Intense Cotton Effects Research Articles

Synthesis and chiroptical properties of optically active poly(ethynylcarbazole) derivatives: Substituent effect on the helix formation

AbstractNovel chiral acetylene monomers containing carbazole, 2‐ethynyl‐9‐[(S)‐2‐methylbutoxycarbonyl]carbazole (1), 3‐ethynyl‐9‐[(S)‐2‐methylbutoxycarbonyl]carbazole (2), 2‐ethynyl‐9‐[(S)‐2‐methylbutyl]carbazole (3), and 2‐ethynyl‐9‐[(S)‐4‐methylhexyl]carbazole (4) were synthesized and polymerized with [(nbd)RhCl]2Et3N. The corresponding polyacetylenes with number‐average molecular weights ranging from 68,700 to 310,000 were obtained in good yields. Poly(1) exhibited a large specific rotation and an intense Cotton effect in toluene, indicating that it formed a helix with predominantly one‐handed screw sense, while the other three polymers showed no evidence for taking a helical structure. Poly(1) largely decreased the CD intensity upon heating from −10 to 60 °C. Poly(1) showed a Cotton effect in film state in a manner similar to solution state. No chiral amplification was observed in the copolymerization of 1 with achiral 2‐ethynyl‐9‐tert‐butoxycarbonylcarbazole (5). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4450–4458, 2007

Preparation of Optically Active Pyridine-Based Conducting Polymer Films Using a Liquid Crystal Electrolyte Containing a Cholesterol Derivative

Optically active poly(1,4-di(2-thienyl)phenylene), poly(terthiophene), poly(bithiophene), and poly(2,5-di(2-thienyl)pyridine) are successfully prepared by asymmetric electrochemical polymerization in a cholesteric electrolyte. The polymers thus synthesized are confirmed by polarizing optical microscopy to assume the double-spiral optical texture of the cholesteric liquid crystal electrolyte. In the reduced state, these polymers exhibit an intense Cotton effect in circular dichroism measurements. This optically active electrochromism is reversible by appropriate adjustment of applied voltage (redox state). Cyclic voltammetry measurements indicate that poly(2,5-di(2-thienyl)pyridine) has an electrochemical n-dopable property.

Synthesis and helicity of optically active poly(N‐propargylphosphonamidates) having chiral phosphorus center

AbstractNovel chiral N‐propargylphosphonamidate monomers (HCCCH2NHP(O)ROmenthyl, 1: R = CH3, 2: R = C2H5, 3: R = n‐C3H7, 4: R = Ph) were synthesized by the reaction of the corresponding phosphonic dichlorides with menthol and propargylamine. Pairs of diastereomeric monomers 1–4 with different ratios were obtained due to the chiral P‐center and menthyl group. One diastereomer could be separated from another one in the cases of monomers 1 and 2. Polymerization of 1–4 with (nbd)Rh+[η6‐C6H5B−(C6H5)3] as a catalyst in CHCl3 gave the polymers with number‐average molecular weights ranging from 5000 to 12,000 in 65–85%. Poly(1)–poly(4) exhibited quantitative cis contents, and much larger specific rotations than 1–4 did in CHCl3. The polymers showed an intense Cotton effect around 325 nm based on the conjugated polyacetylene backbone. It was indicated that the polymers took a helical structure with predominantly one‐handed screw sense, and intramolecular hydrogen bonding between PO and NH of the polymers contributed to the stability of the helical structure. Poly(1a) and poly(2a) decreased the CD intensity upon raising CH3OH content in CHCl3/CH3OH. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1515–1524, 2007

Synthesis and characterization of chiral polymer complexes incorporating polybinaphthyls, bipyridine, and Eu(III)

