General, Efficient Route to Thiophene-1-Oxides and Well-Defined, Mixed Thiophene-Thiophene-1-Oxide Oligomers

Editing of polymer backbones

Polymers are at the epicentre of modern technological progress and the associated environmental pollution. Considerations of both polymer functionality and lifecycle are crucial in these contexts, and the polymer backbone - the core of a polymer - is at the root of these considerations. Just as the meaning of a sentence can be altered by editing its words, the function and sustainability of a polymer can also be transformed via the chemical modification of its backbone. Yet, polymer modification has primarily been focused on the polymer periphery. In this Review, we focus on the transformations of the polymer backbone by defining some concepts fundamental to this topic (for example, 'polymer backbone' and 'backbone editing') and by collecting and categorizing examples of backbone editing scattered throughout a century's worth of chemical literature, and outline critical directions for further research. In so doing, we lay the foundation for the field of polymer backbone editing and hope to accelerate its development.

Tetraazacoronenes and Their Dimers, Trimers and Tetramers

Tetraazacoronenes were synthesized from bay-functionalized tetraazaperylenes by Zr-mediated cyclization and four-fold Suzuki-Miyaura cross coupling. In the Zr-mediated approach, an η4 -cyclobutadiene-zirconium(IV) complex was isolated as an intermediate to cyclobutene-annulated derivatives. Using bis(pinacolatoboryl)vinyltrimethylsilane as a C2 building block gave the tetraazacoronene target compound along with the condensed azacoronene dimer as well as higher oligomers. The series of extended azacoronenes show highly resolved UV/Vis absorption bands with increased extinction coefficients for the extended aromatic cores and fluorescence quantum yields of up to 80 % at 659 nm.

Metallacycle Transfer and its Link to Light-Emitting Materials and Conjugated Polymers

Major advances in optoelectronic technologies (e. g., solar cells, organic light-emitting diodes, etc…) are prefaced by the discovery of new synthetic methodologies. In this review, the key role of the Fagan-Nugent reaction in enabling our team (and others) to gain access to new building blocks for luminescent materials and conjugated polymers bearing p-block elements will be described. The Fagan-Nugent reaction is extremely powerful as a synthetic tool since the efficient zirconium-element atom exchange involved affords a wide range of unsaturated inorganic heterocycles of controllable composition and function.

Negishi's Reagent Versus Rosenthal's Reagent in the Formation of Zirconacyclopentadienes

Zirconacyclopentadienes are versatile precursors for a large number of heteroles, which are accessible by Zr-element exchange reactions. The vast majority of reports describe their preparation by the use of Negishi's reagent, which is a species that is formed in situ. The zirconacyclopentadiene is then formed by the addition of one equivalent of a diyne or two equivalents of a monoyne moiety to this Negishi species. Another route involves Rosenthal's reagent (Cp2 Zr(py)Me3 SiC≡CSiMe3 ), which then reacts with a diyne or monoyne moiety. In this work, the efficiency of both routes was compared in terms of reaction time, stability of the product in the reaction mixture, and yield. The synthetic implications of using both routes are evaluated. Novel zirconacyclopentadienes were synthesized, characterized directly from the reaction mixture, and crystal structures could be obtained in most cases.

Thiophene S-oxides

Methods for the preparation of thiophene S-oxides, their roles in thiophene metabolism, and the structures and chemical reactivity of these compounds, are discussed.

Rapid access to (cycloalkyl)tellurophene oligomer mixtures and the first poly(3-aryltellurophene)

New poly- and oligotellurophenes bearing cycloalkyl and 3-aryl substituents have been reported, with narrow band gaps approaching 1.3 eV observed.

End-Capping π-Conjugated Systems with Medium-Sized Sulfur-Containing Rings: A Route Towards Solution-Processable Air-Stable Semiconductors

The sulfur-containing nine-membered heterocycle thiacyclononene (TN) was evaluated as a new type of end-capping group for π-conjugated systems. A systematic study on TN-capped α-oligothiophenes (TNnTs; n=4-7) revealed that the capping with TN, which adopts a bent conformation, imparts the resulting oligothiophenes with drastically increased solubility at approximately 140 °C and high electrochemical stability, whereas the electronic structure remains virtually unperturbed. The even-numbered oligothiophenes TN4T and TN6T form characteristic offset herringbone-type packing structures on account of the steric repulsion between the TN rings and the presence of intermolecular nonbonding S⋅⋅⋅S interactions. This packing mode in combination with the high solubility enabled the solution-process fabrication of field-effect transistors based on TN6T, which exhibited a high performance without degradation even upon exposure to air.

