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Ene reaction AI simulator
(@Ene reaction_simulator)
Hub AI
Ene reaction AI simulator
(@Ene reaction_simulator)
Ene reaction
In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.
This transformation is a group transfer pericyclic reaction, and therefore, usually requires highly activated substrates and/or high temperatures. Nonetheless, the reaction is compatible with a wide variety of functional groups that can be appended to the ene and enophile moieties. Many useful Lewis acid-catalyzed ene reactions have been also developed, which can afford high yields and selectivities at significantly lower temperatures.
Enes are π-bonded molecules that contain at least one hydrogen atom at the allylic, propargylic, or α-position. Possible ene components include olefinic, acetylenic, allenic, aromatic, cyclopropyl, and carbon-hetero bonds. Usually, the allylic hydrogen of allenic components participates in ene reactions, but in the case of allenyl silanes, the allenic hydrogen atom α to the silicon substituent is the one transferred, affording a silylalkyne. Phenol can act as an ene component, for example in the reaction with dihydropyran, but high temperatures are required (150–170 °C (302–338 °F)); nevertheless, strained enes and fused small ring systems undergo ene reactions at much lower temperatures. Similarly, ene reactions with enols or enolates are classified as Conia-ene and Conia-ene-type reactions. In addition, ene components containing C=O, C=N and C=S bonds have been reported, but such cases are rare.
Enophiles are typically electrophilic olefins. They are dienophiles. A range π-bonded molecules with electron-withdrawing substituents also participate. In addition to carbon-carbon multiple bonds (olefins, acetylenes), enophiles may contain carbon-hetero multiple bonds (C=O in the case of carbonyl-ene reactions, C=N, C=S, C≡P), hetero-hetero multiple bonds (N=N, O=O, Si=Si, N=O, S=O), cumulene systems (N=S=O, N=S=N, N=Se=N, C=C=O, C=C=S, SO2) and charged π systems (C=N+, C=S+, C≡O+, C≡N+).
The reverse process, a retro-ene reaction are observed when thermodynamically stable molecules like carbon dioxide or dinitrogen are extruded. For instance, kinetic data and computational studies indicate that thermolysis of but-3-enoic acid to give propene and carbon dioxide proceeds via a retro-ene mechanism. Similarly, propargylic diazenes decompose readily through a retro-ene mechanism to give allene products and nitrogen gas (see Myers allene synthesis).
The HOMO of the ene and the LUMO of the enophile comprise the main frontier-orbital interaction in an ene reaction. The HOMO of the ene results from the combination of the pi-bonding orbital in the vinyl moiety and the allylic C-H bond. Concerted, all-carbon-ene reactions have, in general, a high activation barrier. A barrier of 138 kJ/mol is calculated in the case of the propylene + ethylene. However, if the enophile becomes more polar (going from ethane to formaldehyde), its LUMO has a larger amplitude on C, yielding a better C–C overlap and a worse H–O one, determining the reaction to proceed in an asynchronous fashion. This translates into a lowering of the activation barrier until 61.5 kJ/mol (M06-2X/def2-TZVPP)[incomprehensible], if S replaces O on the enophile. By computationally examining both the activation barriers and the activation strains of several different ene reactions involving propene as the ene component, Fernandez and co-workers have found that the barrier decreases along the enophiles in the order:
as the reaction becomes more and more asynchronous and/or the activation strain decreases.
The concerted nature of the ene process has been supported experimentally, and the reaction can be designated as [σ2s + π2s + π2s] in the Woodward-Hoffmann notation. The early transition state proposed for the thermal ene reaction of propene with formaldehyde has an envelope conformation, with a C–O–H angle of 155°, as calculated at the 3-21G level of theory[incomprehensible].
Ene reaction
In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.
This transformation is a group transfer pericyclic reaction, and therefore, usually requires highly activated substrates and/or high temperatures. Nonetheless, the reaction is compatible with a wide variety of functional groups that can be appended to the ene and enophile moieties. Many useful Lewis acid-catalyzed ene reactions have been also developed, which can afford high yields and selectivities at significantly lower temperatures.
Enes are π-bonded molecules that contain at least one hydrogen atom at the allylic, propargylic, or α-position. Possible ene components include olefinic, acetylenic, allenic, aromatic, cyclopropyl, and carbon-hetero bonds. Usually, the allylic hydrogen of allenic components participates in ene reactions, but in the case of allenyl silanes, the allenic hydrogen atom α to the silicon substituent is the one transferred, affording a silylalkyne. Phenol can act as an ene component, for example in the reaction with dihydropyran, but high temperatures are required (150–170 °C (302–338 °F)); nevertheless, strained enes and fused small ring systems undergo ene reactions at much lower temperatures. Similarly, ene reactions with enols or enolates are classified as Conia-ene and Conia-ene-type reactions. In addition, ene components containing C=O, C=N and C=S bonds have been reported, but such cases are rare.
Enophiles are typically electrophilic olefins. They are dienophiles. A range π-bonded molecules with electron-withdrawing substituents also participate. In addition to carbon-carbon multiple bonds (olefins, acetylenes), enophiles may contain carbon-hetero multiple bonds (C=O in the case of carbonyl-ene reactions, C=N, C=S, C≡P), hetero-hetero multiple bonds (N=N, O=O, Si=Si, N=O, S=O), cumulene systems (N=S=O, N=S=N, N=Se=N, C=C=O, C=C=S, SO2) and charged π systems (C=N+, C=S+, C≡O+, C≡N+).
The reverse process, a retro-ene reaction are observed when thermodynamically stable molecules like carbon dioxide or dinitrogen are extruded. For instance, kinetic data and computational studies indicate that thermolysis of but-3-enoic acid to give propene and carbon dioxide proceeds via a retro-ene mechanism. Similarly, propargylic diazenes decompose readily through a retro-ene mechanism to give allene products and nitrogen gas (see Myers allene synthesis).
The HOMO of the ene and the LUMO of the enophile comprise the main frontier-orbital interaction in an ene reaction. The HOMO of the ene results from the combination of the pi-bonding orbital in the vinyl moiety and the allylic C-H bond. Concerted, all-carbon-ene reactions have, in general, a high activation barrier. A barrier of 138 kJ/mol is calculated in the case of the propylene + ethylene. However, if the enophile becomes more polar (going from ethane to formaldehyde), its LUMO has a larger amplitude on C, yielding a better C–C overlap and a worse H–O one, determining the reaction to proceed in an asynchronous fashion. This translates into a lowering of the activation barrier until 61.5 kJ/mol (M06-2X/def2-TZVPP)[incomprehensible], if S replaces O on the enophile. By computationally examining both the activation barriers and the activation strains of several different ene reactions involving propene as the ene component, Fernandez and co-workers have found that the barrier decreases along the enophiles in the order:
as the reaction becomes more and more asynchronous and/or the activation strain decreases.
The concerted nature of the ene process has been supported experimentally, and the reaction can be designated as [σ2s + π2s + π2s] in the Woodward-Hoffmann notation. The early transition state proposed for the thermal ene reaction of propene with formaldehyde has an envelope conformation, with a C–O–H angle of 155°, as calculated at the 3-21G level of theory[incomprehensible].
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