The rearrangement of 2-amino-1,3,4-thiadiazines to 3-amino-2-thiazolimines. Part 1. The rates of rearrangement of a series of 5-alkyl- and 5-aryl-2-amino-1,3,4-thiadiazines at 30 and 50 °C

Abstract

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 S_N2 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 (k_r+k_–r) of the forward and backward reactions. Both k_r 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/C_H⁺ against log₁₀a_w. A Yagil plot of log₁₀k₁/C_H⁺ against log₁₀C_w 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 thiadiazine and the product thiazolimine (at position 5). A mechanism is suggested for the rearrangement which involves transannular nucleophilic atack by N(3) at an sp³ carbon atom in the 5 position of the thiadiazines.