Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

Silver-catalyzed ring-opening difluoromethylthiolation/trifluoromethylthiolation of cycloalkanols with PhSO2SCF2H or PhSO2SCF3

A silver-catalyzed ring-opening difluoromethylthiolation/trifluoromethylthiolation of cycloalkanols including cyclopropanols, cyclobutanols, cyclopentanols, cyclohexanols and cycloheptanols was described.

Suzuki–Miyaura coupling of unstrained ketones via chelation-assisted C–C bond cleavage

Herein, we report that unstrained ketones can be efficiently employed as electrophiles in Suzuki–Miyaura reactions via catalytic activation of unstrained C–C bonds assist by an N-containing directing group.

Pd-Catalyzed diastereoselective [3 + 2] cycloaddition of vinylcyclopropanes with sulfamate-derived cyclic imines

The palladium-catalyzed diastereoselective [3 + 2] cycloaddition reaction of vinylcyclopropanes with sulfamate-derived cyclic imines worked efficiently under mild conditions, providing sulfamate-fused pyrrolidines in good to excellent yields.

Gold-catalyzed ring enlargement and cycloisomerization of alkynylamide tethered alkylidenecyclopropanes

A gold(i)-catalyzed ring enlargement and cycloisomerization of alkynylamide tethered alkylidenecyclopropanes has been developed, providing facile access to functionalized polycyclic and spiropolycyclic products based on the use of gold(i) catalysts.

Lewis acid catalyzed asymmetric [4+2] cycloaddition of cyclobutenones to synthesize α,β-unsaturated δ-lactones

Here we report an efficient asymmetric cycloaddition of CO double bonds with cyclobutenones catalyzed by a chiral N,N′-dioxide/Yb(iii) complex.

Ruthenium-catalyzed insertion of adjacent diol carbon atoms into C-C bonds: Entry to type II polyketides

Current catalytic processes involving carbon-carbon bond activation rely on π-unsaturated coupling partners. Exploiting the concept of transfer hydrogenative coupling, we report a ruthenium(0)-catalyzed cycloaddition of benzocyclobutenones that functionalizes two adjacent saturated diol carbon-hydrogen bonds. These regio- and diastereoselective processes enable convergent construction of type II polyketide substructures.

A Visible-Light-Driven Iminyl Radical-Mediated C-C Single Bond Cleavage/Radical Addition Cascade of Oxime Esters

A room-temperature, visible-light-driven N-centered iminyl radical-mediated and redox-neutral C-C single bond cleavage/radical addition cascade reaction of oxime esters and unsaturated systems has been accomplished. The strategy tolerates a wide range of O-acyl oximes and unsaturated systems, such as alkenes, silyl enol ethers, alkynes, and isonitrile, enabling highly selective formation of various chemical bonds. This method thus provides an efficient approach to various diversely substituted cyano-containing alkenes, ketones, carbocycles, and heterocycles.

Thermal Hetero-Diels-Alder Reaction of Benzocyclobutenones with Isatins To Form 2-Oxindole Spirolactones

Benzocyclobutenones 1a-1g undergo cycloreversion at 150 °C in m-xylene solvent to form transient α-oxo-ortho-quinodimethanes or "ortho-quinoid ketene methides", which engage in intermolecular [4+2] cycloadditions with isatins 2a-2f to form 2-oxindole spirolactones 3a-3l. This process tolerates an array of different functional groups and substitution patterns, and is applicable to unprotected isatins 2b-2f bearing free NH-functionalities. The superior performance of isatins compared to other carbonyl based dienophiles was demonstrated and rationalized with the aid of quantum chemical calculations.

Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General

Explosive growth in the use of open shell reactivity, including neutral radicals and radical ions, in the field of synthetic organic chemistry has been observed in the past decade, particularly since the advent of ruthenium complexes in 2008. These complexes generally induce single-electron transfer (SET) processes via visible-light absorption. Additionally, recent significant advancements in organic electrochemistry involving SET processes to provide open shell reactivity offer a complementary method to traditional polarity-driven reactions described by two-electron transfer processes. In this Review, we highlight the importance of intramolecular SET processes in the field of synthetic organic chemistry, which seem to be more elusive than the intermolecular versions, since they are net redox-neutral and thus cannot simply be regarded as oxidations or reductions. Such intramolecular SET processes can rationally be understood in combination with concomitant bond formations and/or cleavages, and are regulated by a structural motif that we call a "redox tag." In order to describe modern radical-driven reactions involving SET processes, we focus on a classical formalism in which electrons are treated as particles rather than waves, which offers a practical yet powerful approach to explain and/or predict synthetic outcomes.

