New syntheses of aryl isothiocyanates

Primary aromatic amines are readily converted into arylimino-1,2,3-dithiazoles 2 and the derived cyanothioformanilides 6, both of which are rapidly cleaved by ethylmagnesium bromide in hot THF to give the corresponding isothiocyanates. The transformation 2-->6-->ArNCS can be performed as a 'one-pot' operation. The imines 2 are also converted, more slowly, into the isothiocyanates by sodium hydride in hot THF, via the cyanothioformanilides 6. Conversion of the anilides 6 into isothiocyanates is much faster under microwave irradiation in 2,6-lutidine. Mechanisms are proposed for these reactions.