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Hub AI
6+4 Cycloaddition AI simulator
(@6+4 Cycloaddition_simulator)
Hub AI
6+4 Cycloaddition AI simulator
(@6+4 Cycloaddition_simulator)
6+4 Cycloaddition
[6+4] Cycloaddition is a type of cycloaddition between a six-atom pi system and a four-atom pi system, leading to a ten-membered ring. Because this is a higher-order cycloaddition, issues of periselectivity arise in addition to the usual concerns about regio- and stereoselectivity. Six-atom pi systems that have been employed in the reaction include tropone and tropone derivatives, fulvenes, and cycloheptatriene cobalt complexes.
[6+4] Cycloaddition is a thermally allowed, higher-order cycloaddition process leading to ten-membered rings. Although most linear, acyclic trienes do not give [6+4] products selectively, cyclic trienes give high yields of [6+4] products in many cases. Both cycloheptatrienes and fulvenes can be employed in this reaction, and electron-deficient tropones in particular work well. The pericyclic and transition-metal-mediated versions of the reaction are stereocomplementary: the former gives exo products, and the latter endo products, with essentially complete selectivity in nearly all cases. The possibility of building complex carbocyclic frameworks efficiently has made this reaction particularly attractive synthetically.
(1)
The metal-free version of the [6+4] cycloaddition takes place through a concerted, pericyclic process. The frontier molecular orbitals involved in reactions of tropone illustrate that a repulsive secondary orbital interaction likely destabilizes the endo transition state, leading to complete selectivity for exo products.
(2)
Fulvenes react similarly, although selective [6+4] reactions require the use of an electron-rich diene. Fulvene's frontier orbitals illustrate that it will only act as a 6π component in LUMO-controlled reactions. The next-highest occupied molecular orbital (NHOMO, not shown) also has the proper symmetry and orbital coefficients to participate in [6+4] cycloaddition; the NHOMO can be activated for reaction by substituting the exo methylene with electron-donating groups.
(3)
The metal-promoted [6+4] cycloaddition using a cycloheptatriene–metal complex is a stepwise, photolytic process that lacks the periselectivity issues of the purely thermal [6+4] cycloaddition. The mechanism has been debated, but likely proceeds through a hapticity change in the metal-triene complex followed by coordination of the diene component and coupling. Dissociation of carbon monoxide has also been invoked instead of hapticity change, but the evolution of carbon monoxide is not observed during the reaction. Complete endo selectivity is observed for this process as a result of templating by the metal.
6+4 Cycloaddition
[6+4] Cycloaddition is a type of cycloaddition between a six-atom pi system and a four-atom pi system, leading to a ten-membered ring. Because this is a higher-order cycloaddition, issues of periselectivity arise in addition to the usual concerns about regio- and stereoselectivity. Six-atom pi systems that have been employed in the reaction include tropone and tropone derivatives, fulvenes, and cycloheptatriene cobalt complexes.
[6+4] Cycloaddition is a thermally allowed, higher-order cycloaddition process leading to ten-membered rings. Although most linear, acyclic trienes do not give [6+4] products selectively, cyclic trienes give high yields of [6+4] products in many cases. Both cycloheptatrienes and fulvenes can be employed in this reaction, and electron-deficient tropones in particular work well. The pericyclic and transition-metal-mediated versions of the reaction are stereocomplementary: the former gives exo products, and the latter endo products, with essentially complete selectivity in nearly all cases. The possibility of building complex carbocyclic frameworks efficiently has made this reaction particularly attractive synthetically.
(1)
The metal-free version of the [6+4] cycloaddition takes place through a concerted, pericyclic process. The frontier molecular orbitals involved in reactions of tropone illustrate that a repulsive secondary orbital interaction likely destabilizes the endo transition state, leading to complete selectivity for exo products.
(2)
Fulvenes react similarly, although selective [6+4] reactions require the use of an electron-rich diene. Fulvene's frontier orbitals illustrate that it will only act as a 6π component in LUMO-controlled reactions. The next-highest occupied molecular orbital (NHOMO, not shown) also has the proper symmetry and orbital coefficients to participate in [6+4] cycloaddition; the NHOMO can be activated for reaction by substituting the exo methylene with electron-donating groups.
(3)
The metal-promoted [6+4] cycloaddition using a cycloheptatriene–metal complex is a stepwise, photolytic process that lacks the periselectivity issues of the purely thermal [6+4] cycloaddition. The mechanism has been debated, but likely proceeds through a hapticity change in the metal-triene complex followed by coordination of the diene component and coupling. Dissociation of carbon monoxide has also been invoked instead of hapticity change, but the evolution of carbon monoxide is not observed during the reaction. Complete endo selectivity is observed for this process as a result of templating by the metal.