Mild synthesis of mercaptonitriles from vinyl nitriles and their cyclization reactions
Thiol–ene addition of thioacetic acid A is widely used in the synthesis of thiols from vinyl precursors, but so far has not been conducted on non-conjugated vinyl nitriles. The challenge when vinyl nitriles are used is to selectively conduct the thiol–ene addition, while avoiding the nucleophilic addition of A to the nitrile group. We have found that vinyl nitriles give selective UV-induced thiol–ene addition in the presence of photoinitiators as long as a stoichiometric amount of A to the vinyl group and sterically unhindered vinyls are used. In contrast, when a sterically hindered vinyl is used, the nucleophilic addition of the nitrile is favoured. The prepared mercaptonitriles can easily undergo cyclization reactions in basic and acidic conditions as well as in the presence of silica gel. This illustrates the high reactivity of nitriles towards thiol addition. 1,2-Ethanedithiol B is presented as an alternative reagent to A as it allows conversion of vinyl nitriles directly into mercaptonitriles under mild and non-acidic reaction conditions.