Ni-Al Bimetallic Catalyzed Enantioselective Cycloaddition of Cyclopropyl Carboxamide with Alkyne

A Ni-Al bimetallic catalyzed enantioselective cycloaddition reaction of cyclopropyl carboxamides with alkynes has been developed. A series of cyclopentenyl carboxamides were obtained in up to 99% yield and 94% ee. The bifunctional-ligand-enabled bimetallic catalysis proved to be an efficient strategy for the C-C bond cleavage of unreactive cyclopropanes.

Gold(I) Catalyzed Dearomative Claisen Rearrangement of Allyl, Allenyl Methyl, and Propargyl Aryl Ethers

Claisen rearrangements of allyl aryl ethers to generate enones bearing all carbon quaternary centers are accelerated by Ph₃PAuNTf₂ under mild conditions in good yields. Multiple C-C bond containing variants of the allyl fragment are viable, including alkylidenecyclopropanes, allenes, and alkynes, which generate all-carbon stereogenic centers substituted with vinyl cyclopropanes, 1,3-butadienyl, and allenyl substituents, respectively, for subsequent synthetic manipulation. With allyl phenyl ethers, the product of the [3,3] rearrangements can be trapped by a tandem [4 + 2] cycloaddition to generate complex molecular scaffolds from readily available, achiral starting materials.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

Factors Controlling the Reactivity and Chemoselectivity of Resonance Destabilized Amides in Ni-Catalyzed Decarbonylative and Nondecarbonylative Suzuki-Miyaura Coupling

N-Glutarimide amides have recently emerged as an exceptional group of compounds with unusually high reactivity in amide C-N bond activation. To understand the key factors that control the remarkable reactivity of these resonance destabilized amides, we explored the Ni-catalyzed decarbonylative and nondecarbonylative Suzuki-Miyaura coupling with N-glutarimide amides through density functional theory calculations. Two leading effects are responsible for the C-N cleavage activity of N-glutarimide amides, the coordinating N-substituents and the geometric twisting. The carbonyl substituent of the N-glutarimide amides provides crucial nickel-oxygen interaction, which essentially acts as a directing group to facilitate the formation of the reactive intermediate for the amide C-N bond cleavage. The geometric twisting weakens the resonance stability by removing the acyl-nitrogen conjugation, which lowers the energy penalty for the C-N bond stretch during oxidative addition. For the chemoselectivity of decarbonylation versus carbonyl retention, we found that the C-C reductive elimination for ketone formation is kinetically faster than that for biaryl formation, while ketone is thermodynamically less stable with respect to the decarbonylated biaryls. The computations also suggest that the nickel catalyst is able to promote the decarbonylation of biaryl ketones via an unexpected C-C bond activation.

Iridium-catalysed hydrosilylation of cyclopropanes via regioselective carbon-carbon bond cleavage

While cyclopropanes have been explored as synthetically valuable building blocks, their transformation without conjugated substituents or directly substituted heteroatoms remains challenging. The current study describes the iridium-catalysed ring-opening hydrosilylation of cyclopropanes. A nitrogen-based directing group was found to control the reactivity of iridium active species as well as the regiochemistry of carbon-carbon bond cleavage and hydrosilylation.

Stereoselective cis-Vinylcyclopropanation via a Gold(I)-Catalyzed Retro-Buchner Reaction under Mild Conditions

A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia–Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones. On the basis of mechanistic investigations, a stereochemical model for the cis selectivity was proposed. An unprecedented gold-catalyzed isomerization of cis- to trans-cyclopropanes has also been discovered and studied by DFT calculations.

Rh-catalyzed regiodivergent hydrosilylation of acyl aminocyclopropanes controlled by monophosphine ligands

A Rh-catalyzed regiodivergent hydrosilylation of acyl aminocyclopropanes has been developed. Acyl aminocyclopropanes were reacted with hydrosilanes in the presence of Rh catalysts to afford ring-opened hydrosilylated adducts through carbon-carbon (C-C) bond cleavage of the cyclopropane ring. The regioselectivity of the addition of silanes (linear or branched) can be switched by changing the monophosphine ligand. This C-C bond cleavage/hydrosilylation methodology is applicable to the synthesis of silanediol precursors.

Ruthenium(II)-Catalyzed C-C Arylations and Alkylations: Decarbamoylative C-C Functionalizations

Ruthenium(II)biscarboxylate catalysis enabled selective C-C functionalizations by means of decarbamoylative C-C arylations. The versatility of the ruthenium(II) catalysis was reflected by widely applicable C-C arylations and C-C alkylations of aryl amides, as well as acids with modifiable pyrazoles, through facile organometallic C-C activation.