Back to Table of Contents

42272-AC3
Parahydrogen Induced Polarization in Studies Relating to Catalysis and Bond Activation

Richard Eisenberg, University of Rochester

In catalysis of asymmetric hydrogenation employing rhodium and iridium catalysts, the activation of hydrogen is thought to be the enantioselective step in the catalytic cycle. To probe this feature of the mechanism further, the diastereoselectivity of H2 oxidative addition to chiral Ir(I) complexes was examined, including with the use of parahydrogen induced polarization (PHIP). Parahydrogen induced polarization is an NMR phenomenon whereby product resonances exhibit enhanced absorption and emission signals when hydrogen enriched in the para spin state adds to a metal center or an organic substrate in a pairwise manner. PHIP can thus provide mechanistic information for the H2 activation step in reactions involving hydrogen. Two chiral iridium(I) (R)-BINAP complexes, IrI(CO)((R)-BINAP) (1) and [Ir(CO)2((R)-BINAP)][SbF6] (2), were synthesized and characterized. A structure determination of complex 2 confirmed the square planar coordination geometry while that of 1 revealed both a disorder between iodide and CO ligands and a significant tetrahedral distortion from the expected planar coordination. The reaction of 1 with H2 proceeds under kinetic control and shows a high degree of kinetic and thermodynamic selectivity; the kinetic product is formed by H2 addition across the P–Ir–CO axis of IrI(CO)((R)-BINAP) and yields two diastereomers differing in metal-centered chirality which then convert over time to two more stable diastereomers corresponding to oxidative addition across the P–Ir–I axis. The kinetically favored diastereomers are formed in an initial ratio of 8.6:1 corresponding to a DDG* of 1.27 kcal/mole. The reaction of H2 with the C2 symmetric complex [Ir(CO)2((R)-BINAP)][SbF6] (2) also led to the formation of two diastereomers with one favored over the other kinetically by a 9.9:1 ratio on extrapolation to t = 0. When these reactions were followed using parahydrogen NMR methods, only one of the initially formed diastereomers in each case was found to exhibit substantial polarization in the hydride resonances at room temperature. A full report of this work was published in Inorg. Chem. 2007, 46, 1196-1204.

Recent studies of achiral Ir(III) complexes containing a diphosphine and a weakly binding chelating ligand such as diiodobenzene (DIB) have shown that these systems are effective for electrocyclization catalysis including that of the Nazarov reaction. This last reaction has been used in the total synthesis of the natural product merrilactone A for construction of a key central ring. To probe the possibility of asymmetric catalysis using such systems, complexes 1 and 2b were employed as precursors for preparing new Ir(III) complexes. A new class of tricationic systems was thus synthesized, and while no asymmetric catalysis was found, the systems were shown to be very electrophilic and highly active for Nazarov cyclization, Diels-Alder additions and an intramolecular Friedel-Crafts alkylation. The results are being finalized for publication.

Back to top