N-Heterocyclic carbene complexes of rhodium: structures, dynamics and catalysis

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.

Synthesis and conformational analysis of heteroaromatic atropisomeric systems

The research work reported in this Thesis was held along two main lines of research. The first and main line of research is about the synthesis of heteroaromatic compounds with increasing steric hindrance, with the aim of preparing stable atropisomers. The main tools used for the study of these dynamic systems, as described in the Introduction, are DNMR, coupled with line shape simulation and DFT calculations, aimed to the conformational analysis for the prediction of the geometries and energy barriers to the trasition states. This techniques have been applied to the research projects about: • atropisomers of arylmaleimides; • atropisomers of 4-arylpyrazolo[3,4-b]pyridines; • study of the intramolecular NO2/CO interaction in solution; • study on 2-arylpyridines. Parallel to the main project, in collaboration with other groups, the research line about determination of the absolute configuration was followed. The products, deriving form organocatalytic reactions, in many cases couldn’t be analyzed by means of X-Ray diffraction, making necessary the development of a protocol based on spectroscopic methodologies: NMR, circular dichroism and computational tools (DFT, TD-DFT) have been implemented in this scope. In this Thesis are reported the determination of the absolute configuration of: • substituted 1,2,3,4-tetrahydroquinolines; • compounds from enantioselective Friedel-Crafts alkylation-acetalization cascade of naphthols with α,β-unsaturated cyclic ketones; • substituted 3,4-annulated indoles.