Reductive Bergman-Type Cyclizations of Cross-Conjugated Enediynes to Fulvene and Fulvalene Anions: The Role of the Substituent

Various cross-conjugated enediynes undergo "Bergman-type" cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives. This new anionic cyclization is considerably more facile than the classic Bergman cyclization with linear enediynes, creating highly reactive diradicals at -78 degrees C. Not all cross-conjugated enediynes yield cyclized dianions upon reduction; some give uncyclized, Y-shaped, cross-conjugated dianions, while others apparently yield radical-anions that either dimerize or persist as monomers. One system yields both a cyclized and an uncyclized dianion. The substituents are thus shown to be a critical factor in determining the outcome of the reduction. Cyclization occurs within a specific "window of opportunity" that is governed by the substituents.

A one-pot synthesis and functionalization of polyynes

[reaction: see text] A one-pot synthesis and derivatization of diynes and triynes is reported. The polyyne framework is formed from a dibromoolefin precursor based on a carbenoid rearrangement, and the resulting Li-acetylide is then trapped in situ with an electrophile to provide functionalized di- and triynes. Alternatively, transmetalation of the Li-acetylide intermediate provides either the Zn- or Sn-acetylide, which then allows for the divergent preparation of diaryl polyynes or aryl ynones via palladium-catalyzed cross-coupling reactions.

Synthesis of Naturally Occurring Polyynes

Over the past fifty years, hundreds of polyyne compounds have been isolated from nature. These often unstable molecules are found in sources as common as garden vegetables and as obscure as bacterial cultures. Naturally occurring polyynes feature a wide range of structural diversity and display an equally broad array of biological properties. Early synthetic efforts relied primarily on Cu-catalyzed, oxidative acetylenic homo- and heterocoupling reactions to assemble the polyyne framework. The past 25 years, however, have witnessed a renaissance in the field of polyyne natural product synthesis: transition-metal-catalyzed alkynylation reactions and asymmetric transformations have combined to substantially expand access to natural polyynes. This Review recounts these synthetic achievements and also highlights both the natural source(s) and biological relevance for many of these compounds.

Polyyne synthesis using carbene/carbenoid rearrangements

Rearrangement of a carbene/carbenoid intermediate to form an acetylene moiety, known as the Fritsch-Buttenberg-Wiechell (FBW) rearrangement, was developed for the formation of polyynes and polyyne frameworks within highly conjugated organic materials. Necessary precursors can be prepared through formation of an alkynyl ketone, followed by dibromoolefination under Corey-Fuchs conditions. The carbenoid rearrangement is brought about by treatment of the dibromoolefin with BuLi under mild conditions. The success of these FBW reactions is quite solvent-dependent, and nonpolar hydrocarbon solvents (e.g., hexanes, toluene, benzene) work quite well, while use of ethereal solvents such as diethyl ether and tetrahydrofuran (THF) does not provide the desired polyyne product. This protocol was successfully applied to the formation of silyl, alkyl, alkenyl, and aryl polyynes, including di-, tri-, and tetrayne products, as well as the construction of two-dimensional carbon-rich molecules. A one-pot variant of this procedure is being developed and is particularly applicable toward the synthesis of polyyne natural products. Formation of a series of triisopropylsilyl end-capped polyynes, from the triyne to decayne, was achieved. Third-order nonlinear optical properties of these polyynes were evaluated. This study shows that the molecular second hyperpolarizabilities for the polyynes as a function of length increase at a rate that is higher than all other nonaromatic organic oligomers.

Chiral, cross-conjugated isopolydiacetylenes

A series of chiral, monodisperse enyne oligomers (isopolydiacetylenes) based on a 1,1-binaphthyl core has been synthesized and characterized spectroscopically.

Characterization of Porosity in Organic and Metal−Organic Macrocycles by Hyperpolarized 129Xe NMR Spectroscopy

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]

Esters of 2-Iodoxybenzoic Acid: Hypervalent Iodine Oxidizing Reagents with a Pseudobenziodoxole Structure

