Versatile Solid-Phase Synthesis of Peptide-Derived 2-Oxazolines. Application in the Synthesis of Ligands for Asymmetric Catalysis

A mild and high-yielding procedure for the solid-phase synthesis of 2-oxazolines from amino acids is described. The two-step protocol is based on the iodination of serine containing peptides, followed by in situ nucleophilic attack of the carbonyl oxygen from the next amino acid. Phosphinylation of the terminal amino group cleanly furnishes a resin-bound phosphine−oxazoline ligand, which upon palladium complexation was applied as catalyst in asymmetric allylic substitution

Thiolated π-Extended Tetrathiafulvalenes: Versatile Multifunctional π-Systems

Derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) have been synthesized by a new synthetic methodology, viz., direct phosphite-mediated cross-couplings of anthraquinone with 1,3-dithiole-2-thione derivatives. These ex-TTFs bear one, two, or four cyanoethyl-protected thiol groups on the dithiole rings. Deprotection (NaOMe, MeOH, DMF, 20 °C) and trapping of the transient thiolates with electrophiles have afforded the new ex-TTF trimer 19, dimeric cyclophanes 22 and 25, the tetrakis(hydroxyethylthio) derivative 23, and the strained cyclophane 24. Solution redox properties have been studied by cyclic voltammetry. For compounds 19, 22, and 25 each ex-TTF unit behaves as an independent 2-electron redox system giving rise to a single, quasi-reversible 6-, 4-, and 4-electron wave, respectively. The Eox value for 24 (0 → 2+ wave) is positively shifted by 290 mV compared to that of its precursor 15 due to the short bridge in 24 obstructing the conformational change which accompanies oxidation. X-ray crystal structures of 23·2.5MeOH, 23·1.5MeCN, 24·CH2Cl2, and 24·1.5CH2Cl2 show the saddle-shape folding (typical of ex-TTF derivatives), which in 24 is enhanced by the pentamethylene chain bridging the dithiole units. Both solvates of 23 show an unprecedented crystal packing motif due to hydrogen bonding

Extreme Conformational Constraints in π-Extended Tetrathiafulvalenes: Unusual Topologies and Redox Behavior of Doubly and Triply Bridged Cyclophanes

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single quasi-reversible two-electron oxidation wave, D0 → D2+), cyclic voltammetry of the cyclophanes reveals two reversible, one-electron oxidation steps (D0 → D•+ → D2+), with differences between the half-wave potentials (E21/2 − E11/2) of 0.22−0.26 V. The conformational changes and gain in aromaticity which drive the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy band (λmax = 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction. The steric constraints imposed by multiple bridging have become so extreme that the π-framework of 15, 16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems

Extreme Conformational Constraints in π-Extended Tetrathiafulvalenes: Unusual Topologies and Redox Behavior of Doubly and Triply Bridged Cyclophanes

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single quasi-reversible two-electron oxidation wave, D0 → D2+), cyclic voltammetry of the cyclophanes reveals two reversible, one-electron oxidation steps (D0 → D•+ → D2+), with differences between the half-wave potentials (E21/2 − E11/2) of 0.22−0.26 V. The conformational changes and gain in aromaticity which drive the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy band (λmax = 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction. The steric constraints imposed by multiple bridging have become so extreme that the π-framework of 15, 16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems

Thiolated π-Extended Tetrathiafulvalenes: Versatile Multifunctional π-Systems

Derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) have been synthesized by a new synthetic methodology, viz., direct phosphite-mediated cross-couplings of anthraquinone with 1,3-dithiole-2-thione derivatives. These ex-TTFs bear one, two, or four cyanoethyl-protected thiol groups on the dithiole rings. Deprotection (NaOMe, MeOH, DMF, 20 °C) and trapping of the transient thiolates with electrophiles have afforded the new ex-TTF trimer 19, dimeric cyclophanes 22 and 25, the tetrakis(hydroxyethylthio) derivative 23, and the strained cyclophane 24. Solution redox properties have been studied by cyclic voltammetry. For compounds 19, 22, and 25 each ex-TTF unit behaves as an independent 2-electron redox system giving rise to a single, quasi-reversible 6-, 4-, and 4-electron wave, respectively. The Eox value for 24 (0 → 2+ wave) is positively shifted by 290 mV compared to that of its precursor 15 due to the short bridge in 24 obstructing the conformational change which accompanies oxidation. X-ray crystal structures of 23·2.5MeOH, 23·1.5MeCN, 24·CH2Cl2, and 24·1.5CH2Cl2 show the saddle-shape folding (typical of ex-TTF derivatives), which in 24 is enhanced by the pentamethylene chain bridging the dithiole units. Both solvates of 23 show an unprecedented crystal packing motif due to hydrogen bonding

Molecular Saddles. 7. New 9,10-Bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene Cyclophanes: Synthesis, Redox Properties, and X-ray Crystal Structures of Neutral Species and a Dication Salt

We report the synthesis of cyclophanes 18−20 by ester-forming macrocyclization reactions of diols 15 and 16 with 1,4-benzenedicarbonyl chloride. Compounds 18 and 19 display a two-electron, quasireversible oxidation wave in the cyclic voltammogram to yield the dication species at Eoxpa 0.52 and 0.47 V, respectively (vs Ag/AgCl in acetonitrile), whereas the 2 + 2 product 20 undergoes a single four-electron oxidation process at Eoxpa 0.51 V. X-ray crystal structures are reported for compounds 18−20 and the dication salt 182+(I3-)2·(I2)0.5. For comparative purposes, the structures are also reported for the precursor diol 15 and its dication salt 152+(ClO4-)2, which was obtained by electrocrystallization. In the neutral cyclophanes 18−20, the 9,10-bis(1,3-dithiol-2-ylidene)-9, 10-dihydroanthracene moieties adopt a saddle-shaped conformation. The overall measure of folding, the dihedral angle (θ) between the S(1)C(16)C(17)S(2) and S(5)C(21)C(22)S(6) planes, is similar in 15 and 18 (87.6° and 83.7°, respectively) whereas this angle is significantly narrower in 19 (61.1°), illustrating the flexibility of the saddle conformation and its dependence on the packing. Dimeric molecule 20 contains two saddle moieties with very similar conformations, θ = 73.4° and 73.1°. The structures of dication salts 152+(ClO4-)2 and 182+(I3-)2·(I2)0.5 reveal that a dramatic conformational change accompanies oxidation of the donor with the dithiolium rings planar and nearly perpendicular to the mean plane of the anthracene moieties. A notable feature of 182+ is that the bridge enforces a fold of 22° along the C(9)...C(10) vector of the anthracene unit. In 152+ there is no fold about this axis, instead the anthracene moiety is slightly twisted with the two (planar) outer rings forming an angle of 7°

Langmuir−Blodgett Films of Amphiphilic Bis(tetrathiafulvalene) Macrocycles with Four Alkyl Chains

Amphiphilic bis(tetrathiafulvalene) [bis(TTF)] macrocycles with four alkyl chains were fabricated as novel electrically active Langmuir−Blodgett (LB) films. Two TTF units were linked via [24]crown-8, [21]crown-7, and [18]crown-6 macrocycles, forming charge-transfer (CT) salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (F4-TCNQ) at the air−water interface and on solid substrates. The CT salt of the amphiphilic bis(TTF)-macrocycle having a [24]crown-8 ring system formed a uniform surface morphology on mica. Using single-crystal X-ray structural analysis, the layer structure between the hydrophobic chains and the one-dimensional π−π stack of the CT salt was confirmed. Our results show that the bis(TTF)-macrocycle was folded at the flexible [24]crown-8 moiety, forming intramolecular π−π dimer structures and one-dimensional intermolecular π−π stacks with F4-TCNQ dimers. The open-shell electronic structure of the LB films was determined by electronic spectra, electrical conductivity, and electron spin resonance analyses. Asymmetry was introduced into the bis(TTF)-macrocycle by changing the ring size from [24]crown-8 to [21]crown-7. The surface morphology of the CT salts with F4-TCNQ was established as two-dimensional round-shape domains on mica. Further reduction of the macrocyclic ring from [21]crown-7 to [18]crown-6 resulted in a CT salt of the bis(TTF)-macrocycle with F4-TCNQ with a leaf-shape domain morphology and a typical dimension of ∼1 μm2 on mica. In general, decreasing the macrocyclic ring size from [24]crown-8 to [21]crown-7 or [18]crown-6 affected the inter- and intramolecular interactions and the surface morphologies of LB films

