Atropselection in planar chiral new polyfunctionalized Cp Rh complexes

There is a great interest in chiral-economy syntheses to form molecules to be used in asymmetric synthesis (1). The cyclopentene oxide [CH(CH2)3CH]O opening by NaCp (Cp=C5H5) forms alcohol-tethered Cyclopentadienyl species Cp* [C5H4(CH(CH2)3CHOH)] in high yield (70-80%) and moderate conditions (DME, rfx). Twofold excess of oxirane, or alternatively two sequenced additions, gives raise to the (1,3)-substituted Cp** [C5H3(CH(CH2)3CHOH)2]. The addition is statistically directed to one of the two vicinal stereocentres belonging to cyclopentene oxide, which central chirality are oppositely defined by geometrical reasons. For steric reason, further incorporated side arm exclusively exhibits opposite chiral configuration with respect to the adjacent one. This discrimination has to be ascribed to steric congestion, due to rigid and demanding wing-span cycloalkyl-substituents. The geometry-constrain prevents further attack at intermediate position. The encumbered Rh(NBD) fragment is driven selectively to the less congested enantioface and among all the possible isomers (16) forms exclusively two epimeric atropisomers. They show identical planar chirality (M), but opposite chiral configuration sequence of the stereocentres, respectively [1S,R 2R,S 4S,R] and [1S,R 2R,S 4R,S]. The enantioface stereoselection is likely responsible for chiroptical properties exhibited in solution by ORD-CD experiments (positive Cotton effect, 25D negative value). The target reaction, starting from racemic reactants constitutes a further example of chiral economy synthesis. Two distinct conformations with identical central and planar chiral configuration, but differing from reciprocal OH groups location, have been frozen by crystallization and structurally determined by X-ray diffraction studies. Analogous structures relative to Cp***RhL2 (L2 = CO, CH2CH2, COD, NBD) are attributed in solution by performing bidimensional and n.O.e. NMR experiments. (1). Zhao H.; Hsu D.C.; Carlier P.R. Synthesis 2005, 1, 1-16; J. Gawroński J.; Gawrońska K.; Kwit M.; Kacprzak K.; Rychlewska U. Chirality 2004, 16, 405-413.