Progress in the chemistry of isobenzofurans

Oxidative generation of isobenzofurans from phthalans: application to the formal synthesis of (±)-morphine

Treatment of phthalan derivatives with p-chloranil in dodecane in the presence of molecular sieves at 160-200 °C allowed the generation of unstabilized isobenzofurans, which underwent intramolecular Diels-Alder reaction to give endo cycloadducts exclusively. The cycloaddition turned out to be reversible, providing an equilibrium mixture of endo adducts when heating a substrate with a stereocenter on the tether. We also demonstrated the regioselective allylation of an oxygen-bridged cycloadduct upon exposure to EtAlCl2 in the presence of allyltrimethylsilane, and the conversion to Rice's intermediate completed a formal synthesis of (±)-morphine.

Photon-upconverters for blue organic light-emitting diodes: a low-cost, sky-blue example

In the research ecosystem's quest towards having deployable organic light-emitting diodes with higher-energy emission (e.g., blue light), we advocate focusing on fluorescent emitters, due to their relative stability and colour purity, and developing design strategies to significantly improve their efficiencies. We propose that all triplet-triplet annihilation upconversion (TTA-UC) emitters would make good candidates for triplet fusion-enhanced OLEDs ("FuLEDs"), due to the energetically uphill nature of the photophysical process, and their common requirements. We demonstrate this with the low-cost sky-blue 1,3-diphenylisobenzofuran (DPBF). Having satisfied the criteria for TTA-UC, we show DPBF as a photon upconverter (I th 92 mW cm-2), and henceforth demonstrate it as a bright emitter for FuLEDs. Notably, the devices achieved 6.5% external quantum efficiency (above the ∼5% threshold without triplet contribution), and triplet-exciton-fusion-generated fluorescence contributes up to 44% of the electroluminescence, as shown by transient measurements. Here, triplet fusion translates to a quantum yield (Φ TTA-UC) of 19%, at an electrical excitation of ∼0.01 mW cm-2. The enhancement is meaningful for commercial blue OLED displays. We also found DPBF to have decent hole mobilities of ∼0.08 cm2 V-1 s-1. This additional finding can lead to DPBF being used in other capacities in various printable electronics.

Molecular Spin State Switching and Photochromism in the Red and Near Infrared with Ni(II) Chlorin and Ni(II) Bacteriochlorin

Molecules or ions are either paramagnetic (unpaired electrons) or diamagnetic (all electrons are paired). Switching between the two states under ambient conditions was considered a typical solid state phenomenon and has been termed spin crossover. The first single-molecule spin state switches operated with light in solution were developed a decade ago and offer a number of technical applications that are not accessible to solid state systems. Magnetic switching in biological environments, however, requires water solubility, and for in vivo applications, switching wavelengths within the bio-optical window (650-950 nm) are needed. We now present molecular spin state switches that are water-soluble and switchable in the far-red and near-infrared region. At the same time, they are photochromic compounds with excellent photophysical properties. trans-cis isomerization is induced with 505 nm radiation, and cis-trans conversion with 620 or 720 nm radiation. The metastable cis isomers are stable at room temperature for at least several weeks. The detailed mechanism of this surprising and unprecedented long wavelength photoisomerization of azobenzenes is still under investigation.

BF3-Etherate-catalyzed tandem reaction of 2-formylarylketones with electron-rich arenes/heteroarenes: an assembly of isobenzofurans

An efficient and BF₃·Et₂O-catalyzed chemoselective synthesis of diversified 1,3-diarylisobenzofuran in a high yield has been described.

Preparation and isolation of isobenzofuran

The synthesis, isolation and characterization of isobenzofuran are described in this publication. Isobenzofuran is of general interest in synthetic and physical organic chemistry because it is one of the most reactive dienes known. A number of synthetic pathways have been published which all suffer from disadvantages such as low yields and difficult purification. We present a synthetic pathway to prepare isobenzofuran in laboratory scale with high yields, from affordable, commercially available starting materials.

