SUBJECTS enantiomorphism

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Some achiral organic compounds form chiral crystals, with each crystal exhibiting one of two possible enantiomorphs [9-13]. These chiral crystals composed of an achiral organic compound may serve as an efficient chiral seed in a prebiotic world, therefore, a study of asymmetric autocatalysis using these chiral organic crystals is an interesting subject. 

Independent synthesis of the crystalline amide LXVII established its identity and its configurational relationship to L-(+)-mandelic acid. The latter acid was converted to ethyl L-(+)-mandelate (LXVIII), and the ether linkage introduced by reaction with ethyl bromoacetate in the presence of silver carbonate, under conditions such that Walden inversion was impossible. The resulting ethyl D-(+)-2-phenyldiglycolate (LXVI), was subjected to ammonolysis, giving a crystalline product, m. p. 174-174.5°, [a]26D 106.2°. This showed no mixed melting point depression and an identical infrared absorption spectrum with the sample of LXVII obtained from /3-D-xylopyranosylbenzene. The enantiomorphic l-(—)-2-phenyldiglycolamide was also prepared by identical synthetic steps from d-( —)-mandelic acid. 

The subject of isomerism is closely concerned with symmetry. Consider, for example, the three molecules (i) O2X2, (ii) N2X2, and (iii) C2X2. In a molecule of type (i), formed by oxygen and sulphur, the two X atoms are not coplanar with the two 0 (S) atoms the molecule is enantiomorphic, that is, it cannot be brought into coincidence with its mirror image it exists in left- and right-handed forms. 

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