Chiral inducers

P-Chirogenic phosphine/sulfide hybrid phosphine-boranes 80 were synthesized from the reaction between (l )-tosylates 79 [94] and sodium thiolate in DMF at ambient temperature as depicted in Scheme 12, or alternatively by a one pot synthesis consisting of the nucleophilic attack of the chirally induced hthium salt of 74 on phenyl disulfide. Both methodologies provided the desired sul-fide/phosphine boranes in excellent yields [10]. 

The same phosphine-borane used for the synthesis of BisP acted as the starting materials of the construction of MiniPHOS, the next smaller analogue to BisP (Scheme 13). The chirally induced lithium salt was treated with alkylphos-phorus dichloride, methylmagnesium bromide, and borane-THF complex to afford enantiomerically pure MiniPHOS-borane 82a. Recrystallization enabled elimination of a small amount of corresponding raeso-diastereomer formed [29]. Yields were generally low, ranging from 13 to 28%. 

It was shown that planar-chiral cyclopentadienyl-phosphine ligands were excellent chirality inducers during the synthesis of chiral-at-metal tetrahedral Ru-phosphine or -phosphite complexes (99% de). 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Chiral-at-metal cations can themselves serve as chirality inducers. For example, optically pure Ru[(bipy)3] proved to be an excellent chiral auxihary for the stereoselective preparation of optically active 3D anionic networks [M(II)Cr(III)(oxalate)3]- n (with M = Mn, Ni), which display interesting magnetic properties. In these networks all of the metalhc centers have the same configuration, z or yl, as the template cation, as shown by CD spectroscopy and X-ray crystallography [43]. 

In a diastereoisomeric complex the two types of chirality inducers might have opposite rather than additional effects, a feature so far impossible to predict... 

Ever since Pasteur s work with enantiomers of sodium ammonium tartrate, the interaction of polarized light has provided a powerful, physical probe of molecular chirality [18]. What we may consider to be conventional circular dichroism (CD) arises from the different absorption of left- and right-circularly polarized light by target molecules of a specific handedness [19, 20]. However, absorption measurements made with randomly oriented samples provide a dichroism difference signal that is typically rather small. The chirally induced asymmetry or dichroism can be expressed as a Kuhn g-factor [21] defined as ... 

Recent developments in chirality induced by ligands, and in efficient synthetic methods for ligands such as diphosphines, chiral diamines, amino-phosphine-phosphinites, phosphino-oxazolines and diimino-diphosphines, are available in the literature.105-107... 

Ray SG, Daube SS, Leitus G, Vager Z, Naaman R (2006) Chirality-induced spin-selective properties of self-assembled monolayers of DNA on gold. Phys Rev Lett 96(3) 036101... 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

By linking the steric factor of the stereogenic center of substrates with the chiral-inducing ability of properly designed ligands, homogeneous diastereoselective hydrogenation can attain levels of stereoselectivity that will enable the industrial... 

Surprisingly, given that many P-chiral ligands are efficient chiral inducers, only one example has been reported of a C2-symmetric, P-chiral bisphosphinamidite. Ligand 54 was prepared by Wills et al. and tested in the Rh-catalyzed en-antioselective hydrogenation of a-acylaminoacrylate to give disappointingly low selectivity (33% ee) and low efficiency (TON = 20, TOF=0.4) [49]. 

This reaction is very sensitive to water because in the presence of water and a metal salt (such as copper salt) the thioacetal tends to decompose, and this may reduce the amount of thioacetal available for epoxidation. When water is excluded from all the reagents, the reaction can be carried out in the presence of a catalytic amount of thioacetal. Otherwise, a stoichiometric amount of thioacetal compound is required. Scheme 4 53 summarizes the epoxidation of aldehydes using 138 as the chiral-inducing reagent. Excellent enantioselectivities are obtained in most cases. 

Fluorine-containing compounds can also be synthesized via enantioselective Reformatsky reaction using bromo-difluoroacetate as the nucleophile and chiral amino alcohol as the chiral-inducing agent.86 As shown in Scheme 8-41, 1 equivalent of benzaldehyde is treated with 3 equivalents of 111 in the presence of 2 equivalents of 113, providing a,a-difluoro-/ -hydroxy ester 112 at 61% yield with 84% ee. Poor results are observed for aliphatic aldehyde substrates. For example, product 116 is obtained in only 46% ee. 

T. B. Adler, N. Borho, M. Reiher, and M. A. Suhm, Chirality induced switch in hydrogen bond topology Tetrameric methyl lactate clusters in the gas phase. Angew. Chem. Int. Ed. 45, 3440 3445 (2006). 

It is worth mentioning that the extent of asymmetric induction in the sulfinate synthesis is comparable with that observed in the reaction of sulfinyl chlorides with optically active alcohols. However, in this case, the sulfinate products contain only one chiral center on sulfur the chiral-inducing amine is very easily recovered as the hydrochloride. 

In these systems, after the crystal chirality induced the chirality of asymmetric carbon in external organic compound, the subsequent asymmetric autocatalysis gives the greater amount of enantiomerically amplified product. These results clearly demonstrate that the crystal chirality of achiral organic compound is responsible for the enantioselective addition of /-Pr2Zn to pyrimidine-5-carbalde-hyde Ic. 

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g... 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

Highly diastereoselective alkylation at C-2 of benzo-l,4-diazepin-3-ones 41 was accomplished in 40-85% chemical yield using an (R)-phenylglycinol moiety at N-4, as the chirality-inducing element (Scheme 10) <2005EJ01590>. The optimum conditions involve deprotonation of 41 with 2 equiv of -BuLi at — 40 °C and alkylation at — 78 °C to give products 42 with 86-96% de. A single crystal X-ray diffraction analysis of the methylated derivative determined that the major product formed was the 2-(R)-isomer, as depicted in the product 42. 

It should be noted that although oxiranes (8) and (9) are chiral, this epoxidation methodology gives the racemate since no chiral inducing agent is present. 

Besides molecular shapes and amphiphilicity, chirality also has a large influence on LC self assembly, leading to series of LC phases with helical superstructures, reduced symmetry, and chirality induced frustration [43-46]. 

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