The formation reaction of chiral racemic and diastereoisomeric title complexes, (Lambda, Delta)-[Cr{(+/-)-Mebdtp}(3)] and Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)], was monitored by absorption, UV/Vis, and Circular Dichroism, CD, spectroscopy, in the presence of either an excess of the racemic, (+/-)-Mebdtp(-), or the enantiopure ligand, (+)-(S)(S)-Mebdtp- = O,O'-bis[(+)-(S)-2methylbutyl] dithiophosphate ion, at 25 degrees C in various solvents (ethanol, tetrahydrofuran, chloroform/ethanol = 9:1) and at various pH values and reactant concentrations. The kinetics of the formation reaction of the racemic complex, (Lambda, Delta)[Cr{(+/-)-Mebdtpl(3)], shows three consecutive reaction steps. The relative pseudo first-order rate constants depend in different ways on the ligand and H+ concentrations. Circular dichroism measurements during the formation reaction using the enantiopure ligand, as well as in the final equilibrium state, show a prevalence of the Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}3] over the Lambda-(+)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)] diastereoisomer in all solvents used. A mechanism is proposed which involves two parallel pathways each consisting of three consecutive reactions of solvent substitution by a chiral chelated ligand, interconnected by some reversible inversion reactions to obtain in solution the thermodynamic and kinetic Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)] as the major diastereoisomeric compound. From the equilibrated solution the more stable solid (Lambda, Delta)-[Cr{(+)-(S)(S)-Mebdtp}3] crystallizes as a ca. 1:1 compound of a pair of the two diastereoisomers, undergoing a crystallization-induced second-order asymmetric transformation. Lambda-[Cr{(+)-(S)(S)-Mebdtp}3] is the thermodynamically stable diastereoisomer in the solid state, though in solution this inverts to the more stable Lambda-(-)(589)-[Cr{(+)-(S)(S)Mebdtp}3].
Transfer of chirality in complexes with D-3 symmetry: Kinetics of the formation reaction of chiral tris-[O,O'-bis-(2-methylbutyl)-dithiophosphato] chromium(III) complexes (Lambda, Delta)-[Cr{(+/-)-Mebdtp}(3)], Delta-(+)(589)- and Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)]
BENEDETTI, MICHELE;
2006-01-01
Abstract
The formation reaction of chiral racemic and diastereoisomeric title complexes, (Lambda, Delta)-[Cr{(+/-)-Mebdtp}(3)] and Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)], was monitored by absorption, UV/Vis, and Circular Dichroism, CD, spectroscopy, in the presence of either an excess of the racemic, (+/-)-Mebdtp(-), or the enantiopure ligand, (+)-(S)(S)-Mebdtp- = O,O'-bis[(+)-(S)-2methylbutyl] dithiophosphate ion, at 25 degrees C in various solvents (ethanol, tetrahydrofuran, chloroform/ethanol = 9:1) and at various pH values and reactant concentrations. The kinetics of the formation reaction of the racemic complex, (Lambda, Delta)[Cr{(+/-)-Mebdtpl(3)], shows three consecutive reaction steps. The relative pseudo first-order rate constants depend in different ways on the ligand and H+ concentrations. Circular dichroism measurements during the formation reaction using the enantiopure ligand, as well as in the final equilibrium state, show a prevalence of the Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}3] over the Lambda-(+)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)] diastereoisomer in all solvents used. A mechanism is proposed which involves two parallel pathways each consisting of three consecutive reactions of solvent substitution by a chiral chelated ligand, interconnected by some reversible inversion reactions to obtain in solution the thermodynamic and kinetic Lambda-(-)(589)-[Cr{(+)-(S)(S)-Mebdtp}(3)] as the major diastereoisomeric compound. From the equilibrated solution the more stable solid (Lambda, Delta)-[Cr{(+)-(S)(S)-Mebdtp}3] crystallizes as a ca. 1:1 compound of a pair of the two diastereoisomers, undergoing a crystallization-induced second-order asymmetric transformation. Lambda-[Cr{(+)-(S)(S)-Mebdtp}3] is the thermodynamically stable diastereoisomer in the solid state, though in solution this inverts to the more stable Lambda-(-)(589)-[Cr{(+)-(S)(S)Mebdtp}3].I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
