Non-linear effects


In enantioselective synthesis, a non-linear effect refers to a process in which the enantiopurity of the catalyst does not correlate linearly with the enantiopurity of the product produced. This deviation from linearity is described as the non-linear effect, NLE. The linearity can be expressed mathematically, as shown in Equation 1. Stereoselection that is higher or lower than the enantiomeric excess of the catalyst is considered non-routine behavior.
For an ideal asymmetric reaction, the eeproduct may be described as the product of eemax multiplied by the eecatalyst. This is not the case for reactions exhibiting NLE's.
In 1976, Wynberg and Feringa observed different chemical behavior in the reaction of an enantiopure and racemic substrate in a phenol coupling reaction. In 1981, Kagan and collaborators described the first non-linear effects in asymmetric catalysis and gave rational explanations for these phenomena. General definitions and mathematical models are essential for understanding nonlinear effects and their application to specific chemical reactions. In recent decades, the study of nonlinear effects has helped elucidate reaction mechanism and guide synthetic applications.

Types of non-linear effects

Positive non-linear effect, (+)-NLE

A positive non-linear effect, -NLE, is present in an asymmetric reaction which demonstrates a higher product ee than predicted by an ideal linear situation. It is often referred to as asymmetric amplification, a term coined by Oguni and co-workers. An example of a positive non-linear effect is observed in the case of Sharpless epoxidation with the substrate geraniol.In all cases of chemical reactivity exhibiting -NLE, there is an innate tradeoff between overall reaction rate and enantioselectivity. The overall rate is slower and the enantioselectivity is higher relative to a linear behaving reaction.

Negative non-linear effect, (−)-NLE

Referred to as asymmetric depletion, a negative non-linear effect is present when the eeproduct is lower than predicted by an ideal linear situation. In contrast to a -NLE, a -NLE results in a faster overall reaction rate and a decrease in enantioselectivity. Synthetically, a -NLE effect could be beneficial with a reasonable assay for separating product enantiomers and a high output is necessary. An interesting example of a -NLE effect has been reported in asymmetric sulfide oxidations.

Hyperpositive and enantiodivergent non-linear effect

Beyond the positive or negative non-linear effects, there are atypical cases which are briefly described in this section.
-A hyperpositive nonlinear effect refers to a case where the chiral catalyst, when not enantiopure, can be more enantioselective than its enantiopure counterpart. This case was first deduced from the theoretical models proposed by Henri Kagan in 1994. The first experimental example of such non-linear effect was only observed in 2020 by S. Bellemin-Laponnaz, but with a mechanism that turns out to be different from Kagan's original proposal.
-A catalytic system that generates either enantiomer of the product by modifying only the enantiomeric excess of the ligand is called an enantiodivergent non-linear effect. The first experimental example was described in 2002.The mechanism that could explain this type of behavior appears to be the same as for hyperpositive non-linear effects.

Modeling non-linear effects

In 1986, Henri B. Kagan and coworkers observed a series of known reactions that followed a non-ideal behavior. A correction factor, f, was adapted to Equation 1 to fit the kinetic behavior of reactions with NLEs.
Equation 2: A general mathematical equation that describes non-linear behavior
Unfortunately, Equation 2 is too general to apply to specific chemical reactions. Due to this, Kagan and coworkers also developed simplified mathematical models to describe the behavior of catalysts which lead to non-linear effects. These models involve generic MLn species, based on a metal bound to n number of enantiomeric ligands. The type of MLn model varies among asymmetric reactions, based on the goodness of fit with reaction data. With accurate modeling, NLE may elucidate mechanistic details of an enantioselective, catalytic reaction.

