a,a -Dichloroazo compounds 9 react with antimony pentachloride to furnish 1-(chloroalkyl)-l-aza-2-azoniaallene salts 10 as reactive intermediates, which are intercepted as 3H-4,5-dihydropyrazolium salts 11 by reaction with electron-rich alkenes. Hydrolysis of salts 11 affords 3H-4,5-dihydropyrazoles 12. 1-Aza-2-azoniaallene salts 1 react with cyclic azoalkanes 12 to produce the N-(azoalkyl)diazenium salts 14 or the formazanium salts 15 depending on the substituents of the starting materials. The structure of 15c is secured by X-ray structural analysis.Recently, we reported the preparation and reactions of 1aza-2-azoniaallene salts 1, a new class of heterocumulenes. Cations 1 were found to undergo cycloadditions to many types of multiple bonds (alkenes, alkynes, nitriles, isocyanates, carbodiimides) affording heterocyclic salts 2, which in most cases rearrange spontaneously to salts 3 and/or isomers (Scheme 1). 1-6 Experimental and theoretical arguments had been advanced suggesting that cycloadditions of 1 to nitriles, isocyanates, as well as to carbodiimides are two-step reactions via nitrilium, and acylium intermediates, respectively. However, cycloadditions of 1-aza-2-azoniaallene cations 1 to C=C double bonds were found to proceed with complete conservation of the configuration of the alkenes. These reactions are probably concerted "1,3-dipolar cycloadditions with reverse electron demand" with 1 serving as a cationic threecenter, four-electron component (1,3-dipole) and the electron-rich alkene as dipolarophile. 3 Similarly, reactions of 1 with alkynes seem to be single-step reactions. 4 Little if any seems to be known about cationic organic 1,3-dipoles. 7Cycloadditions of 1 suffer from the disadvantage that one ends up with salts. For applications, electrically neutral heterocycles would be more desirable. In order to circumvent this problem we tried to prepare heterocumulenes 1 substituted with a leaving group R 3 (e.g. CO 2 Et), which can finally be removed from the cycloadducts 2 and 3. For instance, nitriles can be added to heterocumulenes 4 to afford the carbamates 6, which can be hydrolyzed under mild basic conditions to the triazoles 7. 3 However, it turned out that the reactivity of 4 is rather low, obviously due to the equilibrium 4 s 5. 8,9 Thus cycloadditions of 4 could neither be induced with alkenes nor with alkynes or isocyanates.Better reactivity is observed for heterocumulenes 1 with R 3 = tert-butyl. During or after formation of the cycloadducts 2 and 3 the tert-butyl group is eliminated as isobutene, leaving back a protonated heterocycle, from which the neutral compound can be obtained with base. 3 However, in some cases the elimination of isobutene was difficult to achieve. 5 In other cases preparation of the starting tert-butyl hydrazones proved to be unsatisfactory.In search of 1-aza-2-azoniaallene salts 1 with a more suitable leaving group R 3 , we came across a,a¢-dichloroazo compounds 9 (Scheme 2). Compounds 9 are prepared almost quantitatively by chlorination of azines 8 under mild co...
