Control of Circularly Polarized Luminescence by Orientation of Stacked π‐Electron Systems

Planar chiral building blocks based on 4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were obtained via a synthetic route involving an optical resolution step. Planar chiral enantiomers, comprising two fluorophores that were stacked to form a V‐shaped higher‐ordered structure, were synthesized from these building blocks. The V‐shaped molecules emitted intense circularly polarized luminescence (CPL). Their chiroptical properties were compared with those of X‐shaped molecules bearing the same two fluorophores stacked together. The CPL sign of the X‐shaped molecule was opposite to that of the V‐shaped molecule, which is supported by the theoretical results, indicating that the CPL sign can be controlled by the orientation of the stacked fluorophores.     

Synthesis of Optically Active,X‐Shaped,Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane,Leading to Highly Emissive Circularly Polarized Luminescence

Optically active, Fréchet‐type dendrimers containing an emissive X‐shaped π‐electron system as the core unit were synthesized. Gram‐scale optical resolution and transformations of 4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were also carried out. The high‐generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X‐shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.     

Homochiral [2.2]Paracyclophane Self‐Assembly Promoted by Transannular Hydrogen Bonding

[2.2]paracyclophane (pCp), unlike many π‐building blocks, has been virtually unexplored in supramolecular constructs. Reported here is the synthesis and characterization of the first pCp derivatives capable of programmed self‐assembly into extended cofacial π‐stacks in solution and the solid state. The design employs transannular (intramolecular) hydrogen bonds (H‐bonds), hitherto unstudied in pCps, between pseudo‐ortho‐positioned amides of a pCp‐4,7,12,15‐tetracarboxamide (pCpTA) to preorganize the molecules for intermolecular H‐bonding with π‐stacked neighbors. X‐ray crystallography confirms the formation of homochiral, one‐dimensional pCpTA stacks helically laced with two H‐bond strands. The chiral sense is dictated by the planar chirality (R_p or S_p) of the pCpTA monomers. A combination of NMR, IR, and UV/Vis studies confirms the formation of the first supramolecular pCp polymers in solution.     

Preparation of New Ferrocenylmonophosphine Ligands Containing Two Planar Chiral Ferrocenyl Moieties and Their Use for Palladium‐Catalyzed Asymmetric Hydrosilylation of 1,3‐Dienes

Bis{(R_p)‐2‐[(1S)‐1‐methoxyethyl]ferrocenyl}arylphosphines (S,R_p)‐ 9 (aryl=4‐MeOC₆H₄ ( 9a ), Ph ( 9b ), 4‐CF₃C₆H₄ ( 9c ), 3,5‐(CF₃)₂C₆H₃ ( 9d )), which contain two planar chiral ferrocenyl moieties, were prepared via (R_p)‐1‐bromo‐2‐[(1S)‐1‐methoxyethyl]ferrocene ((S,R_p)‐ 8 ). Asymmetric hydrosilylation of linear 1,3‐dienes such as deca‐1,3‐diene ( 10a ) with trichlorosilane in the presence of a palladium catalyst coordinated with 9d gave allylic silanes of up to 93% ee.     

Twisted Morphologies and Novel Chiral Macroporous Films from the Self‐Assembly of Optically Active Helical Polyphosphazene Block Copolymers

