EVIDENCE OF STEREOELECTRONIC CONTROL IN OSMIUM-CATALYZED CIS-DIHYDROXYLATIONS OF STERICALLY UNBIASED 3,3-DIARYLCYCLOPENTENES

This study provides strong evidence for the stereoelectronic control of diastereoselectivity in the osmium tetraoxide-catalyzed cis-dihydroxylation of sterically unbiased 3-(4-X-phenyl)-3-phenylcyclopentenes 1 (X = NO2, Br, Cl, OCH3, N(CH3)2), producing diastereomeric cis-diols 2 in cis:trans ratios (diol relative to the substituted arene) varying from 70:30 to 36:64 as determined by H-1 NMR and C-13 NMR spectroscopy. The reactions were carried out at room temperature in an 8:1 acetone/water solvent system, utilizing trimethylamine N-oxide as the terminal oxidant. Structural assignments were made by 2D-NOE H-1 NMR spectroscopy, C-13 NMR spectroscopy, and X-ray crystallographic analysis. In all cases the addition occurred opposite the more electron-rich aromatic ring as predicted by Cieplak's theory for explaining stereoelectronic control. The observed ratios correspond to an overall energy difference of 1.1 kcal/mol. A Hammet plot of log (cis:trans) versus the sigma(p), parameter produced a linear relationship with a correlation coefficient of 0.97. An efficient synthesis of the diarylcyclopentenes is described. The diastereomeric diols, or derivatives thereof, were separable by preparatory thin-layer chromatography.