstract
A synthetic strategy based on a combination of a sugar electrophile and a transition metal nucleophile allows a metal acyl functionalization at the C-1 carbon atom of acyclic carbohydrates. Following this methodology the per-O-acetylated acyl iron complexes dicarbonylcyclopentadienyl[2,3,4,5,6-penta-O-acetyl-D-galactonyl]iron(II) (6), dicarbonyl cyclopentadienyl[2,3,4,5,6-penta-O-acetyl-D-gluconyl]iron(II) (7), dicarbonylcyclopentadienyl[2,3,4,5-tetra-O-acetyl-L-arabinonyl]iron(II) (8), and dicarbonylcyclopentadienyl-[2,3,4,5-tetra-O-acetyl-D-ribonyl]iron(II) (9) are accessible in high yields. 1H NMR studies indicate that in solution the galacto and arabino (6, 8) complexes adopt planar zigzag conformations, whereas the gluco and ribo (7, 9) analogs prefer non-extended (“sickle”) conformations. A similar conformational preference is observed for the solid state as established by X-ray studies on the galacto and gluco derivatives 6 and 7, underlining the strong impact of syn-1,3-interactions of the protecting groups on the conformation adopted.