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On the Nature of Rate Acceleration in the Synthesis and Fragmentation of Triazolines by Brønsted Acid:  Secondary Catalysis by Water (Hydronium Triflate)

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posted on 20.02.2008, 00:00 by Ki Bum Hong, Matthew G. Donahue, Jeffrey N. Johnston
Rate acceleration of the addition of benzyl azide to an electron deficient olefin is characterized using in situ IR spectroscopy. Under strictly anhydrous conditions and at depressed temperature (−20 °C), a triazoline intermediate is selectively formed. The stability of this protonated triazoline intermediate at −20 °C is indefinite, but warming of the reaction mixture to 0 °C or above results in its conversion to the β-amino oxazolidine dione observed under conditions used in our earlier report. As an alternative to warming, the same conversion can be effected by the addition of a single equivalent of water. Our experiments collectively demonstrate the metastability of the protonated triazoline intermediate and secondary catalysis of triazolinium ring fragmentation by water. This behavior is attributed to the ability of water to transfer a proton from N3 to N1 of the triazoline, thereby allowing ring fragmentation and nitrogen expulsion.