Oxetane Unit Research Articles

Synthesis of (-)-Mitrephorone A via a Bioinspired Late Stage C-H Oxidation of (-)-Mitrephorone B.

We present a bioinspired late-stage C-H oxidation of the ent-trachylobane natural product mitrephorone B to mitrephorone A. The realization of this unprecedented transformation was accomplished by either an iron-catalyzed or electrochemical oxidation and enabled access to the densely substituted oxetane in one step. Formation of mitrephorone C, which is lacking the central oxetane unit but features a keto-function at C2, was not formed under these conditions.

Initiator-free crosslinking of oxetane functionalized low bandgap polymers: an approach towards stabilized bulk heterojunction solar cells

We report the synthesis of crosslinkable derivatives of the low bandgap polymer PFDTBT, poly(2,7-(9,9-dialkylfluorene)-alt-(5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))), and the stabilization of BHJ solar cells by initiator-free crosslinking of oxetanes.

ChemInform Abstract: Highly Regioselective Lithiation of Pyridines Bearing an Oxetane Unit by N‐Butyllithium.

AbstractThe regioselective ortho‐lithiation of pyridine derivatives containing a 3‐oxetane substituent is achieved for the first time.

Phomopsidone A, a novel depsidone metabolite from the mangrove endophytic fungus Phomopsis sp. A123

One novel pentacyclic depsidone containing an oxetane unit, phomopsidone A (1), together with the reported excelsione (also named as phomopsidone) (2), and four known isobenzofuranones (3–6) were isolated from the mangrove endophytic fungus Phomopsis sp. A123. Their structures were elucidated by 1D and 2D NMR spectroscopic analysis and high resolution mass spectrometry. The bioactivity assays showed that these compounds possess cytotoxic, antioxidant, and antifungal activities.

Highly regioselective lithiation of pyridines bearing an oxetane unit by n-butyllithium.

The first regioselective ortho-lithiation at the 4-position of simple pyridine derivatives containing a 3-oxetane unit has been achieved using n-butyllithium as base. Electrophilic quenching of the resulting lithio species provides a rapid access to a broad range of new functionalized pyridine oxetane building blocks.

Oxetane-functionalized Conjugated Polymers in Organic (Opto)Electronic Devices

π-Conjugated polymers have found applications as active materials in a range of optoelectronic devices due to being intrinsically semiconducting, exhibiting switchable properties in the course of redox processes, and the possibility of being processed from solution. Current developments in molecular design have been extended to include switchable solubility by means of chemical cross-linking via polymerization of photo-reactive groups attached to the polymer backbone. Exploiting the cross-linkable ability of conjugated polymers has allowed extending applications to multilayered devices fabrication, as the cross-linked structure does not re-dissolve upon further deposition of solutions, and also to patterning processes, at the sub-micrometric scale, based on photo-induced methods (photo-lithography) or on phaseseparation in spin cast polymer blends. Among the classes of cross-linkable conjugated polymers, those containing oxetane units as the cross-linking moiety have been the most explored in terms of applications in optoelectronic devices. In this article, we review the recent progress in the field of oxetane-functionalized conjugated polymers, focusing on their molecular design to control electronic and processing properties and their most relevant applications in organic electronics.

Tracing a Moving Thin-Film Reaction Front with Nanometer Resolution

X-ray standing waves at grazing incidence are used to trace the position of the reactive chain ends of a surface-initiated (via protons in the underlying PEDOT:PSS layer) polymerization in a bulk polymer film, a procedure recently used to realize insoluble functional films for mutlilayer organic light-emitting diodes. The reactive chain ends as well as their respective counterions are found within a few nanometers thin layer, which moves away from the surface as the reaction proceeds. This reactive front remains narrow for polymerized films that are significantly thicker than the front width. This result can be rationalized by assuming that all oxetane units in contact with the PEDOT:PSS are initiated before the reaction proceeds at elevated temperature.

Oxetanes as Versatile Elements in Drug Discovery and Synthesis

Sizable resources, both financial and human, are invested each year in the development of new pharmaceutical agents. However, despite improved techniques, the new compounds often encounter difficulties in satisfying and overcoming the numerous physicochemical and many pharmacological constraints and hurdles. Oxetanes have been shown to improve key properties when grafted onto molecular scaffolds. Of particular interest are oxetanes that are substituted only in the 3-position, since such units remain achiral and their introduction into a molecular scaffold does not create a new stereocenter. This Minireview gives an overview of the recent advances made in the preparation and use of 3-substituted oxetanes. It also includes a discussion of the site-dependent modifications of various physicochemical and biochemical properties that result from the incorporation of the oxetane unit in molecular architectures.

