Chiral computational models

In order to explain reactions of chiral alkenes like this, we need to assess which conformations are important, and consider how they will react, just as we have done for chiral carbonyl compounds. Much of the work on alkene conformations was done by K.N. Houk using theoretical computer models, and we will summarize the most important conclusions of these studies. The theoretical studies looked at two model alkenes, shown in the margin. 

A kinetic investigation using 20 in the deprotonation of cyclohexene oxide revealed that the composition of the activated complexes was different from that assumed in the theoretical model. The reaction orders showed that an activated complex is built from one molecule of chiral lithium amide dimer and one molecule of epoxide 1. Such activated complexes have been computationally modeled by the use of PM3 and optimized structures are displayed in Figure A44. 

Gennari, C., Vieth, S., Comotti, A., Vulpetti, A., Goodman, J. M., Paterson, I. Diastereofacial selectivity in the aldol reactions of chiral a-methyl aldehydes a computer modelling approach. Tetrahedron 1992, 48,4439-4458. 

Yoshida et al.86 employed HyperChem and performed MD calculations to verify the recognition mechanism of the MIP they synthesized for the separation of optically active tryptophan methyl ester. The computational modeling proved that the enantiomeric selectivity is conferred by the electrostatic and hydrogen bonding interactions between the functional molecule and the target tryptophan methyl ester along with the chiral space formed on the polymer surface. 

Breu, J., and C.R.A. Catlow. 1995. Chiral recognition among tris(diimine)-metal complexes. 4. Atomistic computer modeling of a monolayer of [Ru(bpy)3]2+ intercalated into a smectite clay. Inorg. Chem. 34 4504-4510. 

Computational models. Easy access to computers and user-friendly program packages as well as the beauty of molecular models have resulted in a plethora of theoretical papers aiming at a rationalization of chiral recognition by CyDs. Computational studies of CyDs and their complexes, and in particular those referring to chiral recognition, are described in Chapter 11 in some detail. Here it should suffice to say that such calculations are mostly treated as operations on a... 

In 2012, Yu, Houk, and eo-workers showed a detailed DFT calculation to understand reactivity and stereoselectivity in the Pd-catalyzed diastereoselec-tive C(sp )—H bond activation process. Characterization of the trinuclear palladium-alkyl complexes discloses a clear picture of the chiral induction model (Scheme 5.2). Computational investigation has revealed that the reactions with Pr- and Pu-substituted oxazolines involve different catalyst resting states before C—H bond activation and that the lower reactivity of an Pr-substituted oxazoline results from greater stability of its catalyst resting state. DFT calculation indicated that C—H bond activation most likely occurs at the monomeric Pd center and the most preferred transition state for C—H bond aetivation contains two sterically bulky Pu groups on the carboxylic acid, and the oxazoline moieties are oriented in anti-positions which leads to the major diastereomer. 

Computational model using beam element and nonlinear truss rod element 1.05 0.05 Studying of elastic behavior of MWCNTs and investigating the influence of diameter, chirality and the number of tube layers on elastic modulus... 

With the most advanced streptavidin variants, the strategy in which a racemic catalyst is converted to a chiral-at-metal complex and then further assisted by residues in the chiral protein has led to the development of both R- and 5-selective synthetic enzymes for imine reduction. Extensive kinetic data has been obtained for these new synthetic enzymes, and computer modelling of the complex stmctures (which contain four interacting subunits) serves to support and understand the results. An induced lock and key where the host protein structure determines the catalyst structure and the reduction selectivity is proposed (Fig. 44) [141]. 

ABSTRACT Zeolite Y modified with chiral sulfoxides has been foimd catal rtically to dehydrate racemic butan-2-ol enantioselectively depending on the chiral modifier used. Zeolite Y modified with R-l,3-dithiane-1-oxide shows a higher selectivity towards conversion of S-butan-2-ol and the zeolite modified with S-2-phenyl-3-dithiane-1-oxide reacts preferentially with R-butan-2-ol. Zeolite Y modified with dithiane oxide demonstrates a significantly higher catalsdic activity when compared to the unmodified zeolite. Computational simulations are described and a model for the catalytic site is discussed. 

---