Functionalised cyclopropenes as synthetic intermediates

Baird, Mark S.

doi:10.1007/bfb0111230

Cited by 53 publications

(2 citation statements)

References 234 publications

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“…The rearrangement of cyclopropenes using transition metals as mediators has been extensively studied as a method for generating transition metal vinyl carbene complexes. − Despite the synthetic utility of this reaction, a complete understanding of the mechanism of the rearrangement remains ambiguous. The ring opening of cyclopropenes may be envisioned as proceeding through a stepwise sequence of cyclopropene → metal η 2 - cyclopropane → metallacyclobutene → metal vinyl carbene (Scheme ).…”

Section: Resultsmentioning

confidence: 99%

Cyclization Reactions of Rhodium Carbene Complexes. Effect of Composition and Oxidation State of the Metal

Padwa

Kassir

1997

J. Org. Chem.

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Treatment of o- (1,7-octadiynyl)benzoyldiazoethane with rhodium(II) octanoate in pentane resulted in a double internal/internal alkyne insertion reaction producing a labile bicyclo[4.1.0]hept-1(7)ene derivative which readily undergoes a Diels-Alder reaction with diphenylisobenzofuran. Changing the solvent from pentane to CH 2 Cl 2 afforded a 2:1 mixture of cis-and trans-alkenylsubstituted indenones. Stepwise cyclization involving a set of dipolar intermediates occurs in CH 2 -Cl 2 whereas metallocyclobutenes are involved when pentane is used as the solvent. The rhodium(II) carboxylate catalyzed reaction of unsymmetrically substituted cyclopropenes gives substituted furans derived from cleavage of the less substituted β-bond. Thus, treatment of 3-benzoyl-3-methyl-1-(nbutyl)cyclopropene with Rh 2 OAc 4 afforded a 26:1 mixture of 2-phenyl-3-methyl-4-(n-butyl)-and 2-phenyl-3-methyl-5-(n-butyl)furan. In contrast, the [ClRh(CO) 2 ] 2 -catalyzed reaction resulted in cleavage of the more substituted σ-bond producing only 2-phenyl-3-methyl-5-(n-butyl)furan. Both reactions involve electrophilic attack of the rhodium metal on the less substituted carbon atom of the cyclopropene π-bond to give the most stabilized cyclopropyl carbocation. Ring opening followed by rapid electrocyclization to the furan occurs with the Rh(II) catalyst. With the Rh(I) catalyst, the ring-opened species preferentially cyclizes to a metallocyclobutene intermediate which then equilibrates with the thermodynamically more stable isomer prior to furan formation. The Rh-(I)-catalyzed reaction of 3-benzoyl-1-propylcyclopropene with various terminal alkynes gives 2-alkyl-4-propyl-7-phenyloxepins in good yield. These reactions involve electrophilic attack of the rhodium metal on the more substituted carbon of the cyclopropene π-bond to give a rhodium carbene complex. This metallo carbenoid undergoes a subsequent [2 + 2] cycloaddition with terminal acetylenes. The resulting rhodacycle rearranges by a formal 1,5-sigmatropic shift, and this is followed by reductive elimination of rhodium to produce the observed oxepin.

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Section: Resultsmentioning

confidence: 99%

Cyclization Reactions of Rhodium Carbene Complexes. Effect of Composition and Oxidation State of the Metal

Padwa

Kassir

1997

J. Org. Chem.

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“…The present paper represents the first comprehensive review specifically focusing on the chemistry of CPAs and their analogues (e.g., nitrogen and sulfur analogues of the acetal) and covers all the experimental data reported before May 2002 on the chemistry of these compounds. Although the chemistry of CPA has been partially addressed in various reviews on strained rings, , cycloadditions, − and organometallic chemistry, − we do not intend to avoid any overlaps, so that this work will be the most comprehensive available.…”

Section: Introductionmentioning

confidence: 99%