AbstractChiral conjugated polymers P‐1 and P‐2 were synthesized by the polymerization of (S)‐3,3′‐diiodo‐2,2′‐bisbutoxy‐1,1′‐binaphthyl and (S)‐6,6′‐dibromo‐2,2′‐bisbutoxy‐1,1′‐binaphthyl, respectively, with 5,5′‐divinyl‐2,2′‐bipyridine through a Heck cross‐coupling reaction. Chiral polymer complexes P‐C‐1 and P‐C‐2 were obtained by the bipyridine chelating coordination of P‐1 and P‐2 with Eu(TTA)3·2H2O (where TTA is 2‐thenoyltrifluoroacetonate). Polymers P‐1 and P‐2 and polymer complexes P‐C‐1 and P‐C‐2 exhibited intense circular dichroism signals, with negative and positive Cotton effects in their circular dichroism spectra. The chiral polymers showed strong green‐blue fluorescence because of the efficient energy migration from the extended π‐electronic structure of the conjugated polymer main to the chiral binaphthyl core. The chiral polymer complexes could have not only polymer fluorescence but also the characteristic fluorescence of Eu(III) (5D0→7F2) at a different excited wavelength. These kinds of chiral polymer complexes incorporating polybinaphthyls, bipyridine, and Eu(III) moieties are expected to provide an understanding of the relationship between the structure and properties of chiral polymer complexes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 650–660, 2007

Copolymerization of N-propargylphosphonamidates. Helicity control of the copolymers by P-chirality

The copolymerization of diastereomeric pairs of N-propargylphosphonamidate monomers 1–3 ((1R,2S,5R)-HCCCH2NHP(O)R-O-menthyl, 1: R=CH3, 2: R=C2H5, 3: R=n-C3H7) carrying P- and C-chiral centers was carried out with (nbd)Rh+[η6-C6H5B−(C6H5)3] as a catalyst in CHCl3 for 24h to afford the copolymers with number-average molecular weights ranging from 1400 to 12 000 in 65–85% yields. The cis contents of the copolymers were above 80%. The copolymers consisting of a pair of diastereomeric monomer units with a large diastereomeric excess showed a large specific rotation and an intense Cotton effect in CHCl3, indicating that they take a helical conformation with predominantly one-handed screw sense. On the other hand, the copolymer with a small diastereomeric excess showed a small specific rotation and a CD signal. The copolymerization of P-(R)-1 (1a) with an achiral N-propargylphosphoramidate monomer, HCCCH2NHP(O)(OPh)2 (4) was also carried out with various feed ratios. Among the obtained copolymers with various compositions, poly(1a95-co-45) showed the largest specific rotation and CD intensity in CHCl3, and poly(1a50-co-450) did so in THF. The copolymers decreased the CD intensity upon raising the temperature.

Helical Sense Selective Polymerization of Bulky Aryl Isocyanide Possessing Chiral Ester or Amide Groups Initiated by Arylrhodium Complexes

Bulky aryl isocyanide monomers possessing tert-butyl groups at the ortho position and chiral ester or amide groups at the para position were prepared and polymerized by arylrhodium complex to give polyisocyanides with narrow polydispersity indexes in good yields. The large specific rotation and the intense Cotton effect at 347 nm suggest that the resulting polymers maintained predominantly one-handed helical conformation in solution. The chiroptical properties were increased with an increase in the polymerization degree and were fixed in the region of the polymerization degree more than 70. A positive nonlinear relationship was observed between the optical purity of the chiral monomer and the helical sense selectivity of the resulting polymers. The helical sense selectivity depends on the structure of the chiral ester groups such as the length of the alkyl chain and the position of the chiral carbon center.

Synthesis and Polymerization of Optically Active N‐Propargylphosphonamidates: A Novel Helical Polymer Carrying a P‐Chiral Center

AbstractSummary: A diastereomeric pair of novel N‐propargylphosphonamidates, HCCCH2NHP(O)(CH3)O‐L‐menthyl was synthesized by the successive condensations of methylphosphonic dichloride with L‐menthol and propargylamine. The (R)‐P‐isomer (1a) was isolated, and the absolute configuration was determined by XRD. Polymerization of 1a, and a mixture of 1a and (S)‐P‐isomer (1b) was carried out with a zwitterionic Rh complex as a catalyst. cis‐Stereoregular polymers with number‐average molecular weights of 5 600–9 800 were obtained in good yields. Poly(1a) and poly(1a29‐co‐1b71) exhibited large specific rotations (+408 and −146°), and intense Cotton effects ([θ] = +2.25 and −0.9 × 104 deg · cm2 · dmol−1) based on the conjugated polyacetylene backbone around 325 nm in CHCl3, indicating that these polymers have helical structures, whose predominant helical senses are opposite.Polymerization of N‐propargylphosphonamidate.magnified imagePolymerization of N‐propargylphosphonamidate.