Synthesis of a Cyclophane Bearing Two Benz[a]anthracene Units Connected at the 5 and 7 Positions with Two Naphth-1,4-diyl Groups

A synthetic pathway to a cyclophane bearing two benz[a]anthracene units connected at the 5 and 7 positions through two naphth-1,4-diyl groups was developed, and its structure was confirmed by X-ray structure analysis. Because of structural constraints, the two naphthyl groups are distorted from planarity and the bonds connecting them to the benz[a]anthracene units are bent significantly. The UV-vis and fluorescence spectra of the cyclophane are red-shifted from those of 7-(1-naphthalenyl)benz[a]anthracene, which is the corresponding monomeric polycyclic aromatic hydrocarbon.

Effects of boryl, phosphino, and phosphonio substituents on optical, electrochemical, and photophysical properties of 2,5-dithienylphospholes and 2-phenyl-5-thienylphospholes

The optical, electrochemical, and photophysical properties of α,α′-linked phosphole–thiophene π-systems bearing boryl, phosphino, or phosphonio groups were studied.

Conversion of zirconacyclopentadienes into metalloles: Fagan-Nugent reaction and beyond

Metalloles are derivatives of cyclopentadiene in which the methylene unit is replaced by a heteroatom, such as S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, and so on. Many metallole derivatives have been widely used as photovoltaic cells, organic light emitting diodes (OLEDs), chemical sensors, electrochromic devices, microelectronic actuators, and organic field effect transistors (OFETs). In the meantime, many of them showed promising biological actives. Due to the similarity to cyclopentadiene, the anionic forms of metalloles were also widely explored in coordination chemistry. As a result, development of a general method for the formation of metalloles from available starting materials is highly desired. In this Account, we outline formation of various p-block element metalloles from zirconacyclopentadienes. The zirconacyclopentadienes can be easily prepared from two molecules of alkynes and a low-valent zirconocene species "Cp2Zr(II)" (Cp = cyclopentadienyl). Fagan and Nugent first reported the formation of main group metalloles from zirconacyclopentadiene, which provided a versatile approach for the construction of metalloles, especially for the formation of metalloles in heavier p-block elements. To further expand the substrate scope, a number of stepwise conversions were developed, which involve 1,4-dimetallo- or dihalo-1,3-butadiene as intermediates from zirconacyclopentadienes. Here, four processes are classified based on direct and indirect conversion of zirconacyclopentadienes into metalloles. Direct reaction of zirconacyclopentadienes with element halides afforded heterocycles of main group elements, which provided a versatile method for the synthesis of metalloles. Nonetheless, the reaction scope was restricted to heavier p-block elements such as S, Se, P, As, Sb, Bi, Ge, Sn, Ga, and In. And these reactions usually suffered low yields and long reaction time. Transmetalation of zirconacyclopentadiene with copper chloride greatly enriched the zirconacyclopentadiene chemistry. The synthesis of stannoles and pyrroles from zirconacyclopentadienes has been developed in the presence of CuCl. The direct reaction of the zirconacyclopentadienes with SiCl4 or R2SiCl2 does not give the desired silacyclopendadiene derivatives, even in the presence of CuCl. It can be circumvented by using dilithiated dienes from diiododienes, which are easily prepared by the iodination of zirconacyclopentadienes using CuCl as an additive. Finally, an umpolung strategy, reaction of electrophilic 1,4-diiodo-1,3-butadiene with nucleophilic amine or sulfide reagents, was successfully used in the formation of pyrroles and thiophenes.

Assembly of macrocycles by zirconocene-mediated, reversible carbon-carbon bond formation

Macrocyclic compounds have attracted considerable attention in numerous applications, including host-guest chemistry, chemical sensing, catalysis, and materials science. A major obstacle, however, is the limited number of convenient, versatile, and high-yielding synthetic routes to functionalized macrocycles. Macrocyclic compounds have been typically synthesized by ring-closing or condensation reactions, but many of these procedures produce mixtures of oligomers and cyclic compounds. As a result, macrocycle syntheses are often associated with difficult separations and low yields. Some successful approaches that circumvent these problems are based on "self-assembly" processes utilizing reversible bond-forming reactions, but for many applications, it is essential that the resulting macrocycle be built with a strong covalent bond network. In this Account, we describe how zirconocene-mediated reductive couplings of alkynes can provide reversible carbon-carbon bond-forming reactions well-suited for this purpose. Zirconocene coupling of alkenes and alkynes has been used extensively as a source of novel, versatile pathways to functionalized organic compounds. Here, we describe the development of zirconocene-mediated reductive couplings as a highly efficient method for the preparation of macrocycles and cages with diverse compositions, sizes, and shapes. This methodology is based on the reversible, regioselective coupling of alkynes with bulky substituents. In particular, silyl substituents provide regioselective, reversible couplings that place them into the α-positions of the resulting zirconacyclopentadiene rings. According to density functional theory (DFT) calculations and kinetic studies, the mechanism of this coupling involves a stepwise process, whereby an insertion of the second alkyne influences regiochemistry through both steric and electronic factors. Zirconocene coupling of diynes that incorporate silyl substituents generates predictable macrocyclic products in very high yields. In the absence of significant steric repulsion, the macrocyclization appears to be entropically driven, thereby providing the smallest strain-free macrocyclic structure. The scope of the reaction has been explored by variation of the spacer group between the alkynyl substituents and by incorporation of functional and chiral groups into the macrocycle. The size and shape of the resulting macrocycles are largely determined by the length and geometry of the dialkyne spacer, especially in the case of terminal trimethylsilyl-substituted diynes. For example, linear, rigid diynes with four or fewer phenylene rings lead to trimeric macrocycles, whereas bent or flexible diynes produce dimers. Depending on the reaction conditions, functional groups (such as N-heterocycles and imines) are tolerated in zirconocene coupling reactions, and in selected cases, they can be used to influence the shape of the final macrocyclic product. More recently, Cp(2)Zr(pyr)(Me(3)SiC≡CSiMe(3)) has been employed as a more general zirconocene synthon; it affords higher yields and increased functional group tolerance. Functional groups can also be incorporated through transformation of the zirconacyclopentadiene products, with acid hydrolysis to the corresponding butadiene being the most efficient derivatization. Furthermore, construction of chiral macrocycles has been accomplished by stereoselective macrocyclizations, and triynes have been coupled into three-dimensional cage compounds. We also discuss various design factors, providing a perspective on the utility of zirconocene-mediated couplings in the assembly of macrocyclic and cage compounds.

