Direct observation of a cationic intermediate in the acid-catalyzed decomposition of (2-hydroxy-, (2-ethoxy-, and (2-phenoxyethyl)cobaloximes

Brown, Kenneth L.; Ramamurthy, Sheela

doi:10.1021/om00062a037

Cited by 23 publications

(4 citation statements)

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“…Assuming that cob(I)alamin indeed initiates the dechlorination process, at least five different mechanisms can be written on the basis of the vast literature on the chemistry of vitamin B 12 (Scheme ) . Four of these (pathways A−D) involve the intermediacy of chlorinated alkyl or alkenylcobalamins formed by attack of the strongly nucleophilic , cob(I)alamin on the electron deficient alkene, possibly after initial π-complexation . Mechanisms A−D differ in the manner in which chloride is eliminated and the Co−C bond is broken.…”

Section: Introductionmentioning

confidence: 99%

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants

2002

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Vitamin B(12) catalyzes the reductive dechlorination of several ubiquitous pollutants such as perchloroethylene (PCE) and trichloroethylene (TCE). Several mechanisms have been proposed for these transformations, some of which involve the intermediacy of chlorinated vinylcobalamins. To evaluate the currently unknown chemical and physical properties of such species, various chlorinated vinylcobaloxime complexes [cobaloxime = bis(dimethylglyoximato)(pyridine)cobalt(III)] were prepared and characterized. X-ray structures are reported for (cis-1,2-dichloroethenyl)cobaloxime (4), (cis-monochloroethenyl)cobaloxime (5), (alpha-chloroethenyl)cobaloxime (6), and vinylcobaloxime (7), and the reactivities of these isolated complexes were investigated. They were stable in the presence of ethanolic NaBH(4) unless external cobaloximes were added. The cob(I)aloxime formed under the latter conditions promoted the conversion of 4 to 5 and 6, and of 5 and 6 into 7. Mechanistic studies of these transformations are consistent with a pathway in which the conversion of 4 into 5 and 6 takes place via chloroacetylene as an intermediate, and the conversion of 6 to 7 involves vinyl chloride as an intermediate. Cyclic voltammetry on the chlorinated vinylcobaloximes resulted in irreversible reduction waves, with 4 displaying the least negative and 7 the most negative peak potential. These results are discussed in the context of the B(12)-catalyzed reductive dechlorination of PCE and TCE.

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Section: Introductionmentioning

confidence: 99%

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants

2002

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“…The most plausible interpretation of the results with 12a and 12b is that the cobaloxime π-cations have two possible fates: trapping with C-nucleophile or irreversible alkene decomplexation . Since the alkene products from reactions of 12a and 12b are too volatile to conveniently isolate and quantitate, a larger substrate with a less volatile alkene elimination product was studied.…”

Section: Resultsmentioning

confidence: 99%

“…Golding discovered that the solvolysis of homochiral (2-acetoxypropyl)cobaloxime ( 2 , Scheme ) with benzyl alcohol to produce 4 proceeded with retention of configuration, although no yield was given. Several studies by Brown, Silverman and Dolphin, and others have investigated the mechanism of this reaction. Brown used low-temperature 13 C NMR to directly observe the cobaloxime π-cation of ethene, formed by acid treatment of (2-hydroxyethyl)- and (2-alkoxyethyl)cobaloximes 3b.…”

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confidence: 99%

“…Several studies by Brown, Silverman and Dolphin, and others have investigated the mechanism of this reaction. Brown used low-temperature 13 C NMR to directly observe the cobaloxime π-cation of ethene, formed by acid treatment of (2-hydroxyethyl)- and (2-alkoxyethyl)cobaloximes 3b. At −30 °C there were two 13 C resonances, 41 ppm apart, for the ethene carbons.…”

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confidence: 99%

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Exploratory Studies on Reactions of Cobaloxime π-Cations with C-Nucleophiles: Irreversible Alkene Decomplexation versus Nucleophilic Capture

Gage

Branchaud

1996

J. Org. Chem.

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(β-Hydroxyalkyl)- and (β-acetoxyalkyl)cobaloximes can undergo facile acid-catalyzed β-heteroatom exchange with oxygen and nitrogen nucleophiles in an SN1-like mechanism via cationic metal−alkene π-complex intermediates (cobaloxime π-cations). The reaction of cobaloxime π-cations with carbon nucleophiles has not been previously reported. The results reported in this paper demonstrate that cobaloxime π-cations are reasonably good electrophiles. They are sufficiently reactive to add to the electron-rich sp2 centers in allyltrimethylsilane and pyrrole. A major side reaction for intermolecular reactions is irreversible alkene decomplexation. An intramolecular pyrrole cyclization (22 to 23, 83% yield) significantly raised the yield for nucleophilic addition compared to that of the analogous intermolecular couplings (12b to 13b, 32% yield and 17 to 18, 29% yield). The results of these studies provide a foundation for the design and evaluation of modified cobalt ligands with the goal of suppressing alkene decomplexation and enhancing reaction of cobaloxime π-cations with C-nucleophiles.

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Chlorobis(dimethylglyoximato)(pyridine)cobalt(III)

Giese

Hartung

Pérez

2008

Encyclopedia of Reagents for Organic Synthesis

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Direct observation of a cationic intermediate in the acid-catalyzed decomposition of (2-hydroxy-, (2-ethoxy-, and (2-phenoxyethyl)cobaloximes

Cited by 23 publications

References 1 publication

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants

Exploratory Studies on Reactions of Cobaloxime π-Cations with C-Nucleophiles: Irreversible Alkene Decomplexation versus Nucleophilic Capture

Chlorobis(dimethylglyoximato)(pyridine)cobalt(III)

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Direct observation of a cationic intermediate in the acid-catalyzed decomposition of (2-hydroxy-, (2-ethoxy-, and (2-phenoxyethyl)cobaloximes

Cited by 23 publications

References 1 publication

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B12-Catalyzed Dechlorination of Priority Pollutants

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B12-Catalyzed Dechlorination of Priority Pollutants

Exploratory Studies on Reactions of Cobaloxime π-Cations with C-Nucleophiles: Irreversible Alkene Decomplexation versus Nucleophilic Capture

Chlorobis(dimethylglyoximato)(pyridine)cobalt(III)

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Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants

Synthesis and Characterization of Chlorinated Alkenylcobaloximes To Probe the Mechanism of Vitamin B₁₂-Catalyzed Dechlorination of Priority Pollutants