Bimolecular homolytic substitution at carbon. Stereochemical investigation

Upton, Charles J.; Incremona, Joseph H.

doi:10.1021/jo00865a024

Cited by 18 publications

(6 citation statements)

References 10 publications

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“…55, 102 Radical attack on cyclopropyl rings is known to cause homolytic opening of the ring. [103][104][105] A reversible reaction like that in Scheme 4 was proposed as the mechanism for the equilibration of 1-methylbut-3-enylcobaloxime and 2-methylbut-3-enylcobaloxime 106 and is consistent with observations made during the isomerizations of cyclopropylmethylcobalamin Exchange reactions between Co(T2,6FBzOPP)CH 3 and Co-(TTP) run in complete darkness were extremely slow. Small amounts of Co(TTP)CH 3 were detected after 13 days of exchange.…”

Section: Discussionsupporting

confidence: 83%

“…The second rationalization is that the 3-butenylcobalt(III) porphyrin is a product of a bimolecular attack of a cobalt(II) porphyrin on a cyclopropylmethylcobalt(III) porphyrin at one of the carbons γ to the cobalt, resulting in opening of the cyclopropyl ring and displacement of the originally attached cobalt porphyrin, Scheme . The reaction is analogous to the S H 2‘ reaction in which an organic radical attacks at the γ-carbon of a 3-substituted allylcobaloxime(III) and displaces a cobaloxime(II) to afford a 3-substituted 1-butene. , Radical attack on cyclopropyl rings is known to cause homolytic opening of the ring. − A reversible reaction like that in Scheme was proposed as the mechanism for the equilibration of 1-methylbut-3-enylcobaloxime and 2-methylbut-3-enylcobaloxime and is consistent with observations made during the isomerizations of cyclopropylmethylcobalamin and cyclopropylmethylcobaloxime to their respective 3-butenyl complexes. A nucleophilic attack of a cobalt(I) porphyrin anion on initially formed cyclopropylmethylcobalt(III) porphyrin analogous to the homolytic attack in Scheme could explain why the alkylation reaction using cyclopropylmethyl bromide invariably produces a mixture that contains (3-butenyl)cobalt(III) porphyrin.…”

Section: Discussionmentioning

confidence: 99%

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Alkyl Exchange Reactions of Organocobalt Porphyrins. A Bimolecular Homolytic Substitution Reaction

Stolzenberg

Cao

2001

J. Am. Chem. Soc.

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Reaction of organocobalt(III) porphyrins with a cobalt(II) complex of a distinguishable porphyrin or tetrapyrrole resulted in the reversible exchange of the organic axial ligand. The exchange reaction was facile in such solvents as benzene, toluene, dichloromethane, chloroform, and pyridine; was unaffected by total exclusion of light; was faster than would be expected for a homolytic process given known Co-C bond dissociation energies; and was of broad scope with respect to the organic ligand. Methyl, benzyl, primary alkyl, secondary alkyl, and acyl groups exchanged, but phenyl groups did not. The position of the exchange equilibrium was independent of the direction of approach and was nonstatistical. The relaxation to equilibrium appeared to be consistent with that of a second-order process. The rate of the reaction varied with the identity of the R group in the order Bzl > or = Me > Et approximately n-Pr > i-Pr > i-Bu > acetyl approximately neopentyl approximately 2-adamantyl. However, the total variation in reaction rates was remarkably small. Attempts to find evidence of free-radical intermediates by trapping with TEMPO or CO or by alkyl group interchange with an excess of an alkyl halide of a distinct alkyl group were unsuccessful over a time scale comparable to multiple half-lives of the exchange reaction. In addition, no rearrangement products were detected in exchange reactions of the 5-hexenyl group. Use of cobalt porphyrin reactants that were sterically encumbered on both faces with groups large enough to prevent formation of a bridged, Co-C-Co structure resulted in a 5 or more order of magnitude decrease in the rate of methyl exchange, if not its outright cessation, when run with total exclusion of light. The decrease in the rate of the thermal exchange process revealed the existence a slow photochemical exchange process that was driven by room lights. All evidence was consistent with a bimolecular S(H)2 mechanism for the thermal exchange mechanism.

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Section: Discussionsupporting

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Alkyl Exchange Reactions of Organocobalt Porphyrins. A Bimolecular Homolytic Substitution Reaction

Stolzenberg

Cao

2001

J. Am. Chem. Soc.

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“…This product is optimized once again in a backside attack, and therefore one can predict that radical cleavage of σ-bonds will proceed with inversion of configuration. All known experimental data [185][186][187][188][189][190] conform to this prediction. Another area where successful predictions have been made involves nucleophilic attacks on radical cations.…”

Section: Figure 14 Near Heresupporting

confidence: 67%

Valence Bond Theory, Its History, Fundamentals, and Applications: A Primer

Shaik¹,

Hiberty²

2004

Reviews in Computational Chemistry

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“…Applications of the classical stereochemical tests to this scheme have been hindered by the scarcity of S H 2 reactions at potentially chiral centers. Heroic experiments were carried out in the 1970s with chiral cyclopropanes such as 1,1-dichloro-2,3-trans-dideuteriocyclopropane (23), ring opening of which by both chlorine and bromine atoms 30 gave >96% of the erythro isomer of product 24 (Scheme 7). This, together with other halogenation studies, 31 established that the displacement step occurred with complete inversion at the attacked center.…”

Section: Stereochemistry Of Homolytic Substitutionsmentioning

confidence: 99%