Intermolecular Chirality Transfer Research Articles

Crystallization-Induced Chirality Transfer in Conformationally Flexible Azahelicene Au(I) Complexes with Circularly Polarized Luminescence Activation.

Flexible and twisted annulated π-systems exhibit numerous unique and desirable features, owing to their ability to display chirality. However, preventing their racemization due to the dynamic nature of their chirality remains a challenge. One promising approach to stabilize homochirality in such systems is chirality transfer from a chiral auxiliary to a moiety displaying dynamic chirality. Herein, we introduce a new approach for dynamic chirality stabilization in conformationally flexible azahelicene species via crystallization-induced intermolecular chirality transfer in Au(I) complexes featuring azahelicene (dibenzo[c,g]carbazole and benzo[c]carbazole) and enantio-pure chiral N-heterocyclic carbene (NHC) ligands with a complementary tailored shape. Crystallization of these azahelicene Au(I) complexes not only suppresses the dynamic chirality of the dibenzocarbazole species but also stabilizes their homochirality through the intermolecular conjunction between the chiral NHC and dibenzocarbazole ligands. In the Au(I) benzocarbazole complexes, the intermolecular conjunction and chirality transfer in the crystals induce chirality in the initially achiral benzocarbazole ligand. Furthermore, the crystallization of the studied complexes activates their circularly polarized luminescence (CPL) properties, which were suppressed in solution. Importantly, chirality transfer leads to significant CPL enhancement; the complexes that feature chirality transfer within the crystal structure exhibit luminescence dissymmetry factors 5 to 10 times higher than those of the complexes without chirality transfer.

Chirogenesis and Pfeiffer Effect in Optically Inactive EuIII and TbIII Tris(β-diketonate) Upon Intermolecular Chirality Transfer From Poly- and Monosaccharide Alkyl Esters and α-Pinene: Emerging Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD).

We report emerging circularly polarized luminescence (CPL) at 4f-4f transitions when lanthanide (EuIII and TbIII) tris(β-diketonate) embedded to cellulose triacetate (CTA), cellulose acetate butyrate (CABu), D-/L-glucose pentamethyl esters (D-/L-Glu), and D-/L-arabinose tetramethyl esters (D-/L-Ara) are in film states. Herein, 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate (fod) and 2,2,6,6-tetramethyl-3,5-heptanedione (dpm) were chosen as the β-diketonates. The glum value of Eu(fod)3 in CABu are +0.0671 at 593 nm (5F1) and −0.0059 at 613 nm (5F2), respectively, while those in CTA are +0.0463 and −0.0040 at these transitions, respectively. The glum value of Tb(fod)3 in CABu are −0.0029 at 490 nm (5F6), +0.0078 at 540 nm (5F5), and −0.0018 at 552 nm (5F5), respectively, while those in CTA are −0.0053, +0.0037, and −0.0059 at these transitions, respectively. D-/L-Glu and D-/L-Ara induced weaker glum values at 4f-4f transitions of Eu(fod)3, Tb(fod)3, and Tb(dpm)3. For comparison, Tb(dpm)3 in α-pinene showed clear CPL characteristics, though Eu(dpm)3 did not. A surplus charge neutralization hypothesis was applied to the origin of attractive intermolecular interactions between the ligands and saccharides. This idea was supported from the concomitant opposite tendency in upfield 19F-NMR and downfield 1H-NMR chemical shifts of Eu(fod)3 and the opposite Mulliken charges between F-C bonds (fod) and H-C bonds (CTA and D-/L-Glu). An analysis of CPL excitation (CPLE) and CPL spectra suggests that (+)- and (–)-sign CPL signals of EuIII and TbIII at different 4f-4f transitions in the visible region are the same with the (+)-and (–)-sign exhibited by CPLE bands at high energy levels of EuIII and TbIII in the near-UV region.

Synthesis and Properties of Modified Amylose Containing an Aryl Spacer at a Regular Interval in Its Main Chain

We describe the synthesis and properties of modified amylose containing an aryl spacer at a regular interval in its main chain as a new host polymer. The modified amylose was efficiently synthesized by copper-catalyzed alkyne–azide 1,3-dipolar cycloaddition polymerization of alkyne-containing permethylated maltohexaosyl azide 3. The polymer exhibited greater hydrophobicity than permethylated amylose. A mixture of the polymer with terthiophene formed an inclusion complex in a 10% dimethyl sulfoxide aqueous solution, leading to intermolecular chirality transfer from the polymer to terthiophene.

