Recognition of axially chiral compounds with soft-base functionalities by the dirhodium method

Helmut Duddeck, ^a; S. Moeller ^a; A. Simon ^b; G. Tóth ^b; J. Drabowicz ^c

and K. Michał Pietrusiewicz^d

^a Institut für Organische Chemie, Universität Hannover, Germany

^b Institute for Gene­ral and Analytical Chemistry of the Budapest University of Technology and Economics, Budapest, Hungary

^c Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Department of Organic Sulfur Compounds, Łódź, Poland

^d Maria Curie-Sklodowska University, Department of Organic Chemistry, Lublin, Poland, and Polish Academy of Sciences, Institute of Organic Chemistry, Warsaw, Poland

Spirochalcogenuranes as shown in the formula scheme contain hypervalent chalcogen atoms with trigonal bipyramidal ge­o­metry and exhibit chirality even if both ligand arms are equal. They act as strong donors and form dia­ste­reo­meric adducts with the chiral enantiopure dirhodium com­plex Rh*. The individual adduct species can be identified and stereo­chemically assigned by low-tempe­ra­ture ¹H NMR spectroscopy. Recording ¹H NMR spectra of these adducts is an excel­lent me­thod for chi­ral dis­crimination in this class of compounds. In 2:1-ligands each ligand molecule attached to one rho­dium atom can re­cog­­nize which enantiomer is at the other rhodium on the back-side of the dirho­di­um com­plex cage. The signs of dispersion effects (non-racemic mixtures) indi­cate that establishing a rule for the deter­mi­na­tion of ab­so­lute configurations is possible in the spirochalcogenurane system. Some additional evi­dence about the structure of the adducts emerges from NOE expe­ri­ments; the spa­tial contact of the ge­mi­nal methyl groups in the spirochalco­ge­nur­anes depends strongly of their overall size.

Alkyl and aryl iodides are unpolar compounds which refuse to establish stable interactions with con­ventonal NMR auxiliaries. However, the soft acid dirhodium complex Rh* is capable of form labile adducts with organoiodine compounds although iodine is only a weak donor. Diastereomeric dispersions of room-temperature ¹H NMR signals can be observed after mix­ing bis(2-iodophenyl) de­ri­vatives with an equimolar of Rh* ranging up to 10 Hz (at 400 MHz field strength).