Allyl Monitorization of the Regioselective Pd-Catalyzed Annulation of Alkylnyl Aryl Ethers Leading to Bismethylenechromanes.

The mechanism for the synthesis of 2,3-bismethylenechromanes obtained by the reaction between silylethynyloxyarenes and allylic pivalates and catalyzed by a palladium complex has been investigated using computational methods rooted in density functional theory. The reaction is promoted by a C-H bond activation and the consequent bond cleavage of both substrates, followed by a novel annulation. The whole mechanism of this reaction is described together with the drawbacks that could block it. The main role played by the allyl rotation, inducing selectivity, together with the lability of the phosphine ligand and base (Cs2CO3) effects are unraveled. Finally, the nature of the substrates was managed, confirming that ortho-allylated silylethynyloxybenzenes lead to the same type of annulated products.

Facile Construction of Furanoacenes by a Three-Step Sequence Going through Disilyl-exo-cyclic Dienes.

Facile synthesis of various benzonaphthofurans was achieved by intramolecular hydroarylation of 1,4-disilyl-2-aryloxy-1,3-enynes followed by cycloaddition with arynes or alkenes and finally desilylaromatization. The three-step transformation can be operated sequentially in one-pot, providing with a range of furanoacenes easily and highly effectively.

Designing 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.

P-C reductive elimination in Ru(ii) complexes to convert triarylphosphine ligands into five- or six-membered phosphacycles.

Rare examples of P-C reductive elimination in ruthenium complexes to generate phosphonium salts are presented. Triarylphosphines are converted into benzophospholium or phosphaphenalenium ligands via cyclometalation and 1,2-insertion of an alkyne followed by P-C reductive elimination. The intermediate in each step was successfully characterized using NMR and X-ray diffraction studies.

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.

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.

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.

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.

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.

Palladium-catalyzed hydrobenzylation of ortho-tolyl alkynyl ethers by benzylic C-H activation: remarkable alkynoxy-directing effect.

It's 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.

Palladium-catalyzed annulation of 1,2-diborylalkenes and -arenes with 1-bromo-2-[(Z)-2-bromoethenyl]arenes: a modular approach to multisubstituted naphthalenes and fused phenanthrenes.

(Z)-1,2-Diaryl-1,2-bis(pinacolatoboryl)ethenes underwent double-cross-coupling reactions with 1-bromo-2-[(Z)-2-bromoethenyl]arenes in the presence of [Pd(PPh(3))(4)] as a catalyst and 3 M aqueous Cs(2)CO(3) as a base in THF at 80 °C. The double-coupling reaction gave multisubstituted naphthalenes in good to high yields. Annulation of 1,2-bis(pinacolatoboryl)arenes with bromo(bromoethenyl)arenes in the presence of a catalyst system that consisted of [Pd(2)(dba)(3)] (dba=dibenzylideneacetone) and 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (SPhos) under the same conditions produced fused phenanthrenes in good to high yields. The first annulation coupling occurred regiospecifically at the bromoethenyl moiety. This procedure is applicable to the facile synthesis of polysubstituted anthracenes, benzothiophenes, and dibenzoanthracenes through a double annulation pathway by using the corresponding dibromobis[(Z)-2-bromoethenyl]benzenes as diboryl coupling partners.

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.

Synthesis and photophysical properties of dimethoxybis(3,3,3-trifluoropropen-1-yl)benzenes: compact chromophores exhibiting violet fluorescence in the solid state.

Dimethoxybis(3,3,3-trifluoropropen-1-yl)benzenes were prepared through palladium-catalyzed double cross-coupling reactions of diiododimethoxybenzenes with CF(3)C≡CZnCl, followed by reduction of CF(3)C≡C groups with LiAlH(4) or H(2) in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2-(MeO)(2)-4,5-(CF(3)CHC=CH)(2) benzenes 1 and 1,3-(MeO)(2)-4,6-(CF(3)CH=CH)(2) benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4-(MeO)(2)-2,5-[(E)-CF(3)CH=CH](2) benzene ((E)-3) ended at 406 nm. These findings indicate that the effective conjugation length of (E)-3 was significantly larger than those of 1 and 2. Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)-3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1-3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1-3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)-3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO-LUMO gaps when compared with the corresponding fluorine-free analogues. In the case of (E)-3, it is suggested that the HOMO-LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp(2))-CF(3) moieties.

Silicon-based cross-coupling reaction: an environmentally benign version.

Much attention has been paid to the cross-coupling reaction of organosilicon compounds due to their stability, non-toxicity, and natural abundance of silicon. In addition, the silicon-based cross-coupling has many advantages over other cross-coupling protocols. Successful examples of the silicon-based cross-coupling reaction are reviewed, focusing especially on the advances made in the last decade. Having had a number of highly effective palladium catalysts developed mainly for other cross-coupling reactions, the development of the silicon-based protocol owes heavily to the design of organosilicon reagents which effectively undergo transmetalation, a key elemental step of the silicon-based cross-coupling reaction. This tutorial review thus classifies various organosilicon reagents depending on substituents on silicon and surveys their cross-coupling reactions with various electrophiles.