Conversion of Aliphatic Amides into Amines with benzene. 2. Kinetics and Mechanism

Article abstract of DOI:10.1021/jo00196a032

The reagent benzene (PIFA), used to prepare amines from amides as described in the preceding paper, dissolves in 50:50 (v/v) aqueous acetonitrile to give an acidic solution.This behavior can be explained quantitatively by the dimerization of PIFA in solution under preparatively significant conditions; the dimer, μ-oxo-I,I'-bis(trifluoroacetato-O)-I,I'-diphenyldiiodine(III), 2, can be isolated from the reaction mixture above pH 3.The rate of hexanamide rearrangement by PIFA was studied as a function of PIFA concentration and shown to display asymtotic behavior.The rate is depressed by added trifluoroacetate and accelerated by increasing pH, but not in a simple way.These observations can be accounted for by a mechanism (eq 13-15) in which the dimer 2 complexes with the amide, releasing acid.It is this released acid that accounts for most of the kinetically significant observations.The rearrangement of the amide-dimer complex is the rate-limiting step.Other kinetically indistinguishable mechanism are also possible.The rate of rearrangement promoted by dimer alone is in agreement with that predicted by the proposed mechanism.The imidic acid (enol) form of the amide is considered as a possible kinetically active form of the amide but is rejected on kinetic grounds.