Cyclic ketene N,X‐acetals 1 are electron‐rich dipolarophiles that undergo 1,3‐dipolar cycloaddition reactions with organic azides 2 ranging from alkyl to strongly electron‐deficient azides, e.g., picryl azide (2L; R1=2,4,6‐(NO2)3C6H2) and sulfonyl azides 2M–O (R1=XSO2; cf. Scheme 1). Reactions of the latter with the most‐nucleophilic ketene N,N‐acetals 1A provided the first examples for two‐step HOMO(dipolarophile)–LUMO(1,3‐dipole)‐controlled 1,3‐dipolar cycloadditions via intermediate zwitterions 3. To set the stage for an exploration of the frontier between concerted and two‐step 1,3‐dipolar cycloadditions of this type, we first describe the scope and limitations of concerted cycloadditions of 2 to 1 and delineate a number of zwitterions 3. Alkyl azides 2A–C add exclusively to ketene N,N‐acetals that are derived from 1H‐tetrazole (see 1A) and 1H‐imidazole (see 1B,C), while almost all aryl azides yield cycloadducts 4 with the ketene N,X‐acetals (X=NR, O, S) employed, except for the case of extreme steric hindrance of the 1,3‐dipole (see 2E; R1=2,4,6‐(tBu)3C6H2). The most electron‐deficient paradigm, 2L, affords zwitterions 16D,E in the reactions with 1A, while ketene N,O‐ and N,S‐acetals furnish products of unstable intermediate cycloadducts. By tuning the electronic and steric demands of aryl azides to those of ketene N,N‐acetals 1A, we discovered new borderlines between concerted and two‐step 1,3‐dipolar cycloadditions that involve similar pairs of dipoles and dipolarophiles: 4‐Nitrophenyl azide (2G) and the 2,2‐dimethylpropylidene dipolarophile 1A (R, R=H, tBu) gave a cycloadduct 13 H, while 2‐nitrophenyl azide (2 H) and the same dipolarophile afforded a zwitterion 16A. Isopropylidene dipolarophile 1A (R=Me) reacted with both 2G and 2 H to afford cycloadducts 13G,J) but furnished a zwitterion 16B with 2,4‐dinitrophenyl azide (2I). Likewise, 1A (R=Me) reacted with the isomeric encumbered nitrophenyl azides 2J and 2K to yield a cycloadduct 13L and a zwitterion 16C, respectively. These examples suggest that, in principle, a host of such borderlines exist which can be crossed by means of small structural variations of the reactants. Eventually, we use 15N‐NMR spectroscopy for the first time to characterize spirocyclic cycloadducts 10–14 and 17 (Table 6), and zwitterions 16 (Table 7).

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RI-R2-H -(CH2)2-Me -(CH2)2-3b, 4b Me -(CH2)3-H -MqC-C&-CMq-H -(CH2)3-H -(CH&-Lithiation of the annulated tetrazoles 6a, c with butyllithium yields the N-lithiotetrazoles 7a, c which are allowed to react with alkyl halides. Alkylation at the a-carbon atom is observed in the reaction with methyl iodide (+ 6b, d), l-bromo-2chloroethane (7c + 14a), and 1,3-dibromopropane (+ 16, 17), while 1,2-dichloro-and 1,2-dibromoethane give other products, viz. 11 -13. Quaternisation of 6 with dimethyl sulphate affords mixtures of 1-methyl-(1) and 2-methyltetrazolium salts (8) (3:l-4:l) from which the hexafluorophosphates 1 .PF6 are obtained by crystallisation. Methyl triflate converts the a-azidonitriles 9 into the N-methylnitrilium triflates 10 which immediately undergo an intramolecular 1,3-dipolar cycloaddition to afford the 1-methyltetrazolium triflates 1 . F3CS03. Cyclisation of 16 by intramolecular N-alkylation furnishes the bisannulated tetrazolium bromide 3a . Br. Attempts to obtain the lower homologue 15, either from 14a or from 17, met with failure. Instead, 17 rearranges via 15b into 14b. The a-branched tetrazole 24 is synthesised from ethyl cyanoacetate and the bromide 18. Double cyclisation of 24 affords the bisannulated tetrazolium chloride 3b . C1. The analogous scheme, envisaged for the synthesis of the lower homologue 32, failed in the last step owing to the high strain of this system. Deprotonation of 1,4-substituted 5-alkyltetrazolium salts affords 5-alkylidenedihydrotetra~oles[~~~] which undergo a number of interesting reactions. Photoextrusion of molecular nitrogen yields diastereoselectively iminoaziridines in the E configuration[*]. Iminoaziridines are also formed on thermolysis, besides products derived from cycloadducts of fragments like methyl azide or 1,3-diazabutadienes to the exocyclic double bond of 5-alkylidenedihydrotetrazole~[~]. This double bond is extremely electron-rich and polari~ed[~] and hence even adds normally unreactive 1,3-dipoles such as alkyl azidesL6]. Electrophilic azides afford zwitterions, that are intermediates of non-concerted 1,3-dipolar cycloaddition reactions['], or 5-imino-l,4,5,6-tetrahydro-l,2,3,4tetrazines via [3 + 21 cycloaddition followed by extrusion of molecular nitrogen with concomitant ring expansion[*]. With the view of exploiting and extending the rich chemistry of 5-alkylidenedihydrotetrazoles, we embarked on the synthesis of a number of annulated tetrazolium salts of type 1 and 3 which yield the annulated alkylidenedihydrotetrazoles 2 and 4, respectively, on deprotonation[']. Three different routes were envisaged: Alkylation of known annulated tetrazoles, cyclisation by intramolecular N-alkylation, and intramolecular [3 + 21 cycloaddition of o-azidonitrilium salts. The results are reported here. . . N=N X-N=N 1 2 3 2 6 H 7 R -7 G Q 4 R a R \ I N=N \ I N=N X-2 1 3 4Annulated Tetrazoles f114,1s1) and hydrogenation of tetrazolo[ 1,5-a]pyridine The annulated tetrazoles 6a, c, and f, which were needed ( 6~' '~~) .The tetramethyl compound 6e was now obtained as starti...

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5-imino-1,4,5,6-tetrahydro-/ Nitrogen 1,2-shift / Ring expansion 5-1mino-1.4.5.6-tetrahydro-1.2.3.4-tetrazines Formed from Azides ') The electrophilic azides 1 react with the 5-alkylidenedihydrotetrazoles 8 already at low temperatures to produce high yields of the novel 5-iminotetrahydro-l,2,3,4-tetrazines 10 besides molecular nitrogen. The configurations of (4-lOa, (E)-lOc, (4- Die Alkylidendihydrotetrazole 8 konnen als Endiamine (Ketenaminale, Keten-N,N-acetale) betrachtet werden und besitzen eine besonders elektronenreiche C = C-Bindung'). Sie addieren glatt Alkylazide",") und Arylazide'.") zu den Spirocyclen 9. Wir berichten hier iiber die Umsetzung von 8 mit Sulfonyl-(la12), 1 b12), 1 c) und Phosphorylaziden (1 e,f) 5-Alkylidene-4,5-dihydro-lH-tetrazoles and Electrophilic pounds 9. Ring opening of their dihydro-l,2,3-triazole ring generates the zwitterions 11 which lose molecular nitrogen with concomitant ring expansion of the dihydrotetrazole ring by a nitrogen 1,2-shift. The tetrazines 1Oa-c and e are also obtained when 8a is generated from the tetrazolium salt 7a and trapped "in situ" by the azides 1 a -c and e.