The Chemo- and Stereoselective Formation of Pallado- and Platinocryptophanes

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 [M-3(dppp)(3)((+)-1)(2)](OTf)(6) and [M-3(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 H-1 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 H-1 NMR spectroscopy and ESI-MS.