Ring-Closing Metathesis as a Basis for the Construction of Aromatic Compounds

Site-Selective Ortho/Ipso C-H Difunctionalizations of Arenes using Thianthrene as a Leaving Group

Site-selective ortho/ipso C-H difunctionalizations of aromatic compounds were designed to afford polyfunctionalized arenes including challenging 1,2,3,4-tetrasubstituted ones (62 examples, up to 97 % yields). To ensure the excellent regioselectivity of the process while keeping high efficiency, an original strategy based on a "C-H thianthenation/Catellani-type reaction" sequence was developed starting from simple arenes. Non-prefunctionalized arenes were first regioselectively converted into the corresponding thianthrenium salts. Then, a palladium-catalyzed, norbornene (NBE)-mediated process allowed the synthesis of ipso-olefinated/ortho-alkylated polyfunctionalized arenes using a thianthrene as a leaving group (revisited Catellani reaction). Pleasingly, using a commercially available norbornene (NBE) and a unique catalytic system, synthetic challenges known for the Catellani reaction with aryl iodides were smoothly and successfully tackled with the "thianthrenium" approach. The protocol was robust (gram-scale reaction) and was widely applied to the two-fold functionalization of various arenes including bio-active compounds. Moreover, a panel of olefins and alkyl halides as coupling partners was suitable. Pleasingly, the "thianthrenium" strategy was successfully further applied to the incorporation of other groups at the ipso (CN/alkyl/H, aryl) and ortho (alkyl, aryl, amine, thiol) positions, showcasing the generality of the process.

Sequential ortho-/meta-C-H functionalizations of N-tosyl-benzamides for the synthesis of polyfunctionalized arenes

Selective one-pot sequential ortho-/meta-C-H functionalizations provided highly desirable polyfunctionalized arenes. Starting from readily available carboxylic acid derivatives, the concomitant formation of C-O and C-halogen bonds was achieved under mild reaction conditions (12 examples, up to 75% yield). The utility of the products was illustrated with post-functionalization reactions and Metiglinid synthesis.

Unconventional Reactivity of a Grubbs Catalyst: Hydroalkylation Overriding Metathesis

Unprecedented reactivity of a Grubbs catalyst has been disclosed in a reaction between vinyl azaarenes and alkenylnitriles under standard metathesis conditions. No metathesis was observed; only hydroalkylation products were obtained in high yields. The practical utility of this method has been demonstrated by the application of the products in useful transformations, e.g., the formation of cyclic iminoesters and highly challenging medium-sized carbocycles.

Deaminative ring contraction for the synthesis of polycyclic heteroaromatics: a concise total synthesis of toddaquinoline

A concise strategy to prepare polycyclic heteroaromatics involving a deaminative contraction cascade is detailed. The efficient deaminative ring contraction involves the in situ methylation of a biaryl-linked dihydroazepine to form a cyclic ammonium cation that undergoes a base-induced [1,2]-Stevens rearrangement/dehydroamination sequence. The presence of pseudosymmetry guides the retrosynthetic analysis of pyridyl-containing polycyclic heteroaromatics, enabling their construction by the reductive cyclization and deaminative contraction of tertiary amine precursors.

Ruthenium-catalysed decarboxylative unsymmetric dual ortho-/meta-C-H bond functionalization of arenecarboxylic acids

Here, we describe a ruthenium-catalysed decarboxylative unsymmetric ortho-C-H azaarylation/meta-C-H alkylation via a traceless directing group relay strategy. The installation of a 2-pyridyl functionality via carboxyl directed ortho-C-H activation is critical to promote decarboxylation and enable meta-C-H bond alkylation to streamline the synthesis of 4-azaaryl-benzo-fused five-membered heterocycles. This protocol is characterized by high regio- and chemoselectivity, broad substrate scopes, and good functional group tolerance under redox-neutral conditions.

Application of Olefin Metathesis in the Synthesis of Carbo- and Heteroaromatic Compounds-Recent Advances

The olefin metathesis reaction has found numerous applications in organic synthesis. This is due to a number of advantages, such as the tolerance of most functional groups and sterically demanding olefins. This article reviews recent advances in the application of the metathesis reaction, particularly the metathetic cyclization of dienes and enynes, in synthesis protocols leading to (hetero)aromatic compounds.

