Discrimination of Enantiofaces and Stereoselective Electrophilic Addition Reactions for Pyrrole Complexes

A series of asymmetric N-substituted pyrroles (1−7) has been synthesized from amino acid derivatives and complexed to the pentaammineosmium(II) fragment. Many of these pyrrole complexes (8−14) show a thermodynamic preference for one coordination diastereomer according to their NMR spectra (−5 to −20 °C). This stereoselective coordination results in stereoselective electrophilic addition at the uncoordinated β carbon (C3) when the complexes are treated with trifluoromethanesulfonic acid (HOTf), methyl triflate, or dimethoxymethane. These stereoselective reactions at C3 are a direct result of differentiation of the pyrrole enantiofaces.