A series of optically active helical polyphosphazene block copolymers of general formula R? [N?P(O₂C₂₀H₁₂)]_n‐b‐[N?PMePh]_m (R‐ 7 a – c ) was synthesized and characterized. The polymers were prepared by sequential living cationic polycondensation of N‐silylphosphoranimines using the mono‐end‐capped initiator [Ph₃P?N?PCl₃][PCl₆] ( 5 ) and exhibit a low polydispersity index (ca. 1.3). The temperature dependence of the specific optical activity ([α]_D) of R‐ 7 a , b relative to that for the homopolymers R‐[N?P(O₂C₂₀H₁₂)]_n (R‐ 8 a ) and the R/S analogues (R/S‐ 7 a , b ), revealed that the binaphthoxy–phosphazene segments induce a preferential helical conformation in the [N?PMePh] blocks through a “sergeant‐and‐soldiers” mechanism, an effect that is unprecedented in polyphosphazenes. The self‐assembly of drop‐cast thin films of the chiral block copolymer R‐ 7 b (bearing a long chiral and rigid R? [N?P(O₂C₂₀H₁₂)] segment) evidenced a transfer of helicity mechanism, leading to the formation of twisted morphologies (twisted “pearl necklace”), not observed in the nonchiral R/S‐ 7 b . The chiral R‐ 7 a and the nonchiral R/S‐ 7 a , self‐assemble by a nondirected morphology reconstruction process into regular‐shaped macroporous films with chiral‐rich areas close to edge of the pore. This is the first nontemplate self‐assembly route to chiral macroporous polymeric films with pore size larger than 50 nm. The solvent annealing (THF) of these films leads to the formation of regular spherical nanostructures (ca. 50 nm), a rare example of nanospheres exclusively formed by synthetic helical polymers.     

Synthesis of Both Enantiomers of Nine‐Membered CF3‐Substituted Heterocycles Using a Single Chiral Ligand: Palladium‐Catalyzed Decarboxylative Ring Expansion with Kinetic Resolution

The two enantiomers of trifluoromethyl‐benzo[c][1,5]oxazonines, (R)‐ 4 and (S)‐ 4 , can be selectively accessed with high enantiopurity by the Pd‐catalyzed ring‐expansion reaction of trifluoromethyl‐benzo[d][1,3]oxazinones ( 1 ) with vinyl ethylene carbonates ( 3 ) using one antipode of a chiral ligand. Initially, the reaction proceeds by a double decarboxylative ring‐expansion with kinetic resolution of 1 in the presence of a Pd‐catalyst/chiral ligand to provide (R)‐ 4 with high enantiopurity. At the same time, the nonreactive antipode of 1 , (S)‐ 1 , which was recovered with an impeccable s factor of up to 713 and an ideal chemical yield, was transferred into the antipode of the products, (S)‐ 4 , with high enantiopurity by a second run of the Pd‐catalyzed double decarboxylation reaction, but this time without any chiral auxiliary. Thus, both antipodes of the chiral trifluoromethyl heterocycles 4 can be obtained in excellent enantiopurity using only a single antipode of the chiral catalyst.     

Enantioselective addition of diethylzinc to aldehydes catalyzed by monosubstituted [2.2]paracyclophane-based N,O-ligands: remarkable cooperative effects of planar and central chiralities

Diastereomeric monosubstituted [2.2]paracyclophane-based N,O-ligands, which unite the planar and central chiral elements, were optimized for the enantioselective diethylzinc addition to aldehydes. (S)-1-{(S_p)-[2.2]Paracyclophan-4-yl}methyl-2-pyrrolidine-α,α-diphenylmethanol (S_p,S)-3 catalyzed the addition to give (R)-1-phenyl-1-propanol in a high yield and with a good enantioselectivity (91% ee).     

A Convenient Approach to C2‐Chiral 1,1,4,4‐Tetrasubstituted Butanetetraols: Direct Alkylation or Arylation of Enantiomerically Pure Diethyl Tartrates

C₂‐Chiral 1,1,4,4‐tetraaryl‐ or 1,1,4,4‐tetraalkyl‐substituted butanetetraols have been conveniently synthesized via arylation or alkylation of unprotected diethyl (2R,3R)‐ and (2S,3S)‐tartrates with Grignard reagent. The chiral 1,1,4,4‐tetrasubstituted butanetetraols were characterized by IR, ¹H‐ and ¹³C‐NMR, as well as LC/MS.     

