Facile Carbon−Carbon Bond Cleavage in an (η<sup>3</sup>-Cyclooctenyl)cobalt Complex Exhibiting Agostic C···H···Co Bonding

Cracknell, Robert B.; Nicholls, Julian C.; Spencer, John L.

doi:10.1021/om950727+

Cited by 16 publications

(10 citation statements)

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“…Since C(2) and C(5) are not involved in the interaction with the agostic hydrogen, no additional splitting of a doublet resonance at δ 69.3 ppm is observed in the 13 C NMR spectrum of 5 . These observations are in accord with the idea that fluxional species 5 undergoes rapid metal-assisted 1,4-hydride shifts in solution . One may reasonably anticipate that an intramolecular fluxional process such as that observed for 5 in solution could likely proceed via an 18-electron diene-hydride intermediate, earlier depicted as B in Chart .…”

Section: Resultssupporting

confidence: 83%

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

et al. 2004

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Agostic (C-H‚‚‚M) complexes [closo-3,3-(σ,η 2 -C 8 H 13 )-1,2-µ-(ortho-xylylene)-3,1,2-IrC 2 B 9 H 9 ] (5) and [closo-3,3-(σ,η 2 -C 8 H 13 )-1,2-µ-(ortho-xylylene)-3,1,2-RhC 2 B 9 H 9 ] (9), stable in the solid state, have been prepared via the reaction of [M(η 4 -COD)Cl] 2 (M ) Ir, Rh) with the K + salt of the [nido-7,8-µ-(ortho-xylylene)-7,8-C 2 B 9 H 10 ]anion and characterized by a combination of analytical (in the case of 5) and multinuclear NMR data, including a single-crystal X-ray diffraction study of 5. The crystallographic study confirmed the agostic structure of 5 and revealed that the orientation of the σ,η 2 -cyclooctenyl moiety relative to the carborane ligand is substantially influenced by the specific intramolecular C-H‚‚‚π interaction between the agostic hydrogen and the π-system of the cage aromatic substituent. In solution, 5 exhibited both "side-to-side" agostic hydrogen migration and reversible interconversion with [closo-3-(η 3 -C 8 H 13 )-1,2-µ-(ortho-xylylene)-3,1,2-IrC 2 B 9 H 9 ] (8). The agostic rhodium complex (9), in contrast, converts irreversibly both in the solid state and in solution to its 11), which thus could be obtained as a pure solid. In solution, complex 11 is fluxional and shows an agostic C-H‚‚‚Rh interaction. The fluxional process involves the exchange between the endo hydrogen atoms, on one hand, and the exo and allyl hydrogens of the C 8 -ring, on the other hand, confirmed by 2D [ 1 H-1 H]-EXSY spectroscopy. Solution structures of the agostic complexes obtained are discussed in detail on the basis of normal and low-temperature 1 H and 13 C/ 13 C{ 1 H} NMR spectroscopic data.

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Section: Resultssupporting

confidence: 83%

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

et al. 2004

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“…Calculations on CoCp(η 3 -C 8 H 13 ), 2 , are consistent with the known crystal structure of 9 . There is no sign of an agostic interaction with a C−H bond adjacent to the Co-allyl unit (Figure ), even though the cationic Cp* analogue [CoCp*(η 3 -C 8 H 13 )] + (i.e., 8 + ) has been established to possess such an interaction . Addition of an electron to 17-electron 2 and optimization of the resulting 18-electron structure reveal that minor structural changes occur on reduction to [CoCp(η 3 -C 8 H 13 )] − ( 2 − ), the most significant of which is a small change in the position of the η 3 -cyclooctenyl ligand, which results in increased accessibility to a region of negative charge at the metal center (Figure ).…”

