Chiral tether-mediated stabilization and helix-sense control of complementary metallo-double helices

A series of novel Pt^II-linked double helices were prepared by inter- or intrastrand ligand-exchange reactions of the complementary duplexes composed of chiral or achiral amidine dimer and achiral carboxylic acid dimer strands joined by trans-Pt^II–acetylide complexes with PPh₃ ligands using chiral and achiral chelating diphosphines. The structure and stability of the Pt^II-linked double helices were highly dependent on the diphosphine structures. An interstrand ligand exchange took place with chiral and achiral 1,3-diphosphine-based ligands, resulting in trans-Pt^II-bridged double helices, whose helical structures were quite stable even in dimethyl sulfoxide (DMSO) due to the interstrand cross-link, whereas a 1,2-diphosphine-based ligand produced non-cross-linked cis-Pt^II-linked duplexes, resulting from an intrastrand ligand-exchange that readily dissociated into single strands in DMSO. When enantiopure 1,3-diphosphine-based ligands were used, the resulting trans-Pt^II-bridged double helices adopted a preferred-handed helical sense biased by the chirality of the bridged diphosphines. Interestingly, the interstrand ligand exchange with racemic 1,3-diphosphine toward an optically-active Pt^II-linked duplex, composed of chiral amidine and achiral carboxylic acid strands, was found to proceed in a diastereoselective manner, thus forming complete homochiral trans-Pt^II-bridged double helices via a unique chiral self-sorting.     

The synthesis of chiral amino diol tridentate ligands and their enantioselective induction during the addition of diethylzinc to aldehydes

A series of C₂-symmetric chiral amino diol tridentate ligands 3a–g were prepared from achiral bulky organolithiums, achiral bulky primary amines, and optically active epichlorohydrin (ECH). The prepared C₂-symmetric chiral amino diol tridentate ligands were capable of inducing enantioselectivity in the model reaction of aromatic and aliphatic aldehydes with diethylzinc with an ee of up to 96%. The enantioselectivity can be modulated by adjusting the steric hindrance of the achiral reagents employed in the synthesis of the chiral ligand. The configuration of the addition product depended on the configuration of the amino diol ligands, which can be simply controlled as desired by using the ECH with the desired configuration during the preparation of the ligand.     

Chemical applications of topology and group theory 13. Chirality and framework groups [1]

Pople has recently introduced the concept of a framework group to specify the full symmetry properties of a molecular structure. Furthermore, Pople has developed powerful algorithms for the use of framework groups to generate all distinguishable skeletons with a given number of sites. This paper studies the systematics of chirality arising from different framework groups. In this connection framework groups can be classified into four different types: linear, planar, achiral, and chiral. Chiral framework groups lead to chiral systems for any ligand partition including that with all ligands equivalent. Linear framework groups are never chiral even for the ligand partition with all ligands different. Planar framework groups are also never chiral since all sites are in the same plane, which therefore remains a symmetry plane for any ligand partition. However, the mirror symmetry of the molecular plane of a planar framework group can be destroyed by a process called polarization; this process can be viewed as the mathematical analogue of complexing a planar aromatic hydrocarbon to a transition metal. The chirality of four-, five-, and six-site framework groups is discussed in terms of the maximum symmetry ligand partitions resulting in removal of all of the symmetry elements corresponding to improper rotations S _n (including reflections S ₁ and inversions S ₂) from achiral and polarized planar framework groups. The Ruch-Schönhofer group theoretical algorithms for the calculation of chiral ligand partitions and pseudoscalar polynomials of lowest degree (“chirality functions”) are adapted for use with these framework groups. Other properties of framework groups relevant to a study of their chirality are also discussed: these include their transitivity (i.e. whether all sites are equivalent or not), their normality (i.e. whether proper rotations correspond to even permutations and improper rotations correspond to odd permutations), and the number of sites in their symmetry planes.     

Palladium-catalysed α-allylation of chiral sulfinimines derived from symmetric cyclic ketones

A diastereoselective mono-allylation reaction at the α-position of symmetric cyclic ketones by using tert-butanesulfinamide as a chiral auxiliary is explored. Excellent yields and high diastereomeric ratios were achieved under palladium(0) catalysis in the presence of a readily available achiral phosphine ligand.     

Enantioselective addition of diethylzinc to N-diphenylphosphinoylimines employing cinchonidine and cinchonine as chiral ligands

The use of the `pseudoenantiomeric pair' of cinchonine and cinchonidine as ligands for the addition of diethylzinc to N-diphenylphosphinoylimines has been investigated. With 1 equiv of cinchonidine as ligand, a series of chiral amines was prepared in good yield and enantiomeric excesses of up to 94%. The use of 2.0 equiv of methanol as an achiral additive was found significantly to improve the selectivity of the addition when using 0.2 equiv of ligand, yielding ee's close to those obtained with a stoichiometric amount of ligand.     