Esters of 2-iodoxybenzoic acid (IBX-esters) were prepared by the hypochlorite oxidation of the corresponding 2-iodobenzoate esters and isolated as chemically stable, microcrystalline products. These hypervalent iodine compounds are potentially valuable oxidizing reagents belonging to a new class of pentavalent iodine compounds with a pseudobenziodoxole structure. Methyl 2-iodoxybenzoate can be further converted to the diacetate or a bis(trifluoroacetate) derivative by treatment with acetic anhydride or trifluoroacetic anhydride, respectively. Single-crystal X-ray diffraction analysis of methyl 2-[(diacetoxy)iodosyl]benzoate 8a reveals a pseudobenziodoxole structure with three relatively weak intramolecular I...O interactions. The dimethyl and diisopropyl esters of 2-iodoxyisophthalic acid were prepared by oxidation of the respective iodoarenes with dimethyldioxirane. Single-crystal X-ray diffraction analysis of diisopropyl 2-iodoxyisophthalate 6b showed intramolecular I...O interaction with the carbonyl oxygen of only one of the two carboxylic groups, while NMR spectra in solution indicated equivalency of both ester groups. IBX-esters, methyl 2-[(diacetoxy)iodosyl]benzoate, and 2-iodoxyisophthalate esters can oxidize alcohols to the respective aldehydes or ketones in the presence of trifluoroacetic acid or boron trifluoride etherate. The bis(trifluoroacetate) derivative can oxidize alcohols to carbonyl compounds without acid catalyst.

Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response

With the Fritsch-Buttenberg-Wiechell rearrangement as a primary synthetic route, a series of conjugated, triisopropylsilyl end-capped polyynes containing 2-10 acetylene units has been assembled. In a few steps, significant quantities of the polyynes are made available, which allow for a thorough analysis of their structural, physical, and optical properties. Molecules in the series have been characterized in detail using (13)C NMR spectroscopy, differential scanning calorimetry, mass spectrometry, and, for four derivatives including octayne 6, X-ray crystallography. UV-vis spectroscopy of the polyynes 1-7 shows a consistent lowering of the HOMO-LUMO gap (E(g)) as a function of the number of acetylene units (n), fitting a power-law relationship of E(g) approximately n(-)(0.379)(+/-)(0.002). The third-order nonlinear optical (NLO) properties of the polyyne series have been examined, and the nonresonant molecular second hyperpolarizabilities (gamma) increase as a function of length according to the power-law gamma approximately n(4.28)(+/-)(0.13). This result exhibits an exponent that is larger than theoretically predicted for polyynes and higher than is observed for polyenes and polyenynes. The combined linear and nonlinear optical results confirm recent theoretical studies that suggest polyynes as model 1-D conjugated systems. On the basis of UV-vis spectroscopic analysis, the effective conjugation length for this series of polyynes is estimated to be ca. n = 32, providing insight into characteristics of carbyne.

Synthesis, Structure, and Nonlinear Optical Properties of Cross-Conjugated Perphenylatediso-Polydiacetylenes

Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray crystallographic analysis demonstrate a non-planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red-shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25, a trend that has saturated by the stage of nonamer 22. Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n. The molecular third-order nonlinearities, gamma, for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n. Molecular modeling and spectroscopic studies suggest that iso-PDA oligomers (n>7) adopt a coiled, helical conformation in solution.

The surprising nonlinear optical properties of conjugated polyyne oligomers

Polyynes represent a unique class of conjugated organic compounds. The third-order nonlinear optical response of polyynes has been extensively modeled theoretically, and it is generally believed that the increase in molecular second hyperpolarizability (gamma) as a function of length for polyynes should be lower than that for polyenes. Experimental evidence to test this prediction, however, has been absent. We have synthesized conjugated polyynes that contain up to 20 consecutive sp-hybridized carbons, and we have determined their nonresonant gamma-values as a function of the number of acetylene repeat units (n). These gamma-values demonstrate a power-law behavior versus n(gamma approximately n(4.28+/-0.13)), with an exponent that is both larger than theoretically predicted for polyynes and substantially higher than that observed for polyenes or polyenynes. Furthermore, no saturation of the linear or nonlinear optical properties is observed.

Preparation and structure of 2-iodoxybenzoate esters: soluble and stable periodinane oxidizing reagentsElectronic Supplementary Information (ESI) available: synthetic and characterization data for all new compounds; general procedures for the oxidation of alcohols with reagent 4c. See http://www.rsc.org/suppdata/cc/b3/b312961f/

Esters of 2-iodoxybenzoic acid (IBX-esters) are potentially valuable oxidizing reagents belonging to a new class of pentavalent iodine compounds with a pseudo benziodoxole structure.

Ammonium Cyanate Shows N−H···N Hydrogen Bonding, Not N−H···O

The transformation of ammonium cyanate into urea, first studied over 170 years ago by Wöhler and Liebig, has an important place in the history of chemistry. To understand the nature of this solid state reaction, knowledge of the crystal structure of ammonium cyanate is a prerequisite. Employing neutron powder diffraction, we demonstrate conclusively that, in the structure of ammonium cyanate, the NH(4)(+) cation forms N-H...N hydrogen bonds to four cyanate N atoms at alternate corners of a distorted cube, rather than our previously proposed alternative arrangement with N-H...O hydrogen bonds to cyanate O atoms at the other four corners.