Section: Resultsmentioning

confidence: 79%

“…It is noteworthy that ESR spectra of 2 do not indicate any coupling to a H-atom that might indicate agostic interactions with C−H bonds adjacent to the allylic moiety to yield a 19-electron species. It has been demonstrated for the [CoCp*(η 3 -C 8 H 13 )] + cation (generated by protonation of 5 ) that an agostic interaction is present . In the [Rh(C 5 Ph 5 )(1,3-COD)], successive oxidations were stabilized by the formation of agostic interactions .…”

Section: Resultsmentioning

confidence: 82%

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Consequences of the Protonation of the 19-Electron Anion [Co(η⁵-C₅H₅)(1,5-C₈H₁₂)]⁻

Shaw¹,

Eilers²,

Geiger³

2009

Organometallics

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The electrochemical and chemical reduction of CoCp(1,5-COD) (1, Cp = η 5 -C 5 H 5 ; COD = cyclooctadiene) has been reinvestigated in THF. Whereas the initially formed monoanion 1 -(E 1/2 = -3.01 V vs Fc, Fc = FeCp 2 0/þ ) has been well-characterized electrochemically and by ESR spectroscopy, the long-term electrolysis product had been previously assigned as the isomerized anion [CoCp(1,3-COD)] -. New magnetic resonance data show that the ultimate product is the 17-electron cyclooctenyl complex CoCp(η 3 -C 8 H 13 ) (2, E 1/2 = -2.17 V), formed by the reaction of 1with adventitious proton donors. The η 1 :η 3 -bonded isomer of 1, CoCp(η 1 :η 3 -C 8 H 12 ) (3), reduces (E 1/2 = -2.97 V) to yield the same product 2 on the cyclic voltammetry (CV) time scale. Deliberate addition of the weak proton donor 2,4,6-trimethylphenol to solutions of 1 facilitates protonation and results in scan-rate-and concentration-dependent formation of 2 on the CV time scale as 1 is reduced in an ECE mechanism. Under these conditions, the reduced 2anion undergoes further reaction with proton donors, which results in the formation of another electroactive product, 4, which is assigned the structure [CoCp(η 3 -C 8 H 13 )H] on the basis of NMR evidence and DFT calculations.

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“…Two decades ago, Spencer et al, showed that upon protonation with HBF 4 Cp*Co(η 4 -cyclopentadiene) and Cp*Co(η 5 -cyclooctadiene) complexes undergo a suspiciously similar C–C bond cleavage reaction, yielding ring-opened cationic η 5 -pentadienyl complexes (eq ) . The possibility that such a bond activation could be embedded in the mechanism of the anomalous [3 + 2 + 2] cycloaddition reaction prompted us to investigate directly the reactivity of alkynes with acyclic pentadienyl complexes of cobalt.…”

Section: Introductionmentioning

confidence: 99%

Cobalt-Mediated η⁵-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η⁴-Cycloheptadiene Demetalation

