Supramolecular hydrogen-bonding tautomeric sulfonamido-phosphinamides: A perfect p-chirogenic memory

P-chirogenic, supramolecular hydrogen-bonding C₁-symmetrical sulfonamido-phosphinamides (METAMORPhos) have been successfully prepared. They were all found to possess a characteristic prototropic equilibrium between the P^III and the P^V tautomers that is slow on the NMR spectroscopic timescale. Despite the dynamic and reversible protonation of the P centre, the P-chirogenic information was found to be retained in the tautomerization process, even in a protic solvent environment. Several strategies to access the corresponding enantiopure compounds were investigated, such as diastereoselective crystallization, or enantioselective synthesis. It was possible to resolve such a class of chiral ligands with up to 99 % ee, and apply them in the Rh-catalyzed asymmetric hydrogenation of alkenes. These ligands are interesting new building blocks in the area of acid/base-type noninnocent ligand catalysis.