Abstract
Primary and secondary isotope effects were determined for the reaction of 4-methyl[1,2,4]triazoline-3,5-dione with 2,5-dimethyl-2,4-hexadiene-1,1,1,2',2',2'-d6. The results are consistent with the formation of an aziridinium imide intermediate in equilibrium with an open zwitterion. At higher temperatures, the aziridinium imide prevails and can be either transformed to the ene adduct in a slow step, or trapped by the solvent if the reaction is carried out in methanol. At lower temperatures, the entropically favored open zwitterion is formed preferentially, that either polymerizes in chloroform or can be trapped by methanol.
Highlights
Primary and secondary isotope effects were determined for the reaction of 4-methyl[1,2,4]triazoline-3,5-dione with 2,5-dimethyl-2,4-hexadiene-1,1,1,2',2',2'-d6
In the light of this theoretical work, we present a study on the isotope effect in the reaction of MTAD with the bulky 2,5-dimethyl-2,4-hexadiene (DMHD)
To elucidate the energy profile, and to shed light on mechanistic details of this intriguing reaction, we determined the primary and secondary isotope effects for the two possible pathways
Summary
4-Methyl- and 4-phenyl[1,2,4]triazoline-3,5-dione (MTAD and PTAD, respectively) readily react with conjugated dienes to form [4+2] adducts. The stereochemistry of the Diels-Alder adducts from the reaction of PTAD with 2,4-hexadienes and alkoxy-substituted 1,3-butadienes is consistent with both concerted and stepwise mechanisms; e.g. (E,E)-2,4-hexadiene affords stereospecifically the cis Diels-Alder adduct following the Woodward-Hoffmann rules. The stereochemistry of the Diels-Alder adducts from the reaction of PTAD with 2,4-hexadienes and alkoxy-substituted 1,3-butadienes is consistent with both concerted and stepwise mechanisms; e.g. (E,E)-2,4-hexadiene affords stereospecifically the cis Diels-Alder adduct following the Woodward-Hoffmann rules. DFT theoretical calculations reported by Houk and Foote on dienes that adopt the s-cis conformation reveal that the concerted pathway is favored over a non-symmetrical transition state. The reaction of DMHD 1 with PTAD 2a was originally reported by Gillis and Hagarty and was found to afford an acyclic addition product containing an acetoxy group. The authors suggest that the adduct is formed by trapping the possible dipolar intermediate with Pb(OAc) used to oxidize 4-phenylurazole to PTAD 2a. Variable amounts of unidentified polymeric material were formed In methanol this reaction was performed furnished a mixture of 1,4- and 1,2-methoxy adducts 5 and 6, respectively (ratio 80:20). Reaction of 2,5-dimethyl-2,4-hexadiene (1) with PTAD 2a in CHCl3 and MeOH
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