Abstract
BackgroundThe nature of the heteroatom substitution in the nitrogen of a 3-aza-Cope system is explored.ResultsWhile N-propargyl isoxazolin-5-ones suffer 3-aza-Cope rearrangements at 60°C, the corresponding N-propargyl pyrazol-5-ones need a higher temperature of 180°C for the equivalent reaction. When the propargyl group is substituted by an allyl group, the temperature of the rearrangement for both type of compounds is less affected by the nature of the heteroatom present. Treatment with a base, such as ethoxide, facilitates the rearrangement, and in the case of isoxazol-5- ones other ring opening reactions take precedence, involving N–O ring cleavage of the 5-membered ring. However when base-catalysed decomposition is prevented by substituents, products arising from a room temperature aza-Cope rearrangement are isolated. A possible mechanistic pathway based on free energies derived from density functional calculations involving cyclic intermediates is proposed.ConclusionsThe nature of the heteroatom substitution in the nitrogen of a 3-aza-Cope system leads to a remarkable difference in the energy of activation of the reaction.
Highlights
The nature of the heteroatom substitution in the nitrogen of a 3-aza-Cope system is explored
Theoretical studies indicate that an oxyanionic substituent on the nitrogen atom reduces substantially the activation energies for these rearrangements [8]. We report in this manuscript the results obtained with the N-propargyl and N-allyl 5-membered heterocycles, pyrazolin-5-ones
In the presence of EtOK and 18-crown-6 ether at r.t., it was found that the propargyl group of 1a isomerized to the corresponding allene 5a as in Scheme 2, which could in turn suffer upon heating a 3,3-sigmatropic rearrangement leading to 6a
Summary
While N-propargyl isoxazolin-5-ones suffer 3-aza-Cope rearrangements at 60°C, the corresponding N-propargyl pyrazol-5-ones need a higher temperature of 180°C for the equivalent reaction. When the propargyl group is substituted by an allyl group, the temperature of the rearrangement for both type of compounds is less affected by the nature of the heteroatom present. Treatment with a base, such as ethoxide, facilitates the rearrangement, and in the case of isoxazol-5- ones other ring opening reactions take precedence, involving N–O ring cleavage of the 5-membered ring. When base-catalysed decomposition is prevented by substituents, products arising from a room temperature aza-Cope rearrangement are isolated. A possible mechanistic pathway based on free energies derived from density functional calculations involving cyclic intermediates is proposed
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