Asymmetric palladium-catalyzed annulation of benzene-1,2-diol and racemic secondary propargylic carbonates bearing two different substituents

The palladium-catalyzed cyclization of benzene-1,2-diol with various racemic secondary propargyl carbonates having no acetylenic hydrogen in the presence of (R)-Binap as the chiral ligand afforded the two regioisomers of the corresponding 2,3-dihydro-1,4-dioxin derivatives in quite good yields, and also in enantioselectivities going from 40 to 97%. The cyclization of benzene-1,2-diol with methyl (R)-1-methyl-3-phenylpro-2-yn-1-yl carbonate in the presence of dppb as the achiral ligand afforded 2-benzylidene-3-methyl-2,3-dihydro-1,4-benzodioxine as the major product with 15% ee. The use of (R)-Binap as the chiral ligand afforded the (+) cyclized compound in 45% ee, when the (−) enantiomer was obtained with 77% ee in the presence of (S)-Binap. All the results suggest that in this case the enantioselective step is the diastereoselective protonation of the palladium-carbene intermediates.     

Asymmetric hydrogenation of quinolines catalyzed by iridium complexes of BINOL-derived diphosphonites

A chiral diphosphonite, derived from BINOL and with an achiral diphenyl ether backbone, is an excellent ligand for the Ir-catalyzed asymmetric hydrogenation of quinolines; achiral P-ligands serving as possible additives (ee = 73-96%).     

Protected amino acids as a nonbonding source of chirality in induction of single-handed screw-sense to helical macromolecular catalysts

Chiral nonbonding interaction with N-protected amino acid methyl esters used as chiral additives in achiral solvents allows dynamic induction of single-handed helical conformation in poly(quinoxaline-2,3-diyl)s (PQX) bearing only achiral substituents. Ac-l-Pro-OMe, for instance, allows induction of energy preference of 0.16 kJ mol⁻¹ per monomer unit for the M-helical structure over the P-helix in t-butyl methyl ether (MTBE). With this new mode of screw-sense induction, homochiral screw-sense has been induced in virtually achiral poly(quinoxaline-2,3-diyl)s 1000-mer containing phosphine pendants (PQXphos). Use of PQXphos as a helically dynamic ligand along with Ac-Pro-OMe (l or d) as a chiral additive in MTBE allowed a highly enantioselective Suzuki–Miyaura coupling reaction with up to 95% enantiomeric excess.

Achiral poly(quinoxaline-2,3-diyl) containing Ar₂P groups undergo dynamic induction of M-helical conformation through nonbonding interaction with protected AA such as Ac-l-Pro-OMe, serving as a chiral ligand in asymmetric cross-coupling with up to 95% ee.     

Alternative chiral thiols for preparation of chiral CdS quantum dots covered immediately by achiral thiols

Developing of alternative chiral thiol stabilizers from the assembly of achiral thiol (e.g. thioglycolic acid) and chiral ligand (e.g. arginine) via both hydrogen bonding and electrostatic interactions was proposed and successfully applied to an efficient preparation of chiral CdS quantum dots (QDs). Chiral CdS QDs capped mainly with achiral thioglycolic acid were also obtained that may allow the chiral QDs to be modified for extended applications.     

Separation mechanism of chiral impurities, ephedrine and pseudoephedrine, found in amphetamine-type substances using achiral modifiers in the gas phase

A new mechanism is proposed that describes the gas-phase separation of chiral molecules found in amphetamine-type substances (ATS) by the use of high-resolution ion mobility spectrometry (IMS). Straight-chain achiral alcohols of increasing carbon chain length, from methanol to n-octanol, are used as drift gas modifiers in IMS to highlight the mechanism proposed for gas-phase separations of these chiral molecules. The results suggest the possibility of using these achiral modifiers to separate the chiral molecules (R,S) and (S,R)-ephedrine and (S,S) and (R,R)-pseudoephedrine which contain an internal hydroxyl group at the first chiral center and an amino group at the other chiral center. Ionization was achieved with an electrospray source, the ions were introduced into an IMS with a resolving power of 80, and the resulting ion clusters were characterized with a coupled quadrupole mass spectrometer detector. A complementary computational study conducted at the density functional B3LYP/6-31g level of theory for the electronic structure of the analyte–modifier clusters was also performed, and showed either “bridged” or “independent” binding. The combined experimental and simulation data support the proposed mechanism for gas-phase chiral separations using achiral modifiers in the gas phase, thus enhancing the potential to conduct fast chiral separations with relative ease and efficiency.     