Chiroptical Activity of BINAP-Stabilized Undecagold Clusters

Undecagold cluster compounds [Au11(BINAP)4X2]+ (X = Cl and Br) were synthesized by chemical reduction of the corresponding precursor complexes, Au2X2(BINAP), where BINAP represents the bidentate phosphine ligand 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. The circular dichroism spectra of Au11 stabilized by the enantiomers [Au11(R-BINAP)4X2]+ and [Au11(S-BINAP)4X2]+ exhibited intense and mirror-image Cotton effect, whereas those of Au11(3+) clusters stabilized by achiral monodentate phosphine ligands did not. The origin of the chiroptical activity of [Au11(BINAP)4X2]+ is discussed in the context of the structural deformation of the Au11(3+) core.

Self‐assembly of multinuclear complexes with enantiomerically pure chiral binaphthoxy imine ligands: Effect of the alkyl spacer connecting two binaphthyl units on the metal binding

Metal complexing behavior of enantiomerically pure alpha,omega-diiminoalkanes possessing the two terminal binaphthyl units (L1 and L2) was studied. The ligands L1 and L2 were prepared by the reaction of optically pure 2'-butoxy-3-formyl-2-hydroxy-1,1'-binaphthyl with propane and pentane diamines. Reactions of L1 and L2 with equimolar amount of Cu(OAc)2 afforded quantitatively multinuclear complexes 1. The structure of 1 was confirmed by MALDI-TOF MS spectroscopy, X-ray single-crystal-structure analysis, and UV/vis and CD spectroscopic analyses. The reaction of L1 having a 1,3-propanediyl spacer resulted in the formation of a self-assembled product, which was assigned as enantiopure trinuclear circular helicate 1a, while the ligand having a 1,5-pentanediyl spacer L2 gave a different self-assembled product, dinuclear side-by-side complex 1b. The circular dichroism (CD) spectrum of 1a in solution showed intense Cotton effects in both the pi-pi* transition of the naphthalene units and the LMCT region of the N,O-chelate moieties. The CD spectrum of 1b was completely different from that of 1a; in particular the Cotton effects in the LMCT region were very weak, contrary to those of 1a. These results suggest that 1a retains some chirality induced on the N,O-chelating moieties even in solution, while the induced chirality on the N,O-chelating moieties in 1b is not very significant, being consistent with the consequences of the X-ray single-crystal-structure studies.

Cyclopolymerization of Amino Acid-Based Diynes and Properties of the Obtained Polymers. Chiral Recognition and Metal Ion Extraction

Cyclopolymerization of amino acid-derived novel dipropargyl ester and dipropargylamide was carried out using a rhodium catalyst. Polymers with cyclized structures were obtained under certain conditions. The dipropargylamide polymer exhibited a large specific rotation and an intense Cotton effect around 300 nm based on the chiral higher order structure. It adsorbed N-benzyloxycarbonyl-L-alanine more than the D-isomer, and extracted Li+, Na+, K+, and Cs+ from water phase into CH2Cl2 phase.

Unambiguous Determination of the Absolute Configurations of Acetylene Alcohols by Combination of the Sonogashira Reaction and the CD Exciton Chirality Method ? Exciton Coupling between Phenylacetylene and Benzoate Chromophores

The CD exciton chirality method was applied to various phenylacetylene alcohols to determine their absolute configurations; the long axis polarized π–π* transition (λmax=252 nm) of the 4-methoxyphenylacetylene chromophore couples with the transition (λmax=257 nm) of the 4-methoxybenzoate group to generate intense exciton split CD Cotton effects, from the signs of which the absolute configurations of phenylacetylene alcohols were unambiguously determined. As an extension of the results, a new methodology for determining the absolute configurations of acetylene alcohols having the HC≡CCH(OH)-moiety by combination of the Sonogashira reaction and the CD exciton chirality method has been developed and applied. Since the π–π* transition of acetylene triple bond is located below 180 nm, it is difficult to observe ideal bisignate CD Cotton effects due to the exciton coupling between acetylene and benzoate chromophores. To observe the ideal exciton split Cotton effects necessary for the unambiguous determination of absolute configuration, the terminal acetylene group was converted, by the Sonogashira reaction, to the 4-methoxyphenylacetylene moiety, which exhibits an intense π–π* absorption band polarized along the long axis of the chromophore at 252 nm. As a partner of exciton coupling, 4-methoxybenzoate showing a π–π* band at 257 nm was introduced into the alcohol moiety, and the benzoates formed showed intense bisignate CD Cotton effects, from the signs of which the absolute configurations of original acetylene alcohols could be determined in an unambiguous manner.

