Haloaldehyde polymers, 40. Stereostructure of chloral oligomers prepared with lithium tert‐butoxide: Helical conformation of the linear isotactic oligomers in crystal and in solution
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Title:
Haloaldehyde polymers, 40. Stereostructure of chloral oligomers prepared with lithium tert‐butoxide: Helical conformation of the linear isotactic oligomers in crystal and in solution
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<jats:title>Abstract</jats:title><jats:p>The stereostructures of linear chloral oligomers were determined by X‐ray single crystal analysis. The chloral oligomers from dimer to pentamer were isolated by distillation and by GPC fractionation from the oligomer mixture prepared with <jats:italic>t</jats:italic>‐BuOLi initiation and acetate endcapping. The dimer fraction was a diastereomeric mixture of the <jats:italic>racemo</jats:italic> (<jats:bold>1</jats:bold>) and <jats:italic>meso</jats:italic> (<jats:bold>2</jats:bold>) isomers, while the trimer, tetramer and pentamer fractions each contained a single diastereomer (<jats:bold>3, 4</jats:bold> and <jats:bold>5</jats:bold>). The single crystals of <jats:bold>2, 3, 4</jats:bold> and <jats:bold>5</jats:bold> grown from methanol solutions belong to the space groups <jats:italic>P1</jats:italic><jats:sup>−</jats:sup>, <jats:italic>P2</jats:italic><jats:sub>1</jats:sub>/<jats:italic>n</jats:italic>, <jats:italic>P2</jats:italic><jats:sub>1</jats:sub>/<jats:italic>a</jats:italic> and <jats:italic>Pca2</jats:italic><jats:sub>1</jats:sub>, respectively. X‐ray analyses showed that the acetal backbones of <jats:bold>2, 3</jats:bold> and <jats:bold>4</jats:bold> consist exclusively of <jats:italic>meso</jats:italic> dyads. The backbone arrangements of <jats:bold>2, 3</jats:bold> and <jats:bold>4</jats:bold>, are essentially identical and approximate the repeat <jats:italic>gauche</jats:italic>(+)‐<jats:italic>skew</jats:italic>(−) sequences from the <jats:italic>t</jats:italic>‐BuO group to the acetyl terminal group (for <jats:italic>S</jats:italic>‐configurations). The conformation of (<jats:italic>S</jats:italic>,<jats:italic>S</jats:italic>,<jats:italic>S</jats:italic>,<jats:italic>S</jats:italic>)‐<jats:bold>4</jats:bold> is very similar to a unit sequence of a 4/1‐helical structure of isotactic polychloral with left‐handed helicity. The axis of the helix is parallel to the <jats:italic>c</jats:italic>‐axis of the crystal, and the repeat distance is 4,81 Å. The conformations of the acetal backbones of <jats:bold>4</jats:bold> and <jats:bold>5</jats:bold> in solution were also investigated by NMR spectroscopy, on the basis of dihedral angle dependence of <jats:sup>3</jats:sup><jats:italic>J</jats:italic><jats:sub>COCH</jats:sub> coupling constants.</jats:p>