Rhenium-Catalyzed Cyclization via 1,2-Iodine and 1,5-Hydrogen Migration for the Synthesis of 2-Iodo-1H-indenes

A rhenium complex catalyzed the formation of 2-iodo-1H-indene derivatives through iodine and hydrogen migration of 3-iodopropargyl ethers. The reaction proceeded via generation of 1-iodoalkenylrhenium carbene species by sequential 1,2-iodine and 1,5-hydrogen shifts with readily available precursors under neutral conditions. The reaction mechanism and the reactivity of the generated alkenylcarbene species were also investigated

Mechanistic Insights into Rhenium-Catalyzed Regioselective C-Alkenylation of Phenols with Internal Alkynes

A (μ-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel-Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon-carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes

Amine-Promoted anti-Markovnikov Addition of 1,3-Dicarbonyl Compounds with Terminal Alkynes under Rhenium Catalysis

Amines have been identified to greatly accelerate the intermolecular anti-Markovnikov addition of carbon nucleophiles to unactivated terminal alkynes. Using a combination of [ReBr(CO)3(thf)]2 and iPr2NEt, construction of cyclic all-carbon quaternary centers was achieved with various 1,3-ketoesters, diketones, and diesters with lower catalyst loading under milder conditions. The type of addition could be easily controlled by choice of additive, highlighting the unique features of rhenium catalysi

Use of Cyclopropane as C1 Synthetic Unit by Directed Retro- Cyclopropanation with Ethylene Release

Cyclopropanation of alkenes is a well-established textbook reaction for the synthesis of cyclopropanes, where a “high-energy” carbene species is exploited to drive the reaction forward. However, little attention has been focused toward molecular transformations involving the reverse reaction, retro-cyclopropanation (RC). This is because of difficulties associated with both cleaving the two geminal C–C single bonds and exploiting the generated carbenes for further transformations in an efficient and selective manner. Here, we report that a molybdenum-based catalytic system overcomes the above challenges and effects the RC of cyclopropanes bearing a pyridyl group with the release of ethylene (alkene) and the subsequent intramolecular cyclization leading to pyrido[2,1-a]isoindoles. The reaction allows for the uncommon use of cyclopropanes as C1 synthetic units in contrast to most conventional reactions in which cyclopropanes are used as C3 synthetic units. We anticipate that this new strategy will pave the way for C1 cyclopropane chemistry

Rhenium-Catalyzed Regioselective ortho-Alkenylation and [3 + 2 + 1] Cycloaddition of Phenols with Internal Alkynes

An operationally simple and direct rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes was developed. The protocol provided ortho-alkenylphenols exclusively, and formation of para- or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products were not observed. The [3 + 2 + 1] cycloaddition of phenols and two alkynes via ortho-alkenylation was also demonstrated, in which the alkynes functioned as both two- and one-carbon units. These reactions proceeded with readily available starting materials under neutral conditions without additional ligands

Chromium carbides and cyclopropenylidenes

Carbon tetrabromide can be reduced with CrBr2 in THF to form a dinuclear carbido complex, [CrBr2(thf)(2))][CrBr2(thf)(3)](mu-C), along with formation of [CrBr3(thf)(3)]. An X-ray diffraction (XRD) study of the pyridine adduct displayed a dinuclear structure bridged by a carbido ligand between 5- and 6-coordinate chromium centers. The carbido complex reacted with two equivalents of aldehydes to form alpha,beta-unsaturated ketones. Treatment of the carbido complex with alkenes resulted in a formal double-cyclopropanation of alkenes by the carbido moiety to afford spiropentanes. Isotope labeling studies using a C-13-enriched carbido complex, [CrBr2(thf)(2))][CrBr2(thf)(3)](mu-C-13), identified that the quaternary carbon in the spiropentane framework was delivered by carbide transfer from the carbido complex. Terminal and internal alkynes also reacted with the carbido complex to form cyclopropenylidene complexes. A solid-state structure of the diethylcyclopropenylidene complex, prepared from 3-hexyne, showed a mononuclear cyclopropenylidene chromium(iii) structure

Deoxygenative Insertion of Carbonyl Carbon into a C(sp3)–H Bond: Synthesis of Indolines and Indoles

A simple deoxygenation reagent prepared in situ from commercially available Mo(CO)6 and ortho-quinone has been developed for the synthesis of indoline and indole derivatives. The Mo/quinone complex efficiently deoxygenates carbonyl compounds bearing a neighboring dialkylamino group and effects intramolecular cyclizations with the insertion of a deoxygenated carbonyl carbon into a C(sp3)–H bond, in which a carbonyl group acts as a carbene equivalent. The reaction also proceeds with a catalytic amount of Mo/quinone in the presence of disilane as an oxygen atom acceptor

Chromium-Mediated Stannylcyclopropanation of Alkenes with (Diiodomethyl)stannanes

A stannyl-group-substituted gem-dichromiomethane species, generated in situ from CrCl2, TMEDA, and tributyl(diiodomethyl)stannane, worked as an efficient stannylcarbene equivalent to promote cyclopropanation of alkenes. The reaction provided synthetically useful stannylcyclopropanes directly from commercially available unactivated alkenes without using potentially flammable alkylzinc and diazo compounds. Structural characterization of stannyl- and germyl-group-substituted gem-dichromiomethane complexes and the effect of group 14 elements containing substituents for cyclopropanation are also described

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(iii) methylene complex

Transmetallation of a zinc methylene complex [ZnI(tmeda)](2)(mu-CH2) with a titanium(iii) chloride [TiCl3(tmeda)(thf)] produced a titanium methylene complex. The X-ray diffraction study displayed a dinuclear methylene structure [TiCl(tmeda)](2)(mu-CH2)(mu-Cl)(2). Treatment of an ester with the titanium methylene complex resulted in methylenation of the ester carbonyl to form a vinyl ether. The titanium methylene complex also reacted with a terminal olefin, resulting in olefin-metathesis and olefin-homologation. Cyclopropanation by methylene transfer from the titanium methylene proceeded by use of a 1,3-diene. The mechanistic study of the cyclopropanation reaction by the density functional theory calculations was also reported

Halogen-sodium exchange enables efficient access to organosodium compounds

With sodium being the most abundant alkali metal on Earth, organosodium compounds are an attractive choice for sustainable chemical synthesis. However, organosodium compounds are rarely used-and are overshadowed by organolithium compounds-because of a lack of convenient and efficient preparation methods. Here we report a halogen-sodium exchange method to prepare a large variety of (hetero)aryl- and alkenylsodium compounds including tri- and tetrasodioarenes, many of them previously inaccessible by other methods. The key discovery is the use of a primary and bulky alkylsodium lacking beta-hydrogens, which retards undesired reactions, such as Wurtz-Fittig coupling and beta-hydrogen elimination, and enables efficient halogen-sodium exchange. The alkylsodium is readily prepared in situ from neopentyl chloride and an easy-to-handle sodium dispersion. We believe that the efficiency, generality, and convenience of the present method will contribute to the widespread use of organosodium in organic synthesis, ultimately contributing to the development of sustainable organic synthesis by rivalling the currently dominant organolithium reagents. Halogen-sodium exchange reactions with neopentyl sodium provides access to a range of aryl and alkenyl organosodium compounds in situ, as an alternative to organolithium reagents