Chiral sensing by nonchiral tetrapyrroles.

Enantiomeric excess (ee) is a measure of the purity of an enantiomer of a chiral compound with respect to the presence of the complementary enantiomer. It is an important aspect of chemistry, especially in the fields of pharmaceuticals and asymmetric catalysis. Existing methods for determination of enantiomeric excesses using nuclear magnetic resonance (NMR) spectroscopy mostly rely on special chiral reagents (auxiliaries) that form two or more diastereomeric complexes with a chiral compound. As a result of this, the NMR spectrum of each enantiomer is different, allowing the determination of enantiomeric excess. In this Account, we describe a molecular design process that has allowed us to prepare prochiral solvating agents for NMR determination of ee of a wide variety of analyte types. At the outset of this work, we initially encountered the phenomenon of NMR peak splitting in the oxoporphyrinogen (OxP) host component of a supramolecular host-guest complex, where the extent of the splitting is apparently proportional to the guests' ee. Upon closer examination of the mechanism of action, it was found that several complicating factors, including prototropic tautomerism, macrocyclic inversion (ring-flipping), and 1:2 host-guest stoichiometry, obstruct potential applications of OxP as a chiral solvating agent. By considering the molecular conformation of the OxP host, a saddle-shaped calix[4]pyrrole, we moved to study the tetraphenylporphyrin (TPP) dication since it has a similar form, and it was found that it could also be used to probe ee. However, although TPP does not suffer from disadvantageous tautomeric processes, it is still subject to macrocyclic inversion and has the additional serious disadvantage of operating for ee sensing only at depressed temperatures. The intrinsic disadvantages of the OxP and TPP systems were finally overcome by covalently modifying the OxP chromophore by regioselective N-alkylation at one face of the molecule. This procedure yields a host Bz2OxP that undergoes 1:1 host-guest interactions, cannot be protonated (and so does not suffer drawbacks due to tautomeric processes), and can interact solely through hydrogen bonding with a much wider range of analyte types, including acids, esters, amines (including amino acid derivatives), and ketones, for the determination of their ee at room temperature. Chiral sensing, in this case, can be understood by considering the breakdown of the host's symmetry when it interacts with a chiral guest under fast exchange. Furthermore, chirality discrimination (i.e., which is the major enantiomer in a sample) can be performed by addition of a small amount of one of the known enantiomers. Adaptation of a symmetrical molecule for ee sensing presents certain intrinsic advantages, including identical binding constants of each enantiomer. Our results indicate that other symmetrical molecules might also be useful as NMR probes of enantiopurity. These systems could provide insights into important chirality principles such as majority rule, intermolecular chirality transfer, and asymmetric reactions. The Bz2OxP system is also of note from the point of view that it does not rely on the formation of diastereomers.

The asymmetric alkylation of dimethylhydrazones; intermolecular chirality transfer using sparteine as chiral ligand.

The asymmetric alkylation of ketones represents a fundamental transformation in organic chemistry. Chiral auxiliaries have been used almost exclusively for this transformation. Herein we describe a strategy for the generation of enantiomerically enriched α-alkylated ketones up to an er of 83 : 17, using a chiral ligand protocol.

Intermolecular Chirality Transfer from Silicon to Carbon

This report allows mechanistic insight into the chirality transfer from silicon to carbon under palladium catalysis. Isotope labeling and crossover experiments led to the discovery that the catalytic cycle involves two silicon atoms, one for addition to the substrate and a second to turn over the catalyst and continue the cycle. One of the important factors is the reversibility of the first steps where K 2 favors the π-bound palladium and K 3 involves a match(m)-mismatch(mm) situation where k m/k mm = 3, which explains the high chirality transfer.

Intermolecular Chirality Transfer from Silicon to Carbon: Interrogation of the Two-Silicon Cycle for Pd-Catalyzed Hydrosilylation by Stereoisotopochemical Crossover

A “two-silicon cycle” for the highly efficient intermolecular chirality transfer from silicon to carbon (98−99% ct) in the palladium-catalyzed hydrosilylation of norbornenes emerges from an investigation involving catalytic crossover experiments with isotopically labeled silicon-stereogenic silanes. A key outcome of these experiments, which are supported by product-distribution modeling, is the conclusion that the chirality transfer arises from thermodynamically controlled reversible silapalladation of the alkene rather than from kinetic control during irreversible σ-bond metathesis of the resulting β-silyl σ-alkyl palladium complex with chiral silane.