Photosensitized Splitting of Thymine Dimer or Oxetane Unit by a Covalently N-Linked Carbazole via Electron Transfer in Different Marcus Regions

Although many similarities exist between the two classes of enzymes, cyclobutane photolyases and (6-4) photolyases have certain important differences. The most significant difference is in their repair quantum yields, cyclobutane photolyases with a uniformly high efficiency (0.7-0.98) and very low repair efficiency for (6-4) photolyases (0.05-0.1). To understand the significant difference, we prepared two classes of model compounds, covalently N-linked dimer- (1) or oxetane-carbazole (2) compounds with a dimethylene or trimethylene group as a linker. Under light irradiation, the dimer or oxetane unit of model compounds can be sensitized to split by the excited carbazole via an intramolecular electron transfer. The splitting reaction of dimer or oxetane unit in model compounds is strongly solvent dependent. In nonpolar solvents, such as cyclohexane or THF, no fluorescence quenching of the carbazole moiety of model compounds relative to a free carbazole, N-methylcarbazole, was observed and thus no splitting occurred. In polar solvents, two classes of model compounds reveal two reverse solvent effects on the splitting quantum yield. One is an inverse relation between the quantum yield and the polarity of the solvent for dimer-model systems, and another is a normal relation for oxetane-model systems. This phenomenon was also observed with another two classes of model compounds, covalently linked dimer- or oxetane-indole. Based on Marcus theory and thermodynamic data, it has been rationalized that the two reverse solvent effects derive from back electron transfer in the splitting process lying in the different Marcus regions. Back electron transfer lies in the Marcus inverted region for dimer-model systems and the normal region for oxetane-model systems. From repair solvent behavior of the two classes of model compounds, we gained some insights into the major difference in the repair efficiency for the two classes of photolyases.

Macromol. Chem. Phys. 7/2009

Cover: The cover image presents the design of a phosphorescent Ir(III) complex equipped with an oxetane unit. Admixed to crosslinkable matrix moieties the emitter can be copolymerized by a photoinduced cationic ring opening polymerization forming an insoluble network. Applied as emissive layer, fully solution processed multi‐layer OLEDs can be obtained. Further details can be found in the article by C. Ulbricht, N. Rehmann, E. Holder, D. Hertel, K. Meerholz,* U. S. Schubert* on page 531.

Photoreactive hole-transporting polymer bearing oxetane moieties and its application to green polymer light-emitting diodes

A new hole-transporting polymer was prepared by the copolymerization of 9-(4-vinyl-benzyl)- 9H-carbazole and 3-methyl-3-(4-vinyl-benzyloxymethyl)-oxetane in the presence of α,α′-azobisisobutyronitrile (AIBN). The latter contains an oxetane unit that is cationically photopolymerizable. It was characterized by NMR, GPC, IR spectroscopy, TGA, and differential scanning calorimetry (DSC). Multilayer polymer light-emitting diodes (PLEDs) were fabricated layer by layer by using the spin-coating method with an organic solution of a photoreactive copolymer. Organic soluble poly[2-(carbazol-9-yl)-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (CzEh-PPV) that carries hole-transporting carbazole pendants was also synthesized and employed as a green-emitting material in PLEDs. After UV illumination for photoreaction in the new photoreactive oxetane-based copolymer (VCz-Ox), we could spin coat the CzEh-PPV solution for the next layer. The hole-transport capability of the copolymer was evaluated in multilayer PLEDs that were fabricated to test the device performances.

Model studies of the (6–4) photoproduct photoreactivation: efficient photosensitized splitting of thymine oxetane units by covalently linked tryptophan in high polarity solvents

Three covalently linked tryptophan-thymine oxetane compounds used as a model of the (6-4) photolyase-substrate complex have been prepared. Under 290 nm light, efficient splitting of the thymine oxetane with aromatic carbonyl compounds gives the thymine monomer and the corresponding carbonyl compounds by the covalently linked tryptophan via an intramolecular electron transfer, and exhibits a strong solvent dependence: the quantum yield (Phi) is ca. 0.1 in dioxane, and near 0.3 in water. Electron transfer from the excited tryptophan residue to the oxetane unit is the origin of fluorescence quenching of the tryptophan residue, and is more efficient in strong polar solvents. The splitting efficiency of the oxetane radical anion within the tryptophan.+-oxetane.- species is also solvent-dependent, ranging from ca. 0.2 in dioxane to near 0.35 in water. Thus, the back electron transfer reaction in the charge-separated species would be suppressed in water, but is still a main factor causing low splitting efficiencies in the tryptophan-oxetane systems. In contrast to the tryptophan-oxetane system, fast nonradiation processes are the main causes of low efficiency in the flavin-oxetane system. Hence, nonradiative processes of the excited FADH-, rather than electron transfer to oxetane, may be an important factor for the low repair efficiency of (6-4) photolyase.

