a b s t r a c t 2,4-Bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide (Woollins' reagent) reacts with benzoic hydrazides in refluxing toluene to give new symmetrical spiro heterocycles in 48–76% isolated yields. However, under identical conditions, treating Woollins' reagent with thiophene-2-carbohydrazide leads to an additional product, 1,3,4-selenadiazole (27% yield) together with the spiro phosphorus heterocycle in 44% yield. Three representative X-ray structures are reported. Phosphorus-containing heterocycles continue to attract consid-erable attention, in part due to their large variety of interesting phar-macological and biological activities, such as herbicidal, insecticidal, antibacterial, antifungal and anticancer. 1–4 2,4-Bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide [{PhP(Se)(l-Se)} 2 , Woollins' reagent (WR)] has become a very useful selenium source or selenation reagent in synthetic chemistry 5–15 because of its ready preparation and ease of handling. 16 As part of our studies aimed at the reactivity of Woollins' reagent towards different organic substrates, herein, we report the preparation of a series of novel spiro organophosphorus heterocycles via the selenation of carbohy-drazides with Woollins' reagent. Woollins' reagent reacted with 2 equiv of benzohydrazides in refluxing toluene for 4 h. During this period the red suspension gradually disappeared and a pale yellow solution was formed with precipitation of small amounts of grey elemental selenium. Upon cooling to room temperature the solution was passed through a silica gel column. The fractions containing the product were combined and concentrated in vacuum to give the air-and mois-ture-stable compounds 1–4 in 48–76% isolated yields (Scheme 1). Interestingly, treating Woollins' reagent with 2 equiv of thiophene-2-carbohydrazide under identical conditions led to 1,3,4-selenadiazole 5 in 27% yield along with spiro phosphorus heterocycle 6 in 44% yield after work-up in air (Scheme 2). We have previously noted that reaction of 1,2 diacylhydrazines with Woollins' reagent leads to 2,5-disubstituted 1,3,4-selenadiazoles 10 and so the formation of 5 is not surprising. A possible reaction pathway for the formation of compounds 1– 4 and 6 proceeds via two step-wise nucleophilic substitutions in which the selenium atoms in Woollins' reagent are replaced twice by nitrogen and oxygen atoms from the carbohydrazide. The lack of selenation of the carbonyl group suggests that the hydrazide at-tacks phosphorus through nitrogen first, rather than through the carbonyl oxygen. At elevated temperatures WR is believed to be in equilibrium with a diselenaphosphorane (PhPSe 2) intermediate A, which can be considered to be the reactive species in refluxing solution. First, the reaction of A with one molecule of carbohydraz-ide affords intermediate B via a nucleophilic substitution followed by an intramolecular proton transfer. Intermediate C (the tautomer of intermediate B) cyclizes and loses a molecule of H 2 Se to afford D, which undergoes another nucleophilic substitution with a second molecule of carbohydrazide, followed by an intramolecular proton transfer to generate E. Intermediate F (the tautomer of intermedi-ate E) cyclizes by eliminating a molecule of H 2 Se to give stable compounds 1–4 or 6 (Scheme 3). Reaction of Woollins' reagent with one equimolar equivalent of benzohydrazine was performed under identical conditions to investigate if intermediate D could be isolated or was stable. Unfortunately, the same compounds 1–4 and 6 were isolated as the only products with much lower yields (ca. 25–35%) rather than intermediate D. The release of H 2 Se in the above reactions was observed in the bubbler (the formation of dark Se due to the decomposition of the