An Obtuse-angled Corner Unit for Fluctuating Carbon Nanohoops

The invention of corner units was the key factor that allowed the synthesis of cyclo-para-phenylenes with strained curved π-systems. Despite only a few scarce instances of the development of corner units to date, a variety of structural congeners have been synthesized. These preceding corner units commonly possessed directing angles of ≤90°, which enabled the macrocyclization of multiple units, up to six. In this study, we introduce an obtuse-angled corner unit for the synthesis of cyclo-para-phenylene congeners. The corner unit with oxanorbornadiene possessed a directing angle of 126° and thus allowed for the macrocyclization of larger structures with up to seven units. Reductive aromatization was applicable to complete the cyclo-para-phenylene structures and afforded the congeners with multiple anthracenylene panels. Structural studies with experimental and theoretical methods revealed a fluctuating structure with an intrinsic non-belt shape.

Polymer Synthesis via a Diels–Alder Reaction between Bis(Isobenzofuran)s and Bis(Phenylacetylene)s Containing Preformed Phthalimide Rings

Polymers have been prepared by the Diels–Alder reaction between novel bis(isobenzofuran)s and bis(phenylacetylene)s containing preformed phthalimide rings. The initial polymer undergoes a thermal rearrangement without volatile evolution that effectively prevents a retro Diels–Alder reaction from occurring. The polymers were soluble in common organic solvents such as chloroform, N-methyl-2-pyrrolidinone, N, N-dimethylacetamide and chlorinated aromatic solvents. The polymers exhibited glass transitions ranging from 251 to 359 °C and a 10%weightloss ranging from 451 to 486 °C in air and 482 to520 °C in helium as determined by thermogravimetric analysis. Thissynthetic approach offers advantages over other Diels–Alder polymerizations since no volatile by-products are generated and a thermal rearrangement eliminates a retro Diels–Alder reaction.

Justicidin B: A Promising Bioactive Lignan

Adverse effects and drug resistance to the current onchopharmacologicals have increased the demand for alternative novel therapeutics. We herein introduce justicidin B, an arylnaphthalen lignan isolated from different plant origins, especially Justicia, Phyllanthus, Haplophyllum and Linum species. This cyclolignan exhibits a wide array of biological properties ranges from piscicidal to antifungal, antiviral and antibacterial activities. Activity against Trypanosoma brucei makes justicidin B a potential antiprotozoal agent for the treatment of neglected tropical diseases. Pharmacological properties like antiplatelet, anti-inflammatory and bone resorption inhibition have been also attributed to justicidin B. This compound is a potent cytotoxic substance on several cell lines, especially chronic myeloid and chronic lymphoid leukemia. Pharmacological values, natural variation, as well as biotechnological production of justicidin B by plant cell, tissue and organ culture are also described in this review. Chemical characteristics and chromatographic methods to identify justicidin B and its biosynthetic pathway have been discussed. Different approaches to the total synthesis of justicidin B are compared. This review would shed light on the role of justicidin B as an intriguing natural compound and provides a chance to optimize conditions for industrial applications.

Selective Halogen-Lithium Exchange of 1,2-Dihaloarenes for Successive [2+4] Cycloadditions of Arynes and Isobenzofurans

Successive [2+4] cycloadditions of arynes and isobenzofurans by site-selective halogen-lithium exchange of 1,2-dihaloarenes were developed, allowing the rapid construction of polycyclic compounds which serve as a useful synthetic intermediates for the preparation of various polyacene derivatives.

Synthesis of 4,5,6-trisubstituted-1,3-dihydroisobenzofurans by virtue of palladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes

An efficient hetero-annulation protocol has been developed for the construction of 4,5,6-trisubstituted-1,3-dihydroisobenzofuransviapalladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes.

An efficient synthetic route to 1,3-bis(arylethynyl)isobenzofuran using alkoxybenzocyclobutenone as a reactive platform

An efficient synthetic method of 1,3-bis(arylethynyl)isobenzofurans is developed.

Facile Sc(OTf)3-catalyzed generation and successive aromatization of isobenzofuran from o-dicarbonylbenzenes

Isobenzofuran can be prepared from o-phthalaldehyde using hydrosilane. The formed isobenzofuran is trapped by an alkene via a Diels-Alder reaction. Further dehydration proceeds to furnish the conjugated aromatic compound. This multistep reaction was promoted by catalytic amounts of Sc(OTf)(3).

