Cross-conjugated biradicals

Gajewski, Joseph J.; Benner, Charles W.; Stahlly, Bruce N.; Hall, Ralph F.; Sato, Ronald I.

doi:10.1016/0040-4020(82)80166-x

Cited by 15 publications

(5 citation statements)

References 20 publications

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“…It is theoretically possible that this reaction is a concerted 1,3-sigmatropic rearrangement proceeding with inversion of stereochemistry at the migrating center . However the accepted mechanism involves a stepwise formation of a singlet biradical intermediate such as 15 , which can close to give the rearranged product 14 …”

Section: Introductionmentioning

confidence: 99%

Super Radical Stabilizers

Creary

2006

Acc. Chem. Res.

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A comprehensive series of substituted 1,1-dimethyl-2-methylenecyclopropanes have been thermally rearranged. These rearrangements proceed via singlet biradical intermediates that can be stabilized by substituents. Rates are greatly enhanced by certain groups that are termed super radical stabilizers. Substituents included 4-pyridyl N-oxide, 2-(1,6-methano[10]annulenyl), and a number of anion-substituted phenyl groups. Simple valence bond theory, as well as more sophisticated computational studies, gives insights into modes of radical stabilization.

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Section: Introductionmentioning

confidence: 99%

Super Radical Stabilizers

Creary

2006

Acc. Chem. Res.

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“…While the thermal methylenecyclopropane rearrangement is a much studied reaction, there are only isolated examples of photochemical variants. , Thus, direct irradiation of 4 under rather harsh conditions gave a competing methylenecyclopropane rearrangement and photofragmentation to 1,1-diphenylethylene and vinylidene . These reactions are believed to occur on the singlet energy surface.…”

Section: Introductionmentioning

confidence: 99%

Triplet Energy Dissipation in Methylenecyclopropane Rearrangement

2007

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Certain 1,1-dimethyl-2-aryl-3-methylenecyclopropanes containing carbonyl substituents undergo rearrangement when irradiated with 350 nm light. These rearrangements occur via the (n,pi*) triplet state, which fragments the strained cyclopropane bond. Intersystem crossing followed by ring closure gives the observed products. No photoreduction is seen in i-PrOH. Potential Norrish type II processes are also bypassed. It is suggested that the cyclopropane bond fragmentation dissipates the triplet energy and that the new intermediates are not energetic enough to abstract hydrogen atoms in an intramolecular fashion or from solvent. Nitro substituted systems undergo analogous photoinitiated rearrangements. Benzophenone sensitization of naphthyl, biphenyl, styrene, and phenylacetylene analogues also leads to rearrangement, presumably via the sensitized generation of triplet states. When triplet states cannot be accessed by direct irradiation or by sensitized processes, methylenecyclopropane rearrangements do not occur. An exception is the ferrocenyl analogue, which does not photorearrange, presumably due to the very short lifetime of the triplet intermediate.

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“…Secondly, the products of intermolecular Heck reactions would give rise to bis(exomethylene)cyclobutanes, which are known to undergo ring opening reactions by formation of a diradical. 23 In the products 7,11,12, the p-orbitals of the conjugated diene systems are not co-planar 24 thus this might be an explanation for the stability of the products.…”

Section: Methodsmentioning

confidence: 99%

Synthesis of Bis(enolnonaflates) and their 4-exo-trig-Cyclizations by Intramolecular Heck Reactions

Bräse

1999

Synlett

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Starting from dimedone, novel 1,3-bis(enolnonaflates) (nonafluorobutanesulfonates) have been prepared in four efficient steps. Cyclization of these 1,5-hexadienes under Palladium catalysis proceeded cleanly to give bicyclo[4.2.0]octadienes and bicyclo[4.2.0]octenones, respectively, by an unprecedented 4-exo-trig process. Insertion in one enolsulfonate moiety, intramolecular cyclization and subsequent second insertion after intermolecular reaction with alkenes (acrylates) or alkynes (3,3-dimethylbutyne) led to novel, highly congested hydrocarbons. Application of a chiral catalyst system resulted in modest asymmetric induction to furnish an enantioselective Heck-type desymmetrization reaction product.

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Cross-conjugated biradicals

Cited by 15 publications

References 20 publications

Super Radical Stabilizers

Super Radical Stabilizers

Triplet Energy Dissipation in Methylenecyclopropane Rearrangement

Synthesis of Bis(enolnonaflates) and their 4-exo-trig-Cyclizations by Intramolecular Heck Reactions

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