Chiral Organic Radical Cation and Dication. A Reversible Chiroptical Redox Switch Based on Stepwise Transformation of Optically Active Tetrakis(p-alkoxyphenyl)ethylenes to Radical Cations and Dications

Optically active tetrakis(p-alkoxyphenyl)ethylenes were found to function as reversible chiroptical switches upon redox transformations. Successive one-electron oxidations of chirally modified tetraarylethylene to the corresponding radical cation and then to the dication led to dramatic changes in the electronic absorption and circular dichroism (CD) spectra. The neutral species showed no color or CD in the visible region, while the radical ion was blue in color and exhibited a weak Cotton effect, with the dication green and giving an intense Cotton effect and a sign opposite that observed for the radical cation, at a longer wavelength. Molecular orbital calculations and X-ray crystallographic studies clearly indicate that the olefinic C=C bond is significantly twisted in the dication to minimize the electrostatic and steric repulsions. By lowering the temperature of the dication, the twist around the double bond is more firmly fixed in either P or M chirality to give a stronger Cotton effect and a larger anisotropy (g) factor. Since the spectral changes are completely reversible and reproducible for multiple redox cycles, this chiral redox system can be used in novel redox-driven chiroptical applications, such as molecular switches and memory devices, in which the information is written/read chiroptically in the ternary mode, giving zero CD signal in the neutral form, positive CD for the radical cation, and negative CD for the dication at a given wavelength.

Syntheses, Hydrogen-Bonding Interactions, Tunable Chain Helicities, and Cooperative Supramolecular Associations and Dissociations of Poly(Phenylacetylene)s Bearingl-Valine Pendants: Toward the Development of Proteomimetic Polyenes

4-Ethynylbenzoyl-l-valine methyl ester (1e), an acetylene−valine adduct, is polymerized by organorhodium catalysts to the corresponding “polyester” (P1e) of high molecular weights (Mw up to 371 000) and high stereoregularities (Z content up to 100%) in high yields (up to ∼95%). The amino acid residues form intrastrand and interstrand hydrogen bonds within and between the polymer chains. The ester groups of P1e are selectively deprotected by base-catalyzed hydrolysis, giving “polyacid” P1a with “free” valine pendants. While 1e is CD-inactive at λ > 300 nm, both P1e and P1a exhibit intense Cotton effects in the long wavelength region where the polyacetylene backbone absorbs, confirming that the chiral valine pendants have induced the polymer chain to take a helical conformation with an excess in one handedness. The helicity of the chain segments is sensitive to the variations in their environmental surroundings. Utilizing this environmental susceptibility, the chain helicity of the polymers is tuned continu...

Helix-Forming Oligoureas: Temperature-Dependent NMR, Structure Determination, and Circular Dichroism of a Nonamer with Functionalized Side Chains

To further investigate the degree of structural homology between γ-peptides A and N,N′-linked oligoureas B, we prepared oligourea nonamer 2 containing Ala, Val, Leu, Phe, Tyr and Lys side chains. Oligomer 2 was synthesized on solid support from activated monomers, i.e., from enantiomerically pure succinimidyl {2-{[(9H-fluoren-9-ylmethoxy)carbonyl]amino}ethyl}carbamates 3a–f that are further substituted at C(2) of the ethyl moiety. These precursors were conveniently prepared from N-Fmoc-protected β3-amino acids with corresponding side chains. Detailed NMR studies (DQF-COSY, TOCSY, and ROESY) in (D5)pyridine revealed that 2 adopts a regular (P)-2.5 helical secondary structure very similar to that previously determined for oligourea heptamer 1 and closely related to the (P)-2.614 helix of γ-peptides. Temperature-dependent NMR further demonstrated the conformational homogeneity and remarkable stability of the structure of 2 in pyridine. The CD spectrum of 2 (0.2 mM) was recorded in MeOH with the aim to gain more information about the conformation of oligoureas. In contrast to 2.6-helical γ-peptides, which display only a weak or no Cotton effect, oligourea 2 exhibits an intense positive Cotton effect at ca. 203 nm ([Θ]=+373000 deg cm2 dmol−1) that decreases only slowly upon increasing the temperature.