Synthesis of Novel Phthalocyanine−Tetrathiafulvalene Hybrids; Intramolecular Fluorescence Quenching Related to Molecular Geometry

A number of silicon phthalocyanine bis-esters have been synthesized and characterized, with axial ligands containing one or more tetrathiafulvalene groups. Variations in the substitution positions around a central aromatic “hinge” within the ligands lead to different molecular geometries, and the fluorescence of the macrocyclic core is subsequently quenched to varying degrees by the electron-rich tetrathiafulvalene moiety, the magnitude of this effect being dependent upon both the relative separation of the two units and the flexibility of the linking group. Pc derivative 24, with a highly flexible linker group, and pc derivative 28, with a dendritic axial ligand, have the intensity of the macrocycle emission reduced by 99% and 96%, respectively, relative to a similar silicon pc reference compound lacking the TTF moieties. Molecular modeling studies of a series of such hybrids allow the degree of this fluorescence quenching to be related to the intramolecular spacing. Additionally, the potential for rapid electrochemical switching of the phthalocyanine fluorescence by oxidation of the appended tetrathiafulvalene units is explored

Langmuir−Blodgett Films of Amphiphilic Bis(tetrathiafulvalene) Macrocycles with Four Alkyl Chains

Amphiphilic bis(tetrathiafulvalene) [bis(TTF)] macrocycles with four alkyl chains were fabricated as novel electrically active Langmuir−Blodgett (LB) films. Two TTF units were linked via [24]crown-8, [21]crown-7, and [18]crown-6 macrocycles, forming charge-transfer (CT) salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (F4-TCNQ) at the air−water interface and on solid substrates. The CT salt of the amphiphilic bis(TTF)-macrocycle having a [24]crown-8 ring system formed a uniform surface morphology on mica. Using single-crystal X-ray structural analysis, the layer structure between the hydrophobic chains and the one-dimensional π−π stack of the CT salt was confirmed. Our results show that the bis(TTF)-macrocycle was folded at the flexible [24]crown-8 moiety, forming intramolecular π−π dimer structures and one-dimensional intermolecular π−π stacks with F4-TCNQ dimers. The open-shell electronic structure of the LB films was determined by electronic spectra, electrical conductivity, and electron spin resonance analyses. Asymmetry was introduced into the bis(TTF)-macrocycle by changing the ring size from [24]crown-8 to [21]crown-7. The surface morphology of the CT salts with F4-TCNQ was established as two-dimensional round-shape domains on mica. Further reduction of the macrocyclic ring from [21]crown-7 to [18]crown-6 resulted in a CT salt of the bis(TTF)-macrocycle with F4-TCNQ with a leaf-shape domain morphology and a typical dimension of ∼1 μm2 on mica. In general, decreasing the macrocyclic ring size from [24]crown-8 to [21]crown-7 or [18]crown-6 affected the inter- and intramolecular interactions and the surface morphologies of LB films

Linear Pentaiodide in the Radical Cation Salt of a Tetrathiafulvalene Bisannulated Macrocycle

Linear Pentaiodide in the Radical Cation Salt of a Tetrathiafulvalene Bisannulated Macrocycl