Mechanistic insights into hydroxy(tosyloxy)iodobenzene-mediated ditosyloxylation of chalcones: a DFT study

In this paper, the mechanism of the hydroxy(tosyloxy)iodobenzene (HTIB)-mediated conversion of chalcones (α,β-unsaturated carbonyl compounds) to ditosyloxy ketones is investigated. Here, at β-carbon of the chalcone, an aryl group with a para-substituent is present. Our study focuses on investigating the effect of different nature of para-substituents on the reaction mechanism. The substituents considered in the study include -OCH₃, -SCH₃, -Cl and -NO₂ groups. For these chalcones, different possible pathways at various steps during the reaction are investigated leading to formation of α,β-ditosyloxy ketones and β,β-ditosyloxy ketones. It is found that the mechanism for the formation of α,β-ditosyloxy ketone involves only electrophilic addition of HTIB, and the mechanism is the same for all studied chalcones, irrespective of whether an electron-donating or electron-withdrawing substituent is present on the aryl ring. However, the detailed mechanism for the formation of β,β-ditosyloxy ketones is different and depends on the nature of the substituent. Broadly, the formation of β,β-ditosyloxy ketones involves electrophilic addition followed by 1,2-aryl migration. Our study shows that the presence of an electron-donating group on the migrating aryl ring favours the formation of β,β-ditosyloxy ketones while in case of electron-withdrawing groups, there are nearly equal chances of the formation of α,β-ditosyloxy ketones and β,β-ditosyloxy ketones.