CYCLOADDITION-ELIMINATION REACTIONS OF 4-METHYL-5-(SUBSTITUTED)IMINO-DELTA-2-1,2,3,4-THIATRIAZOLINES WITH ISOCYANATES

4-Methyl-5-(substituted)imino-1,2,3,4-thiatriazolines 1 (R2 not-equal Me) undergo cycloaddition-elimination reactions with isocyanates to yield 4-methyl-5-(substituted)imino-1,2,4-thiadiazolidine-3-ones 5 via the thermodynamically less stable isomers 4. The latter have not been isolated, except for 4q which was shown to isomerize rapidly into 5q with phenylsulfonyl isocyanate. The reactions of 1 are accelerated by using less bulky R2 substituents and more electrophilic isocyanates, in accordance with the viewpoint that 1 reacts as a masked 1,3-dipole. The products 4i-n (= 5i-n), derived from 1b, add isocyanates reversibly to give 2,3,4,5-tetrahydro-6a-lambda-4-thia-1,3,4,6-tetraazapentalene-2,5-diones 9i-n, which have been isolated and characterized spectroscopically. Such compounds with a hypervalent sulfur atom thus occur as intermediates during the isomerization of 4 to 5 under the influence of isocyanates.