Journal of the Chemical Society. Perkin transactions II p. 890 - 899 (1980)
Update date:2022-08-05
Topics:
Busby, Reginald E.
Dominey, Terence W.
The rates of rearrangement of the series 5-methyl, 5-ethyl, 5-isopropyl-, and 5-t-butyl-2-amino-1,3,4-thiadiazines in strong acid have been followed by observing the u.v. absorptions of the protonated thiadiazine substrates or the product thiazolimines.The rates of rearrangement were found to be first order with respect to the thiadiazine concentration and the rates decreased along the series in a manner characteristic of an SN2 reaction.The rates of rearrangement of 2-amino-1,3,4-thiadiazines with phenyl, p-nitrophenyl, and p-hydroxyphenyl groups in the 5-position were all extremely slow whereas 2-amino-5-benzyl-1,3,4-thiadiazine rearranged much faster than these arylthiadiazines but more slowly than the 5-methyl and 5-ethyl compounds.The rearrangement rates of 2-amino-5-methyl-1,3,4-thiadiazine were followed at various acid strengths within the range 4.47-8.12M-HCl.The reaction ultimately reached equilibrium, and the measured constant, k1, was equal to the sum of the velocity constants (kr + k-r) of the forward and backward reactions.Both kr and k-r were found to be similarly dependent on acidity and water activity.Using H0 values derived by Long and Paul with substituted anilines, a plot of log10k1 against -H0 was found to be linear and of slope equal to 0.92.A slope of -5.4 (w*) was obtained from the Bunnett plot of log10k1/CH(1+) against log10aw.A Yagil plot of log10k1/CH(1+) against log10Cw was linear and of slope equal to -2.95 at acidities less than 6.9M-HCl.Replacement of H by D at position 6 in all the thiadiazines was observed in D2O only in strong acid and D was incorporated both into the unchanged thiadiazaine and the product thiazolimine (at position 5).A mechanism is suggested for the rearrangement which involves transannular nucleophilic attack by N(3) at an sp3 carbon atom in the 5 position of the thiadiazines.
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