Preferred 3D‐Structure of Peptides Rich in a Severely Conformationally Restricted Cyclopropane Analogue of Phenylalanine

Terminally blocked, homo‐peptide amides of (R,R)‐1‐amino‐2,3‐diphenylcyclopropane‐1‐carboxylic acid (c₃diPhe), a chiral member of the family of C^α‐tetrasubstituted α‐amino acids, from the dimer to the tetramer, and diastereomeric co‐oligopeptides of (R,R)‐ or (S,S)‐c₃diPhe with (S)‐alanine residues to the trimer level were prepared in solution and fully characterized. The synthetic effort was extended to terminally protected co‐oligopeptide esters to the hexamer, where c₃diPhe residues are combined with achiral α‐aminoisobutyric acid residues. The preferred conformations of the peptides were assessed in solution by FT‐IR absorption, NMR, and CD techniques, and for seven oligomers in the crystal state (by X‐ray diffraction) as well. This study clearly indicates that c₃diPhe, a sterically demanding cyclopropane analogue of phenylalanine, tends to fold peptides into β‐turn and 3₁₀‐helix conformations. However, when c₃diPhe is in combination with other chiral residues, the conformation preferred by the resulting peptides is also dictated by the chiral sequence of the amino acid building blocks. The (S,S)‐enantiomer of this α‐amino acid, unusually lacking asymmetry in the main chain, strongly favors the left‐handedness of the turn/helical peptides formed.     

One‐Handed Helical Screw Direction of Homopeptide Foldamer Exclusively Induced by Cyclic α‐Amino Acid Side‐Chain Chiral Centers

Chiral cyclic α,α‐disubstituted amino acids, (3S,4S)‐ and (3R,4R)‐1‐amino‐3,4‐(dialkoxy)cyclopentanecarboxylic acids ((S,S)‐ and (R,R)‐Ac₅c^dOR; R: methyl, methoxymethyl), were synthesized from dimethyl L ‐(+)‐ or D ‐(?)‐tartrate, and their homochiral homoligomers were prepared by solution‐phase methods. The preferred secondary structure of the (S,S)‐Ac₅c^dOMe hexapeptide was a left‐handed (M) 3₁₀ helix, whereas those of the (S,S)‐Ac₅c^dOMe octa‐ and decapeptides were left‐handed (M) α helices, both in solution and in the crystal state. The octa‐ and decapeptides can be well dissolved in pure water and are more α helical in water than in 2,2,2‐trifluoroethanol solution. The left‐handed (M) helices of the (S,S)‐Ac₅c^dOMe homochiral homopeptides were exclusively controlled by the side‐chain chiral centers, because the cyclic amino acid (S,S)‐Ac₅c^dOMe does not have an α‐carbon chiral center but has side‐chain γ‐carbon chiral centers.     

α‐Propargyl amino acid‐derived optically active novel substituted polyacetylenes: Synthesis,secondary structures,and responsiveness to ions