Atroposelective Arene-Forming Alkene Metathesis

Alkene metathesis catalyzed by enantiopure metal alkylidene complexes enables exceptionally versatile strategies to products with configurationally-defined stereocenters. Desymmetrization processes thereby provide reliable stereoselective routes to aliphatic structures, while the differentiation of aromatic stereogenic units remained an outstanding challenge. Herein, we describe the feasibility of alkene metathesis to catalytically control stereogenic axes by traceless arene formation. Stereodynamic trienes are selectively converted into corresponding binaphthalene atropisomers upon exposure to a chiral molybdenum catalyst. Remarkably, stereoselective arene-forming metathesis allows enantioselectivities of up to 98 : 2 e.r. and excellent yields. As the disconnection of each bond of an aromatic target is retrosynthetically conceivable, it is anticipated that forging arenes by means of stereoselective metathesis will enable versatile approaches for the synthesis of a broad range of molecular topologies with precisely defined configuration.

Protecting Group-Dependent Synthesis of Densely Substituted Dihydropyrroles v/s Pyrroles via 5-Exo-trig Cascade Radical Cyclization to Alkynyl Vinylogous Carbamates

Densely substituted dihydropyrroles could be synthesized with excellent diastereoselectivity via 5-exo-trig cascade radical cyclization to alkynyl vinylogous carbamates. N-Alkyl/acyl protected alkynyl vinylogous carbamates upon radical cyclization using thiophenol gave substituted pyrroles as against dihydropyrroles, which were formed with N-sulfonyl protecting groups. This enabled a rare example wherein both dihydropyrrole and pyrrole rings are assembled in the same reaction. This strategy could be used for the synthesis of an unprecedented adjacent polyheterocyclic system having a furan-thiophene-pyrrole motif. When vinylogous carbamate is embedded in the isoindole moiety, a pyridoisoindole derivative was formed with excellent diastereoselectivity, instead of the expected pyrroloisoindole product.

Award Lecture Paper: Hypervalent Iodine-Mediated Oxidative Alkene Arylation: A Thorough Analysis

A hypervalent iodine-mediated, oxidative alkene arylation reaction has been developed. Both Koser’s reagent (PhI(OH)OTs) and (diacetoxyiodo)toluene (TolI(OAc)₂) were equally viable as oxidants, which reacted with ortho-vinylbiphenyl derivatives to produce tri-, tetra- and pentacyclic polycyclic aromatic hydrocarbons (PAHs) in yields up to 97%. Comparison of this stoichiometric reaction with a previously-reported catalytic process showed that these protocols were largely complementary, and that they likely operate via the same general mechanistic sequence involving vinyliodonium salts decomposing into vinylene phenonium ions. Various mechanistic control experiments were conducted, which ruled out epoxides as intermediates, and which showed that E- and Z- alkene geometry in 10-substituted ortho-vinylbiphenyls had no impact on the ensuing isomeric product distributions. These experiments strongly supported the formation of E-vinyliodonium ions as initial reaction intermediates, and while the occurrence of 1,2-phenyl shift products was a common phenomenon, we concluded that alkyl substitution on the ortho-vinylbiphenyl was a requirement for this alternate pathway to occur.

Heck Reaction Boosted Heterocycle Ring-Closing and Ring-Opening Rearrangement: A Strategy for the Synthesis of Indolyl-Type Ligands

A novel method for P-involved heterocycle ring-closing-ring-opening rearrangement (HRR) via the Heck reaction is disclosed. The approach enables direct installation of a phosphorus-containing aryl group onto the C2 position of indole. This new rearrangement directly transforms easily prepared indole derivatives into indolyl-derived phosphonates and phosphinic acids with high yields, and many of the products are difficult to obtain by using established methods. This new HRR reaction provides an extremely simple and step-economic method to induce C-C bond formation and P-N bond cleavage for the synthesis of a variety of indolyl-type ligands.

Ring closing metathesis for the construction of carbazole and indole-fused natural products

The synthesis and functionalization of carbazole ring systems have received considerable attention in organic synthesis due to their widespread occurrence in biologically active compounds. One of the classical methods for the synthesis of carbazoles involves C-C bond formation of a biaryl amine moiety by oxidizing agents. Over the last few years, various new strategies have evolved for the synthesis of carbazole ring systems. During the past two decades, ring-closing metathesis (RCM) based approaches have been efficiently employed for the synthesis of nitrogen containing heteroaromatic systems including carbazoles. Herein, we discuss the construction of carbazole ring systems using RCM and the application of RCM based methods in the preparation of other indole-fused heterocycles. The application of these methods in the synthesis of carbazole alkaloids and bioactive indole-fused natural products has been discussed to highlight the importance of RCM in total synthesis.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Cyclopropene-Templated Assembly of Medium Cycles via Ru-Catalyzed Ring-Closing Metathesis

An expeditious click-click cyclize strategy for the assembly of medium-sized heterocyclic rings is described. The sequence involves the reaction of cycloprop-2-ene carboxylic acids with unsaturated amines to furnish amides, which are further subjected to a Cu-catalyzed directed carbomagnesiation and a ring-closing olefin metathesis reaction. This methodology allows for the efficient preparation of lactams with ring sizes up to 10.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