Synthesis of naturally occurring acetylenes via an alkylidene carbenoid rearrangement

Naturally occurring mosquito larvicidal acetylenes 1 and 2, and analogues 3 and 4, each containing either a 1,3-butadiynyl or a 1,3,5-hexatriynyl moiety, are synthesized via a Fritsch-Buttenberg-Wiechell rearrangement. The alkylidene carbenoid intermediate results from lithium-halogen exchange of a suitable dibromoolefin precursor, and the rearrangement is accomplished under mild conditions. Synthesis of the dibromoolefin precursors to acetylenes 1-4 is easily achieved in three steps from commercially available carboxylic acids or aldehydes, making this procedure a viable alternative to conventional methods for the synthesis of naturally occurring acetylenes.

Alkyne migration in alkylidene carbenoid species: a new method of polyyne synthesis

The synthesis of conjugated polyyne structures via a modification of the Fritsch-Buttenberg-Wiechell (FBW) rearrangement is reported. Our adaptation provides for the 1,2-migration of an alkyne in a carbene/carbenoid intermediate that is conveniently effected via lithium-halogen exchange with the appropriate dibromo-olefinic precursor. This rearrangement is quite rapidly accomplished under mild conditions (hexane solution, -78 degrees C), and the seemingly high migratory aptitude of the alkynyl moiety provides for efficient rearrangement. This, in turn, allows for multiple rearrangements in a single molecule, greatly facilitating the construction of highly unsaturated substrates. This procedure is exploited for the rapid synthesis of symmetrical and unsymmetrical 1,3,5-hexatriynes, extended polyynes, and aryl polyyne building blocks. Most significantly, many of these structures have been or would be difficult to access via more traditional transition metal catalyzed homo- or cross-coupling techniques.

Synthesis of unsymmetrically substituted 1,3-butadiynes and 1,3,5-hexatriynes via alkylidene carbenoid rearrangements

Unsymmetrically substituted 1,3-butadiynes and 1,3,5-hexatriynes are synthesized in four steps from commercially available aldehydes or carboxylic acids. The key step in this process involves a Fritsch-Buttenberg-Wiechell rearrangement, in which an alkylidene carbenoid intermediate subsequently rearranges to the desired polyyne. This rearrangement proceeds under mild conditions, and it is tolerant of a range of functionalities. In general, the procedurally facile formation of the dibromoolefinic precursors, in combination with the effectiveness of the rearrangement step, makes this procedure an attractive alternative to traditional methods for di- and triyne synthesis that utilize palladium or copper catalysis.

Synthesis and derivatization of ethynyl alpha,alpha-dibromomethyl ketones: formation of highly functionalized vinyl triflates

[reaction: see text] We describe the synthesis of alpha,alpha-dibromomethyl ynones (8) and their subsequent derivatization to vinyl acetates (10). These vinyl acetates feature a 1,1-dibromo-olefin moiety, which is readily exploited in palladium-catalyzed Sonogashira, Stille, and Suzuki cross-coupling reactions with alkynes, stannanes, and boronic acids, respectively. A novel one-pot process then directly converts the resulting vinyl acetates 11-13 to the vinyl triflate derivatives 14a-j.

Picosecond transient photoconductivity in functionalized pentacene molecular crystals probed by terahertz pulse spectroscopy

We have measured transient photoconductivity in functionalized pentacene molecular crystals using ultrafast optical pump-terahertz probe techniques. The single crystal samples were excited using 800 nm, 100 fs pulses, and the change in transmission of time-delayed, subpicosecond terahertz pulses was used to probe the photoconducting state over a temperature range from 10 to 300 K. A subpicosecond rise in photoconductivity is observed, suggesting that mobile carriers are a primary photoexcitation. At times longer than 4 ps, a power-law decay is observed consistent with dispersive transport.

Preparation, Structure, and Unexpected Chemistry of Phosphoranyl-Derived Benziodoxoles

Novel phosphoranyl-derived benziodoxoles were prepared by the reaction of 1-acetoxybenziodoxole with stabilized phosphonium ylides [1-triphenylphosphoranylidene-2-propanone and (carbomethoxymethyl)triphenylphosphoranylidene acetate]. X-ray crystallographic analysis of these compounds demonstrates the significance of secondary bonding interactions, which link individual molecules into infinite chains. An unusual ligand exchange on an iodine(III) center resulting in the substitution of a carbon ligand with an oxygen ligand was observed in the reaction of these compounds with strong acids.

