Chiral: axis induction

The rearrangement of E and Z N-crotylamines 245 (R, E or Z=Me) gave the corresponding nitriles 247 with 82 and 68% yield, respectively. Disappointingly, the product was obtained as an inseparable mixture of synlanti diastereomers 247 indicating a low simple diastereoselectivity. Obviously, the intermediate ketene imine fitted neither a chair- nor a boat-like conformation. Hence, a low axis-to-center chirality induction was operative, and E and Z reactants gave a... 

Although reasonable asymmetric induction has been observed in the alkylation of a chiral cyclohexanone enamine, it was noted1 in 1977 that in order to obtain higher induction in this type of reaction. Clearly what is needed is an amine with a C2 axis of symmetry . (+ )-trans-... 

Recently, the importance of the structure of chiral metal complexes on the handedness of the mesophases induced in a nematic LC was exemplified [114]. The chiral metal complexes 10 and 11—in which the alkyl substituents are aligned almost perpendicularly to the C2 axis in the former and parallel in the latter—show very different induction phenomena. Not only are the induced helicities in the nematic LC of opposite sense for the two compounds, but the helical twisting power of 10 is much higher than that of 11. The reason for these differences is the way in which the molecules are incorporated into the host nematic phase and exert their force upon it to create the twist between the layers. 

In general, the chiral induction of these ligands has remained low, which is probably due to the rapid internal rotation of the chiral substituents around the C - N axis and the flexibility of the substituents. This may leave the active chiral space at the metal center relatively ill-defined. 

Ketals have also been used as an alternative to link the chiral inductor to the starting reagents. An important asymmetric induction was observed during the cycloaddition of ketals 94 having a C2 axis of symmetry, and a cyclopentenone or cyclohexenone derivative 95. Unfortunately, the observed chemical yields of 96 remain low (Scheme 22) [68]. With ketals of aliphatic enones, the selectivity decreases, and complex mixtures of isomers were observed [69]. 

A particularly interesting case of asymmetric induction occurred with d A-(2-benzoylethyl)-A-tosylglycinamides derived from pyrrolidines 158 havinj C2 axis of symmetry as the only source of chirality. A large chiral discriminati... 

Stoodley and co-workers found retention of chirality in a cyclization reaction of 87.43 The reaction of 87 with triethylamine gave 89 in an enantiomerically pure form. They suggested that axially chiral enolate 88 with a chiral C(4)-N axis may be the origin of the asymmetric induction. Because of the high reactivity of the electrophilic diazo group, the cyclization of 87 would proceed before racemization of the axially chiral enolate. 

An interesting use of removable chiral auxiliaries in photocycloaddition reactions concerns imminium salts. With cyclic enones, the observed asymmetric induction does not result from an approach of the double bonds in parallel planes because of the triplet nature of the reactive excited state. In contrast, the corresponding imminium salts react through their singlet excited state, and an approach of the reactants in parallel planes is now required during the cycloaddition process. For chiral imminium salts 130 derived from a cyclohexenone and a pyrrolidine having a C2 axis of symmetry, the intramolecular [2 -I- 2] photocycloaddition process occurs with a de up to 82%. As expected, the stereochemis-... 

Fenchenko studied free induction decays and transverse relaxation in entangled polymer melts. He considered both the effects of the dipolar interactions between spins in different polymer chains and within an isolated segment along s single chain. Sebastiao and co-workers presented a unifying model for molecular dynamics and NMR relaxation for chiral and non-chiral nematic liquid crystals. The model included molecular rotations/ reorientations, translational self-diffusion as well as collective motions. For the chiral nematic phase, an additional relaxation mechanism was proposed, associated with rotations induced by translational diffusion along the helical axis. The model was applied to interpret experimental data, to which we return below. 

The experimental investigation of the chiral induction of the amino-anthraquinones and binaphthyls showed, on the one hand, that the induction by a dopant molecule depends strongly on the orientation of the orientation axis with respect to its skeleton and, on the other hand, on the orientation of a chiral group with respect to the principal axes of the order tensor of the molecule. It became clear that the quantitative contribution to the HTP of a chiral ligand of a molecule of class A depends on the orientation of this chiral ligand with respect to the skeleton. These properties are typical for the behavior of tensorial quantities and thus a tensorial description has to be developed. 

The application of NHC ligands in this reaction has naturally been extended to asymmetric processes. In general, the chiral induction of NHCs was low, probably due to the rapid internal rotation of the chiral substituents around the C-N axis. Also, metalation procedures involving deprotonation of a chiral azolium precursor with strong bases may cause loss of chirality in the desired complex. In contrast, the silver transmetalation route generally yields the optically active target complex. Since the first reports by Herrmann et al., and shortly after by Enders et al. in 1996-1997 on the synthesis and applications of chiral NHC-Rh complexes in the hydrosilylation of carbonyl compounds, increased efforts were made in this area. ... 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

The role of the trimethylaluminum in the electrocyc-lization is not understood. Other Lewis acids were screened, but all led to the decomposition of the carbamoyl trienes. This work demonstrates respectable levels of 1,6-asymmetric induction. The electrocyclization appears to be under kinetic control because no equilibration of the products was observed under the reaction conditions. The mechanism through which stereochemical information is transmitted from the oxazolidi-none chiral auxiliary group to the reacting termini of the hexatriene is indicated by structure 21 (Figure 19.1). Of the two coplanar rotational isomers about the C-N bond axis of the triene, 21 represents the minimum energy structure. 

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