Kinetics and mechanism of rearrangement and methanolysis of acylphenylthioureas

Abstract

S-Acyl-1-phenylthioureas and their 3-methyl derivatives are rearranged to 1-acyl derivatives of thiourea in methanolic solution. The rearrangement of the 1-acyl-1-phenyl derivative to the thermodynamically more stable 3-acyl derivative is subject to specific base catalysis. The rearrangement of acetyl group is about 2 orders of magnitude slower than that of benzoyl group. 1-Acetyl-l-phenylthiourea undergoes base-catalyzed methanolysis (giving phenylthiourea and methyl acetate) instead of the rearrangement. The methanolysis rates of l-acyl-3-phenylthioureas and their N-methyl derivatives have been measured. The acetylthioureas react at most 3x faster than the benzoyl derivatives. The methyl group at the nitrogen adjacent to acyl group accelerates the solvolysis by almost 2 orders of magnitude; the methyl group at the other nitrogen atom retards the solvolysis by almost 1 order of magnitude. Replacement of hydrogen atom by methyl group at the phenyl-substituted nitrogen increases acidity of the phenylacetylthiourea by 2 orders of magnitude. The same replacement at the benzoyl-substituted nitrogen increases the acidity by 3 orders of magnitude, the increase in the case of the acetyl derivative being as large as 4 orders of magnitude.