Yasunori Minami

Palladium-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes via selective ortho C-H activation.

Alkynyl aryl ethers react with internal alkynes through selective ortho C-H activation by a palladium(0) catalyst to give substituted 2-methylidene-2H-chromenes. The alkynoxy group acts as a directing group to promote ortho C-H functionalization. Deuterium-labeling experiments indicated that the arylpalladium hydride complex is a key intermediate via oxidative addition. Various functional groups tolerate the present transformation to give the corresponding products.

Synthetic Transformations through Alkynoxy-Palladium Interactions and C-H Activation.

Organic synthesis based on straightforward transformations is essential for environmentally benign manufacturing for the invention of novel pharmaceuticals, agrochemicals, and organoelectronic materials in order to ultimately realize a sustainable society. Metal-catalyzed C-H bond-cleaving functionalization has become a promising method for achieving the above goal. For site-selective C-H bond cleavage, so-called directing groups, i.e., ligands attached to substrates, are employed. Commonly utilized directing groups are carbonyls, imines, carboxyls, amides, and pyridyls, which σ-donate electron pairs to metals. On the other hand, unsaturated substrates such as alkenes and alkynes, which participate largely as reactants in organic synthesis, are prepared readily by a wide variety of synthetic transformations and are also employed as reactants in organometallic chemistry. Moreover, such unsaturated groups form complexes with some metals by ligation of their p orbitals via donation and back-donation. However, the use of unsaturated bonds as directing groups has not been studied extensively. We have been involved in the development of methods for the cleavage of C-H bonds by means of transition-metal catalysts to achieve new carbon-carbon bond-forming reactions and incidentally came to focus on the alkynoxy group (-OC≡C-), which shows a ketene-like resonance structure. We expected the alkynoxy group to interact electrophilically with a low-valent transition-metal complex in order to cleave adjacent C-H bonds. In this Account, we summarize our recent achievements on C-H activation based on interactions of palladium with the alkynoxy group in alkynyl aryl ethers. The alkynoxy group plays two roles in the transformation: as a directing group for adjacent C-H bond activation and as an acceptor for the carbon and hydrogen fragments. A typical example is palladium-catalyzed ortho-C-H bond activation in alkynoxyarenes followed by sequential insertion/annulation with internal alkynes and the alkynoxy group to produce 2-methylidene-2H-1-benzopyrans. Mechanistic studies have shown that the presence of both oxygen and alkynyl moieties is essential for selective ortho-C-H bond activation and subsequent annulation. In addition to internal alkynes, norbornene, allenes, isocyanates, and ketenes produce the corresponding oxacycles. It is worthy of note that benzoxadinones formed by the reaction with isocyanates exhibit solid-state luminescence. In addition, 2-methylphenyl alkynyl ethers and 2-alkynoxybiaryls undergo intramolecular annulation at the benzylic γ-position and aryl δ-position via C-H bond activation to give benzofurans and dibenzopyrans, respectively. The disclosed methods allow us to construct useful π-conjugated systems in a straightforward manner.

Palladium‐Catalyzed Hydrobenzylation of ortho‐Tolyl Alkynyl Ethers by Benzylic CH Activation: Remarkable Alkynoxy‐Directing Effect

Its selective: The title reaction involves palladium(0)-catalyzed insertion of C≡C bonds into benzylic C(sp(3))-H bonds, thus providing efficient access to 2-methylene-2,3-dihydrobenzofurans, which transform into benzofurans upon treatment with a weak acid (e.g., AcOH) and electrophiles. The alkynoxy group serves as a directing group in promoting C-H bond functionalization.

Annulation of Alkynyl Aryl Ethers with Allyl Pivalates To Give 2,3-Bismethylenechromanes through Double C−H Bond Cleavage

The treatment of silylethynyloxyarenes with allylic pivalates in the presence of a palladium catalyst led to efficient C-H bond cleavage in both substrates and a novel annulation reaction to give 2,3-bismethylenechromanes. When ortho-allylated silylethynyloxybenzenes were used as the substrates, the same products were obtained. This result shows that site-selective intramolecular hydrovinylation is involved in the annulation reaction. The synthetic utility of the products was demonstrated by the construction of condensed polycycles.

