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The cover picture shows two conformations of the chiral chelate cation [{ฮบ-P(Mes) 2 CH 2 CH(OH)CH 2 -ฮบ-P(Ph 2 }Rh(ฮท 4 -COD)] ฯฉ which is active as an enantioselective hydrogenation catalyst. The ฮด-and ฮป-twist conformations of the six-membered diphosphane chelate cycle are shown together with a projection of the hypersurface relevant to the transition between these enantiomeric forms. The hypersurface is generated by force field calculations. The specific force field is derived by refinement of the relevant force field parameters by genetic algorithms on the basis of eleven compounds of this type. The predictions made by this model are in almost quantitative agreement with the experimental data: The ฮป-form is calculated to be 3.1 kJโขmol ฯช1 more stable than the ฮด-form (experimental: 3.4 kJโขmol ฯช1 ). The ฮด โ ฮป transition is predicted to be accompanied by a coupled rotation of the mesityl groups (color code in the cover picture) in full agreement with the experimental results. The upper threshold of the activation barrier of the transition is calculated to be 69.1 kJโขmol ฯช1 ; the experimental value is 64.4โขkJ mol ฯช1 . This novel approach to molecular modeling is described in the article by G. Huttner et al. on p. 3111 ff. The quantitative NMR analysis of the structure and dynamics of compounds containing six-membered diphosphane chelate cycles is presented in the accompanying article by G. Huttner et al. on p. 3129 ff.
MICROREVIEW Contents
๐ SIMILAR VOLUMES
The cover picture shows the [2ฯซ2] grid, [Cu 1 4 (2ฯฉ2) 2 ) 4ฯฉ , of four copper(I) ions and two (2ฯฉ2) Schiff-base macrocycles, which self-assembles when copper(I) ions are used to template the reaction of 3,6-diformylpyridazine with 1,3-diaminopropane. In each case the two parallel strands of the [2ฯซ2