REGIOSELECTIVITY OF THE ADDITION OF GROUP-IVB ORGANOMETALLIC RADICALS TO SUBSTITUTED BENZO[2,1,3]THIADIAZOLES - AN ESR INVESTIGATION

Silyl, germyl, and stannyl radicals have been reacted with a variety of ring substituted benzo[2,1,3]thiadiazoles, and the paramagnetic adducts resulting from addition to one of the heterocyclic nitrogens have been characterized by means of ESR spectroscopy. Reactions with unsymmetrically substituted substrates lead to the formation of two distinct isomeric adducts, whose relative amounts depend on the steric and electronic effects exerted by the substituents. With compounds bearing a substituent in position 5, the amount of each individual isomeric adduct appears to be directly related to the extent of its resonance stabilization. Substituents in position 4 can be divided into two classes: those exerting only steric effects (Me, Cl) direct the addition to the less hindered nitrogen, while substituents like MeO and, to a lesser extent, CN favour radical attack on the closer, although more hindered nitrogen. Upon increase in the temperature a reversal of the relative stability of the two species is sometimes observed.