2,4-Bisimino-1,3-diazetidines: iminophosphoranes, carbodiimides and related betaines

Abstract

Iminophosphoranes 2 and 3 derived from 4-amino-6-methyl-3-methylthio-4,5-dihydro[1,2,4]triazin-5-one react with primary isocyanates or isothiocyanates to give the betaines 8 and 9, respectively; however, the reaction with isopropyl and tert-butyl isocyanate leads to the corresponding carbodiimides 6. Thermal treatment of betaines 8 affords diazetidines 10.

The crystal structures of betaines 8d and 8i have been determined. The relationship between the rotation about the [P N]⁺bond and the hybridization of the nitrogen is perturbated by the intramolecular phenyl–phenyl ‘stacking type’ interaction in 8d and by the steric hindrance of the isopropyl substituent in 8i. This steric effect precludes the bipyramidalization of P(1) in 8i, which loses the intermediate character between the open-chain betaine and the ring-chain 1,3,2-diazaphosphetidine, present in the other betaines. In contrast, 8d has the lowest P [graphic omitted] N/P–N ratio found. A careful ¹H and ¹³C NMR study of betaines and diazetidines reveals that in betaines 9 carrying a PPh₂Me substituent, the phenyl groups are diastereotopic and present differences in the ¹³C–³¹P coupling constants and that in diazetidines 10 the major isomer is E,E when the 1,3-substituent is either a benzyl or an isopropyl group. We propose, tentatively, how the title compounds are mechanistically related which includes a possible formation of diazetidines different from carbodiimide dimerization, the experimental evidence that betaines result from the reaction of carbodiimides with iminophosphoranes and that this last reaction is reversible.