Novel optically active substituted acetylenes HC? CCH₂CR¹(CO₂CH₃)NHR² [(S)‐/(R)‐ 1 : R¹ = H, R² = Boc, (S)‐ 2 : R¹ = CH₃, R² = Boc, (S)‐ 3 : R¹ = H, R² = Fmoc, (S)‐ 4 : R¹ = CH₃, R² = Fmoc (Boc = tert‐butoxycarbonyl, Fmoc = 9‐fluorenylmethoxycarbonyl)] were synthesized from α‐propargylglycine and α‐propargylalanine, and polymerized with a rhodium catalyst to provide the polymers with number‐average molecular weights of 2400–38,900 in good yields. Polarimetric, circular dichroism (CD), and UV–vis spectroscopic analyses indicated that poly[(S)‐ 1 ], poly[(R)‐ 1 ], and poly[(S)‐ 4 ] formed predominantly one‐handed helical structures both in polar and nonpolar solvents. Poly[(S)‐ 1a ] carrying unprotected carboxy groups was obtained by alkaline hydrolysis of poly[(S)‐ 1 ], and poly[(S)‐ 4b ] carrying unprotected amino groups was obtained by removal of Fmoc groups of poly[(S)‐ 4 ] using piperidine. Poly[(S)‐ 1a ] and poly[(S)‐ 4b ] also exhibited clear CD signals, which were different from those of the precursors, poly[(S)‐ 1 ] and poly[(S)‐ 4 ]. The solution‐state IR measurement revealed the presence of intramolecular hydrogen bonding between the carbamate groups of poly[(S)‐ 1 ] and poly[(S)‐ 1a ]. The plus CD signal of poly[(S)‐ 1a ] turned into minus one on addition of alkali hydroxides and tetrabutylammonium fluoride, accompanying the red‐shift of λ_max. The degree of λ_max shift became large as the size of cation of the additive. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Preparation of C2‐Symmetric Bis[2‐(diphenylphosphino)ferrocen‐1‐yl]‐ methane and Its Use in Rhodium‐ and Ruthenium‐Catalyzed Hydrogenation

The two diphosphine ligands (R_p,R_p)‐ and (S_p,S_p)‐bis[2‐(diphenylphospino)ferrocenyl]methane, (R_p,R_p)‐ and (S_p,S_p)‐ 1 , resp., were prepared in six steps from (S)‐ and (R)‐ferrocenyl tolyl sulfoxide, respectively (Scheme). In the solid state, both the diborane complex (R_p,R_p)‐ 1 ? (BH₃)₂ and the palladium dichloride complex [PdCl₂((R_p,R_p)‐ 1 )] were found to adopt C₂‐pseudosymmetric structures according to X‐ray analyses (Figs. 2 and 3). In the Rh‐ and Ru‐catalyzed hydrogenation of selected alkenes and ketones in the presence of the new ligands, enantioselectivities of up to 55% ee were obtained.     

Strong and Reversible Circularly Polarized Luminescence Emission of a Chiral 1,8‐Naphthalimide Fluorophore Induced by Excimer Emission and Orderly Aggregation

Four chiral 1,2‐diaminocyclohexane (DACH)‐based molecules (R,R/S,S‐ 2 and R,R/S,S‐ 4 ) incorporating 1,8‐naphthalimide fluorophores exhibit strong circularly polarized luminescence (CPL) emission signals in common organic solvents. Interestingly, the reversed CPL signals can be observed in the aggregated state, which is due to the orderly aggregation.     

Reversal Circularly Polarized Luminescence of AIE‐Active Chiral Binaphthyl Molecules from Solution to Aggregation

Four aggregation‐induced emission (AIE)‐active chiral binaphthyl‐based molecules, (R/S)‐ 1 and (R/S)‐ 2 , were designed and synthesized. Interestingly, all of them can exhibit reversal circularly polarized luminescence (CPL) signals from solution to aggregation, which could be attributed to the different dihedral angle of binaphthyl units from cis‐conformation in pure THF solution to trans‐conformation in THF/water mixtures.     

