Isotope effects and syn selectivity in the ene reaction of triazolinedionewith conjugated enones: Aziridinium imide or an open intermediate mechanism?

[GRAPHICS] The ene reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with phorone -d(6) (5-d(6)) shows an inverse beta-secondary isotope effect and with mesi tyl oxide-d(3) (7-d(3)) proceeds with 93% syn selectivity, These results ar e consistent with a mechanism involving the formation of an aziridinium imi de intermediate in the rate-determining step, An open biradical or dipolar intermediate is excluded by them.