Kinetic resolution using enantioimpure catalysts: mechanistic considerations of complex rate laws.

Kinetic resolutions in which the reactions exhibit complex rate laws are discussed. When enantioimpure catalysts are employed, a conversion-dependent selectivity factor k(rel) may in some cases be observed due to "kinetic partitioning" of catalysts within a reaction network. Both asymmetric amplifications and depletions may be observed, and the effects are separate from-and may in some cases be superimposed on-the classic nonlinear effect due to catalyst interactions as those predicted by Kagan's ML(n) models. Consideration of the conversion dependence of the selectivity factor using enantioimpure catalysts reveals significant detail about the reaction mechanism for the enantiopure case and may offer insights for practical application of kinetic resolution. Examples from the literature are analyzed in the context of kinetic partitioning.

Kinetic studies of Heck coupling reactions using palladacycle catalysts: experimental and kinetic modeling of the role of dimer species.

Experimental kinetic studies of the coupling of p-bromobenzaldehyde (1) with butyl acrylate (2) using the dimeric palladacycles complex (4) with chelating nitrogen ligands were carried out together with kinetic modeling using a reaction rate expression based on the mechanism shown in Scheme 2. The oxidative addition product of 1 was found to be the resting state within the catalytic cycle. The formation of dimeric Pd species external to the catalytic cycle helped to rationalize a non-first-order rate dependence on catalyst concentration. Theoretical modeling showed how the relative concentrations of the different intermediate species within the catalystic cycle can influence the observed rate dependence on Pd concentration. It was shown how conventional kinetic studies may give reaction orders in substrates which differ from those which would be observed under practical synthetic conditions. Comparison between phosphine- and nonphosphine-based palladacycles suggests that they follow the same reaction mechanism. The role of water in accelerating the initial formation of the active catalyst species is noted.

Kinetic aspects of nonlinear effects in asymmetric catalysis.

Probing catalyst systems for a nonlinear relationship between product enantioselectivity and catalyst enantiopurity is now commonly being used as a mechanistic tool. We show that in some cases striking consequences for reaction rate can ensue for reactions carried out using non-enantiopure catalysts. We also demonstrate how consideration of the kinetic behavior may serve to enhance the use of nonlinear effects as a diagnostic tool for identifying active catalytic species and for providing mechanistic insight. Kinetic information may also help in the development of efficient synthetic strategies using non-enantiopure systems.

Enantioselective hydrogenation of olefins with phosphinooxazoline-iridium catalysts.

Cationic iridium complexes with chiral phosphinooxazoline ligands are efficient catalysts for the enantioselective hydrogenation of olefins. The complexes are readily prepared, air-stable, and easy to handle. In contrast to chiral rhodium- and ruthenium-phosphine catalysts, they do not require the presence of a polar coordinating group near the C=C bond. In the hydrogenation of unfunctionalized trisubstituted 1,2-diaryl-olefins, high enantioselectivities of >95% ee with full conversion and turnover frequencies of >7,000 h-1 can be achieved, using 0.1 mol% of catalyst with tetrakis[3, 5-bis(trifluoromethyl)phenyl]borate (TFPB or BARF) as the counterion. The corresponding hexafluorophosphate or tetrafluoroborate salts give low conversion due to deactivation of the catalyst during the reaction. Substrates with polar substituents such as allylic alcohols, on the other hand, afford better results with the hexafluorophosphate salts.