The Chemo‐ and Stereoselective Formation of Pallado‐ and Platinocryptophanes

Abstract

Cyclotricyanobenzylene (CTB) 1 was resolved by HPLC on chiral stationary phase and the absolute configurations of its enantiomers were determined. Optically‐active pallado‐ and platinocryptophanes assembled separately from (+)‐1 and (–)‐1, and the optical properties of [M3(dppp)3((+)‐1)2](OTf)6 and [M3(dppp)3((–)‐1)2](OTf)6 (M = Pd and Pt; dppp = 1,3‐bis(diphenylphosphanyl)propane) were studied by polarimetry and electronic circular dichroism (ECD). The enantiomerization barriers of the metallocryptophanes depend only weakly on the nature of the metal. TD‐DFT calculations demonstrated the occurence of induced ECD effects from the optically active CTB 1 to the metal complex fragments. Comparison of the 1H NMR spectra of the optically‐pure and racemic metallocryptophanes confirmed that the latter formed diastereoselectively and enantioselectively. Self‐assembly of metallocryptophanes from two different CTBs (1 and 2) gave clear evidence of chemoselectivity for the process, using 1H NMR spectroscopy and ESI‐MS.