Gold(I)-Catalyzed Aromatization: Expeditious Synthesis of Polyfunctionalized Naphthalenes

A gold-catalyzed 6-endo-dig carbocyclization of alkyne with the pendent diazo group is reported. It provides an expeditious approach for the synthesis of multi-functionalized naphthalene derivatives under mild conditions. Mechanistic studies suggest that a vinyl gold carbene is generated as the key intermediate in this cascade transformation that smoothly delivers naphthalene products through an unprecedented stepwise aromatization or an intermolecular aromatic substitution process. The unique endocyclic vinyl species is inaccessible with other precursors; thus, novel carbene cascade transformations could be envisioned with the current catalytic model. Functional groups, such as alkenyl, hydroxyl, amino, and carboxyl groups, remain untouched under these conditions. In addition, the utility of these generated 2-carboxyl naphthalenes is illustrated by the synthesis of chiral 1,2′-binaphthalene ligands and π-conjugated polycyclic hydrocarbons (CPHs).

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A general method for the construction of multi-functionalized naphthalenes

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First 6-endo-dig diazo-yne carbocyclization leading to the vinyl gold carbene

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Expeditious access to naphthalenes with broad functional group compatibility

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Applications for the synthesis of chiral 1,2′-binaphthalene ligands and CPHs

Recent Advancements in Transition-Metal-Catalyzed One-Pot Twofold Unsymmetrical Difunctionalization of Arenes

Transition-metal-catalyzed direct C-H bond activation reactions have been embraced as a powerful synthetic tool to access diverse functionalized arenes. However, site-selective incorporation of multiple distinct functionalities in an arene has always been a formidable challenge. Recent efforts from the synthetic community have disclosed a few dynamic synthetic approaches to fabricate multifunctionalized arenes in one-pot using a single catalytic system. These reports manifested the immense potential of such approaches to expedite contemporary organic synthesis towards building molecular complexity. In this minireview, we have illustrated the recent progress in this area, highlighting the contribution from several synthetic chemists including our group.

"Close-to-Release": Spontaneous Bioorthogonal Uncaging Resulting from Ring-Closing Metathesis

Bioorthogonal uncaging reactions offer versatile tools in chemical biology. In recent years, reactions have been developed to proceed efficiently under physiological conditions. We present herein an uncaging reaction that results from ring-closing metathesis (RCM). A caged molecule, tethered to a diolefinic substrate, is released via spontaneous 1,4-elimination following RCM. Using this strategy, which we term “close-to-release”, we show that drugs and fluorescent probes are uncaged with fast rates, including in the presence of mammalian cells or in the periplasm of Escherichia coli. We envision that this tool may find applications in chemical biology, bioengineering and medicine.

Tandem Organocatalytic Cycloaromatization/Intramolecular Friedel-Crafts Alkylation Sequence for the Synthesis of Indolizinones and Pyrrolo-azepinone Derivatives

The organocatalytic synthesis of indolizinones and pyrrolo-azepinones has been accomplished in a tandem fashion through a sequence that comprises initial cycloaromatization followed by intramolecular Friedel-Crafts alkylation. The process takes place under Brønsted acid catalysis, giving rise to final products in moderate to good yields. Attempts to carry out the tandem protocol in an enantioselective fashion were performed with chiral (R)-BINOL-derived N-triflyl phosphoramides. After initial optimization, the tandem process took place with moderate levels of enantioselectivity.

Oxidative, Iodoarene-Catalyzed Intramolecular Alkene Arylation for the Synthesis of Polycyclic Aromatic Hydrocarbons

A catalytic, metal-free and chemoselective oxidative intramolecular coupling of arene and alkene C-H bonds is reported. The active hypervalent iodine (HVI) reagent, generated catalytically in situ from iodotoluene and meta-chloroperoxybenzoic acid (m-CPBA), reacts with o-vinylbiphenyls to generate polyaromatic hydrocarbons in up to 95 % yield. Experimental evidence suggests the reactions proceed though vinyliodonium and, possibly, vinylenephenonium intermediates.

Rational design and synthesis of a novel laterally-tetrafluorinated tricyclic mesogen with large negative dielectric anisotropy

Guided by theoretical calculations, we predicted that 5,5,6,6-tetrafluoro-1-methoxycyclohex-1-ene, a new structural motif, exhibits a large negative dielectric anisotropy, and then developed a synthetic strategy for the preparation of the novel tetrafluorinated tricyclic mesogen in twelve reaction steps starting from a commercially available fluorinated starting material. During the evaluation of its liquid-crystalline characteristics, the tetrafluorinated mesogen was found to display low birefringence and a large negative dielectric anisotropy, confirming that its molecular structure is a reasonable design for the further development of novel mesogenic molecules with large negative dielectric anisotropies.

