Deprotonation by sodium hydride of the N-methyl-2-alkylazolium salts lOc, 14-16, and 18 and of the perimidinium salt 20 as well affords the corresponding ketene N,X-acetals l c , 3 -5, 7, and 8 as air-sensitive distillable oils or low-melting crystals. The a-carbon atoms of cyclic ketene N,X-acetals resonate at very high field with the smallest chemical shift of no more than 6 = 39 for the 5methylenedihydrotetrazol 2a. Geometries of l a , b, 2a, and b were optimized on the Hartree-Fock level with polarized double-6 basis sets, followed by IGLO calculations of the NMR chemical shifts. These are in excellent agreement with the unusual carbon-13 shifts observed. The electronic structures of l a , b, 2a, b, 3a, b, 4b, 5b, 6a, and b are investigated by photoelectron spectroscopy and semi-empirical quantum chemical calculations. The observed ionization potentials are assigned to molecular orbitals on the basis of Koopmans' theorem. The order of reactivity in 1,3-dipolar cycloaddition reactions of cyclic ketene N,X-acetals resembles that of the chemical shifts of their a-carbon atoms.Keten-N,X-acetale ohne Akzeptor-Substituenten am a-Kohlenstoff-Atom zeigen eine erhohte nucleophile Reaktivitat, wenn die beiden Heteroatome zu einem Ring verkniipft ~i n d [~-~I .Die dadurch bewirkte Einebnung erlaubt namlich eine besonders effektive Wechselwirkung zwischen den n-Elektronen der Doppelbindung und den einsamen Elektronenpaaren der Heteroatome. Darauf haben zuerst Gruseck und Heuschmann aufmerksam gemacht [lo]. Cyclische Keten-N,N-acetale 2, die sich von Tetrazol ableiten" 'I, reagieren mit organischen Aziden zu sehr unterschiedlichen Prod~kten['~-'~]. Um diese Reaktionen verallgemeinern zu konnen, haben wir aus anderen Heterocyclen cyclische Keten-N,X-acetale hergestellt, woriiber wir hier berichten. Die Ausnahmestellung solcher Keten-N,X-acetale unter Alkenen mit Elektronendonor-Substituenten zeigt sich jetzt auch durch teilweise extreme chemische Verschiebungen in Protonen-und Kohlenstoff-1 3-Spektren sowie durch sehr niedrige Ionisationspotentiale in Photoelektronen-Spektren. Es lag daher nahe zu priifen, ob Zusammenhange zwischen Reaktivitat und spektroskopischen Eigenschaften bestehen. Synthese der cyclischen Keten-N,X-acetaleDie als Vorstufen der cyclischen Keten-N,X-acetale benotigten heterocyclischen Kationen 10, 14 -16, 18 und 20 wurden durch Methylierung der entsprechenden Heterocyclen 9, 11 -13, 17 und 19 hergestellt, soweit sie nicht bereits bekannt waren. Diese wurden nach Standard-Methoden erhalten. Die mit Dimethylsulfat gebildeten Methosulfate, die als o l e anfielen, iiberfiihrte man in gut kristallisierende Tetrafluoroborate (Tab. 1). Die Deprotonierung quartarisierter 2-Alkyl-N-heterocyclen mit starken Basen [2-6~10 -j2] , vorzugsweise Natriumhyd i d , alleine [4%10-121 oder in Gegenwart von Zusatzen wie N,N,N',N'-Tetramethylg~anidin[~~~~], ist die beste und bereits vielfach bewahrte Methode zur Herstellung cyclischer Keten-N,X-acetale. Dabei miissen die Reaktionsbedingungen der Loslichkeit und CH-Aciditat...

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).

Cyclic ketene N,N-acetals derived from imidazolidine (4a, c) and 2,3-dihydrobenzimidazole (6a-c) add methanesulphony1 azide (2a) or picryl azide (2f) to afford the zwitterions 5 and 7, respectively. The structure of 7d is elucidated by X-ray crystallography. Reversibility of formation and thermal stability of the N-sulphonyl zwitterions depend on the substitution pattern at the carbon atom to which the triazenide moiety is attached: In the case of a pair of geminal methyl groups (5a, ?a) formation is irreversible and decomposition by cyclisation and subsequent reactions occurs above -2O"C, while in presence of a single alkyl group (7c, d) these processes require heating to 80°C and are accompanied by partial reversion to 2a and ketene N,N-acetals (6b, c). into N-sulphonylimine 13 and diazo compound 14 or extrude molecular nitrogen to furnish ring-expanded 2-(sulphonylimino)piperazine derivatives (9, 11).According to experimental criteria, the vast majority of 1,3-dipolar cycloaddition reactions proceed by a concerted Ten years ago, the first limiting case of a nonconcerted 1,3-dipolar cycloaddition has been realised by Huisgen and coworkers when they treated (strongly nucleophilic) thiocarbonyl ylides with strongly electrophilic dipolarophiles. The resulting zwitterions were demonstrated to be true intermediates in [3 + 21 cycloaddition reactions"]. More recently, we have disclosed first examples of the second case, which limits the scale of orbital interactions at the opposite side, when electrophilic dipoles, viz. sulphonyl azides 2a-c and nitroaryl azides 2d-f, were allowed to react with strongly nucleophilic dipolarophiles, i.e. alkylidenedihydrotetrazoles (1). Zwitterions 3 arose as primary products, and their thermal stability varied over a wide range[4-61. Conclusive evidence has been adduced for the role of the zwitterions 3a and c as intermediates in twostep 1,3-dipolar cycloaddition~ [~]. We now report that other cyclic ketene NN-acetals, namely alkylidenetetrahydroimidazoles 4 and alkylidenedihydrobenzimidazoles 6, also react with electrophilic azides (2a, f) to afford zwitterions 5, 7 which closely resemble those obtained from alkylidenedihydrotetrazoles 1.Cyclic ketene NN-acetals whose nitrogen atoms are connected by a bridge, like alkylidenedihydvotetrazoles 1, -imidazole~[~I, and -benzimidazoles 6 as well as alkylidenetetrahydroimidazoles 4, possess extremely electron-rich nucleophilic terminal carbon atoms which may be regarded as anionic part of ylidic systems. This property is reflected

The electrophilic azides 2 react with cyclic ketene N,X-acetals of type 7, e.g. 13, 15, 18, 21, 23, 25, and with the alkylidenedihydroindoles 28 as well, to produce, besides molecular nitrogen, ring-expanded products of type 11 and 12, e.g. 14, 16, 19, 22, 24, 26, and 29 (path A), and/or N-sulfonylimines 10, viz.

Triazole derivatives R 0280 5-Alkylamino-1H-1,2,3-triazoles by Base-Mediated Cleavage of Cycloadducts of Azides to Cyclic Ketene N,N-Acetals. -(QUAST*, H.; ACH, M.; HERGENROETHER, T.; REGNAT, D.; Synthesis 2006, 12, 1943-1945; Inst. Org. Chem., Julius-Maximilians-Univ., D-97074 Wuerzburg, Germany; Eng.) -Klein 42-125