Synthesis ofβ3-Peptides and Mixedα/β3-Peptides by Thioligation

Five β-peptide thioesters (1–5, containing 3, 4, 10 residues) were prepared by manual solid-phase synthesis and purified by reverse-phase preparative HPLC. A β-undecapeptide (6) and an α-undecapeptide (7) with N-terminal β3-HCys and Cys residues were prepared by manual and machine synthesis, respectively. Coupling of the thioesters with the cysteine derivatives in the presence of PhSH (Scheme and Fig. 1) in aqueous solution occurred smoothly and quantitatively. Pentadeca- and heneicosapeptides (8–10) were isolated, after preparative RP-HPLC purification, in yields of up to 60%. Thus, the so-called native chemical ligation works well with β-peptides, producing larger β3- and α/β3-mixed peptides. Compounds 1–10 were characterized by high-resolution mass spectrometry (HR-MS) and by CD spectroscopy, including temperature and concentration dependence. β-Peptide 9 with 21 residues shows an intense negative Cotton effect near 210 nm but no zero-crossing above 190 nm, (Figs. 2–4), which is characteristic of β-peptidic 314-helical structures. Comparison of the CD spectra of the mixed α/β-pentadecapeptide (10) and a helical α-peptide (Fig. 5) indicate the presence of an α-peptidic 3.613 helix.

Absolute stereochemistry of chiral C60 fullerene bis-adducts.

To determine the absolute configuration of chiral fullerene bis-adducts, we have studied the double Bingel reaction of C60 with chiral tether (2S,3S)-(-)-9 derived from (R,R)-(+)-tartaric acid, and have succeeded in isolating two possible chiral bis-adducts 10a (5%) and 10b (2%) in addition to the Cs-symmetrically added derivative 10c (40%). The CD spectra of chiral bis-adducts [CD(+)281]-10a and [CD(-)281]-10b show very intense Cotton effects, which are almost of mirror image, indicating that their chiral C60 pi-electron systems are enantiomeric each other. The 1H and 13C NMR spectra of 10a and 10b indicate that they have C2-symmetrical structures, and the vicinal coupling constants between two equivalent protons H-2 and H-2' were determined as 1.2 Hz for 10a and 1.8 Hz for 10b, respectively by the 13C satellite band method. From the conformational analyses, the absolute configurations of these chiral C60 fullerene bis-adducts were unambiguously determined as [CD(+)281]-(S,S,fC)-10a and [CD(-)281]-(S,S,fA)-10b, respectively.

An enantiomerically pure bilirubin. Absolute configuration of (α R,α′ R)-dimethylmesobilirubin-XIIIα

Enantiomerically pure (+)-(α R,α′ R)-dimethylmesobilirubin-XIIIα 1 and its (α S,α′ S) enantiomer ent- 1 were synthesized in ten steps from simple precursors. Resolution was achieved at an early stage in the synthesis, with a racemic monopyrrole precursor rac- 6 being converted to its amides 8 with (1 S)-camphor-2,10-sultam. Resolution of 8 to 99% d.e. was accomplished in three crystallizations, and the absolute configuration of the acid 6 was deduced by X-ray crystallography of the more crystalline, diastereomerically pure amide 7. Circular dichroism spectroscopy of 1 showed intense bisignate Cotton effects: Δ ε max 435=+344, Δ ε max 391=−193 (CHCl 3), as expected for a molecular exciton, and consistent with a P -helical intramolecularly hydrogen-bonded ridge-tile conformation. The Cotton effect magnitudes of 1 match almost exactly those found for (−)-(β S,β′ S)-dimethylmesobilirubin-XIIIα 11 and (+)-(α R,β′ R)-dimethylmesobilirubin-XIIIα. However, the Cotton effect of the pseudo- meso diastereomer (α R,β′ S)-dimethylmesobilirubin-XIIIα 12 is not zero. Its large positive exciton couplet and 1H NMR NOE analysis confirm that an α-CH 3 exerts a greater steric demand than a β-CH 3—by a factor of ∼3.