Cycloaddition of benzo[b]thiophene-S,S-dioxide – a route to substituted dibenzothiophenes and dibenzothiophene S,S-dioxides

Benzo[b]thiophene S,S-dioxide can be transformed by cycloaddition with tetraarylthiophene S-oxides to tetraaryldibenzothiophene S,S-dioxides. An analogous cycloaddition of benzo[b]thiophene S,S-dioxide, albeit at higher temperatures, leads directly to tetraaryldibenzothiophenes.

Ladder Distyrylbenzenes with Silicon and Chalcogen Bridges: Synthesis, Structures, and Properties

[reaction: see text] A cascade-type anionic double cyclization of (o-silylphenyl)(o-halophenyl)acetylenes via lithiation followed by treatment with elemental chalcogen produces silicon and chalcogen-bridged stilbenes. Based on this reaction, a series of silicon and sulfur- or silicon and selenium-bridged ladder distyrylbenzenes have been synthesized. Their chemical modification by oxidation, crystal structures, and photophysical properties are described.

The photochemistry of thiophene S-oxides

Thiophene S-oxides have been prepared and their photoreactivity has been studied. In many cases, the sulfur moiety of the thiophene S-oxides is deoxygenated. Here, the corresponding thiophenes or hydroxylated thiophenes are isolated. In the case of the photoirradiation of tert-butyl substituted thiophene S-oxides, the oxygen of the sulfoxy unit is incorporated into the heterocyclic ring system and the corresponding furans have been isolated. Also, structures with two thiophene cores have been oxidized to the respective thiophene S-oxides. These molecules undergo photodeoxygenation, a reaction, which is catalysed by (non-oxidized) thiophenes.

Lewis acid-promoted reactions of zirconacyclopentadienes with isocyanates. A one-pot three-component synthesis of multiply-substituted iminocyclopentadienes from one isocyanate and two alkynes

Multiply-substituted iminocyclopentadienes were formed from Lewis acid-promoted reactions of zirconacyclopentadienes and isocyanates via a one-pot three-component coupling process; the C=O double bond of the RN=C=O moiety in the isocyanate was cleaved, and the isocyanates behaved formally as a one-carbon unit with Lewis acid-dependent and substituent-dependent reactions being realized.

The sonoelectrooxidation of thiophene S-oxides

Thiophene-S-oxides (thiophene monoxides) are relatively new compounds, less stable than the better-known thiophene-S-dioxides. They are useful as synthons for a range of applications, including in the production of pharmaceuticals. They have interesting photochemical properties, but in this presentation we contrast the electro-oxidative voltammetry of differently substituted derivatives. We also compare carbocyclic compounds such as tetracyclone, the electro-oxidation of which at relatively high potentials has never been reported in silent or insonated conditions.

Lewis acid mediated reactions of zirconacyclopentenes with aldehydes affording homoallyl ketones via oppenauer-type oxidation

Three different components involving alkynes, ethylene, and aldehydes were selectively integrated in a one-pot procedure to afford homoallyl ketones in good yields, via an effective combination of zirconocene-mediated C-C bond forming reactions and Lewis acid mediated organic transformation. Mechanistic studies revealed that a formal Oppenauer oxidation of seven-membered oxazirconacycles, generated in situ from the reactions of zirconacyclopentenes and aldehydes, was promoted by Lewis acid-aldehyde adducts. As a whole, the first aldehyde was incorporated into the product and the second aldehyde was reduced to an alcohol. Multiply deuterated homoallyl ketones could be readily prepared in high yields with more than 98% deuterium incorporation by using this method.