Acetamidinosulphenylation of alkynes via electrophilic addition of 4′-substituted benzenesulphenanilides in acetonitrile
A number of 4′-substituted benzenesulphenanilides and N-methyl-4′-nitrobenzenesulphenanilide are shown to add to simple alkynes in acetonitrile in the presence of boron trifluoride–diethyl ether to give acetamidinovinyl sulphides in varying yields. Addition is largely favoured by dialkyl substitution on the alkyne and by electron-attracting anilide substituents. Moreover, the azasulphenylation adducts are produced with trans-stereospecificity and high regioselectivity (Markovnikov orientation). A likely mechanism involves nucleophilic displacement by the alkyne at the activated sulphur of the anilide complexed with boron trifluoride leading to the initial formation of a thiirenium ion intermediate. Subsequent back-side attack of acetonitrile on the thiirenium ion gives a nitrilium ion which is ultimately trapped by the liberated arylamine.