Dehydrogenative Carbon–Carbon Bond Formation Using Alkynyloxy Moieties as Hydrogen-Accepting Directing Groups

In the presence of a catalyst system consisting of Pd(OAc)2 , PCy3 , and Zn(OAc)2 , the reaction of alkynyl aryl ethers with bicycloalkenes, α,ß-unsaturated esters, or heteroarenes results in the site-selective cleavage of two C-H bonds followed by the formation of C-C bonds. In all cases, the alkynyloxy group acts as a directing group for the activation of an ortho C-H bond and as a hydrogen acceptor, thus rendering the use of additives such as an oxidant or base unnecessary.

Aryl(triethyl)silanes for Biaryl and Teraryl Synthesis by Copper(II)‐Catalyzed Cross‐Coupling Reaction

Aryl(triethyl)silanes are found to undergo cross-coupling with iodoarenes in the presence of catalytic amounts of CuBr2 and Ph-Davephos, as well as cesium fluoride as a stoichiometric base. Because the silicon reagents are readily accessible through catalytic C-H silylation of aromatic substrates, the net transformation allows coupling of aromatic hydrocarbons with iodoarenes via triethylsilylation.

Palladium/Copper Dual Catalysis for the Cross‐Coupling of Aryl(trialkyl)silanes with Aryl Bromides

Whereas aryl(trialkyl)silanes are considered to be ideal organometallic reagents for cross-coupling reactions owing to their stability, low toxicity, solubility, and easy accessibility, they are generally inert under typical cross-coupling conditions. Disclosed herein is a palladium/copper catalytic system that enables the cross-coupling of trimethyl, triethyl, tert-butyldimethyl, and triisopropyl aryl silanes with aryl bromides. This process is applicable to the sequential C-H and C-Si bond arylation of thiophenes and the synthesis of poly(thiophene-fluorene)s.

Synthesis of Benzosiloles by Intramolecular anti-Hydroarylation via ortho-C–H Activation of Aryloxyethynyl Silanes

Straightforward synthesis of benzosiloles was achieved by the invention of Pd/acid-catalyzed intramolecular anti-hydroarylation of aryloxyethynyl(aryl)silanes via ortho-C-H bond activation. The aryloxy group bound to the ethynyl carbon is the key factor for this transformation.

Hydroarylation of 2-Aryloxybut-1-en-3-ynes via Pd/Acid-Catalyzed C−H Bond Activation: A Concise Synthesis of 2,3-Bismethylene-2,3-dihydrobenzofurans

An intramolecular exo-hydroarylation of 2-aryloxy-1,4-disilylbut-1-en-3-ynes via ortho-C-H bond activation under palladium(0) and acid catalysis was found to give 2,3-bis(silylmethylidene)-2,3-dihydrobenzofurans. The two silyl groups present probably promoted the reaction and played a key role in stabilizing the diene moiety in the product. The products readily led to functionalized condensed cycles by a Diels-Alder reaction.

Designing for Cross-coupling Reactions Using Aryl(trialkyl)silanes

Organo(trialkyl)silanes have several advantages, including high stability, low toxicity, good solubility, easy handling, and ready availability compared with heteroatom-substituted silanes. However, methods for the cross-coupling of organo(trialkyl)silanes are limited, most probably because of their exceeding robustness. Thus, a practical method for the cross-coupling of organo(trialkyl)silanes has been a long-standing challenging research target. This article discusses how aryl(trialkyl)silanes can be used in cross-coupling reactions. A pioneering example is CuII catalytic conditions with the use of electron-accepting aryl- or heteroaryl(triethyl)silanes and aryl iodides. The reaction forms biaryls or teraryls. This design concept can be extended to Pd/CuII -catalyzed cross-coupling polymerization reactions between such silanes and aryl bromides or chlorides and to CuI -catalyzed alkylation using alkyl halides.