Reversible cyclopropyl ring opening of 1-aroyl-2-phenylcyclopropane radical anions. Determination of the ring opening and closure rates of the intermediate ketyls

Tanner, Dennis D.; Chen, Jian Jeffrey; Luelo, Christine; Peters, P. M.

doi:10.1021/ja00028a045

Cited by 42 publications

(16 citation statements)

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“…The latter is not subject to hydrosilylation with this catalyst and would be cleaved during workup to yield 8. The reaction rate for the ring opening was estimated using the Arrhenius parameters given in the literature 79 to be at least 4 orders of magnitude faster than the rate-determining step of the hydrosilylation reaction. The product mixture was analyzed using HPLC and 2 H NMR (after quenching with K 2 CO 3 in MeOD), and both confirm that less than 5% of the ring-opened product 8 were formed.…”

Section: ■ Resultsmentioning

confidence: 99%

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Bleith

Gade

2016

J. Am. Chem. Soc.

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A detailed mechanistic study of the catalytic hydrosilylation of ketones with the highly active and enantioselective iron(II) boxmi complexes as catalysts (up to >99% ee) was carried out to elucidate the pathways for precatalyst activation and the mechanism for the iron-catalyzed hydrosilylation. Carboxylate precatalysts were found to be activated by reduction of the carboxylate ligand to the corresponding alkoxide followed by entering the catalytic cycle for the iron-catalyzed hydrosilylation. An Eyring-type analysis of the temperature dependence of the enantiomeric ratio established a linear relationship of ln(S/R) and T(-1), indicating a single selectivity-determining step over the whole temperature range from -40 to +65 °C (ΔΔG(‡)sel, 233 K = 9 ± 1 kJ/mol). The rate law as well as activation parameters for the rate-determining step were derived and complemented by a Hammett analysis, radical clock experiments, kinetic isotope effect (KIE) measurements (kH/kD = 3.0 ± 0.2), the isolation of the catalytically active alkoxide intermediate, and DFT-modeling of the whole reaction sequence. The proposed reaction mechanism is characterized by a rate-determining σ-bond metathesis of an alkoxide complex with the silane, subsequent coordination of the ketone to the iron hydride complex, and insertion of the ketone into the Fe-H bond to regenerate the alkoxide complex.

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Section: ■ Resultsmentioning

confidence: 99%

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Bleith

Gade

2016

J. Am. Chem. Soc.

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“…However, there are several reports that ring opening of the cyclopropylmethyl radical is reversible when the radical is stabilized. This reversibility of ring opening is observed in the single‐electron transfer reductions of cyclopropylphenylketone34 and cyclopropylbenzoquinone 35. The cis / trans isomerization of disubstituted cyclopropane has been used as a probe to identify radical intermediates by several research groups 36.…”

Section: Resultsmentioning

confidence: 99%

Nef Reaction with Molecular Oxygen in the Absence of Metal Additives, and Mechanistic Insights

Umemiya

Nishino

Sato

et al. 2014

Chemistry A European J

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A Nef reaction has been developed that is conducted under mildly basic conditions with molecular oxygen as an oxidant, without the need for metal additives. Whereas nitroalkanes are converted into ketones in good yield, nitroalkenes are transformed into α,β-unsaturated ketones in one-pot by double-bond isomerization followed by the oxygen-mediated Nef reaction. The reaction protocol is both mild and general, and tolerates acid- and base-labile functionality or protecting groups. When oxygen-saturated solvents are employed, the reaction completes within 20 min. Mechanistically, the addition of nitronate ion and molecular oxygen is proposed to proceed initially through a single-electron transfer event, as indicated by radical clock experiments. This ultimately generates a putative 1,1-dioxirane, which reacts further with another nitronate ion to generate the ketone. Involvement of a 1,1-dioxirane is supported by intramolecular trapping experiments with sulfide at the γ-position of the nitro-moiety.

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“…For our purposes, the radical clock 8 was selected, which was comprised of a cyclopropyl unit incorporated into the α position of the ketone substrate (Scheme ). Cyclopropyl ring opening by a potentially formed ketyl radical is known to occur quickly ( k obs = 9 × 10 5 s –1 at 61 °C), followed by hydrogen atom abstraction and formation of an enolate intermediate, which tautomerizes to the corresponding ketone in the presence of a base (Scheme , pathway B) . When a catalytic run was performed with complex 4c as precatalyst and radical clock 8 as substrate (S:C 100:1), only the alcohol product 9 derived from hydrosilylation and desilylation was observed after exposure to methanolic base (pathway A).…”

Section: Resultsmentioning

confidence: 99%

Triazolylidene Iron(II) Piano-Stool Complexes: Synthesis and Catalytic Hydrosilylation of Carbonyl Compounds

Johnson

Albrecht

2017

Organometallics

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A new series of iron(II) piano stool complexes was synthesized that contain monodentate triazolylidene ligands with different aryl and alkyl substituents as well as an example of a C,N-chelating pyridine-substituted triazolylidene iron complex. The electronic and steric effect of wingtip modification was assessed by electrochemical, infrared spectroscopic, and X-ray diffraction analysis. All complexes were active in the catalytic hydrosilylation of aldehydes and ketones. The monodentate systems outperform the chelating triazolylidene analogue by far, reaching turnover frequencies TOF max as high as 14400 h −1 at 0.1 mol % catalyst loading. Mechanistic investigations indicate a radical mechanism for the catalytic H−Si bond activation.

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Reversible cyclopropyl ring opening of 1-aroyl-2-phenylcyclopropane radical anions. Determination of the ring opening and closure rates of the intermediate ketyls

Cited by 42 publications

References 0 publications

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Mechanism of the Iron(II)-Catalyzed Hydrosilylation of Ketones: Activation of Iron Carboxylate Precatalysts and Reaction Pathways of the Active Catalyst

Nef Reaction with Molecular Oxygen in the Absence of Metal Additives, and Mechanistic Insights

Triazolylidene Iron(II) Piano-Stool Complexes: Synthesis and Catalytic Hydrosilylation of Carbonyl Compounds

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