Mechanism of the reaction of Lawesson's reagent with N-alkylhydroxamic acids

The mechanism of the reaction under discussion has been established by investigating the products of the reaction between 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide (Lawesson's reagent, LR) and N-alkylhydroxamic acids HAs 1. The primary intermediate is an adduct, O-dithiophosphonylated hydroxamic acid 19, which decomposes to yield metathiophosphonate (AnsPOS), a sulfur atom, and an amide. At the same time, owing to the co-existence of 19 and metadithiophosphonate (AnsPSS) in equilibrium, the carbonyl group is thionated. It has also been established that monomeric AnsPOS formed in both reduction and thionation processes does not undergo oligomerisation to cyclic trimer 5 and linear oligomers, which is typical for amides reacting with LR. Since there is unreacted HA 1 in the reaction mixture, AnsPOS takes part in a controlled transformation to form a dimer, the corresponding pyrothiophosphonate 3, together with the intermediate O-thiophosphonylated hydroxamic acid 2. A hydrolysed product of AnsPOS, namely (4-methoxyphenyl)thiophosphonic acid 4, participates in the last reaction. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)