Double Bonds in Motion: Bis(oxazolinylmethyl)pyrroles and Their Metal‐Induced Planarization to a New Class of Rigid Chiral C2‐symmetric Complexes

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Abstract
The synthesis of a new class of chiral C2‐symmetric tridentate Ndonor 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 3 a–c (3 a: bis[2‐(4,4′‐dimethyl‐5‐hydrooxazolyl)methyl]pyrrole; 3 b: (S,S)‐bis[2‐(4‐isopropyl‐4,5‐dihydrooxazolyl)methyl]pyrrole; 3 c: (S,S)‐bis[2‐(4‐tertiobutyl‐4,5‐dihydrooxazolyl)methyl]pyrrole). Metallation of 3 a–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 4 a–c. Whereas the arrangement of the Ndonor atoms in the crystallographically characterized complex 4 a 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 4 a–c led to the planarization of the 2,5‐bis(oxazolinylmethyl)pyrrolide ligands in the palladium(II) complexes 5 a–c, 6 b, and 6 c, and to the formation of rigid chiral C2‐symmetric systems as shown by Xray 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 π*←π transition and a characteristic vibrational progression of ν≈1350 cm−1. Complexes 4 b and 4 c were tested in the catalytic asymmetric Michael addition of ethyl 2cyanopropionate to methyl vinylketone (catalyst loading: 1 mol %) and were found to give maximum ee values of 43 % (4 b) and 21 % (4 c) at low conversions.

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