Non‐iterative Asymmetric Synthesis of C15 Polyketide Spiroketals

The 2,2′‐methylenebis[furan] ( 1 ) was converted to 1‐{(4R,6S))‐6‐[(2R)‐2,4‐dihydroxybutyl]‐2,2‐dimethyl‐1,3‐dioxan‐4‐yl}‐3‐[(2R,4R)‐tetrahydro‐4,6‐dihydroxy‐2H‐pyran‐2‐yl)propan‐2‐one ((+)‐ 18 ) and its (4S)‐epimer (?)‐ 19 with high stereo‐ and enantioselectivity (Schemes 1–3). Under acidic methanolysis, (+)‐ 18 yielded a single spiroketal, (3R)‐4‐{(1R,3S,4′R,5R,6′S,7R)‐3′,4′,5′,6′‐tetrahydro‐4′‐hydroxy‐7‐methoxyspiro[2,6‐dioxabicyclo[3.3.1]nonane‐3,2′‐[2H]pyran]‐6′‐yl}butane‐1,3‐diol ((?)‐ 20 ), in which both O‐atoms at the spiro center reside in equatorial positions, this being due to the tricyclic nature of (?)‐ 20 (methyl pyranoside formation). Compound (?)‐ 19 was converted similarly into the (4′S)‐epimeric tricyclic spiroketal (?)‐ 21 that also adopts a similar (3S)‐configuration and conformation. Spiroketals (?)‐ 20 , (?)‐ 21 and analog (?)‐ 23 , i.e., (1R,3S,4′R,5R,6′R)‐3′,4′,5′,6′‐tetrahydro‐6′‐[(2S)‐2‐hydroxybut‐3‐enyl]‐7‐methoxyspiro[2,6‐dioxabicyclo[3.3.1]nonane‐3,2′‐[2H]pyran]‐4′‐ol, derived from (?)‐ 20 , were assayed for their cytotoxicity toward murine P388 lymphocytic leukemia and six human cancer cell lines. Only racemic (±)‐ 21 showed evidence of cancer‐cell‐growth inhibition (P388, ED₅₀: 6.9 μg/ml).     

Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

The self‐assembly of triangular‐shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X‐ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ～20° (λ=Cu_Kα), the higher crystallinity of OPE 3 , endowed with three linear decyl chains, results in a diffractrogram with a number of well‐resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)‐ 1 a and (R)‐ 1 b —endowed with (S)‐ and (R)‐3,7‐dimethyloctyloxy chains—transfer their chirality to the supramolecular structures formed upon their self‐assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)‐ 2 decorated with (S)‐2‐methylnonyloxy chains which forms right‐handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)‐ 1 a that self‐assembles into left‐handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self‐assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)‐ 1 a and (R)‐ 1 b self‐assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)‐ 2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.     

Two modes of asymmetric polymerization of phenylacetylenes having an L‐amino alcohol residue and two hydroxy groups

Four novel chiral phenylacetylenes having an L ‐amino alcohol residue and two hydroxymethyl groups were synthesized and polymerized by an achiral catalyst ((nbd)Rh⁺[η⁶‐(C₆H₅)B^?(C₆H₅)₃]) or a chiral catalytic system ([Rh(nbd)Cl]₂/(S)‐ or (R)‐phenylethylamine ((S)‐ or (R)‐PEA)). The two resulting polymers having an L ‐valinol or L ‐phenylalaninol residue showed Cotton effects at wavelengths around 430 nm. This observation indicated that they had an excess of one‐handed helical backbones. Positive and negative Cotton effects were observed only for the polymers having an L ‐valinol residue produced by using (R)‐ and (S)‐PEA as a cocatalyst, respectively, although the monomer had the same chirality. Even when the achiral catalyst was used, the two resulting polymers having an L ‐valinol or L ‐phenylalaninol residue showed Cotton effects despite the long distance between the chiral groups and the main chain. We have found the first example of a new type of chiral monomer, that is, a chiral phenylacetylene monomer having an L ‐amino alcohol residue and two hydroxy groups that was suitable for both modes of asymmetric polymerization, that is, the helix‐sense‐selective polymerization ( HSSP ) with the chiral catalytic system and the asymmetric‐induced polymerization ( AIP ) with the achiral catalyst. The other two monomers having L ‐alaninol and L ‐tyrosinol were found to be unsuitable to neither HSSP nor AIP because of their polymers' low solubility. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Chiral (Diphosphonite)platinum Complexes in Asymmetric Hydroformylation

