アリルパラジウム中間体を求核剤として利用した環化反応の開発

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  • Umpolung Type-I and -II Cyclizations of Aldehyde-Containing Allylpalladium Intermediates
  • アリルパラジウム チュウカンタイ オ キュウカクザイ ト シテ リヨウ シタ カンカ ハンノウ ノ カイハツ

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<p>Allylpalladium intermediates usually act as an electrophile due to the property of the η3-complex, which is much more stable than the nucleophilic η1-complex. We have developed type-I and -II umpolung cyclizations of aldehyde-containing allylpalladium intermediates, where the unstable η1-complexes are effectively captured by the intramolecular electrophile. The diastereoselectivities of the type-I cyclization highly depend on how to generate the allylpalladium intermediates either by oxidative addition of allyl acetate or carbopalladation of allenes. In the former case, the constituent elements and length of the tether also affect the diastereoselectivities. The enantioselectivities achieved by using SEGPHOS or DM-SEGPHOS ligand are useful to rationale the reaction mechanism via common cationic Zimmerman-Traxler transition states consisting of the cationic η1-allylpalladium intermediates. A sterically demanding substituent at the center of the allyl moiety is necessary for high enantioselectivity. On the other hand, the diastereoselectivities of the type-II cyclization using an achiral diphosphine ligand show that the reaction proceeds through a chair-chair transition state, based on which we have designed substrates applicable to asymmetric variant. In addition, we have developed one-pot type-II cyclizations by combining palladium-catalyzed reactions using allylic and propargylic carbonates, which can provide a solution to the complexity of substrate preparation.</p>

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