Design, synthesis and evaluation of new fluorinated liquid crystals bearing a CF2CF2 fragment with negative dielectric anisotropy

We designed unique negative dielectric molecules, which were tricyclic compounds with a 5,5,6,6-tetrafluorocyclohexa-1,3-diene moiety in the mesogen, and then for the first time developed an effective synthetic protocol from commercially available 4-bromo-3,3,4,4-tetrafluorobut-1-ene. We also evaluated their physical properties for new mesogenic compounds, such as birefringence, dielectric anisotropy and shear viscosity, which are important characteristics for flat LC display device applications. The molecules clearly showed not only a large negative dielectric anisotropy and birefringence but also high viscosity.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Iterative C-H Functionalization Leading to Multiple Amidations of Anilides

Polyaminobenzenes were synthesized by the ruthenium-catalyzed iterative C-H amidation of anilides using dioxazolones as an amino source. This strategy could be implemented by the sequential activation of C-H bonds of formerly generated compounds by cascade chelation assistance of newly installed amide groups. Computational studies provided a rationale.

Polycyclic Aromatic Hydrocarbons via Iron(III)-Catalyzed Carbonyl-Olefin Metathesis

Polycyclic aromatic hydrocarbons are important structural motifs in organic chemistry, pharmaceutical chemistry, and materials science. The development of a new synthetic strategy toward these compounds is described based on the design principle of iron(III)-catalyzed carbonyl-olefin metathesis reactions. This approach is characterized by its operational simplicity, high functional group compatibility, and regioselectivity while relying on FeCl3 as an environmentally benign, earth-abundant metal catalyst. Experimental evidence for oxetanes as reactive intermediates in the catalytic carbonyl-olefin ring-closing metathesis has been obtained.

Thermodynamic synthesis of solution processable ladder polymers

The synthesis of a carbazole-derived, well-defined ladder polymer was achieved under thermodynamic control by employing reversible ring-closing olefin metathesis. This unique approach featured mild conditions and excellent efficiency, affording the ladder polymer backbone with minimum levels of unreacted defects. Rigorous NMR analysis on a 13C isotope-enriched product revealed that the main-chain contained less than 1% of unreacted precursory vinyl groups. The rigid conformation of the ladder-type backbone was confirmed by photophysical analysis, while the extended rod-like structure was visualized under scanning tunneling microscope. Excellent solubility of this polymer in common organic solvents allowed for feasible processing of thin films using solution-casting techniques. Atomic force microscopy and grazing incident X-ray scattering revealed a uniform and amorphous morphology of these films, in sharp contrast to the polycrystalline thin films of its small molecular counterpart.

(Hetero)aromatics from dienynes, enediynes and enyne–allenes

Recent advances in the synthesis of (hetero)aromatics starting from polyenes, such as dienynes, enediynes and enyne–allenes, are discussed.

Enantioselective synthesis of planar-chiral ferrocene-fused 4-pyridones and their application in construction of pyridine-based organocatalyst library

A couple of planar-chiral ferrocene-fused 4-pyridone derivatives 2a and 2b were synthesized in enantiomerically pure form by scalable asymmetric transformations. Pyridones 2 are versatile precursors to various ferrocene-fused pyridine derivatives, which are useful nucleophilic asymmetric organocatalysts.

Z-Selective olefin metathesis on peptides: investigation of side-chain influence, preorganization, and guidelines in substrate selection

Olefin metathesis has emerged as a promising strategy for modulating the stability and activity of biologically relevant compounds; however, the ability to control olefin geometry in the product remains a challenge. Recent advances in the design of cyclometalated ruthenium catalysts has led to new strategies for achieving such control with high fidelity and Z selectivity, but the scope and limitations of these catalysts on substrates bearing multiple functionalities, including peptides, remained unexplored. Herein, we report an assessment of various factors that contribute to both productive and nonproductive Z-selective metathesis on peptides. The influence of sterics, side-chain identity, and preorganization through peptide secondary structure are explored by homodimerization, cross metathesis, and ring-closing metathesis. Our results indicate that the amino acid side chain and identity of the olefin profoundly influence the activity of cyclometalated ruthenium catalysts in Z-selective metathesis. The criteria set forth for achieving high conversion and Z selectivity are highlighted by cross metathesis and ring-closing metathesis on diverse peptide substrates. The principles outlined in this report are important not only for expanding the scope of Z-selective olefin metathesis to peptides but also for applying stereoselective olefin metathesis in general synthetic endeavors.