X-ray structures of cyclophanes derived from naphtho[1,2-c:5,6-c]difuran and the synthesis, structure, and reaction kinetics of its 1,3,6,8-tetrasilylated derivative

A silylated derivative of naphtho[1,2-c:5,6-c]difuran, 1,3,6,8-tetrakis(tert-butyldimethylsilyl)naphtho[1,2-c:5,6-c]difuran, has been isolated and its X-ray crystal structure determined. Bond localization confirms the polyene character of this isobenzofuran ring system. This molecule undergoes two successive Diels-Alder reactions with second-order rate constants differing by over 2 orders of magnitude, consistent with predictions based on their structure-count ratios and with the reactivity of the novel 1,3-bis(tert-butyldimethylsilyl)isobenzofuran. Crystal structures of two cyclophanes derived from the reaction of naphtho[1,2-c:5,6-c]difuran and bis(imide) or bis(ester) dienophiles show marked differences in the conformation of the aliphatic chain found in the solid state.

Stable phenylene- and biphenylene-bis(isobenzofuran)s related to diphenylisobenzofuran

A series of phenyl-substituted bis(isobenzofuran)s in which the furan moieties are linked by 1,3-phenylene, 1,4-phenylene, or 4,4′-biphenylene linkers have been prepared in only two steps. They are comparable to diphenylisobenzofuran in their absorption and luminescent properties, their electrochemistry, and their reactivity. Their degree of similarity to diphenylisobenzofuran was found to depend significantly on the nature of the linking group. Diels–Alder reaction of the title compounds and subsequent aromatization gave very rapid access to penta- or hexa-aryl products.

4,7-Diphenylisobenzofuran: A Useful Intermediate for the Construction of Phenyl-Substituted Acenes

The formation and subsequent reactivity of previously unknown 4,7-diphenylisobenzofuran, 5, is reported. The Diels-Alder reaction between 5 and p-benzoquinone in boiling glacial acetic acid yields an unprecedented exo,exo anti dual cycloaddition product, 16b, in excellent yield and with 100% diastereoselectivity. Differences between the reactivities of 5 and the more common 1,3-diphenylisobenzofuran are highlighted. Reactive 5 is utilized to form new three-, four-, and five-ring acenes, and the latter compound is reacted with [60]fullerene to produce new [60]fullerene-acene adducts.

Approaches to Open Fullerenes: Synthesis and Thermal Stability of cis-1 Bis(isobenzofuran) Diels−Alder Adducts of C60

In a quest to form wider openings within the cage of the fullerene C60 through controlled bond-breaking reactions, we have examined the double saturation of adjacent C=C bonds within a six-membered ring of C60. We have investigated the double Diels-Alder cycloaddition of two tethered isobenzofurans to the fullerene C60. We obtained cis-1 adducts in good yields after reacting the methylene- or quinoxaline-tethered bis(isobenzofuran) precursors 2a-k with parent 3,6-dihydro-1,2,4,5-tetrazine (3b). The X-ray structure of the methylene-tethered bis(isobenzofuran)-C60 adduct 4b has been obtained; four-eclipsed substituents are held rigidly by the bicyclic addends. The cis-1 bis(isobenzofuran) bisadducts 4b and 4e-j are kinetically far more stable toward thermal retro-Diels-Alder fragmentation than are mono(isobenzofuran) adducts of C60, in solution and in the solid state as determined by 1H NMR spectroscopy or thermogravimetric analysis. A methodology for the reversible solubilization of other fullerene derivatives based on this work is also presented.

