Heterotetracenes: Flexible Synthesis and in Silico Assessment of the Hole-Transport Properties

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.

Intramolecular activation of strong Si-O bonds by gold(I): regioselective synthesis of 3-bromo-2-silylbenzofurans

The high reactivity of gold-vinylidene complexes, generated in situ by [1,2]-bromine shift from the corresponding 1-bromoalkynes, allows the activation of one of the strongest bonds in organic chemistry (Si-O), strategically placed at the ortho-position. In this way, the synthesis of 3-bromo-2-silylbenzofuranes is achieved in good yields. Several substituents with different electronic properties and substitution patterns are well tolerated on the tethering aromatic ring as well as a number of silyl groups on the O-atom. A preliminary mechanistic study is compatible with the participation of gold vinylidene intermediate species. The synthetic applicability of the obtained scaffolds was preliminarily showcased by orthogonal C-C bond forming transformations.

Thiophene-fused polyaromatics: synthesis, columnar liquid crystal, fluorescence and electrochemical properties

Efficient syntheses that incorporate thiophene units into different extended conjugation systems are of interest as a result of the prevalence of sulfur-rich aromatics in organic electronics. Self-organization by using liquid crystal properties is also desirable for optimal processing of organic electronics and optical devices. In this article, we describe a two-step process to access extended regioisomers of polyaromatics with different shapes. This method involves an efficient single or double benzannulation from an alkyne precursor followed by Scholl cyclization. In spite of their unconventional nondiscoid shape, these materials display stable columnar liquid crystal phases. We examine the photophysical and electrochemical properties and find that structurally very similar thiophene-fused polyaromatics display significant differences in their properties.

Cyclization of ortho-ethynylbiaryls as an emerging versatile tool for the construction of polycyclic arenes

Cyclization and cycloisomerization of ortho-aryl(ethynyl)arenes provide an easy direct access to fused polycyclic aromatic carbo- and heterocycles. This methodology has demonstrated an impressive progress in the recent years. The goal of this review is to give a comprehensive outlook on the synthetic potential, scope, limitations, and mechanistic aspects of the cyclization reactions. The material is arranged according to the activation method that can be used to induce cyclization: pyrolysis, metal catalysis, electrophilic activation, radical induction, base catalysis. Particular attention is paid to the specificity of ortho-ethynylbiaryls with a heterocyclic central core.

The bibliography includes 257 references.

Hypervalent iodine reactions utilized in carbon–carbon bond formations

This review covers recent developments of hypervalent iodine chemistry in dearomatizations, radicals, hypervalent iodine-guided electrophilic substitution, arylations, photoredox, and more.

Cyclic Hypervalent Iodine Reagents: Enabling Tools for Bond Disconnection via Reactivity Umpolung

The efficient synthesis of organic compounds is an important field of research, which sets the basis for numerous applications in medicine or materials science. Based on the polarity induced by functional groups, logical bond disconnections can be deduced for the elaboration of organic compounds. Nevertheless, this classical approach makes synthesis rigid, as not all bond disconnections are possible. The concept of Umpolung has been therefore introduced: by inverting the normal polarity of functional groups, new disconnections become possible. Among the tools for achieving Umpolung, hypervalent iodine reagents occupy a privileged position. The electrophilicity of the iodine atom and the reactivity of the hypervalent bond allow access to electrophilic synthons starting from nucleophiles. Nevertheless, some classes of hypervalent iodine reagents can be too unstable for many applications, in particular involving metal catalysis. In this context, cyclic hypervalent iodine reagents, especially benziodoxolones (BXs), have been known for a long time to be more stable than their acyclic counterparts, yet their synthetic potential had not been fully exploited. In this Account, we report our efforts since 2008 on the use of BX reagents in the development of new transformations in organic synthesis, which showed for the first time their versatility as synthetic tools. Our work started with electrophilic alkynylation, as alkynes are one of the most important functional groups in organic chemistry, but are usually introduced as nucleophiles. We used ethynylbenziodoxolones (EBXs) in the direct alkynylation of nucleophiles, such as keto esters, thiols, or phosphines. The reagents could then be applied to the gold- and palladium-catalyzed alkynylation of C-H bonds on (hetero)arenes, leading to a more efficient alternative to the Sonogashira reaction. More complex reactions were then developed with formations of several bonds in a single transformation. Gold- and platinum-catalyzed cyclization/alkynylation domino processes gave access to new types of alkynylated heterocycles. Multifunctionalization of olefins became possible through intramolecular oxy- and amino-alkynylations. (Enantioselective) copper-catalyzed oxy-alkynylation of diazo compounds led to stereocenters with perfect atom economy. Finally, EBXs were also used for the alkynylation of radicals generated under photoredox conditions. Since 2013, we then extended the use of BX reagents to other transformations. Azidobenziodoxol(on)ess (ABXs) were used in the azidation of keto esters, enol silanes, and styrenes. New more stable derivatives were introduced. Cyanobenziodoxolones (CBXs) enabled the cyanation of stabilized enolates, thiols, and radicals. Finally, new BX reagents were developed for the Umpolung of indoles and pyrroles. They could be used in metal-catalyzed directed C-H functionalizations, as well as in Lewis acid mediated oxidative coupling to give functionalized bi(hetero)arenes. In the past decade, our group and others have shown that BX reagents are not only "structural beauties", but also extremely useful reagents in synthetic chemistry. A toolbox of cyclic hypervalent iodine reagents is now available to achieve Umpolung-based disconnections. We are convinced that the field is still in its infancy, and many new reagents and transformations still remain to be discovered.

Indium(III)-Catalyzed Synthesis of Benzo[ b]furans by Intramolecular Hydroalkoxylation of ortho-Alkynylphenols: Scope and Mechanistic Insights

Indium(III) halides catalyze the hydroalkoxylation reaction of ortho-alkynylphenols to afford benzo[ b]furans in good yields. The reaction proceeds with 5- endo- dig regioselectivity with a variety of phenols functionalized at the arene and alkyne moieties in high yields using InI₃ (5 mol %) in DCE. Experimental and computational studies support a mechanism based on the indium(III) π-Lewis acid activation of the alkyne followed by nucleophilic addition of the phenol and final protodemetalation to afford the corresponding benzo[ b]furan. DFT calculations suggest that dimer In₂I₆ is the catalytic species through a novel double coordination with the alkyne and the hydroxyl group.

Hydrazone–Pd-catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates

The hydrazone–palladium catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates gave the corresponding 2-substituted-3-allylbenzofuran derivatives at room temperature.

Synthesis of azulene-substituted benzofurans and isocoumarins via intramolecular cyclization of 1-ethynylazulenes, and their structural and optical properties

The preparation of azulene-substituted benzofurans and isocoumarins was established by two types of intramolecular cyclization reaction of 1-ethynylazulenes.