The chiral diphosphonite ligand (11bR,11′bR)‐4,4′‐(9,9‐dimethyl‐9H‐xanthene‐4,5‐diyl)bis[dinaphtho[2,1‐d:1′,2′‐f][1,3,2]dioxaphosphepin] ((R,R)‐XantBino; (R)‐ 1 ), based on a rigid xanthene backbone, was applied in the Pt/Sn‐catalyzed hydroformylation of styrene ( 4a ), 4‐methylstyrene ( 4b ), vinyl acetate ( 4c ), and allyl acetate ( 4d ), by using a Pt/Sn ratio of 1 : 1. High ee of up to 80% were observed, along with good regioselectivities towards the desired branched aldehydes. For styrene, an interesting inversion in the stereoselection process was observed at elevated temperatures, and a mechanism is proposed considering the temperature dependence of the regioselectivity. The complex [PtCl₂{(S,S)‐XantBino}] ((S)‐ 2 ) was characterized by X‐ray crystal‐structure analysis, revealing an unusual out‐of‐plane ligand coordination of the metal fragment. The complex [PtCl(SnCl₃){(R,R)‐XantBino}] ((R)‐ 3 ) was characterized by means of ³¹P‐NMR spectroscopy.     

Enantioselective Synthesis of β‐Amino Acids,Part 13

Inexpensive acryloyl chloride was converted in 91% overall yield to two derivatives of β‐alanine, (R,R,R)‐ 6 and (R,R,S)‐ 6 , containing two chiral auxiliaries. C‐Alkylation of (R,R,R)‐ and (R,R,S)‐ 6 via a dianion derivative, was performed by direct metallation with 2.2 equiv. of lithium hexamethyldisilazane (LHMDS) in THF at ?78°. C‐Alkylation of (R,R,S)‐ 6 ‐Li₂ (‘matched' pair of chiral auxiliaries) afforded the mono‐alkylated products 8 – 11 in 29–96% yield and 54–95% stereoselectivity. Employment of LiCl as an additive generally increased stereoselectivities, whereas the effect of HMPA as a cosolvent was erratic. Chemical correlation of the major diastereoisomer from the alkylation reactions with (S)‐α‐alkyl‐β‐alanine ( 12 – 15 ) showed that addition of the electrophile preferentially takes place on the enolate's Si‐face. This conclusion is also supported by molecular‐modeling studies (ab initio HF/3‐21G), which indicate that the lowest‐energy conformation for (R,R,S)‐ 6 ‐Li₂ presents the more sterically hindered Re‐face of the enolate. The theoretical studies also predict a determining role for N? Li? O chelation in (R,R,S)‐ 6 ‐Li₂, giving rise to an interesting ‘ion‐triplet' configuration for the dilithium dianion.     

Synthesis of dendronized,chiral conjugated polymers with appendant Fréchet‐type dendrons

New chiral binaphthyl‐based polyarylenes [(S)‐ 3a and (S)‐ 3b ] with appendant Fréchet‐type poly(aryl ether) dendrons (first generation and second generation) were synthesized with Suzuki polycondensation from chiral (S)‐6,6′‐dibromo‐2,2′‐didendron‐substituted 1,1′‐binaphthyl derivatives and p‐phenylene diboronic acid. The polymers were studied with circular dichroism, fluorescence, and ultraviolet–visible spectra. Laser light scattering measurements of (S)‐ 3a and (S)‐ 3b showed that their weight‐average molecular weights were 2.39 × 10⁵ and 1.09 × 10⁴, respectively. The specific optical rotation [α]_D was ?59.6 for (S)‐ 3a and ?62.7 for (S)‐ 3b . These dendronized conjugated polymers exhibited good thermal stability. The glass‐transition temperatures and the initial decomposition temperatures were 187.5 and 265.3 °C for (S)‐ 3a and 173.8 and 308.9 °C for (S)‐ 3b , respectively. (S)‐ 3a and (S)‐ 3b had high fluorescence quantum efficiencies, 87 and 91%, respectively, in tetrahydrofuran. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1167–1172, 2002