Ionic and Radical Reactions of π‐Bonded Cobalt Complexes

Melikyan, G. G.; Artashyan, Elen

doi:10.1002/9783527814855.ch6

Cited by 5 publications

(1 citation statement)

References 103 publications

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“…Organometallic complexes have found extensive use in organic chemistry due to the unique properties they impart on the coordinated organic ligands. A notable transformation that showcases this altered reactivity is the Nicholas reaction [ 1 , 2 , 3 , 4 , 5 , 6 ]. It involves the reaction of dicobalt hexacarbonyl-stabilized propargylic carbocations with nucleophiles.…”

Section: Introductionmentioning

confidence: 99%

Intramolecular Nicholas Reaction Enables the Stereoselective Synthesis of Strained Cyclooctynes

et al. 2021

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Cyclic products can be obtained through the intramolecular version of the Nicholas reaction, which requires having the nucleophile connected to the alkyne unit. Here, we report the synthesis of 1-oxa-3-cyclooctynes starting from commercially available (1R,3S)-camphoric acid. The strategy is based on the initial preparation of propargylic alcohols, complexation of the triple bond with Co2(CO)8, and treatment with BF3·Et2O to induce an intramolecular Nicholas reaction with the free hydroxyl group as nucleophile. Finally, oxidative deprotection of the alkyne afforded the cyclooctynes in good yields. Notably, large-sized R substituents at the chiral center connected to the O atom were oriented in such a way that steric interactions were minimized in the cyclization, allowing the formation of cyclooctynes exclusively with (R) configuration, in good agreement with theoretical predictions. Moreover, preliminary studies demonstrated that these cyclooctynes were reactive in the presence of azides yielding substituted triazoles.

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Section: Introductionmentioning

confidence: 99%

Intramolecular Nicholas Reaction Enables the Stereoselective Synthesis of Strained Cyclooctynes

et al. 2021

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Cobalt‐Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Glasel,

Hapke

2023

Patai's Chemistry of Functional Groups

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Cobalt‐catalyzed [2 + 2 + 2] cycloaddition reactions are an important tool for the assembly of carbo‐ and hetero‐cyclic ring systems from simple precursor molecules such as alkynes, alkenes, nitriles, and other heterocumulenes such as isocyanates. Cyclopentadienyl (Cp)‐cobalt complexes are still one of the most important classes of catalysts, but in situ generated cobalt catalyst systems are becoming increasingly important for the assembly of achiral as well as chiral target molecules from a large variety of substrates. Substrates comprise simple terminal as well as internal alkynes, diynes, and higher oligoynes and also allow target‐oriented syntheses of unusually large as well as highly substituted molecules as well as natural product synthesis with a cyclotrimerization as the key step of the synthetic sequence. The utilization of olefins as cyclization partners allows the synthesis of products with lesser degree of unsaturation such as cyclohexadienes, cyclohexenes, and dihydropyridines, just to name a view examples. Chiral catalyst systems allow the enantioselective synthesis of biaryls and products with central chirality, and chiral starting materials allow the synthesis of diastereomeric helicenes in an efficient manner. Novel catalysts and reaction engineering allow reaction conditions such as conventional thermal or microwave heating, photochemically assisted cyclizations, and the use of plug‐flow reactors.

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Cobalt-Mediated Radical Cyclizations: Stereoselective Synthesis of Cyclopentanes, Cyclohexanes, and Tetralins

et al. 2023

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A stereoselective method for assembling five- and six-membered, mono- and binuclear carbocyclic compounds by intramolecular cyclization of Co2(CO)6-complexed propargyl radicals was developed. The formation of requisite bis-propargyl cations A is enabled by a stabilizing metal cluster, while converging transient radicals are stereodirected by a bulky metal core and substituents at the acetylenic termini. With the flexible three- or four-carbon tethers, formation of 1,2-dialkynylcyclopentanes and 1,2-dialkynylcyclohexanes occurred with a high stereoselectivity (91–100% trans-), noticeably enhanced by a γ-TMS group that exclusively directs the cobalt-alkyne cores into diaxial positions (1,3-steric induction). The main cyclization parameters were fully preserved with an aromatic ring as a rigid tether, thus providing rapid access to trans-tetralins with terminal and internal acetylenic moieties. The observed stereoselectivity in cyclization reactionswith both flexible and rigid tetherswas interpreted in terms of the selective stereomutation of the alpha stereocenters occurring in intermediate cations or radicals. Organic ligands were separated from metal cores under oxidative conditions (Ce4+), providing access to vicinally substituted bis-alkynyl cyclopentanes, cyclohexanes, and tetralins of trans-configurations, prone to secondary transformations, and of immediate relevance to the angularly and linearly fused estrogen mimics.

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Ionic and Radical Reactions of π‐Bonded Cobalt Complexes

Cited by 5 publications

References 103 publications

Intramolecular Nicholas Reaction Enables the Stereoselective Synthesis of Strained Cyclooctynes

Intramolecular Nicholas Reaction Enables the Stereoselective Synthesis of Strained Cyclooctynes

Cobalt‐Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Cobalt-Mediated Radical Cyclizations: Stereoselective Synthesis of Cyclopentanes, Cyclohexanes, and Tetralins

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