Divergent Reactivity of Homologue ortho-Allenylbenzaldehydes Controlled by the Tether Length: Chromone versus Chromene Formation

The divergent behavior of two homologue allenals, namely, 2‐(buta‐2,3‐dienyloxy)‐ and 2‐(propa‐1,2‐dienyloxy)benzaldehydes, as cyclization substrates is described. 2‐(Buta‐2,3‐dienyloxy)benzaldehydes suffers a formal allenic carbocyclization reaction to afford chromenes, whereas 2‐(propa‐1,2‐dienylo...

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Bibliographic Details
Published in:Chemistry : a European journal 2015-01, Vol.21 (4), p.1533-1541
Main Authors: Alcaide, Benito, Almendros, Pedro, Fernández, Israel, Martínez del Campo, Teresa, Naranjo, Teresa
Format: Article
Language:English
Subjects:
allenes
Bearing
Carbenes
Chemistry
cyclization
density functional calculations
Density functional theory
Formations
heterocycles
Homology
Mathematical analysis
Tethers
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Summary:The divergent behavior of two homologue allenals, namely, 2‐(buta‐2,3‐dienyloxy)‐ and 2‐(propa‐1,2‐dienyloxy)benzaldehydes, as cyclization substrates is described. 2‐(Buta‐2,3‐dienyloxy)benzaldehydes suffers a formal allenic carbocyclization reaction to afford chromenes, whereas 2‐(propa‐1,2‐dienyloxy)benzaldehydes react to yield chromones. The formation of chromenes is strictly a formal hydroarylation process divided into two parts, namely, allenic Claisen‐type rearrangement and oxycyclization. An unknown N‐heterocyclic carbene (NHC)‐catalyzed allenic hydroacylation reaction must be invoked to account for the preparation of chromones. ortho‐Allenylbenzaldehydes bearing either electron‐donating substituents or electron‐withdrawing substituents worked well to afford both the hydroarylation and hydroacylation products. This unexpected difference in reactivity can be rationalized by means of density functional theory calculations. Separate ways: The divergent behavior of two homologue allenals as cyclization substrates is described. 2‐(Buta‐2,3‐dienyloxy)benzaldehydes underwent a formal allenic hydroarylation reaction to afford chromenes, whereas 2‐(propa‐1,2‐dienyloxy)benzaldehydes reacted to yield chromones by means of N‐heterocyclic carbene (NHC)‐catalyzed allenic hydroacylation (see scheme). To understand this unanticipated difference in reactivity, a DFT study has been performed.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.201404516