Solvent‐Dependent Changes in the Triazolinedione–Alkene Ene Reaction Mechanism

The influence of the solvent on the triazolinedione–alkene ene reaction mechanism has been investigated. Both inter‐ and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2‐(trideuterio)methyl‐7‐methyl‐2,6‐octadiene‐[D3]‐1,1,1 provide, for the first time, strong evidence for c...

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Bibliographic Details
Published in:Chemistry : a European journal 2008-10, Vol.14 (31), p.9697-9705
Main Authors: Vougioukalakis, Georgios C., Roubelakis, Manolis M., Alberti, Mariza N., Orfanopoulos, Michael
Format: Article
Language:English
Subjects:
ene reaction
kinetic isotope effects
reaction mechanisms
solvent effects
triazolinediones
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Summary:The influence of the solvent on the triazolinedione–alkene ene reaction mechanism has been investigated. Both inter‐ and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2‐(trideuterio)methyl‐7‐methyl‐2,6‐octadiene‐[D3]‐1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non‐protic to polar protic solvents. In non‐protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate‐determining step of the reaction, which is followed by fast hydrogen ion. On the contrary, in polar protic solvents, hydrogen ion is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents. The mechanism of the ene reaction of triazolinedione (RTAD) with simple alkenes depends on solvent polarity (see scheme). In aprotic solvents, a tight aziridinium imide intermediate with insignificant equilibration to its open intermediate is suggested. In protic solvents, large, non‐stereochemically‐dependent kinetic isotope effects suggest a loose aziridinium imide intermediate that equilibrates extensively with its open intermediate and the starting reagents.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.200800920