An unexpected and easy way of freezing the configuration of a triaryl phosphane oxide

Configurationally stable triaryl phosphane oxides are important for reactions with transfer of chiral information. Apart from introducing bulky substituents to suppress fast inversion of helicity at room temperature, the use of a second chiral element which induces chirality in the triaryl phosphane oxide, so that it adopts only one configuration, is suitable. With regard to chirality transfer, C(2)-symmetric imidazole cyclopeptides were tested for obtaining a configurationally stable phosphane oxide. Density functional calculations showed almost equal energies of the three possible triaryl phosphane oxides (MMM)-1, (PPP)-1, and (MP)-1. Surprisingly, after synthesis only the MMM conformer is present in solution, and its configurational stability was proved by variable-temperature and 2D NMR experiments as well as CD measurements. In view of the results of the DFT calculations, formation of stable (MMM)-1 cannot be explained thermodynamically but by kinetic reaction control. This concept of freezing the conformation of a triaryl phosphane oxide can in future be used to easily prepare configurationally stable stereoisomeric propellerlike compounds.