ML2 model

General description

The simplest model to describe a non-linear effect, the ML2 model involves a metal system with two chiral ligands, LR and LS. In addition to the catalyzed reaction of interest, the model accounts for a steady state equilibrium between the unbound and bound catalyst complexes. There are three possible catalytic complexes at equilibrium. The two enantiomerically pure complexes are referred to as homochiral complexes. The possible heterochiral complex, MLRLS, is often referred to as a meso-complex.
The equilibrium constant that describes this equilibrium, K, is presumably independent on the catalytic chemical reaction. In Kagan's model, K is determined by the amount of aggregation present in the chemical environment. A K=4 is considered to be the state at which there is a statistical distribution of ligands to each metal complex. In other words, there is no thermodynamic disadvantage or advantage to the formation of heterochiral complexes at K=4.
Obeying the same kinetic rate law, each of the three catalytic complexes catalyze the desired reaction to form product. As enantiomers of each other, the homochiral complexes catalyze the reaction at the same rate, although opposite absolute configuration of the product is induced. The heterochiral complex, however, forms a racemic product at a different rate constant.

Mathematical model for the ML2 Model

In order to describe the ML2 model in quantitative parameters, Kagan and coworkers described the following formula:
In the correction factor, Kagan and co-workers introduced two new parameters absent in Equation 1, β and g. In general, these parameters represent the concentration and activity of three catalytic complexes relative to each other. β represents the relative amount of the heterochiral complex as shown in Equation 3. It is important to recognize that the equilibrium constant K is independent on both β and g. As described by at , "the parameter K is an inherent property of the catalyst mixture, independent of the eecatalyst. K is also independent of the catalytic reaction itself, and therefore independent of the parameter g."
Equation 3: The correction factor, β, may be described as z, the heterochiral complex concentration, divided by x and y, the respective concentrations of the complex concentration divided by x and y, the respective concentrations of the homochiral complexes
The parameter g represents the reactivity of the heterochiral complex relative to the homochiral complexes. As shown in Equation 5, this may be described in terms of rate constants. Since the homochiral complexes react at identical rates, g can then be described as the rate constant corresponding to the heterochiral complex divided by the rate constant corresponding to either homochiral complex.
Equation 4: The correction parameter, g, can be described as the rate of product formation with the heterochiral catalyst MLRLS divided by the rate of product formation of the homochiral complex.

Interpretation of the mathematical results of the ML2 Model

iv. Reaction Kinetics with the ML2 Model: Following H.B. Kagan's publication of the ML2 model, Professor Donna Blackmond at Scripps demonstrated how this model could be used to also calculate the overall reaction rates. With these relative reaction rates, Blackmond showed how the ML2 model could be used to formulate kinetic predictions which could then be compared to experimental data. The overall rate equation, Equation 6, is shown below.
In addition to the goodness of fit to the model, kinetic information about the overall reaction may further validate the proposed reaction mechanism. For instance, a positive NLE in the ML2 should result in an overall lower reaction rate. By solving the reaction rate from Equation 6, one can confirm if that is the case.

M*L2 Model

General description

Similar to the ML2 model, this modified system involves chiral ligands binding to a metal center to create a new center of chirality. There are four pairs of enantiomeric chiral complexes in the M*L2 model, as shown in Figure 5.
In this model, one can make the approximation that the dimeric complexes dissociate irreversibly to the monomeric species. In this case, the same mathematical equations apply to the ML*2 model that applied to the ML2 model.

ML3 model

General description

A higher level of modeling, the ML3 model involves four active catalytic complexes: MLRLRLR, MLSLSLS, MLRLRLS, MLSLSLR. Unlike the ML2 model, where only the two homochiral complexes reacted to form enantiomerically enriched product, all four of the catalytic complexes react enantioselectively. However, the same steady state assumption applies to the equilibrium between unbound and bound catalytic complexes as in the more simple ML2 model. This relationship is shown below in Figure 7.

Mathematical modeling

Calculating the eeproduct is considerably more challenging than in the simple ML2 model. Each of the two heterochiral catalytic complexes should react at the same rate. The homochiral catalytic complexes, similar to the ML2 case, should also react at the same rate. As such, the correction parameter g is still calculated as the rate of the heterochiral catalytic complex divided by the rate of the homochiral catalytic complex. However, since the heterochiral complexes lead to enantiomerically enriched product, the overall equation for calculating the eeproduct becomes more difficult. In Figure 8., the mathematical formula for calculating enantioselectivity is shown.
Figure 8: The mathematical formula describing an ML3 system. The eeproduct is calculated by multiplying the eemax by the correction factor developed by Kagan and co-workers.