et al. 2015

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The preparation of seven-membered carbocycles via traditional organic synthesis is difficult, yet essential, due to the prevalence of these moieties in bioactive compounds. As we report, the Co-mediated pentadienyl/alkyne [5 + 2] cycloaddition reaction generates kinetically stable η 2 ,η 3 -cycloheptadienyl complexes in high yield at room temperature, which isomerize to the thermodynamically preferred η 5 -cycloheptadienyl complexes upon heating at 60−70°C. Here we describe an extended investigation of this reaction manifold, exploring substituent effects and extending the reaction to tandem cycloaddition/nucleophilic cyclizations, generating fused bicyclic compounds. We also describe a new high-yielding photolytic method for the decomplexation of organic cycloheptadienes from Co(I) complexes. Both C 5 Me 5 (Cp*) and C 5 H 5 (Cp) halfsandwich complexes are active in [5 + 2] cycloaddition with alkynes, with Cp* generally providing higher yields of cycloheptadienyl complexes. Cp cycloheptadienyl complexes, however, are resistant to thermal η 2 ,η 3 → η 5 isomerization. The reaction remains limited to open pentadienyl complexes incorporating substituents in the terminal (1 and 5) positions, except for the unsubstituted CpCo(η 5 -cycloheptadienyl) + complex, which is modestly reactive. Incorporation of tethered latent nucleophiles allows cyclization onto the intermediate cycloheptadienyl cations, producing bicyclo[5.3.0]decadiene and bicyclo[5.4.0]-undecadiene systems with complete diastereocontrol. A selection of intermediate complexes have been crystallographically characterized. Addition of tethered malonate nucleophiles occurs reversibly with equilibration to a thermodynamic elimination product, while enolate nucleophiles cyclize reliably under kinetic control. The resulting bicyclic products are decomplexed in high (>90%) yield by UV photolysis in the presence of allyl bromide to provide the organic bicyclic diene with complete retention of ring fusion geometry and without double-bond isomerization. ■ INTRODUCTIONSince the first report of the Diels−Alder reaction, cycloaddition methodology has become a key component of the synthetic organic chemist's toolbox. This utility is largely due to the formation of multiple carbon−carbon bonds in a single step, often with excellent control of regiochemistry and stereoselectivity. Thermal, photochemical, and metal-mediated cycloaddition reactions for the synthesis of small-and medium-sized carbocycles have been developed, with four-, five-, and sixmembered rings receiving most of the attention (i.e., [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions). In contrast, the preparation of seven-membered rings by cycloaddition remains less developed, 1 despite the almost continuous discovery of bioactive natural products with seven-membered-ring structures in the core. 2 One important pathway to access the sevenmembered ring is the formal [5 + 2] cycloaddition reaction, which has a rich and varied history. 3 Among unsolved problems in [5 + 2] cycloaddition is the cyclization...

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Facile Carbon−Carbon Bond Cleavage in an (η³-Cyclooctenyl)cobalt Complex Exhibiting Agostic C···H···Co Bonding

Abstract: The reaction of [(η5-C5Me5)Co(η4-1,5-cyclooctadiene)] with HBF4·Me2O affords, at room temperature, an agostic η3-cyclooctenyl complex which undergoes a facile ring opening to afford anti-[(η5-C5Me5)Co(η5-1-propylpentadienyl)]BF4.

Cited by 16 publications

References 57 publications

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

Consequences of the Protonation of the 19-Electron Anion [Co(η⁵-C₅H₅)(1,5-C₈H₁₂)]⁻

Cobalt-Mediated η⁵-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η⁴-Cycloheptadiene Demetalation

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Facile Carbon−Carbon Bond Cleavage in an (η3-Cyclooctenyl)cobalt Complex Exhibiting Agostic C···H···Co Bonding

Abstract: The reaction of [(η5-C5Me5)Co(η4-1,5-cyclooctadiene)] with HBF4·Me2O affords, at room temperature, an agostic η3-cyclooctenyl complex which undergoes a facile ring opening to afford anti-[(η5-C5Me5)Co(η5-1-propylpentadienyl)]BF4.

Cited by 16 publications

References 57 publications

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η2-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η2-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-IrC2B9H9

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η2-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η2-C8H13)-1,2-μ-(ortho-xylylene)-3,1,2-IrC2B9H9

Consequences of the Protonation of the 19-Electron Anion [Co(η5-C5H5)(1,5-C8H12)]−

Cobalt-Mediated η5-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η4-Cycloheptadiene Demetalation

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Facile Carbon−Carbon Bond Cleavage in an (η³-Cyclooctenyl)cobalt Complex Exhibiting Agostic C···H···Co Bonding

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

Stable Agostic (C−H···M)closo-Irida- andcloso-Rhodacarboranes with σ,η²-Cyclooctenyl Ligands. Crystal and Molecular Structure ofcloso-3,3-(σ,η²-C₈H₁₃)-1,2-μ-(ortho-xylylene)-3,1,2-IrC₂B₉H₉

Consequences of the Protonation of the 19-Electron Anion [Co(η⁵-C₅H₅)(1,5-C₈H₁₂)]⁻

Cobalt-Mediated η⁵-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η⁴-Cycloheptadiene Demetalation