Coordination-driven self-assembly: solids with bidirectional porosity

Coordination-driven self-assembly reactions have been used in the preparation of a variety of discrete supramolecular species, some of which have shown promise as synthetic receptors. Many highly ordered coordination polymers and porous networks have been prepared in a similar fashion. While a few of these solids are capable of the uptake of small organic molecules in the resultant molecular channels, the formation of truly porous structures has frequently been thwarted by lattice interpenetration. A strategy for the formation of porous solids that may circumvent this problem is based on the covalent construction of nanoscale macrocycles which, when eclipsed in the solid state, may lead to porous, tubular assemblies. We have incorporated these concepts toward the realization of a bidirectionally porous solid. The metal-directed, self-assembly of a conjugated, macrocyclic ligand provides a discrete, supramolecular entity in solution and the solid state. X-ray crystallographic analysis establishes that this assembly packs such that bidirectional channels are realized, and the incorporation of only ClCH2CH2Cl into the crystal lattice demonstrates that these channels are potentially suitable for the selective uptake of small organic guests.

Structural and electronic characteristics of thienyl(aryl)iodonium triflates

A general strategy toward the synthesis of aryliodonium triflate salts has been exploited to afford derivatives that incorporate thiophene and bithiophene components. Both mono- and bis(iodonium) salts have been realized, and a series of bithienyl(aryl)iodonium triflates with increasingly electron-withdrawing substituents on the aryl moiety have been synthesized. X-ray crystallographic analysis of four derivatives (4a, 4b, 6b, and 8c) demonstrates that the solid-state organization of these salts incorporates extensive networks of secondary bonding interactions between the cationic iodonium centers and the triflate counterions. UV-vis spectroscopic analysis shows that the electronic interactions between pendent aryl and heteroaryl groups across the iodonium center can be dictated by substitution. Furthermore, the energy of the HOMO-LUMO gap decreases substantially in weakly or noncoordinating solvents.

Study of cross-conjugated iso-polytriacetylenes and related oligoenynes

Two series of monodisperse cross-conjugated oligomers based on enyne repeat units have been realized. The first class of molecules, iso-polytriacetylenes (iso-PTAs, 2), was divergently synthesized using an iterative sequence of palladium-catalyzed cross-coupling reactions of vinyl triflate 5 with terminal alkynes. The second series of oligoenynes (17-20) are based on an octatetrayne backbone, and result from homocoupling of the differentially protected iso-PTA oligomers 8-11. The longest member of this series, 20, spans ca. 5.6 nm from Si atom to Si atom and is composed of a contiguous sequence of 44 sp and sp(2) carbons. The lowest energy electronic absorption band for iso-PTA dimers in the progression 13 --> 9 --> 16 is consistently red-shifted as a result of extending the cross-conjugated structure. A similar comparison within each series (i.e., 16, 6-7, or 17-20), however, suggests little effect on the electronic characteristics of these molecules as oligomer length is increased. The solid-state properties of one derivative, 17, are also described.

Functionalized macrocyclic ligands for use in supramolecular chemistry

The synthesis of three cross-conjugated macrocycles bearing pyridine functionality is described. The ability of two of these molecules, 5a and 5c, to behave as 4,4'-bipyridine mimics in self-assembly reactions is demonstrated by axial coordination to a metalloporphyrin. The directed coordinative ability of the pyridine rings ensures predictable self-assembly into well-ordered nanoscale systems, both in solution and the solid state. 1H, 13C, and HMQC NMR spectroscopic experiments and ESI MS have been used to characterize the supramolecular complexes in solution. X-ray crystallographic analysis of solid-state assemblies 7a and 7c provides insight into the scope and flexibility of these macrocyclic ligands as supramolecular building blocks. UV-vis and fluorescence spectroscopies provide a description of their electronic characteristics.

Iterative synthesis and characterization of cross-conjugated iso-polydiacetylenes

An iterative synthesis of cross-conjugated iso-polydiacetylenes (iso-PDAs) is reported based on three fundamental building blocks: vinyl triflates 8, 9, and 25. An efficient sequence of palladium-catalyzed cross-coupling reactions of vinyl triflates and terminal alkynes has been employed to extend the chain length of these oligomers. The longest member of the series, nonamer 19, spans 3.4 nm from Si to Si atom. The stability and solubility of the conjugated oligomers have been evaluated as a function of pendant substitution. Assessment of solid-state structural properties was achieved via X-ray crystallographic analyses of monomer 12 and trimer 14. The electronic characteristics of the monodisperse oligomers have been fully analyzed by UV-vis spectroscopy in solution and as thin films, and these studies suggest that pi-electron communication is present along the enyne framework, but quickly reaches saturation by the stage of nonamer 19.

Highly functionalized dimeric tetraethynylethenes and expanded radialenes: strong evidence for macrocyclic cross-conjugation

A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.