Approaches to Open Fullerenes: Synthesis and Kinetic Stability of Diels−Alder Adducts of Substituted Isobenzofurans and C60

We have examined the reactions of 1,3-disubstituted isobenzofurans with the fullerene C60 in the context of an approach to open a large orifice on the fullerene framework. A variety of substituted isobenzofurans (6a-h), generated from the reaction of 1,4-substituted 1,4-epoxynaphthalenes 3a-h with 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (4a) or 1,2,4,5-tetrazine (4b), were added to C60 to afford the Diels-Alder adducts 7a-h. The thermal stability of these adducts toward retro-Diels-Alder fragmentation differs greatly in solution from that in the solid state. In solution, the relatively facile retro-Diels-Alder fragmentation of monoadducts 7a and 7c, to give C60 and the free isobenzofurans 6a and 6c, have rate constants (and activation barriers) of k=9.29x10(-5) s-1 at 70 degrees C (Ea=32.6 kcal mol-1) and k=1.36x10(-4) s-1 at 40 degrees C (Ea=33.7 kcal mol-1), respectively, indicating that the addition of isobenzofurans to C60 is readily reversible at those temperatures. In the solid state, thermogravimetric analysis of adduct 7a indicates that its decomposition occurs only within the temperature range of 220-300 degrees C. As a result, these compounds can be stored at room temperature in the solid state for several weeks without significant decomposition but have to be handled within several hours in solution.

Directed growth of poly(isobenzofuran) films by chemical vapor deposition on patterned self-assembled monolayers as templates

This paper describes a method to direct the formation of microstructures of poly(isobenzofuran) (PIBF) by chemical vapor deposition (CVD) on chemically patterned, reactive, self-assembled monolayers (SAMs) prepared on gold substrates. We examined the growth dependence of PIBF by deposition onto several different SAMs each presenting different surface functional groups, including a carboxylic acid, a phenol, an alcohol, an amine, and a methyl group. Interferometry, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), and optical microscopy were used to characterize the PIBF films grown on the various SAMs. Based on the kinetic and the spectroscopic analyses, we suggest that the growth of PIBF is surface-dependent and may follow a cationic polymerization mechanism. Using the cationic polymerization mechanism of PIBF growth, we also prepared patterned SAMs of 11-mercapto-1-undecanol (MUO) or 11-mercaptoundecanoic acid (MUA) by microcontact printing (microCP) on gold substrates as templates, to direct the growth of the PIBF. The directed growth and the formation of microstructures of PIBF with lateral dimensions of 6 microm were investigated using atomic force microscopy (AFM). The average thickness of the microstructures of PIBF films grown on the MUO and the MUA patterns were 400 +/- 40 nm and 490 +/- 40 nm, respectively. SAMs patterned with carboxylic acid salts (Cu2+, Fe2+, or Ag+) derived from MUA led to increases in the average thickness of the microstructures of PIBF by 10%, 12%, or 27%, respectively, relative to that of control templates. The growth dependence of PIBF on the various carboxylic acid salts was also investigated using experimental observations of the growth kinetics and XPS analyses of the relative amount of metal ions present on the template surfaces.

Highly Functionalized and Stereocontrolled Syntheses of 2-(2-Methylenecycloalkyl)-furan Derivatives by Pd-Catalyzed Cycloreduction

[structure: see text]. We have discovered a new method employing Pd-catalyzed cycloreduction of conjugated enynals 1 bearing an alkyne unit leading to the corresponding 2-(2-methylenecycloalkyl)-furans 2a-j and a related compound 7 in good to excellent yields.

Rearrangements of the Diels−Alder Cycloadducts Obtained from Acetylenic Sulfones and 1,3-Diphenylisobenzofuran

1,3-Diphenylisobenzofuran afforded Diels-Alder cycloadducts 4a,b with n-butyl- and phenyl-substituted acetylenic sulfones 3a,b, respectively. The products underwent various types of rearrangements under pyrolytic, acid-catalyzed, and photochemical conditions. In the presence of acid, or upon heating in xylenes, they afforded the ketones 5a,b. In addition, the dehydration product 7a was produced from the pyrolysis of 4a, and the unexpected transposed ketone 6b was generated under acid-catalyzed or pyrolytic conditions from 4b via a postulated epoxide intermediate. The photolysis of 4a afforded ketone 5a as the sole isolated product, whereas 5b afforded oxepin 8b and indenyl phenyl ketone 9b. The formation of the latter two products can be rationalized by a series of pericyclic reactions. These include an intramolecular [2+2] cycloaddition, followed by a 1,3-dipolar cycloreversion, for the transformation of 4b to 8b and a series of electrocyclic and [1,3]sigmatropic reactions to convert 8b into 9b.