Divergent Reactivity of Homologue ortho‐Allenylbenzaldehydes Controlled by the Tether Length: Chromone versus Chromene Formation |
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| Authors: | Prof Dr Benito Alcaide Dr Pedro Almendros Dr Israel Fernández Dr Teresa Martínez del Campo Teresa Naranjo |
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| Institution: | 1. Grupo de Lactamas y Heterociclos Bioactivos, Departamento de Química Orgánica I, Unidad Asociada al CSIC, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid (Spain);2. Instituto de Química Orgánica General, Consejo Superior de Investigaciones Científicas, IQOG‐CSIC, Juan de la Cierva 3, 28006 Madrid (Spain);3. Departamento de Química Orgánica I, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid (Spain) |
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| Abstract: | 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. |
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| Keywords: | allenes carbenes cyclization density functional calculations heterocycles |
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