Combining Covalent and Noncovalent Cross-Linking: A Novel Terpolymer for Two-Step Curing Applications

A terpolymer of poly(butyl acrylate) bearing terpyridine as well as oxetane units was synthesized by free radical polymerization and characterized using NMR, UV−vis, and GPC. Subsequently, UV−vis experiments indicated clearly a noncovalent cross-linking of the terpyridine moieties by addition of iron(II) ions. Moreover, the ability of covalent cross-linking was studied by polymerizing the oxetane rings utilizing Lewis acids. IR spectroscopy and DSC experiments clearly revealed the success of the combination of both steps when utilizing iron(II) ions and AlCl3.

Synchrotron X‐Ray Study of Liquid Crystalline Polyoxetanes Containing Two Mesogenic Groups Connected by a Flexible Spacer in the Side Chain

AbstractThe study of the mesophase behaviour of a family of new side chain liquid crystalline polymers by means of synchrotron X‐ray diffraction is reported. These systems contain two bibenzoate rigid units in the side chain that are interconnected through a flexible spacer. A polymerisable oxetane unit is linked to one end of the dimer structure, and a flexible terminal chain is linked to the other end. All the polymers form phases with variable degree of order (from low ordered smectic to crystalline) depending on the chemical constitution of the different segments. The influence of the length, parity, and lateral substitution of the spacers on the transitional parameters and the symmetry of the mesophases that are formed is analysed. Close similarities were found between the phase behaviour of the monomer precursors and the derived polyoxetanes. The arrangement of the mesogens, the flexible groups and the polymeric backbone in the layered structure is discussed with respect to the L/d ratio. Different values were obtained depending on the parity of the central spacer and on the degree of order of the mesophase. Interpenetrated structures, where the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when building up highly ordered mesophases. image

Novel Polymer Electrolytes Based on Mono- and Bis-Oxetane Monomers with Oligo(Ethylene Oxide) Units

New types of polymer electrolytes were prepared by cationic ring-opening copolymerization of bis-oxetane (R-O-(CH2CH2O)m-R, R = 3-ethyl-3-methylene oxetane unit, DDOE (m = 2), TrDOE (m = 3), TeDOE (m = 4)) and mono-oxetane (R-O-(CH2CH2O)n-CH3, TrMOE (n = 3), NoMOE (n = 9), DoMOE (n = 12)) with lithium salts as a catalyst, and were characterized by differential scanning calorimetry (DSC) and alternating impedance spectroscopy. The poly(oxetane)-based electrolytes had cross-linked networks with oligo(ethylene oxide) and 2-ethyltrimethylene oxide main chains and methoxyoligo(ethylene oxide) side chains. The polymer electrolytes prepared with LiBF4 revealed high conductivity, compared to those done with LiPF6 or LiN(C2F5SO2)2. The conductivities of the poly(oxetane)-LiBF4 complexes depended on the mono-oxetane content and the length of oligo(ethylene oxide) in the mono- and bis-oxetanes. The oligo(ethylene oxide) side chains in the complexes acted as efficient plasticizing agents, particularly using NoMOE or DoMOE. Maximum conductivities of the polymer electrolytes with LiBF4([Li]/[O] = 0.045) revealed 9.1 × 10−6 (TrMOE/DDOE mole ratio = 3.0) and 1.0 × 10−4 S cm−1 (NoMOE/DDOE = 1.72 and DoMOE/DDOE = 1.29).

Preparation and Characterization of Side-Chain Liquid-Crystalline Polyoxetanes Anchoring a Pendant Spacer-Separated Mesogen at the Tertiary-Like C-3 Carbon of the Oxetane Unit

Abstract Polyoxetanes anchoring a pendant spacer-separated mesogen at the tertiary-like C-3 carbon of the oxetane unit were prepared by cationic ring-opening polymerization of the corresponding oxetane derivatives. The resulting polymers were liquid-crystalline substances showing textures assignable to the nematic or smectic mesophase over a wide temperature range from about 260 °C to room temperature. These mesogens also behaved in the manner similar to that of the mesogens in the analogous polyoxetanes attached by a methyl side chain at the quaternary C-3 carbon, although the less bulky polymer backbone in the present work indicated a somewhat higher isotropic phase-transition temperature than that of the methyl-substituted analogs. In both types of the polyoxetanes, their mesophase patterns were influenced by the core structure and alkoxy tail length in the pendant mesogen, but not by the bulkiness of the second side chains, H and CH3, attached at the C-3 carbon of the polyoxetane unit.

Photopolymerization via the Paterno‐Büchi reaction. I. Reaction of aromatic diketones with tetramethylallene

AbstractAromatic diketones of the benzophenone type undergo photopolymerization on irradiation at 350 nm with tetramethylallene in benzene solution. The polymers produced contain oxetane units in the main chain, and spectroscopic analysis demonstrates that the distribution of structural units parallels that found in the reaction between benzophenone and tetramethylallene. The polymers are rapidly degraded thermally and by mineral acids.