Synthesis and CD Spectra in MeCN, MeOH, and H2O ofγ-Oligopeptides with Hydroxy Groups on the Backbone, Preliminary Communication

γ4-Tripeptides and γ4-hexapeptides, 1 – 4, with OH groups in the 2- or 3-position on each residue have been prepared. The corresponding 2-hydroxy amino acids were obtained by Si-nitronate (3+2) cycloadditions to the acryloyl derivative of Oppolzer's sultam and Raney-Ni reduction of the resulting 1,2-oxazolidines (Scheme 1). The 3-hydroxy amino acid derivatives were prepared by chain elongation via Claisen condensation of Boc-Ala-OH, Boc-Val-OH, and Boc-Leu-OH, and NaBH4 reduction of the methyl 4-amino 3-oxo carboxylates formed (Scheme 2). The N-Boc hydroxy amino acids were coupled in solution to give the γ-peptides. CD Spectra of the new types of γ-peptides were recorded and compared with those of simple γ2-, γ3-, γ4-, and γ2,3,4-peptides (Figs. 3, 4, and 5). An intense Cotton effect at ca. 200 nm ([Θ]=−2⋅105 deg⋅cm2⋅dmol−1) indicates that the hexapeptide built of (3R,4S)-4-amino-3-hydroxy acids (with the side chains of Val, Ala, Leu) folds to a secondary structure so far unknown. The stability of peptides from β- and γ-amino acids, which carry heteroatoms on their backbones is discussed (Fig. 1). Positions on the γ-peptidic 2.614 helix are identified at which non-H-atoms are `allowed' (Fig. 2).

A Chirally Stable, Atropoisomeric, C-Tetrasubstituted -Amino Acid: Incorporation into Model Peptides and Conformational Preference

A variety of model peptides, including four complete homologous series, to the pentamer level, characterized by the recently proposed binaphthyl-based, axially chiral, Cα-tetrasubstituted, cyclic α-amino acid Bin, in combination with Ala, Gly, or Aib residues, was synthesized by solution methods and fully characterized. The solution conformational propensity of these peptides was determined by FT-IR absorption and 1H-NMR techniques. Moreover, the molecular structures of the free amino acid (S)-enantiomer and an Nα-acylated dipeptide alkylamide with the heterochiral sequence -(R)-Bin-Phe- were assessed in the crystal state by X-ray diffraction. Taken together, the results point to the conclusion that β-bends and 310 helices are preferentially adopted by Bin-containing peptides, although the fully extended conformation would also be adopted in solution by the short oligomers to some extent. We also confirmed the tendency of (R)-Bin to fold a peptide chain into right-handed bend and helical structures. The absolute configuration of the Bin residue(s) was correlated with the typically intense exciton-split Cotton effect of the 1Bb binaphthyl transition near 225 nm.

A Chirally Stable, Atropoisomeric,Cα-Tetrasubstitutedα-Amino Acid: Incorporation into Model Peptides and Conformational Preference

A variety of model peptides, including four complete homologous series, to the pentamer level, characterized by the recently proposed binaphthyl-based, axially chiral, Cα-tetrasubstituted, cyclic α-amino acid Bin, in combination with Ala, Gly, or Aib residues, was synthesized by solution methods and fully characterized. The solution conformational propensity of these peptides was determined by FT-IR absorption and 1H-NMR techniques. Moreover, the molecular structures of the free amino acid (S)-enantiomer and an Nα-acylated dipeptide alkylamide with the heterochiral sequence -(R)-Bin-Phe- were assessed in the crystal state by X-ray diffraction. Taken together, the results point to the conclusion that β-bends and 310 helices are preferentially adopted by Bin-containing peptides, although the fully extended conformation would also be adopted in solution by the short oligomers to some extent. We also confirmed the tendency of (R)-Bin to fold a peptide chain into right-handed bend and helical structures. The absolute configuration of the Bin residue(s) was correlated with the typically intense exciton-split Cotton effect of the 1Bb binaphthyl transition near 225 nm.