Efficient NMR enantiodiscrimination of bridge fluorinated paracyclophanes using lanthanide tris β-diketonate complexes

A selection of amino-substituted 1,1,2,2,9,9,10,10 octafluoro[2.2]paracyclophanes were tested for enantiodiscrimination by ¹H and ¹⁹F NMR spectroscopy via their interaction with different lanthanide tris β-diketonate chiral shift reagents. The amino-, and the pseudo-ortho di-amino substituted octafluoro[2.2]paracyclophanes, both of which exhibit planar chirality, revealed significant shifts and splittings of various ¹H and ¹⁹F NMR signals upon the addition of the chiral shift reagents, which allowed the easy determination of the enantiomeric purity. When the chiral shift reagent was added to an inseparable mixture of the (chiral) pseudo-meta, and (achiral) pseudo-para diamino analogues, both the chiral and achiral molecules revealed NMR doubling. In the case of the achiral molecule, this NMR behavior is due to the meso nature of the pseudo-para species.     

Spontaneous resolution of a 3D chiral polyoxometalate-based polythreaded framework consisting of an achiral ligand

Two enantiomerically 3D chiral POM-based architectures have been constructed based on the achiral ligand bbi, [V10O26](4-) polyoxoanion and mixed valence Cu(I/II) without a chiral auxiliary, and they represent the first examples of enantiomerically 3D POM-based compounds using achiral ligands.     

Synthesis, curing kinetics and thermal properties of novel difunctional chiral and achiral benzoxazines with double chiral centers

Novel difunctional chiral and achiral benzoxazine monomers were synthesized from the reaction of bisphenol A with paraformaldehyde and primary amines, including S-(+)-3-methyl-2-butylamine and rac-(±)-3-methyl-2-butylamine, by solventless method. The chemical structures of chiral and achiral benzoxazines were identified by fourier transform infrared, nuclear magnetic resonance (¹H NMR and ¹³C NMR). The curing behavior and non-isothermal curing kinetics of chiral and achiral benzoxazine monomers were investigated by differential scanning calorimeter (DSC). Isoconversional methods based on Friedman and Kissinger–Akahira–Sunose were applied to analyze the curing process of chiral and achiral benzoxazines. The thermal properties of cured polymers were characterized by DSC and thermogravimetry. The results suggested that the optical purity and stereo-configuration for chiral and achiral benzoxazines have definite influence on curing behavior and thermal properties despite the same chemical structure. Chiral benzoxazine displayed typical characteristics of difunctional benzoxazines. Achiral benzoxazine showed distinctly double peaks in DSC exotherms due to the presence of racemic and mesomeric isomers. The thermal properties of achiral polybenzoxazine were slightly higher than those of chiral polybenzoxazine, and were much higher than those of other bisphenol A-C₃–C₈ linear aliphatic amine-based polybenzoxazines because of tight packing, low free volume, and abundant intramolecular and intermolecular hydrogen bonds in network structure of polymers.     

Enantioselective synthesis of 1-vinyltetrahydroisoquinolines through palladium-catalysed intramolecular allylic amidation with chiral PhthalaPhos ligands

The PhthalaPhos ligands, chiral BINOL monophosphites endowed with a phthalamide group, have been screened in the synthesis of 1-vinyltetrahydroisoquinolines by intramolecular palladium-catalysed asymmetric allylic amidation (AAA) of achiral tosylamidocarbonates. Identification of the best ligand followed by optimisation of the reaction conditions allowed the desired product to be obtained with up to 83% ee. Remarkably, the reaction is stereoconvergent, affording the same enantiomer of the desired product regardless of the geometry of the allylic carbonate’s double bond, which allows, in principle, the use of E/Z mixtures.     

Heteroleptic [Bis(oxazoline)](dipyrrinato)zinc(II) Complexes: Bright and Circularly Polarized Luminescence from an Originally Achiral Dipyrrinato Ligand

Heteroleptic zinc(II) complexes synthesized using achiral dipyrrinato and chiral bis(oxazoline) ligands show bright fluorescence with quantum efficiencies of up to 0.70. The fluorescence originates from the ¹π–π^* photoexcited state localized exclusively on the dipyrrinato ligand. Furthermore, the luminescence is circularly polarized despite the achirality of the dipyrrinato ligand. Single‐crystal X‐ray structure analysis discloses that the chiral bis(oxazoline) ligand undergoes intramolecular π–π stacking with the dipyrrinato ligand, inducing axial chirality in the dipyrrinato moiety.     

