Inducing models, chiral

Figure 10 Chiral inducing models for Ni-catalyzed three-component coupling reactions...

Finally a fourth way to achieve asymmetric induction in the Passerini reaction is by way of a chiral catalyst, such as a Lewis acid. This approach is not trivial since in most cases the Lewis acid replaces the carboxylic acid as third component, leading to a-hydroxyamides or to other kinds of products instead of the classical adducts 7 (vide infra). After a thorough screening of combinations of Lewis acids/ chiral ligands, it was possible to select the couple 13 (Scheme 1.6), which affords clean reaction and a moderate ee with a model set of substrates [17]. Although improvements are needed in order to gain higher ees and to use efficiently sub-stoichiometric quantities of the chiral inducer, this represents the first example of an asymmetric classical Passerini reaction between three achiral components. 

A theoretical explanation for the chirality induced upon formation of inclusion or intercalation complexes was given by Schipper and Rodger, who developed symmetry rules based on a coupled-oscillator model [26]. These workers were able to obtain a correlation of the sign of the induced CD with... 

Continuum shell models used to study the CNT properties and showed similarities between MD simulations of macroscopic shell model. Because of the neglecting the discrete nature of the CNT geometry in this method, it has shown that mechanical properties of CNTs were strongly dependent on atomic structure of the tubes and like the curvature and chirality effects, the mechanical behavior of CNTs cannot be calculated in an isotropic shell model. Different from common shell model, which is constmcted as an isotropic continuum shell with constant elastic properties for SWCNTs, the MBASM model can predict the chirality induced anisotropic effects on some mechanical behaviors of CNTs by incorporating molecular and continuum mechanics solutions. One of the other theory is shallow shell theories, this theory are not accurate for CNT analysis because of CNT is a... 

The experiments described above have led to the conclusion that the chiral-induced equilibrium shift could be induced in the lanthanide (III) complex by a combination of electrostatic and hydrophobic interactions. Hydrogen bonding effects appear to be less important as suggested by experiments carried out under variable pH, temperature, concentration, solvent type, and solution dielectric constant conditions. By analogy to the associated/dissociated equilibrium shift models of Schipper (1978) in which the source of the equilibrium perturbation is attributed to a free energy of mixing, Brittain (1981) and Wu et al. (1989) attempted to ascertain the complex mechanism responsible for this type of Pfeiffer effect. However, their conclusions were opposite as they concluded that the [Tb(DPA)3] complex interacted with the... 

In conclusion, the model shown in Fig. 5.42 provides an experimentally validated picture of the structure of the lyo-SmC phase. However, it does not answer how the correlation of the director tilt takes place from one bilayer to the next across the intermediating layers of solvent molecules. Furthermore, it does not explain how the chirality-induced subtle precession of the director is transmitted, which corresponds to only 0.2° per lamella in the example shown. Thus, to address these two issues further considerations have to be taken into account. 

If a chiral aldehyde, e.g., methyl (27 ,4S)-4-formyl-2-methylpentanoate (syn-1) is attacked by an achiral enolate (see Section 1.3.4.3.1.), the induced stereoselectivity is directed by the aldehyde ( inherent aldehyde selectivity ). Predictions of the stereochemical outcome are possible (at least for 1,2- and 1,3-induction) based on the Cram—Felkin Anh model or Cram s cyclic model (see Sections 1.3.4.3.1. and 1.3.4.3.2.). If, however, the enantiomerically pure aldehyde 1 is allowed to react with both enantiomers of the boron enolate l-rerr-butyldimethylsilyloxy-2-dibutylboranyloxy-1-cyclohexyl-2-butene (2), it must be expected that the diastereofacial selec-tivitics of the aldehyde and enolate will be consonant in one of the combinations ( matched pair 29), but will be dissonant in the other combination ( mismatched pair 29). This would lead to different ratios of the adducts 3a/3b and 4a/4b. 

An attractive alternative to these novel aminoalcohol type modifiers is the use of 1-(1-naphthyl)ethylamine (NEA, Fig. 5) and derivatives thereof as chiral modifiers [45-47]. Trace quantities of (R)- or (S)-l-(l-naphthyl)ethylamine induce up to 82% ee in the hydrogenation of ethyl pyruvate over Pt/alumina. Note that naphthylethylamine is only a precursor of the actual modifier, which is formed in situ by reductive alkylation of NEA with the reactant ethyl pyruvate. This transformation (Fig. 5), which proceeds via imine formation and subsequent reduction of the C=N bond, is highly diastereoselective (d.e. >95%). Reductive alkylation of NEA with different aldehydes or ketones provides easy access to a variety of related modifiers [47]. The enantioselection occurring with the modifiers derived from NEA could be rationalized with the same strategy of molecular modelling as demonstrated for the Pt-cinchona system. 

Figure 1.14 (a) STM image (lOnmx lOnm, tip bias +0.52V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(l 1 0) surface terrace, (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag l 0 0 microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)... 

The transition-state model of this reaction has been proposed as (1), based on X-ray analyses of single crystals prepared from Ti(OPr )4, (R,R)-diethyl tartrate (DET), and PhCON(OH)Ph and from Ti(OPr )4, and (R,R) N,/V -dibenzyltartramide.30-32 The Z-substituent (R2), located close to the metal center, destabilizes the desired transition state and decreases enantioselectivity (vide supra). When the Z-substituent is chiral, face selection induced by the substituent strongly affects the stereochemistry of the epoxidation, and sometimes reversed face selectivity is observed.4 In contrast, the. E-substituent (R1) protrudes into an open space and E -allylic alcohols are generally good substrates for the epoxidation. 

From a cholesteric induction experiment, one can obtain chiral information on the induced cholesteric (namely, pitch and handedness) and therefore the helical twisting power of the dopant in that solvent (at a certain temperature). If a model or molecular theory relating molecular chirality to mesophase chirality is available, one can infer stereochemical information about the dopant (absolute configuration, preferred conformation). 

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

Recently, a palladium(II) complex with a chiral (S )-BINAP ligand was shown to induce an excellent enantioselectivity in the model reaction of /V-acryloyl-l,3-oxazolidin-2-onc with cyclopentadiene88. 

The efficiency with which modified Cinchona alkaloids catalyze conjugate additions of a-substituted a-cyanoacetates highlights the nitrile group s stereoselective role with the catalyst. Deng et al. [60] utilized this observation to develop a one-step construction of chiral acyclic adducts that have non-adjacent, 1,3-tertiary-quatemary stereocenters. Based on their mechanistic studies and proposed transition state model, the bifimctional nature of the quinoline C(6 )-OH Cinchona alkaloids could induce a tandem conjugate addition-protonation reaction to create the tertiary and quaternary stereocenters in an enantioselective and diastereoselective manner (Scheme 18). 

Closing naphthyl ring. The induced dipole moment is then chirally disposed in relation to the inducing NH2 dipole moment. This mechanism, referred to as the dynamic polarization model (45), is shown to explain most of the observed VCD intensity in the synunetric NH2 stretching mode, >>nh2- The anisotropy ratio for this VCD band is ca. 10. Since an NH. . . x type hydrogen bond is possible in this molecule, a description based on vibrationally induced charge flow (currents) may also be riuitfiil, similar to that proposed for a-phenyleth-ylamine. Sect. FV-B-2. 

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