Search

A series of imidazolium salts of the type [BocNHCH2CH2ImR]X (Boc = t-Bu carbamates; Im = imidazole) (R = Me, X = I, 1a; R = Bn, X = Br, 1b; R = Trityl, X = Cl, 1c) and [BnImR’]X (R’ = Me, X = Br, 1d; R’ = Bn, X = Br, 1e; R’ = Trityl, X = Cl, 1g; R’ = tBu, X = Br, 1h) bearing increasingly bulky substituents were synthetized and characterized. Subsequently, these precursors were employed in the synthesis of silver(I)-N-heterocyclic (NHC) complexes as transmetallating reagents for the preparation of rhodium(I) complexes [RhX(NBD)(NHC)] (NHC = 1-(2-NHBoc-ethyl)-3-R-imidazolin-2-ylidene; X = Cl; R = Me, 4a; R = Bn, 4b; R = Trityl, 4c; X = I, R = Me, 5a; NHC = 1-Bn-3-R’-imidazolin-2-ylidene; X = Cl; R’ = Me, 4d, R’ = Bn, 4e, R’ = Trityl, 4g; R’ = tBu, 4h). VT NMR studies of these complexes revealed a restricted rotation barriers about the metal-carbene bond. While the rotation barriers calculated for the complexes in which R = Me, Bn (4a,b,d,e and 5a) matched the experimental values, this was not true for the complexes 4c,g, bearing a trityl group for which the values are much smaller than the calculated ones. Energy barriers for 4c,g, derived from a line shape simulation, showed a strong dependence on the temperature while for 4h the rotational energy barrier is stopped at room temperature. The catalytic activity of the new rhodium compounds was investigated in the hydrosilylation of terminal alkynes and in the addition of phenylboronic acid to benzaldehyde. The imidazolium salts 1d,e were also employed in the synthesis of new iron(II)-NHC complexes. Finally, during a six-months stay at the University of York a new ligand derived from Norharman was prepared and employed in palladium-mediated cross-coupling.

N-Heterocyclic Carbene Rhodium Complexes: Conformational Analysis and Catalytic Studies G. Solinasa, M. A. Bruckaa, M. C. Cassania, C. Femonia, R. Mazzonia, M. Mancinellib, A. Mazzantib aDipartimento di Chimica Fisica ed Inorganica, Università di Bologna, viale Risorgimento, 4, Bologna bDipartimento di Chimica Organica “A. Mangini”, Università di Bologna, viale Risorgimento, 4 Bologna gavino.solinas2@unibo.it N-Heterocyclic carbenes (NHCs) have attracted considerable attention as a new class of ligands over the past decade1 and recently our research in this field has been dealing with the syntheses of rhodium(I) complexes presenting axial chirality and catalitically active in the hydrosilylation of terminal alkynes and in the addition of arylaldehydes with phenylboronic acid.2 We now present the synthesis, characterization (X-ray, VT NMR, DFT calculation) and catalysis of the new NHC-Rh(I) complexes shown below. Preliminary studies on NHC-Fe(II) complexes will be also presented and discussed. References 1. Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. (b) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. 2, Busetto L.; Cassani, M.C.; Femoni, C.; Mancinelli, M.; Mazzanti, A.; Mazzoni, R.; Solinas, G. Organometallics 2011, 30, 5258.

Metal complexes of nitrogen-donor ligands exhibit some of the most interesting stoichiometric and useful catalytic transformations described in the chemical literature. 1H-pyridin-(2E)-ylidene and related N- donor ligands (Figure 1) have been described recently and their coordination chemistry and catalytic chemistry investigated. [1] A related class of compound can be derived from norharman, a β-carboline, that has wide biological relevance, but no metal coordination chemistry has been reported. Here we describe the preparation of norharman derived ligands and their metal complexes. IR spectroscopic and X-ray structural data (Figure 1) indicate that this class of ligand is a greater electron-donor than the N-heterocyclic carbenes.[2,3] Preliminary catalytic data will also be presented.

Imidazolium salts incorporating functional groups represent a class of compounds having great versatility due to their wide range of applications as task specific ionic liquids (ILs)1 or N-heterocyclic carbene precursors (NHCs)2. Recently our research in this field has been dealing with the high-purity synthesis of t-butyloxylcarbonyl (Boc)-protected and deprotected amino-functionalized imidazolium cation [NHR1CH2CH2ImR2]+ ([1]+, R1 = Boc, H; R2 = Me, Bz, Allyl) and related NHC-Ag(I) silver complexes have been obtained by reacting [1]+ with Ag2O.3 Here we report the synthesis of new NHC-Rh(I) (2) complexes that contain an axis of chirality. The enantiomerical inversion has been investigated by variable-temperature NMR spectroscopy, DFT calculation and X-ray diffraction. The catalytic behaviour of 2 in the 1,2 addition reaction of aryl aldehydes with boronic acid will be also presented and discussed.