No abstract

COMMENTIn a recent study we were involved in the synthesis and structural characterization of model complexes of the antitumour-active titanocene dichloride (TDC) with essential α-amino acids bearing a sulfur atom in their side chain, i.e. Cys, S-substituted Cys and Met. Herein, we present the structure of one of these compounds. The molecular structure of the TDC-L-Met complex 1 (Fig. 1) shows that α-amino acid ligands are coordinated to the central titanium atom exclusively through the oxygen of the carboxylic group 1,2 and that no Ti-S interaction is present. Neighbouring cations are connected through intermolecular hydrogen bonds between NH 3 + -protons of α-amino acid ligands and chloride anions. Carboxyl group structural features and C-O bond lengths and angles compare well with those found in esters. 3 Compared with TDC, 4 shortening of titanocene core bond lengths and Ti-L bond lengths, as well as changes in appropriate bond angles, was observed regarding the exchange of ligands in the cis-position; average bond distances Ti-Cp(c), Ti-L: complex 1 2.0482, 1.9694Å; TDC 2.058, 2.364Å; bond angles Cp1(c)-Ti-Cp2(c), L-Ti-L: complex 1 132. 33, 89.10; TDC 130.89, 94.43 • (L Cl or L-OOCCHNH 3 CH 2 CH 2 SCH 3 ; Cp(c) = ring centre). During the preparation of suitable monocrystals for X-ray

COMMENTThe very first example of the crystal structure of a DMTDC-α-amino acid complex is reported (DMTDC = Cp 2 TiCl 2 , where Cp = η 5 -(CH 3 )C 5 H 4 ). The α-amino acid ligands are coordinated to titanium atom exclusively 'via' the oxygen of the carboxylic group. The coordinated carboxyl group has similar types of C-O bond length as found in esters:1,2 Neighbouring cationic units are attached through H · · · O and H· · · Cl bonds within the crystal cell (Fig. 1). A close intramolecular H · · · O bond between the oxygen of the carboxylic group of one ligand and the ammonium group hydrogen of the second one represents a unique example of such interaction for this class of compounds.3 -5 Only a slight effect on the titanium-ligand bonding angles was observed, regarding exchange of chloride ligands in the cis-position for α-amino

The cyclization reactions of substituted 2-(N-benzoyl-N-methyl)aminoalkanamides 1a-1g have been studied in aqueous medium. The Hammett reaction constant is ρ = 1.4 for the cyclization reactions of compounds 1a-1e in sodium hydroxide solutions. 2-[N-Methyl- N-(4-nitrobenzoyl)amino]-2-(4-nitrophenyl)propanamide (1g) is cyclized to imidazolinone 2g in aqueous amine buffers of pH 9-11.5; the reaction is subject to specific base catalysis in these media, and the rate-limiting step is the formation of a tetrahedral intermediate. In sodium hydroxide solution, the primary cyclization product is hydrolyzed to give an intermediate M which is subsequently decomposed to sodium 4-nitrobenzoate and 2-methylamino- 2-(4-nitrophenyl)propanamide. At low sodium hydroxide concentration, the rate-limiting step of the opening of imidazoline ring of compound 2g is non-catalyzed decomposition of the intermediate. At higher sodium hydroxide concentrations, the other reaction path begins to make itself felt: hydroxide-ion-catalyzed decomposition of the intermediate. The dependence of observed rate constant of cyclization of compound 1f on sodium hydroxide concentration was used to determine kinetically the value of pKa = 13.5 ± 0.1. The kinetic deuterium isotope effect of cyclization of compounds 1f giving 2f (kCH/kCD = 1.7) was determined in solutions of NaOD in D2O.

No abstract