Asymmetric crystallization of bisguanidinobenzene derivatives

We show that the mono-N-methylated and -ethylated derivatives of the achiral compound bisguanidinobenzene undergo spontaneous asymmetric crystallization into a chiral form with chiral space group P2₁2₁2₁. The absolute configurations of the chiral crystals were determined by X-ray crystallography and correlated with circular dichroism (CD) spectra recorded in the solid state. The corresponding protonated and isopropylated derivatives, by contrast, afforded achiral crystals.     

Discovery of chiral catalysts by asymmetric activation for highly enantioselective diethylzinc addition to imines: using racemic and achiral diimines as effective activators

A library of chiral zinc complexes formed in situ by the combination of achiral and racemic diimines with 3,3′-di(3,5-ditrifluoromethylphenyl)-BINOL and diethylzinc were evaluated in the asymmetric addition of diethylzinc to N-acylimines. In the presence of 10 mol % of chiral ligand 4 and racemic diimine 5, high enantioselectivities of up to 97% ee and yields of up to 96% were achieved for a wide range of aromatic imines in dichloromethane at −30 °C.     

Linear trimer analogues of calixarene as chiral coordinating ligands: X-ray crystallographic and NMR spectroscopic characterization of chiral and achiral trisphenolates complexed to titanium(IV) and aluminum(III)

Achiral and chiral linear trisphenol analogues of calixarene (HOArCH(2)Ar'(OH)C(R)HArOH, Ar = 4,6-di-tert-butylphenyl; Ar' = 4-tert-butylphenyl; R = H (achiral), Me (chiral)) were prepared in anticipation of their adoption of a chiral conformation upon coordination to Lewis acidic metal centers. The trisphenols react with 1 equiv of Ti(OR')(4) (R' = i-Pr or t-Bu) to yield complexes with molecular formula Ti(2)(OArCH(2)Ar'(O)C(R)HArO)(2)(OR')(2) (R = H, Me; R' = i-Pr or t-Bu). An X-ray crystal structure of the titanium complex of the achiral trisphenol (R = H; R' = t-Bu) reveals that the trisphenolate ligand adopts an unsymmetrical (and therefore chiral) conformation, with eta(2)-coordination to one metal center and eta(1)-coordination to the second metal center. The chiral trisphenol, which contains a stereogenic center (indicated as C in the shorthand notation used above), coordinates titanium in an analogous fashion to produce only one diastereomer (out of four possible); therefore, the configuration of the stereogenic center controls the conformation adopted by the bound ligand. The reaction of achiral trisphenol with AlMe(3) produces a compound with molecular formula Al(2)(OArCH(2)Ar'(O)CH(2)ArO)(2). (1)H NMR spectroscopy and X-ray crystallography reveal that the trisphenolate ligand adopts an asymmetric, C(2) conformation in this complex, where the central phenolate oxygen bridges the aluminum centers and the terminal phenolate oxygens each coordinate a separate aluminum center. Because these trisphenolate ligands adopt chiral conformations when coordinated to metal centers, they may be useful for developing diastereo- or enantioselective catalysts and reagents.     

Synthesis of chiral iridium complexes immobilized on amphiphilic polymers and their application to asymmetric catalysis

This article details the enantioselective catalytic performance of crosslinked, polymer immobilized, Ir‐based, chiral complexes for transfer hydrogenation of cyclic imines to chiral amines. Polymerization of the achiral vinyl monomer, divinylbenzene, and a polymerizable chiral 1,2‐diamine monosulfonamide ligand followed by complexation with [IrCl₂Cp*]₂ affords the crosslinked polymeric chiral complex, which can be successfully applied to asymmetric transfer hydrogenation of cyclic imines. Polymeric catalysts prepared from amphiphilic achiral monomers have high catalytic activity in the reaction and can be used both in organic solvents and water to give chiral cyclic amines with a high level of enantioselectivity (up to 98% ee). The asymmetric reaction allows for reuse of the heterogeneous catalyst without any loss in activity or enantioselectivity over several runs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3037–3044     

First study on the enantioselective palladium-catalyzed C-P cross-coupling reaction between an alkenyltriflate and a phosphine-borane

In this article, we report the first example of a palladium-catalyzed asymmetric C-P cross-coupling reaction between a racemic secondary phosphine-borane, methylphenylphosphine-borane 1, and an achiral triflate. The influence of various parameters such as the structure of the chiral ligand, the temperature and the nature of the solvent on the activity and the selectivity of the reaction is reported. Enantiomeric ratios up to 78:22 were obtained using (S,S)-Me-DUPHOSPdCl₂ as catalyst. A kinetic resolution process is proposed to account for this selectivity.