Search

This article is a continuation of our previous research on the catalytic capability of a chiral copper complex based on commercially available ( S )-2-aminomethylpyrrolidine and 3,5-di- tert -butylsalicylaldehyde with various counter-anions in the asymmetric Henry reaction. Our findings indicate that depending on the type of base used, chiral nitroalcohols with yields up to 98% and ee values up to 77%, as well as β-nitrostyrenes with yields up to 88%, can be produced. Additionally, it has been found that the outcome of the reaction and the catalytic properties of copper (II) complexes ( S )- Cu1 and ( S )- Cu2 are influenced by the structure of the aldehyde used.

Chiral copper(II) and cobalt(III) complexes ( 1 - 5 and 6 , respectively) derived from Schiff bases of ( S )-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic study of the Henry reaction-type condensation of nitromethane and o -nitrobenzaldehyde in CH 2 Cl 2 (CD 2 Cl 2 ), containing different amounts of water. The reaction kinetics was monitored by 1 H and 13 C NMR. The addition of water had a different influence on the activity of the two types of complexes, ranging from a crucial positive effect in the case of the copper(II) complex 2 to insignificant in the case of the stereochemically inert cobalt(III) complex 6 . No experimental support was found by 1 H NMR studies for the classical Lewis acid complexation of the carbonyl group of the aldehyde by the central copper(II) ion, and, moreover, density functional theory (DFT) calculations support the absence of such coordination. On the other hand, a very significant complexation was found for water, and it was supported by DFT calculations. In fact, we suggest that it is the Brønsted acidity of the water molecule coordinated to the metal ion that triggers the aldehyde activation. The rate-limiting step of the reaction was the removal of an α-proton from the nitromethane molecule, as supported by the observed kinetic isotope effect equaling 6.3 in the case of the copper complex 2 . It was found by high-resolution mass spectrometry with electrospray ionization that the copper(II) complex 2 existed in CH 2 Cl 2 in a dimeric form. The reaction had a second-order dependence on the catalyst concentration, which implicated two dimeric forms of the copper(II) complex 2 in the rate-limiting step. Furthermore, DFT calculations help to generate a plausible structure of the stereodetermining transition step of the condensation.

A (salen)titanium catalyst has been found to induce the asymmetric addition of potassium cyanide and acetic anhydride to aldehydes, giving enantiomerically enriched cyanohydrin esters with up to 92% enantiomeric excess using just 1 mol% of the catalyst. This is the first report of the asymmetric synthesis of cyanohydrin derivatives using a cyanide source which is non-volatile and inexpensive.

The first two examples of the successful application of chiral salen ligands for diastereoselective syntheses of dinuclear Ti(IV), (L1Ti)2O2 and (PdI2L2Ti)2O2 species with defined chirality and geometry at the metal centre are presented. The structure of the complexes was established by X-ray structural analysis. Both complexes have similar structural features: (1) chiral salen ligands have cis-beta configuration around Ti centres, (2) (R,R) chirality of ligands induces delta helicity of ligand, and (3) syn arrangement of ligand donor atoms in relation to the TiO2Ti plane. Stereoselective effects of the dimers are discussed engaging available literature data. A model has been proposed to rationalise high syn selectivity in the formation of dimers.