Haloaldehyde polymers, 43.. Structure of 2‐bornyloxy‐terminated chloral oligomers

Abstract

Abstract Oligomerization of trichloroacetaldehyde (chloral) was carried out with the lithium salt of (1 S , 2 R , 4 S )‐(–)‐borneol as the initiator. The unimer and dimer fractions consisted of two and four diastereomers, respectively; the individual isomers were isolated from the fractions by crystallization from methanol or by high performance liquid chromatography. The X‐ray single‐crystal analysis showed that the major isomer in the unimer fraction is the ( R )‐form which has the acetal carbon in ( R )‐configuration. The diastereomeric ratios were determined by 1 H NMR as R / S = 57/43 for the unimers and RS/SR/RR/SS = 34/25/23/18 for the dimers. The conformations of the meso‐dimers [( R , R )‐ and ( S , S )‐isomers] in the crystalline form corresponded to a half turn of the 4 1 ‐helix of isotactic polychloral, whereas the conformations of the racemo‐dimers (the ( R , S )‐ and ( S , R )‐isomers) were different from the helical structure in the crystal and in solution. The trimer fraction consisted of six of the eight possible diastereomers; among these isomers, the mm isomers (the ( R , R , R )‐ and ( S , S , S )‐isomers) were highly predominant. The amount of the ( R , R , R )‐trimer formed was only slightly larger than that of the ( S , S , S )‐isomer. (a) Part 42, cf. G. D. Jaycox, O. Vogl, J. Am. Chem. Soc. , submitted.