ChemInform Abstract: Helical Twisting Power and Circular Dichroism as Chirality Observations. Development of a Sector Rule for the Intermolecular Chirality Transfer in a Liquid Crystal Phase

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Ab initio and PM3 studies of hydrogen bonding of acetoin (E)- and (Z)-oxime dimers. Cooperativity and competition

Structural properties of hydrogen-bonded open and cyclic dimers of acetoin (E)- and (Z)-oximes were investigated at the ab initio HF/6-31G ∗∗ and at the PM3 level of theory. For the (E)-oxime stereoisomer, the planar, cyclic specimen, like the classic oxime dimer, is first postulated. At the ab initio level the cohesive energy per bond is − 6.95 kcal mol −1 with R ( O) H … O = 193 pm for the open (E)-dimer as compared to − 3.42 kcal mol −1 ( R ( O) H … N = 216 pm) for the cyclic dimer. For the (Z)-stereoisomer the hydrogen bonding energy increases to − 7.55 kcal mol −1 ( R ( O) H … O = 190 pm) and − 5.19 kcal mol −1 per bond ( R ( O) H… N = 205 pm ). The results convincingly prove that hydrogen bonding in hydroxy oxime dimers is a highly cooperative phenomenon both from the structural and energetic points of view. Comparison of the (E)-cyclic with the (Z)-cyclic dimer structure shows that in the former the participation of an external hydrogen bond dramatically changes the intramolecular hydrogen bond pattern (from 207 to 240 pm), providing evidence of competition between the two proton donors for the nitrogen acceptor. The structural data obtained at the semiempirical level (PM3) show trends which are in agreement with those referred to above. Intermolecular chirality transfer is also discussed on the basis of the PM3 Hamiltonian results.

Helical twisting power and circular dichroism as chirality observations: The intramolecular and intermolecular chirality transfer

The helical twisting power (HTP) of a chiral guest in a chiral or achiral mesophase as host system is a chirality observation and, therefore, should be a suitable quantity for determination of the absolute configuration. Rules correlating the sign of the HTP and the absolute configuration of the guest are not known or, as sometimes proposed, do not exist at all. The sign and size of the HTP of compounds with one or more chiral centers are determined by the effects of an intramolecular and an intermolecular chirality transfer. The intramolecular chirality transfer is a well known phenomena in the development of sector or helicity rules for circular dichroism spectroscopy. Due to the strong omisotrophy of the intermolecular chirality transfer in a liquid crystal phase the development of rules for the determination of the absolute configuration from the sign of the HTP is called in question. By analysis of the HTP and the circular dichroism (CD) of mono- and bis-aminoanthraquinones we now show that the development of rules in comparison to CD measurements does seem to be possible if some specialities of the anisotropic mesophases are taken into account. Chirality 9:407–423, 1997. © 1997 Wiley-Liss, Inc.

Helical twisting power and circular dichroism as chirality observations: The intramolecular and intermolecular chirality transfer

Helical twisting power and circular dichroism as chirality observations: The intramolecular and intermolecular chirality transfer

Computer simulation of chiral liquid crystal phases. IV. Intermolecular chirality transfer to rotamers in a cholesteric phase

Abstract Intermolecular chirality transfer was studied by investigating the conformational distribution of rotamers in a cholesteric guest-host phase using Monte Carlo (MC) simulations in the NVT ensemble. The guest-host system under investigation was given by N c = 238 rigid, chiral Gay-Berne atropisomers as host molecules and N a = 18 flexible Gay-Berne rotamers as guest molecules. The rigid, chiral Gay-Berne atropisomers of point symmetry group D2 were defined by joining two Gay-Berne particles through a bond with a suitable fixed dihedral angle. The possibility of internal rotation about the bond axis without a rotational barrier was introduced as an internal degree of freedom for the guest molecules, for convenience denoted as Gay-Berne rotamers. Starting from an isotropic configuration, cholesteric phases were obtained on equilibrating the guest-host systems, whereby left-handed and right-handed cholesterics were formed depending on the M- and P-helicity of the atropisomers, respectively. Analysing ...