Circular dichroism, XCIII determination of the absolute configuration of alcohols, olefins, epoxides, and ethers from the CD of their “in situ” complexes with [Rh2(O2CCF3)4].

A new, very convenient and sensitive “in situ” method has been developed for the determination of the absolute configuration of chiral compounds, which can bind to the axial position of [Rh2(O2CCF3)4] (5) and induce circular dichroism within its electronic absorption bands. The CD spectra of the “in situ” complexes of chiral alcohols, ethers (including epoxides), olefins, and ketons with 5 show up to 5 Cotton effects (A – E) in the 600 – 300 nm range. This method works well for chiral secondary alcohols and monoolefins. For these compounds tentative rules are proposed, which correlate the absolute stereochemistry with the sign of certain Cotton effects of their complexes. A rule for the sign of band E for alcohols and a quadrant rule for the sign of band C for olefins can be applied. Olefins induce unexpected strong Cotton effects in presence of 5 giving evidence for a relatively stable π-bond between olefin and rhodium atom.