Double bonds in motion:: Bis(oxazolinylmethyl)pyrroles and their metal-induced planarization to a new class of rigid chiral C2-symmetric complexes

The synthesis of a new class of chiral C-2-symmetric tridentate N-donor ligands, a series of 2,5-bis(2-oxazolinylmethyl)pyrroles, was achieved in four steps starting from the known 2,5-bis(trimethylammoniomethyl)pyrrole diiodide (1). Reaction of 1 with NaCN in dimethyl sulfoxide gave 2,5-bis(cyanomethyl)pyrrole (2) cleanly, which was then cyclized with amino alcohols to give the 2,5-bis(2-oxazolinylmethyl)pyrroles 3a-c (3a: bis[2-(4,4'-dimethyl-5-hydrooxazolyl)methyl]pyrrole; 3b: (S,S)-bis[2-(4-isopropyl-4,5-dihydrooxazolyl)methyl]pyrrole; 3c: (S,S)-bis[2-(4-tertiobutyl-4,5-dihydrooxazolyl)methyl]- pyrrole). Metallation of 3a-c with one molar equivalent of tBuLi and their subsequent reaction with a stoichiometric amount of [PdCl2(cod)] (cod = cyclooctadiene) gave the palladium((II)) complexes 4a-c. Whereas the arrangement of the N-donor atoms in the crystallographically characterized complex 4a is almost ideally square planar, all three heterocycles in the ligand are twisted out of the coordination plane, leading to a chiral conformation of the complex. Attempts to freeze out these two conformers in solution at 200 K (NMR) failed, and this suggests that the activation barrier for conformational racemization is significantly below 10 kcal mol(-1). The palladium-induced shift of two double bonds as well as the porphyrinogen/porphyrin-type oxidation of the complexes 4a-c led to the planarization of the 2,5-bis(oxazolinylmethyl)pyrrolide ligands in the palladium(n) complexes 5a-c, 6b, and 6c, and to the formation of rigid chiral C-2-symmetric systems as shown by X-ray diffraction studies. The formation of the conjugated system of double bonds in this transformation is accompanied by the emergence of an intra-ligand chromophore. This is evident in the absorption spectrum of 6 c which displays an intense band with a maximum at 485 nm attributable to an intra-ligand pi* <-- pi transition and a characteristic vibrational progression of v approximate to 1350 cm(-1). Complexes 4b and 4c were tested in the catalytic asymmetric Michael addition of ethyl 2-cyanopropionate to methyl vinylketone (catalyst loading: 1 mol %) and were found to give maximum ee values of 43 % (4b) and 21 % (4c) at low conversions.